The immune system helps our bodies fight pathogens, control cancer growth, and stay healthy. If it is not properly controlled, the immune system can also cause disease. Starting with a basic review of the immune cells, this session provides a general overview of the immune system and its activation. It includes a comparison of the innate and adaptive immune systems, and a characterization of T cell activation via MHC-I and MHC-II molecules.
Note: Access an infographic summary of the immune system here.
00:00:07.12 The immune system is responsible
00:00:09.06 for fighting infection and disease.
00:00:10.29 It is comprised of many specialized cell types,
00:00:13.16 all which work together to keep your body healthy.
00:00:16.24 In this short video,
00:00:18.11 you will be introduced to the major cellular players of the immune system.
00:00:23.10 Let’s start by introducing the two major arms of the immune system:
00:00:26.00 innate and adaptive immunity.
00:00:28.25 Innate immunity serves as the first line of defense
00:00:31.14 and is a more general immune response.
00:00:33.27 You might recognize some of the classic symptoms of innate immunity,
00:00:36.24 such as fever and inflammation.
00:00:39.09 Adaptive immunity allows for very specific detection
00:00:42.16 and elimination of pathogens.
00:00:45.04 However, it takes a longer time to ramp up than innate immunity.
00:00:48.14 Importantly, the adaptive immune system provides immunological memory --
00:00:52.23 the ability of our immune cells to remember previous infections and clear them more quickly in the future.
00:01:00.17 All immune cells develop from a single pluripotent cell
00:01:03.07 in the bone marrow,
00:01:04.20 the hematopoietic stem cell.
00:01:06.07 Hematopoietic stem cells give rise to
00:01:08.09 lymphoid and myeloid progenitors,
00:01:09.24 each of which differentiate into a range of immune cell types.
00:01:13.27 We refer to the cells on the top as the lymphoid lineage,
00:01:16.18 whereas the cells on the bottom are classified as the myeloid lineage.
00:01:20.27 The myeloid progenitors also give rise to red blood cells and platelets,
00:01:24.09 although we won’t focus on those here.
00:01:27.25 Let’s now look closely at the lymphoid lineage.
00:01:31.08 The lymphoid progenitor differentiates into three cell classes.
00:01:35.09 We’ll start by introducing B cells.
00:01:38.02 Upon activation, mature B cells differentiate into memory cells or plasma cells.
00:01:43.09 Plasma cells are the immune cells that are responsible for secreting antibodies,
00:01:47.10 an important component of adaptive immunity.
00:01:52.05 Natural killer cells are cytotoxic cells of the innate immune system.
00:01:56.10 They detect virus-infected cells and kill them.
00:02:00.29 T cells arise from a common progenitor.
00:02:03.22 There are many types of mature T cells,
00:02:05.25 but the best known ones -- those which we’ll cover here -- include:
00:02:10.08 memory T cells, cytotoxic T cells,
00:02:12.11 and helper T cells.
00:02:14.22 Like memory B cells,
00:02:16.15 a fraction of mature T cells remain in the body as memory T cells
00:02:19.09 to help the body mount a faster immune response in future infections.
00:02:24.01 Cytotoxic T cells recognize antigen,
00:02:26.19 or small pathogen-derived particles,
00:02:28.28 on infected cells and kill them in a pathogen-specific manner.
00:02:32.25 Lastly, upon activation by specialized cells in the body,
00:02:37.18 helper T cells secrete cytokines that boost the adaptive immune response.
00:02:41.18 For example, helper T cells play an important role in B cell activation.
00:02:47.28 Now let’s take a look at the myeloid lineage.
00:02:52.01 The myeloid lineage produces most cells of the innate immune system
00:02:54.29 as well as important antigen-presenting cells that prime the adaptive immune response.
00:03:00.25 The myeloid progenitor gives rise to neutrophils,
00:03:03.22 which are innate immune cells that specialize in the capture and killing of microorganisms
00:03:07.27 throughout the body;
00:03:09.20 eosinophils, which are a type of granulocyte
00:03:12.10 that releases cytokines to defend against parasites;
00:03:15.24 and monocytes, which further differentiate into dendritic cells and macrophages.
00:03:21.02 Dendritic cells are a specialized type of phagocytic cell
00:03:24.10 that bridges innate and adaptive immunity.
00:03:27.14 Macrophages are tissue resident phagocytic cells.
00:03:30.21 They patrol the body and assist in cleaning up infection
00:03:33.12 and activating other immune cells.
00:03:37.08 Other cells that arise from the myeloid progenitor
00:03:40.06 include mast cells, another type of granulocyte that are implicated in allergy;
00:03:45.02 as well as basophils,
00:03:47.11 which are a less well-understood cell type that is involved in the immune response to parasites.
00:03:54.06 So how are immune cells activated?
00:03:56.28 Here, we’ll briefly cover the molecular paradigms
00:03:59.29 for innate and adaptive immune activation.
00:04:02.16 Cells of the innate immune system
00:04:05.13 express molecules known as pattern recognition receptors at their surface.
00:04:09.07 These receptors bind pathogens or parts of pathogens,
00:04:12.04 which induce intracellular signals
00:04:14.29 that activate an innate immune response.
00:04:18.17 The particles recognized by these receptors
00:04:20.20 are common amongst pathogens.
00:04:23.14 To activate adaptive immunity,
00:04:25.12 cells present antigen
00:04:28.06 -- small peptide fragments of pathogens --
00:04:30.01 to T cells to inform a specific immune response.
00:04:33.01 Antigen is presented by two types of surface molecules:
00:04:36.22 MHC class I and MHC class II.
00:04:39.27 MHC class I molecules are expressed by all cells in the body
00:04:43.24 and are used in defense against intracellular pathogens such as viruses.
00:04:49.00 They do this by presenting endogenous, or intracellular, antigens
00:04:53.18 to cytotoxic T cells.
00:04:56.02 MHC class II molecules present exogenous antigen,
00:04:59.02 which is antigen found on pathogens outside of cells,
00:05:02.09 and activate helper T cells.
00:05:05.21 MHC class II molecules are expressed
00:05:07.14 by what are known as professional antigen-presenting cells,
00:05:11.27 which include dendritic cells, macrophages, and B cells.
00:05:15.02 There are safeguards in place to ensure that the immune system
00:05:18.01 isn’t activated against self antigens.
00:05:23.15 The mammalian immune system
00:05:25.09 is comprised of a complex set of cells
00:05:27.04 that contribute to innate and adaptive immunity.
00:05:30.16 The foundational understanding you’ve gained from this video
00:05:32.26 serves as a starting point for a deeper look at this topic.
00:00:06.19 Hi, my name is Ira Mellman. I'm a scientist at Genentech,
00:00:10.02 which is a biotechnology company here in San Francisco.
00:00:12.24 I study cancer, but I also study the immune system, so I'm happy to be here today
00:00:17.00 to tell you something about the immune system, what it is, and how it works.
00:00:20.11 It is really not as complicated as a lot of us fear.
00:00:23.16 Now, what is the function of the immune system?
00:00:26.05 Now, it turns out that in our daily lives we are surrounded by a wide variety of pathogens.
00:00:32.00 Bacteria, viruses, other little organisms that cause an almost limitless number of infections in us,
00:00:38.26 in humans, and we need to be protected against these environmental pathogens,
00:00:42.16 and toxins that they make, on an almost continuous basis. Every time we breath, we take in bacteria or viruses,
00:00:49.15 every time we drink or eat something,
00:00:51.29 also, we have the potential of taking in viruses and bacteria
00:00:56.01 and obviously we don't get infected and sick as a consequence of just daily life on a routine basis.
00:01:01.11 And the reason that happens is because we have immune systems to protect us
00:01:04.22 against all of these pathogens and toxins.
00:01:07.27 Now, the immune system has to work in a very special way though.
00:01:10.24 Although it's very powerful at being able to rid us of all sorts of noxious organisms,
00:01:15.16 it has to be able to understand who is foreign, in other words, who are the pathogens
00:01:20.13 that are trying to infect us, and distinguish those pathogens from our own cells.
00:01:24.15 So, all of this has to occur, without avoiding injury to the host, which is us.
00:01:29.04 To do that, what the immune system has to do, and this is the complicated part,
00:01:34.13 is to distinguish self from non-self, in other words, distinguish the invaders from that which it is trying to protect,
00:01:42.15 which, again, is us, so this is a very, very important feature of the immune system,
00:01:47.01 and means that it's actually really highly specialized, and in many ways, really smart
00:01:51.24 to be able to tell the differences, and I'll touch on some of the ways in which the immune system does this.
00:01:56.28 Now, it turns out that all multicellular organisms have immune systems,
00:02:00.16 whether they are animals, humans, fish, insects or plants.
00:02:05.16 The degree of complexity of the immune system, as you go through different types of organisms
00:02:10.29 that you find on earth, is entirely different, where, you know, humans and dogs and cats
00:02:17.07 have much more complicated ways protecting themselves than do plants and insects,
00:02:23.00 but nevertheless, the same basic principle is conserved throughout evolution,
00:02:27.12 and in fact, you can find even the most primitive types of immune responses, and immune systems,
00:02:32.19 that one can see in even organisms as simple as insects,
00:02:37.04 and also playing a very, very important role in protecting us against the very same pathogens that insects have to be protected against.
00:02:43.19 Now, as I said, immune response is complicated,
00:02:47.18 yep, that's true, but it's really not that much more complicated than anything else in biology
00:02:51.25 and if you break it down in terms of the cells that are involved, that actually make it work,
00:02:57.13 it's a lot easier to understand.
00:02:59.18 Now, it is important to understand, not only because, as I already told you,
00:03:04.02 on a day-to-day basis we have to protect ourselves against invading pathogens,
00:03:07.15 but also, many, many important diseases, such as infectious disease, as we've already been discussing,
00:03:13.07 or AIDS, or asthma or lupus, autoimmune disorders, or even cancer, and various allergies,
00:03:19.10 are caused by breakdowns, or at least are aided by breakdowns of the immune system
00:03:24.17 and as a result, in order to really understand the basis for these diseases
00:03:29.29 and understand their biology, we have to know something about how the immune system works.
00:03:34.22 So, what is the immune system anyway?
00:03:38.04 It's very, very simple, in very, very simple terms, it's a system of specialized cell types
00:03:43.16 that are mostly derived from the bone marrow. I'm sure all of you know what the bone marrow is,
00:03:47.21 especially in long bones, such as shown here,
00:03:49.26 you find a factory of cells that produce many, many types of cells that are found throughout the body.
00:03:57.11 These are called, in the first instance, the most popular and populous of them are called lymphocytes,
00:04:02.22 which come in two basic flavors, T cells and B cells.
00:04:06.20 There are also cells known as macrophages, and dendritic cells and monocytes
00:04:11.06 and K cells, granulocytes, each one of them has a very, very specific function
00:04:15.22 in helping us to protect ourselves against invading pathogens.
00:04:19.22 All of these cells together are called the immune system
00:04:24.00 and they're found all over the place.
00:04:26.10 So, they may form themselves in the bone marrow, but they then span out through the blood
00:04:31.15 and into virtually every tissue that we find in the body.
00:04:34.24 Here, what you're looking at is a picture of skin, which has one type of immune cell in it, called a dendritic cell,
00:04:41.18 which basically sits in the skin waiting for invading pathogens to come,
00:04:45.14 for example, after you cut your finger or something like that,
00:04:49.12 a bacteria will enter into the cut and that bacteria will be detected and recognized by the dendritic cells.
00:04:55.21 The dendritic cells, as I'll show you in a minute, will leave the skin
00:04:59.05 and then migrate elsewhere, through a system of little tubes and vessels, totally separate from your blood vessels,
00:05:06.09 but basically doing the same sort of thing.
00:05:08.14 And this is called the lymphatic system, or lymphatic vessels.
00:05:12.20 These are small tubes, as I said, that are found literally everywhere,
00:05:18.15 lymphoid cells enter into the lymphatics, and start migrating.
00:05:24.08 Where do they go? They migrate and then congregate in a series of small lymphoid organs,
00:05:30.07 well, some of them are not so small, like the spleen, which is found in your abdomen,
00:05:33.19 but the small ones, and in fact probably the most important ones,
00:05:37.11 are called lymph nodes. Now, lymph nodes are little aggregates, and I'll show you this in a minute,
00:05:42.11 of cells of the immune system that have very, very special functions
00:05:47.00 and very, very important activities to perform, but you can find them on yourself, throughout your body,
00:05:52.23 on your arms, legs, chest, everywhere else, but probably most typically,
00:05:57.25 if you just feel on your neck, particularly after you have any type of an infection,
00:06:02.29 such as a sore throat, you can find these little bumps that are there.
00:06:06.26 And I'm sure you probably, many of you know that these are lymph nodes,
00:06:10.11 but this is basically what they do, they're just not there sitting there, they're actually all connected together
00:06:15.13 in a very complex system that is again totally parallel to what's going on in the blood.
00:06:20.12 So, they really exist only to transport cells of the immune system from your peripheral tissues,
00:06:28.14 from your fingers, back to the lymph nodes where they can find other types of immune cells.
00:06:35.12 Now, this is a blow-up of what actually happens in a lymph node.
00:06:40.29 Precisely what happens here at the moment is not really important,
00:06:43.20 but I want you to look at is the fact that you see a great and high concentration
00:06:49.01 of all different types of cells of the immune system, particularly lymphocytes and dendritic cells,
00:06:54.01 that all are talking to each other and basically what they do
00:06:57.16 is they are communicating and trading information on what they encountered in the tissues:
00:07:04.07 what types of pathogens were there and decide among themselves essentially what to do about them.
00:07:10.22 So, these are centralized processing centers in which information about invading pathogens
00:07:18.05 is communicated to different cells that have different functions than actually mounting a protective response
00:07:24.28 against a particular pathogen type.
00:07:27.18 Now, the immune system system consists of two interconnected arms
00:07:31.22 that we call the innate immune system and the adaptive immune system.
00:07:36.01 Innate immunity is the most evolutionarily ancient and is found in insects,
00:07:42.27 in fact, it was first found in insects,
00:07:45.07 and is responsible for detecting components that are shared by virtually all pathogens.
00:07:51.06 So, although there are many, many different types of bacteria,
00:07:53.25 they do have a lot of very fundamental things in common,
00:07:56.24 and the innate immune system evolved to be able to recognize those things that are fundamentally the same
00:08:01.15 from one bacteria to another or from one virus to another or one protozoan parasite to another.
00:08:08.25 Regardless of what their species actually is.
00:08:12.13 The adaptive immune system, though, recognizes those things that are really very specific
00:08:17.08 and very special to an individual pathogen and requires a greatly more amplified
00:08:23.26 and complex series of events to take place at the cellular level in order to allow that type of specificity to take place.
00:08:31.08 These two systems work very, very closely with each other, hand in glove,
00:08:36.04 and how they connect one to the next has only recently been worked out
00:08:40.27 and it turns out to reveal yet another fundamental part of the immune system
00:08:45.12 which is the missing link between the adaptive and the innate immune responses
00:08:49.24 and we'll get to that just in a minute.
00:08:51.02 But just to give you a hint, that's what dendritic cells do.
00:08:55.07 Now, our understanding of how the immune system works really dates back into the 19th century,
00:09:02.16 really to the work of two scientists, the first of whom is shown here, this is Ellie Metchnikoff,
00:09:08.22 who really made a very, very important conceptual understanding,
00:09:12.11 which is that when you see an infection, particularly infection that would occur in the skin,
00:09:17.14 the swelling and the redness and the heat and the pain, and all of this stuff that occurs,
00:09:22.25 that we know is characteristic of infections, is not really an indication of tissue being destroyed,
00:09:29.05 but actually is an indication of the immune system, particularly the innate immune system,
00:09:34.08 trying to do its job, and trying to kill of the invading bacteria.
00:09:38.00 One of the cells, and perhaps the most important one, that Metchnikoff found is the cell called a macrophage
00:09:44.04 and this is one of his original drawings showing what a macrophage looks like.
00:09:48.02 You can see a cell with these long tentacles hanging off the ends of it,
00:09:53.03 plus a lot of structures inside the cell, some of which are colored red,
00:09:57.19 which are actually the intracellular destructive bodies
00:10:00.14 that are to a very large extent responsible for killing and then destroying invading bacteria.
00:10:07.19 And all of this Metchnikoff found out just with very, very crude microscopes
00:10:11.04 and very, very simple techniques, in fact, probably a lot simpler than what's available now
00:10:19.02 in even junior high schools and high schools everywhere,
00:10:22.23 he was still able to work this out.
00:10:24.20 As I said, microorganisms such as bacteria are recognized by macrophages
00:10:29.28 and they are killed, they are taken up, they're ingested by macrophages
00:10:33.14 and this process all enhances the protective immune aspect of what macrophages do
00:10:41.20 and starts the process of inflammation,
00:10:44.15 and inflammation is really the process whereby one cell, after detecting a pathogen,
00:10:49.23 signals to its neighbors that there's something going on, and as a consequence, more cells come in to help out.
00:10:56.25 Part of the reason that inflammation takes place
00:11:00.16 is because the act of killing bacteria involves a lot of agents that really serve as signals
00:11:07.10 that attract other cells of the immune system.
00:11:09.28 So, I've listed some of the more important ones here,
00:11:12.02 cytotoxic agents occur, so that macrophages can make large amounts of hydrogen peroxide
00:11:17.18 which actually is quite effective at killing most bacteria.
00:11:20.23 They secrete enzymes or expose the bacteria to enzymes that will degrade the bacteria themselves,
00:11:29.05 and then also, finally, they will release these inflammatory components,
00:11:33.19 which in the business we call cytokines, that are small proteins or hormones
00:11:39.06 that are released by the macrophage, rather, that attract other immune cells to the site.
00:11:44.29 Now, why is it that macrophages and in fact other similar immune cells are able to do all of this?
00:11:51.23 How can they even know when a bacterium is present,
00:11:54.28 so as not to turn on all of this inflammatory activity when only normal host cells are around?
00:12:01.29 Well, that's because they have a series of receptors
00:12:05.02 on the surface that are really very specialized for being able to understand when bacteria are present.
00:12:12.13 These are called Toll-like receptors which, rather interestingly,
00:12:17.10 were first discovered not because they had anything to do with bacteria,
00:12:20.18 but because they had something very important to do with the earliest stages of fruit fly development.
00:12:26.21 And so these are some images of just what a developing embryo of Drosophila looks like
00:12:32.09 that has mutations in Toll receptors and you can see a normal one in panel A, up on the left,
00:12:39.29 showing what a normal embryo should look like at this stage,
00:12:42.22 and here down in the lower right, you can see a mutant embryo that is not able to form
00:12:47.20 because it's absent of this Toll receptor.
00:12:50.26 But by performing genetic tricks, you can actually get these embryos to form real flies
00:12:56.26 and these real flies are defective in Toll receptors and what you see when those real flies grow up
00:13:02.07 is something like this.
00:13:03.13 So, here, you have a Drosophila blown up at a very high magnification using something called a scanning electron microscope,
00:13:12.25 and Drosophila don't normally have hair, or beards, in quite this way,
00:13:18.21 what you're looking at here is a very serious fungal infection that this fruit fly could not fend off against
00:13:29.29 because the fly is defective in one of these Toll receptors.
00:13:34.16 This is one of the earliest indications that Toll receptors are important in fly
00:13:41.10 for being able to protect them against fungal infections, and indeed, as it turns out, other infections as well.
00:13:47.02 Turns out that Toll receptors, this is just a molecular diagram of what they look like,
00:13:51.20 the details are not important, these are the ones that are found in fly,
00:13:55.14 which is D. melanogaster is Drosophila melanogaster;
00:14:00.02 these are the similar ones that are found in human.
00:14:01.27 And as soon as they were found, just by scanning the genome,
00:14:05.24 it became very, very clear that maybe they were performing a very similar type of function,
00:14:13.20 and in fact, a series of laboratory studies over the years, all fairly recent, by the way,
00:14:18.17 has really illustrated that these Toll-like receptors that one finds in humans and mice and dogs and cats
00:14:26.04 and everything else, really perform not so much of an important function in development,
00:14:32.12 of the human embryo or fetus, but rather in protecting the human adult,
00:14:39.10 and in fact all animals, against bacteria and indeed, all sorts of pathogens.
00:14:44.09 How, again, by serving as the sensor that macrophages and other similar cells in the immune system use
00:14:50.29 in order to detect when a bacterium is present and in order to allow those cells to know when to activate the immune response,
00:14:59.13 the innate immune response, and when to activate inflammation.
00:15:04.13 Now, again, the way these work is that there are many, many different types of Toll receptors,
00:15:09.13 maybe as many as 14 of them now, and they as a consequence of being so many,
00:15:14.22 they really can bracket the entire universe of different types of pathogens and bacteria.
00:15:19.17 I'm showing here a bacterium is releasing a portion of its cell wall,
00:15:24.16 which it can't avoid doing, and there's specific Toll-like receptors that can actually detect those components,
00:15:30.16 things with names like LPS, or lipopolysaccharide.
00:15:34.03 So, these activate the receptors, which then turn on a typical signaling cascade,
00:15:39.14 again, the details of this are not important, but I just wanted to indicate that something that happens outside the cell
00:15:46.00 stimulates a Toll-like receptor, that then generates an event that occurs inside the cell
00:15:51.20 that basically then licenses the cell, in this case a macrophage, to start the inflammatory process
00:15:57.12 and start protection.
00:15:59.06 Now, here what you're looking at is macrophages in action.
00:16:02.07 These are cells that detect bacteria; actually, what they're detecting is a small yeast particle,
00:16:08.17 such as there, and what you can see very quickly, that macrophage, as soon as I pointed at it,
00:16:12.25 came and ate it. The reason it was able to do that was because there are molecules that yeast make,
00:16:19.25 like bacteria, that can bind to specific Toll-like receptors present on the macrophage
00:16:24.25 and track the macrophage towards, in this case, the yeast particle,
00:16:28.21 and then the macrophage both eats and kills the yeast particle at the same time.
00:16:34.02 Now, what you're not seeing in that movie is that the macrophage also was releasing all sorts of cytokines
00:16:40.20 that would stimulate the surrounding macrophages,
00:16:43.18 but nevertheless, it's a pretty clear indication of exactly what happens.
00:16:47.08 I'd like to show you one more video of just what this process looks like in a bit more resolution.
00:16:52.17 So, here you're looking at 3 macrophages that have been stained with fluorescent dyes,
00:16:57.09 the green dye stains the surface of the plasma membrane with the macrophage
00:17:01.05 and the red dye stains these intracellular digestive elements,
00:17:04.23 exactly the same ones the Metchnikoff colored red in his early diagrams,
00:17:08.06 a hundred or so years earlier.
00:17:10.02 These are the lysozomes, which are responsible for both killing and digesting the bacteria that are being eaten.
00:17:17.09 So, the cell on your left, you can see this little crescent shaped thing there,
00:17:21.21 that's where a particle has bound and I just want you to watch what happens
00:17:27.11 when that particle is internalized, you'll see it first surrounded by green,
00:17:31.28 then rapidly, the green turns red.
00:17:34.08 So, what that means is the bacterium is eaten in a piece of membrane that is derived from the cell surface of the macrophage
00:17:42.12 and that intracellular vacuole then fuses, physically, coalesces with these lysozomes,
00:17:49.23 exposing the internalized bacterium to the cytotoxic and digestive enzymes that are found within the lysozomes
00:17:56.16 that are responsible for killing the internalized bacteria.
00:17:59.06 Ok, just to summarize: innate immunity, discovered by Ellie Metchnikoff, he got a Nobel prize for this work in 1908,
00:18:06.18 and what innate immunity does, is to, remember, to recognize shared pathogen derived components
00:18:13.00 that are recognized by Toll-like receptors, and related receptors, but the most important ones for today are Toll-like receptors.
00:18:19.15 The main cell of the innate immune system are phagocytes,
00:18:24.08 are the cells that eat the bacteria that are recognized.
00:18:27.24 These are cells such as macrophages, and another cell type closely related that we didn't talk about, called neutrophils.
00:18:33.18 These are the cells that are the main effectors, as we say, of the innate immune response
00:18:38.17 by protecting us against bacteria and other types of organisms by eating them and killing them.
00:18:45.11 Ok, so let's go on to the next system.
00:18:48.25 This is adaptive immunity, or the adaptive immune system, which was really discovered about the same time
00:18:55.16 as Metchnikoff discovered the innate immune system.
00:18:58.11 The individual really responsible for this was Paul Ehrlich, who also won a Nobel prize for his work, together with Metchnikoff, in 1908.
00:19:06.15 Here he is in his office. And what Ehrlich found was that when you immunize people
00:19:13.25 with foreign proteins, such as a bacterial toxin, or a protein from a cow or a sheep,
00:19:19.06 those individuals make what Ehrlich called protective antibodies in the blood.
00:19:25.01 And you could actually confer protection from individual to the next.
00:19:29.12 These antibodies were very specific to the individual pathogen or the individual protein that was added
00:19:36.22 and, unlike the innate immune system, you just, any protein wouldn't do, but had to be a very specific protein
00:19:46.00 that would be recognized by a different antibody
00:19:48.23 and finally, also, in this realm, although this really was beyond what Ehrlich himself did,
00:19:55.04 it turns out that the antibodies are not made by the macrophages,
00:20:00.05 but they're rather made by antigen specific lymphocytes,
00:20:03.19 which were the other major cell type that I told you about at the very beginning that really comprise the immune system.
00:20:08.17 So, lymphocytes figure out how to make antibodies against these individual proteins,
00:20:14.01 against these individual pathogens, and use those antibodies to help kill the infected cells.
00:20:20.02 Now this is what an antibody molecule looks like in a 3-dimensional structure.
00:20:24.18 It really consists of two major parts:
00:20:26.07 on top, you can see the so called Fab region and on the bottom you can see the Fc region.
00:20:32.00 Now, both have different functions, the Fab region, which actually consists of two arms,
00:20:37.06 is really where the specificity lies in antibody molecules, so it's this portion of the antibody
00:20:44.21 that is capable of recognizing and binding to any one of the thousands, if not millions,
00:20:50.29 of different proteins that are pathogen derived that can come into our bloodstream at a moment's notice
00:20:58.08 as a consequence of breathing in the wrong stuff or cutting ourselves in the presence of the wrong bacteria.
00:21:04.02 Now, this is just a diagram of what this looks like,
00:21:07.26 again, on the top, you can see these Fab domains, or Fab regions,
00:21:11.29 which are responsible for the specificity of the antibodies
00:21:16.03 and the Fc regions, which have a different function that we'll come to just in a moment.
00:21:21.04 And I think it's important to understand this a bit.
00:21:23.29 This illustrates what the different functions of the Fab and Fc regions are.
00:21:28.16 So, here, in green, is a bacterium. It's being recognized by an antibody that's specific to a protein on that bacterium
00:21:36.05 and you can see here we've drawn that the Fab regions are up, attached to the bacteria,
00:21:42.00 because it's the Fab regions that are responsible for understanding and decoding the specificity in the process.
00:21:48.14 The Fc regions are waving off in the breeze, but they have a very specific function.
00:21:52.15 The first instance they'll recruit another protein that one finds in the blood,
00:21:55.27 called complement. What complement does is basically bind to the Fc portion of an antibody molecule
00:22:02.01 and stick a hole in the surface of the bacteria.
00:22:06.08 It's very hard to live, if you're a cell, if you've got holes stuck in you,
00:22:10.13 so as a consequence, this is one way in which antibody molecules all by themselves can help kill bacteria.
00:22:17.07 Another thing that can happen though is that these antibodies can work in conjunction with Metchnikoff's macrophages
00:22:25.02 so that the Fc regions, that fixed complement to put holes in bacteria
00:22:31.03 also will bind to specific receptors that are present on the surface of macrophages and related cells, called Fc receptors,
00:22:38.22 and these Fc receptors have two functions.
00:22:42.24 One is that they will help mediate the uptake of the bacteria by this process of phagocytosis
00:22:48.13 that I've already shown you in the two videos that we looked at,
00:22:51.09 but also, like Toll receptors, binding of bacteria that are coated with antibody to these Fc receptors
00:22:58.18 will also help aid the process of inflammation by enabling the macrophage and related cells
00:23:04.15 to secrete the hormones and the inflammatory cytokines and other components
00:23:10.00 that will basically indicate to the rest of the immune system that an infection is taking place
00:23:14.24 and help is needed at the site where the macrophages detected these bacteria.
00:23:19.28 Where do antibodies come from? As I already mentioned, they come from lymphocytes,
00:23:24.12 but they come from a very specific lymphocyte, which is called the B-lymphocyte,
00:23:28.22 or more simply, B-cell.
00:23:30.13 These are cells that have the capacity to be able to actually molecularly generate
00:23:37.11 this incredibly broad specificity array of antibodies that one finds,
00:23:44.12 and in fact, that one needs, in order to maintain proper immunity in the blood stream.
00:23:48.29 B-cells will continuously mutate the genes that encode for antibodies
00:23:55.15 and these are called immunoglobulin genes and as a consequence of this continuous process of mutation
00:24:00.17 which is very, very carefully controlled, B-cells can generate the type of diversity that they need,
00:24:06.25 in terms of recognition, to be able to provide and secrete antibodies, or release antibodies,
00:24:13.05 that can interact with virtually any type of pathogen that we are exposed to in life.
00:24:19.16 Now, although B-cells can make antibodies on their own, it turns out, of course, the system is more complicated than that
00:24:26.26 because the best antibodies that B-cells can make
00:24:29.06 are only made are only made when they are, as we say, "helped"
00:24:32.12 by a second major type of lymphocyte and these are called the T-cell.
00:24:36.15 So, T-cells interact with B-cells, while B-cells are interacting with the specific proteins derived from a given pathogen
00:24:45.07 and help the B-cells do a better job making even better antibodies than they could possibly have done on their own.
00:24:52.02 Now, how do they do this?
00:24:54.10 Well, turns out that T-cells have their own receptor, they don't make antibodies,
00:24:58.24 in fact, they make nothing that will go into the bloodstream and directly kill a pathogen like this,
00:25:04.27 under normal circumstances, but they will see another, a little bit of the pathogen,
00:25:10.22 or the pathogenic protein that's come in and use another receptor,
00:25:14.03 which not surprisingly is called the T-cell receptor to recognize that small bit,
00:25:19.14 and then, as a consequence of that, release its own hormones that then help the B-cell do its job
00:25:25.03 at making antibodies. This just shows this process in a little bit more clear fashion, I think,
00:25:30.27 so you see over here, the antigen, or the bacterium coming in,
00:25:34.25 it binding to a receptor on B-cells, which is actually antibody molecule that's embedded in the B-cell.
00:25:40.28 A small piece of that antigen is broken off and put on another receptor
00:25:46.00 on the surface of the B-cell, which interacts with this so-called T-cell receptor
00:25:50.03 that then turns on the T-cells, allowing these hormones, or cytokines, to be secreted by the T-cell,
00:25:56.03 basically telling the B-cell what to do, or at least to do its job better.
00:25:59.16 Finally, I'd just like to address the problem of where do T-cells come from?
00:26:03.19 And how do they know what to do?
00:26:05.19 Turns out, of course, that T-cells come in multiple components, or multiple types.
00:26:11.00 The two basic flavors of T-cell are called CD4 and CD8.
00:26:16.21 It's not really important to be able to distinguish between the two.
00:26:20.04 One of them, the CD4 T-cell, is actually the one that's actually responsible for helping B-cells,
00:26:25.18 CD8 T-cells, which we won't talk about today, have an additional property to that,
00:26:30.15 which is that they can actually kill stuff on their own
00:26:33.00 and form a very important component of anti-viral immunity.
00:26:37.18 But we can take that up another time.
00:26:39.14 Now, the way that T-cells develop their own ability to recognize these small pieces
00:26:46.05 of antigen that are derived from various bacteria or other pathogens,
00:26:51.00 is not because they interact with the B-cells, necessarily,
00:26:54.25 but because they interact with another cell type which I mentioned early on called the dendritic cell.
00:27:00.25 Dendritic cells have the special property of being able to also take up bacteria
00:27:06.12 and they don't really kill the bacteria, they analyze the bacteria.
00:27:10.20 And they ask, what type of bacterium it is, and then display small bits of that bacterium on their surfaces,
00:27:19.02 on molecules here that are called MHC class I or class II molecules
00:27:23.20 that have the very, very special property of being able to detect the appropriate recognition sequences on T-cells
00:27:33.24 such that the right T-cells are generated for the right type of bacterial infection that's taking place.
00:27:41.05 Now, the way this actually works is shown in this little cartoon,
00:27:44.12 and so, once again, you maybe can understand it a little bit better.
00:27:48.27 In the left, you see a happy dendritic cell taking up a not-so happy bacteria.
00:27:55.20 Parts of that bacteria are then generated as a consequence of the dendritic cell
00:28:00.27 having the ability to break it down, placing some parts of, small little bits of the bacterium
00:28:06.09 on the surface, small peptides derived from bacterial proteins, for those of you who know what a peptide is,
00:28:11.15 bound to these MHC class I or class II molecules.
00:28:15.11 T-cells will recognize this, the dendritic cell will make additional cytokines
00:28:21.14 and that T-cell, if it sees its right little bit, it becomes activated,
00:28:26.17 very happy, as you can see here and then runs off to find B-cells that it can help in the antibody generation process.
00:28:35.01 So, it's all a nicely closed loop.
00:28:37.13 I would like to show you a video that tries to put all of this together.
00:28:40.21 So, here you're looking at an animation of someone who's just gotten a sore throat,
00:28:44.18 so you can see the sore throat.
00:28:46.12 What happens in a sore throat, of course, is you have, in most cases, a bacterial infection.
00:28:52.21 So, here, you see a bunch of green bacteria that are colonizing, that have infected the throat
00:28:57.26 and are colonizing the surface of it.
00:29:00.04 Now, those bacteria are covered with specific proteins and that's what's shown in green
00:29:06.02 and the proteins then are sloughed off, or released from the bacteria,
00:29:11.03 and enter the circulation and then also enter into the lymphatics.
00:29:18.20 So, these, remember, are these small conduits that cells in the immune system travel through
00:29:24.18 in order to go from the peripheral tissues into lymph nodes.
00:29:27.27 So, cells of the immune system, such as dendritic cells,
00:29:31.02 have taken up these proteins, B-cells have taken up these proteins,
00:29:35.27 and then come back to these central congregating sites that are connected to all of these lymphatic vessels
00:29:42.21 called lymph nodes. We talked about these in the very beginning.
00:29:46.17 So, here, you see, in this particular video, not the cells, but the bacterial protein entering into the lymph nodes,
00:29:52.23 where it encounters all of these lymphocytes and dendritic cells
00:29:56.17 that begin to take up the bacterial protein
00:29:59.09 and whatever bacteria that come in and begin to interrogate what has happened,
00:30:04.23 respond via Toll-like receptors to these proteins and other components and start to be activated.
00:30:12.09 So here you see, now, a blow-up of the surface of the B-cell,
00:30:15.21 these are the receptors on the surface of the B-cell, this one seems to be specific for this particular bacterial protein
00:30:22.07 because you can see as these proteins come down,
00:30:26.00 they bind to these receptors, one and then a second and a third and a fourth.
00:30:31.12 And as a consequence of all of these receptors accumulating together, particularly if a T-cell is around,
00:30:37.12 these B-cells will then generate a signal, as you can see here,
00:30:43.01 that then tells the B-cell it has recognized the right antigen, the one that it was born to recognize,
00:30:50.05 and it basically gets activated and as a consequence of getting activated,
00:30:56.11 it not only starts moving, but as you'll see in a second, it starts to grow.
00:31:01.14 There, it's dividing two and four and eight, 16, et cetera, et cetera,
00:31:06.13 you wind up with a clone of B-cells, in other words, they're all identical to the first one,
00:31:11.17 they just expand, make more and more of themselves,
00:31:14.18 and the reason for that is so that the immune system can then generate a large amount of the antibody
00:31:21.01 that it already had that can then neutralize the bacteria.
00:31:26.20 So, here you see these B-cells after they've grown up, and replicated themselves,
00:31:30.23 now secreting large quantities of these antibodies that enter not only back into the lymphatics,
00:31:38.07 but now also permeate into the blood stream and can circulate throughout the body,
00:31:43.00 including going back to the original bacteria that had colonized the throat,
00:31:48.29 creating the sore throat in the first place, binding to it, complement will fix to it at that point,
00:31:55.14 and here, in this last image, a macrophage or something similar to that is coming in
00:32:01.02 and as a consequence of recognizing the antibody bound to the bacterium,
00:32:04.16 being attracted by the bacterium itself, you can see this macrophage eating these cells and kill them.
00:32:10.12 And eventually, we get better, as a consequence of all this.
00:32:15.17 Ok, just one last word just to make sure that you at least get the basic concept
00:32:22.01 of how the immune system works, what its logic is, what its function is.
00:32:26.12 I want you to remember that the immune system consists of two basic components:
00:32:30.25 the innate immune system, discovered by Metchnikoff, and the adaptive immune system, discovered by Ehrlich.
00:32:36.02 The innate immune system exists and itâ€™s a very primitive form of the immune response.
00:32:41.04 It exists to recognize components that are found on virtually all pathogens,
00:32:46.21 without really distinguishing so well one pathogen from the next.
00:32:49.27 The adaptive immune system, on the other hand, is highly specific
00:32:53.07 and makes antibodies through the activity of T-cells and B-cells that can specifically identify very, very highly individual pathogens
00:33:02.01 and very highly individual viruses and help, working together with the innate immune system,
00:33:09.00 killing them off.
00:33:10.08 Now, as I mentioned, one of the big problems that we've had until fairly recently,
00:33:16.22 just really within the last 15 years or so is really understanding
00:33:20.17 how the innate and the adaptive immune system work together.
00:33:23.28 In fact, Metchnikoff or Ehrlich didn't really understand this either
00:33:27.13 but this is a realization that was achieved by another scientist, just in 2011,
00:33:36.23 received a Nobel prize, tragically died just days before learning of the award,
00:33:42.10 this is Ralph Steinman, who worked at the Rockefeller University in New York,
00:33:47.23 and Ralph was really the person who is responsible for our understanding that dendritic cells exist
00:33:54.01 and that dendritic cells provide this missing link.
00:33:56.27 They are like macrophages in the sense that they have all of the Toll-like receptors macrophages have,
00:34:01.08 they have the capacity of taking up bacteria and other pathogens,
00:34:04.10 and to some extent killing them.
00:34:06.29 But that's not really what their major role in the immune system is.
00:34:10.11 What their role is is to carry the information, small pieces of the bacteria that are preserved on the surface of the dendritic cell,
00:34:18.03 they take that information from the periphery, where the dendritic cell first encountered the bacterium,
00:34:25.21 into lymph nodes and into the lymphoid organs,
00:34:28.14 basically instructing the cells of the adaptive immune response, the B-cells and the T-cells,
00:34:34.01 as to exactly what type of bacterium is present,
00:34:37.12 stimulating their activities, stimulating their growth, and ultimately leading to the protective antibody responses
00:34:44.22 that you saw in the video.
00:34:46.18 Ok, I hope you enjoy understanding something about the immune system
00:34:52.21 because it really is very important and there are a lot of resources,
00:34:56.12 both online and in print, that you can go to in order to be able to learn more
00:35:00.21 and test your own knowledge of this very, very important and very elemental part of biology.
00:35:06.09 Thank you.
00:00:04.01 Hi, my name is Ira Mellman. I'm Vice President of Research on Oncology at Genentech,
00:00:09.09 a large biotechnology company here in San Francisco. Prior to that though,
00:00:13.12 in fact up to 2007 I was a professor at the Yale University School of Medicine,
00:00:17.29 chairman of the Department of Cell Biology and a member of the Ludwig Institute for Cancer Research.
00:00:22.01 During that time and in fact up until today, my laboratory has been
00:00:26.15 very interested in understanding the fundamental mechanisms of membrane traffic.
00:00:30.02 In other words, understanding the movement of cells and membranes
00:00:33.27 and the proteins and lipids that go to comprise those membranes
00:00:37.00 allowing them to work together to make cell shapes and cell functions of various sorts.
00:00:43.00 The last, more recent period of time though, we've become increasingly interested
00:00:47.27 in trying to understand how membrane traffic acts in highly specialized cells to allow them
00:00:53.16 to carry out the variety of important and specialized functions that are associated with specific
00:00:59.05 and more complex systems. The system that we have chosen to work with
00:01:03.02 over the last several years is the immune system, trying to understand how membrane traffic conspires
00:01:09.06 to get cells of the immune system to work and work together to generate an immune response.
00:01:14.02 It's a terrific problem of cell biology which almost inexplicably to me, at least,
00:01:18.05 has been relatively inaccessible to most cell biologists almost as inaccessible
00:01:22.17 as cell biology has been to immunologists. But nevertheless, there is an enormous amount
00:01:26.29 of very important cell biology to be learned by looking at cells of the immune system,
00:01:30.20 but it also provides a lot more. It provides you with the opportunity to actually understand
00:01:35.27 how events that occur at the molecular level and at the cellular level
00:01:39.22 can integrate with each other at the systems level to understand a very complex
00:01:44.09 and important phenomenon, which has a great great deal of relevance with respect to
00:01:49.13 understanding important and very devastating human diseases.
00:01:53.02 Now in order to introduce you to the topic, and let you know what it is you can indeed find out,
00:02:00.00 I have to first review with you some of the basic features of what the immune response is,
00:02:06.05 and how it's organized in its cellular level of organization.
00:02:10.03 So, what is the basic function of the immune system? It seems to be to be able to provide us with
00:02:15.14 protection against a limitless number of pathogens and toxins
00:02:19.07 that one finds in the environment. It's basically conserved in it's most elemental forms across evolution.
00:02:25.16 Invertebrates have a type of immune system and certainly we vertebrates
00:02:29.23 have a type of immune system, which provides us with a terrific amount of protection
00:02:34.16 under normal circumstances to viruses, bacteria, protozoa, and other environmental poisons and allergens.
00:02:42.00 Now, the immune system is extremely good at this and it's also extremely clever.
00:02:46.02 It can kill incoming viruses or incoming bacteria, but it does so in a fashion which avoids the injury
00:02:53.13 to the host, in other words, us, which provides a really important conundrum in
00:02:59.09 understanding how the immune system works. How is it that cells in the immune system
00:03:03.15 can distinguish self from non-self? How can they distinguish
00:03:07.01 the normal cells and tissues of our bodies from the proteins and components
00:03:13.02 that are associated with the protozoa and bacteria and other microbial invaders
00:03:17.07 that are coming in from the outside. This is probably the major fundamental
00:03:21.29 and conceptual problem in understanding the immune system
00:03:24.25 and it's one that undoubtedly will have the cell biological solution associated with it.
00:03:30.02 Now is this complicated? It's absolutely very complicated, but in my view itâ€™s not
00:03:35.03 really any more complicated than anything else,
00:03:37.00 and if you try and understand how the immune system works in cellular terms,
00:03:40.29 through the eyes of the cell biologists, in fact I believe it does become quite accessible.
00:03:45.02 Now, it's worth the effort, as I've said already, because it's not only interesting,
00:03:50.28 but it's of profound importance to understanding and controlling a wealth of
00:03:55.03 important human disorders, infectious disease, is obvious, arthritis,
00:03:59.10 Crohn's disease, asthma, autoimmunity, and quite possibly even cancer.
00:04:04.01 Now the immune system, or immune response rather, consists of two interconnected
00:04:08.08 arms, the innate immune response, or innate immunity, and adaptive immunity.
00:04:14.02 Well innate immunity refers to that portion of the immune response,
00:04:18.15 which is responsible for detecting components shared by all pathogens
00:04:24.27 that we have to deal with as invaders.
00:04:28.00 An interesting concept, in terms of how this might work, and not immediately
00:04:32.15 obvious how it works, but indeed it turns out to be the case.
00:04:35.01 Adaptive immunity is something different. Adaptive immunity
00:04:39.24 refers to the type of immune response that is molecularly crafted to mold and detect
00:04:45.21 individual antigens that are specific to individual pathogens and individual types of pathogens.
00:04:50.21 These two work together in some rather interesting and mysterious ways that we'll
00:04:55.14 go into later, but nevertheless it is important to keep it in mind that there are
00:04:59.10 these two equal and equally important aspects of the immune response:
00:05:03.04 innate and adaptive immunity. Innate immunity was really first discovered
00:05:07.01 by this great scientists of the past, Elle Metchnikoff, shown here working in his
00:05:12.20 laboratory about 100 years ago at the Institut Pasteur in Paris.
00:05:16.00 Metchnikoff's great conceptual advance was the understanding that inflammation,
00:05:22.27 which is what happens after you become infected by the introduction of a bacterium or a virus,
00:05:30.02 this is why your cuts become red and inflamed and hot and painful if they go untreated,
00:05:37.09 this process of inflammation is a protective process of immunity
00:05:41.26 and not a manifestation of the tissues destruction. In other words, as bad as it looks,
00:05:46.01 this is actually a representation of how the body is responding to invading microorganisms,
00:05:52.05 to rid the body of those microorganisms, rather than a reflection
00:05:57.00 of the microorganisms capacity to meet out damage on the hosts tissue.
00:06:03.01 Prior to this point, this idea was completely foreign to scientists.
00:06:10.01 Now the story goes that one of the most important observations that Metchnikoff
00:06:17.09 made was actually made while on vacation on the beach with his family,
00:06:22.03 and like many modern day scientists, who bring their computers with them on vacation,
00:06:27.03 Metchnikoff in the age before computers brought his microscope,
00:06:31.10 and studied the reaction of starfish larvae to the introduction of foreign materials
00:06:38.16 such as bacteria introduced experimentally into the larvae.
00:06:42.02 What Metchnikoff found was that a series of cells would come in
00:06:46.00 and surround the bacterium and in fact, quite visibly, engulf these microorganisms
00:06:52.00 shown here as these red oblong creatures, contained within this cell.
00:06:56.02 He named these cells macrophages, because they were large cells that ate bacteria,
00:07:02.08 macrophage: â€œlarge eater". The bacteria were taken into the interior of the cell,
00:07:08.00 and Metchnikoff further found that if he stained these cells with a dye
00:07:10.02 that partitioned into acidic elements within cells and other things,
00:07:17.00 the bacteria stained red. Further indicating to him, and in fact correctly to us,
00:07:22.26 that the bacteria after they entered into the cell would be delivered to
00:07:26.28 an acidic intracellular compartment, which we now understand and know as lysosomes.
00:07:32.01 Now bear in mind, this was well over 100 years ago,
00:07:35.24 and long before even membranes were truly discovered. Now these bacteria eventually stopped growing,
00:07:42.18 they died after being taken up by the cells, and eventually disappeared
00:07:49.00 and Metchnikoff further understood that as a process of degradation.
00:07:51.02 So what actually happens when a macrophage eats a bacteria or encounters a bacteria is shown here.
00:07:58.01 The response is not only limited to the uptake and intracellular killing of the bacteria, but also
00:08:05.01 these cells secrete a variety of important cytotoxic agents,
00:08:09.03 degradative enzymes as well as protein hormones referred to as cytokines.
00:08:14.01 Cytotoxic agents include important mediators, such as hydrogen peroxide,
00:08:18.28 which is a common disinfectant that is used, but macrophages do this on their own,
00:08:25.05 as well as other types of active forms of oxygen. They secrete an enzyme called
00:08:30.06 lysozyme, which is quite adept at degrading the cell walls of incoming bacteria,
00:08:34.17 and a series of lysosomal enzymes contained within the lysosomal structures of
00:08:39.09 these cells that basically degrade all sorts of other things. As I mentioned,
00:08:43.11 the cytokines that are released by cells, by macrophages after encountering
00:08:48.17 bacteria, some of which you may already know, such as the molecule interferon,
00:08:54.03 macrophages are a prime source of interferon have the effect of recruiting more
00:08:59.03 macrophages and other cells of the immune system to the site of bacterial
00:09:03.01 invasion, increasing the overall level of inflammation, but again,
00:09:06.24 not for the purpose of tissue destruction, but rather to recruit
00:09:10.24 more and more cells of greater complexity and of increased levels of sophistication
00:09:17.00 that help deal with the infection and hopefully eradicate it before it
00:09:20.27 gets out of hand. Now why is it that macrophages and other cells
00:09:26.07 of the innate immune system, such as neutrophils,
00:09:28.17 know to respond to microorganisms? What is it that they are recognizing?
00:09:33.18 So here the story turns to a relatively recent observation
00:09:38.27 having to do with a class of membrane protein receptors found on macrophages
00:09:43.02 as well as other cells, called Toll-like receptors. What are these?
00:09:47.18 The story actually starts a little bit earlier than that, in the work of
00:09:52.07 Catherine Anderson and Carl Hashimoto, who identified the activities associated
00:09:56.25 with a well known gene in Drosophila called Toll. Now Toll was originally identified
00:10:05.26 as acting in early embryonic development in Drosophila, specifying the dorsal
00:10:11.13 from the ventral pole of the early Drosophila embryo. Toll was known to bind to a ligand, called Spaetzle,
00:10:19.07 which activated the Toll-receptor, sending a signal to the embryo
00:10:23.01 allowing this dorsal ventral polarity axis to be established. Here you can see some embryos
00:10:29.16 that either do or do not express wild-type Toll, or members of the pathway.
00:10:35.01 In panel A, all the way on your right, you can see what the normal embryo looks like,
00:10:39.26 and in the absence of functional Toll-receptor or an active Toll-like pathway
00:10:43.03 you can see that these embryos completely disorganized and they don't develop
00:10:49.01 and it is known as a lethal defect early in embryogenesis. Now Toll it turns out also
00:10:57.17 has a role in the adult fly, a quite different role, but one that is no less important
00:11:05.06 in so far as the fly is concerned. Here a different group of investigators,
00:11:10.23 lead by Bruno Lemaitre and Jules A. Hoffmann found that Toll was absolutely
00:11:16.18 required in order to protect adult Drosophila from their pathogens,
00:11:21.20 in this case what you are looking at is a poor dead fly that was unable to combat
00:11:26.00 the fungal infection that you can see here, growing almost as a beard, or a beard of death
00:11:34.01 around this fly's torso because of the absence of a functional Toll-receptor
00:11:41.05 in the fly as the adult. This is really one of the very very first indications
00:11:46.19 that Toll has a role outside of early morphogenesis and also plays an important role in host defense.
00:11:55.01 Now how does it do this? Here is a diagram of what the Toll-receptor looks like in flies,
00:12:01.24 it has a nice extracellular domain with some repeats,
00:12:04.22 it has an intracellular domain that one can presume has signaling molecules
00:12:09.01 or signaling features associated with it, but one of the most interesting features
00:12:14.06 that fell out of the analysis of what this receptor looked like,
00:12:17.18 was that it was very very similar to a receptor for cytokine, called IL-1.
00:12:22.02 Again, another product of the macrophage. And specifically where it was similar
00:12:27.17 was in the cytoplasmic domain, here in the case of Toll, and here in the case of IL-1 receptor.
00:12:34.03 Since it was known that stimulation of IL-1 receptor turned on an inflammatory
00:12:38.21 response from other sorts of studies, it was surmised that
00:12:42.21 indeed the Toll-receptor did much the same thing. So in subsequent years,
00:12:48.11 when Charlie Janeway and his colleagues started looking for the expression of these receptors
00:12:54.21 in vertebrates, they found indeed that there is a whole family of Toll-receptors,
00:12:59.08 and indeed well over a dozen of them are now known that have the same basic organization as
00:13:03.26 Toll does in Drosophila with an extracellular domain that is similar in many ways,
00:13:09.07 but most importantly the cytoplasmic domains that have a large degree of homology
00:13:15.14 to the Toll-receptor in Drosophila as well as to the IL-1 receptor.
00:13:20.01 Now what these receptors do, and the reason they are so many of them
00:13:24.17 is that each one has a specificity for a different microbial component.
00:13:29.01 In each case, the microbe itself needs those components to survive.
00:13:34.00 So here Toll-like receptors are expressed on the plasma membrane,
00:13:40.05 such as Toll-receptor four, or TRL-4 and TRL-5, see various important
00:13:46.00 components associated with bacteria. So TRL-4 sees components associated
00:13:52.15 with the peptidoglycan, a lipopolysaccharide component of bacterial coats.
00:13:58.01 TRL-5 sees a major protein associated with bacterial flagella,
00:14:02.01 both of which are components that are absolutely needed by these bugs in order to survive.
00:14:07.03 They are shared by great many types of bacteria, hence these Toll-receptors
00:14:12.01 recognize these conserved molecular patterns and act as the innate immune system sensor.
00:14:18.03 Another set of Toll-receptors are actually found in intracellular vesicles, endosomes
00:14:23.09 and lysosomes found within macrophages and other cells, examples here are TLR-7 and TLR-8, and -9.
00:14:30.01 And they have a tendency to recognize nucleic acid components
00:14:34.09 such as double-stranded RNA, DNA, and single-stranded RNA,
00:14:38.29 which are associated with other types of incoming pathogens
00:14:41.03 such as both enveloped and non-enveloped viruses. So here you have a complete pattern
00:14:48.20 of conserved receptors that have this remarkable capacity to be able to detect,
00:14:54.19 again, the shared components that can be found with a wide variety
00:14:58.13 of different types of incoming and invading microbes, turning on the macrophages
00:15:03.26 and in fact any other cell that happens to be expressing them.
00:15:06.26 Now the way that this turn-on takes place, or the way that this signaling occurs,
00:15:13.06 perhaps not surprisingly, is similar to the way that stimulation of IL-1 receptor,
00:15:19.26 the cytokine receptor, does it, turning on a series of signaling intermediates leading into the nucleus
00:15:27.03 activating I think the most important and simplest element, the so-called Nf-kappa-B pathway,
00:15:33.01 which leads to in turn a great number of events that prime
00:15:38.27 almost everything important that happens in the context of an innate immune response.
00:15:44.02 So intracellular signaling pathways are all activated by TLR receptors,
00:15:49.21 the pathways themselves can be different from TLR to TLR as they can from cytokine to cytokine,
00:15:56.13 but this really summarizes the basic feature of how the system works.
00:15:59.02 Now the consequences of it are shown here, here what you are looking at is a video
00:16:04.24 of macrophages that have been given yeast to play with, and what you can see
00:16:11.11 is that the macrophages move towards the yeast and then engulf them.
00:16:16.00 So here is one that is moving directly towards it, and engulfs it. Here is another yeast
00:16:20.19 that is here, which will eventually be attacked and eaten by another macrophage,
00:16:24.10 shown right there, so in much the same way that Metchnikoff found
00:16:28.03 these invading microorganisms activated the macrophage, turned it on,
00:16:35.28 increased its capacity for migration, its capacity for cytotoxic killing,
00:16:41.14 and as shown here its capacity for actually killing and eating the invading microorganisms.
00:16:49.00 This is another movie showing that the process of phagocytosis of these yeast,
00:16:57.17 or in fact any other large particle that a macrophage encounters,
00:17:00.15 is intimately associated with the ability of the macrophage to polymerize actin
00:17:08.03 as a motile cytoskeletal component directly underneath where the particle binds
00:17:14.02 and turns on itself, growing out long pseudopods that engulf the particle,
00:17:20.12 capturing it and bringing it inside the cell, and eventually getting it to fuse with lysosomes,
00:17:25.09 again shown here. So what you are looking at in this particular video is a particle
00:17:31.02 that is entering a macrophage whose surface is stained green and shortly after internalization,
00:17:38.12 this green membrane turns red as a consequence of fusing with these
00:17:43.06 red endocytic vesicles that are found inside the macrophage.
00:17:47.02 Again, showing that very high tech components which Metchnikoff knew over 100 years
00:17:53.22 ago, which is that macrophages can detect, can bind, internalize,
00:17:58.24 and to deliver to lysosomes for purposes of degradation those incoming bacteria and
00:18:05.03 other sorts of pathogens that are found in the outside world against which we need protection.
00:18:09.02 So to summarize then, Metchnikoff really has to be credited with providing us
00:18:16.02 a great deal of not only conceptual understanding, but in many ways even
00:18:19.25 experimental detail that has defined for us the innate arm of the immune system.
00:18:25.03 The system that recognizes shared pathogen-derived components that are recognized
00:18:30.04 by Toll-like receptors as well as a variety of related receptors that
00:18:33.27 we won't have a chance to talk about. And it is powered to a very large extent by cells
00:18:40.16 which are referred to collectively as phagocytes and these include
00:18:44.19 the macrophage, which are the large cells that Metchnikoff first identified
00:18:48.02 as well as neutrophils, or granulocytes, polymorphonucleocytes,
00:18:53.15 which are actually smaller than macrophages, and indeed Metchnikoff had seen these too,
00:18:58.12 he referred to them as microphages. These mediated protection by
00:19:02.05 activating cytotoxic mechanisms, and as you have seen, quite graphically,
00:19:05.19 by actually physically eating, clearing, and degrading extracellular bacteria
00:19:10.24 and other types of microorganisms. Now, almost at exactly the same time
00:19:15.05 that Metchnikoff was working, elsewhere in Europe another
00:19:19.07 terrific scientist, Paul Ehrlich, was also hard at work
00:19:24.01 and you can tell from this image that Ehrlich was a terrific and great scientist
00:19:29.06 because he has one of the hallmarks of great scientists, which is an incredibly messy office.
00:19:34.00 So here he is sitting, reviewing some notes, and what Ehrlich's contribution was,
00:19:41.20 was to show for us how the other main arm of the immune system was organized and what it did.
00:19:49.03 This is called "adaptive immunity". What Ehrlich identified was that individuals,
00:19:56.10 whether it be the humans, it be the animals, that are immunized with various types of microorganisms
00:20:02.03 or bacterial derived toxins, make a response in the blood that is inherently protective,
00:20:10.06 and in fact Ehrlich coined the term "antibodies" to describe this response,
00:20:15.01 which has the ability, obviously, of protecting us against whatever it is that had entered
00:20:20.27 into our systems or into our bloodstreams.Antibodies see individual antigens
00:20:26.27 that are specific to individual pathogens types, so if you immunize with one type of bacteria,
00:20:33.02 you will not necessarily make antibodies that will recognize and protect a second individual
00:20:38.02 against another type of bacteria. So in this way, the system is fundamentally different
00:20:43.21 from the innate immune system. It's crafted in a molecular fashion
00:20:47.22 to the molecular entity that happens to be coming in. The way the cellular
00:20:54.00 mechanism, whereby the adaptive immune system works, is using not so much
00:20:58.11 macrophages, as we know now, but rather making use of antigen-specific
00:21:02.21 lymphocytes that indeed make these antibodies that recognize and kill the infected target cells
00:21:08.03 either separately from, or as you will see in a minute, in conjunction with the macrophages.
00:21:14.01 This is what an antibody molecule looks like in crystal structure. It consists of two major parts
00:21:21.04 a Fab fragment and a Fc fragment, both of which are linked together,
00:21:26.01 the molecular weight is about 150,000 Daltons and consists of four chains:
00:21:31.28 two light chains, shown in red, and two heavy chains shown in yellow and in blue.
00:21:37.01 The antigens themselves actually bind to the so called Fab regions of antibody
00:21:45.26 molecules, demonstrated in this diagram, out near the tips. So here you find a
00:21:50.26 great deal of sequence variation that allows the antibody to form complementary structures
00:21:56.02 allowing it to interact quite specifically with whatever antigen comes into an organism,
00:22:04.03 against which an antibody can be made. Now how antibodies work.
00:22:10.01 After recognizing their antigen, they are made, and as shown here they'll bind
00:22:16.03 to the antigen and this particular example that antigen is present on the surface of the bacterium,
00:22:21.20 and one of the simplest ways in which the antibody can work to kill
00:22:25.29 the bacterium is to recruit another set of proteins that are found in our plasma
00:22:31.00 called complement, which actually is a number of proteins, not just one. Complement is
00:22:37.00 recruited by antibodies, inserted into the bacterial membrane causing
00:22:41.23 the bacterium to lyse. Now in addition, these antibodies will work together
00:22:46.02 with element of the innate immune system, macrophages in particular,
00:22:49.26 but also neutrophils. Macrophages are probably more important
00:22:53.00 since macrophages actually have receptors for the Fc domains of these antibodies,
00:22:58.04 which have some of the same effects that Toll-receptors do,
00:23:02.01 in other words binding of antibody coated microorganisms to macrophages will trigger
00:23:06.24 the release of the same types of cytotoxic compounds that we've already discussed
00:23:11.01 and also trigger the ability of the macrophage to mediate phagocytosis
00:23:15.17 and intracellular killing and destruction of these antibody associated
00:23:21.09 or antibody coated microorganisms. So here you have this nice example of the innate system
00:23:26.12 working together with the adaptive immune system. Now antibodies are not made by
00:23:31.20 macrophages, although that was an idea that was originally thought of by
00:23:36.01 Metchnikoff and by Ehrlich who by the way shared the Nobel Prize
00:23:39.11 for their discoveries back in 1908, 100 years ago this year. Antibodies instead
00:23:45.16 are made by lymphocytes, a particular class of lymphocytes, as many of you know I'm sure,
00:23:49.27 called B lymphocytes. B lymphocytes exist in great number found throughout
00:23:57.22 a variety of lymphoid tissues in the body, and have a repertoire
00:24:04.03 of potential interactions, in other words, due to inherent and preexisting variability in
00:24:10.11 that small Fab region, or complementarity determining region
00:24:15.01 found in the Fab region of antibody molecules, incoming antigens can find or B cells
00:24:23.13 rather with the right specificity can find and actually bind to
00:24:27.00 antigens associated with bacteria, or antigens that are simply with soluble proteins,
00:24:32.09 and the simple act of binding of the antigen to the antibody,
00:24:37.03 which at this point in a B-cell's development is actually a membrane protein receptor
00:24:41.02 on the B-cell, is sufficient to simulate development of the B-cell and finally the
00:24:46.22 production of an amplification of more and more of these antibody molecules.
00:24:51.03 Now this is a video that shows you how the system works, when you get a sore throat,
00:24:57.27 which is indicated here in red. What that's again a manifestation of,
00:25:02.27 as Metchnikoff told us, a protective response, not a destructive response.
00:25:07.01 So here you see bacteria lining the surface of your throat following an infection
00:25:14.02 and these bacteria are putting off a variety of protein antigens that are associated with these bacteria,
00:25:22.03 so not only have you generated an innate immune response,
00:25:25.24 but now these bacterial specific antigens are being released and what happens to them is that they begin
00:25:33.13 to drain into the intercellular spaces, that one finds in all of your tissues
00:25:39.08 referred to as the lymph. The lymph drains into the lymphatics,
00:25:42.25 which then comes into these nodes, or specifically lymph nodes that monitor every 24 hours a day
00:25:52.04 what proteins, what antigens, what bacteria have entered into the lymph.
00:25:57.03 So here you see our bacterial protein entering into the lymph node, and what they
00:26:02.07 encounter here are the wide array of lymphocytes and other cells and here you can
00:26:08.01 see them in a higher magnification and as our bacterial derived proteins come in,
00:26:12.26 they filter through these millions or billions of cells as you can imagine,
00:26:20.22 and cells that are B-cells that may detect a component of that antigen will bind to it,
00:26:29.23 based on these antibody molecules or via these antibody molecules
00:26:33.07 that are intercalated as membrane proteins on the surface of these early B-cells shown here
00:26:38.19 clicking on, and this will then hit onto another B-cell receptor, and a third
00:26:45.05 and a fourth, until enough of them come together to actually generate a signal
00:26:50.00 to the B-cell, which says that I have now detected the antigen I was born to detect
00:26:57.10 and as a result, it is now time to start making more of myself. So these B-cells
00:27:03.23 become very active and begin to develop and divide.
00:27:10.29 The affinity that they have for the individual antigens, in essence, increases as B-cells with increasingly
00:27:20.24 large affinities, increasingly great affinities for antigens,
00:27:24.07 compete with each other to bind more and more. So here you see a clone of B-cells that gives rise
00:27:31.07 to a larger number of itself and then as you can see here, in this representation,
00:27:39.13 antibody molecules begin to be released into the extracellular space
00:27:44.19 and here is just an indication of the secretory event that takes place very similar
00:27:51.10 to that which is described in another of the iBioseminar science series given by Randy Schekman.
00:27:59.01 Now here the antibody molecules at this stage in development are actually pentamers
00:28:04.00 coming back homing to the bacteria, coating them with antibody molecules,
00:28:09.11 recruiting complement, and, as shown here also recruiting macrophages,
00:28:13.28 which again, just as Metchnikoff has already shown us, will come and eat those
00:28:18.08 bacteria that are coated with antibody taking them up by phagocytosis,
00:28:21.27 killing them and degrading them. Now it turns out that things are not that simple,
00:28:29.28 of course. What I've just shown you in that video isn't in every sense correct,
00:28:37.20 but it turns out that B lymphocytes make the best antibodies when they are helped
00:28:41.02 by another type of lymphocyte, referred to T, or thymus-derived, lymphocytes.
00:28:46.18 And the way that works is that here you can see an extracellular antigen coming,
00:28:50.28 binding to the surface antibody molecule of B-cells and then it turns out that
00:28:56.09 the antigen is internalized by endocytosis into the B-cell, and a small fragment
00:29:01.23 of this antigen is cleaved off and is bound to a molecule referred to as a major
00:29:07.02 histocompatability class II molecule, or MHC class II molecule. This complex of the
00:29:13.17 peptide plus the MHC class II molecule is then recognized by a preexisting T-cell,
00:29:19.26 which has a different type of antigen receptor referred to as the T-cell receptor,
00:29:24.11 again which has a great deal of diversity built into it as a consequence
00:29:30.24 of the development of T-cells in the early development of the immune system
00:29:37.09 and what the T-cell then does, is to secrete again our friends the cytokines,
00:29:41.06 in this case a class of cytokines, which helps the B-cell develop, helps them
00:29:47.28 divide even more, and helps them create antibodies with every increasing affinity.
00:29:54.22 Now the way the T-cell system works, is illustrated in a little bit more detail here.
00:30:00.08 T-cells come in two general flavors, and we'll come back to this later,
00:30:04.28 CD4 T-cells, as well as CD-8 T-cells. We've been talking about CD-4 T-cells,
00:30:09.27 which recognize bacterial or other derived peptides bound to these so-called MHC class II molecules,
00:30:16.23 and they do this by the virtue of having these T-cell receptors, which again have
00:30:22.14 their own specificities associated with them, such that by random chance,
00:30:28.08 the appropriate T-cell receptor is made for this particular complex of peptide and MHC class II.
00:30:36.00 Diversity in the T-cell receptors, rather than being so much generated by mutation,
00:30:41.00 which is what occurs in the antibody in the B-cell system, occurs by
00:30:45.11 rearrangement of the T-cell receptor gene, again rearrangements that cause a greater number of
00:30:53.15 potential combinatorial sequences that can recognize a wide array
00:30:57.27 of these peptide MHC complexes. So that said CD4 T-cell sees MHC class II molecules to a large
00:31:06.04 extent because it makes the second membrane protein called CD4,
00:31:09.17 which also recognizes the MHC class II molecules, but recognizes an invariable portion
00:31:16.01 of those molecules. CD8 T-cells express exactly the same type of T-cell receptors,
00:31:22.02 but they're targeted to a different type of MHC molecule, in this case a MHC class I molecule
00:31:28.01 and the reason this particular targeting takes place is because the CD8 membrane
00:31:33.01 protein found on the CD8 T-cell, recognizes the class I molecule and not the class II molecule.
00:31:41.01 So very complicated and this is obviously a simplistic view of how this works,
00:31:47.10 but at its most elemental form, this is how T-cells cooperate with B-cells
00:31:52.27 and in fact with a variety of other cells as well. Now CD8 restricted T-cells
00:32:00.28 or those T-cells that see antigens bound to class I in general don't see antigens
00:32:07.07 that are coming in from the outside. In much the same way,
00:32:11.11 or at least I should say, analogous to what happens in the case of Toll-like receptors
00:32:16.06 that are expressed either on the surface of a cell or inside, the CD4/CD8 system, or class I and class II
00:32:23.08 MHC systems are adapted to incoming pathogens that reveal themselves
00:32:28.04 either on the outside world or on the inside world. The CD8 system and the class I system
00:32:33.28 take care of those things that are going on in the inside world.
00:32:37.01 So when viruses infect our cells, those viruses almost invariably work by binding to
00:32:43.14 receptors on the surface of the cell and fusing, or entering, the cell
00:32:48.25 either at the level of the plasma membrane or following entry into endocytic vesicles,
00:32:53.22 and then breaking out into the cytoplasm. That type of endogenous pathogen,
00:33:00.16 because you can now refer to it as endogenous because it's found inside the cell,
00:33:06.15 that type of pathogen is treated by the immune system to generate peptides
00:33:11.29 that are then loaded onto class I as opposed to class II molecules. Now the way
00:33:17.13 these T-cells work, again, is somewhat analogous to what one finds in the
00:33:22.09 innate immune system, except that it is all very specific and very antigen driven.
00:33:26.01 So here you are looking at a cytotoxic CD8 positive (+) T-cell,
00:33:30.15 this is work of Julian Griffin at the University of Cambridge in England, and what Julian
00:33:36.18 has noticed as well as others over the years is that these cytotoxic CD8+ T-cells, when they recognize
00:33:44.07 a virus-infected target in fact polarize all of their lysosomes,
00:33:50.08 which actually now are differentiated to contain a wide variety of cytotoxic compounds,
00:33:55.27 towards the infected T-cell. These granules line up at the interface, or synapse,
00:34:02.19 between the T-cell and the target, and are eventually released, killing the target.
00:34:08.01 This is what you are looking at here in an electron micrograph also from Julian's work.
00:34:12.12 Here you can see these cytotoxic T-cell, or CD8 T-cell, or CTL granules
00:34:18.01 in the region of the cytoplasm that is very very close to the infected target cell.
00:34:24.02 These granules contain lysosomal enzymes, various lytic enzymes, ligands
00:34:30.04 such as Fas and receptors that will induce apoptosis or cell death in the target,
00:34:36.01 and also perforating molecules called perforins among others that will punch holes
00:34:43.10 in the target cell. All leading to its rapid death. So in other words, the virus has
00:34:48.15 infected this cell, think its safe by existing within the cell and then the cytotoxic
00:34:52.16 T-cell comes along and recognizes a peptide derived from the virus,
00:34:56.17 releasing the granules at the site of the infected cell, thereby killing the cell.
00:35:02.02 This is just the diagram showing how this process works. In fact over the last several
00:35:07.16 years, as a consequence of interrogating some important mutations
00:35:11.13 that lead to defective CD8 T-cell responses, which Julian and others have shown,
00:35:17.00 is that these granules actually have to physically polarize by moving on microtubule tracks
00:35:22.19 from diverse places within the cytoplasm to this interface, this synapse,
00:35:27.14 between the T-cell and its target. In the absence of genes that are required
00:35:33.10 for the translocation of these granules to this point in the cell, Rab proteins,
00:35:40.02 microtubule binding proteins, etc. these T-cells are incapable of docking
00:35:47.11 with or incapable of fusing with the plasma membrane of the T-cell close to the target.
00:35:54.02 Again, emphasizing one critical element of where membrane traffic
00:35:58.25 plays a major role in allowing the immune system to conduct its job.
00:36:03.00 Now how does this all work, we've discussed already a bit how TLR's activate
00:36:11.16 cells such as macrophages in the innate immune system. T-cells become activated
00:36:16.20 in two very important ways. The T-cell receptor itself is a signaling molecule,
00:36:23.01 recruiting tyrosine kinases to its cytoplasmic domain after it recognizes its ligand,
00:36:29.22 which as we've been discussing is a complex of peptides plus MHC molecules.
00:36:34.00 But also a variety of other interactions take place so-called co-stimulatory receptors
00:36:41.02 which are other receptors that are present on the surface of T-cells,
00:36:45.02 recognize a variety of other molecules that can be found on the surface of infected cells
00:36:49.28 or on B-cells or on other cells of the immune system, that send further signals
00:36:54.24 to the T-cell, enabling it to more effectively do its job and also amplify its own
00:37:01.03 numbers in a way that is selective and antigen driven,
00:37:05.12 quite analogous to what happens in the case of B-cells. Further, selected cells in the immune system
00:37:12.05 which are referred to as antigen-presenting cells or APCs,
00:37:15.22 which we'll come back to in a later lecture are capable of secreting their own sets of cytokines,
00:37:22.13 which have the effect of further accentuating and enhancing T-cell responses.
00:37:27.01 So here we have two aspects, two critical aspects of the immune system.
00:37:32.04 Innate immunity, which is characterized by direct cellular responses
00:37:37.09 to pathogens by detecting shared microbial patterns that is not antigen specific,
00:37:43.09 working together hand in glove with Ehrlich's observation of the adaptive immune response,
00:37:50.20 which generates specific antibodies to variable antigens,
00:37:54.01 and occurs as a consequence of an antigen stimulation of T-cells and B-cells. These two things,
00:38:01.09 as I've already intimated to you are connected, but they're connected in even a more intimate
00:38:06.01 fashion, which we'll turn to in the next lecture, provided by a recently identified
00:38:11.10 cell type referred to as the dendritic cell, which really we now understands
00:38:15.16 provides the long sought after missing link between the innate and adaptive immune system.
Dr. Brittany Anderton obtained her PhD in biomedicine from UCSF in 2015. After that, she did a non-traditional postdoc at UC Davis where she studied the teaching and communication of biotechnology. Brittany has served as adjunct faculty at UC Davis and CSU Sacramento, where she taught introductory biology courses. At iBiology, she seeks to improve… Continue Reading
Ira Mellman is Vice President of Research Oncology at Genentech. Ira is a cell biologist with a long standing interest in membrane traffic. His lab is reponsible for key observations leading to the initial discovery of endosomes, the mechanisms of epithelial cell polarity, and the cellular basis of dendritic cell function. Until 2007, Ira was… Continue Reading