Intracellular Protozoan Parasites: Trypanosoma cruzi and Leishmania
Transcript of Part 2: Leishmania spp and Leishmaniasis
00:00:02.04 My name is Norma Andrews, I'm a professor at Yale University, 00:00:05.18 and what I'm going to be talking to you about in this second part of my lecture 00:00:09.12 is about Leishmania, this parasite, and the disease that it causes in humans 00:00:15.00 which is called Leishmaniasis. 00:00:17.06 This is also a protozoan parasite from the order Kinetoplastida, 00:00:23.04 family Trypanosomatidae, and inside this family, we find several protozoan organisms, 00:00:30.09 and the two groups that are medically important because they cause serious diseases in man, 00:00:35.26 Trypanosoma which includes Trypanosoma cruzi that I already talked about 00:00:39.23 in the first part of this lecture. 00:00:42.03 And also the African trypanosome that causes sleeping sickness in Africa, Leishmania, 00:00:47.08 which I'm going to introduce to you now is closely related to trypanosomes, 00:00:53.09 and it's also an intracellular parasite. 00:00:58.12 So, here in the distribution of Leishmania throughout the world, 00:01:02.25 we can see that it's not only restricted to South America like Chagas' disease, 00:01:07.21 but is also found in many regions of Africa, the Middle East, and also around the Mediterranean. 00:01:15.09 So, similar to Chagas' disease, Leishmaniasis is also largely a disease of poverty. 00:01:21.07 It is mostly found in underdeveloped regions of the world, 00:01:27.22 but it has some characteristics that actually make it very serious in the sense that 00:01:34.17 development is proving not to be sufficient to stop transmission, 00:01:39.01 and actually there's several signs that the disease is recently moving 00:01:44.23 from rural areas to urban areas of the world. 00:01:48.18 So, this is a very serious development that indicates that this number of people 00:01:54.19 which are currently around 12 million that are already infected by this parasite... 00:01:58.15 There are 350 million people at risk worldwide. 00:02:02.17 So, the instance of Leishmaniasis may actually increase over time. 00:02:07.28 Here we see an example of one of the clinical manifestations of Leishmaniasis. 00:02:14.27 This is a child with a cutaneous lesion, which... The form of the disease in Leishmaniasis 00:02:22.10 is directly associated to the species of Leishmania that causes the infection. 00:02:27.07 So, these three species here, L. major, L. tropica, and L. mexicana 00:02:31.18 are frequently associated with the cutaneous form of the disease 00:02:35.07 which can be very disfiguring, although these lesions most of the time, they are self-healing. 00:02:41.10 There's a more serious form of this cutaneous disease 00:02:45.21 which is the form that attacks the mucosal membranes 00:02:49.06 in which this can really be seriously disfiguring and this is usually 00:02:53.09 associated with some of the species, for example Leishmania braziliensis. 00:02:58.06 The most severe form of Leishmaniasis is by far the visceral form 00:03:03.24 which is caused by these three species, mostly, 00:03:08.28 Leishmania donovani, Leishmania infantum, and Leishmania chagasi, 00:03:12.21 and this disease is characterized by periodic fever, 00:03:18.13 severe enlargement of the liver and the spleen, and also weight loss and anemia. 00:03:25.19 This is significant cause of morbidity and mortality 00:03:30.02 in the endemic areas for visceral Leishmaniasis. 00:03:33.27 This disease was already known to be present. 00:03:38.02 It was called early on Calazar, but what was not known is what was the organism causing it. 00:03:46.27 It was not even clear that it was an infectious disease. 00:03:49.10 So, it was this British physician, William Leishman 00:03:54.03 which analyzed the material from patients in India and also simultaneously, 00:04:01.26 by the Irish investigator Charles Donovan, 00:04:05.26 they realized that macrophages present in the spleen 00:04:10.11 of these patients were heavily loaded with these organisms 00:04:15.26 which they recognized as similar to trypanosomes. 00:04:19.13 So this was the first indication that this was a parasitic disease, 00:04:24.03 and another very important finding in the history of Leishmaniasis 00:04:29.00 was made by Rogers and Nicolle a few years later, 00:04:32.28 in which they took this material from the spleen of the patients and put it into culture 00:04:37.26 and they were able to show that a second form of the parasite appeared in these cultures 00:04:43.19 indicating directly the existence of a life cycle and most likely an insect vector. 00:04:49.17 So this actually was only decades later that the first hint for which could be 00:04:56.09 the insect involved in this transmission was made by John Sinton when he realized that 00:05:02.05 the incidence of visceral Leishmaniasis in Eastern India 00:05:05.01 actually coincided with the distribution 00:05:07.26 of a specific type of sandfly, the silvery sandfly. 00:05:12.05 This rapidly led to a lot of work in which this picture is what emerged 00:05:19.14 in which these regions that I already showed 00:05:22.08 that we can see Leishmaniasis transmitted to humans. 00:05:27.22 In each case, a specific species of Leishmania 00:05:30.05 is associated with a specific species of the sandfly 00:05:33.06 which is responsible for the transmission. 00:05:35.15 And it was only more recently, decades later after these original observations, 00:05:42.09 that there was a formal demonstration that sandflies 00:05:46.25 could transmit Leishmania to humans. 00:05:49.04 Although earlier, it had already been detected in sandflies 00:05:53.13 that had been feeding on an infected animal, but also in humans. 00:06:02.07 It was then only this time around the 40s that 00:06:08.01 the life cycle of Leishmania was really understood, 00:06:12.21 with this replication of the parasites happening inside the sandfly 00:06:17.14 and this is similar to Trypanosoma cruzi in the sense 00:06:23.03 that these parasites also don't gain access to the salivary gland. 00:06:25.23 They are transmitted in this case by regurgitation. 00:06:30.06 So, after a blood meal, this fly delivers these parasites 00:06:36.06 which replicate abundantly in its digestive tract, 00:06:40.06 and this is how they penetrate the mammalian host, 00:06:43.26 which in addition to humans, dogs, and rodents play a major role in this cycle, 00:06:49.17 and it is inside macrophages of the mammalian host that the intracellular replication occurs. 00:06:56.03 More recently, David Sacks at the NIH made another very important observation 00:07:02.23 and contribution to the understanding of the biology of Leishmania parasite 00:07:08.00 which was by identifying an infective stage. 00:07:10.26 So, similar to what I discussed in the first part of this lecture, for Trypanosoma cruzi, 00:07:16.00 Leishmania also has an infective stage, so the parasites that replicate inside the insect vector, 00:07:22.22 they replicate into these infective stages which David Sacks called metacyclic promastigotes. 00:07:29.07 During this differentiation, they also acquire that resistance to lysis by complement, 00:07:34.26 which allows them to survive inside the mammalian host. 00:07:38.01 David Sacks and other groups have made 00:07:42.13 very important contributions in linking this differentiation 00:07:46.13 into the metacyclic form to this surface molecule, LPG, 00:07:50.10 which is a major component of the surface coat of these parasites. 00:07:54.09 What is shown here in this immunogold electron microscopy, 00:07:59.23 these black dots that we can see here in this enlarged segment 00:08:03.20 represent antibodies reacting with this surface molecule 00:08:08.07 and what David Sacks and others showed 00:08:12.05 is that these molecules are lipid-anchored, 00:08:16.05 and they have these repeated disaccharide-phosphate units... 00:08:20.03 This molecule undergoes significant elongation during differentiation 00:08:25.11 and these side groups which are usually terminated in galactose 00:08:29.00 in the epimastigote insect form 00:08:31.13 acquire these side chains terminated by arabinose. 00:08:36.06 This is a key change in one of the specific species, Leishmania major 00:08:41.28 that David Sacks showed that is responsible 00:08:46.01 for a very important part of this transmission cycle, 00:08:48.24 which is the detachment of the parasites from the midgut of the insect, 00:08:54.04 allowing them to be delivered to the infected host. 00:08:57.12 So this change in the structure of this LPG molecule 00:09:01.22 is what mediates detachment and then facilitates transmission 00:09:07.10 during or after the blood meal by the sandfly. 00:09:11.10 Another very important point that emerged recently 00:09:16.13 also from the work of David Sacks at the NIH 00:09:18.28 is that the vectorial competence 00:09:21.06 --the ability of these sandflies to transmit specific species of Leishmania-- 00:09:26.18 is related to polymorphisms on this LPG molecule, so only now, very recently, 00:09:33.25 a much more complete picture has emerged of what are the molecular properties of this parasite 00:09:40.16 that really create these very specific associations between vector 00:09:46.19 and cycles of transmission throughout the world. 00:09:50.24 What is the next frontier now, now that we really understand 00:09:55.20 quite well what is happening inside the sandfly 00:09:58.03 with the delivery of these infective stages, 00:10:00.17 the next frontier is understanding what's happening in the mammalian host. 00:10:04.15 We know that Leishmania mostly replicates inside macrophages, 00:10:08.14 since the original observations of Leishman and Donovan, 00:10:12.12 but we don't understand completely how the parasites actually survive in these cells. 00:10:18.10 Macrophages are cells that are actually designed to destroy intracellular pathogens, 00:10:23.01 and the entry of Leishmania into these macrophages 00:10:26.09 is known to be mediated by phagocytic receptors 00:10:29.01 so in this case, different from Trypanosoma cruzi. 00:10:32.11 There's no indication of an active mechanism of invasion, 00:10:35.26 but it is pretty clear that at least the complement receptors, 00:10:40.14 CR1 and CR3 play an important role 00:10:42.26 because these parasites are not lysed by complement, 00:10:46.06 but they are opsinized by complement, 00:10:48.05 which allows them to be efficiently internalized by macrophages. 00:10:52.03 Inside the macrophages, the really remarkable aspect of the life cycle of Leishmania 00:10:58.04 is that these parasites can survive in compartments 00:11:01.12 that have many properties of lysosomes. 00:11:03.26 They are acidic, they contain a full set of hydrolases 00:11:08.23 and also of lysosomal membrane proteins, 00:11:11.16 and we don't really understand how the parasites are able to spend their whole cycle 00:11:16.28 which also lasts several days--they have a doubling time of around 12 hours-- 00:11:22.01 and what is indicated in this slide here is that even MHC class II, 00:11:27.02 which is a protein expressed by activated macrophages 00:11:31.01 is present on the membrane of these vacuoles containing the parasites 00:11:35.14 that replicate in spite of these harsh conditions. 00:11:40.09 In addition to this remarkable adaptation to survive inside cells of the immune system, 00:11:47.12 which in normal circumstances actually play a role in the elimination of pathogens, 00:11:51.19 another very challenging aspect of Leishmaniasis 00:11:56.22 is the difficulty in limiting exposure of humans to these insect vectors. 00:12:02.11 In this case, this is a very small insect. 00:12:06.16 What this picture here shows is one sandfly sitting on the knuckle of a finger, 00:12:11.24 just to give an idea of how small they are. 00:12:14.24 The simple measures that have been shown to be so effective 00:12:17.25 in containing malaria transmission, for example, 00:12:22.00 just bed nets, really are not effective 00:12:25.11 because these are very small flies that can go through these regular bed nets 00:12:30.15 that are normally used in these endemic areas. 00:12:33.23 What is very clear for Leishmaniasis is that there is a strong need 00:12:37.17 for development of new drugs 00:12:40.01 and new drugs with less toxicity, because again here the drugs currently in use 00:12:44.25 in large parts of the world are still very toxic. 00:12:49.19 The good news is that the recent developments are leading rapidly 00:12:54.19 to the identification, implementation, 00:12:57.10 and clinical trials of new and better drugs 00:13:00.25 with several philanthropic organizations and also non-profit institutions participating actively 00:13:08.00 in this concerted effort to make this very important need in public health met 00:13:17.21 by taking advantage of current state-of-the-art technology. 00:13:24.03 Thank you. In the next segment, I'm going to discuss more the cell biology of infection 00:13:30.07 of both Trypanosoma cruzi and Leishmania.