A new mechanism of immune evasion by Leishmania parasites

Posted by Kasra

It is always pleasant to read papers published by friends and old colleagues.  Also it was about time I would write another post about Leishmania. This new paper published in Cell host and microbe (pubmed index) discusses a new mechanism that Leishmania parasites use to evade the host immunity.

 A quick reminder, the promastigote forms of Leishmania enter the mammalian host via bite of the sandfly. They are readily phagocytosed by macrophages. Yet, they manage to evade to propagate within the macrophage thanks to multiple mechanisms of immune modulation and evasion. 

Previous publications of this group that showed how Leishmania is able to impair maturation of the phagolysosome (for example delay acidification) by interfering with phagolysosome associated proteins. Their previous work had pointed to Leishmania‘s surface lipophosphoglycan LPG. Following their work on the phagosome, they look to see if there are other molecules that are also altered after Leishmania infection, stumbling upon VAMP3 and VAMP8, two SNAREs that are cleaved after infection. They find that this time this time this cleavage is due to another important virulence factor of Leishmania namely surface protease GP63.  This protease has been shown to have many immunomodulatory properties and cleaving many important macrophage proteins (phosphatases, transcription factors…) and now there is a new one on the list.  The authors show that VAMP8 is important for cross-presentation of antigens from MHCII to MHCI. To show this they create ovalbumin expressing expressing L. major and see how presence or absence of GP63 could affect activation of OT-II (ovalbumin-reactive T cells) after coculture with macrophages. Importance of VAMP8 in cross presentation is also shown by using VAMP8-/- cells.

SNAREs are important in vesicle transport and fusion. Therefore they can be targeted by pathogens like Leishmania to impair effective pathogen killing (Image from Wikipedia).

 

What is nice about this study is that by studying host-parasite interactions and immune modulation, it also helps us learn more about the innate immune mechanisms and communication of the innate and adaptive immune systems. 

 

Matheoud D, Moradin N, Bellemare-Pelletier A, Shio MT, Hong WJ, Olivier M, Gagnon E, Desjardins M, & Descoteaux A (2013). Leishmania Evades Host Immunity by Inhibiting Antigen Cross-Presentation through Direct Cleavage of the SNARE VAMP8. Cell host & microbe, 14 (1), 15-25 PMID: 23870310

Olivier M, Atayde VD, Isnard A, Hassani K, & Shio MT (2012). Leishmania virulence factors: focus on the metalloprotease GP63. Microbes and infection / Institut Pasteur, 14 (15), 1377-89 PMID: 22683718
 

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Macrophages commit ‘defensive suicide’ after Adenovirus and Listeria infection

Posted by Kasra

Cells often kill themselves for the benefit of their lot. New forms of cell suicide are being discovered every day now.  I wrote about apoptosis, which is a rather clean form of cell suicide recently. However, necrosis which until recently seemed to be a an uncontrolled form of cell death, is now being looked at again as a form of controlled suicide. A recent publication by  Di Paolo et al in the new journal of Cell Reports sheds some light on on of these rather unusual forms of cell death. The authors call it ‘defensive suicide’.

Di Paolo et al. intravenously injected Adenovirus into the mice. They observed that the macrophages (specifically in this paper, liver macrophages) capture the virus particles. However, shortly after the macrophages died of necrosis. Interestingly, they find this phenomenon to be independent from normal mediators of cell death such as various Caspases, as well as inflammatory mediators such as MyD88, TRIF and ASC. They finally point to IRF3,  a transcription factor normally activated after certain infections. Macrophages from IRF3-/- mice did not go through necrotic death after Adenovirus infection. The authors next show the proteins upstream of IRF3 are dispensable for the necrotic death of macrophages and that IRF3 is not phosphorylated at the time of macrophage necrosis, further adding to the enigma of the mechanism. The only clue we get so far is that this mechanism is dependent on escape the of the pathogen from the phagolysosome into the cytosol. They show this by using Adenovirus and also Listeria monocytogenes  mutants that cannot escape the phagolysosome. Compared to their wildtype counterparts, the mutant intracellular pathogens do not induce necrotic death of the macrophages.

Finally, to see if this necrotic death actually has a benefit for the host, the authors deplete mice from macrophages and infect them again with Adenovirus or L. monocytogenes. They observe that without the macrophages the virus or bacterial burden is a lot higher in the liver. Thus, this mechanism could be a way of slowing down the systemic spread of infection. The macrophages might collect the pathogens that would be otherwise infecting other defenseless cells and destroy them via necrotic death. Would this mean that necrotic death better kills the intracellular pathogens compared to other forms of programmed death? Or they just go through this pathway because other pathways of programmed death are blocked by the pathogens? Considering that necrosis occurs very rapidly (within minutes), the first one seems more likely.

The possible role of IRF3 in induction of necrotic death in macrophage following intracellular infection. From Di Paolo et al. , Cell Reports, Volume 3, Issue 6, 1840-1846, 13 June 2013

The possible role of IRF3 in induction of necrotic death in macrophage following intracellular infection. From Di Paolo et al. , Cell Reports, Volume 3, Issue 6, 1840-1846, 13 June 2013

This mass suicide of macrophages is a very interesting phenomenon. It also raises many questions that have not yet been addressed. The most obvious question is the signaling mechanism by which IRF3 induces this special form of necrosis. The authors did not find any dependence on the proteins that are usually known to be upstream of IRF3. So there might be a novel mechanism involved. Another question concerns the route of infection. The authors have used intravenous injection both for Adenovirus as well as L. monocytogenes infection. However, these pathogens usually enter the body from the gut or the lungs and then reach the circulation system. Would this defensive necrosis extend to the immune cells in other tissues such the lung or the gut macrophages? Would the route of infection affect the intensity/quality of macrophage necrosis? We will hopefully get the answers in the near future!

Di Paolo NC, Doronin K, Baldwin LK, Papayannopoulou T, & Shayakhmetov DM (2013). The Transcription Factor IRF3 Triggers “Defensive Suicide” Necrosis in Response to Viral and Bacterial Pathogens. Cell reports, 3 (6), 1840-6 PMID: 23770239

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A new murine model of Giardia infection: linking pathogenesis to malnutrition

Posted by Kasra

Giardia is a very successful parasite.   It’s highly durable cysts enter the body via contaminated food. Once inside the small intestine, the cysts hatch and the trophozoites start swimming with their multiple flagella. They attach to the intestine’s surface and enjoy the nutrient rich environment of the small intestine.  Shortly after, they start producing cysts that leave the body via feces.  Their presence may or may not cause severe symptoms. In many cases people carrying Giardia in their body – thus shedding cysts –  might not feel anything. On the other hand, Giardia infection can lead to severe diarrhea that could stick around for weeks if untreated. Our immune system usually manages to control the infection and get rid of the parasite. But in any case, the parasite can come, live and go undetected.

Similar to many other parasitic diseases, since Giardia infection does not kill a significant number of its victims, is not rampant in industrialized countries and is more or less readily treatable, it is not funded and studied by many researchers. Unfortunately, despite unique biological features such as lack of mitochondria, presence of two nuclei and an anti-inflammatory host-pathogen interaction, Giardia remains largely understudied.  We don’t fully understand the host and pathogen factors that could lead to disease or just sub-clinical infection. Nor we know much about how Giardia gets detected by the immune system and what is the nature of the immune response that kicks the parasite out of the body.

A scanning electron micrograph of the surface of the small intestine of a gerbil infested with Giardia sp. protozoa. The intestinal epithelial surface is almost entirely obscured by the attached Giardia trophozoites. (Source Wikipedia)

One of the strong incentives for studying Giardia is its higher prevalence in children. Infectious diarrhea is still the most common of cause of child death (Cryptosporidium is another understudied parasite and causative agent of diarrhea in children, which I discussed in another post). A recent study by Bartelt et al. published in Journal of Clinical Investigation, presents a new model of Giardia infection, focusing on malnutrition and young age. They argue that many children in areas where Giardia infection is common are undernourished. This malnutrition could contribute to development of a persistent Giardia infection with severe symptoms rather than a shorter non-symptomatic infection. To study the effect of children malnutrition on Giardia infection, they set their model on 3-week old recently weaned (taken away from mother, eating solid food) mice. They show that although healthy mice are able to clear the infection, Giardia parasites manage to stay longer in the small intestine of malnourished mice and also cause more growth impairment. It can be thought that this is a vicious cycle, where infectious diarrhea causes further weakening of the individual and thus further difficulty in fighting the infection, leading to severe weight loss and persistence of the parasite. Interestingly, the load of parasite in the intestine remains unchanged when comparing healthy and malnurished mice. However, the authors describe their model by pointing to other differences such as immune response and small intestine pathology. More studies on this model can help us better understand and hopefully better treat Giardia infection in children.

Highlight in Nature reviews in Gastroenterology and Hepatology

Bartelt, L., Roche, J., Kolling, G., Bolick, D., Noronha, F., Naylor, C., Hoffman, P., Warren, C., Singer, S., & Guerrant, R. (2013). Persistent G. lamblia impairs growth in a murine malnutrition model Journal of Clinical Investigation, 123 (6), 2672-2684 DOI: 10.1172/JCI67294

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