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


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


Apoptosis, not so quiet after all

Posted by Kasra

Apoptosis has been conventionally regarded as a quiet and non-inflammatory event, compared to necrosis which results in release of alarmins and other danger signals inducing inflammation and immune cell recruitment. However, a recent report by Cullen et al. published in Molecular Cell suggests otherwise. They suggest that at least in one form of apoptosis, pro-inflammatory mediators are released by the apoptotic cells and can act as ‘find-me’ signals to the phagocytes to find and clear them.

Fas receptor or CD95 is among the famous apoptosis receptors. It is a member of the TNF receptor family and it induces apoptosis through Caspase-8 activation. Interestingly, Cullen et al. show that different cell types produce pro-inflammatory chemikones such as MCP-1, CXCL1 and MIP-2 when they go through apoptosis via Fas-pathway. They show that this chemokine release is NF-kappa-B mediated and independent of Caspase-8 activation. It is possible be that somewhere during the evolution, the apoptotic pathway cross-linked with the pro-inflammatory signaling pathway and found benefit in it. Accordingly, Cullen et al. show that the Fas-induced pro-inflammatory cytokine/chemokine production still occurs  even if the apoptotic pathway is inhibited showing that these pathways are separate.

Next the authors show that the supernatant from the apoptotic cells can induce migration of macrophages and neutrophils. They also pinpoint the responsible chemokine by depleting them one-by-one. They show that MCP-1 induces macrophage migration and IL-8 recruits neutrophils.

At this point it cannot be said really how inflammatory these apoptotic cells would be in vivo. There are parts of the body where apoptosis occurs constantly,  so this could potentially lead to an unwanted constant inflammation in those areas. Therefore, either different cells would have different levels of apoptotic-proinflammatory chemokine release, or local mechanisms would compensate and counteract the inflammation. More studies will help us understand how apoptotic cells can send their ‘find-me’ signals without causing too much turbulence in their tissue.

Schematic diagram of model proposed by Cullen et al. From

From Cullen et al. Molecular cell, 49 (6), 1034-48

Cullen SP, Henry CM, Kearney CJ, Logue SE, Feoktistova M, Tynan GA, Lavelle EC, Leverkus M, & Martin SJ (2013). Fas/CD95-induced chemokines can serve as “find-me” signals for apoptotic cells. Molecular cell, 49 (6), 1034-48 PMID: 23434371


An intracellular receptor for antibodies

Posted by: Kasra

We usually consider exiting the phagolysosome and entering the cell cytoplasm to be a immune evasion mechanism for pathogens. The pathogens inside the phagolysosome can be processed and presented via MHCII to the adaptive immune system, but once free of that compartment, the pathogen could potentially ‘hide’ from the immune system, well apparently not that much! Apart from the intracellular pattern recognition receptors (NLRs), researchers have found another receptor that responds to intracellular presence of antibodies. McEwan et al. showed that if antibody coated viruses or bacteria have entered the cytosol, presence of the Fc part of the antibody can be sensed by a protein called TRIM21. This could in turn result in an inflammatory and anti-viral response by activating NF-κB and AP-1 and production of cytokines. To me, this is an excellent example that shows how the host and the pathogens have evolved together for many years becoming more and more complex through an arms race.  A newly developed strategy by one party is followed by a counter strategy by the other party.


From Geijtenbeek TB, & Gringhuis SI (2013). An inside job for antibodies: tagging pathogens for intracellular sensing. Nature immunology, 14 (4), 309-11 PMID: 23507635

McEwan WA, Tam JC, Watkinson RE, Bidgood SR, Mallery DL, & James LC (2013). Intracellular antibody-bound pathogens stimulate immune signaling via the Fc receptor TRIM21. Nature immunology, 14 (4), 327-36 PMID: 23455675

Geijtenbeek TB, & Gringhuis SI (2013). An inside job for antibodies: tagging pathogens for intracellular sensing. Nature immunology, 14 (4), 309-11 PMID: 23507635


Phosphatases for and against: Trichuris vs. Leishmania

Posted by Kasra

Trichuris trichiura adult male

Trichuris trichiura adult male – Image taken from DPDx

Trichuris, is an intestinal roundworm, also known as whipworm, that can be transmitted through ingestion of food contaminated with its eggs. The larvae hatch inside the small intestine and complete their life cycle to adults in the cecum. After maturation, which can take about 3 months, the female worm lays thousands of eggs per day. The parasite can stay in the intetine between 1-5 years. Trichuris trichiura is a parasite of humans, while Trichuris muris is a mouse parasite, used usually as the animal model to study its infection.

In contrast to intracellular pathogens, a Th1 response is non-protective in infection with large extracellular pathogens such as intestinal helminths. For instance, during infection with Trichuris muris, a Th2 response comprising IL-4 and Ig-E production leads to resolution of infection, while a Th1 response comprising IFN-gamma, IL-12 and IL-18 is not protective.

S Hadidi et al. look at regulation of the immune response to T. muris and focus on the importance of the macrophage lipid phosphatase Ship1. Ship1 or Sh-2 containing inositol 5′ phosphatase 1 is a regulator of the PI3K pathway. Hadidi et al. show that Ship1 expression is upregulated steadily following T. muris infection. Ship1-/- mice have higher parasite burden and IFN-gamma while lower levels of IL-13. Also, Ship1-/- macrophages produce more IL-12. Blocking IL-12 or IFN-gamma by blocking antibodies rescued the phenotype by reducing worm burden and increase in IL-13. Thus, they found how activity of this phosphatase can direct the immune response against T. muris infection. It would be very interesting now to see what stimuli induce upregulation of Ship1 and also what are this enzyme’s substrates, which are so important for production of IL-12 by macrophages.

Similar to this story, a few years ago, Abu-Dayyeh et al. and Gomez et al. showed that activating phosphatases is important for Leishmania to establish its infection. Being an intracellular parasite, a Th1 response, with large amounts of IFN-gamma would be protective against Leishmania. So in this context, Leishmania-mediated activation of many phosphatases (most importantly SHP-1) leading to inhibition of IL-12 production leads to disease progression, because it skews the immune response towards Th2. In this situation, Leishmania takes advantage of the phosphatase’s function.

Hadidi S, Antignano F, Hughes MR, Wang SK, Snyder K, Sammis GM, Kerr WG, McNagny KM, & Zaph C (2012). Myeloid cell-specific expression of Ship1 regulates IL-12 production and immunity to helminth infection. Mucosal immunology, 5 (5), 535-43 PMID: 22535180

Abu-Dayyeh I, Shio MT, Sato S, Akira S, Cousineau B, & Olivier M (2008). Leishmania-induced IRAK-1 inactivation is mediated by SHP-1 interacting with an evolutionarily conserved KTIM motif. PLoS neglected tropical diseases, 2 (12) PMID: 19104650
Gomez MA, Contreras I, Hallé M, Tremblay ML, McMaster RW, & Olivier M (2009). Leishmania GP63 alters host signaling through cleavage-activated protein tyrosine phosphatases. Science signaling, 2 (90) PMID: 19797268

Can KTIM be a regulatory site widely used by cytosolic kinases?

Identification of Key Cytosolic Kinases Containing Evolutionarily Conserved Kinase Tyrosine-based Inhibitory Motifs (KTIMs).

Posted by: Issa Abu-Dayyeh

I have posted an earlier article to talk about our PLoS NTD paper where we have described a novel strategy by which Leishmania was able to inhibit TLR-mediated macrophage activation through its ability to inhibit IRAK-1 kinase activity by activating the protein tyrosine phosphatase  (PTP) SHP-1.

We have identified the site of binding between SHP-1 and IRAK-1 to be an evolutionarily conserved ITIM-like motif, which we called a kinase tyrosine-based inhibitory motif (KTIM). In this newly-published paper in Developmental and Comparative Immunology, Abu-Dayyeh et al. present evolutinary as well as experimental data that propose that KTIMs could potentially represent a novel regulatory site involved in the control of the kinase activity of many key kinases involved in siganlling pathways of immune cells. Although this work awaits to be further explored by other researches, I believe this work could open various doors towards many important discoveries in the field of immunology.

Here is the abstract of the paper:

We previously reported that SHP-1 regulates IRAK-1 activity by binding to an ITIM-like motif found within its kinase domain, which we named Kinase Tyrosine-based Inhibitory Motif (KTIM). Herein, we further investigated the presence, number, location, and evolutionary time of emergence of potential KTIMs in many cytosolic kinases, all known to play important roles in the signalling and function of immune cells. We unveil that several kinases contain potential KTIMs, mostly located within their kinase domain and appearing predominantly at the level of early vertebrates becoming highly conserved thereafter. Regarding the KTIMs that were found conserved in both vertebrates and invertebrates, we provide experimental data suggesting that such motifs may have constituted readily-available sites that performed new regulatory functions as soon as their binding partners (e.g. SHP-1) appeared in vertebrates. We thus propose KTIMs as novel regulatory motifs in kinases that function through binding to SH2 domain-containing proteins such as SHP-1. Copyright © 2009. Published by Elsevier Ltd.

PMID: 20043942