How insects fight infections: Kill as much as you can first, AMPs take care of the rest

Posted by: Kasra Hassani

Recently antimicrobial peptides (AMPs) have received a lot of attention due to their ubiquitous presence in defence systems along with diversity of structure and function and of course putative commercial and therapeutics usages. Organisms as diverse as bacteria, fungi, insects and vertebrates possess a ‘personalized’ set of AMPs that fight invaders with low or no effect on hosts; interestingly, AMPs seems to be highly tolerant to emergence of resistance.

A study in Science by Haine et al. has suggested that insects use a two step mechanism in fighting infections. Firstly, up to 99.5% of the bacteria are killed by the phagocytic haematocytes and other immune mechanisms of the insects within the first few hours. Secondly and interestingly, the remaining low percentage which have been selected due resistance to the first immune response are ‘mopped up’ by a load of AMPs secreted from the host for the following days (up to two weeks). Because different AMPs with different properties and functions (pore forming, modulatory, inhibitory…) are secreted at the same time, very low chances for emergence of resistance remains for the surviving bacteria. The authors have highlighted that when thinking of AMPs for therapeutic purposes, their exact ecological role in nature has to be kept in mind.

Picture from Schneider and Chambers, Science 2008

Leishmania-induced IRAK-1 inactivation via host SHP-1 activation

Posted by Issa Abu-Dayyeh

I will hereby post the author summary of my most recent publication regarding modulation of macrophage signaling by the Leishmania parasite. The paper has been published on the 23rd of December 2008 in the Public Library of Science (PLoS) Neglected Tropical Diseases, and demonstrates for the first time the ability of Leishmania to inactivate IRAK-1 through its ability to activate host PTP-SHP-1.

Citation: Abu-Dayyeh I, Shio MT, Sato S, Akira S, Cousineau B, et al. (2008) Leishmania-Induced IRAK-1 Inactivation Is Mediated by SHP-1 Interacting with an Evolutionarily Conserved KTIM Motif. PLoS Negl Trop Dis 2(12): e305. doi:10.1371/journal.pntd.0000305

Here is the author summary of the paper:

Leishmania developed several methods to seize control of macrophage signalling pathways in an effort to inactivate their killing abilities. One effective method utilized by the parasite is the activation of host protein tyrosine phosphatases, specifically SHP-1. This increased phosphatase activity contributes to the inactivation of signalling molecules involved in critical macrophage functions such as NO and cytokine production. Interestingly, the absence of SHP-1 results in stronger macrophage inflammatory responses to a bacterial cell wall component known as LPS, a molecule detected by macrophages through Toll-like receptors (TLRs). This observation suggested a role for SHP-1 in the regulation of TLR signalling. Our study reveals that upon Leishmania infection, SHP-1 is able to rapidly bind to and inactivate a critical kinase (IRAK-1) in this pathway. This regulatory binding was shown to be mediated by an evolutionarily conserved motif identified in the kinase. This motif was also present in other kinases involved in Toll signalling and therefore could represent a regulatory mechanism of relevance to many kinases. This work not only reports a unique mechanism by which Leishmania can avoid harmful TLR signalling, but also provides a platform on which extensive investigation on host evasion mechanisms and regulation of cellular kinases can be gained.

For further information please check the citation below to read the whole paper. PLoS is an open-access journal and therefore offers free access to its contents to anybody anywhere in the world!

P.S.: Abstract translations are also available in the following languages: Arabic, French, Spanish, and Farsi.

MHC I and MHC II, simple and clear, once and for all!

posted by: Issa Abu-Dayyeh

Every couple of months I ultimately get into a discussion about the role of MHC class I and II in the activation of immune functions. What drives their transcription/translation, what cells produce them, how are the peptides found and loaded on them; these are many questions that do not seem to go in the long-term memory section of my brain as well as many others.

Here I decided -once and for all- to summarize the dogmas in the aspects mentioned previously.

MHC class I are molecules produced by all nucleated cells, their production is augmented by IFN-alpha, IFN-beta, IFN-gamma, and TNF-alpha. They are loaded with peptides that result from proteasome-mediated degradation of proteins found in the cytosol, they are transported to be expressed on the surface of cells, and MHC I molecules bound to foreign peptides activate CD8+ T cells by binding to their TCR.

In other words, these MHC molecules keep the internal contents of molecules in check. This is consistent with the fact that such a mechanism is very useful against cancer cells and cells infected with viruses that are actively producing their proteins inside the cells.

On the other hand, MHC class II molecules are produced by antigen presenting cells of the immune system: mainly dendritic cells, macrophages, and B cells. Their production is primarily driven by IFN-gamma (and not the other interferons), they are loaded with peptides that are generated by peptidases found inside phagolysosomes, and upon their translocation to the cell surface, they activate CD4+ T cells.

Two main comments come to my mind upon writing those “facts”:

1- It is impressive how those two MHC types seem to complement each other’s function. MHC I screens for internal “problems”: viruses, cancer, and other antigens that are hiding inside cells away from immune detection, while MHC class II are directed against obvious intruders that go inside the cells by phagocytosis (or any surface detection mechanism ex: TLR) such as:  bacteria and parasites. Together, those two molecules work hand in hand in keeping the organism as alert as possible to all sorts of invaders.

2-Although it is interesting per se that interferons and other pro-inflammatory cytokines upregulate the production of those molecules, it is even more interesting to see that molecules such as IFN-gamma which is not typically involved in counter-acting viral infections can upregulate MHC type I molecules. To me, this suggests that an invasion of the immune system by viruses must somehow lead to the production of danger signals recognized by IFN-gamma producing cells (or their activators) and ultimately leads to a response against those viruses through MHC I upregulation (Future work in the field will be the judge!)

In the end, it is as if the immune system is on “code red” and asks each cell to rapidly disply its ID card to the immune police….Normal cells will show the good ID and pass, suspicious cells will display a “wanted” ID and are destined to be eliminated.

Isn’t it amaizing???

We know Nirtic Oxide is produced when SHP-1 is absent, but why??

Posted by: Issa Abu-Dayyeh

The exact role played by the protein tyrosine phosphatase (SHP-1) in the negative regulation of macrophage functions has been an active area of research for many years. In fact, SHP-1 deficient mice are hyper-inflammatory. They lose their hair “for God’s sake” due to exaggerated inflammatory responses in the skin area! (hence their name motheaten). But what does this tell us? It tells us a lot of pathways are simply “on fire”. To dissect every single pathway controlled by this PTP is a humongous job, and the best approach in my opinion is to try to focus, and dissect a pathway at a time and a function at a time. So, what did we attempt to do in our most recent publication (Blanchette, J. et al.) in Immunology (2008)?

The paper explores the signaling pathways that seem to be major contributors to NO production in SHP-1 deficient macrophages. NO production is driven by a gene known as iNOS whose expression is driven by several transcription factors, most importantly: Nf-kB, STAT, and AP-1. One of those transcription factors “AP-1″ is activated by a MAP kinase called JNK.

This work utilizes inhibitors of many of these members to see which of them will be able to suppress that excess NO production observed in SHP-1 deficient macrophages in an effort to understand how SHP-1 causes this increased NO production.

To save you the dull experimental details…Results showed that the exaggerated NO production in SHP-1-/- macrophages seems to be due to an increased JNK/AP-1 and not NF-kB activity.

And so what? some people might ask!

Well…I agree a finding like this might not find a cure to leishmaniasis. Nevertheless, bearing in mind that NF-kB translocation is increased in the absence of SHP-1, this paper then suggests something rather important. This increased NF-kB activity is not contributing to iNOS transcription. What is it doing then? and how can iNOS be differentially regulated? These are questions that await answers. (If somebody has answers, I will be glad to hear from them).

This work simply broadens our knowledge about where SHP-1 exerts its effects, and by knowing how, we can probably try to eventually revert some of those actions during the course of a Leishmania infection and help find an effective drug against leishmaniasis that is not as toxic as the ones available nowadays…

A block added to the wall. that is how I see it.

If you are interested in viewing the paper, please visit it here

Enjoy,

Issa

Altruism in Leishmania: apoptotic parasites are required for infectivity of metacyclic promastigotes

Posted by Kasra Hassani

Suppression of the innate immune response and inhibition of activation of phagocytes that would otherwise kill the parasites has long been established as mechanisms of immune evasion and persistence among Leishmania parasites.

In their paper, van Zandbergen et al. have indicated presence of a high ratio (more than 40%) of apoptotic cells in the metacyclic/stationary phage parasites. They have characterized these cells by occurrence of phosphatidyl serine (PS) in the outer leaflet of plasma membrane as well as PS-binding protein Anexin A5(AnxA5). The majority of AnxA5+ cells have been shown to be apoptotic and different in morphology to infective parasites and they have shown that depletion of these apoptotic cells from the infective population substantially abrogates infectivity.

Apoptotic cells induce production of TGF-beta and IL-10 which are anti-inflammatory cytokines; these cytokines are produced as well by neutrophils when they phagocyte apoptotic Leishmania. Apoptotic parasites also hamper secretion of TNF-alpha, all of which results in inactivation of neutrophils and later macrophages and their inability to kill the phagocytosed parasites.

This is an interesting example of altruism among single-cell populations; the authors have suggested that apoptosis is probably triggered in late log phase and stationary phase promastigotes in the sandfly midgut due to nutrient depletion prior to their entry into the mammalian host.