Helminths release anti-microbial peptide-like molecules that are immunomodulatory

Posted by Kasra

In this paper, the authors have studied peptides that are found secreted by helminths Schistosoma mansonai and Fasciola hepatica and closely resemble mammalian antimicrobial peptidescathelidins to be precise.

First, the backgrounds: S. mansonai and F. hepatica are both trematodes or flukes. Their cyst form can be ingested via contaminated food or water and in the gut they hatch and can migrate to the liver. Like many other parasites, they don’t kill but cause morbidity. They are categorized as neglected tropical diseases and their infections are treatable and obviously preventable.

Anti-microbial peptides are very diverse in sequence and functions among different organisms, but there are similarities in their secondary and tertiary structures. They are produced by many multicellular organisms and their activities can range from bacteriocidal to immune modulatory. Some peptides can have both functions simultaneously,

Thivierge et al. start their paper by an interesting notion: the similarity of the innate immune response to helminths with the immune response to wounds and tissue injury. They are both anti-inflammatory and pro-Th2. Skewing the immune response from Th1 to Th2 and thus leading to less pathology as well as parasite chronicity is a recurring theme in parasite immunology. Read a full review here.

The peptides studied in this paper have similar secondary structure to alpha helical mammalian antimicrobial peptides (Cathelicidins) such as LL-37 and BMAP-28. A feature of these peptides is presence of amphi-pathic helices (hydrophobic on one side and hydrophilic on another side). This was also seen in the predicted secondary structures of peptides that were found to be secreted from S. mansonai and F. hepatica. 

Following this, the authors studied the peptides for a variety of anti-microbial and toxicity activities that are seen with mammalian peptides and found none to be present even at high doses (things such as pore formation, . However, what they did find was the peptides’ ability to modulate functions of immune cells. In this particular case they report inhibition of TNF secretion by macrophages and alteration of antibody secretion by B cells.

Similar secondary structure among mammalian and helminth peptides. (A) shows mammalian peptides with hydrophilic areas marked green and hydrophobic areas marked red. The dotted line and arrows in (B) show hydrophobic patches in the helmnith peptides. From Thivierge et al. 2013. PLoS Negl Trop Dis 7(7): e2307. doi:10.1371/journal.pntd.0002307

What the authors argue from their results is that the similar structure of these peptides to mammalian peptides and yet lack of toxicity allows them to effectively manipulate the immune response in their favor. These modulations could help in blunting of a strong Th1 response with lots of damage to the parasite as well as the host tissue and a milder response leading to parasite chronicity. Knocked-out parasites will better show the extent of importance of these peptides. Nontheless, longterm co-evolution of host and parasites has given rise to these peptides: they are nontoxic and modulatory at least in vitro.  This means plenty of potential in biotech and pharmaceutics!

Thivierge K, Cotton S, Schaefer DA, Riggs MW, To J, Lund ME, Robinson MW, Dalton JP, & Donnelly SM (2013). Cathelicidin-like Helminth Defence Molecules (HDMs): Absence of Cytotoxic, Anti-microbial and Anti-protozoan Activities Imply a Specific Adaptation to Immune Modulation. PLoS neglected tropical diseases, 7 (7) PMID: 23875042


Control of cytokine production in vivo

Last week I attended a talk by Dr. Markus Mohrs where he introduced the idea of a dual-reporter mouse model that they had developed some years ago to measure cytokine production in vivo. It fascinated me so much that I decided to go through his research a bit and read about their exciting findings, especially because they used parasites such as Heligmosomoides polygyrus, Toxoplasma gondii and Schistosoma mansoni as their infection models.

Th2 immune response is highlighted by IL-4 production. But when looking in vivo, it is impossible to find the source of the IL-4 that is present in the biological fluids. So in their study published in Immunity in 2005, the authors used an intestinal infection by nematode  H. polygyrus  as their model. This worm resides solely in the intestine and induces a robust Th2 response. they engineered a GFP sequence preceded by  an internal ribosome entry site (IRES) after the final exon of the IL-4 gene. In this way, when the IL-4 mRNA gets transcribed, the ribosome can bind separately to the IRES and translate GFP. Therefore, cells that are producing IL-4 mRNA could be identified. What they surprisingly found was that, CD4+ GFP+ T cells were not always producing IL-4 unless secondarily stimulated. So, they replaced the IL-4 allele on the other chromosome with a Human CD2 gene. Presence of huCD2 on cell surface would now be indicative of IL-4 production.

They therefore showed that CD4+ Tcells could be either ”IL-4 competent” or ”IL-4 producing”. They observe a very strong post-transcriptional control and selective secretion of IL-4 . Various types of cells start producing IL-4 mRNA but only select ones in certain areas actually produce the cytokine after receiving the proper secondary signal. This idea was tried on the key Th1 cytokine IFN-gamma in another study using influenza virus and Toxoplasma as infection models and similar results were found. Interestingly, the iconic pro-inflammatory cytokine IL-1beta also takes a similar route, but in this case, both control levels are post-translational: First signal induces pro-IL-1beta production and second signal induces its maturation via the inflammasome complex. These key cytokines have receptors on plenty of cells all through the body and their aberrant release can cause serious problems, no wonder multiple controls levels are set for their release.  Studies of this kind should really alert us on interpretation of microarray or qRT-PCR data.

Mohrs K, Wakil AE, Killeen N, Locksley RM, & Mohrs M (2005). A two-step process for cytokine production revealed by IL-4 dual-reporter mice. Immunity, 23 (4), 419-29 PMID: 16226507