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


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

Picture from Schneider and Chambers, Science 2008