In vivo demonstration of polymorphisms in antimicrobial peptides shaping host-pathogen interactions
Authors: Mark Hanson; Lena Grollmus; Bruno Lemaitre
Affiliation: EPFL
Keywords: e. anti-microbial peptides; h. selection
Antimicrobial peptides (AMPs) are key players in innate defence against infection. In Drosophila, a large array of immune peptides contribute to host defence downstream of the Toll and Imd NF-κB pathways. Our recent work using single and compound AMP mutations confirmed that AMPs can additively or synergistically contribute to combat pathogens in vivo. For instance, Drosocin, Attacin, and Diptericins collectively contribute to defence against Providencia burhodogranariea. However we also revealed a high degree of specificity wherein one AMP can play a major role in combatting a specific pathogen. We found a specific importance of Drosocin for defence against Enterobacter cloacae, and previous work has shown a primary role for Diptericins in defence against Providencia rettgeri. We also recently showed how AMPs shape the fly microbiome over aging, wherein AMP-deficient flies suffered increased microbiota load and community diversity mimicking flies lacking Imd immune signalling. Finally, we recently described how furin cleavage enables a polypeptide AMP gene to produce multiple distinct peptides under the regulation of a single promoter.
Here we take advantage of newly-available mutations to dissect the specificity of these interactions in an isogenic genetic background. We confirm the Drosocin peptide specifically confers defence against Enterobacter cloacae, but also investigate a previously uncharacterized Drosocin peptide downstream of its furin cleavage site. This undescribed peptide specifically contributes to defence against P. burhodogranariea, while the classic Drosocin peptide does not in fact contribute to this defence. Moreover a Threonine/Alanine polymorphism in this uncharacterized Drosocin peptide mirrors the effect of gene deletion in defence against P. burhodogranariea. Thus the Drosocin gene confers defence against different pathogens using two distinct peptide products.
At the same time, we dissect the contribution of the two Diptericin genes in defence against P. rettgeri and other bacteria. We confirmed previous findings that Diptericin A is specifically important in defence against P. rettgeri. Surprisingly, we also uncovered a highly specific interaction between Diptericin B and systemic infection by a member of the gut microbiome. This finding of alternate specificites for the two Diptericin genes in defence against environmentally-relevant microbes paints a picture for how the immune system encodes innate defences. We show not only how AMPs and their alleles are critical to defence against specific natural enemies, but also provide a needed example for how specificity in defence can be derived from the evolution of AMP gene sequence and structure.