The Peritrophic Matrix Mediates Differential Infection Outcomes in the Tsetse Fly Gut following Challenge with Commensal, Pathogenic, and Parasitic Microbes

Abstract

The insect gut is lined by a protective, chitinous peritrophic matrix (PM) that separates immunoreactive epithelial cells from microbes present within the luminal contents. Tsetse flies (Glossina spp.) imbibe vertebrate blood exclusively, and can be exposed to foreign microorganisms during the feeding process. We employed RNAi-based reverse genetics to inhibit the production of a structurally robust PM, and then observed how this procedure impacted infection outcomes following per os challenge with exogenous bacteria (Enterobacter sp. and Serratia marcescens strain Db11) and parasitic African trypanosomes. Enterobacter and Serratia proliferation was impeded in tsetse that lacked an intact PM because these flies expressed the antimicrobial peptide gene, attacin, earlier in the infection process than did their counterparts that housed a fully developed PM. Following challenge with trypanosomes, attacin expression was latent in tsetse that lacked an intact PM and these flies were thus highly susceptible to parasite infection. Our results suggest that IMD signaling pathway effectors, as opposed to reactive oxygen intermediates, serve as the first line of defense in tsetse’s gut following the ingestion of exogenous microorganisms. Furthermore, tsetse’s PM is not a physical impediment to infection establishment, but instead serves as a barrier that regulates the fly’s ability to immunologically detect and respond to the presence of these microbes. Collectively, our findings indicate that effective insect antimicrobial responses depend largely upon the coordination of multiple host and microbe-specific developmental factors.

Mammalian African trypanosome VSG coat enhances tsetse’s vector competence

“…1A). The Imd pathway produced antimicrobial peptides that have been shown to be important trypanolytic effectors (14,(16)(17)(18). The induction of gut immune-signaling pathways likely results from recognition of parasite-associated molecular patterns by the gut epithelia.…”

“…Tsetse that present an intact PM fail to immunologically detect this microbe, which thus proliferates rapidly in the gut lumen and translocates into the hemolymph and kills the fly by sepsis (18). Conversely, when PM structure is experimentally compromised, the fly's immune system can detect the presence of Serratia in the gut early in the infection process and express robust antimicrobial immunity that limits pathogen replication and increases host survival (18). Thus, tsetse's survival upon acquisition of Serratia reflects the functional integrity of the PM.…”

“…We addressed this question by feeding different groups of tsetse a blood meal supplemented with heat-killed extracts prepared from bloodstream-form Trypanosoma brucei brucei, T. b. rhodesiense, and T. congolense or from procyclic forms of the same T. b. brucei and T. b. rhodesiense parasites, respectively. To determine the physiological effect of these treatments, we subsequently used a highly sensitive microbial infection assay that has been used as an indicator of PM functional integrity in tsetse and Drosophila (18,35). All extract-treated flies were provided a blood meal inoculated with entomopathogenic Serratia marcescens.…”

Modulation of Midgut Peritrophins' Expression during <i>Plasmodium Infection</i> in <i>Anopheles stephensi</i> (Diptera:Culicidae)

“…In a recent study, it was shown that PM influences parasitic microbial infection outcome in the Tsetse flies (Glossina sp.) 11 . Our data provides the first definitive evidence of variations in peritrophin expression during Plasmodium infection.…”

VSG overcomes an early barrier to survival of African trypanosomes in tsetse flies

“…Moreover, unraveling the mechanism by which VSG is specifically internalized by cells of the cardia can potentially shed light on how to block an event critical to trypanosome survival in the gut. Although it is well recognized that susceptibility to infection is multifactorial, influenced not only by age of the fly but also by other factors, such as the composition of its gut microbiome (13,20), it remains that interruption of trypanosome development in the tsetse at such an early step in gut colonization represents an attractive target for parasite control. The work by Aksoy et al (14) represents a major advancement in our understanding of tsetse-trypanosome molecular interactions, revealing how the parasites migrate from the gut lumen into the ectoperitrophic space.…”