Gut bacteria induce heterologous immune priming in Rhodnius prolixus encompassing both humoral and cellular immune responses
Insects lack the adaptive, antibody mediated responses of vertebrates, yet they possess a robust innate immune system capable of defending the host against pathogens. Immune priming has been observed in multiple insect species, wherein exposure to a pathogen provides protection against subsequent infections by the pathogen. Less frequently, heterologous immune priming has been observed where exposure to one bacterial species provides protection against other species. We determined that Rhodococcus rhodnii, a gut symbiont of the kissing bug Rhodnius prolixus, induces a strong heterologous immune priming effect, while axenic bugs lacking any gut bacteria are highly susceptible to pathogens in their hemolymph. Commensal Escherichia coli provides a less robust protective effect than R. rhodnii. R. rhodnii must be alive within the insect as dead bacteria do not stimulate immune priming and pathogen resistance. Removal of R. rhodnii from the gut reduces resistance to pathogens while restoring it to otherwise axenic bugs improves resistance to pathogens, though not completely. R. rhodnii and E. coli activate both the Imd and Toll pathways, indicating cross-activation of the pathways and demonstrating the canonical Drosophila immune response has diverged in Hemiptera. Silencing of either pathway leads to a loss of the protective effect. Several antimicrobial peptides are induced in the fat body by presence of gut bacteria. When E. coli is in the gut, expression of antimicrobial peptides is often higher than when R. rhodnii, though R. rhodnii stimulates proliferation of hemocytes and induce a stronger melanization response. Hemolymph from R. rhodnii bugs has a greater ability to convert the melanin precursor DOPA to melanization products than axenic or E. coli-harboring bugs. These results demonstrate that R. rhodniis benefits to its host extend beyond nutritional provisioning, playing an important role in the host immune system. Author SummaryInsects often form beneficial relationships with bacteria allowing them to eat nutritionally deficient diets. In insects that only consume blood, symbionts are necessary to provide B vitamins absent in the host diet. There is a growing appreciation that in some of these symbiotic associations, the bacteria provide services beyond nutrition. We show that in kissing bugs, which feed exclusively on vertebrate blood and require bacterial symbionts for development, these symbiotic bacteria are important in activating the insect immune system. Insects with no gut bacteria are highly susceptible to infection and cannot mount an effective immune response. The bacteria reside exclusively in the insect gut yet protect against infections in the rest of the insects body. The bacteria must be alive to prime the immune system, and the response is dependent on the species of bacteria in the gut, with symbiotic bacteria providing stronger protection against infection and inducing a broader array of immune responses than commensal bacteria. This study expands our understanding of the role of beneficial bacteria in insect immunity and demonstrates that immune systems differ between major groups of insects.