Gut bacteria induce heterologous immune priming inRhodnius prolixusencompassing both humoral and cellular immune responses
Authors: Gilliland Carissa A. , Vogel Kevin J. 
Published: February 2025

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 thatRhodococcus rhodnii, a gut symbiont of the kissing bugRhodnius prolixus,induces a strong heterologous immune priming effect, while axenic bugs lacking any gut bacteria are highly susceptible to pathogens in their hemolymph. CommensalEscherichia coliprovides a less robust protective effect thanR. rhodnii.R. rhodniimust be alive within the insect as dead bacteria do not stimulate immune priming and pathogen resistance. Removal ofR. rhodniifrom the gut reduces resistance to pathogens while restoring it to otherwise axenic bugs improves resistance to pathogens, though not completely.R. rhodniiandE. coliactivate both the Imd and Toll pathways, indicating cross-activation of the pathways and demonstrating the canonicalDrosophilaimmune 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. WhenE. coliis in the gut, expression of antimicrobial peptides is often higher than whenR. rhodnii, thoughR. rhodniistimulates proliferation of hemocytes and induce a stronger melanization response. Hemolymph fromR. rhodniibugs has a greater ability to convert the melanin precursor DOPA to melanization products than axenic orE. coli-harboring bugs. These results demonstrate thatR. rhodnii’sbenefits to its host extend beyond nutritional provisioning, playing an important role in the host immune system.

Author Summary

Insects 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 insect’s 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.