Gut bacteria induce heterologous immune priming in Rhodnius prolixus encompassing both humoral and cellular immune responses Public Library of Science 12/01/2025 10660069 10.1371/journal.ppat.1012947 PLOS Pathogens 1553-7374 21 12 Carissa A GillilandLoretta Mugo-KamiriSara MartinKevin J VogelPedro F Vale Insects lack the adaptive, antibody mediated responses of vertebrates, yet they possess a robust innate immune system capable of defending themselves against pathogens. Immune priming has been observed in multiple insect species, wherein exposure to a pathogen provides protection against subsequent infections by the pathogen. Heterologous immune priming has also been described, where presence of one bacterial species provides protection against another. We determined thatRhodococcus rhodnii, a gut symbiont of the kissing bugRhodnius prolixus,induces strong heterologous immune priming, while axenic bugs lacking gut bacteria are highly susceptible to pathogens. CommensalEscherichia coliprovides less robust protection.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 axenic bugs improves pathogen resistance, though not completely. Unlike most other examples of symbiont-mediated immune priming, we find no evidence thatR. rhodniiever leaves the gut, despite activating a potent immune response in the hemocoel and fat body.R. rhodniiandE. coliactivate both the IMD and Toll pathways indicating cross-activation of the pathways, while 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. rhodniiis present, whileR. rhodniiinduces proliferation of hemocytes and induces a stronger melanization response thanE. coli. 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.

]]>