IgA facilitates the persistence of the mucosal pathogen Helicobacter pylori

IgA antibodies have an important role in clearing mucosal pathogens. In this study, we have examined the contribution of IgA to the immune control of the gastrointestinal bacterial pathogens Helicobacter pylori and Citrobacter rodentium. Both bacteria trigger a strong local IgA response that results in bacterial IgA coating in mice and in gastritis patients. Class switching to IgA depends on Peyer's patches, T-cells, eosinophils, and eosinophil-derived TGF-β in both models. In the case of H. pylori, IgA secretion and bacterial coating also depend on a functional bacterial type IV secretion system, which drives the generation of Th17 cells and the IL-17-dependent expression of the polymeric immunoglobulin receptor PIGR. IgA^-/- mice are hypercolonized with C. rodentium in all examined tissues, suffer from more severe weight loss and develop more colitis. In contrast, H. pylori is controlled more efficiently in IgA^-/- mice than their WT counterparts. The effects of IgA deficiency of the offspring can be compensated by maternal IgA delivered by WT foster mothers. We attribute the improved immune control observed in IgA^-/- mice to IgA-mediated protection from complement killing, as H. pylori colonization is restored to wild type levels in a composite strain lacking both IgA and the central complement component C3. IgA antibodies can thus have protective or detrimental activities depending on the infectious agent.