High microbiota reactivity of adult human intestinal IgA requires somatic mutations

Abstract

<jats:p>The gut is home to the body’s largest population of plasma cells. In healthy individuals, IgA is the dominating isotype, whereas patients with inflammatory bowel disease also produce high concentrations of IgG. In the gut lumen, secretory IgA binds pathogens and toxins but also the microbiota. However, the antigen specificity of IgA and IgG for the microbiota and underlying mechanisms of antibody binding to bacteria are largely unknown. Here we show that microbiota binding is a defining property of human intestinal antibodies in both healthy and inflamed gut. Some bacterial taxa were commonly targeted by different monoclonal antibodies, whereas others selectively bound single antibodies. Interestingly, individual human monoclonal antibodies from both healthy and inflamed intestines bound phylogenetically unrelated bacterial species. This microbiota cross-species reactivity did not correlate with antibody polyreactivity but was crucially dependent on the accumulation of somatic mutations. Therefore, our data suggest that a system of affinity-matured, microbiota cross-species–reactive IgA is a common aspect of SIgA–microbiota interactions in the gut.</jats:p>

Mutations make gut antibodies promiscuous

“…demonstrated that mutations of the antigen-binding Fab domain of gut antibodies from control donors or patients with Crohn's disease (CD) confer crossspecies reactivity to the commensal microbiota (Kabbert et al, 2020). Intestinal antibodies include immunoglobulin A (IgA) dimers and IgM pentamers, which accumulate in the gut lumen as a result of a close cooperation between plasma cells and epithelial cells (Chen et al, 2020b).…”

“…To address this question, a wellestablished methodology (Tiller et al, 2008) was used to generate recombinant monoclonal antibodies from the terminal ileum of control or CD donors (Kabbert et al, 2020). These antibodies were obtained by PCR-amplifying Ig variable heavy chain (VH) and variable light chain (VL) gene products from single-cell sorted gut IgA + or IgG + plasma cells (see panel A in the figure).…”

“…This strategy allowed authors to measure the Fab-mediated binding of IgA or IgG clones to gut bacteria from lymphocytedepleted Rag2-deficient mice (Kabbert et al, 2020). Such mice lack antibodies, which avoided microbiota binding competition between exogenous recombinant and endogenous gut-derived antibodies.…”

“…Such mice lack antibodies, which avoided microbiota binding competition between exogenous recombinant and endogenous gut-derived antibodies. DNA sequencing combined with flow cytometry demonstrated that all microbiota-binding antibodies displayed mutated V(D)J genes (Kabbert et al, 2020). In addition, these antibodies showed cross-species reactivity, as they could recognize a diverse set of commensals from nonrelated taxa (Kabbert et al, 2020).…”

“…DNA sequencing combined with flow cytometry demonstrated that all microbiota-binding antibodies displayed mutated V(D)J genes (Kabbert et al, 2020). In addition, these antibodies showed cross-species reactivity, as they could recognize a diverse set of commensals from nonrelated taxa (Kabbert et al, 2020). Of note, most antibodies lost microbiota reactivity when their mutated (Kabbert et al, 2020).…”