Immunomolecular and reactivity landscapes of gut IgA subclasses in homeostasis and inflammatory bowel diseases.

The human gut includes plasma cells (PCs) expressing immunoglobulin A1 (IgA1) or IgA2, two structurally distinct IgA subclasses with elusive regulation, function and reactivity. We show here that intestinal IgA1+ and IgA2+ PCs co-emerged early in life, comparably accumulated somatic mutations, and were enriched within short-lived CD19+ and long-lived CD19− PC subsets, respectively. IgA2+ PCs were often clonally related to IgA1+ PCs and a subset of them presumably emerged from IgA1+ precursors. Of note, secretory IgA1 (SIgA1) and SIgA2 dually coated a large fraction of mucus-embedded bacteria, including Akkermansia muciniphila. Disruption of homeostasis by inflammatory bowel disease (IBD) increased newly formed and actively proliferating IgA1+ plasmablasts, depleted long-lived IgA2+ PCs, and increased SIgA1+SIgA2+ gut microbiota. Such increase featured enhanced IgA1 reactivity to pathobionts, including Escherichia coli, combined with depletion of beneficial Akkermansia muciniphila. Thus, gut IgA1 and IgA2 emerge from clonally related PCs and show unique changes of both frequency and reactivity in IBD. scRNA and V(D)J sequencing of alive lamina propria cells from an histological normal human ileal sample.