Luminal bacteria coated with IgA and IgG during intestinal inflammation as a new and abundant stimulus for colonic macrophages.

In the gut, secretory immunoglobulin A is the predominant humoral response against commensals, although healthy hosts also produce microbiota-specific IgG antibodies. During intestinal inflammation, the content of IgG in the lumen increases along with the proportion of commensal bacteria coated with this antibody, suggesting signalling through the IgG-CD64 axis in the pathogenesis of inflammatory bowel diseases. In this work, we evaluated day by day the frequency of faecal bacteria coated with IgA and IgG during the development of DSS colitis. We studied the phenotype and phagocytic activity of F4/80 CD64 colonic macrophages, as well as the production of cytokines and nitric oxide by lamina propria or bone marrow-derived macrophages after stimulation with IgA , IgG and IgA IgG bacteria. We found that the percentage of faecal IgA IgG double-coated bacteria increased rapidly during DSS colitis. Also, analysis of the luminal content of mice with colitis showed a markedly superior ability to coat fresh bacteria. IgA IgG bacteria were the most potent stimulus for phagocytic activity involving CD64 and Dectin-1 receptors. IgA IgG bacteria observed during the development of DSS colitis could represent a new marker to monitor permeability and inflammatory progression. The interaction of IgA IgG bacteria with CD64 F4/80 macrophages could be part of the complex cascade of events in colitis. Interestingly, after stimulation, CD64 colonic macrophages showed features similar to those of restorative macrophages that are relevant for tissue repair and healing.