Teleost IgDIgM B cells in gills and skin have a plasmablast profile, but functionally and phenotypically differ from IgMIgD B cells in these sites.

Although most B cells in teleost systemic compartments co-express IgM and IgD on the surface, cells exclusively expressing either of the two Igs are common in fish mucosal tissues, providing us with a unique opportunity to further characterize IgDIgM B cells, an intriguing B cell subset. Hence, we compared the phenotype of IgDIgM cells to that of IgMIgD B cells in rainbow trout gills and skin, also establishing the response of these subsets to immune stimulation. The transcriptional profile and secreting capacity of IgDIgM B cells corresponded to that of cells that have started a differentiation program toward plasmablasts, similarly to IgMIgD B cells. Yet, IgMIgD B cells retained high levels of surface MHC II and antigen-processing abilities, while these were much lower in IgDIgM cells, suggesting important differences in their antigen-presenting capacities. Our findings contribute to a deeper understanding of the enigmatic role of IgD in mucosal surfaces.