Different IgM+ B cell subpopulations residing within the peritoneal cavity of vaccinated rainbow trout are differently regulated by BAFF.

In teleost fish, IgM+ B cells are one of the main responders against inflammatory stimuli in the peritoneal cavity, as IgM+ B cells dominate the peritoneum after intraperitoneal stimulation, also increasing the levels of secreted IgM. BAFF, a cytokine known to play a major role in B cell biology, has been shown to be up-regulated along with its receptors in the peritoneum of rainbow trout upon antigenic exposure, however, the regulatory mechanisms underneath this response remain unclear. In this study, we have identified two different IgM+ B cell types residing in the peritoneal cavity of previously vaccinated rainbow trout (Oncorhynchus mykiss): IgD+IgMhiMHCIIhi cells, resembling naïve B cells, and IgD-IgMloMHCIIlo cells, resembling antibody-secreting cells. Based on their membrane IgM levels, these cell types were named IgMhi and IgMlo B cells, respectively. As each of these B cell populations showed a distinct expression pattern for the different BAFF receptors, we studied the effect of BAFF individually on each cell subset. Recombinant BAFF promoted the survival of IgMlo but not IgMhi B cells in vitro, resulting in increased levels of IgM-secreting cells. In contrast, BAFF increased the levels of membrane MHC II only on IgMhi B cells, suggesting different functions on these B cell subsets. Moreover, we also showed that peritoneal IgMhi B cells expressed BAFF at levels comparable to those seen on myeloid cells. These results point to BAFF as a main regulator of B cell homeostasis in the peritoneal cavity, suggesting that this cytokine can trigger different signals on different peritoneal B cell subsets in a specific manner.