Bacterial and viral infections promote proliferation and local aggregation of IgM+ B cells in fish spleen.

Humoral immunity is a central component of host defense in teleost fish, yet the dynamics of IgM+ B cells during infection remain poorly understood. In this study, we generated and characterized a monoclonal antibody (mAb) against native IgM protein purified from grass carp (Ctenopharyngodon idella) sera. This mAb specifically recognized the recombinant grass carp IgM heavy chain expressed in the HEK293 cells and showed cross-reactivity with IgM molecules from multiple cyprinid species, including rare minnow, zebrafish, black carp, and common carp, but not with IgM from largemouth bass. Flow cytometric analysis showed that IgM+ B cells were abundant in the spleen and head kidney of grass carp, accounting for 35.40 ± 3.15% and 27.63 ± 3.97% of total leukocytes, respectively. Using this antibody, we further investigated IgM+ B-cell responses in grass carp and rare minnow after infection with Aeromonas hydrophila or grass carp reovirus genotype II. Both bacterial and viral infections significantly enhanced the proliferation and local aggregation of splenic IgM+ B cells. Moreover, EdU+ proliferating cells were preferentially enriched within localized IgM+ B-cell clusters in the spleens of infected fish. Taken together, our work provides a useful antibody tool for studying B-cell responses and reveals that bacterial and viral infections induce proliferation and localized aggregation of splenic IgM+ B cells in teleost fish, highlighting infection-induced remodeling of splenic IgM+ B-cell responses.