Regulatory role of Fc receptor in mIgM+ B lymphocyte phagocytosis in common carp (Cyprinus carpio).

Fc receptors (FcRs) are crucial opsonin receptors expressed on the surface of immune cells and play an indispensable role in antibody-dependent cellular phagocytosis (ADCP). Our previous study demonstrated that FcRs exhibits a significant transcriptional response in phagocytic IgM+ B lymphocytes of common carp (Cyprinus carpio), suggesting its potential involvement in immunoglobulin-opsonized phagocytosis. In the present study, to elucidate the functional role of FcRs in mediating phagocytosis by IgM+ B lymphocytes, we generated common carp antiserum against Aeromonas hydrophila and employed heat-inactivated serum for opsonization assays, using serum from healthy carp as a control. Flow cytometric analysis revealed that the phagocytic rates of mIgM+ B lymphocytes against antiserum-opsonized A. hydrophila in peripheral blood, spleen, and head kidney after 2 h of incubation were 58.7 ± 0.8%, 55.7 ± 0.9%, and 54.3 ± 0.6%, respectively, all of which were significantly higher than those observed in the control group (37.5 ± 0.5%, 37.2 ± 0.6%, and 34.2 ± 0.8%, respectively). Moreover, following phagocytosis of antiserum-opsonized bacteria, the proportions of mIgM+ B lymphocytes exhibiting high intracellular reactive oxygen species (ROS) levels in peripheral blood, spleen, and head kidney reached 24.9 ± 0.8%, 23.3 ± 0.9%, and 22.9 ± 0.8%, respectively, significantly exceeding those in the control group (13.9 ± 0.4%, 12.4 ± 0.5%, and 12.7 ± 0.4%). In addition, the expression levels of ADCP-related genes, as well as genes associated with myosin and dynein, were significantly upregulated in mIgM + B lymphocytes following phagocytosis of opsonized A. hydrophila. Notably, treatment with the Syk inhibitor markedly reduced the phagocytic capacity of mIgM+ B lymphocytes against antiserum-opsonized bacteria compared to untreated controls. Collectively, these findings indicate that FcRs play a pivotal role in mediating phagocytosis and potential microbicidal response in mIgM+ B lymphocytes, providing new insights into the regulatory role of FcRs in B-cell phagocytosis in teleost.