Characterization of monoclonal antibody against IgM and phagocytic function of IgM+ B cells in snakehead (Channa argus).

Immunoglobulin (Ig) M (IgM) expressed on B lymphocytes functions as a transmembrane antigen receptor within the B-cell receptor (BCR) complex, and secreted IgM is an essential component of adaptive humoral immune response. Monoclonal antibody (mAb) capable of recognizing IgM serves as a powerful tool for studying B lymphocyte development and humoral immune response. In this work, serum IgM was purified from snakehead (Channa argus) and used to develop mAb. Western blotting confirmed that the mAb specifically recognizes the heavy chain of snakehead IgM, and indirect immunofluorescence assay (IFA) demonstrated its binding to membrane-bound IgM (mIgM) on lymphocyte surface. Flow cytometric analysis (FCA) demonstrated that this mAb recognizes a lymphocyte population distinct from Zap70+ cell population, and IgM transcripts were detected exclusively in sorted IgM+ lymphocytes. The highest frequencies of IgM+ B cells were found in spleen (20.94 ± 2.77 %) and head kidney (21.04 ± 1.87 %), followed by peripheral blood (15.29 ± 2.90 %), while mucosal tissues contained few IgM+ cells (0.46 ± 0.14 % in gills; 0.95 ± 0.17 % in intestines). Moreover, IgM+ B lymphocytes in snakehead were shown to phagocytose fluorescence microspheres and bacteria. Following the stimulation with infectious hemorrhagic syndrome virus (IHSV) or the bacterial pathogen Nocardia seriolae, the proliferation of IgM+ B cells increased significantly in spleen, head kidney and blood, accompanied with a notable upregulation of mIgM, CD79a and CD79b. Collectively, a mAb developed against snakehead IgM was employed to investigate the distribution, proliferation and phagocytosis of IgM+ B lymphocytes in snakehead, which should provide a valuable tool for investigating further the function of IgM and IgM+ B lymphocytes in snakehead.