Preparation of the monoclonal antibody against Nile tilapia Ig and study on the Ig⁺ B cell subset in Nile tilapia

Immunoglobulins (Igs) are important effector molecules that mediate humoral immunity. A typical Ig consists of two heavy and two light chains. In teleosts, three Ig heavy chain isotypes (Ig mu, Ig delta and Ig tau) and three Ig light chain isotypes (Ig kappa, Ig lambda and Ig sigma) have been identified. Compared to the heavy chains, teleost Ig light chains have been poorly studied due to the lack of antibodies. In this study, a mouse anti-Nile tilapia Ig lambda monoclonal antibody (mAb) was prepared, which could specifically recognize Ig lambda in serum and Ig lambda(+) B cells in tissues. Further, the composition of IgM(+) and Ig lambda(+) B cell subsets was analyzed using this antibody and a mouse anti-tilapia IgM heavy chain mAb. The ratio of IgM(+)Ig lambda(+) B cells to total IgM(+) B cells in head kidney and peripheral blood was about 30%, while that in spleen was about 50%; the ratio of IgM(-)Ig lambda(+) B cells to total Ig lambda(+) B cells in head kidney and peripheral blood was about 45%, while that in spleen was about 25%. The IgM(-)Ig lambda(+) B cells was speculated to be IgT(+) B cells. Finally, we detected an increase in the level of specific antibodies against the surface antigen-Sip of Streptococcus agalactiae in serum after S. agalactiae infection, indicating that mouse anti-tilapia Ig lambda mAb can be used to detect the antibody level after immunization of Nile tilapia, which lays a foundation for the evaluation of immunization effect of tilapia vaccine.