Immunological process of T cells and IgM+ B cells in head kidney and peripheral blood in early vertebrate Nile tilapia

• T and B cells in HKL and PBL involve in anti-bacterial response of tilapia. • T and B cells in HKL and PBL involve in the secondary immune response of tilapia. • T cells in HKL and PBL perform effector function in tilapia. T and B lymphocytes are the primary effectors of the immune system's specific anti-infective functions, abundantly concentrated in immune-related tissues and organs. The two cell types work in different mechanisms to combat pathogens through precision guidance and jointly accomplish the immune response. Previous studies have elucidated the critical role of splenic T cells in antimicrobial infections in Nile tilapia ( Oreochromis niloticus ). On this basis, this study aimed to further reveal the distribution and effector functions of T and B lymphocytes in the head kidney and peripheral blood of teleosts. Here, the distribution and effector functions of T cells and IgM + B cells were examined using a model of Edwardsiella piscicida infection in Nile tilapia. The results verified that T and B cells were distributed in the peripheral blood and spleen of Nile tilapia, with primary and secondary infection induced remarkable increases of T cells and IgM + B cells in the head kidney and peripheral blood. Notably, T cells and IgM + B cells proliferated rapidly in secondary infection than initial immune response, implying that immune memory as a hallmark feature of adaptive immunity was already present in early vertebrates. More importantly, the inhibition of T cells during bacterial infection impaired the expression of perforin A and granzyme B . Thus, this study indicated the distribution and effector functions of T cells and IgM + B cells during specific immunity in Nile tilapia, and provided theoretical support for understanding the evolutionary basis of the adaptive immune system.