In Vitro Analysis of the Tissue Distribution, Lifespan, and Antigen-Dependent Maintenance of Specific Antibody-Secreting Cells in Nile Tilapia

This study aimed to elucidate the key characteristics of the humoral immune response and the tissue distribution of specific antibody-secreting cells (ASCs) in Nile tilapia (Oreochromis niloticus). A specific immune model was established by immunizing fish with human IgG. Lymphocytes were isolated from the head kidney, spleen, and peripheral blood and subjected to antigen stimulation in vitro. The MTT assay, reflecting cell metabolic activity and viability, identified the optimal culture conditions as a cell concentration of 2.5 × 106 cells/mL, an antigen concentration of 2 μg/mL, and a culture duration of 72 h. Under these conditions, peripheral blood lymphocytes exhibited the most increase in metabolic activity, followed by head kidney lymphocytes, while splenic lymphocytes showed no significant response. Subsequent dynamic monitoring of antibody-secreting cells (ASCs) using ELISPOT revealed that, in the absence of antigen stimulation, ASC numbers from all three tissues declined over time. Notably, head kidney ASCs retained approximately 50% of their initial number by day 5, whereas ASCs in peripheral blood and spleen decayed to barely detectable or completely undetectable levels, respectively. These findings suggest that the head kidney may serve as a primary site for ASC persistence during the effector phase, potentially contributing to sustained humoral immunity. Although antigen stimulation did not induce significant ASC expansion, it significantly slowed their decay rate (p < 0.05), indicating an antigen-dependent maintenance role. ELISA detection of antibody levels in the culture supernatants showed a consistent trend with the ELISPOT results, further confirming the sustained functional support of antigen for ASCs. Additionally, LPS stimulation experiments demonstrated that all three tissues contained plasmablasts activatable by non-specific stimuli, with peripheral blood showing the highest proliferation fold (4–6 times). In conclusion, this study provides insights into the tissue-specific distribution, in vitro persistence, and antigen-dependent maintenance of ASCs in Nile tilapia, providing insights into the cellular basis that may contribute to humoral immune memory and laying a theoretical foundation for the rational design and application of tilapia vaccines.