B Cell Heterogeneity in the Teleost Kidney: Evidence for a Maturation Gradient from Anterior to Posterior Kidney

The fish immune system is quite different from the mammalian system because the anterior kidney forms the main site for hematopoiesis in this species. Using transcription factor-specific Abs derived from the murine system, together with anti-trout Ig Abs and Percoll gradient separation, we analyzed B cells from trout kidney sections and compared them to those from spleen and blood. For this study, immune cells were separated by Percoll gradients, and the resulting subpopulations were defined based on expression of B cell-specific transcription factors Pax-5 and B lymphocyte-induced maturation protein-1, as well as proliferative and Ig-secreting properties. Comparison of kidney, blood, and spleen B cell subsets suggest that 1) the anterior kidney contains mostly proliferating B cell precursors and plasma cells; 2) posterior kidney houses significant populations of (partially) activated B cells and plasmablasts; and 3) trout blood contains resting, non-Ig-secreting cells and lacks plasma cells. After LPS induction of resting B cells in vitro, the kidney and spleen have a high capacity for the generation of plasma cells, whereas the blood has virtually none. Our results indicate that trout B cell subsets are profoundly different among blood, anterior kidney, posterior kidney, and spleen. We hypothesize that developing B cells mature in the anterior side of the kidney and then migrate to sites of activation, either the spleen or the posterior kidney. Lastly, our data support the notion that the trout kidney is a complex, multifunctional immune organ with the potential to support both hemopoiesis as well as humoral immune activation.