Evolution of the lymphoid system. I. Evidence for lymphocyte heterogeneity in rainbow trout revealed by the organ distribution of mitogenic responses.

Leukocytes from the various lymphoid tissues of rainbow trout (RBT) were tested for their capacity to respond to the lymphocyte mitogens concanavalin A (Con A), lipopolysaccharide (LPS), and purified protein derivative of tuberculin (PPD). Thymocytes responded to Con A but not to LPS or PPD. In contrast, leukocytes from anterior kidney were stimulated with LPS but not with Con A or PPD. Cells from spleen and peripheral blood were stimulated by each mitogen. However, the degree of stimulation at optimally stimulatory concentrations of each mitogen was distinctive. The finding that the patterns of mitogenic responses of cells from each tissue were significantly different suggested that there is lymphoid heterogeneity in the RBT with a unique tissue distribution. The species source of serum utilized as a medium supplement appeared to be capable of markedly affecting mitogenesis. Thus, LPS and PPD stimulation occurred in medium supplemented with rainbow trout serum (RBTS). On the other hand, LPS and PPD stimulation was not observed in medium supplemented with fetal bovine serum (FBS), with the exception of peripheral blood leukocytes which were stimulated by LPS in culture medium supplemented with FBS. Con A stimulated leukocytes from each lymphoid tissue in medium supplemented with RBTS and, with the exception of cells from anterior kidney, also stimulated cells from each tissue in medium supplemented with FBS. The kinetic profiles of the responses of peripheral blood leukocytes to Con A, LPS, and PPD suggested that the extent as well as the time required for maximal stimulation was dependent on the dose of mitogen.