Phylogeny of lymphocyte heterogeneity: The thymus of the channel catfish

The number of thymocytes (approximately 3 × 107) that were recoverable from fingerling channel catfish remained constant from about 3 to 10 months of age, i.e. from September to April following hatching the previous June. Between 11 and 12 months, i.e. May and June, the thymus dramatically increased in size with 3 × 109 thymocytes being recoverable from the tissue of individual fish. The thymus remained enlarged for several months (throughout the summer) but at about 15 months (in September) began to involute such that by 17 months (November) no thymus tissue could be seen macroscopically. This natural involution could be accelerated by subjecting the fish to handling and transport stress. Thymocytes of channel catfish aged 4 to 16 months exhibited reactivity with monoclonal antibodies against peripheral T cells but not B cells. Thymocytes responded to the mitogen Concanavalin A only in the presence of added accessory cells (peripheral blood monocytes) or a monocyte-derived supernatant (presumably containing IL-1) at permissive temperatures (27°C). Thymocytes could also be induced to divide at nonpermissive temperatures (17°C) when incubated in the presence of the following combinations of stimulants, a) the phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate (TPA) and the calcium ionophore A23187, b) TPA and ConA, or c) A23187 and ConA. In those cases where TPA or A23187 were used, accessory cells or their products were not needed. Collectively, these results support the notion that channel catfish thymocytes functionally mimic those lymphocytes in the peripheral blood previously designated as T cells.