Ontogeny of T cell function. I. Acquisition of helper cell activity by the thymus

The ontogeny of the ability of thymus cells to "help" the response to a T-dependent antigen was studied in a cell transfer system. Lethally irradiated, thymectomized mice were reconstituted with thymus cells from donors of various ages together with rabbit anti-mouse brain antiserum and complement- (C) treated bone marrow cells. Mice were immunized with TNP-BGG and the magnitude and affinity distributions of their splenic plaque-forming cell (PFC) responses were assayed 3 weeks later. The results indicate that neonatal thymic cells are capable of helping a direct PFC response but cannot mediate the shift to indirect plaque formation. The ability to mediate the switch to an indirect PFC response is a separate maturation event that occurs between birth and 2 to 4 days of age. The thymus cell population from 2-day-old donors is already capable of mediating selection of high affinity PFC. Neonatal cells residing in an irradiated, thymectomized or nonthymectomized adult recipient for 7 days, even in the presence of adult rabbit anti-mouse brain and C-treated syngeneic spleen cells, did not mature to be capable of mediating an indirect PFC response.