The xenogeneic effect. II. Requirement for unactivated murine T cells during restoration of immune responsiveness with xenogeneic reconstitution factor.

Studies were conducted on the mechanism of action of a soluble mediator which was generated in human mixed lymphocyte cultures and assayed for helper activity in T cell-deficient murine spleen cell cultures. The mediator, termed xenogeneic reconstitution factor or XFR, restored the anti-sheep erythrocyte plaque-forming cell response of spleen cells from thymectomized, lethally irradiated, and syngeneic bone marrow transplated (TxB) mice. It was found that in order to obtain a maximal antibody response, the XRF had to be present sometime during the first 24 to 40 hr of the induction process. XRF was also required during the last 24 hr of the incubation period. A striking synergistic effect was obtained by combined exposure of the T cell-deficient spleen cell cultures to XRF during the first and last days of culture. These data suggested a bi-modal mechanism of action of XRF, and raised the possibility that in addition to providing a signal to the B cells, XRF-mediated activation of the residual TxB splenic T cells was crucial to the successful restoration of the antibody response. Treatment of the nonadherent splenocytes from the TxB mice with anti-T cell serum and guinea pig complement completely abrogated the antibody response of these cells in the presence of adherent spleen cells, sheep erythrocytes, and the helper factor XRF. The antibody response of this combination of cells, antigen, and XRF was reconstituted by a population of unimmunized murine T cells which, in the absence of XRF, were totally unable to restore responsiveness. These results suggest that B cell activation to the formation of antibody involves the mandatory co-participation of two functionally distinct helper activities.