Regulation of the class of immune response induced by antigen. I. Specific T cells switch the in vivo response from a cell-mediated to humoral mode.

Unprimed murine spleen cells, when administered intravenously to irradiated recipients together with antigen for 7 days, are induced to display either DTH reactivity or to mount a humoral (IgM and IgG) response. The class induced depends on the number of spleen cells given to the irradiated host. A low number of cells does not support the induction of any response, a medium number only gives rise to substantial DTH reactivity, whereas a high number only mounts a humoral (IgM and IgG) response. Observations show that the higher number of T cells in a large inoculum of spleen cells, compared to the number present in a medium one, is responsible for the absence of DTH reactivity and the mounting of a humoral response. This finding suggests that the induction of DTH precursor cells may occur when fewer antigen-specific helper-T-cell-dependent signals are generated than the number of signals required to induce B-cell precursors of the IgM and IgG classes. This possibility is favored by further observations. The administration of in situ irradiated, primed helper T cells to mice reconstituted with a medium number of normal spleen cells, results both in the specific suppression of the DTH response that occurs in the absence of these primed cells and in the mounting of a humoral response.