Characterization of an inhibitory allogeneic effect on humoral responsiveness in vitro.

Evidence is presented that the induction of a humoral response is inhibitable by a thymus-derived cell (TI) that acts on the antigen-sensitive precursors of both the thymus-derived cooperating and the bone marrow-derived antibody-secreting cell-that is, the tC and B cell respectively. The inhibition of induction of the tC and B cell by the TI cell is shown to be reversed by increasing the effective level of cooperation. This competitive interaction between the inhibitory (TI) and cooperating (TC) systems is postulated to be part of the mechanism for regulating the class of the response, cell-mediated or humoral. The following properties of the inhibitory system were demonstrated: [1] The tI cell--the antigen-sensitive precursor of the TI cell--is both paralyzable and inducible. [2] The TI cell appears during the induction of a cell-mediated response and, if not identical to the effector cytotoxic ('killer') TK cell, the TI cell is induced in parallel with it. [3] The effector function of the TI cell, like that of the TK cell, is H-2-restricted.