Restriction of the Capacity to Respond to Two Antigens by Single Precursors of Antibody-producing Cells in Culture

Experiments were designed to determine whether or not precursors of antibody-producing cells are restricted in the number of antigens to which they can respond. An in vitro culture system was used, in which the successful production of hemolysin PFC was dependent on the presence of a large number of heavily irradiated spleen cells which did not themselves give rise to PFC, but which supported the production of PFC by a small number of normal spleen cells. All spleen cells were obtained from unimmunized CBA mice. The cells were mixed with either sheep or chicken erythrocytes, or both, cultured for 4 days and analyzed for hemolysin PFC. By reducing the number of unirradiated spleen cells to limiting dilution it was shown that normal spleen cell suspensions contain approximately three times as many precursors capable of responding to chicken erythrocytes as to sheep erythrocytes. In cultures containing both antigens, the number of precursors responding to one antigen was not affected by the presence of the other antigen. In addition, some cultures were positive for PFC-producing hemolysin against chicken erythrocytes, but not against sheep erythrocytes, and vice versa. This pattern of response was independent of the concentration of antigen in the cultures. Thus, the antigen-sensitive precursors for these non-cross-reacting antigens responded independently of each other, indicating that each precursor was restricted in its capacity to respond to more than one antigen prior to stimulation.