The effect of soluble Concanavalin A (Con A) on the primary antibody response in vitro against sheep red cells (SRC) by mouse spleen cells was studied. Stimulation of normal spleen cells with Con A caused a slight increase of the background number of plaque-forming cells (PFC). Cultures stimulated with SRC did not show an increased PFC response after Con A treatment, except in experiments where the PFC response against SRC was rather low in the absence of Con A. Concentrations of Con A higher than 0·5 μg/ml were inhibitory to the immune response of SRC-treated cultures also early during the culture period. In contrast, T cell depleted spleen cultures, incapable by themselves to respond to SRC, were reconstituted by addition of Con A at concentrations of 0·5 μg/ml or more. Presumably, residual T cells were activated by Con A. Con A activated T cells could reconstitute the PFC response in T cell-deficient cultures. In contrast, it was not possible to obtain more than a marginal stimulation of the antibody response in cultures of spleen cells depleted of adherent cells by addition of soluble Con A or Con A-activated thymocytes.
The results suggest that Con A may stimulate the antibody response by activation of T cells, and support the concept that activated T cells can non-specifically stimulate the antibody response of B cells. Large numbers of activated T cells were inhibitory to the immune response and it is suggested that this phenomenon is analagous to antigenic competition. Furthermore, activated T cells do not seem capable of substituting for adherent cells in the primary immune response in vitro, suggesting that adherent cells are important for the functions of B cells.