Suppressor cells in levamisole-treated mice: a possible role of T-cell-mediated feedback suppression in the drug-induced suppression of the humoral immune response.

The suppressive effect of levamisole (LMS) on the primary IgM plaque-forming cell (PFC) response to sheep erythrocytes in mice was studied. The suppressive effect of LMS on day 4 PFC response was found to be modified by dose, the timing of drug administration, and the amount of antigen injected. The experiments, using an inhibitory dose of 10 mg/kg LMS, showed that: 1) T cells play an essential role in LMS-induced suppression; 2) antigen-specific suppressor cells are induced in the spleen of LMS-treated mice; 3) the spleen cells from LMS-treated donors either suppress or enhance the PFC response in the recipients, depending on the amount of antigen injected and the number of spleen cells transferred; and 4) a dose of 10 mg/kg LMS, which is inhibitory when assessed on day 4 PFC response, actually enhances the response three and a half days after the immunization. These results suggest that LMS induces precursors of both helper and suppressor cells, and their differentiation and/or maturation to LMS-primed antigen-specific suppressor T cells are modulated, at least in part, by T-cell-mediated feedback suppression.