The effect of Toxoplasma
infection on primary antibody responses to both T-dependent and
T-independent antigens was examined in mice. Drastic suppression of primary responses to sheep erythrocytes (SRBC) occurred when mice were immunized 7 days after
infection. The suppression was observed in both 2-mercaptoethanol-sensitive and -resistant
hemagglutinin responses. Anti-dinitrophenol (DNP)
immunoglobulin E and G1 responses to DNP-conjugated
keyhole limpet hemocyanin were also suppressed by
infection. It was suggested that the suppressive effect is nonspecific for the
antigens and
immunoglobulin classes produced. Anti-DNP responses to
DNP-Ficoll, a T-independent
antigen, were suppressed by
infection, but the suppressive effect was weaker than that on the responses to SRBC. This suggests that both T and B cells are suppressed by
infection. In vitro responses of infected mouse spleen cells to SRBC and
DNP-Ficoll confirmed the results of in vivo experiments. In addition, the presence of
plastic-adherent suppressor cells was demonstrated in the spleen cells of infected mice, which suppressed the ability of normal mouse spleen cells to mount an SRBC-specific plaque-forming cell response. These
plastic-adherent suppressor cells appeared to be a major cause of nonspecific suppression of primary antibody responses in Toxoplasma-infected mice.