Depressed antibody responses to a thymus-dependent antigen in toxoplasmosis

The immunodepressive effect of Toxoplasma gondii infection in mice was studied, using sheep erythrocytes (SRBC) as the testing antigen and serum hemagglutinins, hemolysins, and both direct and indirect splenic plaque-forming cells (PFC) to SRBC as assays. In the primary antibody response, immunoglobulin M (IgM), hemagglutinins, and hemolysins and both IgM- and IgG-secreting PFC were depressed in animals immunized after infection. Maximum immunodepression occurred during the first 3 weeks of Toxoplasma infection. When the secondary antibody response was studied, results varied. Mice immunized with SRBC after being infected with T. gondii had a depression in both IgM and IgG PFC. Mice immunized with SRBC before being infected with T. gondii and then given a challenge dose of SRBC had a delay, but no an actual depression, in IgG hemagglutinins and hemolysins and IgG-secreting PFC. These studies show that the immunodepression associated with Toxoplasma infection is complicated, and they provide no definitive explanation for the mechanism.