IgG Antibody Suppression of the IgM Antibody Response to Toxoplasma Gondii in Newborn Rabbits

Studies were performed in an experimental rabbit model in an attempt to explore the possibility that the absence of an IgM antibody response in human infants congenitally infected with Toxoplasma may be caused by a suppressive effect of IgG antibody passively transferred from the mother. The IgM antibody response to Toxoplasma gondii infection in newborn rabbits previously injected with IgG Toxoplasma antibody was significantly suppressed as determined by an immunofluorescent antibody assay (IgM-IFA). In newborn rabbits infected with Toxoplasma, IgM-IFA antibody was first demonstrated 15 days after infection and reached a peak titer of 1:128 in two animals on days 20 and 25. In newborn rabbits injected with IgG Toxoplasma antibody and then infected with Toxoplasma, the Igm-IFA antibody and then infected with Toxoplasma, the Igm-IFA antibody response was considerably lower; the highest titer was 1:16 and occurred 25 and 30 days after infection. The results of these studies lend support to the concept that maternally transmitted IgG may suppress the IgM antibody response in the fetus and newborn infant.