Pregnancy, immunosuppression and reactivation of latent toxoplasmosis.

Toxoplasmosis is a chronic, latent infection which can be reactivated in the presence of immunosuppression. The critical question in obstetrics is whether toxoplasmosis may be reactivated in the presence of the physiological "immunosuppression" of pregnancy. Standard in vitro tests, done in 24 healthy pregnant women and compared with the literature, show no significant changes in humoral and cellular immunity during pregnancy. However, the fact that some infections occur more frequently and more severely than in non-pregnant women (e.g. those due to cytomegalovirus (CMV) and human papilloma virus (HPV) points to a degree of pregnancy-associated immunosuppression. Non-rejection of the semiallogenic fetus is achieved in presence of maternal immunocompetence and is explained mainly by local changes in immune function, mediated by inhibitors of decidual, placental and fetal origin, and by the absence of class II histocompatibility antigens at the fetomaternal interface. Immune status allowing reactivation of toxoplasmosis was studied in a selected group of (predominantly male) AIDS patients from the Swiss HIV Cohort study. Shortly before (cerebral) reactivation of toxoplasmosis, 92% of these patients had very low CD4 lymphocyte counts (mean 50 cells/microliters, i.e. lower than ever recorded in a normal uncomplicated pregnancy). In a larger population of 48 women receiving immunosuppressive therapy after organ transplantation, not a single case of cerebral toxoplasmosis was observed during pregnancy, while in the 105 HIV-positive women in the Swiss HIV and Pregnancy study, there was only one case of cerebral toxoplasmosis during pregnancy and the puerperium (20 CD4/microliters), even though some 17% of those sampled (18/105) had CD4 levels below 200 cells/microliters on at least one occasion during pregnancy. These findings explain why latent toxoplasmosis is not reactivated in normal pregnancy, and why it is only likely in an immunosuppressed mother when her CD4 lymphocyte count is very low (< 200 cells/microliters). In such cases, a prophylactic treatment to prevent maternal reactivation and vertical transmission of toxoplasmosis may be useful.