Monoclonal Antibodies against Murine Neonatal and Pregnancy‐Associated Natural Suppressor Cells Induce Resorption of the Fetus

Pregnant and neonatal/fetal mice have been shown to harbour naturally occurring inhibitory cells of both T and non‐T type. Non‐T suppressor cells present in the spleen of primiparous pregnant and newborn animals inhibit proliferative responses in autologous and allogeneic mixed lymphocyte reactions. Such cells can be positively selected for by agglutination with the B cell‐specific lectin soybean agglutinin (SBA). We generated rat IgG monoclonal antibodies against unique cell surface structures on the non‐T inhibitory cells, and cylotoxic pretreatment of spleen cells from pregnant or neonatal/felal mice largely abrogates their suppressive activity on proliferative responses. Furthermore, in vivo administration of such antibodies to pregnant inbred and outbred mice results in termination of the pregnancy or decreased litter size. Since injection of anti‐T cell IgG monoclonal antibodies does not interfere with the delivery of normal sized litters it is evident from these studies that the non‐T immunoregulatory cells, in contrast to T‐inhibitory cells, are of great importance in ensuring immunological homeostasis in the fetal‐placental environment during pregnancy in mice.