Maternal Immunocompetence II. Proliferative Responses of Maternal Lymphocytes In Vitro and Inhibition by Serum from Pregnant Rats

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Abstract
Peripheral blood and splenic lymphocytes from interstrain (L .times. BN) or intrastrain (L .times. L) primigravida rats were equivalent to those from virgin L females in their in vitro DNA synthetic responses to paternal strain cells (BN), to unrelated allogeneic cells (ACI) and to the mitogen phytohemagglutinin (PHA). Heat-inactivated serum from pregnant L rats, when compared to serum from virgin or postpartum L rats, regularly suppressed the in vitro response of L lymphocytes to paternal and allogeneic cells. The response of L cells to PHA was not suppressed. The degree of inhibition was related to the final concentration of pregnant serum in culture, concentrations above 2% producing more than 80% inhibition of the mixed lymphocyte reaction. The inhibiting sera were not cytotoxic by a sensitive 51Cr release assay. Histoincompatibility between mother and fetus is not required for production of this inhibitory effect, since it is consistently present in intrastrain (L .times. L) pregnant rats. Among interstrain pregnant rats, the degree of inhibition is influenced by, but not specific for paternal-strain alloantigens. The proliferative function of thymus-derived (T) lymphocytes from pregnant rats is intrinsically normal, but the response to allogeneic cells can be altered by factors present in pregnant serum.