An inhibitor of lymphocyte proliferation and lymphokine production released by unstimulated foetal monocytes.

Supernatants obtained from 3-day cultures of cord blood monocytes inhibited normal lymphocyte activation by either PHA or in a two-way MLC. Both lymphocyte transformation and lymphokine production was significantly inhibited by these supernatants but not by those derived from adult mononuclear cell cultures. The inhibitory material produced by foetal monocytes was dialysable and was non-cytotoxic to target cells. It is postulated that this factor contributes to the depressed maternal cell-mediated immune response observed in pregnancy.