Fetal suppressor cells. Their influence on the cell-mediated immune responses.

The cell-mediated immune responses are significantly reduced in fetal and newborn mice, furthermore, spleen cells of these animals are able to suppress the immune responses of adult lymphocytes in a variety of situations. Newborn mice sensitized to picryl chloride within 24 hr after birth fail to develop contact sensitivity reaction when tested several weeks later, and fetal mice do not develop graft-versus-host reaction when given injections of parental lymphocytes. Spleen cells of fetal or newborn mice are able to suppress the passive transfer of contact sensitivity and the local graft-versus-host reaction elicited by immunized parental cells in F1 hybrid mice, and reduce significantly the severity of graft-versus-host reaction and mortality rate in cyclophosphamide-treated F1 recipients. In no experiment were thymus cells of either fetal or newborn mice found to be inhibitory. The possible mechanisms of action and biological significance of fetal suppressor cells are discussed.