Human Cord-Blood Granulocytes Do Not Further Modulate Autologous Natural Killer Cell Activity in vitro *

We have previously reported that mature adult peripheral blood granulocytes (polymorphonuclear leukocytes, or PMN) have the capacity to downregulate the in vitro cytolytic activity of autologous and allogeneic lymphocytes in in vitro assays that measure natural killer (NK) and antigen-specific cytotoxic T-cell functional activities. Because several PMN functions in the neonate have been found to be immature (or under some as yet undefined regulatory control), we investigated whether neonatal PMN could express a capacity in vitro to modulate the NK cytolytic activity of autologous or allogeneic lymphocyte suspensions. Thirty-five cord-blood samples were studied. We found that NK activity by cord-derived lymphocytes was always 50-90% lower than that expressed by adult lymphocytes isolated and tested on the same day. Neither adult nor cord-derived PMN were found to significantly downregulate the alreadyreduced level of NK activity expressed by cord lymphocytes. While adult PMN exhibited the capacity in vitro to suppress the cytolytic activity of either allogeneic or autologous adult NK cells, cord PMN only occasionally exhibited the capacity to influence allogeneic (adult) NK cell activities. That the low level of neonatal NK activity is not further downregulated by autologous cord-blood PMN is a phenomenon which may have beneficial implications at a time when natural immunologic defenses of the neonate are still immature.