The inhibitory effect of normal immunosuppressive protein on lymphocytes mediating natural killing activity.

Normal immunosuppressive protein (Nip) was found to have a marked in vitro inhibitory effect on lymphocytes mediating natural cytotoxic activity against the myeloid cell line K-562. The inhibitory activity of Nip was most marked when tested at an effector: target cell ratio of 10:1. When Nip was washed away, a significant recovery of the cytotoxic activity of the natural killer (NK) cells occurred. In control experiments in which human albumin was employed instead of Nip, no inhibitory activity was observed; on the contrary, a stimulatory effect of the NK activity was encountered. The present study shows that Nip suppresses the functions of NK cells just as it inhibits the function of other lymphocytes.