Inhibition of T- and B-lymphocyte functions by normal immunosuppressive protein.

Normal immunosuppressive protein (NIP) isolated from human plasma was studied in two well defined systems. (1) Spontaneous rosettes of sheep red blood cells with human peripheral blood lymphocytes and PHA-induced lymphocyte cytotoxicity as indicators for T-cell function. (2) Rosette formation tests of human lymphocytes with antibody-coated erythrocytes or erythrocytes coated with antibody and complement as well as antibody-induced lymphocyte-mediated cytotoxicity represented non-T-cell activity. While NIP did not inhibit the formation of any of the above mentioned rosettes, it practically prevented both PHA-induced and antibody-mediated lymphocyte cytotoxicity. Relatively small amounts of NIP inhibited PHA-induced cytotoxicity while higher doses were required for the inhibition of antibody-mediated lymphocyte cytotoxicity. Possible mechanisms of its suppressive activity are discussed. NIP was found to be heat-stable and did not show any species specificity, as NIP preparations from human plasma were immunosuppressive in human, mouse and guinea-pig systems.