Characterization of Human Null Cells Isolated from Peripheral Lymphocytes by a Simultaneous Double‐Rosetting Procedure

Human null cells were isolated from peripheral blood lymphocytes (PBL) by differential centrifugation on a Ficoll‐Hypaque gradient of the PBL following simultaneous rosetting with erythrocyte indicators specific for B and T lymphocytes. Specifically, the T lymphocytes were rosetted with 2‐aminoethylisothiuroniuum bromido‐treated sheep erythrocytes (ShE), whereas the B lymphocytes were either rosetted with ShE coated with xenoantibodies against human gamma globulin or first sensitized with monoclonal antibodies to human Ia‐like antigens and then rosetted with ShE coated with xenoantibodies against mouse gamma globulin. Approximately 90% of the lymphocytes isolated were null cells that did not bear detectable B‐cell markers—that is, surface immunoglobulin and/or Ia‐like antigens—or T‐cell markers—that is, ShE receptors. The large majority of the null cells expressed receptors for the IgG Fc fragment (53–92%), C3 component (65–92%), and monkey erythrocytes (60–91%) but lacked receptors for the IgM Fc fragment and murine erythrocytes. The null cells exhibited high natural killer cell activity and antibody‐dependent cellular cytotoxicity and were two‐ to four‐fold more active than the total PBL and the T‐enriched cell fractions. The null cells, however, did not respond to stimulation with phytohaemagglutinin and failed to function as either stimulation or responders in an unidirectional mixed lymphocyte reaction.