Human peripheral Null lymphocytes I. Isolation, immunological and functional characterization

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Abstract
A Null lymphocyte‐enriched population was isolated from human peripheral blood of healthy donors using a combination of Ficoll density gradient centrifugation followed by elimination of mononuclear phagocytes, passage through Ig‐anti‐Ig columns and sedimentation of E rosettes. After each separation step lymphocyte fractions were examined for morphology, cell surface markers, mitogen responsiveness and effector functions in antibody‐dependent (ADCC) and spontaneous cellular cytotoxicity (SCMC) reactions against an allogeneic melanoma cell line. The final Null lymphocyte preparation was recovered at a rate of 1 to 3 % from the population passed through Ig‐anti‐Ig columns (fraction FFF‐C). The marker analysis revealed over 99 %of surface Ig‐negative lymphoid cells; 50 to 60 % of these cells were ‘real Null’ cells lacking immunological cell surface markers, 7 % formed EA, 13 % EAC and 24 %E rosettes.Regarding the mitogen responses, passage through Ig‐anti‐Ig columns drastically reduced concanavalin A (Con A), pokeweed mitogen (PWM)and tuberculin (PPD) responses, whereas the phytohemagglutinin (PHA) response was reduced in absolute counts but not in the stimulation index. Compared to the T cell‐enriched lymphocyte fraction, the Null cells showed significantly diminished proliferative responses to PHA and Con A and slightly increased reactivity to PWM and PPD.Although depleted of high affinity Fc receptor lymphocytes, the Null cell fractions exhibited good ADCC and SCMC activities being about 4 to 6 times higher than in the T cell fraction.