Limiting dilution analysis of the frequency of human T cells and large granular lymphocytes proliferating in response to interleukin 2. I. The effect of lectin on the proliferative frequency and cytotoxic activity of cultured lymphoid cells.

Human peripheral blood LGL that mediated NK and small T cells were isolated in high purity (98% by morphology) by density sedimentation on discontinuous Percoll gradients. The proliferative frequency of these subpopulations in the presence of lectin-free conditioned media containing IL 2 was determined by limiting dilution analysis. LGL showed a 20-fold greater frequency of proliferative cell precursors than small T cells (1/200 and 1/4970, respectively). The NK-like nature of cells expanded from LGL preparations in IL 2 was confirmed by parallel testing of the cytotoxicity against K562. Whereas T cell microcultures showed no lytic activity against K562 (cytotoxic precursor frequency less than 1/10,000), LGL cultures showed frequencies of cytotoxic precursors (1/170) comparable to those of proliferative precursors. Neither responder cell type gave rise to detectable lytic activity against NK-insusceptible mouse lymphoma RL male 1 or alloblasts. LGL proliferation was only minimally affected by the presence of PHA at the onset of culture (rise to 1/74 with 2 micrograms/ml PHA). By contrast, small T cells showed a dose-dependent increase of proliferative frequency, to reach 1/11 with 2 micrograms/ml PHA, provided accessory cells in the form of PBMC, monocytes, or LGL but not T cells were present. The cytotoxic activity of LGL and small T cells expanded in IL 2 was confirmed in bulk cultures. LGL-CLC showed high lytic activity against NK-susceptible cell lines and a majority of freshly isolated allogeneic human tumor targets. T cell-CLC showed little activity against cell line targets (K562, Raji, L1210, RL male 1) but were lytic for some fresh tumor cells. These data establish optimal conditions for the growth of human LGL in IL 2-dependent culture and suggest that a major contributor to lysis of allogeneic human tumors by CLC is likely to be NK cells. The data indicate that large numbers of activated T cells cannot be detected in vivo and that in vitro induction of IL 2 receptors by lectin/antigen is necessary for the establishment of antigen-reactive T cell lines. In contrast, a proportion of LGL appear to be spontaneously activated and susceptible to IL 2-dependent growth. Thus, in the absence of stimulation, culture of unfractionated lymphoid cells in the presence of IL 2 is likely to select for the growth of LGL with NK activity.