Interleukin 2 promotes conjugate formation by purified LAK precursors and T lymphocytes: evaluation of conjugates using flow cytometric techniques

The capacity of null cells, enriched in natural killer cells and lymphokine-activated killer (LAK) cell precursors, and T cells to conjugate with tumor targets was analyzed using a flow cytometric assay. Both purified null and T lymphocytes had a similar capacity to conjugate with uncultured and cultured tumor targets prior to interleukin-2 (IL-2) stimulation. In addition, the frequency of conjugation did not correlate with the cytotoxicity expressed by these purified lymphocyte subpopulations, as null but not T cells were highly lytic for the K562 target. Following incubation in IL-2, however, the level of conjugation with tumor targets of both null and T lymphocytes was increased. Both effector populations formed stable conjugates within 6 min at 37°C. The promotion of conjugation was associated with the induction of LAK activity by null but not T lymphocytes. No differences were apparent between the capacity of either IL-2-activated null or IL-2-activated T lymphocytes to conjugate with tumor targets, although only the former efficiently lysed them. Conjugation of both null and T effectors with tumor targets required the presence of Mg²⁺ cations because it was inhibited by the presence of EDTA (38–72% inhibition) but not EGTA. Conjugation was also inhibited by an antibody (MHM 23) recognizing the β-chain shared by LFA-1, MAC-1, and P150/95 molecules. These observations demonstrate that both null and T lymphocytes responded to IL-2 with an increase in their ability to conjugate with tumor targets. The frequent formation of stable conjugates under these conditions by cells with both cytolytic and noncytolytic capacity failed to define conjugation as the only major condition proximate to LAK lysis.