Pivotal Role of Endogenous TNF-α in the IL-2-Driven Activation and Proliferation of the Functionally Immature NK Free Subset

Highly purified peripheral blood-derived NK cells can be separated into three functionally defined subpopulations, namely, non-target binding free cells, binders, and killers. The free cell subset is the least mature and was examined for its response to IL-2-mediated activation and the role of endogenous secretion of TNF-α in its maturation and differentiation. The findings demonstrate that endogenous TNF-α secretion is prerequisite for the initiation of IL-2-mediated activation of free cells into killer cells. The addition of IL-2 to free cells upregulated the surface expression of IL-2R (TAC), TNF-R (p75), CD69, and ICAM-1 antigens and also stimulated cell proliferation. Furthermore, the addition of IL-2 to free cells resulted in the induction of cytotoxic activity and stimulation of free cells to become binder and killer cells. All of these IL-2-mediated manifestations are shown to be downregulated by the addition of anti-TNF-α antibody. However, IL-2-mediated TNF-α secretion was not affected by the addition of anti-TNF-α antibody. The specificity of the anti-TNF-α antibody-mediated inhibition was corroborated by the failure of the antibody to inhibit interferon-α-mediated activation of free cells into killer cells. Most of the events associated with the inhibitory activity of anti-TNF-α antibody were mimicked by the addition of IL-4 to IL-2-treated free cells. These findings suggest that the IL-2-mediated maturation and differentiation of the immature free cells to become cytotoxic and proliferate are the result of a sequence of events that are initiated by the secretion of endogenous TNF-α.