Macrophage activation for tumor cytotoxicity: characterization of priming and trigger signals during lymphokine activation.

Abstract Macrophage activation by lymphokine- (LK) rich supernatants for nonspecific tumor cytotoxicity requires multiple reactions. Stable, noncytotoxic cellular intermediates between the immature mononuclear phagocyte and the fully activated tumoricidal macrophage can be identified. For example, bacterial endotoxins (LPS) at ng/ml concentrations synergistically increase cytotoxicity by macrophages treated with low concentrations (1 /500) of LK. That cells can be activated by LK alone at high concentrations (1/20), however, suggests an LPS-like signal may be present in LK. We have characterized this LK signal in this report. Macrophages cultured for 4 hr in low concentrations of LK (1/500) developed cytotoxic activity only after subsequent exposure to LPS or 1 /20 LK for 1 hr; macrophages treated with LPS or LK alone were not cytotoxic. Thus, cells exposed to 1 signal (low concentrations of LK) enter into a receptive, primed state in which they are not yet active but can then be triggered by another signal (LPS or LK) to develop full cytotoxic activity. Trigger signals in LK were not contaminating LPS: unlike LPS, LK trigger signals were heat-labile (60°C/30 min) and not affected by polymyxin B. Activation by LK signals required a defined treatment sequence: optimal cytotoxicity occurred with cells treated (primed) 4 hr with low concentrations of LK and then triggered 1 hr with high concentrations. The reverse was ineffective. Macrophages cultured continuously for 36 hr in 1 /320 LK show no cytotoxic activity at any time. Yet cells cultured for various times in 1/320 LK, then exposed to 1/10 LK for 1 hr behave comparably, in terms of time course and levels of cytotoxicity, to cells continuously incubated in 1/10 LK. The optimal time for LK priming was 4 hr; LK triggering required only 20 min. Both LK effects were temperature dependent. LK priming and trigger signals form the basis of a regulatory system that sets the threshold and determines the onset of macrophage effector function.