Different patterns of cytokine regulation of phagocytosis and bacterial killing by human neutrophils

We describe here the capacity of human recombinant interleukin-1-beta (IL-1β), interferon-gamma (INF-γ), interferon-alpha (INF-α), tumor necrosis factor-alpha (INF-α), interleukin-6 (IL-6) and interleukin-2 (IL-2) to modulate phagocytosis and bacterial killing by human neutrophils. IL-1β, INF-γ and TNF-α enhance phagocytosis of bacteria but do not have consistent effect on bacterial killing. In contrast, IL-6 augments bacterial killing but not phagocytosis of bacteria by neutrophils. INF-α augments and IL-2 depresses both antibacterial reactions of neutrophils. A direct effect of TNF-α (inhibition) and IL-2 (promotion) upon bacterial growth has been observed. It is concluded that (a) phagocytosis and intracellular bacterial killing are independent reactions of neutrophil-mediated antibacterial defence on the basis of different patterns of up- and down-regulation by cytokines, and (b) cytokines can exhibit direct effects on bacterial growth.