Tumor Necrosis Factor-α and Interleukin-6 Selectively Regulate Neutrophil Function in Vitro

Abstract

The neutrophil is an important effector cell of the host response to sepsis. Tumor necrosis factor-α (TNF-α), a cytokine mediator of the septic response, is rapidly released following endotoxemia or gram-negative bacteremia. Interleukin-6 (IL-6) is another cytokine mediator of the host response to sepsis whose role is less well understood than that of TNF-α. It is known to be elevated in gram-negative sepsis, where peak levels have been correlated with mortality, This study examined the effect of IL-6 alone and in combination with TNF-α on three neutrophil functions—CD18 adhesion receptor expression, phagocytosis, and superoxide anion generation. Neutrophils from human volunteers were incubated with amounts of IL-6 ranging from 10 to 1000 ng/ml. At a concentration of 1000 ng/ml, IL-6 increased neutrophil phagocytesis of opsonized bacteria (826 ± 255 × 103 MESF vs 552 ± 103 MESF, P < 0.05) and also increased neutrophil superoxide anion generation (18.41 ± 1.86 vs 12.6 nmol O-2/106 PMN/10 min, P < 0.05). Lesser amounts of IL-6 had no effect on phagocytesis or superoxide generation. IL-6 did not increase neutrophil CD18 adhesion receptor expression. Combining IL-6 with TNF-α at doses of 100 ng/ml and 100 U/ml, respectively, neutrophil phagocytosis (221 ± 455 MESF vs 552 ± 103 MESF) and superoxide generation (23.18 ± 1.86 vs 12.6 nmol O-2/106 PMN/10 min) were significantly (P < 0.05) increased above control by an amount similar to that seen with 1000 U/ml TNF-α alone. IL-6 and TNF-α combined did not further increase CD18 expression over that produced by 100 U/ml TNF-α alone. We conclude that IL-6 selectively influences neutrophil function so that, with regard to phagocytosis and superoxide generation, the response of neutrophils to a combination of 100 ng/ml IL-6 and 100 U/ml TNF-α equals that produced by 1000 U/ml of TNF-α alone. This synergy between IL-6 and TNF-α may represent an important mechanism by which IL-6 potentiates the effect of TNF-α in vivo.