Abstract
Tumor necrosis factor-α (TNF-α) is a potent proinflammatory cytokine produced predominantly by activated macrophages, and plays a central role in the protective immunity against intracellular pathogens and the pathogenesis of autoimmune and inflammatory diseases. While both the soluble and transmembrane forms of TNF-α (sTNF-α and tmTNF-α) are biologically functional, the latter but not the former acts as a receptor besides as a ligand, and transmit a retrograde signal in a cell-to-cell contact manner. The production of TNF-α by macrophages under Th2-type (allergic) inflammatory conditions has been ill defined, compared to that under Th1-type inflammatory conditions. Here we examined the effect of representative Th2-inducing cytokines IL-4 and IL-33 on the TNF-α expression in macrophages. IL-4 induced the production of neither sTNF-α nor tmTNF-α while IL-33 promoted the production of sTNF-α with no detectable tmTNF-α. Notably, the combination of IL-4 and IL-33 elicited the tmTNF-α expression on macrophages, in addition to the enhanced production of sTNF-α and IL-6. The IL-4/IL-33-elicited tmTNF-α expression was not observed in IL-6-deficient macrophages, suggesting the involvement of macrophage-derived IL-6 in the tmTNF-α expression. Indeed, the stimulation of macrophages with the combination of IL-4 and IL-6 induced the tmTNF-α expression with no detectable production of sTNF-α. Thus, IL-4 and IL-33 synergistically elicit the tmTNF-α expression on macrophages through the autocrine action of IL-6.