IL-12 completely blocks ultraviolet-induced secretion of tumor necrosis factor α from cultured skin fibroblasts and keratinocytes

Abstract

Interleukin-12 is an important regulator of other cytokines. Although interleukin-12 is considered to act primarily on lymphocytes, provoking a shift from T helper 2 to T helper 1 cells and an increase in lymphocyte-derived tumor necrosis factor α, we hypothesized that interleukin-12 might also affect tumor necrosis factor α secretion from skin cells. In this study, keratinocytes were treated with ultraviolet-B, ultraviolet-A, or sham irradiation, without or with exogenous interleukin-12. Remarkably, the exogenous interleukin-12 totally blocked ultraviolet-B-induced tumor necrosis factor α production. Both ultraviolet-A and ultraviolet-B were capable of inducing interleukin-12 production. To determine the molecular mechanism of this effect, we used a chloramphenicol acetyl transferase reporter under the control of a 1.2 kb fragment of the wild-type (-308G) human tumor necrosis factor α promoter and found significant suppression of promoter activity with interleukin-12. Studies using the -308A variant of the human tumor necrosis factor α promoter showed much higher promoter activity overall, but also a greater sensitivity to suppression by interleukin-12. The mechanism did not involve blockage of the interleukin-1 receptor, because interleukin-12 did not suppress interleukin-1-mediated induction of collagenase mRNA. To determine the role of endogenous interleukin-12, we found that anti-interleukin-12 antibodies enhanced ultraviolet-B-induced tumor necrosis factor α secretion. Thus, interleukin-12 strongly inhibits tumor necrosis factor α production by noninflammatory skin cells, mostly or entirely through inhibition of gene transcription via an element within the first 1.2 kb of the tumor necrosis factor α promoter. The result is a shift in tumor necrosis factor α production from noninflammatory cells to T helper 1 cells. Because tumor necrosis factor α is central to the pathogenesis of several photosensitive skin diseases and certain forms of immune suppression, interleukin-12 may have important physiologic, pathophysiologic, and therapeutic roles.