Induction of lymphotoxin‐α by interleukin‐12 p40 homodimer, the so‐called biologically inactive molecule, but not IL‐12 p70

Abstract

Interleukin-12 (IL-12) p70 (p40:p35) is a bioactive cytokine and its biological functions are becoming clear. On the other hand, the IL-12 p40 homodimer (p40₂) was considered an inactive or inhibitory molecule and its functions are poorly understood. It has been reported that increased expression of lymphotoxin-α (Lt-α) in the central nervous system as well as in peripheral immune cells is associated with multiple sclerosis and experimental allergic encephalomyelitis. Here we describe that p40₂ induces the expression of Lt-α in primary mouse and human microglia, BV-2 microglial cells, splenic macrophages, RAW 264.7 cells and splenic T cells. Interestingly, IL-12 p70 was either unable to induce Lt-α or was a very weak inducer of Lt-α in these cell types. Consistently, p40₂, but not p70, induced Lt-α promoter-driven luciferase activity in microglial cells. Among various stimuli tested, p40₂ emerged as the most potent followed by IL-16, lipopolyaccharide and double-stranded RNA in inducing the activation of Lt-α promoter in microglial cells. Furthermore, an increase in Lt-α messenger RNA expression by overexpression of p40, but not p35, complementary DNA and induction of Lt-α expression by p40₂ in microglia isolated from IL-12p35^−/− mice confirm that p40, but not p35, is responsible for the induction of Lt-α. Finally, by using primary microglia from IUL-12 receptor β1 deficient (IL-12Rβ1^−/−) and IL-12Rβ2^−/− mice, we demonstrate that p40₂ induced the expression of Lt-α in microglia and macrophages via IL-12Rβ1, but not IL-12Rβ2. These studies delineate a novel biological function of p40₂ that is absent in IL-12.