Elevated cyclic AMP Acts through PKA type I to Inhibit Mycobacterium tuberculosis‐induced IFN‐γ Secretion by T cells

Abstract

IFN‐γ secretion is reduced in human tuberculosis (TB), but the underlying mechanisms are unclear. Intracellular cAMP in PBMC from TB patients was higher (1182±93 fmol, n=12) than that in healthy tuberculin reactors (499±91 fmol, n=7, p<0.001), as measured by ELISA, and cAMP levels correlated inversely with M. tb‐induced IFN‐γ production (r=−0.535, p< 0.05). Using selective cAMP agonists, cAMP‐mediated inhibition of M. tb‐induced IFN‐γ secretion by PBMC from healthy tuberculin reactors was found to depend on PKA type I but not PKA type I II or EPAC (early exchange protein directly activated by cAMP). When electrophoretic mobility shift assays were performed to evaluate binding of nuclear extracts of M. tb‐stimulated PBMC to the IFN‐γ proximal promoter, only the PKA type cAMP analogue markedly inhibited M. tb‐induced DNA‐binding activity of the transcription factors, cAMP response element binding protein (CREB), activating transcription factor (ATF)‐2 and c‐Jun, which positively regulate IFN‐γ transcription. Western blotting confirmed reduced expression of CREB, ATF‐2 and c‐Jun in cells treated with M. tb and the PKA type I agonist cAMP analogue. We conclude that increased cAMP utilizes the PKA type I pathway to reduce M. tb‐induced IFN‐γ secretion by inhibiting binding of transcription factors to the IFN‐γ proximal promoter. (NIH AI063514)