Synergistic action of vitamin D and retinoic acid restricts invasion of macrophages by pathogenic mycobacteria.

Phagosomal maturation arrest is known to play a central role in the survival of pathogenic mycobacteria within macrophages. The maturation arrest of mycobacterial phagosome results from the retention of tryptophan-aspartate-containing coat protein (TACO) on this organelle, enabling successful replication of the pathogen. We have shown earlier that vitamin D(3) and retinoic acid (RA) down-regulate TACO gene transcription in a dose-dependent manner.In this study, we analyzed the promoter region of TACO gene using bioinformatics tools and observed that the vitamin D receptor (VDR)/retinoid-X-receptor (RXR) response sequence was highly functional. We also evaluated the effect of treatment with vitamin D(3)/RA on Mycobacterium tuberculosis entry and survival in cultured human macrophages.TACO gene down-regulation observed with vitamin D(3)/RA treatment occurred through modulation of this gene via the VDR/RXR response sequence present in the promoter region of TACO gene. Treatment of macrophages with vitamin D(3)/RA allows maturation of mycobacterial phagosome, leading to degradation of the pathogen.Our results elucidate the mechanism of TACO gene down-regulation observed with vitamin D(3)/RA. Furthermore, the results revealed that vitamin D(3)/RA treatment inhibits mycobacterial entry as well as survival within macrophages, possibly through rescue of phagosome maturation arrest. The developing knowledge in this area suggests that vitamin D(3)/RA may be of importance in the treatment of intracellular infection, particularly tuberculosis.