Indoleamine 2,3-dioxygenase-1 (IDO1) activity protects M. tuberculosis in macrophages from antibiotics

<b>Introduction:</b><i>Mycobacterium tuberculosis</i> (<i>M. tuberculosis</i>) can persist in macrophages in granulomas. IDO1 is expressed in tuberculoid granuloma, catabolizes essential tryptophan and may halt metabolic activity of macrophages and <i>M. tuberculosis</i>. As antibiotics require metabolically active bacteria we postulated that antibiotics cannot eliminate <i>M. tuberculosis</i> in IDO1-expressing macrophages. <b>Method:</b> THP-1 cell, differentiated to macrophages, were infected with <i>M. tuberculosis</i> after which IDO1 was induced by interferon gamma (IFN-γ). At day 2, antibiotics (+Rifampicin+Isoniazid) were added for 24 h or 72 h and surviving<i> M. tuberculosis</i> were enumerated in lysed macrophages by CFU via serial dilution onto Middlebrook 7H10 solid medium plates. IDO1 and tryptophanyl tRNA-synthetase (TtS) protein expression was determined by Western blotting. <b>Result:</b> IDO1 expression in differentiated THP1 is weak after infection, slightly enhanced by IFN-γ, but IFN-γ and <i>M. tuberculosis</i> synergize in IDO1 expression. IFN-γ enhanced the expression of TtS and induced mini TtS with a proposed higher affinity for tryptophan, but this was not further enhanced by combining IFN-γ and <i>M. tuberculosis</i>. Preliminary studies indicate that <i>M. tuberculosis</i> (mc² 6030) that stays within the phagolysosome from differentiated and IDO1-expressing (thus IFN-γ-exposed) THP1 cells are relatively protected from antibiotics. <i>M. tuberculosis</i> (mc² 6020) that translocates to the cytoplasm apparently is not protected by IDO1. <b>Conclusion:</b> We propose that IDO1 activity may be pronounced in infected macrophages that are exposed to IFN-γ, and attenuates effect of antibiotics. <b>Acknowledgement:</b> LG is funded by a CSC grant (China).