Autophagy Gene Variant IRGM −261T Contributes to Protection from Tuberculosis Caused by Mycobacterium tuberculosis but Not by M. africanum Strains

The human immunity-related GTPase M (IRGM) has been shown to be critically involved in regulating autophagy as a means of disposing cytosolic cellular structures and of reducing the growth of intracellular pathogens in vitro. This includes Mycobacterium tuberculosis, which is in agreement with findings indicating that M. tuberculosis translocates from the phagolysosome into the cytosol of infected cells, where it becomes exposed to autophagy. To test whether IRGM plays a role in human infection, we studied IRGM gene variants in 2010 patients with pulmonary tuberculosis (TB) and 2346 unaffected controls. Mycobacterial clades were classified by spoligotyping, IS6110 fingerprinting and genotyping of the pks1/15 deletion. The IRGM genotype −261TT was negatively associated with TB caused by M. tuberculosis (OR 0.66, CI 0.52–0.84, Pnominal 0.0009, Pcorrected 0.0045) and not with TB caused by M. africanum or M. bovis (OR 0.95, CI 0.70–1.30. P 0.8). Further stratification for mycobacterial clades revealed that the protective effect applied only to M. tuberculosis strains with a damaged pks1/15 gene which is characteristic for the Euro-American (EUAM) subgroup of M. tuberculosis (OR 0.63, CI 0.49–0.81, Pnominal 0.0004, Pcorrected 0.0019). Our results, including those of luciferase reporter gene assays with the IRGM variants −261C and −261T, suggest a role for IRGM and autophagy in protection of humans against natural infection with M. tuberculosis EUAM clades. Moreover, they support in vitro findings indicating that TB lineages capable of producing a distinct mycobacterial phenolic glycolipid that occurs exclusively in strains with an intact pks1/15 gene inhibit innate immune responses in which IRGM contributes to the control of autophagy. Finally, they raise the possibility that the increased frequency of the IRGM −261TT genotype may have contributed to the establishment of M. africanum as a pathogen in the West African population.
Author Summary Autophagy is a process in which cell components are degraded by the lysosomal machinery. It has recently been described that activation of autophagy reduces the viability of M. tuberculosis in phagosomes due to an intimate autophagy-phagocytosis interaction. M. tuberculosis may also be directly accessible to autophagy, as M. tuberculosis was found to translocate into the cytoplasm. The immunity-related GTPase IRGM is a mediator of innate immune responses and induces autophagy. We have studied genetic variants of the human IRGM gene in a Ghanaian tuberculosis case-control group and found that the IRGM variant −261T provides relative protection against disease when the infection is caused by the Euro-American lineage of M. tuberculosis. This lineage is characterized by the pks1/15 seven base-pair (bp) deletion. The product of an intact pks1/15 gene, phenolic glycolipid-tb, might contribute to mycobacterial virulence by suppressing innate immune responses. It is, therefore, conceivable that only the Euro-American lineage is exposed to IRGM-triggered innate defence mechanisms. Our observations suggest that the increased frequency of the IRGM −261TT genotype may have allowed the establishment of M. africanum as a pathogen in West Africa.