Mycobacterium leprae Phenolglycolipid-1 Expressed by Engineered M. bovis BCG Modulates Early Interaction with Human Phagocytes

The species-specific phenolic glycolipid 1 (PGL-1) is suspected to play a critical role in the pathogenesis of leprosy, a chronic disease of the skin and peripheral nerves caused by Mycobacterium leprae. Based on studies using the purified compound, PGL-1 was proposed to mediate the tropism of M. leprae for the nervous system and to modulate host immune responses. However, deciphering the biological function of this glycolipid has been hampered by the inability to grow M. leprae in vitro and to genetically engineer this bacterium. Here, we identified the M. leprae genes required for the biosynthesis of the species-specific saccharidic domain of PGL-1 and reprogrammed seven enzymatic steps in M. bovis BCG to make it synthesize and display PGL-1 in the context of an M. leprae-like cell envelope. This recombinant strain provides us with a unique tool to address the key questions of the contribution of PGL-1 in the infection process and to study the underlying molecular mechanisms. We found that PGL-1 production endowed recombinant BCG with an increased capacity to exploit complement receptor 3 (CR3) for efficient invasion of human macrophages and evasion of inflammatory responses. PGL-1 production also promoted bacterial uptake by human dendritic cells and dampened their infection-induced maturation. Our results therefore suggest that M. leprae produces PGL-1 for immune-silent invasion of host phagocytic cells.
Author Summary Mycobacterium leprae, the causative agent of leprosy, is a chronic human disease responsible for irreversible peripheral nerve damage and deformities. Lepromatous leprosy, the most severe form of the disease, is accompanied by T-cell unresponsiveness, suggesting that M. leprae has evolved strategies to modulate host immune responses. However, the molecular mechanisms of M. leprae infection remain poorly understood, mainly because this bacterium has been to date impossible to grow in vitro. The present study reports an innovative approach to study the contribution of a phenolic glycolipid (PGL-1) specific of M. leprae in the cross-talk of the pathogen with host cells. We reprogrammed a biosynthetic pathway in a surrogate host, M. bovis BCG, to make it synthesize and display PGL-1 in the context of a mycobacterial envelope. Using this novel microbial tool, we found that PGL-1 production enhances the cellular invasiveness of BCG and promotes the entry via complement receptor 3-mediated phagocytosis. Bacterial uptake via this route was associated with reduced inflammatory responses in infected human macrophages. In addition, we showed that PGL-1 production inhibited the infection-induced maturation of human dendritic cells. Our findings thus provide new insights into the contribution and molecular mechanisms of action of PGL-1 in leprosy pathogenesis.