The Mycobacterium tuberculosis cell wall component mycolic acid elicits pathogen-associated host innate immune responses

Recognition of conserved pathogen-associated molecular patterns constitutes a crucial step in the initiation of innate immune responses. We studied the contribution to the host-pathogen interaction of mycolic acid (MA), a major lipid component of the cell envelope of the macrophage intracellular pathogen Mycobacterium tuberculosis and other mycobacteria. MA administered to the peritoneal cavity or to the airways induced a unique macrophage morphotype, similar to the foamy macrophage derivatives observed in tuberculous granulomas and characterized by intracellular accumulation of neutral lipids and entry into mitosis. When assayed for production of inflammatory mediators, a conditioning rather than a direct activation of the MA-elicited foamy macrophages was observed. MA enabled production of IFN-gamma and myeloperoxidase, enhanced TNF-alpha production and suppressed IL-10 upon renewed exposure to innate triggers. Intratracheal instillation of MA mimicked additional features of the airway response to M. tuberculosis infection, namely a rapid but transient neutrophil influx and IL-6 production and a chronic IL-12 production. These MA-elicited cellular innate defenses and the accompanying formation of foamy macrophages identify for the first time the foamy macrophage morphotype as part of the host response to a pathogen associated structure. Furthermore, these results characterize MA as a direct trigger of innate immunity, distinct from Toll-like receptor ligands.