Mycobacterium tuberculosis is a leading cause of death by infectious disease in the world. Mycobacterial cell wall components are potent stimulators of the immune system, and have long been as adjuvants. The host systems that recognize these components are poorly defined, but several studies implicate CD14, a component of LPS recognition pathway that utilizes Toll- like receptor-4 (TLR4). TLRs appear to be early and important inducers of tumor necrosis factor-alpha (TNFalpha). TNFalpha is a critical factor for protective immunity to tuberculosis. We hypothesize that innate recognition of mycobacterial cell wall components by TLRs is important for host defense. TLRs have a signaling pathway that has similarities to the IL-1 receptor pathway, and include MyD88 as a proximal component. Using murine macrophage cell line RAW 264.7, we have identified three cell wall fractions that stimulate MyD88-dependent TNFalpha production. This implies TLR-dependent recognition of mycobacterial cell wall components. The mechanism of stimulatory ligand recognition by TLRs is unknown and may require direct or indirect involvement of CD14. Phagocytosis of mycobacterium involves another set of receptors including the complement and mannose receptors. Complement opsonization increases the infectivity of mycobacteria in vitro. In addition, phagocytic receptors recognize mycobacterium and potentially activate an overlapping set of signaling pathways. This suggests that cross- talk may exist between phagocytic and inflammatory signaling pathways.
The aims of this proposal are to 1) identify the specific mycobacterial cell wall components recognized by TLRs, 2) identify the corresponding receptors that are responsible for MyD88-dependent recognition of mycobacteria, 3) characterize mycobacterial cell wall ligand-recognition by TLRs, and 4) characterize the consequences of complement opsonization and TLR- dependent recognition on phagocytosis of mycobacteria. Understanding the innate recognition of mycobacteria will greatly advance our understanding of tuberculosis, point to possible sites of host genetic variation that alter responses to M. tuberculosis, and disclose fundamental principles of innate immunity in infectious and inflammatory diseases.
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