摘要：

Mycobacteria are multifaceted pathogens capable of causing both acute disease as well as an asymptomatic latent infection. Host immune responses during mycobacterial infection involve potent cell effector functions including that of CD4+, CD8+ and γδT cells, macrophages and dendritic cells (DCs). Further, the critical regulators of protective immunity to mycobacterial infection include IFN-γ, IL-12, IL-23, TNF-α, lymphotoxins, CD40, nitric oxide and reactive oxygen species. However, the success of mycobacterial infection often relies in its ability to evade immune surveillance mechanisms mediated by sentinels of host immunity by modulating host signal transduction pathways and expression of immunoregulatory molecules. Therefore, the key to control mycobacterial growth and limit pathogenesis lies in the understanding the interactions between Mycobacterium and primary responders like macrophages and DCs. In this scenario, the role of pattern recognition receptors (PPRs) in orchestrating host immune responses assumes central importance. The cell surface receptors play crucial role in influencing overall immune responses. Of the PRRs, the Toll-like receptors (TLRs) form key immune surveillance mechanisms in recognition as well as control of mycobacterial infection. Among them, TLR2 is the primary interacting receptor on antigen presenting cells that recognize the invading mycobacteria. Mycobacterial cell wall constituents such as LAM, LM, PIM and 19-kDa protein have been shown to activate TLR2 signaling leading to proinflammatory responses. Recent reports have suggested that PE_PGRS antigens of M. tuberculosis interact with TLR2. For example, RV0754, Rv0978c, RV1917c have been implicated in modulation of human DCs. The 19-kDa lipoprotein, LpqH (Rv3763) and LprG (Rv1411c) utilize TLR2 signaling to inhibit macrophage responsiveness to IFN-γ triggered MHC class II expression and mycobacterial antigen presentation. Interestingly, recognition and amplification of pathogenic-specific signaling events play important roles in not only discriminating the invading microbes, but also in regulating explicit immune responses. In this context, integration of key signaling centers, which modulate host immunity to pathogenic mycobacterial infections, remains unexplored. In accordance to above observations, signal transduction pathways downstream to TLRs play a critical role in modulation of battery of host cells genes in terms of expression and production of immune modulatory cytokines and chemokines, recruitment of cellular machineries to site of infections etc. This suggests the decisive role for TLRs in modulation of host cell fate decisions. However, during the ensuing immunity to invading pathogens, beside TLR signaling pathways, various other signaling molecules are thought to execute specific functions in divergent cellular contexts. Recent studies from our laboratory have clearly demarcated a novel cross talk of TLR2-NOTCH1 and TLR2-Wnt signaling pathways during mycobacterial infections. The current study primary focuses on the broad range of cross talk of TLR2 and Sonic hedgehog (SHH) signaling pathways and its functional significance. The present investigation demonstrates that M. bovis BCG, a vaccine strain, triggers a robust activation of SHH signaling in macrophages compared to infection with diverse Gram-positive or Gram-negative microbes. This observation was further evidenced by the heightened SHH signaling signatures during in vivo scenario in cells /tissues from pulmonary tuberculosis (TB) individuals as well as tuberculous meningitis (TBM) patients. Furthermore, we show that the sustained TNF-α secretion by macrophages upon infection with M. bovis BCG is a critical necessity for SHH activation. Significantly, perturbation studies implicate a vital role for M. bovis BCG stimulated TLR2/PI3K/PKC/MAPK/NF-κB axis to induce TNF-α, that contributes to enhance SHH signaling. The TNF-α driven SHH signaling downregulates M. bovis BCG induced TLR2 signaling events leading to modulation of battery of genes that regulate various func

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