Later, immunoreactivity to this receptor disappeared. It has been proposed that TLR4 plays a fundamental role in the recognition and fight against infectious agents, but a consensus has not been reached on this issue. Some studies report that TLR4 plays a protective role in experimental pulmonary tuberculosis: in mice RG7204 with nonfunctional TLR4, an increased susceptibility, mortality, and mycobacterial load in the lungs has been found (Abel et al., 2002; Branger
et al., 2004). We speculate that N. brasiliensis downregulates TLR4 expression in the later stages of actinomycetoma, inducing an imbalance between the host immune response and the bacterial load present in the infection site, which favours chronicity. In contrast, other authors show that TLR4-deficient mice do not differ from wild-type controls in a model of Mycobacterium avium MG-132 cell line infection (Feng et al., 2003). Some studies report that phosphatidylinositol mannosides, a component of the M. tuberculosis cell wall, inhibit the TLR4 pathway, disturbing the release of cytokines and chemokines by lipopolysaccharide-stimulated macrophages; this effect was independent of the presence of TLR2 (Doz et al., 2009). We do not know whether a similar interaction could be present between N. brasiliensis and TLR4. The sudden and early decrease in TLR2 and TLR4 expression
that was observed in both the ISSI-MG and the CI-MG, along with the recovery of this expression after 8 h, indicates that both mechanical (trauma with a needle) and chemical (carrageenan as an irritant Sulfite dehydrogenase substance) injuries are capable of modifying the expression of TLR2 and TLR4. However, these findings indirectly underline the importance of N. brasiliensis
in the maintenance of TLR2 expression and in TLR4 downregulation. In addition to recognizing and responding to microbial pathogens, TLR2 and TLR4 sense tissue integrity by binding danger-associated molecular patterns – endogenous ligands including some extracellular matrix components, hyaluronidase, and necrotic cell debris released during infectious and inflammatory processes – thereby increasing the tissue damage. A vicious cycle of inflammation–tissue damage–inflammation and its molecular mediators could be the basis of chronic inflammation (Jiang et al., 2005; Mollen et al., 2006; Drexler & Foxwell, 2010). A consequence of the inflammatory process in actinomycetoma is the production of huge quantities of tissue debris. The increased TLR2 expression observed in the present work could be associated with the recognition of both these damage signals and N. brasiliensis participating in the maintenance of inflammatory processes, and in consequence, in the chronic evolution of disease. This is the first report describing the in situ expression of TLR2 and TLR4 during the acute and chronic inflammatory processes following experimental N. brasiliensis infection. The N.
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