Innate Immune Responses to Rhodococcus equi

Abstract

We examined innate immune responses to the intracellular bacterium Rhodococcus equi and show that infection of macrophages with intact bacteria induced the rapid translocation of NF-κB and the production of a variety of proinflammatory mediators, including TNF, IL-12, and NO. Macrophages from mice deficient in MyD88 failed to translocate NF-KB and produced virtually no cytokines in response to R. equi infection, implicating a TLR pathway. TLR4 was not involved in this response, because C3H/HeJ macrophages were fully capable of responding to R. equi infection, and because RAW-264 cells transfected with a dominant negative form of TLR4 responded normally to infection by R. equi. A central role for TLR2 was identified. A TLR2 reporter cell was activated by R. equi, and RAW-264 cells transfected with a dominant negative TLR2 exhibited markedly reduced cytokine responses to R. equi. Moreover, macrophages from TLR2−/− mice exhibited diminished cytokine responses to R. equi. The role of the surface-localized R. equi lipoprotein VapA (virulence-associated protein A), in TLR2 activation was examined. Purified rVapA activated a TLR2-specific reporter cell, and it induced the maturation of dendritic cells and the production of cytokines from macrophages. Importantly, TLR2−/−-deficient but not TLR4−/−-deficient mice were found to be compromised in their ability to clear a challenge with virulent R. equi. We conclude that the efficient activation of innate immunity by R. equi may account for the relative lack of virulence of this organism in immunocompetent adults.

Lack of galectin‐3 alters the balance of innate immune cytokines and confers resistance to Rhodococcus equi infection

“…A recent study has identified TLR2 as the main receptor mediating the innate immune response of macrophages to R. equi [27]. Therefore, we investigated the expression of TLR2 and MyD88 mRNA in gal3 1/1 and gal3 À/À peritoneal macrophages.…”

“…Gal3 À/À mice produce higher IL-12 levels than gal3 1/1 mice Based on the previous demonstration that galectin-3 regulates IL-12 production by APCs [29] and that R. equi is an efficient inducer of IL-12 and TNF-a production by macrophages [27], we measured the serum levels of both cytokines in gal3 1/1 and Immunohistochemical staining for galectin-3 in organs of R. equi-infected wild-type mice. Gal3 1/1 and gal3 À/À mice were intravenously inoculated with R. equi (1 Â 10 6 ), and splenic and liver immunohistochemical studies were performed at several intervals postinfection.…”

“…The absence of a direct binding of galectin-3 to the surface of R. equi (data not shown) may explain at least in part why bacterial entry is not altered in gal3 À/À macrophages. Although it may not be the only mechanism, the higher production of macrophage-derived cytokines by R. equi-infected gal3 À/À [27] reported that R. equi-infected macrophages produce a variety of proinflammatory mediators, including IL-12. In our study, the higher IL-12p40 mRNA levels found in gal3 À/À macrophages did not lead to detectable IL-12p70 protein levels in the culture supernatants of infected macrophages.…”

“…Recently, it has been shown that VapA can interact with TLR2 and trigger one of the major mechanisms by which macrophages respond to R. equi, culminating in NF-kB activation. Furthermore, in the absence of TLR2, but not TLR4, in vivo clearance of R. equi is impaired [27].…”

“…Recently, it has been shown that VapA can interact with TLR2 and trigger one of the major mechanisms by which macrophages respond to R. equi, culminating in NF-kB activation. Furthermore, in the absence of TLR2, but not TLR4, in vivo clearance of R. equi is impaired [27].The present work was undertaken to examine the role of galectin-3 in the innate immune defense mechanism during the early course of R. equi infection. We show that the gal3 À/À mice are more resistant to R. equi infection and this resistance is associated with higher production of cytokines by the gal3 À/À macrophages.…”