B. abortus RNA is the component involved in the down-modulation of MHC-I expression on human monocytes via TLR8 and the EGFR pathway

Abstract

Despite eliciting a potent CD8+ T cell response, Brucella abortus is able to persist and establish a chronic infection inside its host. We have previously reported that the infection of human monocytes/macrophages with B. abortus inhibits the IFN-γ-induced MHC-I cell surface expression down-modulating cytotoxic CD8+ T cell responses. MHC-I down-modulation depends on bacterial viability and results from the capacity of B. abortus to retain the MHC-I molecules within the Golgi apparatus. Furthermore, we recently demonstrated that epidermal growth factor receptor (EGFR) pathway is involved in this phenomenon and that this is an early event during infection. However, the components and mechanisms whereby B. abortus is able to down-modulate MHC-I remained to be elucidated. In this study we demonstrated that the down-modulation of MHC-I expression is not mediated by well-known Brucella virulence factors but instead by B. abortus RNA, a PAMP associated to viability (vita-PAMP). Surprisingly, completely degraded RNA was also able to inhibit MHC-I expression to the same extent as intact RNA. Accordingly, B. abortus RNA and its degradation products were able to mimic the MHC-I intracellular retention within the Golgi apparatus observed upon infection. We further demonstrated that TLR8, a single-stranded RNA and RNA degradation products sensor, was involved in MHC-I inhibition. On the other hand, neutralization of the EGFR reversed the MHC-I inhibition, suggesting a connection between the TLR8 and EGFR pathways. Finally, B. abortus RNA-treated macrophages display diminished capacity of antigen presentation to CD8+ T cells. Overall, our results indicate that the vita-PAMP RNA as well as its degradation products constitute novel virulence factors whereby B. abortus, by a TLR8-dependent mechanism and through the EGFR pathway, inhibits the IFN-γ-induced MHC-I surface expression on human monocytes/macrophages. Thus, bacteria can hide within infected cells and avoid the immunological surveillance of cytotoxic CD8+ T cells.

Prokaryotic RNA activates endothelial cells promoting neutrophil transmigration

“…Our controls demonstrate that pRNA‐mediated effects were not the consequence of contaminants (DNA, proteases or LPS) present in our samples, and this was consistent with the fact that both pRNA and its degraded products have biological effects. In this sense, we and others have previously demonstrated that degradation products of RNA derived from bacteria are able to trigger biological responses in immune cells …”

“…In this sense, we and others have previously demonstrated that degradation products of RNA derived from bacteria are able to trigger biological responses in immune cells. 5,10 Upregulation of ICAM-1 was higher when pRNA was used as a stimulus compared with degraded pRNA, but this differential upregulation was not translated to dissimilar biological effects of both types of molecules (entire versus degraded pRNA) regarding the interaction of EC with PMN. However, an interesting conclusion can be drawn if this differential upregulation is considered together with the results obtained when using the inhibitors of endocytosis.…”

“…Discrimination between self (host) and nonself RNA is crucial as damaged host cells can release nucleic acids that might induce inappropriate inflammation. In recent years, several investigators have reported the specific recognition of pRNA by different immune cells . Considering the central role of EC–PMN interactions in the resolution of infections, in this work, we explored whether EC could also sense pRNA, and modulate the response of PMN, an aspect that has never been previously investigated.…”

“…In recent years, several investigators have reported the specific recognition of pRNA by different immune cells. 5,9,10 Considering the central role of EC-PMN interactions in the resolution of infections, in this work, we explored whether EC could also sense pRNA, and modulate the response of PMN, an aspect that has never been previously investigated. We found that activation of HUVEC was induced by the entire molecule of pRNA and also by its degradation products.…”

Immunoproteasome Subunits Are Required for CD8 + T Cell Function and Host Resistance to Brucella abortus Infection in Mice

“…Previously, others reported that B. abortus infection of human monocytes/macrophages inhibited IFN-␥-induced MHC-I cell surface expression (53). This phenomenon was dependent on metabolically active viable bacteria, and MHC-I downmodulation correlated with the development of diminished CD8 ϩ cytotoxic T cell responses (54). Taken together, our results suggest that TKO animals have an impaired ability to express MHC-I-Brucella peptides on the cell surface, since the levels of MHC-I molecules were reduced on DCs after bacterial infection.…”