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

M Ayelén Milillo,Mónica Vermeulen,Paula Barrionuevo,Sonia L Espindola,Luciana Balboa,Sergio Costa Oliveira,M Victoria Delpino,Nahuel Rodriguez-Rodrigues,Aldana Trotta,Guillermo H Giambartolomei,Lis N Velásquez,Fábio V Marinho,Gabriela C Fernández,Brian K Coombes

doi:10.1371/journal.ppat.1006527

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.

Highlights

Host control of brucellosis requires a set of cells and components of the immune system which together promote a complex response against Brucella spp. [1]
Our study describes a novel immune evasion strategy exploited by B. abortus: the down-modulation of the IFNγ-induced expression of the molecules responsible for antigen presentation to CD8+ T cells on the surface of monocytes, Major Histocompatibility Complex Class I (MHC-I) molecules
These results led us to think that human monocytes/macrophages could be able to detect a component associated with bacterial viability independently of its virulence factors

Summary

Introduction

Host control of brucellosis requires a set of cells and components of the immune system which together promote a complex response against Brucella spp. [1]. From the many defensive resources that adaptive immunity brings into play, cytotoxic CD8+ T cells are determinant to restrain Brucella infection. Mice and bovines have shown that specific CD8+ T cells are developed during Brucella infection [1, 4], confirming the ability of Brucella-infected macrophages to present bacterial antigens on MHC-I molecules and activate cytotoxic CD8+ T cell responses. Despite this immune response, Brucella is able to persist inside these cells establishing a chronic infection.

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