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 ; LIS N. VELÁSQUEZ; ALDANA TROTTA; M. VICTORIA DELPINO; FÁBIO V. MARINHO; LUCIANA BALBOA; MÓNICA VERMEULEN; SONIA L. ESPINDOLA; NAHUEL RODRIGUEZ-RODRIGUES; GABRIELA C. FERNÁNDEZ; SERGIO C. OLIVEIRA; GUILLERMO H. GIAMBARTOLOMEI; PAULA BARRIONUEVO

PLOS PATHOGENS

PUBLIC LIBRARY SCIENCE

Lugar: San Francisco; Año: 2017

1553-7366

Host control of brucellosis requires a set of cells andcomponents of the immune system which together promote a complex responseagainst Brucella spp. [1]. Yet, from the many defensive resources that adaptive immunitybrings into play, cytotoxic CD8+ T cells are determinant to restrainBrucella infection. The importance ofthese cells resides in their capacity to eliminate Brucella-infected target cells [2,3]. Previous studies in humans, mice and bovines have shown thatspecific CD8+ T cells are developed during Brucella infection [1,4], confirming the ability of Brucella-infectedmacrophages to present bacterial antigens on MHC-I molecules and activatecytotoxic CD8+ T cell responses. Despite this immune response, Brucella is able to persist inside thesecells establishing a chronic infection. Therefore, as a successful chronic andpersistent pathogen Brucella must ownan effective strategy to subvert the obnoxious challenge of highly outfittedCD8+ T cells. In this regard, we have recently reported thatinfection of human monocytes/macrophages with B. abortus inhibits the IFN-γ-induced MHC-I cell surface expression[5]. As a consequence, B. abortus-infected macrophages display diminishedcapacity of antigen presentation to CD8+ T cells [5]. Moreover, our results demonstrate thatMHC-I down-modulation emerges from the capacity of B. abortus to retain the MHC-I molecules within the Golgi apparatus[5]. However, the components of B.abortus involved in this phenomenon remained unknown. Interestingly, B.abortus-mediated MHC-I down-modulation was dependent on bacterial viabilityas was demonstrated by the incapacity of heat-killed B. abortus to inhibit the expression of such molecules [5]. Furthermore, we have recently reported that two B. abortus mutant strains in keyvirulence factors, B. abortus RB51 (a rough LPS mutant) and B.abortus virB10- (aVirB type IV secretion system mutant), were capable of inhibiting the IFN-g-induced MHC-Isurface expression to the same extent as wild-type B. abortus [5]. It is a well-known fact that these B. abortus mutantstrains are unable to persist inside human monocytes for a long period despite theirpreserved capacity of infecting cells [6-8]. Consistentwith this, we observed that the phenomenon of MHC-I inhibition was triggeredearly in time and could be observed at 8 h post-infection. At 24 h and 48 h post-infectionit became even stronger [9]. Overall these results led us to think that the components involvedin the inhibition of IFN-g-induced MHC-I surfaceexpression should be associated with bacterial viability. In turn, our resultswith the mutant strains gave us the idea that these bacterial components shouldbe expressed early during infection.It has been recently demonstrated that the immune systemis capable of sensing the most essential characteristic of microbialinfectivity, microbial viability itself [10]. It was shown that what the immune system particularly detects arepathogen-associated molecular patterns (PAMPs) which are found in live bacteriabut rapidly eliminated when bacteria lose their viability [10]. These PAMPs are lost since they are intimately linked to themetabolic activity and replicative capacity of the microorganisms. In order todifferentiate them from traditional PAMPs, structural components that arepreserved after loss of bacterial viability (such as LPS, lipoproteins and DNA,among others), this special class of PAMPs were named viability-associatedPAMPs (vita-PAMPs), among which theprokaryotic RNA is found [10, 11]. Recognition of nucleicacids in general and of RNA in particular by receptors of the innate immunesystem is a complicated and interesting field of investigation. The immunesystem must differentiate between ?self? (host) and ?foreign? (invadingpathogen) nucleic acids [12]. This principle is based on threecriteria: the availability of nucleic acid ligands, the localization of suchnucleic acids and the structure of nucleic acid ligands (characterized bysequence motifs, conformation and chemical modification). In most cases, a combinationof these aspects contributes to the proper recognition of foreign nucleic acidsand the induction of adequate immune responses [12]. Most of the receptors involved inthe immune sensing of nucleic acids have been identified. Among them, the TLRslocated in endosomes/phagolysosomes are the most studied: TLR9 senses CpG DNAmotifs; TLR3 and TLR7 are capable of recognizing double-stranded RNA andsingle-stranded RNA respectively and TLR8 is also not only able to recognizesingle-stranded RNA but it has recently been described a RNA degradationproducts sensor as well [13, 14].Taking into account our preliminary results, we hypothesizedthat the components of B. abortusinvolved in the inhibition of MHC-I could be vita-PAMPs such as B. abortusRNA, since they are found exclusively in live bacteria and are activelyexpressed during early stages of infection. Thus, the aim of this study was to characterize the components, signallingpathways and mechanisms implicated in MHC-I down-modulation. Overall, ourresults indicate that the vita-PAMPRNA as well as its degradation products constitute novel virulence factors wherebyB. abortus, by a TLR8-dependentmechanism and through the EGFR pathway, inhibits the IFN-g-induced MHC-I surface expression on humanmonocytes/macrophages.