B. Abortus RNA: a novel vita-pamp involved in the down-modulation of mhc-i expression on human monocytes

Abstract

Brucella abortus elicits a strong Th1 immune response whichactivates cytotoxic T lymphocytes. However, this pathogen is ableto survive inside macrophages and generate a chronic infection.Previously we reported that infection of human monocytes/macrophageswith B. abortus inhibits the IFN-γ-induced MHC-I cellsurface expression. More importantly, we have recently demonstratedthat B. abortus RNA, described as a viability-associated(vita)-PAMP, is the bacterial component involved in this phenomenon.Thus, the aim of this study was to further characterize thecomponent, signalling pathways and mechanisms implicated inMHC-I down-modulation. For this, RNases-treated B. abortus RNAwas employed to stimulate human monocytic THP-1 cells in thepresence of IFN-γ for 48 h. Then, the expression of MHC-I moleculeswas evaluated by flow cytometry. Surprisingly, completelydegraded RNA was still able to inhibit MHC-I expression (p<0.05)and it also induced the intracellular retention of these moleculeswithin the Golgi apparatus into the same extent as intact RNA. Onthe contrary, DNase- and Proteinase K-treated RNA as well aseukaryotic RNA controls elicited no effect. Furthermore, B. abortusRNA also inhibited MCH-I expression on human primary monocytesand murine bone-marrow derived macrophages (p<0.05).TLR3 is one of the best known RNA immune receptors, thereforewe evaluated whether it could be involved in this phenomenon.Yet, in the presence of a TLR3 inhibitor, B. abortus RNA downregulated MHC-I expression. On the other hand, neutralization ofthe EGFR resulted in partial recovery (p<0.05) of RNA-mediatedMHC-I inhibition. Overall, these results indicate that the vita-PAMP RNA as well as its degradation products constitute a novelvirulence factor whereby B. abortus, by a TLR3 independentmechanism and through the EGFR pathway, inhibit MHC-I expression.Thus, bacteria can hide within infected cells and avoid theimmunological surveillance of cytotoxic CD8+ T cells.