Inhibition of major histocompatibility complex class I (MHCI) by Brucella abortus is an early event during infection and involves the EGF receptor pathway

VELÁSQUEZ L. N., DELPINO M. V., MILILLO M. A., GIAMBARTOLOMEI G. H., BARRIONUEVO P

Congreso; LXII Reunión Científica de la Sociedad Argentina de Inmunología; 2014

Brucella abortus is able to persist inside the host despite the development of a potent CD8+ T cell response. We have recently reported the ability of B. abortus to inhibit the IFN-γ-induced cell surface expression of MHC-I molecules on human monocytes. MHC-I down-modulation resulted in a diminished CD8+ cytotoxic T cell response and was dependent on bacterial viability. The aim of this study was to further characterize this phenomenon by studying its kinetics and if it elicits a bystander effect. For this, THP-1 cells were infected with a virulent strain of B. abortus (S2308), a rough strain (RB51) or a mutant strain lacking the VirB Type IV secretion system (virB10-) in the presence of IFN-γ for 48 h. MHC-I expression was then evaluated by flow cytometry. All 3 strains were able to diminish the IFN-γ-induced expression of MHC-I molecules (p<0.05). However, only the S2308 strain was capable of surviving intracellularly and establishing a successful infection. Thus, we evaluated the kinetics of MHC-I down-modulation. The phenomenon occurred early in time and was observable at 8 h post-infection (p<0.01). At 24 h and 48 h it was even more pronounced. Interestingly, even though few cells were infected in culture, MHC-I down-regulation occurred in the whole population. Thus, we infected cells with B. abortus-GFP and evaluated MHC-I expression in infected as well as non-infected cells. In both populations MHC-I expression was inhibited (p<0.01). Moreover, supernants from B. abortus-infected cells were also able to inhibit MHC-I expression in non-infected THP-1 cells. Finally, neutralization of the EGF receptor by a monoclonal antibody (Cetuximab) resulted in partial recovery (p<0.01) of MHC-I expression indicating that EGF-like ligands may be the soluble mediators involved. Overall, these results describe how B. abortus evades CD8+ T cell responses early during infection and generates a bystander effect to better escape from the immune system and favor chronicity.