The down-modulation of IFN-Y-induced MHC-I expression by Brucella abortus RNA in CALU-6, HMEC and A-549 cells shares features with monocytes/macrophages'

Abstract

Brucella abortus (Ba) is an intracellular pathogen capable of surviving inside macrophages. Since the disease is presented in multiple forms, many different cells are susceptible to be infected by Ba. We previously demonstrated that Ba RNA is a vita-PAMP involved in the immune evasion mediated by this pathogen. One of the mechanisms displayed by Ba is the down-modulation of MHC-I on monocytes/macrophages when Th1 response is being held, i.e., in the presence of IFN-γ. Moreover, MHC-I total expression is not altered, instead these proteins are retained within the Golgi Apparatus (GA). More recently, we demonstrated that Ba RNA diminishes the IFN-γ-induced MHC-I surface expression in other cells able to be infected with Ba. However, we do not know if this phenomenon is due to the retention of MHC-I within the GA by Ba RNA, as occurs in human macrophages, their preferential niche. To evaluate this, we stimulated the human bronchial epithelium cell line (Calu-6), the human alveolar epithelium cell line (A549) and the endothelial microvasculature cell line (HMEC) with 10 µg/ml of Ba RNA in the presence of IFN-γ. After 48 h, MHC-I expression and GA marker GM130 were detected by confocal microscopy. We observed that Ba RNA induces colocalization of MHC-I and GM130 in Calu-6 and HMEC cells. However, no colocalization was detected in A-549 cells. Then, we evaluated the effect of Ba RNA on the secretion of IL-8, IL-6 and MCP-1. For this, Calu-6, A549 and HMEC cells were stimulated with Ba RNA (1, 5 and 10 μg/ml) in the presence of IFN-γ for 48 h. Afterwards, supernatants were collected and the secretion of IL-8, IL-6 and MCP-1 was quantified by sandwich ELISA. We did not observe any changes in MCP-1 in Ba RNA-treated cells. Conversely to what we expected, Calu-6, HMEC and A-549-Ba RNA-treated cells had higher IL-8 and IL-6 levels compared to those from untreated cells (p<0.05). In addition, our previous results indicate that Ba RNA inhibits the IFN-γ-induced MHC-I surface expression on human monocytes/macrophages by a TLR8-dependent mechanism and through the Epidermal Growth Factor Receptor (EGFR) pathway. In order to extend this finding Calu-6, A-549 and HMEC cells were stimulated with 10 µg/ml of Ba RNA in the presence of IFN-γ for 48 h. TLR-8 expression was confirmed by flow cytometry in all cell lines. Next, cells were stimulated with 10 µg/ml of Ba RNA in the presence of an EGFR ligand-blocking antibody (Cetuximab). Neutralization of the EGFR partially reversed the inhibition of MHC-I surface expression mediated by Ba RNA in HMEC and A549 cells. Overall, these results show that the down-modulation of MHC-I expression by Ba RNA in different cells susceptible to be infected by Ba would allow the bacteria to persist successfully within the host, remaining unnoticed and evading CD8+ T cell surveillance.