Interferon regulatory factor 5 activation in monocytes of systemic lupus erythematosus patients is triggered by circulating autoantigens independent of type I interferons

Objective Genetic variants of interferon regulatory factor 5 (IRF‐5) are associated with susceptibility to systemic lupus erythematosus (SLE). IRF‐5 regulates the expression of proinflammatory cytokines and type I interferons (IFNs) believed to be involved in the pathogenesis of SLE. The aim of this study was to determine the activation status of IRF‐5 by assessing its nuclear localization in the immune cells of SLE patients and healthy donors, and to identify SLE‐associated triggers of IRF‐5 activation. Methods IRF‐5 nuclear localization in subpopulations of peripheral blood mononuclear cells from 14 genotyped SLE patients and 11 healthy controls was assessed using imaging flow cytometry. The activation and function of IRF‐5 were examined after ex vivo stimulation of healthy donor monocytes with SLE serum or components of SLE serum. Cellular localization was determined by ImageStream flow cytometry, and cytokine expression was analyzed by quantitative polymerase chain reaction and enzyme‐linked immunosorbent assay. Results IRF‐5 was activated in a cell type–specific manner; monocytes from SLE patients had constitutively elevated levels of nuclear IRF‐5, as compared to natural killer cells and T cells. SLE serum was identified as a trigger for IRF‐5 nuclear accumulation; however, neither IFNα nor SLE immune complexes could induce nuclear localization. Instead, autoantigens composed of apoptotic/necrotic material triggered IRF‐5 nuclear accumulation in monocytes. Production of the cytokines IFNα, tumor necrosis factor α, and interleukin‐6 in monocytes stimulated with SLE serum or autoantigens was distinct, yet showed a correlation with the kinetics of IRF‐5 nuclear localization. Conclusion This study provides the first formal proof that IRF‐5 activation is altered in the monocytes of SLE patients, which can be attributed, in part, to the SLE blood environment.