Cellular and molecular dysregulations of neutrophils in systemic lupus erythematosus

Abstract

Systemic lupus erythematosus (SLE) is a female-biased chronic autoimmune disorder presenting a broad spectrum of clinical manifestations in multiple organ systems. As the most abundant immune cells in innate immunity, neutrophils are actively involved in the regulation of inflammation and immune responses in SLE. However, the molecular and cellular dysfunctions of neutrophils in lupus remain ambiguous. The aim of this study was to investigate the expression and functional significance of two molecular targets, namely interleukin-18 receptor accessory protein (IL18RAP) and peptidylarginine deiminase 4 (PADI4), in neutrophils of SLE patients.

IL18RAP is an indispensable subunit for the IL-18 receptor (IL-18R) complex’s ability to mediate high-affinity IL-18 binding and signaling transduction. Research in IL-18 in SLE has been mostly focused on its role as a type 1 T helper cell-driving cytokine. The functional significance of IL18RAP in mediating the IL-18-driven response in myeloid cells in SLE remains largely unexplored. Elevated IL18RAP expression in association with neutrophil activation network was initially identified in a pilot transcriptome profile analysis of SLE leukocytes. In a larger patient cohort, an increased expression of IL18RAP was observed in neutrophils from SLE patients, particularly those with a history of nephritis. IL18RAP expression was positively correlated with SLE disease activity. The increased IL18RAP expression in SLE neutrophils could be attributed to the elevated type I interferon level in sera. Functionally, neutrophils from SLE patients showed higher IL-18-mediated enhancement in reactive oxygen species (ROS) generation, which showed positive correlation with IL18RAP expression and could be neutralized by anti-IL18RAP blocking antibodies. The findings suggest that IL-18 could contribute to SLE pathogenesis through mediation of neutrophil dysfunction via the upregulation of IL18RAP expression.

PADI4 is a calcium dependent enzyme that catalyzes the conversion of positively charged arginine into neutrally charged citrulline. PADI4 is mainly expressed in neutrophils, is responsible for histones citrullination and was shown to correlate with chromatin decondensation during neutrophil extracellular traps (NETs) formation. However, the effects of PADI4 on lupus progression were contradictory in different lupus mouse models. In this study, elevated expressions of PADI4 were observed in neutrophils of SLE patients. Functionally, SLE neutrophils were prone to form NETs. Significantly decreased NETs formation was exhibited in PADI4 specific inhibitor GSK484-treated neutrophils and PADI4-knockdown neutrophil-like HL-60 cells (dHL-60). Neutrophils from SLE patients produced more ROS upon stimulation and expressed higher levels of the NADPH oxidase complex subunits. Furthermore, PADI4-deficient dHL-60 cells showed reduced chromatin accessibility in gene regions of NADPH subunits, which could account for their suppressed expression and led to reduced ROS induction. Hence, dysregulated PADI4 expression in SLE neutrophils could mediate enhanced NETs formation and ROS induction, thereby contributing to SLE pathogenesis through providing excessive autoantigens and facilitating oxidative damage.

Taken together, these findings reveal the overactivity of IL18RAP and PADI4 in SLE neutrophils and provide expression and functional bases for their use as potential therapeutic targets or biomarkers for lupus.