Abstract

BackgroundOverexpression of type I interferon (IFN-I)-induced genes is a common feature of systemic lupus erythematosus (SLE) and its experimental models, but the participation of endogenous overproduction of IFN-I on it is not clear. To explore the possibility that abnormally increased IFN-I receptor (IFNAR) signaling could participate in IFN-I-induced gene overexpression of SLE, we examined the phosphorylation status of the IFNAR-associated signaling partners Jak1 and STAT2, and its relation with expression of its physiologic inhibitor SOCS1 and with plasma levels of IFNα and IFN-like activity.Methodology/Principal FindingsPeripheral blood mononuclear cells (PBMC) from SLE patients with or without disease activity and healthy controls cultured in the presence or in the absence of IFNβ were examined by immunoprecipitation and/or western blotting for expression of the two IFNAR chains, Jak1, Tyk2, and STAT2 and their phosphorylated forms. In SLE but not in healthy control PBMC, Jak1 and STAT2 were constitutively phosphorylated, even in the absence of disease activity (basal pJak1: controls vs. active SLE p<0.0001 and controls vs. inactive SLE p = 0.0006; basal pSTAT2: controls vs. active and inactive SLE p<0.0001). Although SOCS1 protein was slightly but significantly decreased in SLE in the absence or in the presence of IFNβ (p = 0.0096 to p<0.0001), in SOCS1 mRNA levels were markedly decreased (p = 0.036 to p<0.0001). IFNβ induced higher levels of the IFN-I-dependent MxA protein mRNA in SLE than in healthy controls, whereas the opposite was observed for SOCS1. Although there was no relation to increased serum IFNα, active SLE plasma could induce expression of IFN-dependent genes by normal PBMC.Conclusions/SignificanceThese findings suggest that in some SLE patients IFN-I dependent gene expression could be the result of a low IFNAR signaling threshold.

Highlights

  • Systemic lupus erythematosus (SLE) is a chronic multiorgan autoimmune disease with multiple defects of the immune system [1,2]

  • We found that systemic lupus erythematosus (SLE) patients display constitutive phosphorylation of Jak1 and STAT2 compared to healthy controls, regardless of disease activity

  • Constitutive Phosphorylation of Jak1 and STAT2 in SLE We first examined the status of the IFNAR associated signal transduction molecules in SLE patients

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Summary

Introduction

Systemic lupus erythematosus (SLE) is a chronic multiorgan autoimmune disease with multiple defects of the immune system [1,2]. Some SLE-associated polymorphisms occur in genes related to the IFN-I pathway [6,16,17]. Transcription of SOCS genes occurs in a classic cytokine-Jak-STAT-induced negative feedback loop [31]. SOCS1-KO mice die neonatally with peripheral T cell activation and generalized T cell infiltrates [32], whereas partial SOCS1 deficiency in lymphoid cells is not lethal, but leads to increased sensitivity to cytokine signaling and a SLE-like autoimmune phenotype [32,33,34]. As SLE is characterized by what appears to be an increased sensitivity to otherwise normal levels of cytokines, we examined the basal phosphorylation levels of the IFNAR signaling pathway and its relationship with expression of its inhibitor SOCS1 in SLE patients. We found decreased SOCS1 protein and mRNA expression in SLE, but with not clear relationship with IFNAR phosphorylation

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