Altered signaling in systemic juvenile idiopathic arthritis monocytes

Highlights

• sJIA CD14 monocytes show specific, defective STAT1 phosphorylation downstream of IFNs.

• sJIA monocytes express higher transcript levels of the IFN signaling inhibitor SOCS1.

• Recent changes in medication use in sJIA may alter the IFN hyporesponsiveness.

• Impaired IFN/pSTAT1 signaling consistent with monocyte skewing away from M1 phenotype

• Impaired IFN/pSTAT1 signaling may contribute to disease pathology in sJIA.

Abstract

Systemic juvenile idiopathic arthritis (sJIA) is characterized by systemic inflammation and arthritis. Monocytes are implicated in sJIA pathogenesis, but their role in disease is unclear. The response of sJIA monocytes to IFN may be dysregulated. We examined intracellular signaling in response to IFN type I (IFNα) and type II (IFNγ) in monocytes during sJIA activity and quiescence, in 2 patient groups. Independent of disease activity, monocytes from Group 1 (collected between 2002 and 2009) showed defective STAT1 phosphorylation downstream of IFNs, and expressed higher transcript levels of SOCS1, an inhibitor of IFN signaling. In the Group 2 (collected between 2011 and 2014), monocytes of patients with recent disease onset were IFNγ hyporesponsive, but in treated, quiescent subjects, monocytes were hyperresponsive to IFNγ. Recent changes in medication in sJIA may alter the IFN hyporesponsiveness. Impaired IFN/pSTAT1 signaling is consistent with skewing of sJIA monocytes away from an M1 phenotype and may contribute to disease pathology.

Introduction

Systemic juvenile idiopathic arthritis (sJIA) is a childhood rheumatic condition that has been classified as one of the categories of JIA. sJIA is typically characterized by spiking fever in a quotidian pattern, transient rash, and arthritis. Patients may alternate between periods of disease activity (flare) and inactivity (quiescence). The etiology of sJIA is unknown, but it is hypothesized that infectious agents are triggers of disease in genetically susceptible individuals [1]. The presence of macrophage activation syndrome (MAS) in a subset of sJIA patients and the efficacy of anti-IL-1 and anti-IL-6 therapies highlight a key role for dysfunction of the innate immune system [1], [2]. Absence of typical characteristics of autoimmunity, such as autoantibodies, also implies a less prominent role for altered adaptive immunity, although it may contribute, especially to persistent disease with polyarticular arthritis [3], [4].

Among the cells of the innate immune system, cells of the monocytic lineage are thought to be drivers of sJIA pathology, as these innate cells (monocytes and macrophages) are among the producers of several mediators implicated in sJIA, including the cytokines IL-1, IL-6 and IL-18 and S100 proteins [1]. Monocytes are found in infiltrates of the sJIA rash [5], and monocytosis during sJIA disease activity has been described [6], [7]. Furthermore, gene expression studies of circulating cells show increased levels of transcripts reflective of monocyte/macrophage-associated activation in sJIA [6], [7], [8].

Different patterns of monocyte/macrophage activation have been described, with the classical, or M1-type, associated with pro-inflammatory functions, and the alternative, or M2-type (and its subtypes), associated with immunosuppression and tissue repair [9]. Although it is likely that clear polarized profiles may be only observed in vitro, the M1/M2 paradigm is a useful framework to examine the roles of monocytes/macrophages in various conditions.

In sJIA, contrary to expectations of a pro-inflammatory monocyte profile, we previously observed reduced proportions of circulating M1-like monocytes and increased mixed M1/M2-like monocytes in active sJIA patients [7]. One possible explanation for these results is that in sJIA, drivers of M1-polarization, such as IFNγ, are decreased. IFNγ has been shown to play an important role in sJIA patient with MAS [1], and levels of plasma IFNγ are comparable or even modestly increased in sJIA in relation to healthy controls during active disease [10], [11]. Production of IFNγ from sJIA PBMC in vitro most often is observed to be similar to controls [[12], [13] and Nguyen, Mellins, unpublished]. Interestingly, independent microarray studies using PBMCs [6], [14] or whole blood [15], have failed to detect an IFN-induced signature in sJIA, with one exception, in which a Japanese population was analyzed [16]. Another IFN, IFNα, has been less studied in sJIA, but as for IFNγ, no evidence of deficient levels has been found [17], [18]. These findings raise the possibility that sJIA cells may not respond normally to M1-driving stimuli and that a defective response of sJIA cells to IFN, rather than a deficit in availability of IFN, may contribute to sJIA pathology.

In this work, we analyze the status of IFN signaling in peripheral monocytes from sJIA patients, using a flow cytometry-based technique, termed “phospho-flow” [19]. We measured the responses of monocytes from patients during sJIA flare or quiescence and compared these responses to those of monocytes from age-matched, immunologically normal children.