The limited role of interferon‐γ in systemic juvenile idiopathic arthritis cannot be explained by cellular hyporesponsiveness

Systemic juvenile idiopathic arthritis (SJIA) is an autoinflammatory disease characterized by spiking fevers, arthritis, polyserositis, evanescent rash, and lymphadenopathy (1). Evidence suggests that one of the effector cells in the pathogenesis of this disease is of the monocyte/macrophage lineage. Patients with active SJIA not only have an increased proportion of CD14+ monocytes (2), but also increased myelomonocytic precursors within their peripheral blood (3,4). In addition, one of the feared complications of SJIA is the macrophage activation syndrome (MAS) where affected patients experience widespread activation and expansion of T lymphocytes and cells of the monocyte-macrophage lineage, leading to unremitting fever, hyperferritinemia, hypertriglyceridemia, hepatosplenomegaly, a consumptive coagulopathy, and cytopenias. One of its prominent characteristics is an expansion of macrophages actively engaged in the hemophagocytosis of multiple cellular elements within the bone marrow and other lymphoid tissue (5,6). Yet, many of these features can be appreciated in severe SJIA too, in an intermediate phenotype called subclinical or occult MAS, thus suggesting MAS and SJIA may be different extremes of the same clinical spectrum (7,8). The activated macrophage phenotype can be divided into two general groups, M1 and M2, depending on the stimuli they receive and their subsequent differentiation pathway (9–11). In this model, M1 macrophage differentiation is mainly driven by interferon-γ (IFN-γ) and M2 macrophages result from mostly anti-inflammatory signals, such as IL-4, IL-10, M-CSF, TGF-β, or glucocorticoids. M1 macrophages excel at intracellular pathogen eradication, producing high amounts of IL-12 and TNF-α. Due to their highly phagocytic properties, M2 macrophages are mostly scavengers that are heavily involved in wound healing and tissue remodeling. In contrast to M1 macrophages, M2 macrophages are a prominent source of IL-10. Currently, the activated macrophage in SJIA and MAS has yet to be characterized into the M1/M2 polar model. SJIA and MAS are both inflammatory conditions, thus it’s likely that the macrophages therein are of the M1 phenotype. However, the data supporting IFN-γ, which is the key facilitator of M1 macrophage differentiation, as a mediator in active SJIA is scant. Gattorno et al. demonstrated elevated serum levels of both IFN-γ and IP-10, regardless of disease activity (12). In contrast, Lasiglie at al. showed that upon overnight stimulation of PBMC populations, no difference in the absolute number of IFN-γ-producing cells between active SJIA patients and healthy controls could be appreciated (13). Congruent with this finding, three independent gene expression studies have failed to detect an IFN-induced genetic signature within the peripheral blood mononuclear cells (PBMCs) of active SJIA patients (3,14,15). In contrast to SJIA, IFN-γ is believed to have a prominent role in the pathogenesis of hemophagocytic lymphohistiocytosis (HLH), both a hereditary and acquired hemophagocytic syndrome clinically similar to MAS, where IFN-γ is found to be highly elevated in the serum (16–18). In fact, blockade of IFN-γ has been suggested as a potential treatment for both inherited and acquired forms of active HLH (19). To this end, the goals of this study were to characterize the status of an IFN-induced signature within affected tissues and to gauge the integrity of IFN-signaling pathways within the peripheral monocytes of patients with SJIA. IFN-γ is a pleotropic cytokine that canonically signals via the Janus kinase (Jak)-signal transducer and activator of transcription (STAT) pathway, machinery that is common to the many different cytokines (20). Specifically, IFN-γ will lead to the recruitment of Jak1 and Jak2 to the intracellular portion of the IFN-γ receptor, which then becomes phosphorylated on a tyrosine residue. This allows recruitment, docking, and subsequent phosphorylation of STAT1. In its active form, STAT1 will then form a homodimer and translocate into the nucleus in order to bind to a promoter region IFN-γ activation site (GAS) to initiate specific IFN-responsive genes. Given the inflammatory nature of active SJIA, it seems paradoxical that an IFN-induced genetic signature cannot be detected within the PBMCs of active SJIA. In this study, we explore the role of IFN-γ in the pathogenesis of SJIA. We confirm a restricted IFN-induced genetic signature in not only the peripheral mononuclear cells, but also isolated peripheral monocytes therein. Furthermore, we demonstrate a markedly constrained expression of IFN-responsive genes within peripherally affected SJIA synovial tissue in comparison to extended oligoarticular JIA, a subtype of JIA that is known to express an IFN-induced genetic signature within its synovial cells (21). Upon exploring the ability of SJIA peripheral monocytes to even respond to IFN-γ, we did not find their overall reactivity to be significantly different from healthy control peripheral monocytes, which suggests that the absence of an IFN-induced genetic signature is not due to monocyte hyporesponsiveness to IFN-γ, but likely due to a limited exposure to it. Finally, we also report an increased basal interferon signal and augmented reactivity to IFN-γ within the peripheral monocytes of inactive SJIA patients on an IL-1β blocking agent, a finding that may have an impact on the ability of these patients to appropriately respond to viral illnesses.