A complicated liaison: IL-33 and IL-33R in arthritis pathogenesis

Interruption of cytokine signaling, by targeting either the cytokine itself or its cellular receptor, is a mainstay in the therapy for patients with rheumatic diseases. Interleukin (IL)-33, a member of the IL-1 cytokine family, has emerged as an important mediator of inflammatory responses. In a side-by-side examination of IL-33-deficient and IL-33 receptor (IL-33R)-deficient mice in the K/BxN serum transfer model, arthritis was ameliorated in the IL-33R knockout (KO) mice but not in the IL-33 KO mice. These findings complement previous knowledge on IL-33R signaling, demonstrating that the IL-33R cross-activates other signaling pathways in addition to IL-33-mediated signals. The results reported by Martin and colleagues in a previous issue of Arthritis Research & Therapy underline the clinical relevance of IL-33R cross-signaling and further illustrate that targeting a cytokine receptor (IL-33R) can have completely different clinical outcomes than targeting the respective cytokine.

In a previous issue of Arthritis Research & Th erapy, Martin and colleagues [1] reported a puzzling fi nding: in a side-by-side comparison of interleukin-33 (IL-33)defi cient mice and mice that lacked the receptor for IL-33 (IL-33R), K/BxN serum transfer arthritis was ameliorated only in the IL-33R-defi cient but not the IL-33-defi cient mice. IL-33 is an IL-1 family member that has emerged as a key regulator of protective and pathogenic immune responses [2,3]. It is also a chromatin-associated nuclear protein acting as a transcriptional regulator [2,3]. IL-33 is expressed by fi broblasts and epithelial, endothelial, mast, and innate immune cells. Similar to other IL-1 family members IL-33 signals through a heterodimeric complex consisting of the binding receptor (T1/ST2) and the IL-1R accessory protein (IL-1RAcP) [2,3]. T1/ST2 occurs in membrane-bound or soluble (sIL-33R) isoforms. Expres sion of membrane-bound IL-33R is restricted to hemato poietic cells, most prominently in T helper 2 (Th 2) lymphocytes, mast cells, eosinophils, basophils, and innate lymphoid cells. Signaling via IL-33R is relevant in type 2 immune responses as it induces the expression of IL-4, IL-5, and IL-13 in Th 2 lymphocytes, independently of T-cell receptor triggering [4]. Soluble T1/ST2 functions as a decoy receptor that can block IL-33/IL-33R interaction [2,3].
IL-33 is expressed in the synovium of patients with rheumatoid arthritis (RA), and IL-33 serum concentrations are elevated in patients with RA [3]. Th ese fi ndings suggest that IL-33 is involved in RA pathogenesis. Th is concept is supported by data obtained in collageninduced arthritis (CIA). Treatment with a monoclonal antibody against T1/ST2 or genetic ablation of T1/ST2 ameliorated arthritis [5,6]. In contrast, injection of IL-33 exacerbated CIA [6].
IL-33 and IL-33R have also been studied in the human tumor necrosis factor-alpha transgenic [7] and the K/ BxN serum transfer models of arthritis. It was reported, in line with the conclusions from the CIA model, that K/ BxN serum transfer arthritis was ameliorated in IL-33R (T1/ST2) knockout (KO) mice [8]. As expected from this fi nding, injection of IL-33 exacerbated K/BxN serum transfer arthritis [8]. Another study, however, yielded contradicting fi ndings. Th ere, the injection of IL-33 ameliorated K/BxN serum transfer arthritis and this was dependent on IL-4 signaling [9].
It is against this background that Martin and colleagues [1] aimed to compare the contributions of IL-33 and its receptor directly within the same model (K/BxN serum transfer) in the same laboratory. Th e authors found both the incidence and severity of arthritis to be reduced in ST2 KO mice in comparison with wild-type (WT) controls. In striking contrast, arthritis incidence and severity did not diff er in IL-33 KO mice and WT mice. Moreover, the authors found that sT1/ST2, the decoy receptor for IL-33, had no eff ect on arthritis [1].

Abstract
Interruption of cytokine signaling, by targeting either the cytokine itself or its cellular receptor, is a mainstay in the therapy for patients with rheumatic diseases. Interleukin (IL)-33, a member of the IL-1 cytokine family, has emerged as an important mediator of infl ammatory responses. In a side-byside examination of IL-33-defi cient and IL-33 receptor (IL-33R)-defi cient mice in the K/BxN serum transfer model, arthritis was ameliorated in the IL-33R knockout (KO) mice but not in the IL-33 KO mice. These fi ndings complement previous knowledge on IL-33R signaling, demonstrating that the IL-33R cross-activates other signaling pathways in addition to IL-33-mediated signals. The results reported by Martin and colleagues in a previous issue of Arthritis Research & Therapy underline the clinical relevance of IL-33R crosssignaling and further illustrate that targeting a cytokine receptor (IL-33R) can have completely diff erent clinical outcomes than targeting the respective cytokine.
How can these contradictory fi ndings be explained? Mouse models of infl ammatory diseases, including arthritis, are sometimes aff ected by environmental factors such as the diff erent microbial colonization of experimental animals in diff erent animal facilities. By performing the experiments in IL-33 and IL-33R KO mice within one group and the same facilities, Martin and colleagues could rule out such explanations for their fi ndings. Th e il-33 −/− and t1/st2 −/− mice were made from genetically modifi ed embryonic stem cells of the 129/Sv mouse strain and then back-crossed to C57BL/6 mice, in which the experiments were performed. Martin and colleagues used a 377 SNIP panel to determine that the il-33 −/− mice were 100% C57BL/6. In contrast, the t1/st2 −/− mice shared only 89% of 55 tested markers with the C57BL/6 mice that were used as WT controls [1]. Th e possibility, therefore, remains that subtle genetic polymorphisms may form the basis for the observed diff erences [10]. An obvious solution to this problem is to use littermate controls for each genetically modifi ed strain [10].
Alternative explanations for the opposite fi ndings in il-33 −/− and t1/st2 −/− mice stem from the IL-1R family signaling pathways. IL-1R family members share the IL-1RAcP. It is, therefore, conceivable that, in the absence of one receptor (for example, T1/ST2), the formation of other receptor complexes (IL-1R1/IL-1RAcP and IL-18Rα/ IL-1RAcP) is increased, allowing enhanced IL-1 or IL-18 signaling. In fact, macrophages from T1/ST2-defi cient mice were reported to produce more proinfl ammatory cytokines than cells from WT mice in response to IL-1 or lipo polysaccharide [11]. Both CIA and K/BxN arthritis depend on IL-1 [12]. Increased responsiveness toward IL-1 would be expected to increase incidence and severity of arthritis. Th erefore, enhanced formation of other receptor complexes is unlikely to explain why mice lacking T1/ST2 −/− are protected from arthritis whereas IL-33 −/− mice are susceptible [1].
Earlier fi ndings from other disease models resemble those reported by Martin and colleagues [1] and have been reported for IL-18, another IL-1 family member, and its receptor. IL-18Rα-defi cient mice were resistant to experi mental autoimmune encephalomyelitis, a mouse model for multiple sclerosis. In contrast, IL-18-defi cient mice were susceptible to experimental autoimmune encepha lo myelitis [13]. Th e published data on the role of IL-33 and T1/ST2 in Leishmania infection allow the conclusion that T1/ST2 + eff ector cells, but not IL-33induced signaling, are relevant for disease pathogenesis [3]. Consequently, T1/ST2 may exert functions in addition to the transduction of IL-33-induced signals.
We found that IL-33-induced signaling via T1/ST2 cross-activates the receptor tyrosine kinase c-Kit, the receptor for stem cell factor in human and murine mast cells. T1/ST2-induced activation of c-Kit is required to elicit optimal eff ector functions in response to IL-33 [14]. Th e structural basis for this cross-activation is the ligandinduced complex formation between c-Kit, T1/ST2, and IL-1R accessory protein (IL-1RAcP). Th e requirement for T1/ST2 for optimal signal transduction from other receptors could provide one mechanistic explanation for the fact that T1/ST2-defi cient mice, but not IL-33defi cient mice, are protected from arthritis. Together with the data reported by Martin and colleagues [1], these fi ndings indicate that targeting IL-33 or its receptor might result in fundamentally diff erent outcomes in patients (with arthritis).
To date, the cross-activation of c-Kit by T1/ST2 has been reported [14]. Mast cells are probably involved in arthritis pathogenesis, but most cell lineages relevant for arthritis pathogenesis do not express c-Kit. It remains to be elucidated whether T1/ST2 cooperates with other receptor tyrosine kinases in other eff ector cells that do not express c-Kit. A detailed understanding of such interactions between T1/ST2 with other signaling pathways seems to be mandatory for rational therapeutic mani pula tion of this system. More surprises, in addition to the puzzling fi ndings reported by Martin and colleagues [1], regarding T1/ST2 signaling in health and disease are likely to be discovered.