A potential new target for autoinflammatory bone disease

Abstract

Chronic recurrent multifocal osteomyelitis (CRMO) is an autoinflammatory bone disease mediated by the inflammatory cytokine, IL-1β. Although IL-1β is known as the key driver of bone lesions in CRMO, the signaling events leading to pathogenic levels of the cytokine are not fully understood. Using a genetic mouse model of CRMO, Dasari et al. find a role for the nonreceptor spleen tyrosine kinase (SYK) in upstream signaling leading to IL-1β up-regulation. Their findings suggest that SYK may constitute a new therapeutic target for CRMO.

Introduction

IL-1β is a key pro-inflammatory cytokine that aids the immune response during infection. However, when IL-1β is produced in excess, the resulting chronic inflammation can lead to tissue destruction and recurrent inflammatory disease (1). In the case of chronic recurrent multifocal osteomyelitis (CRMO),² patients suffer bone lesions, joint swelling, and debilitating pain that is managed in a nonspecific way with anti-inflammatory drugs and corticosteroids (2, 3). The potential for pathogenic levels of IL-1β to cause inflammation and tissue damage means that the signaling events leading to production of the cytokine are usually tightly regulated; understanding these steps is thought to be pivotal to the development of new therapies to relieve the symptoms of CRMO. The penultimate step of this process is the cleavage of pro-IL-1β by caspase-1 or caspase-8, which act in a redundant manner to elicit mature IL-1β. The upstream events leading to caspase-1–mediated cleavage of IL-1β are well-established and result from activation of the canonical NLRP3 inflammasome (4). In contrast, the events leading to caspase-8 cleavage of IL-1β are not as well-defined. The findings from Dasari et al., seeking to shed light on the pathways leading to caspase-8 processing of IL-1β, unexpectedly implicate the nonreceptor tyrosine kinase, SYK, in mediating chronic osteomyelitis (5).

CRMO in humans can be modeled using mice that carry a missense mutation (L98P) in the Pstpip2 gene 9 (Pstpip2^cmo). Pstpip2 encodes a proline-serine-threonine phosphatase-interacting protein 2, which regulates membrane and cytoskeletal dynamics. The disease condition in mice is termed chronic multifocal osteomyelitis (CMO), and Pstpip2^cmo mice suffer an autoinflammatory disease that is characterized by osteomyelitis, bone deformities, and elevated levels of IL-1β. This has led to the identification of IL-1β and the IL-1 receptor as essential for disease and implicated redundant roles for NLRP3/caspase-1 and caspase-8. Although caspase-8 is mostly known for driving death receptor–driven extrinsic apoptosis, activated caspase-8 can also cleave pro-IL-1β (6). In CMO, the factors that lead to caspase-8 activation have not been defined.

Activation of the nonreceptor tyrosine kinase, SYK, which is involved in Toll-like receptor and NOD-like receptor signaling, can also lead to activated caspase-8 (7). Hence, to examine the factors upstream of caspase-8 activation, Dasari et al. first crossed Pstpip2^cmo mice with Nlrp3^−/− mice to create a strain deficient for caspase-1–mediated activation of IL-1β, leaving only the caspase-8 pathway. By crossing Pstpip2^cmo Nlrp3^−/− mice with other mice deficient for selected inflammasome or innate immune signaling factors, Dasari et al. showed that none of RIPK3, AIM2, or CARD9 played a role in CMO disease progression. In contrast, Pstpip2^cmo Nlrp3^−/−Syk^fl/flLysM^cre were rescued for disease. In these mice, the nonreceptor tyrosine kinase, SYK, was absent in myeloid cells. Interestingly, Pstpip2^cmoSyk^fl/flLysM^cre mice were also rescued for disease, suggesting that SYK acted upstream of both caspase-1– and caspase-8–mediated processing of IL-1β in CMO disease. Hence, SYK is an attractive target for therapeutic intervention because the tyrosine kinase was sufficient and required for the induction of disease in the Pstpip2^cmo model (5).

To understand the mechanism behind SYK involvement in CMO, Dasari et al. compared SYK activation and pro-IL-1β production in the footpads of WT, Pstpip2^cmo, and Pstpip2^cmoSyk^fl/flLysM^cre mice by immunoblotting. This showed that myeloid depletion of SYK restored WT levels of pro-IL-1β expression to Pstpip2^cmo mice. Further experiments with LPS-stimulated bone marrow–derived macrophages confirmed that SYK deficiency in Pstpip2^cmo macrophages restored pro-IL-1β production and SYK activation to WT levels, suggesting that SYK acted upstream of pro-IL-1β expression. Upon studying the activation state of other inflammatory signaling factors (extracellular signal–regulated kinase, p38, c-Jun N-terminal kinase, and NF-κB), Dasari et al. found that SYK deficiency reversed the activation of NF-κB in Pstpip2^cmo macrophages, whereas the expression of Asc, Nlrp3, caspase-1, and caspase-8 as well as the cleavage of caspase-1 and caspase-8 were not affected by SYK deficiency. In other words, SYK activation in Pstpip2^cmo mice led to NF-κB–dependent enhanced expression of pro-IL-1β, and this was critical for the development of disease (Fig. 1).

Figure 1.

In a mouse model of CRMO, activation of SYK leads to enhanced NF-κB activity and IL1β expression. Pro-IL-1β is cleaved by caspase-1 or caspase-8, which act redundantly, to generate high levels of mature IL-1β, leading to inflammation and bone lesions.

The receptor signaling events leading to SYK activation remain to be identified. Possible candidates include integrins, CD74, C-type lectins, and Fc receptors (8), and this knowledge would help to clarify the pathways that lead to overproduction of IL-1β. Another point to resolve fully is the cell type involved in mediating disease. However, given the contribution of neutrophils to IL-1β production in CMO disease (9), Dasari et al. propose that neutrophils are likely to be the mediators of bone lesions, acting in a SYK-dependent manner.

Given that the current treatment for CRMO with anti-inflammatory agents is fairly nonspecific, many patients would benefit from a more precise approach that reduces the risk of permanent bone damage and deformities by preventing the activation of inflammatory signaling. In this study, the identification of SYK in the development of CMO disease in Pstpip2^cmo mice offers an opportunity to target a single factor in the development of therapeutic inhibitors to treat an autoinflammatory condition with greater precision. With greater understanding of the upstream events leading to SYK activation, more possibilities will emerge for the development of tailored approaches to combat autoinflammatory diseases arising from the overproduction of IL-1β.