Heidi Schiøler Schultz:
Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease that primarily affects the peripheral diarthroidal joints. The joint inflammation in RA is characterized by hyperplasia of the synovium and pannus formation as well as by the presence of pro-inflammatory cytokines and chemokines which causes a massive influx of immune cells into the synovial tissue (ST). A hallmark of RA pathogenesis is the enhanced formation and activation of osteoclasts (OC). The progressive inflammation leads to remodeling of the extracellular matrix (ECM), and ultimately results in destruction of cartilage and bone in the joint. RA can to some degree be suppressed by non-biologics disease-modifying anti-rheumatic drugs (nbDMARDS) or biologics (bDMARDS) such as e.g. TNF-α inhibitors. However, the efficacy of the current drugs is limited as they are only suppressing inflammation, but are not able to halt joint destruction completely. Identification of factors that regulate inflammation and bone/cartilage loss is crucial for development of a new generation of drugs to treat RA.
The OSteoClast-Associated Receptor (OSCAR) was proposed as a new target for treatment of RA. OSCAR has been reported to be up-regulated in the synovium of RA patients and on the circulating monocytes where its expression level has been shown to positively correlate with the disease activity. In mice, OSCAR expression is limited to OC. It was proposed that OSCAR provides a co-stimulatory signaling in RANK-dependent osteoclastogenesis. In humans, OSCAR is expressed in OC, DCs, monocytes, macrophages and neutrophils. We and others have identified the ligands of OSCAR to be collagen type I (ColI) and –type II (ColII). In RA, excessive degradation of ECM by enzymes secreted by activated immune cells, fibroblasts and OC leads to collagens exposure and makes them available for interaction with the immune cells. Thus, we hypothesize that at such pathological conditions ColI/II serve as naturally occurring activators of the OSCAR signaling.
In the present study, we have investigated the functional outcome of the OSCARcollagen interaction in human myeloid cells. We have observed that OSCAR engagement by ColI/II induced activation and maturation of DCs characterized by up-regulation of cell surface markers and secretion of pro-inflammatory cytokines. These collagen-matured DCs (Col-DCs) were efficient drivers of naïve T cell proliferation. The T cells expanded by Col-DCs secreted cytokines with no clear T-cell polarization pattern. Global RNA profiling revealed that multiple proinflammatory mediators, including cytokines and cytokine receptors, components of the stable immune synapse (namely, CD40, CD86, CD80, ICAM-1), as well as components of TNF- and TLR signaling, are transcriptional targets of OSCAR in DCs.
The functional role of the OSCAR-collagen interaction was further addressed in monocytes and macrophages. We have shown that the OSCAR-ColII signaling supports survival of monocytes under conditions of growth factor withdrawal. Moreover, ColII stimulated release of pro-inflammatory cytokines by monocytes from healthy donors, and this release could be completely blocked by an antagonistic anti-OSCAR mAb. Similarly, mononuclear cells from synovial fluid of RA patients cultured on ColII secreted TNF-α and IL-8 in an OSCAR-dependent manner. Global profiling of gene expression in monocytes showed that components of multiple signaling pathways relevant for the RA pathogenesis are transcriptional targets of OSCAR. Surprisingly, OSCAR engagement by ColII in macrophages, which were differentiated in vitro from monocytes of healthy donors, did not lead to secretion of cytokines. Microarray analysis did not reveal any gene regulation by the OSCAR-ColII signaling in macrophages. However, degradation of collagen-rich matrix, gelatin, by macrophages was significantly abolished by an antagonistic anti-OSCAR mAb, indicating that OSCAR modulates the activity of proteolytic enzymes on a posttranscriptional level.
Taken together, our findings indicate the existence of a novel pathway mediating a sustained inflammation in the RA joints where collagens become exposed during tissue remodeling and can thus activate the OSCAR signaling in monocyte-derived cells. We hypothesize that the OSCAR-collagen pathway can potentially contribute to the RA pathogenesis on multiple levels and may thus represents a new target for therapeutic intervention.