THU0053 Anti-fractalkine monoclonal antibody ameliorates joint destruction in collagen-induced arthritis model by inhibition of osteoclast precursor cell survival and migration

Background In the Phase 1/2 clinical study, E6011, a novel humanised anti-fractalkine (FKN) mAb demonstrated a promising efficacy in active RA patients who were inadequately controlled by MTX and/or TNF-α inhibitors (NCT 02196558). FKN is expressed on endothelial cells and fibroblast-like synoviocytes in synovium and also expressed on osteoblasts. CX3CR1 is expressed on monocytes/macrophages and osteoclast precursor cells (OPCs). Therefore, FKN-CX3CR1 interaction could play pivotal roles in migration, differentiation and activation of those cells. However, the precise mechanism(s) of FKN-CX3CR1 axis in RA, especially on joint destruction remains to be elucidated. Objectives We examined the roles of FKN-CX3CR1 axis in joint destruction, particularly focused on osteoclast precursor cells in in vitro and in vivo by using anti-mouse FKN mAb (anti-mFKN mAb). Methods DBA/1J mice were immunised with intradermal injections of bovine type II collagen to induce arthritis(CIA). Anti-mFKN mAb or control IgG were intraperitoneally injected twice a week. The clinical arthritis score was monitored, and joint destruction was evaluated by soft X-ray and histopathology. Plasma levels of joint destruction markers were assessed by ELISA. FKN expression in joint tissues were assessed by immunohistochemistry. Cell survival of bone marrow-derived OPCs without or with immobilised FKN was also assessed by FACS. In in vivo , OPCs were labelled by fluorescein and transferred to CIA mice to evaluate migration of OPCs into inflamed synovium. Anti-mFKN mAb or control IgG were injected before the cell transfer. The number of fluorescein-labelled OPCs that migrated into the CIA joint tissue were analysed. Results In both prophylactic and therapeutic treatments, anti-mFKN mAb clearly reduced the clinical arthritis score, soft X-ray score. Plasma levels of cartilage oligomeric matrix protein (COMP) and matrix metalloproteinase-3 (MMP-3) were also decreased after treatment with anti-mFKN mAb. Histological analysis revealed that anti-mFKN mAb inhibited synovitis, pannus formation, cartilage destruction, and that it suppressed bone damage with marked reductions in the number of tartrate-resistant acid phosphatase (TRAP)-positive cells. In in vitro , RANKL-induced osteoclast differentiation was enhanced by immobilised FKN, and anti-mFKN mAb suppressed FKN-dependent enhancement of osteoclast formation. FKN also enhanced cell survival of OPCs and eventually increased the number of OPCs. In in vivo , increasing expression of FKN was detected on endothelium in synovial tissue of CIA mice, and fluorescein-labelled OPCs migrated into the affected synovium. Anti-mFKN mAb clearly abrogated their migration into synovium. Conclusions Anti-mFKN mAb remarkably ameliorated the joint destruction with the marked reduction of osteoclasts by the inhibition of both OPC survival and OPC migration in inflamed joint tissues These results strongly indicate that inhibition of FKN-CX3CR1 axis by a humanised anti-FKN mAb, E6011, is an attractive and affected joints-selective therapeutic strategy for the treatment of both inflammatory synovitis and joint destruction in RA patients. Disclosure of Interest None declared