Background/Purpose: In the Phase 1/2 clinical study, E6011, a novel humanized anti-fractalkine (FKN) mAb demonstrated a promising efficacy in active RA patients who were inadequately controlled by MTX and/or TNF-α inhibitors. In RA joint tissue, increased expression of FKN and abundant infiltration of CX3CR1-positive cells were observed. However, the precise mechanism(s) of FKN-CX3CR1 axis in RA, especially on joint destruction remains to be elucidated. 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. Thus, 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 immunized with intradermal injections of bovine type II collagen to induce arthritis. 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. Blood parameters were measured using ELISA. In in vitro, effect of immobilized FKN on RANK ligand (RANKL)-induced osteoclast differentiation was examined. Cell survival of bone marrow-derived OPCs without or with immobilized FKN was also assessed by FACS. In in vivo, OPCs were labeled 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-labeled OPCs that migrated into the CIA joint tissue were counted.

Results: In both prophylactic and therapeutic treatments, anti-mFKN mAb clearly reduced the clinical arthritis score, soft x-ray score and histopathological changes (synovitis, bone erosion and cartilage destruction). The number of TRAP-positive cells in the joint was clearly decreased with anti-mFKN mAb treatment. Interestingly, anti-mFKN mAb suppressed plasma levels of COMP and MMP-3 without affecting those of IL-6, TNF-α and SAA. In in vitro, RANKL-induced osteoclast differentiation was enhanced by immobilized FKN, and anti-mFKN mAb suppressed FKN-dependent enhancement of osteoclast formation. FKN enhanced cell survival of OPCs and eventually increased the number of OPCs. In in vivo, fluorescein-labeled OPCs migrated into inflamed joint tissues, and anti-mFKN mAb clearly abrogated their migration into synovium.

Conclusion: 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 without affecting systemic inflammatory parameters. These results strongly indicate that inhibition of FKN-CX3CR1 axis by a humanized 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.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/anti-fractalkine-monoclonal-antibody-ameliorates-joint-destruction-in-collagen-induced-arthritis-model-through-suppression-of-osteoclast-precursor-cell-survival-and-migration/