Background Tumor necrosis factor-alpha (TNF) contributes to the pathological osteolysis observed in rheumatoid arthritis either directly by stimulating the development of osteoclasts, or indirectly through the induction of the pro-osteoclastogenic cytokine RANKL. However, the role of TNF in promoting human osteoclast differentiation and/or recruitment of osteoclast precursors (OCP) through the secretion of chemokines in the presence of RANKL is poorly understood.

Objectives To determine the impact of TNF on RANKL-induced human osteoclast development and then assess the contribution of TNF-induced chemokines to promote the chemotaxis of monocytes, potential precursors of osteolasts, to inflammatory sites, both in the absence or presence of various anti-TNF biologics.

Methods Human OCP were cultured for up to 6 days with M-CSF and RANKL +/− 100 ng/mL TNF pre-treated for 30 min with increasing concentrations of hIgG, adalimumab (Ada) or etanercept (Etn). Osteoclast maturation was determined by measuring tartrate-resistant acid phosphatase (TRAP5b) activity. Collagen degradation was assessed by measuring release of plate-bound Europium-labeled collagen (Eu-col). Chemokines were identified by 9-plex MSD. Chemotaxis was assessed in 5 mm pore size plate with human monocytic THP-1 cells in upper chamber, and culture supernatant harvested on day 5 from OCP in lower chamber +/− pre-treatment for 30 min with anti-chemokine antibodies. Cell migration was detected with CellTiter Fluor.

Results TNF promoted greater maturation (3-fold increased TRAP5b) and enhanced collagen degradation (3-fold increased Eu-col levels) of osteoclasts as compared to RANKL alone. Ada restored osteoclast differentiation (at a 10-fold lower concentration than Etn, 0.74 μg/mL and 20 μg/mL, respectively) as well as collagen degradation to levels comparable to RANKL alone. Chemokines including MDC, IL-8, MCP-1 and MIP-1b were detected at higher levels in TNF-exposed OCP cultures as compared to RANKL alone, with a 2-fold reduction in each only in the presence of Ada except for MDC, which was equally inhibited by both Ada and Etn. Culture supernatants from OCP treated only with combination of TNF and RANKL significantly induced chemotaxis of THP-1 (p<0.0001) in an MCP-1 dependent manner as demonstrated by a 3-fold reduction in presence of anti-MCP-1 neutralizing antibody. Addition of Ada, but not Etn, to TNF-exposed OCP cultures led to a significant reduction (p<0.001) in THP-1 chemotaxis in response to TNF-induced chemokines secreted from the OCP.

Conclusions Overall, the results of our investigation demonstrate that TNF can enhance RANKL-induced osteoclast development and promote secretion of chemokines such as IL-8, MIP-1b and MCP-1. In addition, TNF stimulation of OCP led to the chemotaxis of human monocytic cells, suggesting that additional precursors, such as monocytes, may be recruited to the erosion site through the increased chemokine expression. Interestingly, Ada was more effective than Etn to inhibit all three TNF-mediated events, suggesting that Ada may be more effective in preventing TNF-induced OCP population development and expansion in areas of osteolysis.

Acknowledgement Authors thank Drs. Jochen Salfeld, Anna Yarilina and Jing Min for their critical review.

Disclosure of Interest B. Harvey Shareholder of: AbbVie Inc., Employee of: AbbVie Inc., Z. Kaymakcalan Shareholder of: AbbVie Inc., Employee of: AbbVie Inc.