CHARACTERISTICS OF TUMOR NECROSIS FACTOR-ALPHA AND INTERLEUKIN-6-INDUCED OSTEOCLASTS IN PERIPHERAL BLOOD AND BONE TISSUE FROM PATIENTS WITH RHEUMATOID ARTHRITIS

BackgroundWe have previously reported that stimulation of mouse bone marrow–derived macrophages with tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) induces differentiation of osteoclast-like cells having bone resorption ability1. Recently, we have shown that the combination of TNF-α and IL-6 can induce osteoclasts from human peripheral blood mononuclear cells (PBMCs) via RANKL-independent pathways, and that there are functional differences between TNF-α and IL-6-induced osteoclasts (T6-OCs) and RANKL-induced, conventional osteoclasts (cOCs). In particular, the number of T6-OCs differentiated from PBMCs in patients with rheumatoid arthritis (RA) positively correlated with the modified total Sharp score (mTSS)2. On the other hands, no such correlation was observed between the number of cOCs from RA and mTSS.ObjectivesObjectives of this study were to compare the differentiational potential into T6-OCs of PBMCs from RA patients with those from healthy donors, to clarify mRNA and protein expressions of T6-OCs derived from PBMCs from patients with RA, and to identify tartrate resistant acid phosphatase (TRACP) positive multinuclear cells with the same characters as T6-OCs histologically in the sub-chondral bone tissues from patients with RA and osteoarthritis (OA).MethodsPBMCs and CD14+ monocytes derived from RA patients and healthy volunteers were stimulated with TNF-α and IL-6 or RANKL. Real-time quantitative PCR and immunofluorescence staining were used to measure expression levels of osteoclast-associated mRNA and protein. Consecutive sections of the proximal tibial bone tissue from patients with RA and OA (n=6 each) were stained by TRACP, and analyzed expression levels of osteoclast-associated molecules by immunohistochemistry.ResultsThe number of T6-OCs differentiated from PBMCs in RA patients was significantly increased compared to that in healthy volunteers. Expression levels of RANK mRNA and protein were clearly up-regulated in cOCs differentiated from CD14+ monocytes and were down-regulated in T6-OCs. In contrast, expression levels of MMP-3 mRNA and protein were obviously up-regulated in T6-OCs and down-regulated in cOCs. Therefore, we believe T6-OCs and cOCs were differently identified on bone tissue as TRACP+RANK-/MMP-3+ cells and TRACP+RANK+/MMP-3- cells, respectively. The numbers of TRACP+ osteoclasts in subchondral cancellous bone were significantly increased in RA patients compared to those in OA patients. Interestingly, numerous TRACP+/RANK-/MMP-3+ osteoclasts were present in the subchondral bone from patients with RA, on the other hands, no such cells observed in OA patients.ConclusionThe PBMCs of RA patients have definitely increased differentiation capacity into T6-OCs, which have potential of degrading chondral tissue. Additionally, cells having same characteristics with T6-OCs are observed in subchondral bone of patients with RA. These results suggest that novel T6-OCs may be involved in the pathogenic mechanisms of inflammatory bone destruction in patients with RA.References[1]Yokota K, Sato K, Miyazaki T, Kitaura H, Kayama H, Miyoshi F, Araki Y, Akiyama Y, Takeda K, Mimura T. Combination of Tumor Necrosis Factor α and Interleukin-6 Induces Mouse Osteoclast-like Cells With Bone Resorption Activity Both in Vitro and In Vivo. Arthritis & Rheumatology Jan;66(1):121-9, 2014.[2]Yokota K, Sato K, Miyazaki T, Aizaki Y, Tanaka S, Sekikawa M, Kozu N, Kadono Y, Oda H, Mimura T. Characterization and Function of Tumor Necrosis Factor alpha and Interleukin-6-Induced Osteoclasts in Rheumatoid Arthritis. Arthritis & Rheumatology Jul;73(7):1145-1154, 2021.AcknowledgementsWe are grateful to H. Kajiyama and Y. Araki (Saitama Medical University) for helpful discussion.Disclosure of InterestsNone declared