Objective: To clarify the function of osteoclast‐like multinuclear cells differentiated from bone marrow–derived macrophages (BMMs) by a combination of tumor necrosis factor α (TNFα) and interleukin‐6 (IL‐6), and to investigate the molecular mechanisms underlying the differentiation.Methods: BMMs were stimulated by TNFα and/or IL‐6. The cells were then compared with conventional osteoclasts differentiated in vitro by RANKL. An in vitro pit formation assay on dentine slices and an in vivo resorption assay of calvarial bones were performed. We also evaluated the activities and expression levels of NF‐κB, c‐Fos, and NF‐ATc1, which are essential to the differentiation of conventional osteoclasts. Small interfering RNA was used to knock down c‐Fos. The effects of genetic ablation of STAT‐3 and pharmacologic inhibitors of NF‐AT, JAK, and ERK were also studied.Results: Osteoclast‐like cell differentiation depended on TNFα and IL‐6 and was not inhibited by osteoprotegerin. These differentiated cells were associated with both in vitro and in vivo bone resorption activity. TNFα and IL‐6 had a synergistic effect on the activity and expression of c‐Fos. Knockdown of c‐Fos inhibited the expression of NF‐ATc1 and the differentiation of osteoclast‐like cells. All of these inhibitors blocked differentiation of the cells in vitro, but surprisingly, the conditional knockout of STAT‐3 did not. Tofacitinib also inhibited the bone destruction caused by TNFα and IL‐6 in vivo.Conclusion: Our results demonstrate that a combination of the inflammatory cytokines TNFα and IL‐6 can induce osteoclast‐like cells that have in vitro and in vivo bone‐resorptive activity.

Until recently, effector T helper (Th) cells have been classified into two subsets, Th1 and Th2 cells. Since the discovery of Th17 cells, which produce IL-17, much attention has been given to Th17 cells, mainly because they have been implicated in the pathogenesis of various inflammatory diseases. We have performed transcriptome analysis combined with factor analysis and revealed that the expression level of c-Maf, which is considered to be important for Th2 differentiation, increases significantly during the course of Th17 differentiation. The IL-23 receptor (IL-23R), which is important for Th17 cells, is among putative transcriptional targets of c-Maf. Interestingly, the analysis of c-Maf transgenic Th cells revealed that the overexpression of c-Maf did not lead to the acceleration of the early stage of Th17 differentiation but rather to the expansion of memory phenotype cells, particularly with Th1 and Th17 traits. Consistently, mouse wild-type memory Th cells expressed higher mRNA levels of c-Maf, IL-23R, IL-17, and IFN-γ than control cells; in contrast, Maf(-/-) memory Th cells expressed lower mRNA levels of those molecules. Thus, we propose that c-Maf is important for the development of memory Th cells, particularly memory Th17 cells and Th1 cells.

Rheumatoid arthritis is a chronic disease that affects multiple joints. It is considered to be an autoimmune disease in which a T helper (Th)-1 type response has been implicated to play an important pathogenetic role. As osteoclasts, cells that resorb bone, play a crucial part in the bone destruction that occurs in RA, we and others have investigated the pathophysiology of these cells. The findings that interferon (IFN)-gamma strongly inhibits osteoclastogenesis and that interleukin (IL)-17 has the ability to enhance osteoclast differentiation have cast doubt on the hypothesis that RA is a Th1 disease. In this review, I describe the relationship between Th cells, the so-called "commander" of the immune response, and RA, mainly from the viewpoint of the environments Th cells create for the excessive differentiation and function of osteoclasts, resulting in the destruction of bone.

During osteoclast differentiation, the expression of the transcription factor nuclear factor of activated T cell 1 (Nfatc1) increases in an autoproliferative manner. Nfatc1 isoforms are of three sizes, and only the short isoform increases during osteoclast differentiation. Genetic ablation of the whole Nfatc1 gene demonstrated that it is essential for osteoclastogenesis; however, the specific role of the Nfatc1 short form (Nfatc1/αA) remains unknown. In this study, we engineered Nfatc1 short form-specific knockout mice and found that these mice died in utero by day 13.5. We developed a novel osteoclast culture system in which hematopoietic stem cells were cultured, proliferated, and then differentiated into osteoclasts in vitro. Using this system, we show that the Nfatc1/αA isoform is essential for osteoclastogenesis and is responsible for the expression of various osteoclast markers, the Nfatc1 short form itself, and Nfatc1 regulators.

Full Papers; Authors: A. Kokuzawa, J. Nakamura, Y. Kamata, K. Sato