['TNFα and IL-17 have been shown to be the major inflammatory cytokines involved in the pathogenesis of rheumatoid arthritis (RA). Here, we examined the effect of these cytokines on spontaneously occurring RA in our newly established arthritis-prone FcγRIIB- deficient C57BL/6 (B6) mice, designated KO1, by introducing genetic deficiency of TNFα and IL-17 into KO1 mice.', 'KO1.TNFα(-/-) and KO1.IL-17(-/-) mice were established by crossing KO1 with TNFα-deficient and IL-17-deficient B6 mice, respectively. The incidence and severity of RA, cartilage and bone destruction, immunological abnormalities, and transcription levels of receptor activator of NF-κB ligand (RANKL)/osteoprotegerin (OPG) and inflammatory cytokines/chemokines in ankle joints were compared among KO1, KO1.TNFα(-/-), and KO1.IL-17(-/-) mice.', 'The development of RA was completely inhibited in KO1.TNFα(-/-) mice. In contrast, KO1.IL-17(-/-) mice unexpectedly developed severe RA comparable to KO1. Compared with those in KO1 and KO1.IL-17(-/-) mice, frequencies of peripheral monocytes, known to be containing osteoclast precursors, were significantly decreased in KO1.TNFα(-/-) mice. Intriguingly, while RANKL expression levels in ankle joints did not differ among the three strains, OPG expression levels were drastically decreased in arthritis-prone, but not arthritis-free, mice. The expression levels of inflammatory cytokines/chemokines, such as MCP-1, IL-6, and TNFα, were up-regulated in arthritis-prone mice.'] Is tNFα but not IL-17 critical in the pathogenesis of rheumatoid arthritis spontaneously occurring in a unique FcγRIIB-deficient mouse model?