Induction of arthritis by high mobility group box chromosomal protein 1 is independent of tumour necrosis factor signalling

Abstract

IntroductionTNFα and high mobility group box chromosomal protein 1 (HMGB1) are two potent proinflammatory cytokines implicated as important mediators of arthritis. Increased levels of these cytokines are found in the joints of rheumatoid arthritis patients, and the cytokines trigger arthritis when applied into the joints of naïve mice. HMGB1 is actively released from immune cells in response to TNFα; once released, HMGB1 in turn induces production of several proinflammatory cytokines – including IL-6 and TNFα – by macrophages. Whether HMGB1-induced arthritis is mediated via the TNFα pathway, however, is unknown. The purpose of the present study was to investigate whether the arthritis-inducing effect of HMGB1 is dependent on TNFα expression in vivo and to assess whether TNFα deficiency affects a proinflammatory cytokine response to HMGB1 in vitro.MethodsTNFα knockout mice and backcrossed control animals on a C57Bl6 background were injected intraarticularly with 5 μg HMGB1. Joints were dissected 3 days after intraarticular injection and were evaluated histologically by scoring the frequency and severity of arthritis. For in vitro studies, mouse spleen cultures from TNFα knockout mice and from control mice were incubated with different doses of HMGB1, and cell culture supernatants were collected at different time points for analysis of IL-6.ResultsIntraarticular injection of HMGB1 into healthy mouse joints resulted in an overall frequency of 32% to 39% arthritic animals. No significant differences were found with respect to the severity and incidence of synovitis between mice deficient for TNFα (seven out of 18 mice with arthritis) in comparison with control TNFα+/+ animals (six out of 19). No significant differences were detected between spleen cells from TNFα+/+ mice versus TNFα-/- mice regarding IL-6 production upon stimulation with highly purified HMGB1 after 24 hours and 48 hours. Upon stimulation with a suboptimal dose of recombinant HMGB1, however, the splenocytes from TNFα+/+ animals released significantly more IL-6 than cells from the knockout mice (602 ± 112 pg/ml and 304 ± 50 pg/ml, respectively; P < 0.05).ConclusionOur data show that HMGB1-triggered joint inflammation is not mediated via the TNF pathway. Combined with our previous study, we suggest that HMGB1-triggered arthritis is probably mediated through IL-1 activation.