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

Introduction TNF alpha 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 nave mice. HMGB1 is actively released from immune cells in response to TNF alpha; once released, HMGB1 in turn induces production of several proinflammatory cytokines including IL-6 and TNF alpha - by macrophages. Whether HMGB1-induced arthritis is mediated via the TNF alpha pathway, however, is unknown. The purpose of the present study was to investigate whether the arthritis-inducing effect of HMGB1 is dependent on TNF alpha expression in vivo and to assess whether TNF alpha deficiency affects a proinflammatory cytokine response to HMGB1 in vitro. Methods TNF alpha knockout mice and backcrossed control animals on a C57Bl6 background were injected intraarticularly with 5 mu g 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 TNF alpha 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. Results Intraarticular 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 TNF alpha (seven out of 18 mice with arthritis) in comparison with control TNF alpha(+/+) animals (six out of 19). No significant differences were detected between spleen cells from TNF alpha(+/+) mice versus TNF alpha(-/-) 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 TNF alpha(+/+) 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). Conclusion Our 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.