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