Analysis of combined deficiency of interleukin-1 and -6 versus single deficiencies in TNF-mediated arthritis and systemic bone loss

AIMS: Insufficient treatment response in rheumatoid arthritis (RA) patients requires novel treatment strategies to halt disease progression. The potential benefit of combination of cytokine-inhibitors in RA is still unclear and needs further investigation. To explore the impact of combined deficiency of two major cytokines, namely interleukin (IL)-1 and IL-6, in this study double deficient mice for IL-1αβ and IL-6 were investigated in different tumour necrosis factor (TNF)-driven inflammatory bone disorders, namely peripheral arthritis and sacroiliitis, as well as systemic bone loss. METHODS: Disease course, histopathological features of arthritis, and micro-CT (µCT) bone analysis of local and systemic bone loss were assessed in 15-week-old IL1(-/-)IL6(-/-) hTNFtg in comparison to IL1(-/-) hTNFtg, IL6(-/-) hTNFtg, and hTNFtg mice. µCT bone analysis of single deficient and wild-type mice was also performed. RESULTS: Combined deficiency of IL-1/IL-6 markedly ameliorated TNF-mediated arthritis and bilateral sacroiliitis, but without additive benefits compared to single IL-1 deficiency. This finding confirms the important role of IL-1 and the marginal role of IL-6 in TNF-driven pathways of local joint damage, but questions the efficacy of potential combinatorial therapies of IL-1 and IL-6 in treatment of RA. In contrast, combined deficiency of IL-1/IL-6 led to an additive protective effect on TNF-driven systemic bone loss compared to single IL-1 and IL-6 deficiency. This finding clearly indicates a common contribution of both IL-1 and IL-6 in TNF-driven systemic bone loss, and points to a discrepancy of cytokine dependency in local and systemic TNF-driven mechanisms of inflammatory arthritis. CONCLUSION: Combinatorial treatments in RA might provide different benefits to inflammatory local arthritis and systemic comorbidities. Cite this article: Bone Joint Res 2022;11(7):484–493.