title: Focal bone involvement in inflammatory arthritis: the role of IL17.
abstract: Conditions such as rheumatoid arthritis (RA) and spondyloarthritis (SpA, such as psoriatic arthritis, PsA, and ankylosing spondylitis, AS) are characterized by an imbalance between osteoclast (OC) bone resorption and osteoblast (OB) bone formation. The two conditions present substantial differences in bone involvement, which is probably related to the different expression of IL17 and TNFα, two cytokines that strongly promote osteoclastogenesis and focal bone erosions. TNFα is the major inflammatory cytokine in RA. It acts by both triggering OC bone erosion via the RANK-RANKL system, and suppressing OB bone formation through the overexpression of DKK1, a powerful inhibitor of the WNT bone anabolic signaling pathway. Differing from TNFα, IL17 promotes also osteogenesis, particularly at inflamed sites undergoing mechanical stress, such as entheses. Therefore, in RA, where overexpression of TNFα is higher than IL17, OC bone resorption largely prevails upon bone formation. In PsA and AS, the prevailing inflammatory cytokine is IL17, which promotes also osteogenesis. Given the prevalent involvement of entheses poor of OC, excess bone formation may even prevail over excess bone resorption. The results of clinical trials support the different pathophysiology of bone involvement in chronic arthritis. Inflammation control through anti-TNFα agents has not resulted in incomparable effects on radiographic progression and excess bone formation in both AS and PsA. Clinical trials investigating IL17 inhibitors, such as secukinumab, in patients with psoriatic disease are underway. The preliminary results on inflammation and symptoms appear positive, while long-term studies are required to demonstrate an effect on excess bone formation.
authors.affiliation: Rheumatology Section, Department of Medicine, University of Verona, Verona, Italy. maurizio.rossini@univr.it.|Rheumatology Unit, University of Verona, Policlinico Borgo Roma, Piazzale Scuro, 10, 37134, Verona, Italy. maurizio.rossini@univr.it.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Rossini
authors.affiliation: Rheumatology Section, Department of Medicine, University of Verona, Verona, Italy.
authors.forename: Ombretta
authors.initials: O
authors.lastname: Viapiana
authors.affiliation: Rheumatology Section, Department of Medicine, University of Verona, Verona, Italy.
authors.forename: Silvano
authors.initials: S
authors.lastname: Adami
authors.affiliation: Rheumatology Section, Department of Medicine, University of Verona, Verona, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Idolazzi
authors.affiliation: Rheumatology Section, Department of Medicine, University of Verona, Verona, Italy.
authors.forename: Elena
authors.initials: E
authors.lastname: Fracassi
authors.affiliation: Rheumatology Section, Department of Medicine, University of Verona, Verona, Italy.
authors.forename: Davide
authors.initials: D
authors.lastname: Gatti
mesh_terms: D000818:Animals; D000893:Anti-Inflammatory Agents / Q000627:therapeutic use; D001172:Arthritis, Rheumatoid / Q000188:drug therapy / Q000276:immunology / Q000378:metabolism* / Q000473:pathology; D016723:Bone Remodeling / Q000187:drug effects; D001842:Bone and Bones / Q000187:drug effects / Q000276:immunology / Q000378:metabolism* / Q000473:pathology; D006801:Humans; D018836:Inflammation Mediators / Q000037:antagonists & inhibitors / Q000378:metabolism*; D020381:Interleukin-17 / Q000037:antagonists & inhibitors / Q000276:immunology / Q000378:metabolism*; D007596:Joints / Q000187:drug effects / Q000276:immunology / Q000378:metabolism* / Q000473:pathology; D053245:RANK Ligand / Q000378:metabolism; D025241:Spondylarthritis / Q000188:drug therapy / Q000276:immunology / Q000378:metabolism* / Q000473:pathology; D060449:Wnt Signaling Pathway
chemical_list: D000893:Anti-Inflammatory Agents; D018836:Inflammation Mediators; D020381:Interleukin-17; D053245:RANK Ligand; C505688:TNFSF11 protein, human
keywords: Bone formation; Bone resorption; IL17; RANK–RANKL system; Rheumatoid arthritis; Spondyloarthritis