Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease of the axial skeleton, characterised by systemic osteoporosis along with new local bone formation. Previous studies have shown that serum levels of TNF, IL-6 and IL-17 are increased in AS patients and may be implicated in the development of secondary osteoporosis, since these cytokines are able to induce osteoclast (OC) differentiation and bone resorption. In this work we aimed to assess the effects of TNF-blocking therapy in the systemic inflammatory environment of AS patients as well as in OC differentiation and activity. Patients with AS starting TNF-blocking therapy were recruited for this study and blood was collected at baseline and 6 months after the first treatment. We performed ELISAs in the serum of patients to assess cytokine levels. We also characterised RANKL surface expression in circulating leukocytes and monocyte subpopulation frequency and phenotype by flow cytometry. We cultured circulating monocytes from AS patients, before and after therapy under osteoclastogenic conditions and we performed TRAP staining and resorption pit assay. We found no differences (before and after treatment) in any of the circulating monocytes subpopulations regarding frequency or cell death. Phenotype analysis only revealed differences in the classical monocyte subpopulation where there was a significant decrease in HLA-DR expression after treatment. The frequency of RANKL positive B lymphocytes was reduced after treatment. No changes were observed on RANKL expression in neutrophils or T lymphocytes after treatment. Before TNF-blocking treatment AS patients have increased levels of pro-inflammatory cytokines when compared with healthy subjects. After TNF-blocking therapy IL-17, TGF-β and osteoprotegerin were significantly decreased. Interestingly, we observed that after TNF-blocking therapy differentiated OCs have increased resorption activity. Our results suggest that in AS patients there might be a paradoxical effect of TNF blocking therapy that induces OC activity.
Disclosure: The authors declared no competing interests. This work was supported by a Fundação para a Ciência e Tecnologia personal grant (SFRH/BD/70533/2010) and in part by a research grant from Investigator-Initiated Studies Program of Merck Sharp & Dohme Corp (Merck_P08574).