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, therefore, bone resorption.
In this work we aimed to assess the effects of TNF-blocking therapy in the systemic inflammatory environment of AS patients with active disease 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 administration. We performed ELISAs in the serum of these patients to assess cytokine levels and bone turnover markers. We also characterised RANKL surface expression in circulating neutrophils, B and T lymphocytes and monocyte subpopulation frequency and phenotype by flow cytometry. We cultured circulating monocytes from AS patients, before and after therapy, and from healthy controls under osteoclastogenic conditions. We performed two functional assays (TRAP staining and resorption pit assay) and analysed the expression of osteoclast specific genes.
We found no differences (before and after treatment) in any of the circulating monocytes’ subpopulations regarding frequency or cell death assessed by annexin staining. Phenotype analysis only revealed differences in the classical monocyte subpopulation where there was a significant decrease in HLA-DR surface 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, gene expression was favouring osteoclastogenesis and that 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.