Background Peripheral blood (PB) monocyte pool contains cells capable of differentiating into osteoclasts (OCs). These osteoclast progenitors (OCPs) contribute to osteoresorption in inflammatory arthritides under influence of the cytokine milieu and chemokine mediated trafficking.

Objectives Our study aimed to define chemokine receptor profile of peripheral OCPs in rheumatoid arthritis (RA), with comparison to psoriatic arthritis (PSA), as well as their susceptibility to chemotactic signals.

Methods 129 RA, 53 PSA and 110 control patients were enrolled after Ethical approval. PB samples and synovial fluid (SF) samples, with clinical data of disease activity, inflammation and autoantibody levels were collected. Patients starting anti-TNF therapy were followed up 6 months. TNF-α and CTX serum levels were measured by ELISA. Frequency of OCP-rich subpopulation (CD3-CD19-CD56-CD11b+CD14+), expression of OC differentiation (CD115, RANK) and chemokine (CCR1, CCR2, CCR4, CXCR4) receptors was assessed by flow cytometry. OCPs were sorted using FACS, cultured with M-CSF and RANKL, stained for TRAP enzyme and mature OCs counted. Levels of CCL2, CCL3, CCL4, CCL5, CXCL9 and CXCL10 were measured using cytometric bead array, and of CXCL12 by ELISA. Osteoclastogenic effects of CCL2, CCL5 and CXCL10 were analyzed in cell culture, and chemotactic effects on OCPs were studied by cell migration assay using Transwell, with count of number of migrated cells and subsequently differentiated mature osteoclasts.

Results OCP population was moderately enlarged in PB, further expanded in SF and correlated with TNF-α and rheumatoid factor (RF) levels in patients with RA. However, sorted OCPs generated similar number of mature OCs as control. RANK+ subpopulation was enlarged in SF vs PB and correlated with number of tender joints. In PSA, the OCP population was not enlarged, but had a higher RANK expression. OCPs in RA and PSA had higher expression of CCR1, CCR2, CCR4, CXCR4, and all except CCR4 showed positive PB-to-SF gradient. RA had higher levels of CCL2, CCL3, CCL4, CCL5, CXCL9 and CXCL10, with a positive PB-to-SF gradient for all except CCL5 and CXCL9. OCP frequency correlated with levels of CCL2 and CCL5. Subset expressing CXCR4 was associated with TNF-α, CTX and RF levels and was lower in patients treated with DMARD, who at the same time had lower osteoresorption (CTX). Subset expressing CCR4 showed significant negative trend during anti-TNF treatment. CCL2, CCL5 and CXCL10 showed significant osteoclastogenic effect. CCL5 showed greatest chemotactic effect, attracting the highest number of cells in the migration assay. At the same time, attracted cells possessed greater osteoclastogenic potential.

Conclusions Our study provides evidence of the specific importance of certain chemokine signals for stimulation of OCP mobilization, subsequent tissue homing, and maturation, explaining local as well as generalized bone loss seen in RA. Novel insights in regards to migratory behavior and functional properties of PB OCPs in response to chemotactic signals could open way to new therapeutic targets in RA.

Acknowledgements This work was supported by a grant from the Croatian Science Foundation (project number 5699).

Disclosure of Interest None declared