Background Rheumatoid arthritis (RA) is characterized by chronic inflammatory response as well as enhanced bone destruction. Development of bone erosions is critically dependent on osteoclasts, which are highly specialized bone cells capable of resorbing the mineralized matrix. Osteoclast progenitors (OCPs), contained among myeloid hematopoietic lineage, could be found in the peripheral blood and synovial tissue of patients with RA, mediating bone loss locally, in the form of bone erosions and joint osteolysis, and systemically, with loss of skeletal bone density.
Objectives To define these chemotactic signals by analyzing expression of several chemokines and chemokine receptors on T lymphocytes and OCPs in the peripheral blood of RA patients, to measure the levels of their respective ligands in serum and synovial fluid of RA patients, and to assess the osteoclastogenic potential of OCPs.
Methods Mononuclear cells were isolated from peripheral blood of healthy controls and RA patients. The phenotype of isolated mononuclear cells was determined using flow cytometry. OCPs (CD3-CD19-CD56-CD11b+CD14+) were analyzed for the expression of the following chemokine receptors: C5AR1, CCR1, CCR2, CCR4, CXCR4. T lymphocytes (CD3+CD4+ or CD3+CD8+) were analyzed for expression of CXCR5, CCR4, CCR6 chemokine receptors. Chemokine ligand concentrations (MIP-1α/CCL3, MIP-1β/CCL4, MCP-1/CCL2, RANTES/CCL5) were measured in serum and synovial fluid of RA patients using flow cytometry bead based array. OCPs were sorted and plated into cell culture with M-CSF and RANKL. After two weeks, the cells were stained for TRAP enzyme and positive, mature, osteoclasts were counted.
Results Human peripheral blood OCPs similarly expressed chemokine receptors CCR1, CCR2, CCR4 and CXCR4 in RA and healthy subjects. However, MCP1/CCL2, MIP1a/CCL3 and MIP1b/CCL4 concentrations were significantly higher in synovial fluid (and blood for CCL2 and CCL4). Cell culture revealed no significant differences in mature osteoclast count between RA and control group. The proportion of T lymphocytes expressing CCR4 was two-fold higher in RA patients compared to controls. T lymphocyte expression of CXCR5 and CCR6 was similar between RA and control group.
Conclusions Although OCPs in RA have a differentiation potential similar to controls, levels of several chemokines are upregulated, indicating a possible chemotactic mechanism of OCP migration to affected joints. These results may help to reveal migration mechanism of T lymphocytes and OCPs specifically associated with RA in order to develop more efficient therapeutic approaches.
Acknowledgements This work has been supported by Croatian Science Foundation (project number 5699).
Disclosure of Interest None declared