Background: Rheumatoid arthritis (RA) is a chronic joint disease marked by persistent inflammation and osteodestruction. The mechanisms leading to joint destruction involve infiltration of osteoclasts, multinucleated cells derived from monocyte/macrophage lineage. Human osteoclast progenitors (OCPs) are contained among peripheral blood monocytes at low frequency even in healthy subjects. OCPs exhibit chemotaxis and, furthermore, synovial compartment of RA patients highly expresses different chemokines. The aim of our study was to define these chemotactic signals by analysing expression of several chemokine receptors on OCPs in the peripheral blood, the levels of their respective ligands in serum and synovial fluid of RA patients and to assess differentiation potential of isolated OCPs.
Methods: Mononuclear cells were separated from peripheral blood of healthy controls and RA patients. The phenotype of OCPs (CD3-CD19-CD56-CD11b+CD14+) was determined using flow cytometry for the following chemokine receptors: C5AR1, CCR1, CCR2, CCR4, CXCR4. 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 cultured 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, MCP-1/CCL2, MIP1a/CCL3 and MIP1b/CCL4 concentrations were significantly higher in synovial fluid, as well as CCL2 and CCL4 in serum. Cell culture revealed no significant differences in mature osteoclast count 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 a migration mechanism of OCPs specifically associated with RA in order to develop more efficient therapeutic approaches.
Disclosure: The authors declared no competing interests. This work was supported by Croatian Funding Agency (grant no 5669).