Plasmacytoid dendritic cells (pDCs) have been found to be accumulated in synovial tissues in rheumatoid arthritis (RA). Since pDCs originate from bone marrow (BM), we explored the differentiation of pDC in BM in RA and osteoarthritis (OA).
BM mononuclear cells (BMMNCs) of the posterior ileac crest from 25 RA patients and 22 OA patients were examined for the expression of BDCA2 and CD34 by flow cytometry. The degree of synovial proliferation was assessed on light microscopy in 10 of 25 RA patients.
There were no significant differences in percentages of CD34 + cells or BDCA2 + cells within BMMNC between RA and OA. However, RA BMMNC contained higher percentages of BDCA2 + CD34 + cells (pre-pDCs) than OA BMMNCs. Accordingly, percentages of BDCA2 + CD34+ cells within BM CD34 + cells were significantly higher in RA than in OA. Finally, the percentages of BDCA2 + CD34+ cells within BM CD34 + cells were significantly correlated with the degree of synovial proliferation in RA.
These results indicate that the generation of pre-pDC from BM CD34 + cells is increased in RA compared with OA. Moreover, the data suggest that the increased output of pDC from BM might be involved in the synovial proliferation in RA.