Presence of cells with myeloid-derived suppressor cell (MDSC)-like phenotype and morphology in synovial fluid (SF) from rheumatoid arthritis (RA) patients. (A) Flow cytometry profile of RA SF cells using a combination of antibodies against the common myeloid marker CD11b, the “immature” myeloid cell marker CD33, MHC class II (HLA-DR), the monocytic subset marker CD14, and the granulocytic subset marker CD15 (gating strategy is indicated by red arrows). The example shown (1 of 11 RA SF samples with similar profiles) demonstrates the dominance of CD11b+CD33lo/hiHLA-DRlo/-CD14−CD15+ (granulocytic) MDSC-like cells in RA SF. (B) Using the same gating strategy on the 11 RA SF samples, the mean frequency of MDSC-like myeloid cells was 85.03% (range: 76.1-97.9%) among the SF cells (left-side graph). The granulocytic subset represented 95.2% (range: 72.9-99.7%) and the monocytic subset represented 1.3% (range: 0.1-5.3%) of the MDSC-like SF cell population (bars in right-side graph). The data shown are the means ± SEM. (C) The morphology of SF cells in Wright-Giemsa-stained cytospin preparations from 3 RA patients also indicated the dominance of the polymorphonuclear granulocytic subset, although the shape of the nuclei of these neutrophil-like cells varied among the patients.