Phenotyping of bone marrow-derived mensenchymal stem cells for typical mensenchymal stromal cell surface markers. Single-parameter histograms for (A1) to (A3), (B1) to (B3), (C1) to (C3) individual mensenchymal stromal cell (MSC) markers and (A4) to (A7), (B4) to (B7), (C4) to (C7) MSC exclusion markers, representative of samples from patients with ankylosing spondylitis (AS) and from healthy donors (blue lines). Red lines indicate background fluorescence obtained with isotype control IgG. x axis, fluorescence intensity; y axis, cell counts. BMSC, bone marrow-derived mensenchymal stem cell; HD1, HLA-B27-negative healthy donors; HD2, HLA-B27-positive healthy donors.