BM cell replacement between WT and IL-1Ra^–/– mice. BM cells from IL-1Ra^–/– mice induced arthritis in WT mice, while those from WT mice suppressed arthritis in IL-1Ra^–/– mice. (A) Incidence of arthritis in IL-1Ra^–/– BM cell_transferred WT (squares, n = 11) or IL-1Ra^–/– mice (circles, n = 9). (B) Arthritic severity score of IL-1Ra^–/– BM cell_transferred mice. (C) Incidence of arthritis in WT BM cell_transferred IL-1Ra^–/– mice with (closed triangle, n = 9) or without (open triangle, n = 13) arthritis. (D) Arthritic severity score of WT BM_transferred IL-1Ra^–/– mice. (E_H) Histopathology of the ankle joints of IL-1Ra^–/– BM cell_transferred IL-1Ra^–/– mice (E) and IL-1Ra^–/– BM cell_transferred WT mice (F_H) at the end of the experiments (32 weeks and 34 weeks after the cell transfer, respectively). (G) Infiltration of inflammatory cells (indicated by arrowheads) into articular space and proliferation of the lining cells of the synovial membrane. (H) Pannus formation and erosive destruction of the bone (indicated by arrowheads). (I and J) Histology of the normal ankle joints of WT BM cell_transferred nonarthritic IL-1Ra^–/– mice (I) and arthritic IL-1Ra^–/– mice (J) at 31 weeks after the cell transfer. Magnification, ×40 (E, F, I, and J); ×100 (G and H).