Immunofluorescence staining of the affected joints revealed that, similar to what was observed in RA synovium, ORM2 was strongly colocalized with CD55+ and F4/80+ cells in the affected joints of mice with CIA (Fig. 6b and Supplementary Fig. 9), confirming that synovial fibroblasts and macrophages are the major cells that produce ORM2. Here, CD55 is linked to rheumatoid arthritis.