Immunophenotypic Analysis of Mononuclear Phagocyte Antigens in the Arthritic Synovium of the Hip Joint

The antigenic phenotype of lining and subintimal mononuclear phagocytes (MPs) of the synovium in osteoarthritis (OA) and rheumatoid arthritis (RA) was characterised, using monoclonal antibodies directed against a range of monocyte/macroph-ageassociated antigens, in order to determine the role of MPs in the pathobiology of OA and RA of the hip. A major proportion of synovial lining cells (SLCs) and subintimal cells in RA and OA consisted of CD68 positive MPs which expressed a wide range of macrophage-associated antigens. The monocyte marker, CD14, was downregulated on SLCs in RA while OA and early markers of MP activation predominated in the OA synovium which was independent of the level of inflammation. In contrast, synovial MPs in RA showed increased expression of activation, maturation and functional antigens such as CD11a, CD11b, CD11c, CD16, CD32, CD54, CD64. Moreover, MP recruitment did not correlate with the degree of lymphocytic infiltration in RA. Thus, synovial MPs in RA are rapidly and fully activated with upregulation of activation, differentiation, and functional antigens occurring in situ. There was also no difference in antigenic phenotype of synovial MPs in inflammatory and non-inflammatory OA.