EXPRESSION OF ICAM-R (ICAM-3), A NOVEL COUNTER-RECEPTOR FOR LFA-1, INRHEUMATOID AND NONRHEUMATOID SYNOVIUM - COMPARISON WITH OTHER ADHESION MOLECULES

Objective. To study the distribution of intercellular adhesion molecul e receptor (ICAM-R, or ICAM-3), a novel ligand for the leukointegrin l ymphocyte function-associated antigen 1 (LFA-1), in normal and rheumat oid synovial membranes and to compare this with the distribution of IC AM-1, ICAM-2, vascular cell adhesion molecule 1 (VCAM-1), and endothel ial leukocyte adhesion molecule 1 (ELAM-1). Methods. We performed immu nohistochemical analyses of frozen sections of normal and rheumatoid s ynovial tissue using monoclonal antibodies to the molecules examined. Results. ICAM-1 staining was detectable on the vascular endothelium an d the synovial lining cells of both normal and rheumatoid synovial mem branes. A variable proportion of lymphocytes infiltrating rheumatoid t issues expressed ICAM-1. ICAM-2 staining was demonstrable in the vascu lar endothelium of both normal and inflamed tissues, the latter demons trating a significantly higher proportion of positive vessels. ELAM-1 staining was not detectable in normal synovial membranes but was seen on the endothelium of a limited number of rheumatoid synovial vessels, usually close to the synovial lining cell layer. VCAM-1 staining was intense in both normal and rheumatoid synovial lining cells, but vascu lar staining was weak in both. In contrast, ICAM-R staining was not de tected in association with any synovial blood vessels, but was widely expressed by lymphocytes and macrophages. Cells of the lining layer di d not stain for ICAM-R. Conclusion. Although ICAM-R is a ligand for LF A-1 and shares considerable sequence homology with ICAM-1 and ICAM-2, it does not appear to be expressed by the endothelium of normal or inf lamed synovial vessels. Intense expression of ICAM-R by rheumatoid syn ovial lymphocytes and macrophages suggests that it may play a role in processes requiring cell-cell contact, such as antigen presentation an d homotypic aggregation.