Objective. To study the distribution of intercellular adhesion molecule receptor (ICAM-R, or ICAM3), a novel ligand for the leukointegrin lymphocyte function-associated antigen 1 (LFA-l), in normal and rheumatoid synovial membranes and to compare this with the distribution of ICAM-1, ICAM-2, vascular cell adhesion molecule 1 (VCAM-l), and endothelial leukocyte adhesion molecule 1 (ELAM-1).Methods. We performed immunohistochemical analyses of frozen sections of normal and rheumatoid synovial tissue using monoclonal antibodies to the molecules examined.Results. ICAM-1 staining was detectable on the vascular endothelium and the synovial lining cells of both normal and rheumatoid synovial membranes. A variable proportion of lymphocytes infiltrating rheumatoid tissues expressed ICAM-1. ICAM-2 staining was demonstrable in the vascular endothelium of both normal and inflamed tissues, the latter demonstrating 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 vascular staining was weak in both. In conHani El-Gabalawy, MD: The University of Manitoba, Winnipeg, Manitoba, Canada; Michael Gallatin, PhD: ICOS Corporation, Seattle, Washington; Rosemay Vazeux, PhD: ICOS Corporation; Gary Peterman, PhD: ICOS Corporation; John Wilkins, P h D University of Manitoba.Address reprint requests to Hani El-Gabalawy, MD, Rheumatic Disease Unit, 800 Sherbrook Street, Winnipeg, Manitoba R3A 1M4, Canada.Submitted for publication March 31, 1993; accepted in revised form November 30, 1993. trast, ICAM-R staining was not detected in association with any synovial blood vessels, but was widely expressed by lymphocytes and macrophages. Cells of the lining layer did not stain for ICAM-R.Conclusion. Although ICAM-R is a ligand for LFA-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 inflamed synovial vessels. Intense expression of ICAM-R by rheumatoid synovial lymphocytes and macrophages suggests that it may play a role in processes requiring cell-cell contact, such as antigen presentation and homotypic aggregation.Chronic inflammatory lesions such as rheumatoid synovitis require ongoing recruitment and retention of leukocytes from the vascular space. A critical initial step involves the adhesion of leukocytes to vascular endothelium (1-3). This is currently thought to be regulated by a cascade of molecular interactions occurring between leukocyte membrane receptors and endothelial counter-receptors, many of which are inducible in their expression. Several endothelial adhesion molecules have been well characterized and shown to participate in leukocyte attachment. These include the endothelial leukocyte adhesion molecule 1 (ELAM-1, or E-selectin) (4), intercellular adhesion molec...