Expression of the EGF-TM7 family members EMR-2 and CD97 in rheumatoid synovial tissue

Background

Fibroblast-like synoviocytes (FLS) express decay-accelerating factor (CD55) at high levels. One of its ligands, CD97, is a member of the EGF-TM7 family, a group of classB seven-span transmembrane receptors, which are prominently expressed by activated immune cells. Previous work has suggested a close association between CD55⁺ FLS and CD97⁺ intimal macrophages in rheumatoid arthritis (RA) synovium. These previous studies were performed with an antibody that binds both CD97 and EMR2, which is another member of the EGF-TM7 family. Recently, monospecific antibodies against EMR2 and CD97 were developed.

Objective

To determine the expression of CD97 and EMR2 using novel, monospecific antibodies to provide more insight into the factors that might be involved in leukocyte activation in rheumatoid synovial tissue.

Methods

Synovial tissue samples were obtained by arthroscopy from 19 RA patients, 17 inflammatory osteoarthritis (OA) patients, and 11 reactive arthritis (ReA) patients. Immunohistologic analysis was performed using the following antibodies: CLB-CD97/1 (which recognizes both CD97 and EMR2), CLB-CD97/3 (specific for CD97), and 2A1 (specific for EMR2). Bound antibody was detected according to a 3-step immunoperoxidase method. In addition, double immunofluorescence was performed. Stained sections were analyzed by digital image analysis using a standardized program and compared by nonparametric statistical analysis.

Results

CD97 was shown to be expressed on activated leukocytes in the intimal lining layer and in the synovial sublining in all forms of arthritis. Of interest, we observed a specific increase in the expression of EMR2 positive cells of myeloid lineage in RA compared with ReA and OA synovium, even after correction for cell numbers. These differences were statistically significant (RA versus ReA and OA for both lining and sublining: all P values < 0.03). Double immunofluorescence revealed that 40–60% of the macrophages in RA synovium expressed EMR2.

Conclusion

The increased expression of various members of the EGF-TM7 family in inflamed synovial tissue suggests a role in the formation of the architecture of the intimal lining layer as well as in the maintenance and amplification of synovial inflammation. EMR-2 might be involved in the specific activation of macrophages in RA.