Expression of the αEβ7 integrin by mast cells in rheumatoid synovium

Objective. Rheumatoid synovitis is characterized by a mast cell response in which tryptase containing mast cells (MC(T)) associate with T lymphocyte infiltration, and tryptase and chymase containing mast cells (MC(TC)) correlate more closely with tissue damage or repair events. We investigated expression of the αEβ7 integrin and its ligand E-cadherin in rheumatoid and normal synovium and compared this expression to synovial mast cell responses. Methods. Immunohistochemical analysis was used to determine the expression of αEβ7 and E-cadherin in rheumatoid (n = 17) and normal (n = 6) synovium. The density of MC(T) and MC(TC) mast cell subsets was compared to the density of αEβ7 positive mast cells. Results. The mean density of αEβ7 positive cells in rheumatoid synovia was 25.2 cells/mm² (range 0.3-102.9), of which 26.7% (range 0-68.6%) were mast cells. A mean of 11.9% (range 0-30.4%) of rheumatoid synovial mast cells expressed αEβ7 compared to 0% in normal synovium (p < 0.0001). There was a strong correlation between the density of αEβ7 positive cells and the total mast cell density in rheumatoid synovium (r² = 0.74). αEβ7 positive mast cell density correlated significantly with the MC(T) subset density (r² = 0.5, p = 0.014), but not with the MC(TC) subset density. E-cadherin expression was increased in rheumatoid compared with normal synovium, but did not colocalize or correlate with αEβ7 expression. Conclusion. These results indicate a role for αEβ7 in the mast cell response that occurs in rheumatoid synovitis, in particular the MC(T) mast cell subset expansion associated with inflammatory events and interactions with infiltrating lymphocytes.