Background: CD60 is a recently described T cell subset marker that is expressed on the surface of most T lymphocytes in synovial tissue and fluid and on a smaller proportion of peripheral T cells. Activation of T lymphocytes can be triggered through CD60. CD60 is also expressed by neuroectodermally derived cells in thymic epithelium and in skin.
Experimental design: Immunohistologic analysis of CD60 expression in synovium and thymus was performed using formalin-fixed tissue samples. Nonlymphoid cell lines grown from similar tissues were analyzed by flow cytometry.
Results: CD60 was readily identified in formalin-fixed, paraffin-embedded tissues. Simultaneous examination of CD60 distribution and cell morphology demonstrated that, in addition to its presence on T cells, CD60 was also expressed by a variety of nonlymphoid cells in synovium, including synovial lining cells, vascular endothelium, and dendritic-appearing cells deep within synovial tissue. Synovial tissue expression of CD60 was similar in rheumatoid arthritis and in other forms of inflammatory arthritis. In addition, it was strongly expressed by giant cells in pigmented villonodular synovitis. Surface expression of CD60 was detected by flow cytometry on cultured synoviocytes and on other CD60+ nonlymphoid cells, thus excluding adsorption of CD60 shed by T cells as a sufficient explanation of the immunohistologic findings.
Conclusions: These results define the T cell-activating CD60 determinant as a broadly distributed Ag within synovial tissue, with a possible functional role in the activation of a variety of cellular populations. CD60 may also be a marker for previously undescribed cell subsets in the synovial compartment, possibly including a cell population of neuroectodermal origin.