CD31 (PECAM1) is a member of the immunoglobulin (Ig) superfamily that is expressed on the surface of circulating platelets, monocytes, neutrophils, and particular T-cell subsets. CD31 is also a major constituent of the endothelial cell intercellular junction, where up to an estimated 1 million molecules are concentrated. Because of this cellular expression pattern, CD31 is implicated in several functions, including transendothelial migration of leukocytes, angiogenesis, and integrin activation. Ig superfamily members are known to mediate cell adhesion, or antigen recognition, e.g.
The CD3 antibody recognizes the 17-19 kD ε-chain of CD3 within the CD3 antigen/T cell antigen receptor (TCR) complex. The CD3 antigen is expressed in the cell cytoplasm during the early stage of T cell development and is expressed on the cell membrane at the late stage. CD3 antigen is displayed on 60-80% of normal peripheral blood lymphocytes and 60-70% of thymocytes and plays an important role in signal transduction after antigen recognition by TCR. NK cells also express the CD3 antigen in the cytoplasm.
Immune costimulatory protein B7-H4 (B7-H4) is expressed on the surface of a variety of immune cells and functions as a negative regulator of T cell response. By arresting cell cycle, B7-H4 inhibits growth, cytokine secretion and cytotoxicity development. When expressed on the cell surface of tumor macrophages, B7-H4 plays an important role, together with regulatory T-cells (Treg), in the suppression of tumor-associated antigen-specific T-cell immunity. B7-H4 is also involved in promoting epithelial cell transformation. B7-H4 can be a predictive marker for renal cell carcinoma.