2B4: An NK cell activating receptor with unique specificity and signal transduction mechanism

2B4 is a cell surface glycoprotein of the Ig-superfamily structurally related to CD2-like molecules such as CD2, CD48, CD58, CD84, Ly-9, and SLAM. Engagement of 2B4 on NK cells with specific antibodies or with its ligand CD48 enhances NK cell-mediated cytotoxicity. 2B4 is also expressed on both CD8⁺ T cells and myeloid cells, but its function in these cells remains unknown. Signal transduction through 2B4 involves recruitment of the SH2- containing adapter molecule SAP to cytoplasmic tyrosines. SAP is deficient in patients affected by X-linked lymphoproliferative disorder (XLP), which is triggered following EBV infection. Thus, an interruption of signaling through 2B4 and related molecules may impair NK cell recognition of virally infected cells and contribute to XLP.