The X-linked lymphoproliferative disease gene product SAP is expressed in activated T and NK cells.

The unique manifestation of the inherited immunodeficiency, X-linked lymphoproliferative disease (XLP), is the impaired control of EBV infection. The gene, which carries mutations or is deleted in the patients, has been identified (Xq25). The encoded protein (SAP, 128 aa) contains a single SH2 domain and binds to signaling lymphocytic activation molecule (SLAM) and to other related surface molecules that are expressed on activated T, B and NK cells. SAP modifies signal transduction through its association with these molecules. Initially it was assumed that SAP acts passively by interfering and blocking active interactions involving other SH2 carrying molecules. We demonstrated that SAP protein is expressed in activated T and NK, but not in activated B cells. This finding is in line with the fact that in vitro performance of effector cells derived from XLP patients is impaired. However, it is still not known why the severe symptoms (fatal mononucleosis or malignant lymphoproliferation in the survivors of the primary infection) are elicited by EBV. We studied SAP expression in several Burkitt lymphoma (BL) derived lines. In contrast to normal B cells, certain lines expressed SAP. These were all type I cells in the Burkitt line nomenclature: they expressed only one of the EBV encoded proteins (EBNA-1) and their phenotype corresponded to resting B cells. Lymphoblastoid cell lines and type III BLs, whose phenotype resembled activated B cells and expressed all nine EBV encoded proteins, were devoid of SAP. The relationship between cell activation and SAP expression is reciprocal in T and B cells i.e. BL lines, activated T and NK cells express SAP, while BL blasts do not express SAP. This opposite relationship may be exploited for studies about the function of SAP.