X-linked lymphoproliferative (XLP) disease is a disorder characterized by an inability to eliminate Epstein-Barr virus (EBV), eventually leading to fulminant infectious mononucleosis and the development of B cell tumors and associated hypogammaglobulinemia. In about 80% of cases, the disease is due to mutations in the gene encoding an adaptor molecule called SAP (SLAM-associated protein) that binds to a family of cell surface molecules involved in the activation of NK cells and T and B lymphocytes, including the signaling lymphocyte activation molecule (SLAM). SAP links the membrane proteins SLAM and 2B4 (see Chapter 7) to the Src family kinase Fyn. Defects in SAP contribute to attenuated NK and T cell activation and result in increased susceptibility to viral infections. As discussed in Chapter 11, SAP is required for TFH cell development, and the inability of XLP patients to generate germinal centers and high-affinity antibodies also likely contributes to susceptibility to viral infection. In about 20% of cases of XLP, the genetic defect resides not in SAP but in the gene encoding XIAP (X-linked inhibitor of apoptosis). The resulting enhanced apoptosis of T cells and NKT cells leads to a marked depletion of these cell types. This immunodeficiency is most commonly manifested by severe EBV infections, which probably arise opportunistically because of the ubiquitous nature of EBV.
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