Characterization of defective natural killer cell function in X-linked lymphoproliferative disease

X-linked Lymphoproliferative Disease (XLP) is a primary immune deficiency characterized by a profound susceptibility to Epstein-Barr Virus (EBV). The clinical phenotypes ascribed to XLP implicate a gene involved in the regulation of cell-mediated immunity. Recently, the molecular basis for disease has been attributed to mutations of SH2D1A, or SLAM-associated protein (SAP), an intracellular signal transduction molecule known to associate with the co-stimulatory receptors SLAM and 2B4. Molecular evidence now suggests that SAP may modulate the activation of T lymphocytes and natural killer (NK) cells through association with the cytoplasmic regions of SLAM and 2B4 respectively. However, precisely how SAP participates in immune signaling remains unknown. Because NK cells are critical in anti-tumour and anti-viral immune responses, and because reduced NK cytotoxicity has previously been demonstrated in this disease, we sought to assess the role of mutant SAP (Arg55Leu) in NK cells derived from XLP patients. Reverse transcription-PCR analysis ascertained that SAP mRNA is expressed in primary NK and lymphokine-activated killer (LAK) cells. Functional assays indicated that the cytotoxic activity of both NK and LAK cell subsets were significantly deficient in two XLP patients relative to normal controls. Furthermore, ligation of 2B4 failed to augment cytotoxicity and secretion of gamma-interferon (IFN-y) by NK cells derived from XLP patients, while enhancing these functions in NK cells obtained from healthy controls. In conclusion, these findings suggest that the association of SAP with 2B4 is necessary for effective NK / LAK immune functions and moreover, imply that alterations in SAP/2B4 signaling may contribute to the immune deficiency observed in XLP.Medicine, Faculty ofPathology and Laboratory Medicine, Department ofGraduat