CD244 is expressed on all NK cells, gd T cells, and monocytes, on 30%–70% of resting CD8 T cells, and on up to 5% 64 • JID 200 (1 July) • Williams et al. The in vitro triggering of CD244 on NK cells results in increased cytotoxicity and cytokine secretion, including IFN-g .This role has been demonstrated in vivo by the excessive production of IFN-g by CD4 and CD8 T cells after lymphocytic choriomeningitis virus infection in SAP-deficient mice [27, 28], and this may provide important clues about the molecular mechanisms of immune activation in IM.Fatal IM is generally associated with the inherited Xlinked lymphoproliferative syndrome (XLP) .Primary EBV infection in affected male infants results in T Cell Activation in Mononucleosis • JID 200 (1 July) • 63 Table 1.
and down-regulation of oncogenes promoting cell cycle arrest and apoptosis. IM is believed to be an immunopathological disease, and it is characterized by dramatic antigen-driven clonal expansions of CD8 T lymphocytes in response to primary Received 27 August 2003; accepted 11 December 2003; electronically published 8 June 2004. After primary infection, the virus establishes life-long persistence in host B lymphocytes .
Defined as housebound at the worst point during illness. of copies of Epstein-Barr virus/106 peripheral blood mononuclear cells at the time of diagnosis.
overwhelming cytotoxic T cell activation and death either from fulminating IM with multiorgan failure or EBV-driven lymphoma .
The abnormal gene product, signaling lymphocyte activation molecule (SLAM)–associated protein (SAP)  or src homology-2 (SH2D1A) [15–16], is a small intracytoplasmic protein with an SH2 domain and is most abundantly expressed in T lymphocytes and NK cells [16–17].
In vitro, SAP inhibits signal-transduction pathways that are initiated by at least 4 lymphocyte cell-surface receptors: SLAM/CD150, 2B4/CD244, CD84, and LY9 [18–21]. Surface lymphocyte activation marker (SLAM)–associated protein (SAP) regulates lymphocyte activation via signals from cell-surface CD244 (2B4) and SLAM (CD150).