SAP is an adaptor protein expressed in T cells and natural killer cells. It plays a critical role in immunity, as it is mutated in humans with X-linked lymphoproliferative syndrome (XLP), a fatal immunodeficiency characterized by an abnormal response to Epstein-Barr virus (EBV) infection. SAP interacts with the SLAM family receptors and promotes transduction signal events by these receptors through its capacity to recruit and activate the Src kinase FynT. Because it has been previously established that FynT is selectively required for the development of NKT cells, we examined NKT cells in SAP-deficient mice and in humans with XLP. In the absence of SAP, the development of NKT cells is severely impaired both in mice and in humans. These results imply that SAP is a potent regulator of NKT cell development. They also identify for the first time a defect in NKT cells associated with a human primary immunodeficiency, revealing a potential role of NKT cells in the immune response to EBV.

Whether cancer is maintained by a small number of stem cells or is composed of proliferating cells with approximate phenotypic equivalency is a central question in cancer biology. In the stem cell hypothesis, relapse after treatment may occur by failure to eradicate cancer stem cells. Chronic myeloid leukaemia (CML) is quintessential to this hypothesis. CML is a myeloproliferative disorder that results from dysregulated tyrosine kinase activity of the fusion oncoprotein BCR-ABL. During the chronic phase, this sole genetic abnormality (chromosomal translocation Ph(+): t(9;22)(q34;q11)) at the stem cell level causes increased proliferation of myeloid cells without loss of their capacity to differentiate. Without treatment, most patients progress to the blast phase when additional oncogenic mutations result in a fatal acute leukaemia made of proliferating immature cells. Imatinib mesylate and other tyrosine kinase inhibitors (TKIs) that target the kinase activity of BCR-ABL have improved patient survival markedly. However, fewer than 10% of patients reach the stage of complete molecular response (CMR), defined as the point when BCR-ABL transcripts become undetectable in blood cells. Failure to reach CMR results from the inability of TKIs to eradicate quiescent CML leukaemia stem cells (LSCs). Here we show that the residual CML LSC pool can be gradually purged by the glitazones, antidiabetic drugs that are agonists of peroxisome proliferator-activated receptor-γ (PPARγ). We found that activation of PPARγ by the glitazones decreases expression of STAT5 and its downstream targets HIF2α and CITED2, which are key guardians of the quiescence and stemness of CML LSCs. When pioglitazone was given temporarily to three CML patients in chronic residual disease in spite of continuous treatment with imatinib, all of them achieved sustained CMR, up to 4.7 years after withdrawal of pioglitazone. This suggests that clinically relevant cancer eradication may become a generally attainable goal by combination therapy that erodes the cancer stem cell pool.

2B4 is a SLAM-related receptor expressed on natural killer (NK) cells and cytotoxic T cells. It can regulate killing and gamma interferon secretion by NK cells, as well as T-cell-mediated cytotoxicity.There are conflicting data regarding the mechanism of action of 2B4. In these studies, we attempted to understand better the nature and basis of 2B4 signaling. Our studies showed that engagement of 2B4 on NK cells triggered a tyrosine phosphorylation signal implicating 2B4, Vav-1, and, to a lesser extent, SHIP-1 and c-Cbl. Structure-function analyses demonstrated that this response was defined by a series of tyrosine-based motifs in the cytoplasmic region of 2B4 and was not influenced by the extracellular or transmembrane segment of 2B4. In addition, the 2B4-induced signal was absolutely dependent on coexpression of SAP, a Src homology 2 (SH2) domaincontaining adaptor associating with SLAM-related receptors and mutated in X-linked lymphoproliferative disease. It was also observed that 2B4 was detectably associated with the Src-related protein tyrosine kinase FynT in an immortalized NK cell line. Mutation of arginine 78 of SAP, a residue critical for binding of SAP to FynT, eliminated 2B4-mediated protein tyrosine phosphorylation, implying that SAP promotes 2B4 signaling most probably by recruiting FynT. Finally, despite the similarities in the signaling modalities of 2B4 and its relative SLAM, the natures of the tyrosine phosphorylation signals induced by these two receptors were found to be different. These differences were not caused by variations in the extent of binding to SAP but rather were dictated by the tyrosine-based sequences in the cytoplasmic domain of the receptors. Taken together, these data lead to a better understanding of 2B4 signaling. Furthermore, they provide firm evidence that the signals transduced by the various SLAM-related receptors are unique and that the specificity of these signals is defined by the distinctive arrays of intracytoplasmic tyrosines in the receptors.2B4, also named CD244, is a member of the SLAM family of immune cell-specific receptors that also includes SLAM, CD84, Ly-9, NTB-A/Ly-108, and CRACC (12,23,32,34). It is expressed in natural killer (NK) cells, cytotoxic CD8ϩ T cells, ␥␦ T cells, and mast cells. Like its relatives, 2B4 possesses immunoglobulin (Ig)-like domains in its extracellular region, a single transmembrane domain, and an intracellular segment bearing several tyrosine-based motifs. The ligand for 2B4 is CD48, a glycosylphosphatidylinositol-linked receptor expressed on diverse types of immune cells (5, 16). While an analysis of mice lacking 2B4 has yet to be reported, engagement of 2B4 by anti-2B4 antibodies or CD48 was shown to promote cytotoxicity and gamma interferon (IFN-␥) secretion by NK cells (24). Furthermore, it augments antigen-induced IFN-␥ secretion by activated CD8 ϩ T cells (14, 21). Like most other members of the SLAM family, 2B4 interacts through its cytoplasmic domain with SAP (or SH2D1A), a small intracellular Src homology 2 (SH2) domain-c...