Signaling lymphocytic activation molecule (SLAM)Cassociated protein (SAP) mutations in X-linked lymphoproliferative disease (XLP) lead to defective NKT cell development and impaired humoral immunity. in patients with XLP. Introduction CellCcell contacts between hematopoietic cells are critical for immune cell selection and differentiation, including the double-positive (DP) selection of natural killer T (NKT) cells in Dovitinib Dilactic acid (TKI258 Dilactic acid) the thymus and the generation of follicular helper T (TFH) cells within germinal centers (GCs; Veillette et al., 2007). NKT cells are specialized T lymphocytes expressing the invariant TCR chain (mouse V14 and human V24) and chain (V8.2, V7, or V2 in mouse and V11 in human; Borg et al., 2007). These cells diverge from the conventional T cell lineage at the DP stage of thymocyte development. In contrast to conventional T cells, which are selected by heterotypic interactions between hematopoietic thymocytes and nonhematopoietic thymic epithelial cells, NKT precursor cells undergo DP selection between two hematopoietic thymocytes (Bendelac, 1995; Wei et al., 2005). Long-term humoral immunity depends on GC responses, in which TFH cells provide help to B cells (Crotty, 2011); consequently, the differentiation of TFH cells also demands contact between hematopoietic cells, that is, T and B cells. The pivotal roles of these two types of contact between immune cells are highlighted by the finding that human signaling lymphocytic activation molecule (SLAM)Cassociated protein (SAP) mutations resulting in X-linked lymphoproliferative disease (XLP) give rise to the nearly complete loss of NKT cells in the thymus and impaired humoral immunity characterized by reduced TFH cell differentiation (Sayos et al., 1998; Morra et al., 2001; Nichols et al., 2005; Pasquier et al., 2005). SAP is an adaptor protein with only one SH2 domain, through which SAP can be recruited by SLAM family receptors (SFRs). These hematopoietic cellCspecific receptors are composed of seven receptors, namely 2B4, Ly9, CRACC, CD48, SLAM, CD84, and Ly108 (Ma et al., 2007; Cannons et al., 2011). SFRs possess two to four unique immunoreceptor tyrosine-based switch motifs to which SAP and/or its homologue EAT2 (EWS-activated transcript 2) is recruited. Moreover, SFRs are also able to signal through other SH2 domainCcontaining molecules, particularly in SAP deficiency. Thus, SFRs can mediate SAP-dependent and SAP-independent signaling (Veillette et al., 2009; Dong and Veillette, 2010). It has Dovitinib Dilactic acid (TKI258 Dilactic acid) been well established that SAP has dual functions in immune regulation (Dong and Veillette, 2010). On one hand, SAP functions as an active signaling molecule. Upon SFR engagement driven by the interaction between two hematopoietic cells, SFR can be phosphorylated by Src family kinases; thus, phosphorylated immunoreceptor tyrosine-based switch motifs have the ability to recruit SAP, which enables recruitment of SH3 domainCcontaining molecules, such as Fyn kinase (Chan et al., 2003; Latour et al., 2003). Mutation of arginine 78 to alanine (R78A) can abolish Fyn recruitment, which leads to a 50% reduction in NKT cell number (Nunez-Cruz et al., 2008), suggesting that SAP regulates NKT cell development in part via its intrinsic signaling activity. This conclusion was further confirmed by the finding that Fyn deficiency causes impairments in NKT cell development (Nichols et al., 2005; Pasquier et al., 2005). SAPR78A mutant mice also exhibit a defect in Th2 cytokine production (Davidson et al., 2004). However, Fyn-deficient mice and Dovitinib Dilactic acid (TKI258 Dilactic acid) SAPR78A mutant mice have unaltered humoral immunity (Cannons et al., 2006; McCausland et al., 2007). Thus, how SAP-mediated positive signaling plays different role in both types of T cell development and differentiation needs to be examined. Dovitinib Dilactic acid (TKI258 Dilactic acid) On the RICTOR other hand, SAP acts as natural blocker to prevent other SH2 domainCcontaining phosphatases from associating with SFRs. These molecules mainly include SH2 domain phosphatase 1 (SHP-1), SHP-2, and SH2 domain inositol phosphatase 1 (SHIP-1; Sayos et al., 1998; Li et al., 2003; Eissmann et al., 2005; Dong et al., 2012; Le Borgne and Shaw, 2012; Zhao et al., 2012; Wu et al., 2016). They are potentially coupled by SFRs, particularly when SAP is absent. It remains to be understood whether SFR-mediated inhibitory signaling initiated by the engagement of hematopoietic cell contact is capable of inhibiting immune cell development and differentiation. The specificity of SAP binding to SFRs suggests the indispensable role of this family in regulating NKT cell development and TFH cell differentiation. In contrast to SAP-deficient mice, mice lacking a single SFR member exhibit only mild or even no loss of NKT cells. By generating pseudoCdouble KO chimeras in which SLAM- and Ly108-mediated homotypic interactions are functionally disrupted, Griewank et al. (2007) discovered that NKT cell selection in the thymus was moderately compromised. Recent studies.