X-linked lymphoproliferative disease 1 (XLP1) is a rare congenital immunodeficiency caused by SH2D1A (Xq25) mutations resulting in lack or dysfunction of SLAM-associated protein adaptor molecule. In XLP1 patients, upon ligand (CD48) engagement, 2B4 delivers inhibitory signals that impair the cytolytic activity of NK (and T) cells. This causes the selective inability to control EBV infections and the occurrence of B-cell lymphomas. Here, we show that in the absence of SLAM-associated protein, co-engagement of 2B4 with different activating receptors, either by antibodies or specific ligands on target cells, inhibits different ITAM-dependent signaling pathways including activating killer Ig-like receptors. In XLP1 NK cells, 2B4 affected both the cytolytic and IFN-γ production capabilities, functions that were restored upon disruption of the 2B4/CD48 interactions. Notably, we provide evidence that 2B4 dysfunction does not affect the activity of DNAM-1 and NKG2D triggering receptors. Thus, while CD48 + B-EBV and lymphoma B cells devoid of NKG2D and DNAM-1 ligands were resistant to lysis, the preferential usage of these receptors allowed XLP1 NK cells to kill lymphomas that expressed sufficient amounts of the specific ligands. The study sheds new light on the XLP1 immunological defect and on the cross-talk of inhibitory 2B4 with triggering NK (and T) receptors. [6][7][8]. In general, virus-infected or neoplastic cells become susceptible to NK-cell-mediated lysis because they downregulate HLA-class I ("missing self-recognition") and upregulate ligands for activating NK receptors ("induced self-recognition") [9]. X-linked lymphoproliferative disease 1 (XLP1, Duncan disease, OMIM#308240) is a rare congenital immunodeficiency caused by mutations in SH2D1A (Xq25), the gene encoding the signaling lymphocyte activation molecule (SLAM) associated protein (SAP) [10,11]. SAP is a cytoplasmic adaptor protein that associates with members of the SLAM family, which includes SLAM (CD150), LY9 (CD229), CRACC (CD319), CD84, NTB-A (CD352), and 2B4 (CD244) [12]. 2B4 is a surface molecule expressed in NK and T lymphocytes specifically recognizing CD48, which is present solely on hematopoietic cells. 2B4 engagement causes tyrosine phosphorylation of immunoreceptor tyrosine-based switch motifs present in its cytoplasmic tail and recruitment of SAP, which transduces activating signals via Fyn-dependent processes [13,14]. In the presence of SAP, 2B4 potentiates the NCR-mediated signaling thus acting as a co-receptor molecule [15]. In the absence of SAP, 2B4 associates with protein tyrosine phosphatases and delivers inhibitory signals [16,17]. In NK cells, this occurs in physiological conditions such as during NK-cell development [18] and pregnancy [19]. On the other hand, in XLP1, the lack (or dysfunction) of SAP causes the selective inability to control infection by EBV, a γ-herpes virus that infects and sequesters itself in B cells [16,[20][21][22]. This causes severe manifestations including fulminant mononucleosis and B-cell lymphomas [...

Study of SAP expression is specific but may have insufficient sensitivity for screening XLP1 as a single tool. Combination with 2B4 functional assay allows identification of all cases.

X-linked lymphoproliferative disease 1 (XLP1) is an inherited immunodeficiency, caused by mutations in SH2D1A encoding Signaling Lymphocyte Activation Molecule (SLAM)-associated protein (SAP). In XLP1, 2B4, upon engagement with CD48, has inhibitory instead of activating function. This causes a selective inability of cytotoxic effectors to kill EBV-infected cells, with dramatic clinical sequelae. Here, we investigated the NK cell education in XLP1, upon characterization of killer Ig-like receptor (KIR)/KIR-L genotype and phenotypic repertoire of self-HLA class I specific inhibitory NK receptors (self-iNKRs). We also analyzed NK-cell cytotoxicity against CD48 or CD48 KIR-ligand matched or autologous hematopoietic cells in XLP1 patients and healthy controls. XLP1 NK cells may show a defective phenotypic repertoire with substantial proportion of cells lacking self-iNKR. These NK cells are cytotoxic and the inhibitory 2B4/CD48 pathway plays a major role to prevent killing of CD48 EBV-transformed B cells and M1 macrophages. Importantly, self-iNKR defective NK cells kill CD48 targets, such as mature DCs. Self-iNKR NK cells in XLP1 patients are functional even in resting conditions, suggesting a role of the inhibitory 2B4/CD48 pathway in the education process during NK-cell maturation. Killing of autologous mature DC by self-iNKR defective XLP1 NK cells may impair adaptive responses, further exacerbating the patients' immune defect.