XLP1 inhibitory effect by 2B4 does not affect DNAM‐1 and NKG2D activating pathways in NK cells

Abstract

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 [...

Roles of CD48 in regulating immunity and tolerance

“…In patients with XLP-1, NK cells are unable to kill EBV+ B cells or to control EBV infection, due to a lack of activation of SAP-deficient NK cells upon engagement of CD48+ targets [137, 138]. Notably, EBV+ B cells do not express ligands for the activating NK receptors DNAM-1 and NKG2a, rendering these cells particularly sensitive to CD244-mediated lysis by NK cells [139]. Similarly, SAP-deficient CD8+ T cells can be activated by most types of APCs, but fail to be activated by antigen-presenting B cells—and EBV selectively infects B cells [140].…”

Familial Hemophagocytic Lymphohistiocytosis: When Rare Diseases Shed Light on Immune System Functioning

“…In contrast, the 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 XLP-1 NK cells to kill lymphomas that expressed sufficient amounts of the specific ligands (57). Better knowledge of the underlying dysfunction could be turned into a diagnostic tool.…”

“…Thus, differential diagnosis is relevant. To this issue, immunological screening for intra-cytoplasmic SAP expression and rapid assays to examine 2B4 receptor function, which is inhibitory instead of activating in SH2D1A mutated patients (53, 57), may be applied (Manuscript in preparation).…”

Microenvironment abnormalities and lymphomagenesis: Immunological aspects

“…[1, 9, 10] In the absence of SAP inhibitory signals impaired NK and T cell cytolytic activity leads to an inability to control EBV infections and the development of B cell lymphomas. [11]…”