2B4 dysfunction in XLP1 NK cells: More than inability to control EBV infection.

X-linked lymphoproliferative disease 1 (XLP1) is a monogenic disorder caused by mutations in SH2D1A, resulting in the absence/dysfunction of the signaling lymphocyte activation molecule (SLAM)-associated protein (SAP). Consequently, SLAM receptors as 2B4 (CD244) and NTB-A (SLAMF6), upon ligand engagement, exert inhibitory instead of activating function. This causes an immune dysfunction that is worsened by the selective inability of NK and T cells to kill EBV-infected B cells with dramatic clinical sequelae (e.g. fulminant mononucleosis, hyperinflammation, lymphoma). Here we outline recent findings on the interplay between inhibitory 2B4 and the various activating receptors in NK cells. 2B4 engagement selectively blocks ITAM-dependent activating receptors as NCR and CD16, while it does not affect NKG2D and DNAM-1. Furthermore, inhibitory 2B4 participates to NK cell education, as highlighted by the existence in XLP1 patients of a large subset of fully functional NK cells that lack self-HLA specific inhibitory receptors and exert autoreactivity against mature dendritic cells.