WhatSAP — 2B4 sends mixed messages in the absence of SAP

2B4 (CD244), a member of the SLAM‐related receptor family, has important immuno‐regulatory functions including coactivating the cytotoxicity and cytokine secretion of NK cells. Immune modulation by 2B4 is dependent on the small intracellular signaling molecule SAP. In patients suffering from X‐linked lymphoproliferative disease (XLP1), SAP is nonfunctional, not only abolishing the activating function of 2B4, but rendering this receptor inhibitory. In this issue of European Journal of Immunology, Meazza et al. [Eur. J. Immunol. 2014. 44: 1526–1534.] demonstrate that 2B4‐mediated inhibition in NK cells from XLP1 patients is selective. While the activation of NK cells via ITAM‐based receptors is blocked by inhibitory 2B4, DNAM‐1, and NKG2D‐dependent NK‐cell activation is not affected by SAP deficiency. These findings provide an important insight into the different defective NK‐cell functions in XLP1 patients and demonstrate the differential integration of redundant receptor signaling pathways in NK cells.