Modulation of natural killer cell functions by interactions between 2B4 and CD48 in cis and in trans

SLAM-related receptors (SRRs) are important modulators of immune cell function. While most SRRs are homophilic, 2B4 (CD244) interacts with CD48, a GPI-anchored protein expressed on many haematopoietic cells. Here we show that natural killer (NK) cell-expressed 2B4 not only binds in trans to CD48 on neighbouring cells but also interacts in cis with CD48 on the same cell. 2B4 uses the same binding site to interact with CD48 in cis and in trans and structural flexibility of 2B4 is necessary for the cis interaction. Furthermore, the cis interaction is sufficient to induce basal phosphorylation of 2B4. However, cis interaction reduces the ability of 2B4 to bind CD48 in trans. As a consequence, stimulation-dependent phosphorylation of 2B4 upon binding to CD48 positive target cells is reduced. Interfering with the cis interaction therefore enhanced the lysis of CD48-expressing tumour cells. These data show that the density of 2B4 and CD48 on both the NK cell and the potential target cell modulates NK cell activity.