Sorting nexin 27-dependent regulation of Lck and CD4 tunes the initial stages of T-cell activation.

Sorting nexin 27 is a unique member of the sorting nexin family of proteins that mediates the endosome-to-plasma membrane trafficking of cargos bearing a PSD95/Dlg1/ZO-1 (PDZ)-binding motif. In brain, sorting nexin 27 regulates synaptic plasticity, and its dysregulation contributes to cognitive impairment and neuronal degeneration. In T lymphocytes, sorting nexin 27 partners with diacylglycerol kinase ζ to facilitate polarized traffic and signaling at the immune synapse. By silencing sorting nexin 27 expression in a human T-cell line, we demonstrate that sorting nexin 27 is a key regulator of the early T-cell tyrosine-based signaling cascade. Sorting nexin 27 transcriptionally controls CD4 abundance in resting conditions and that of its associated molecule, Lck. This guarantees the adequate recruitment of Lck at the immune synapse, which is indispensable for subsequent activation of tyrosine phosphorylation-regulated events. In contrast, reduced sorting nexin 27 expression enhances NF-κB-dependent induction of CXCR4 and triggers production of lytic enzymes and proinflammatory cytokines. These results provide mechanistic explanation to previously described sorting nexin 27 function in the control of immune synapse organization and indicate that impaired sorting nexin 27 expression contributes to CD4 T-cell dysfunction.