Molecular mechanisms required for TCR-mediated formation of TCR-CXCR4 heterodimers

Abstract

Abstract T cell immune activation is frequently cross-regulated by CXCR4, which regulates T cell migration through binding its ligand SDF-1 (CXCL12). Here, we define a novel molecular mechanism by which CXCR4 directly impacts T cell immune function even in the absence of SDF-1. Our lab recently reported that, even in the absence of SDF-1, T cell receptor (TCR) stimulation causes CXCR4 to physically associate with the TCR and form a TCR-CXCR4 heterodimer. Moreover, blocking formation of TCR-CXCR4, with the CXCR4 antagonist AMD3100 or by CXCR4 siRNAs, selectively impairs TCR-stimulated signaling, inhibiting T cell production of IL-2, IL-4, IL-10, and TNF but not IFN-λ. Here, we investigate the molecular mechanisms required for TCR-induced TCR-CXCR4 formation in the absence of SDF-1. Using fluorescence resonance energy transfer (FRET) to detect TCR-CXCR4, we discovered important roles for specific TCR-induced kinases and also particular CXCR4 structural motifs, leading us to propose that a kinase-driven receptor trafficking mechanism is responsible for formation of TCR-CXCR4 heterodimers in response to TCR ligation. Furthermore, we have identified CXCR4 structural motifs and an E3 ligase that act as negative regulators of TCR-CXCR4 heterodimer formation. Thus, TCR ligation triggers formation of TCR-CXCR4 complexes that mediate T cell functions. Targeting formation of TCR-CXCR4 complexes may be a novel approach to modulate T cell-mediated immunity.