Chemokines and their receptors direct leukocyte migration among blood, lymph and tissues. Evidence has recently accumulated that, besides their chemotactic functions, chemokine receptors are highly versatile players that fine tune immune responses. During human T cell activation by antigen-presenting cells, the chemokine receptors CCR5 and CXCR4 are recruited into the immunological synapse, where they deliver costimulatory signals. However, the molecular mechanisms allowing signaling versatility of chemokine receptors are unknown. Here, we describe the functional interaction between CXCR4 and CCR5 to exert specific biological functions and modulate T lymphocyte responses. We demonstrate that simultaneous expression and cooperation between CCR5 and CXCR4 are required for chemokine-induced T cell costimulation at the immunological synapse. In addition, we provide evidence for a physical association of the two receptors in a signaling complex that activates distinct T cell functions. We suggest that cooperation between receptors represents one key strategy for the functional plasticity of chemokines.chemokine receptors ͉ heterodimerization ͉ T cell costimulation T he immune system is able to mount an immune response against antigens present in the body at very low concentrations and, at the same time, to discriminate precisely between an infectious stimulus and a noninfectious one. During T cell activation, this sensitivity and specificity are achieved by mechanisms of sustained interactions with antigen-presenting cells (APCs) as well as by tunable activation thresholds and signal modulation (1). Thus, in addition to the interaction between the T cell receptor (TCR) and its ligand, T cell activation depends on accessory signals delivered by costimulatory molecules. CD28 is one of the most important costimulatory receptors for T cell priming in lymph nodes, but the costimulatory signals for effector T cells in the inflammatory microenvironment are less defined.Chemokines are small cytokines with selective chemoattractant properties coordinating tissue homeostasis and inf lammation. Besides their chemotactic functions, chemokines are involved in several biological and physiopathological processes. Thus, deregulated expression of chemokines and their receptors is involved in the development of autoimmunity, chronic inf lammation, immunodeficiency and cancer (2, 3). The broad range of activities displayed by chemokines is the consequence of multiple signaling pathways induced by chemokine receptors-seven-transmembrane molecules coupled to heterotrimeric G proteins (4).In T lymphocytes, the chemokine receptor CXCR4 is constitutively expressed and regulates T cell migration along gradients of the chemokine CXCL12. In contrast, CCR5 is expressed in activated T cells only and directs their migration along CCL3, CCL4 and CCL5 gradients. CXCR4 and CCR5 receptors are involved in several pathological processes, including autoimmunity, cancer, and HIV infection (3). We recently demonstrated that during T cell stimulation CCR5 ...

During physiological T-cell stimulation by antigen presenting cells (APCs), a major T-cell membrane rearrangement is known to occur leading to the organization of 'supramolecular activation clusters' at the immunological synapse. A possible role for the synapse is the generation of membrane compartments where signalling may be organized and propagated. Thus, engagement of the costimulatory molecule CD28 at the immunological synapse promotes the organization of a signalling compartment by inducing cytoskeletal changes and lipid raft accumulation. We identified the actin-binding protein Filamin-A (FLNa) as a novel molecular partner of CD28. We found that, after physiological stimulation, CD28 associated with and recruited FLNa into the immunological synapse, where FLNa organized CD28 signalling. FLNa knockdown by short interfering RNA (siRNA) inhibited CD28-mediated raft accumulation at the immunological synapse and T-cell costimulation. Together, our data indicate that CD28 binding to FLNa is required to induce the T-cell cytoskeletal rearrangements leading to recruitment of lipid microdomains and signalling mediators into the immunological synapse.

During T-cell migration, cell polarity is orchestrated by chemokine receptors and adhesion molecules and involves the functional redistribution of molecules and organelles towards specific cell compartments. In contrast, it is generally believed that the cell polarity established when T cells meet antigen-presenting cells (APCs) is controlled by the triggered T-cell receptor (TCR). Here, we show that, during activation of human T lymphocytes by APCs, chemokines and LFA-1 establish cell polarity independently of TCR triggering. Chemokine-induced LFA-1 activation results in fast recruitment of MTOC and mitochondria towards the potential APC, a process required to amplify TCR Ca 2 þ signalling at the upcoming immunological synapse, to promote nuclear translocation of transcriptional factor NFATc2 and boost CD25 expression. Our data show that the initial adhesive signals delivered by chemokines and LFA-1 shape and prepare T cells for antigen recognition.