Control of immunological synapse formation in regulatory T cells

Abstract

Regulatory T cells (Tregs) are a CD4+ T cell subset, which maintains tolerance towards self and harmless foreign antigens and is characterized by expression of the master transcription factor Foxp3. Besides this well studied Treg-specific feature, several effector molecules have been identified as preferentially but not exclusively expressed in Tregs. Activation via the T cell receptor (TCR) is crucial for T cell functionality, and there is accumulating evidence that TCR signaling is differentially organized in Tregs as compared to their counterparts, conventional T cells (Tconv). Accordingly, formation and composition of the immunological synapse (IS), built at the interface of interacting immune cells, differs between Tregs and Tconv. Quantitative phosphopeptide sequencing of Tregs and Tconv, which was recently performed in our laboratory, revealed a pre-activated phenotype of Tregs as compared to Tconv and identified differentially regulated phosphorylation sites between the two T cell subsets. Those might serve as promising candidates for targeted intervention of Treg activation. The present study (i) addressed the functional role of Themis, which is strongly under-represented in Tregs, for their suppressive capacity. Furthermore, in order to expand current knowledge about differential TCR signaling in Tregs and Tconv, (ii) phosphorylation kinetics of selected sites were performed. (iii) The functional impact of a novel phosphorylation site at Y523 of CalDAG GEFI (Calcium and DAG regulated guanine nucleotide exchange factor I), which was identified via the aforementioned phosphoproteomic approach, and (iv) the role of the protein itself for development and function of Tregs was investigated. In summary, selective targeting of Tregs by intervention at the signaling level might serve as promising future therapeutic strategy. Up to date, the major obstacle is the lack of specific tools, both to investigate intracellular signaling at the bench and to target selected signaling processes cell subset-specifically at the bedside.