How crosstalk at the immune synapse shapes T cell and dendritic cell biology

In the context of adaptive immunity, T cells are activated by professional antigen-presenting cells (APCs) in a process that begins with peptide-MHC complexes on the APC being recognized by T cell receptor and CD3 co-receptor complexes on the T cell. This triggers a reorganization of T cell morphology, formation of an immune synapse, and the delivery of signals that ultimately culminate in nuclear activation. The interaction between T cells and APCs, such as dendritic cells (DCs), was originally viewed as a unidirectional information highway in which the APC instructs the T cell. It is now clear that bidirectional crosstalk occurs at the immune synapse and that T cells also shape APC functions. The concept of 'DC licensing' originally suggested an instructive role for T cells in modifying DC functions. More recent studies have provided important insight into the changes that occur in DCs during antigen-driven contacts with T cells at the immune synapse. In this Review, we discuss our current understanding of the bidirectional T cell-DC crosstalk that occurs at the IS and its relevance for immune responses and immunotherapies.