The immunological synapse (IS) refers to the junction between antigen presenting cells (APCs) and CD4+ T cells formed by their cell surface receptors. The synapse is comprised of T cell receptors bound to MHC class II, and costimulatory proteins CD80 (B7-1) and CD86 (B7-2) bound to CD28. Protein kinase C-θ (PKC-θ) is the primary signaling protein that is recruited to the IS when these receptors engage, and this recruitment is facilitated by the Lck tyrosine kinase and the second messenger DAG. Phosphorylation of PKC-θ leads to the induction of signaling pathways that result in activation of transcription factors (e.g. NF-κB, NFAT). These ultimately turn on production of cytokines such as IL-2 and expression of T cell activation markers like CD69 and CD25.
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