T cell receptor and costimulatory receptor (TCR (T cell receptor); specific costimulatory receptors include CD28, ICOS, CTLA-4, and others)

The **T cell receptor (TCR)** is a membrane-bound protein complex, most commonly composed of α and β chains (less frequently γ and δ chains), that recognizes peptide antigens bound to major histocompatibility complex (MHC) molecules on antigen-presenting cells. Upon antigen recognition, TCR associates with CD3 subunits harboring immunoreceptor tyrosine-based activation motifs (ITAMs), which trigger intracellular signaling cascades via kinases (e.g., Lck, ZAP70) and adaptor proteins, leading to T cell activation, proliferation, cytokine release, and effector functions[1][2][4][5][6][7]. **Costimulatory receptors** such as CD28, ICOS, and CTLA-4 are essential for full T cell activation or regulation. CD28, for example, delivers a necessary second signal for T cell activation when engaged with its ligands (CD80/CD86), while CTLA-4 and PD-1 inhibit activation to maintain tolerance and prevent autoimmunity[3][6]. The proper balance of stimulatory and inhibitory costimulatory signals determines whether T cells mount a robust immune response or remain tolerant. Both families are crucial in regulating adaptive immunity, and their manipulation is central to immunotherapeutic strategies for cancer, autoimmune diseases, and transplantation[3][5][6]. Both TCR and costimulatory receptors are among the most intensively studied therapeutic targets in modern immunology. Their pathways are exploited by multiple drug classes, primarily monoclonal antibodies and fusion proteins, with applications in cancer (checkpoint inhibitors), autoimmunity (blockade or agonism), and transplantation[3][5][6]. If detailed structured information is required, “T cell receptor” and individual costimulatory receptors (e.g., “CD28”, “CTLA-4”, “PD-1”) should be separated for annotation.

Other names

TCRCD3–TCR complexCostimulatory receptors (main subtypes: CD28, ICOS, CTLA-4, PD-1, etc.)

Blockade or agonism of costimulatory receptors to promote or inhibit T cell activation Inhibition of immune checkpoints (CTLA-4, PD-1) to boost anti-tumor T cell responses (checkpoint blockade immunotherapy) Direct TCR modulation (anti-CD3 antibodies induce or suppress activation)