CTLA-4 (Cytotoxic T-lymphocyte associated protein 4), also referred to CD152, Celiac disease 3, CD, CELIAC3, or GSE, is a 24.7 kDa T cell surface protein with an essential role in downregulation of immune responses. This protein with 223 amino acids encoded by the CTLA4 gene located at the chromosome 2q33.2. It belongs to the immunoglobulin (Ig) gene superfamily and as the first immune checkpoint receptor targeted in clinic. CTLA-4 is exclusively expressed in T lymphocytes, where it mainly regulates the abundance of the early phases of T-cell activation. CTLA4, homologous to the CD28 molecule, both are ligands binding co-stimulatory molecules, CD80 and CD86. But in contrast to CD28, it is a negative feedback signal for T cell activation.
The glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR), a member of the TNF receptor superfamily, comprises multiple functional domains critical for immune modulation. Structurally, it contains extracellular cysteine-rich domains mediating ligand binding, a transmembrane domain, and intracellular regions with TRAF-binding motifs essential for downstream signaling cascades. Functionally, GITR exerts dual roles in immunoregulation: it enhances effector T-cell activation and survival while suppressing regulatory T-cell (Treg) immunosuppressive activity, thereby tipping the balance toward antitumor immunity. This receptor-ligand interaction triggers NF-κB and MAPK pathways, amplifying inflammatory cytokine production (e.g., IL-2, IFN-γ) and promoting cytotoxic lymphocyte proliferation. Recent therapeutic strategies exploit GITR agonism through monoclonal antibodies to overcome tumor immune evasion, particularly when combined with PD-1/PD-L1 checkpoint inhibitors, demonstrating synergistic efficacy in preclinical models. The receptor's structural complexity necessitates sophisticated antibody engineering to achieve optimal agonistic activity without excessive systemic immune activation. Current clinical development focuses on optimizing epitope specificity and Fc receptor interactions to enhance tumor microenvironment targeting while minimizing off-tumor effects.