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.
LAG-3 (Lymphocyte-Activation Gene 3), a type I transmembrane protein, comprises an extracellular region with four immunoglobulin (Ig)-like domains (D1-D4), a transmembrane segment, and a cytoplasmic tail. The D1 domain contains a unique 30-amino acid "extra loop" that mediates interactions with major histocompatibility complex class II (MHC II) molecules through distinct binding mechanisms compared to its structural homolog CD4, despite sharing approximately 20% sequence homology in extracellular regions. This loop, conserved across humans and mice, contributes to LAG-3's stronger MHC II affinity while exhibiting species-specific conformational plasticity. The D2-D4 domains harbor multiple N-glycosylation sites that facilitate interactions with alternative ligands like Galectin-3 and LSECtin through carbohydrate recognition. Notably, the "connecting peptide" between D4 and the transmembrane region serves as a cleavage site for ADAM10/17 metalloproteases in mice, potentially regulating surface LAG-3 expression, though human relevance remains unclear. Functionally, LAG-3 acts as a negative immune checkpoint regulator, primarily expressed on activated T cells (CD4+/CD8+), NK cells, and dendritic cells. Its cytoplasmic region features three conserved motifs: a KIEELE sequence with unknown function, a serine phosphorylation site (S454) for kinase signaling, and EP repeats critical for intracellular signal transduction. Through multiple ligands including MHC II, FGL1, and Galectin-3, LAG-3 suppresses T cell activation and promotes regulatory T cell (Treg) immunosuppressive activity.