HLA class I histocompatibility antigen heavy chain (HLA class I heavy chain (HLA-I heavy chain))

The HLA class I histocompatibility antigen heavy chain is the polymorphic α chain (approximately 44–47 kDa) of human MHC class I molecules that pairs noncovalently with β2‑microglobulin to form a cell-surface heterodimer on nearly all nucleated cells. The heavy chain contains three extracellular domains (α1, α2, α3), a transmembrane segment, and a cytoplasmic tail; the α1/α2 domains form the peptide-binding groove that accommodates mainly 8–10–mer peptides via six specificity-defining pockets (A–F), with primary anchors in pockets B and F. The α3 domain interacts with CD8, facilitating cytotoxic T lymphocyte recognition of peptide–HLA complexes. Peptides are generated from cytosolic proteins, loaded in the endoplasmic reticulum with assistance from chaperones and the peptide editor tapasin, and displayed to CD8+ T cells; HLA-I also serves as an inhibitory ligand for NK cells, and its reduced expression can trigger NK-mediated killing. The principal classical human HLA-I loci producing specific heavy chains are HLA-A, HLA-B, and HLA-C. The folding and assembly of the heavy chain with β2‑microglobulin and peptides is chaperone-dependent; partially folded or peptide-deficient intermediates are less stable until properly loaded. The extensive polymorphism of the heavy chain dictates peptide repertoire, influences immunodominance, and contributes to disease associations and transplant compatibility.

Other names

Human leukocyte antigen class I heavy chainMHC class I heavy chainHLA class I α chainHLA-A/HLA-B/HLA-C heavy chain (locus-specific forms)

Therapies that increase HLA-I expression (e.g., interferons) enhance antigen presentation to CD8+ T cells, improving cytotoxic recognition of infected or malignant cells. Strategies targeting the antigen-processing pathway (e.g., modulating tapasin or proteasomal generation of peptides) affect peptide loading and stability of HLA-I–peptide complexes.