The Major Histocompatibility Complex (MHC) and Antigen Presentation
The Major Histocompatibility Complex (MHC), known as the Human Leukocyte Antigen (HLA) system in humans, represents the most polymorphic gene cluster in the entire human genome, located on the short arm of chromosome 6. Its core function is to mediate adaptive immunity by recognizing and distinguishing between “self” and “non-self” antigens. The primary role of MHC molecules is to bind peptide fragments derived from pathogens or altered self-proteins and display them on the cell surface for scrutiny by T lymphocytes. This process, termed antigen presentation, is indispensable for triggering an immune response, where MHC Class I molecules present endogenous antigens to CD8+ T cells, and MHC Class II molecules present exogenous antigens to CD4+ T cells.
MHC Class I: Structure, Distribution, and the Endogenous Pathway
MHC Class I molecules are universally expressed on the surface of virtually all nucleated cells and platelets in the body, which allows the immune system to monitor for intracellular threats like viruses or cancerous transformation. The structure of a Class I molecule is a heterodimer composed of a polymorphic heavy chain (alpha chain) and a smaller, non-covalently associated invariant protein called beta-2 microglobulin (β2m). The heavy chain consists of three external domains (α1, α2, and α3). The N-terminal domains, α1 and α2, fold together to create the deep peptide-binding cleft, which is closed at both ends by conserved residues. This structural constraint restricts the size of the bound peptide to a relatively short length, typically between 8 and 10 amino acids. The membrane-proximal α3 domain is crucial as it serves as the interaction site for the CD8 T cell co-receptor.
Antigen processing for MHC Class I is referred to as the endogenous pathway. It begins in the cytosol, where intracellular proteins—which can be self-proteins, or non-self proteins derived from viruses or tumors—are degraded into short peptides. The primary machinery for this degradation is the cytosolic proteasome, which produces peptides suitable for Class I binding. These peptides are then actively translocated from the cytosol into the lumen of the endoplasmic reticulum (ER) by the Transporter associated with Antigen Processing (TAP). Within the ER, the nascent MHC Class I heavy chain and β2m associate with various chaperones, including calnexin and calreticulin. This complex then joins with TAP and tapasin to form the Peptide-Loading Complex (PLC). Tapasin acts as a peptide editor, helping the MHC I molecule bind to the optimal peptide; in some cases, the ER aminopeptidase (ERAAP) may trim peptides for a better fit. Once a stable MHC Class I-peptide complex is formed, the chaperones are released, and the complex travels through the Golgi apparatus to be displayed on the cell surface, ready for recognition by CD8+ T cells.
MHC Class II: Structure, Distribution, and the Exogenous Pathway
MHC Class II molecules exhibit a restricted tissue distribution, primarily expressed only on professional antigen-presenting cells (APCs), which include dendritic cells, B lymphocytes, and macrophages. Unlike Class I, the Class II molecule is composed of two similarly sized transmembrane polypeptide chains, an alpha chain and a beta chain. The N-terminal domains of both chains, α1 and β1, interact to form the peptide-binding cleft. This groove is open at both ends due to the non-covalent association of the two chains, allowing it to accommodate significantly longer peptides, typically 13 to 25 amino acids in length. The membrane-proximal domains of the MHC Class II molecule provide the structural basis for interaction with the CD4 co-receptor.
The exogenous pathway is the mechanism for MHC Class II presentation, focusing on antigens acquired from the extracellular environment, such as bacteria or toxins. APCs internalize these antigens through endocytosis, phagocytosis, or macropinocytosis, placing them into a membrane-bound compartment called an endosome. As the endosome matures and fuses with lysosomes to form phagolysosomes, the pH gradually decreases, activating a panel of proteolytic enzymes (cathepsins) that degrade the ingested proteins into peptide fragments. Meanwhile, the MHC Class II molecules are synthesized in the ER, where they associate with a chaperone called the Invariant Chain (I chain or CD74). The I chain serves three roles: it promotes the folding of the MHC II chains, prevents premature binding of endogenous peptides, and directs the complex toward the endocytic pathway. As the MHC II complex translocates into the late endosome, the I chain is cleaved, leaving only a small segment called CLIP (Class II-associated invariant chain peptide) blocking the binding groove. Another non-polymorphic MHC-like molecule, HLA-DM, catalyzes the exchange of CLIP for an antigenic peptide. Once a stable and immunogenic peptide is loaded, the MHC Class II-peptide complex is trafficked to the cell surface for presentation to CD4+ T helper cells.
Cross-Presentation and T Cell Interaction
While the Class I and Class II pathways are classically separate, a special mechanism known as cross-presentation provides an exception, mainly executed by specific subsets of dendritic cells. Cross-presentation allows an APC to take up exogenous antigens (typically presented by MHC Class II) and redirect their derived peptides for loading onto MHC Class I molecules. This is a critical function for initiating a cytotoxic CD8+ T cell response against viruses or tumors when the professional APC itself is not directly infected. The precise details remain complex, but key proposed routes involve transporting the exogenous proteins from the phagosome into the cytosol for proteasomal degradation (the endosome-to-cytosol pathway) or directly loading the peptides onto MHC I molecules within a vesicular compartment (the vacuolar pathway).
The recognition of the MHC-peptide complex is the culmination of antigen presentation. T cells utilize their clonotypic T cell Receptors (TCRs) to bind to the complex in a diagonal fashion. The T cell co-receptors, CD8 and CD4, solidify this interaction and dictate the MHC class specificity: CD8 molecules on cytotoxic T cells interact with the α3 domain of MHC Class I, while CD4 molecules on T helper cells interact with the membrane proximal domains of MHC Class II. This precise, composite binding of the TCR and co-receptor to the MHC-peptide complex is the foundation of MHC restriction and is essential for the adaptive immune system’s ability to mount a focused and effective defense against both intracellular and extracellular pathogens, as well as mutated self-cells.