MHC Class I Assembly and Antigen Presentation Pathway

Cytotoxic T cells (CD8+ T cells) kill infected cells through a mechanism involving perforin and granzymes. The T cell recognizes infected cells through dual recognition of the MHC I-peptide complex. Upon recognition, the T cell releases perforin, which forms channels in the target cell membrane, and granzymes, which enter through these channels and trigger apoptosis. This mechanism allows the immune system to eliminate cells infected with intracellular pathogens.

CD8+ T-cells recognize antigens presented by MHC Class I molecules. The TCR must simultaneously recognize both MHC and its associated peptide. Neither component alone is sufficient for activation. Upon recognition, TCR undergoes conformational changes triggering signaling through CD3 molecules, initiating cytotoxic responses. During T-cell development, cells that strongly recognize self-peptides undergo negative selection (central tolerance) to prevent autoimmunity. This allows recognition of altered self-antigens on tumor cells while avoiding self-tissue damage.

CD8+ cytotoxic T cells are activated by MHC Class I molecules presenting antigens. They require two signals: TCR binding to MHC I-antigen complex and CD28 binding to B7. Activated CD8+ T cells kill target cells through perforin-granzyme release causing apoptosis and Fas-Fas ligand interaction. These mechanisms ensure precise elimination of infected or abnormal cells.

Cytotoxic T cells (CD8+ T cells) kill infected or malignant cells through a multi-step process: they recognize target cells via MHC Class I molecules, undergo cytoskeleton rearrangement to align cytotoxic granules containing perforin and granzymes at the immune synapse, release perforin which creates pores in the target cell membrane, and then granzymes enter the cell to trigger apoptosis through two pathways—activating caspase-3 to degrade IAPs and liberate CAD for DNA degradation, and cleaving BID to release cytochrome c from mitochondria.

Cytotoxic T cells (CD8+ T cells) require dual recognition to activate: they must simultaneously recognize both the MHC Class I molecule and the foreign peptide presented by it. This dual recognition ensures that T cells only attack cells presenting genuine foreign antigens. Upon activation, cytotoxic T cells undergo proliferation and differentiation, with some becoming memory cells for long-term immune memory. The activated cells express perforin and granzymes, which enable them to kill infected cells by forming channels in the target cell's membrane, allowing these molecules to enter and destroy the cell's contents.

All nucleated cells express MHC class I molecules presenting intracellular antigens to CD8 cytotoxic T cells. Viral proteins are processed by the proteasome, transported by TAP proteins, and loaded onto MHC class I for surface display. CD8 T cells recognize viral peptides and kill infected cells through granule exocytosis. Viruses evade this surveillance through multiple mechanisms: cytomegalovirus and adenovirus block MHC class I synthesis or retention; herpes simplex virus blocks TAP protein function; cytomegalovirus produces MHC class I isca molecules that deactivate NK cells. These evasion strategies allow viruses to persist despite immune surveillance.

Many viruses have evolved sophisticated strategies to evade immune detection by disrupting MHC class I antigen presentation. Epstein-Barr virus blocks proteasomal degradation of its EBNA-1 protein. Herpes simplex virus and human cytomegalovirus block TAP function to prevent peptide transport. Adenovirus displaces tapasin from MHC class I complexes. KSHV redirects MHC class I molecules to lysosomal degradation. Persistent pathogens like HIV escape through molecular evolution, acquiring single-nucleotide changes that eliminate anchor residues required for MHC binding. This escape is strongly selected during persistent infections, explaining why different individuals with different MHC types respond differently to the same pathogen. Natural killer cells provide complementary surveillance by detecting cells that have lost MHC class I expression.

MHC class I molecules present intracellular antigens to cytotoxic T cells through a complex peptide loading mechanism involving TAP proteins that form ATP-dependent pores accepting peptides from the proteasome. Multiple viruses have evolved distinct strategies to inhibit this pathway: herpes simplex ICP47 physically blocks peptide entry into TAP; adenovirus E19 prevents MHC-β2m interaction; HCMV US6 inhibits ATP binding to TAP; HCMV US3 and MK3 cause MHC degradation through ubiquitination. These diverse mechanisms demonstrate how viruses attack every step of the MHC class I presentation pathway to evade immune surveillance and establish persistent infections.

MHC class I molecules are found on most cells because most can be infected. Virus-infected cells process viral proteins through the proteasome, generate peptides, and load them onto MHC class I molecules via the TAP transporter. Viruses interfere at multiple levels: herpes viruses (HSV, CMV, EBV) make proteins that block TAP function, bind ATP needed for transport, or inhibit the proteasome. CMV has numerous proteins that retain class I in the ER or push it into the cytoplasm, allowing persistent lifelong infections because infected cells evade detection.

Viruses have evolved mechanisms to escape immune detection. Large DNA viruses like herpes family members encode immune evasion molecules. Some block the TAP transporter, others eliminate MHC class 1 molecules. HIV's Nef protein takes MHC class 1 molecules loaded with peptides and detours them to lysosomes for degradation, preventing CD8 T cells from recognizing infected cells. This co-evolution between viruses and the immune system demonstrates the ongoing arms race in host defense.

MHC class I molecules are crucial for tumor surveillance by presenting tumor-associated antigens to cytotoxic T cells. Failure of MHC class I presentation allows tumor cells to evade immune detection. In transplantation, donor cells expressing different MHC molecules are recognized as foreign by recipient T cells, triggering graft rejection. The degree of MHC matching determines graft acceptance rates.

Tumors generate neoantigens through mutations that create novel peptide sequences not recognized by the immune system during development. These neoantigens can be presented by MHC molecules and recognized by T cells with high specificity. Even single amino acid changes can create recognizable neoantigens—for example, mutations in p53 create peptides that T cells can specifically recognize and destroy tumor cells expressing them. Modern cancer immunotherapy includes checkpoint blockade (antibodies blocking PD-1/PD-L1 or CTLA-4), adoptive cell transfer (ex vivo expansion of tumor-infiltrating lymphocytes), and CAR-T cell therapy (genetically modifying T cells to express chimeric antigen receptors targeting tumor antigens). These approaches have revolutionized treatment for certain cancers.

Cancer immunotherapy trains the immune system to recognize and destroy cancer cells by identifying unique surface proteins. Cancer cells evade detection by presenting altered peptides on Major Histocompatibility Complex (MHC) Class I molecules. These complexes present 9-11 amino acid peptides to T cell receptors, which determine whether cells are recognized as self or non-self. The challenge is identifying cancer-specific peptides that trigger immune recognition while avoiding autoimmunity.

MHC (Major Histocompatibility Complex) molecules are essential for immune recognition, with Class I molecules presenting antigens to CD8+ T cells and Class II molecules presenting to CD4+ T cells; during transplantation, mismatched MHC molecules between donor and recipient trigger immune rejection responses, as the immune system recognizes foreign MHC as non-self antigens.

The immune system distinguishes self from nonself through antigen presentation via two distinct pathways: MHC class I presents endogenous antigens (from inside the cell, such as viral components or cancer neoantigens) to CD8 T cells, while MHC class II presents exogenous antigens (from outside the cell, such as extracellular bacteria) to CD4 T helper cells; both pathways involve specific molecular mechanisms where proteins are processed into peptides, loaded onto MHC molecules, and displayed on the cell surface for immune surveillance.