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Antigen Cross-Presentation – Definition & Function
Antigen cross-presentation is an immunological process in which exogenous antigens are presented on MHC class I molecules to activate cytotoxic CD8+ T cells.
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Antigen cross-presentation is an immunological process in which exogenous antigens are presented on MHC class I molecules to activate cytotoxic CD8+ T cells.
What is Antigen Cross-Presentation?
Antigen cross-presentation is a specialised immunological mechanism by which certain immune cells – most notably dendritic cells – capture antigens from the extracellular environment (exogenous antigens) and present them via MHC class I molecules to cytotoxic CD8+ T lymphocytes. Under classical rules, exogenous antigens are processed and displayed on MHC class II molecules, activating helper T cells. Cross-presentation circumvents this rule, enabling a cytotoxic immune response against pathogens or tumour cells that do not directly infect professional antigen-presenting cells.
Biological Significance
Cross-presentation is central to immunity against a wide range of threats:
- Viral infections: Viruses infecting non-immune cells (e.g., muscle cells) can still trigger cytotoxic T cell responses through cross-presentation by dendritic cells.
- Tumour immunity: Tumour antigens released from dying cancer cells are taken up by dendritic cells and cross-presented to CD8+ T cells, enabling targeted killing of malignant cells.
- Vaccine development: Understanding cross-presentation is critical for designing vaccines that induce robust cellular immunity.
- Transplant medicine: Cross-presentation of foreign tissue antigens can contribute to transplant rejection.
Mechanism
The process of antigen cross-presentation involves several key steps:
1. Uptake of Exogenous Antigens
Dendritic cells internalise extracellular antigens through phagocytosis, macropinocytosis, or receptor-mediated endocytosis. Proteins or protein fragments from the extracellular space are transported into intracellular compartments.
2. Antigen Processing
Internalised antigens are degraded within endosomes and phagosomes. Two main pathways are involved:
- Cytosolic pathway: Antigen or peptide fragments escape from the endosome into the cytoplasm, where they are further processed by the proteasome. The resulting peptides are then transported by the TAP transporter (Transporter Associated with Antigen Processing) into the endoplasmic reticulum (ER), where they are loaded onto MHC class I molecules.
- Vacuolar pathway: Antigens are processed entirely within the endosomal or phagosomal compartment without reaching the cytoplasm. Resulting peptides are loaded directly onto MHC class I molecules within these vesicles.
3. Presentation on MHC Class I Molecules
Processed peptides are displayed on MHC class I molecules at the cell surface. These complexes are recognised by CD8+ cytotoxic T lymphocytes, triggering their activation, clonal expansion, and targeted destruction of infected or malignant cells.
Key Cell Types Involved
Not all immune cells are capable of effective cross-presentation. The most important include:
- Conventional dendritic cells type 1 (cDC1): Considered the most potent cross-presenting cell type, highly efficient at activating CD8+ T cells.
- Macrophages: Can perform cross-presentation under certain conditions.
- Plasmacytoid dendritic cells: Have a limited but demonstrable capacity for cross-presentation.
Clinical Relevance
Antigen cross-presentation has far-reaching implications for medicine and research:
- Cancer immunotherapy: Checkpoint inhibitors and dendritic cell-based vaccines aim to enhance cross-presentation of tumour-specific antigens, directing cytotoxic T cells against cancer cells.
- mRNA vaccines: Modern vaccine platforms, including mRNA vaccines, can promote cross-presentation to induce robust CD8+ T cell responses.
- Autoimmune diseases: Dysregulated cross-presentation may contribute to autoimmune reactions, in which self-antigens are mistakenly recognised as foreign.
- Chronic infections: Pathogens such as Mycobacterium tuberculosis and certain viruses actively inhibit cross-presentation to evade the immune system.
References
- Joffre, O. P. et al. (2012): Cross-presentation by dendritic cells. In: Nature Reviews Immunology, 12(8), pp. 557–569. DOI: 10.1038/nri3254.
- Blum, J. S., Wearsch, P. A. & Cresswell, P. (2013): Pathways of Antigen Processing. In: Annual Review of Immunology, 31, pp. 443–473. DOI: 10.1146/annurev-immunol-032712-095910.
- Alberts, B. et al. (2022): Molecular Biology of the Cell. 7th edition. W. W. Norton & Company.
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