Immunobiology Unit 4 Review: Antigen Processing and Presentation

Key Concepts and Terminology

• Antigens are substances that trigger an immune response by binding to specific receptors on lymphocytes
• Epitopes refer to the specific regions on an antigen that are recognized by antibodies or T cell receptors
• Antigen processing involves the degradation of proteins into smaller peptides that can be presented on the surface of antigen-presenting cells (APCs)
• Major Histocompatibility Complex (MHC) molecules are cell surface proteins that play a crucial role in presenting processed antigens to T cells
  • MHC class I molecules present peptides derived from intracellular proteins to CD8+ T cells
  • MHC class II molecules present peptides derived from extracellular proteins to CD4+ T cells
• Antigen presentation is the process by which processed antigens are displayed on the surface of APCs in conjunction with MHC molecules for recognition by T cells
• Cross-presentation allows exogenous antigens to be presented on MHC class I molecules, enabling CD8+ T cell activation
• Immunodominance refers to the phenomenon where the immune system focuses on a limited number of epitopes within a complex antigen

Types of Antigens

• Exogenous antigens are derived from extracellular sources (bacteria, toxins) and are typically processed through the MHC class II pathway
• Endogenous antigens originate from intracellular proteins (viral proteins, tumor antigens) and are processed through the MHC class I pathway
• Lipid antigens are presented by specialized molecules called CD1 and activate a subset of T cells known as natural killer T (NKT) cells
• Carbohydrate antigens, such as bacterial polysaccharides, can elicit T cell-independent antibody responses
• Hapten antigens are small molecules that become immunogenic when conjugated to a larger carrier protein
• Superantigens are bacterial or viral proteins that bypass conventional antigen processing and directly activate a large proportion of T cells, leading to massive cytokine release
• Tumor-associated antigens (TAAs) are proteins expressed by tumor cells that can be targeted by the immune system for cancer immunotherapy

Antigen Processing Pathways

• MHC class I pathway processes endogenous antigens for presentation to CD8+ T cells
  • Endogenous proteins are degraded by the proteasome into peptides
  • Peptides are transported into the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP)
  • In the ER, peptides are loaded onto MHC class I molecules and the complex is transported to the cell surface
• MHC class II pathway processes exogenous antigens for presentation to CD4+ T cells
  • Exogenous antigens are internalized by APCs through endocytosis or phagocytosis
  • Antigens are degraded into peptides within endosomes or lysosomes
  • MHC class II molecules are synthesized in the ER and transported to the endosomal compartment
  • Peptides are loaded onto MHC class II molecules and the complex is transported to the cell surface
• Cross-presentation allows exogenous antigens to be presented on MHC class I molecules
  • Dendritic cells can uptake exogenous antigens and route them to the MHC class I pathway
  • This process is important for generating CD8+ T cell responses against viruses and tumors

Major Histocompatibility Complex (MHC)

• MHC molecules are highly polymorphic, meaning they have multiple allelic variants within a population
• MHC class I molecules are expressed on all nucleated cells and consist of a heavy chain and a light chain (β2-microglobulin)
• MHC class II molecules are primarily expressed on professional APCs (dendritic cells, macrophages, B cells) and consist of an α chain and a β chain
• The peptide-binding groove of MHC molecules accommodates peptides of specific lengths
  • MHC class I molecules typically bind peptides of 8-10 amino acids
  • MHC class II molecules bind longer peptides of 13-18 amino acids
• The diversity of MHC alleles within a population contributes to the ability to recognize a wide range of pathogens
• Certain MHC alleles are associated with increased susceptibility or resistance to specific diseases (autoimmune disorders, infectious diseases)

Antigen Presentation Mechanisms

• Professional APCs, such as dendritic cells, macrophages, and B cells, specialize in capturing, processing, and presenting antigens to T cells
• Dendritic cells are the most potent APCs and play a crucial role in initiating adaptive immune responses
  • Immature dendritic cells patrol tissues and capture antigens
  • Upon encounter with pathogen-associated molecular patterns (PAMPs) or danger signals, dendritic cells mature and migrate to lymph nodes
  • Mature dendritic cells express high levels of MHC molecules and co-stimulatory molecules (CD80, CD86) for efficient T cell activation
• Macrophages and B cells can also function as APCs, but their primary roles are in innate immunity and antibody production, respectively
• T cell recognition of peptide-MHC complexes occurs through the T cell receptor (TCR)
  • The TCR interacts with both the peptide and the MHC molecule
  • Co-receptors (CD4 or CD8) stabilize the TCR-MHC interaction and transduce activation signals
• Co-stimulatory molecules provide additional signals necessary for full T cell activation and prevent anergy or tolerance

Cellular Players in Antigen Presentation

• Dendritic cells are the primary APCs involved in initiating adaptive immune responses
  • Conventional dendritic cells (cDCs) are specialized in antigen processing and presentation
  • Plasmacytoid dendritic cells (pDCs) produce large amounts of type I interferons in response to viral infections
• Macrophages are phagocytic cells that can present antigens to T cells and secrete cytokines to modulate immune responses
• B cells can internalize antigens through their B cell receptor (BCR) and present them to T cells
  • B cells require T cell help for full activation and differentiation into antibody-secreting plasma cells
• Thymic epithelial cells (TECs) present self-antigens during T cell development for the establishment of central tolerance
• Stromal cells in secondary lymphoid organs (lymph nodes, spleen) can present antigens to T cells and provide survival signals

Clinical Relevance and Disorders

• Defects in antigen processing and presentation can lead to immunodeficiencies and increased susceptibility to infections
  • Bare lymphocyte syndrome is caused by mutations in MHC class II genes, resulting in a lack of CD4+ T cell development and function
  • TAP deficiency impairs the transport of peptides into the ER, leading to reduced MHC class I expression and CD8+ T cell defects
• Autoimmune diseases can arise from the presentation of self-antigens and breakdown of tolerance mechanisms
  • In type 1 diabetes, pancreatic β cell antigens are presented to autoreactive T cells, leading to the destruction of insulin-producing cells
  • Rheumatoid arthritis involves the presentation of citrullinated proteins, triggering an autoimmune response in the joints
• Vaccines exploit antigen presentation to generate protective immune responses
  • Subunit vaccines contain purified antigens that are processed and presented by APCs to stimulate T and B cell responses
  • Attenuated viral vaccines mimic natural infection and induce both humoral and cellular immunity
• Cancer immunotherapy strategies aim to enhance the presentation of tumor antigens and activate tumor-specific T cells
  • Checkpoint inhibitors (anti-CTLA-4, anti-PD-1) block inhibitory signals and promote T cell activation
  • Chimeric antigen receptor (CAR) T cell therapy involves genetically modifying T cells to recognize tumor antigens and eliminate cancer cells

Research Frontiers and Applications

• Developing novel vaccine platforms that optimize antigen presentation and generate robust immune responses
  • Nanoparticle-based vaccines can deliver antigens and adjuvants to specific subsets of APCs
  • mRNA vaccines (COVID-19 vaccines) enable rapid production and induce both humoral and cellular immunity
• Harnessing the power of antigen presentation for cancer immunotherapy
  • Identifying and targeting neoantigens, which are tumor-specific antigens arising from mutations
  • Combining checkpoint inhibitors with other therapies (chemotherapy, radiation) to enhance antigen presentation and T cell activation
• Investigating the role of antigen presentation in the pathogenesis of autoimmune diseases and developing targeted therapies
  • Modulating antigen presentation to promote tolerance and prevent autoimmune responses
  • Identifying disease-associated MHC alleles and developing personalized treatment strategies
• Exploring the cross-talk between innate and adaptive immunity in the context of antigen presentation
  • Understanding how innate immune signals shape the quality and magnitude of adaptive immune responses
  • Targeting innate immune receptors (TLRs, NLRs) to enhance antigen presentation and vaccine efficacy
• Advancing our understanding of the molecular mechanisms underlying antigen processing and presentation
  • Elucidating the structure and function of MHC molecules and their interactions with peptides and TCRs
  • Investigating the role of post-translational modifications (ubiquitination, phosphorylation) in regulating antigen processing and presentation