Major Histocompatibility Complex (MHC)

Major Histocompatibility Complex (MHC)

Introduction to MHC

MHC is a critical component of the immune system, facilitating the presentation of antigen fragments to T-cells, thus playing a pivotal role in the body’s immune response. The structure and function of MHC molecules are paramount in determining an organism's susceptibility to diseases, whether infectious or autoimmune.

Author and Source References

• Primary References:

  • Bjorkman, P.J., Saper, M.A., Samraoui, B., Bennett, W.S., Strominger, J.L., Wiley, D.C. (1987) - Structure of the human class histocompatibility antigen HLA-A2, Nature, 329:508.

  • Tizard, I.R. (2013) Veterinary Immunology. Elsevier.

• Author: Dr. Ashutosh Verma, BVSC, MVSc, PhD, DipACVM, Professor and Associate Dean of Basic Sciences and Research, Richard A. Gillespie College of Veterinary Medicine, Lincoln Memorial University, Harrogate, TN, USA

Disclaimer

• The material in this presentation has been compiled from various sources, including books, published papers, and online resources. It is intended for educational purposes only.

• Most of the material is derived from:

  • Veterinary Immunology by Tizard, 11th ed.

  • Basic Immunology by Abbas et al., 4th and 5th eds.

  • Janeway’s Immunobiology.

Learning Objectives

Upon completing this topic, students should be able to:

1. List the steps of antigen presentation through endogenous and exogenous pathways.

2. Compare and contrast the types of antigens predominantly presented via endogenous and exogenous pathways.

3. Explain MHC restriction and the heterozygote advantage.

4. List examples of MHC-associated resistance or susceptibility to various diseases.

MHC-Mediated Antigen Presentation

Process Overview

1. Antigen Processing: In an adaptive immune response, antigen molecules must be fragmented; these fragments are then presented on antigen-presenting receptors (MHC).

2. MHC Molecule Description: MHC molecules are glycoproteins encoded by the MHC gene cluster, essential for the presentation of antigen fragments to T-cells.

3. Interaction Requirement: An antigen fragment must be bound to an MHC molecule to trigger an immune response; consequently, this complex binds to a T-cell, leading to activation.

4. Role in Disease Susceptibility: MHC controls antigen presentation, determining susceptibility to infectious or autoimmune diseases.

MHC Gene Cluster

Classification of MHC Genes

1. MHC Class I:

   • Located on all nucleated cells, responsible for presenting endogenous antigens.

2. MHC Class II:

   • Restricted to professional antigen-presenting cells (APCs), presenting exogenous antigens.

3. MHC Class III:

   • Contains a diverse mixture of proteins important for the innate immune response, such as complement proteins.

Arrangement of Mouse MHC Loci

• The arrangement of gene loci in mice includes Class I (K, D, Q) and Class II molecules.

MHC I and MHC II Characteristics

Comparison of MHC I and MHC II

Nomenclature in Different Species

• HLA: Human leukocyte antigen

• DLA: Dogs

• RLA: Rabbits

• BoLA: Cattle

• ELA: Horses

• SLA: Pigs

• H-2: Mouse

• B: Chicken

MHC Class I Molecules

Structural Composition

• Composed of two chains:

  1. Heavy α Chain (45 kDa): Contains three extracellular domains, one transmembrane domain, and one cytoplasmic domain.

  2. β2-Microglobulin (12 kDa): Attaches to the extracellular region of the α chain, stabilizing the structure.

• The α chain features a peptide-binding groove formed by the α1 and α2 domains, enabling the binding of antigenic peptides.

• The α chain is polymorphic while the β2-microglobulin is monomorphic, providing structural stability.

Characteristics of Peptide Binding

• Length: Peptides binding to MHC I typically range from 8-10 amino acids.

• Binding Specificity: The binding motif requires specific amino acid conformations at both ends, allowing variability in the central region.

• Peptide bulging is possible for correct accommodation within the groove.

Endogenous Pathway for MHC I Presentation

1. Protein Production: Proteins from within the cytosol are targeted by ubiquitin for degradation in the proteasome, resulting in short peptides.

2. Transport: The Transporter for Antigen Processing (TAP) carries these peptides to the endoplasmic reticulum (ER).

3. Peptide Enhancement: On reaching the ER, peptides are trimmed to 9 amino acids by ERAP and bound to empty MHC I.

4. MHC-I Movement: The resulting MHC I-peptide complex proceeds to the Golgi apparatus, ultimately reaching the cell surface for T-cell recognition.

Display of Peptides by MHC I

• A typical cell can display about 100,000 MHC I molecules on its surface at any time, showcasing several different alleles, thus allowing for diverse antigen representation.

MHC Class II Molecules

Structural Details

1. Construction: Each Class II molecule consists of two chains of similar size:

   • α Chain: Comprising three domains (extracellular α1 and α2, transmembrane, and cytosolic).

   • β Chain: Comprised of three domains (extracellular β1 and β2, transmembrane, and cytosolic).

• The binding groove is formed by α1 and β1 chains, allowing for peptide accommodation.

Characteristics of Peptide Binding to MHC II

• The peptide-binding groove is relatively open, accommodating peptides that range from 13-20 amino acids in length.

• Only 9 out of the total length are accommodated within the groove, while the rest extend outward.

• Approximately 12 MHC II alleles are needed to adequately present all potential antigens.

Exogenous Pathway of MHC II Presentation

1. Internalization: APCs internalize extracellular proteins via endocytosis or phagocytosis.

2. Processing: Intracellular enzymes digest the antigens into peptides within endosomal/lysosomal vesicles.

3. MHC II Synthesis: MHC II is synthesized in the rough ER while α and β chains assemble to create the peptide-binding groove.

4. Invariant Chain Role: During transport, an invariant chain occupies the groove, which is digested to display CLIP (Class II-associated invariant peptide) before the peptide binds.

5. Final Stage: The peptide-MHC II complex is transported to the cell surface, where it is presented to CD4+ T cells.

Additional MHC Classes

MHC Class III

• Composition: Includes genes for various proteins crucial in the immune response, such as complement components and cytokines.

• Function: Involves both innate and adaptive immunity mechanisms.

MHC Restriction

• Concept: Only antigen fragments that can bind to an MHC molecule can prompt an immune response; termed MHC restriction.

• Implication: There is a correlation between MHC alleles and susceptibility to diseases. Specific MHC alleles dictate an individual’s immune reactivity against pathogens.

Heterozygote Advantage

• Heterozygous individuals benefit from increased immune responsiveness to a broader range of antigens due to the presentation capabilities of different MHC alleles (6 MHC I and 12 MHC II alleles per individual is typically sufficient for an effective immune response).

Disease Associations

• Examples of MHC-Associated Resistance or Susceptibility:

  1. Bovine leukosis resistance associated with BoLA-Aw7.

  2. Mastitis resistance linked to BoLA-A*16.

  3. Dermatophilus sp. resistance attributed to the BoLA DR locus.

  4. Equine recurrent uveitis susceptibility tied to ELA-A9.

  5. Sarcoid tumors development linked to ELA-A3, ELA-A15, and ELA-Dw13, likely spurred by bovine papillomavirus.

Key Concepts

1. Antigen-Presenting Cells and MHC: APCs utilize MHC molecules for binding and presenting antigens.

2. MHC Genes: MHC molecules are encoded by genes located within the MHC gene complex.

3. Polymorphism: Classical MHC molecules exhibit significant genetic variability, enhancing individual immune diversity.

4. MHC Class I Function: Present within all nucleated cells for endogenous antigen presentation to CD8+ T cells.

5. MHC Class II Restriction: Found mainly on professional APCs for expressing exogenous antigens to CD4+ T cells.

6. MHC Class III Components: Comprises multiple genes, including those for complement components involved in immune response regulation.

Introduction to MHC

MHC is a critical component of the immune system, facilitating the presentation of antigen fragments to T-cells, thus playing a pivotal role in the body’s immune response and the discrimination between "self" and "non-self." The structure and function of MHC molecules are paramount in determining an organism's susceptibility to diseases, whether infectious or autoimmune. The MHC region is the most polymorphic part of the mammalian genome, ensuring a diverse range of responses within a population.

Author and Source References

• Primary References: - Bjorkman, P.J., Saper, M.A., Samraoui, B., Bennett, W.S., Strominger, J.L., Wiley, D.C. (1987) - Structure of the human class histocompatibility antigen HLA-A2, Nature, 329:508.

  • Tizard, I.R. (2013) Veterinary Immunology. Elsevier.

• Author: Dr. Ashutosh Verma, BVSC, MVSc, PhD, DipACVM, Professor and Associate Dean of Basic Sciences and Research, Richard A. Gillespie College of Veterinary Medicine, Lincoln Memorial University, Harrogate, TN, USA

Disclaimer

• The material in this presentation has been compiled from various sources, including books, published papers, and online resources. It is intended for educational purposes only.

• Most of the material is derived from:- Veterinary Immunology by Tizard, 11th ed.

  • Basic Immunology by Abbas et al., 4th and 5th eds.

  • Janeway’s Immunobiology.

Learning Objectives

Upon completing this topic, students should be able to:

1. List the steps of antigen presentation through endogenous and exogenous pathways.

2. Compare and contrast the types of antigens predominantly presented via endogenous and exogenous pathways.

3. Explain MHC restriction and the heterozygote advantage.

4. List examples of MHC-associated resistance or susceptibility to various diseases.

5. Understand the role of chaperone proteins in MHC assembly.

MHC-Mediated Antigen Presentation

Process Overview

1. Antigen Processing: In an adaptive immune response, antigen molecules must be fragmented into small peptides; these fragments are then loaded onto antigen-presenting receptors (MHC).

2. MHC Molecule Description: MHC molecules are cell-surface glycoproteins encoded by a large gene cluster. They act as "scaffolds" that display peptide fragments for surveillance by T-lymphocytes.

3. Interaction Requirement: A T-cell receptor (TCR) does not recognize a whole protein; it only recognizes a peptide fragment bound to an MHC molecule. This dual recognition of MHC + peptide is the basis of T-cell activation.

4. Role in Disease Susceptibility: Because different MHC alleles bind different sets of peptides, MHC determines whether an individual can mount an immune response against a specific pathogen.

MHC Gene Cluster

Genetic Characteristics

• Polygenic: The MHC contains several different MHC Class I and Class II genes, so every individual possesses a set of MHC molecules with different ranging specificities.

• Polymorphic: Multiple variants (alleles) of each gene exist in the population, making it unlikely for two unrelated individuals to have the same MHC profile (important in transplant rejection).

• Codominance: Alleles inherited from both parents are expressed equally on the cell surface.

Classification of MHC Genes

1. MHC Class I: - Located on all nucleated cells. In humans, these are designated as HLA-A, HLA-B, and HLA-C. They present endogenous (internal) antigens to $CD8^{+}$ Cytotoxic T-cells.

2. MHC Class II: - Restricted to professional antigen-presenting cells (APCs) such as Dendritic cells, Macrophages, and B-cells. In humans, these are HLA-DR, HLA-DP, and HLA-DQ. They present exogenous (external) antigens to $CD4^{+}$ Helper T-cells.

3. MHC Class III: - Contains genes for complement proteins ($C2$, $C4$, Factor B) and cytokines like Tumor Necrosis Factor ($TNF-\alpha$), which are involved in inflammation but do not present antigens.

MHC I and MHC II Characteristics

Comparison and Result

Nomenclature in Different Species

• HLA: Human ($Human \ leukocyte \ antigen$)

• DLA: Dogs ($Dog \ leukocyte \ antigen$)

• BoLA: Cattle ($Bovine \ leukocyte \ antigen$)

• ELA: Horses ($Equine \ leukocyte \ antigen$)

• H-2: Mouse

MHC Class I Molecules

Structural Composition

• Heavy \alpha Chain ($45 \text{ kDa}$): An integral membrane protein with three domains ($\alpha<em>1, \alpha</em>2, \alpha<em>3$). The $\alpha</em>1$ and $\alpha<em>2$ domains form the peptide-binding cleft. The $\alpha</em>3$ domain is highly conserved and serves as the binding site for the $CD8$ coreceptor.

• \beta_{2}-Microglobulin ($12 \text{ kDa}$): A non-covalently associated, monomorphic protein that is essential for the folding and surface expression of the Class I molecule.

• The Binding Cleft: The ends of the cleft are "closed," restricting the size of the bound peptide.

Endogenous Pathway (Cytosolic Pathway)

1. Degradation: Proteins (viral or self) in the cytosol are tagged with ubiquitin and degraded into small peptides by the 26S Proteasome.

2. Transport: Peptides are transported from the cytosol into the Lumen of the Endoplasmic Reticulum (ER) by the TAP1/TAP2 (Transporter associated with Antigen Processing) complex.

3. Chaperone Assembly: Inside the ER, empty MHC I molecules are stabilized by chaperones like Calnexin, Calreticulin, and Tapasin.

4. Loading & Expression: Once a peptide (usually $8-10$ amino acids) binds to the MHC I molecule, the complex becomes stable, moves through the Golgi, and is displayed on the plasma membrane.

MHC Class II Molecules

Structural Details

• \alpha Chain ($33 \text{ kDa}$) and \beta Chain ($28 \text{ kDa}$): Two non-identical transmembrane chains. Both contribute to the peptide-binding cleft, which is formed by the $\alpha<em>1$ and $\beta</em>1$ domains.

• The Binding Cleft: The ends of the cleft are "open," allowing longer peptides ($13-20+$ amino acids) to hang off the ends like a hotdog in a bun.

Exogenous Pathway (Endocytic Pathway)

1. Endocytosis: Extracellular antigens are internalized into endosomes.

2. Proteolysis: Endosomes fuse with lysosomes, and acid-dependent proteases (cathepsins) degrade the protein into peptides.

3. Invariant Chain (Ii): In the ER, newly synthesized MHC II is bound to the Invariant Chain ($Ii$), which blocks the binding cleft to prevent premature loading of ER peptides.

4. CLIP and HLA-DM: As MHC II moves to the endosome, $Ii$ is degraded, leaving a small fragment called CLIP (Class II-associated Invariant Chain Peptide) in the cleft. The molecule HLA-DM then catalyzes the exchange of CLIP for a high-affinity antigenic peptide.

5. Surface Display: The MHC II-peptide complex is exported to the cell surface for recognition by $CD4^{+}$ T-cells.

MHC Restriction and Immunity

• MHC Restriction: T-cells are "restricted" in their recognition; they only recognize an antigen if it is presented by an individual's own "self" MHC molecules. This was discovered by Zinkernagel and Doherty (Nobel Prize, 1996).

• Heterozygote Advantage: An individual who is heterozygous at all MHC loci can present a much wider variety of peptides than a homozygous individual, providing greater protection against diverse pathogens.

• Cross-Presentation: A unique ability of Dendritic Cells to take exogenous antigens and present them on MHC I to activate $CD8^{+}$ T-cells, crucial for mounting responses against viruses that do not infect APCs directly.

Disease Associations

• Veterinary Examples:

  1. Bovine Leukosis: Resistance associated with BoLA-Aw7.

  2. Equine Recurrent Uveitis: Susceptibility linked to ELA-A9.

  3. Chickens: The B-locus (MHC) is strongly linked to resistance to Marek's Disease (a viral lymphoma).

• Human Examples:

  • HLA-B27: Strongly associated with Ankylosing Spondylitis (an inflammatory spine disease).

  • HLA-DQ2/DQ8: Linked to Celiac disease.