Modulation of Antigen Processing/ Presentation Pathways

Modulation of Antigen Processing/Presentation Pathways

Why is antigen processed/presented?

References

Why is antigen processed/presented?

In order to initiate a specific immune response to an infectious agent, the immune system must be able to wade through the sea of molecules that are associated with pathogenic invasion and isolate particular protein products that will hone the efforts of host defense. Implicit to this model of counteraction is the processing of an immunogenic peptide epitope and its presentation on the surface of a team of cells. The result of these actions is the induction of a T-cell response that recruits and engages the other molecular participants of the immune response.

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What are the essential characteristics of this component of the immune system?

At the core of this immune system element is the Major Histocompatibility Complex (MHC). Located on human chromosome 6, the MHC is a highly polymorphic set of genes that encode for molecules essential to self/non-self discrimination and antigen processing and presentation. The power of this multigenic complex lies in its polymorphism, which enables different allelic class I and class II products to bind an almost infinite array of peptides.

Source: The Principles of Protein Structure '97

The nature of the Major Histocompatibility Complex suggests the now fundamental concept of self-MHC restriction. CD4⁺ T_H cells are activated only by antigen presenting cells that share class II MHC alleles with them; that is, antigen recognition by CD4⁺ T_H cells is class II MHC restricted. Antigen recognition by CD8⁺ T_C cells, on the other hand, is class I MHC restricted. Such division of labor is a response to the endogenous and exogenous origin of pathogenic proteins and a consequence of a design paradigm that allows for routing of the immune response.

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How is antigen normally processed/presented?

Source: Infection and Immunity

Endogenous Antigens and the Cytosolic Pathway

Exogenous Antigens and the Endocytic Pathway

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Endogenous Antigens and the Cytosolic Pathway

Source: Bioscience

Endogenous antigens are produced by viruses replicating within a nucleated host cell, processed by the proteolytic system acting on host intercellular proteins, and presented by class I MHC molecules.

Proteasome Action

Intercellular proteins are cut into short peptides in the cell’s multifunctional protease complex, the proteasome. Proteins bound for degradation are targeted to the proteasome by covalent linkage with a small protein known as ubiquitin. A ubiquitinating enzyme conjugates several ubiquitin molecules to a lysine amino group near the amino terminus of the protein. Two subunits to the proteasome, LMP2 and LMP7, encoded by the MHC gene cluster induce the proteolytic complex to generate peptides that bind especially well to class I MHC molecules. The actual cleaving of protein occurs in the channel of the proteasome molecule.

Peptide Transport

Peptides generated in the cytosol must be transported into the rough endoplasmic reticulum (RER) to enable interaction with MHC molecules. TAP (transporters associated with antigen processing) is a transmembrane heterodimeric protein that allows for the crucial step of peptide translocation. TAP trasnsports peptides into the ER that are ideally suited for binding with class I MHC molecules—peptides consisting of 8-13 amino acids and those with basic or hydrophobic residues.

MHC-Peptide Binding

A class I MHC molecules actually requires the presence of a peptide to achieve stability. First, The class I a chain and b ₂-microglobulin associate with the ER membrane protein calnexin. Calnexin belongs to a group of proteins known as molecular chaperones which aid in the folding of polypeptides. Binding of this complex to TAP then initiates peptide capture by the MHC heterodimer and subsequent dissociation from calnexin. The stabilized MHC-antigen complex is finally transported to the cell membrane via the Golgi complex. At the cell surface, CD8⁺ T_C cells recognize antigen.

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Exogenous Antigens and the Endocytic Pathway

Source: Histocompatibility Molecules

Exogenous antigens are internalized by antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B lymphocytes. APCs can phagocytose and/or endocytose antigen, endosomally process it, and present it in association with class II MHC molecules.

Endosome Action

Internalized antigen is processed within three increasingly acidic endosomal environments: early endosomes (pH 6.0-6.5), late endosomes (pH 5.0-6.0), and lysosomes (pH4.5-5.0). Acid-dependent hydrolytic enzymes degrade antigen into peptides consisting of 13-18 amino acids.

MHC transport

Class II a and b chains associate within the RER, the site of interaction between endogenous antigens and MHC class I molecules of the cytosolic pathway. A protein known as the invariant (Ii) chain binds to the peptide-binding cleft of the MHC class I molecule and thereby prevents its binding with endogenous antigens. The Ii chain seems to provide other important functions for the class II MHC molecule, too: it is involved in the folding of a and b chains, the exit of the complex from the ER, and its targeting to the endocytic compartments.

MHC-Peptide Binding

The class II MHC-invariant chain complex is transported to early endosomes. As proteolysis increases in the successive endosomal compartments, the invariant chain is degraded and the MHC molecule takes on an open conformation. A fragment of the invariant chain known as CLIP (class II-associated invariant chain peptide), however, remains bound to the peptide-binding cleft to prevent premature interaction with partially-processed antigen. In the lysosome, HLA-DM, a class II MHC-like molecule, mediates the removal of CLIP and the binding of antigen. The MHC-antigen complex then moves to the plasma membrane where the neutral environment stabilizes it. Here, at the cell surface, antigen is presented to CD4⁺ T_H cells.

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Infectious Agents: How do they subvert this system?

Viruses, bacteria, and parasites have exploited the fact that antigen processing and presentation involves multiple steps as seen in the previous section. By modulating one particular step, these infectious agents can corrupt the whole pathway. Their means of usurping control is by way of immunomodulatory proteins that have been referred to as mimics, pirates, and exploiters. These proteins interrupt essential molecular interactions, redirect cellular products, and abolish cellular protein function.

Antigen Processing/Presentation Targets for Subversion
Proteosome Proteolysis	Endocytic Trafficking/Proteolysis
Peptide Transport into ER	Expression of MHC class II molecules
MHC-Peptide Transport out of ER	Back to the Top
Expression of MHC class I and other Ag-Presenting Molecules

Interference with Proteosomal Proteolysis

Epstein-Barr Virus (EBV): Viral EBNA-1 protein disrupts the cleaving action of the proteosome. EBNA-1 is an important latency-associated protein (LAP) in EBV, a virus that enters a truly latent state in which no viral replication occurs. It is the only LAP against which no CTLs have ever been detected. (Hill, 1996)

Mouse Leukemia Virus (MLV): A single amino acid substitution can eliminate the proteolytic cleavage site needed for the generation of a MLV-derived epitope, preventing CTL recognition. The MHC polymorphism evolved in part to counteract this type of evasion strategy. (Ploegh, 1998)

Human Cytomegalovirus (HCMV): Viral phosphoprotein, pp65, inhibits the generation of the principal immediate early protein p72, an HCMV-specific T-cell epitope. (Hengel, 1998)

Other Subversion Targets

Interference with Peptide Transport

Herpes Simplex Virus 1 and 2 (HSV): Immediate early protein, ICP47, competes with antigens for the TAP peptide-binding site. TAP polymorphisms may have been selected in response to this type of modulation. (Ploegh, 1998)

Source: Infection and Immunity

Adenovirus: Viral protein downregulates transcription of TAP 1 and 2 genes. (Hill, 1996)

Cervical Cancer: Malignantly transformed cells downregulate transcription of TAP 1. [Cancer evasion strategies]

Other Subversion Targets

Retention and Destruction of Class I Molecules

Adenovirus: Viral protein E19 also binds to MHC class I molecules and retains them in the ER. Studies have revealed more detailed knowledge about E19. Tumor necrosis factor (TNF) usually stimulates MHC genes, thereby enhancing T-cell recognition; in E19-expressing cells, however, TNF stimulates the production of viral proteins that neutralize the lytic activity of CTLs. This strategy assures more efficient viral reproduction and survival in the early phase of the immune response when TNF is produced. (Burgert, 1996)

HCMV: US 2 and US 11 products bind to MHC class I molecules and redirect the heavy chains to the cytosol through a process known as dislocation. In uninfected cells, dislocation functions as a mechanism for purging misfolded ER-resident proteins. The viral proteins accelerate the rate constant for the dislocation reaction. (Hengel, 1998)

Other Subversion Targets

General Regulation of MHC I Expression and that of Other Antigen-Presenting Molecules

HIV-1: Nef protein-mediated dowregulation of MHC class I molecules. (Collins, 1998)

Foot and Mouth Disease Virus: Upon infection of cell, rapid reduction of MHC I surface expression. (Sanz-Parra, 1998)

Leishmania: Inhibition of MHC I expression. (Bogdan, 1998)

Mycobacterium Tuberculosis: Downregulation of Ag-presenting molecule CD-1. MHC class I and II expression is not influenced. (Stenger, 1998)

Head and Neck Cell Squammous Carcinoma: Altered expression of HLA I (Human Leukocyte Antigen class I). (Vora, 1997)

Other Subversion Targets

Interference with Endocytic Trafficking/Proteolysis

Bovine Papilloma Virus (BPV): Viral protein E6 interacts with components of the AP-1 adaptor complex, affecting the intercellular distribution of class II products and their associated antigens. The E5 protein can replace a subunit on the enzyme responsible for endosomal and lysosomal acidification, preventing some proteolysis. (Ploegh, 1998)

Helicobacter pylori: The H. pylori VacA toxin also inhibits acidification by modulating the vacuolar enzyme. (Ploegh, 1998)

Other Subversion Targets

General Regulation of MHC Class II Expression

Toxoplasma gondii: Downregulation of MHC class II molecules and inability to upregulate class I molecules. (Luder, 1998)

EBV: Viral protein BZLF2 binds the HLA-DR b chain and inhibits class II-dependent antigen presentation. (Davis-Poynter, 1996)

HCMV: Viral protein prevents upregulation of class II moelcules. Virus targets epithelial cells, which normally promote transcription of class II genes in response to IFN-g and other select cytokines. (Hengel, 1998)

Other Subversion Targets

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What are the consequences of studying the modulation of antigen processing/presentation pathways?
Elucidation of Antigen Processing/Presentation Mechanisms

Studies of the adenoviral E19 protein, which retains MHC I molecules in the ER, led to greater understanding of MHC-antigen binding. Researchers discovered that the polymorphic a 1 and a 2 domains associate to form the MHC I peptide-binding pocket. (Burgert, 1996)

The E5 product of bovine papilloma virus (BPV), which inhibits endosomal proteolysis, helped identify a subunit of the vacuolar H⁺ adenosine triphosphatase enzyme required for the acidification of endosomal compartments. (Ploegh, 1998)

Identification of Immune Counteractivity

In many viral systems in which antigen presentation is downregulated, cells produce large amounts of IFN g and TNF a that stimulate production of MHC class I molecules. Even though many class I MHCs are blocked by viral mechanisms, a surplus of the molecules allows for efficient antigen presentation.

Thorough study of viral evasion mechanisms reveals certain cell-types that are resistant to such exploitation. For example, macrophages resist the subversive effects of cytomegalovirus (CMV). Further research could identify the mechanism of resistance and offer possible therapeutic strategies. (Hengel, 1998)

Recognition of Pathogenic Influences on Immune System Evolution

Analyzing a model in which certain HLA antigens are associated with protection against malaria, a 1991 study provides some the best evidence for the role of infectious agents in driving MHC polymorphisms. (Damian, 1997)
The evasion mechanisms of Murine Leukemia Virus (MLV) and Herpes Simplex Virus (HSV) also shed light on the selective advantage of maintaining MHC and TAP polymorphisms, respectively.

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Case Study: Cytomegalovirus (CMV)

Cytomegalovirus modulates the cytokine network and the complement pathway, but its main target for immune exploitation is the antigen processing/presentation pathway. Link to this part of the site for more detailed information on the CMV immunomodulatory proteins mentioned above and others. Also, see discussion of vaccine development and the implications of antigen processing/presentation modulation.

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