This chapter discusses the process by which the antigen is processed to form an antigen peptide-MHC molecule complex and the antigen is presented to the αβ T cell receptor. As shown in the figure above, the intracellular transport pathways of exogenous antigens and endogenous antigens are different. This chapter will focus on describing the different characteristics of these pathways mediated by MHC class I molecules or class II molecules.
As discussed earlier, the human body has formed a very effective barrier to prevent foreign antigens (such as bacteria or viruses) from entering and causing disease. If a foreign body or antigen (such as a toxin) enters the body, the innate immune system's defensive barriers (such as phagocytes) may destroy B or T cells before they can encounter the antigen and trigger an adaptive immune response. Few foreign antigens survive intact within the host defense system of innate immunity. If so, only a very small number of B or T cell antigen receptors are needed to initiate a protective adaptive immune response.
B cell antigen receptors can directly interact with invading microorganisms, but αβ TCRs can only recognize "processed" antigen peptides. Furthermore, recognition of foreign antigens by αβ TCRs can only be accomplished when the foreign antigens are attached to the surface of other cells. (Antigen-presenting cells APCs or target cells; Fig 10.1)
APCs include macrophages, B cells, and various dendritic cells (Chapters 2, 12, and 20). Dendritic cells are the preeminent professional antigen-presenting cells among APCs (BOX 10.1; see Chapters 12 and 20). These APCs are very efficient in phagocytosis, processing and presentation of extracellular antigens, often together with cell-stimulating molecules to complete the immune activation process. Antigenic peptides are bound to MHC class II molecules in APCs and presented to T cells (Chapter 8).
Target cells are any nucleated cells infected with intracellular pathogens. Malignant cells may also become target cells. Antigens from intracellular pathogens or tumors (endogenous antigens) are processed into peptides, which are then bound to MHC class I molecules and presented to T cells.
The recognition of antigens by T lymphocytes is MHC-restricted. The major subsets of T lymphocytes, helper T cells (CD4+) and cytotoxic T lymphocytes (CD8+), have different MHC restrictions. The interaction of CD4+ T cells with APCs is MHC class II-restricted, and the interaction of CD8+ T cells with target cells is MHC class I-restricted (Chapters 7, 8, and 15). The process of converting antigens into polypeptides of appropriate length and binding to self-MHC molecules is called antigen processing.
Antigenic peptides produced in the cytoplasm of cells, such as viruses and bacteria replicating in the cytoplasm, bind to class I MHC molecules and are presented to CD8+ T cells (Chapter 7). Antigenic peptides produced in endosomes are produced by the endocytic uptake of extracellular antigens (such as toxins) or by microorganisms captured in endosomes (such as macrophages engulfing certain bacteria) and can interact with class II MHC molecules. Binds and then presents to CD4+ T cells. This means that CD8+ T cells can monitor the intracellular environment, while CD4+ T cells can monitor the extracellular environment.
The intracellular processing pathway through which the antigen undergoes is the primary factor that determines whether the antigen is presented via class I or class II MHC molecules, rather than any special properties of the antigen itself.
Extracellular or exogenous antigens may be extracellular proteins, such as protein vaccines, or they may be proteins produced by digestion of pathogens after being taken into endosomes. These antigens are processed and finally combined with MHC class II molecules and presented to CD4+ T lymphocytes (Fig 10.2A). First, they must be internalized by the APC. Soluble antigens are endocytosed intact. Pathogens are internalized through specialized phagocytic processes (Chapter 21), through which they enter the endosomal pathway. Some organisms have evolved to survive in endosomes, such as Mycobacterium tuberculosis. These organisms are intracellular but are still extracellular in the sense that they do not exist in the cytoplasm.
Subsequently, the endosomes containing the antigen will be acidified and fused with lysosomes (see Fig 10.2), and then degraded by cellular proteases into polypeptides of different sizes and finally into amino acids. During this process, peptides (9 to 3 amino acid residues) are produced that can bind to MHC class II molecules.
APC also synthesizes new MHC class II molecules in the endoplasmic reticulum. These molecules pass through the Golgi and eventually become part of vesicles, which detach from the Golgi and can fuse with endosomal vesicles containing antigenic peptides of extracellular or vesicular origin. In the pathway from the endoplasmic reticulum to the Golgi apparatus, the empty binding sites of MHC class II molecules are "protected" by invariant chain molecules, preventing them from binding to other polypeptides (such as self-peptides). This protection is removed by proteolysis in the acidic environment of the endosome, where any suitable peptide can bind to the class II binding site. Endosomes containing occupied (antigen-bound) MHC class II molecules are then transported into the exosome pathway and fuse with the cell membrane. In this way, foreign antigens can be presented to the corresponding TCR and induce the corresponding T cells to proliferate (see Figure 10.2A).
Intracellular or endogenous antigens, such as viral proteins, are processed by "target cells" and ultimately presented on MHC class I molecules to CD8+ T cells (cytotoxic T lymphocytes, see Fig 10.2B ). In this case, the exogenous antigenic polypeptide is produced in the cytoplasm. For example, viral proteins synthesized and assembled in the cytoplasm of virus-infected cells may be broken down through protein degradation pathways in the cell. In Figure 10.2B, viral proteins are being synthesized in the cytoplasm. Cellular degradation mechanisms, such as the proteasomal pathway, may cleave viral protein molecules until 8- to 11-residue peptides are formed; peptides of this length are capable of binding to MHC class I molecules. Many peptides are further degraded, rendering them non-immunogenic. However, some peptides of appropriate length that enter the endoplasmic reticulum can bind to MHC class I molecules.
Cells contain a variety of proteasomes, which continuously degrade and utilize cellular proteins. The genes encoding the proteasome components responsible for degrading pathogen proteins are located in the MHC large multifunctional protease (LMP) gene (see Figure 8.1). During infection, the cytokine interferon gamma is released, which increases LMP transcription and, therefore, pathogen protein breakdown is increased during infection.
Polypeptides are carried into the endoplasmic reticulum by double-chain molecules of the transporter associated with antigen presentation (TAP), which allow the polypeptide to pass through the double-membrane structure of the endoplasmic reticulum and bind to the protein synthesized in the endoplasmic reticulum. in the peptide-binding groove of nascent MHC class I molecules. The binding of these small antigenic peptides is critical for the final stages of assembly of MHC class I molecules. In the absence of peptides, class I molecules do not fold correctly and cannot be transported to the cell surface. MHC class I molecules are biosynthesized in the Golgi apparatus and move to the cell membrane through the extracellular pathway. After the fusion of the Golgi apparatus and the cell membrane, the antigen peptide-MHC class I molecule complex can interact with T lymphocytes (CD8+ or cytotoxic T lymphocytes) carrying receptors that bind this MHC-antigen complex.
It is worth noting that, as shown in Figure 10.2, the possibility of an antigen binding to an MHC class I or class II molecule is entirely determined by its transport pathway through the cell, not by some special properties of the antigen. . This type of antigen processing also explains why polysaccharides, lipids, and nucleic acids are not recognized by αβ T cells; because they are not processed so that they can bind to the binding grooves of MHC molecules.
Antigens derived from cell cytoplasm and endosomes combine with MHC class I or class II molecules respectively, leading to the activation of different T cell subsets (Fig 10.3 and TABLE 10.1). Extracellular antigens presented via MHC class II molecules activate CD4+ T cells. CD4+ T cells are helper; for example, CD4+ T cells can provide help by activating macrophages or stimulating B cells to produce antibodies (see Fig 10.3A). In some cases, CD4+ T cells provide help to the cells carrying the antigen. In some cases, MHC class I molecule/intracellular antigen complexes can activate CD8+ T cells to form cytotoxic T lymphocytes that can inhibit infection. If unsuccessful, the CD8+ T cells will kill the target cells by inducing apoptosis or cell lysis.
If pathogens can avoid antigenic peptides being recognized by MHC molecules, they can avoid detection by the adaptive immune system. Therefore, many pathogens are able to interfere with the process of antigen processing.
For example, Mycobacterium tuberculosis has acquired the ability to inhibit phagosome-lysosome fusion. This inhibits their access to lysosomal proteases, reducing the possibility of mycobacterial peptides binding to MHC molecules for presentation to the cell surface. Among viruses, several have been found to interfere with antigen processing by interfering with binding to MHC class I molecules. For example, herpes simplex virus (HSV) can bind to TAP, thereby inhibiting the entry of peptides into the endoplasmic reticulum, resulting in fewer HSV peptides that can bind to MHC class I molecules. Certain adenovirus strains express a protein that inhibits the transcription of MHC class I molecules, thereby reducing the number of MHC class I molecules that present adenoviral peptides to CD8+ lymphocytes.
The ways in which some pathogens have been discovered to evade detection by host defense systems reflect dynamic interactions between hosts and microorganisms as both attempt to survive or thrive. Likewise, medical research is leveraging knowledge of the antigen presentation process to develop better treatments for pathogen-mediated diseases (BOX 10.2).
Dendritic cells (DC) constitute a cell system that is crucial for immune responses, especially T cell-mediated immunity. DCs can present antigens to T cells more efficiently than other cell types. Classical DCs (cDCs) and plasmacytoid DCs (pDCs) are two subtypes of DCs. In order to become good antigen presenters, DCs undergo the following adaptive adjustments:
1. DCs have extensive “dendrites” that are constantly forming and shrinking (see Fig2.5). This increases the surface area for extracellular antigen uptake and contact with T cells.
2. DCs are mobile and migrate from the bone marrow to surrounding tissues, where they acquire extracellular antigens. DCs utilize Toll-like receptors to detect infection. When infection is detected, DCs migrate from peripheral organs to lymphoid organs, specifically the T cell regions of the organs, such as lymph nodes.
3. DC express very high levels of major histocompatibility complex (MHC) class II, which helps them present antigens to CD4+ T cells.
4. DCs can secrete cytokines, such as cDCs secreting interleukin 12 (IL-12) to activate the T-helper 1 subset (TH1) of T cells. pDC secretes interferon-α, which has antiviral effects.
The importance of DCs as antigen-presenting cells has prompted the development of relevant clinical studies to explore their application in tumor antigen vaccines. For example, DCs loaded with tumor antigens are used in clinical trials to treat melanoma patients.
The peptide transporter TAP found on the endoplasmic reticulum membrane is encoded by two genes, TAP-1 and TAP-2, which are located in the MHC class II region. The transporter is a heterodimer of two proteins, TAP-1 and TAP-2. Some rare mutations can alter the function of TAP, preventing the peptide from efficiently entering the ER lumen. In the absence of polypeptide antigens, MHC class I molecules are unstable, and only a small part can be transported to the cell surface through the extracellular pathway, resulting in reduced expression of MHC class I molecules and interference with cytotoxic T lymphocytes. form.
Chronic upper respiratory tract infections can be observed in humans carrying TAP-1 or TAP-2 mutations. The humoral immunity of these patients is intact, and some aspects of cellular immunity are also normal. For example, the patient's CD4+ T cells can respond to antigens. However, lack of expression of class I MHC molecules leads to a reduction in the number of cytotoxic T lymphocytes, making it difficult to mount an appropriate immune response to certain respiratory viruses.
A limitation of most vaccines is that unless a live organism is included in the vaccine, the antigen is injected into the extracellular space, which means that antigen presentation by MHC class I molecules, CD8+, does not occur T cells will not activate either. DNA vaccines are an alternative to standard vaccine technology and consist of complementary DNA (cDNA) sequences encoding protein antigens (such as viral proteins or tumor antigens) that activate a protective immune response. A "gene gun" is used to inject DNA into cells in subcutaneous tissue. The cDNA is transcribed and translated, and the protein molecules are eventually broken down into antigenic peptides. Some peptides enter the endoplasmic reticulum and bind to MHC class I molecules. After being transported to the cell surface, T cells can detect them on the surface of APCs. DNA vaccine therapies are currently being evaluated to find new treatments for HIV.
Early findings suggest this approach can generate strong and long-lasting immune responses to certain antigens. In addition, since the protein encoding the cDNA is synthesized in the cytoplasm, this may provide a way to introduce the antigen into the cell for processing, thereby triggering the presentation of class I MHC molecules and stimulating CTL responses. Standard vaccine approaches, such as intramuscular injection of a protein vaccine, will result in the protein being introduced into the endocytic/class II pathway and ultimately presented to CD4+ T cells, potentially stimulating an antibody response.
In addition, cDNAs that can produce other immune system enhancers such as cytokines are relatively easy to construct. It may be a good idea to combine cDNAs that produce these cytokines with cDNAs that produce antigenic proteins. The treatment may be widely promoted and applied in the future. DNA vaccines are entering clinical trials in breast, colon, and prostate cancer patients, and other tumor-related DNA vaccines will undoubtedly follow.