Immunologie II 22. Oktober 2007

Antigen Processing and Presentation to T Lymphocytes (Chapter 6; Cellular and Molecular Immunology; 6th ed. Abul K. Abbas, Andrew H. Lichtman, Shiv Pillai) Wesentliche Aspekte, die in dieser Doppelstunde besprochen werden

1.Central roles for T lymphocytes in all adaptive immune responses against protein antigens: • In cell-mediated immunity, CD4+ T cells activate macrophages to destroy phagocytosed microbes. • CD8+ T cells kill cells infected with intracellular microbes. • In humoral immunity, CD4+ helper T cells interact with B lymphocytes and stimulate the proliferation and differentiation of these B cells. Both the induction phase and the effector phase of T cell responses are triggered by the specific recognition of antigen. 2. Antigen-presenting cells (APCs). Cells that display MHC-associated peptides. Certain APCs present antigens to naive T cells during the recognition phase of immune responses to initiate these responses (professional APC’s), and some APCs present antigens to differentiated T cells during the effector phase to trigger the mechanisms that eliminate the antigens. • Characteristics of the APCs that form and display these peptide-MHC complexes and how protein antigens are converted by APCs to peptides that associate with MHC molecules. • Importance of MHC-restricted antigen presentation in immune responses.

Figure 6 -1 MHC restriction of cytolytic T lymphocytes. Virus-specific cytolytic T lymphocytes (CTLs) generated from virus-infected strain A mice kill only syngeneic (strain A) target cells infected with that virus. The CTLs do not kill uninfected strain A targets (which express self peptides but not viral peptides) or infected strain B targets (which express different MHC alleles than does strain A). By use of congenic mouse strains that differ only at class I MHC loci, it has been proved that recognition of antigen by CD8+ CTLs is self class I MHC restricted. Figure 6-2 Antigen-presenting cells are required for T cell activation. Purified CD4+ T cells do not respond to a protein antigen by itself but do respond to the antigen in the presence of an antigen-presenting cell (APC). The function of the APCs is to present a peptide derived from the antigen to the T cell. APCs also express costimulators that are important for T cell activation; these are not shown Figure 6-3 Functions of different antigen-presenting cells. The three major types of antigen-presenting cells for CD4+ T cells function to display antigens at different stages and in different types of immune responses. Note that effector T cells activate macro- phages and B lymphocytes by production of cytokines and by expressing surface molecules Figure 6-4 Dendritic cells. A. Light micrograph of cultured dendritic cells derived from bone marrow precursors. B. A scanning electron micrograph of a dendritic cell, showing the extensive membrane projections. C, D. Dendritic cells in the skin, illustrated schematically (C) and in a section of the skin stained with an antibody specific for Langerhans cells (which appear blue in this immunoenzyme stain) (D). E, F. Dendritic cells in a lymph node, illustrated schematically (E) and in a section of a mouse lymph node stained with fluorescently labeled antibodies against B cells in follicles (green) and dendritic cells in the T cell zone (red) (F). Dendritic cell subsets Figure 6-5 Routes of antigen entry. Microbial antigens commonly enter through the skin and gastrointestinal and respiratory tracts, where they are captured by dendritic cells and transported to regional lymph nodes. Antigens that enter the blood stream are captured by antigen-presenting cells in the spleen. Figure 6-6 Role of dendritic cells in antigen capture and presentation. Immature dendritic cells in the skin (Langerhans cells) capture antigens that enter through the epidermis and transport the antigens to regional lymph nodes. During this migration, the dendritic cells mature and become efficient antigen-presenting cells. The table summarizes some of the changes during dendritic cell maturation that are important in the functions of these cells GFP-MHC II DC + specific Ag + CD4 T cell Figure 5-7 (5th ed) Cross-presentation of antigens to CD8+ T cells. Cells infected with intracellular microbes, such as viruses, are captured by professional antigen- presenting cells (APCs), particularly dendritic cells, and the antigens of the infectious microbes are broken down and presented in association with the MHC molecules of the APCs. T cells recognize the microbial antigens and costimulators expressed on the APCs, and the T cells are activated. This example shows CD8+ T cells recognizing class I MHC-associated antigens; the same cross- presenting APC may display class II MHC-associated antigens from the microbe for recognition by CD4+ helper T cells. Figure 5-7 Pathways of antigen processing and presentation. In the class II MHC pathway extracellular protein antigens are endocytosed into vesicles, where the antigens are processed and the peptides bind to class II MHC molecules. In the class I MHC pathway protein antigens in the cytosol are processed by proteasomes, and peptides are transported into the endoplasmic reticulum (ER), where they bind to class I MHC molecules. TAP, transporter associated with antigen processing. Figure 6-8 Presentation of extracellular and cytosolic antigens. When a model protein ovalbumin is added as an extracellular antigen to an antigen-presenting cell that expresses both class I and class II MHC molecules, ovalbumin-derived peptides are presented only in association with class II molecules (A). When ovalbumin is synthesized intracellularly as a result of transfection of its gene (B), or when it is introduced into the cytoplasm through membranes made leaky by osmotic shock Figure 6-8 Presentation of extracellular and cytosolic antigens. (C), ovalbumin-derived peptides are presented in association with class I MHC molecules. The measured response of class II-restricted helper T cells is cytokine secretion, and the measured response of class I-restricted CTLs is killing of the antigen- presenting cells. Figure 6-9 The class II MHC pathway of antigen presentation. The numbered stages in processing of extracellular antigens correspond to the stages described in the text. APC, antigen-presenting cell; CLIP, class II-associated invariant chain peptide; ER, endoplasmic reticulum; Ii, invariant chain. Figure 6-10 Antigen processing requires time and cellular metabolism and can be mimicked by in vitro proteolysis. If an antigen-presenting cell (APC) is allowed to process antigen and is then chemically fixed (rendered metabolically inert) 3 hours or more after antigen internalization, it is capable of presenting antigen to T cells (A). Antigen is not processed or presented if APCs are fixed less than 3 hours after antigen uptake (B). Fixed APCs bind and present proteolytic fragments of antigens to specific T cells (C). The artificial proteolysis therefore mimics physiologic antigen processing by APCs. Effective antigen presentation is assayed by measuring a T cell response, such as cytokine secretion. (Note that this type of experiment is done with populations of antigen-specific T cells, such as T cell hybridomas, which respond to processed antigens on fixed APCs, but that normal T cells require costimulators that may be destroyed by fixation. Also, the time required for antigen processing is 3 hours in this experiment, but it may be different with other antigens and APCs.) Figure 6-11 The functions of class II MHC-associated invariant chains and HLA-DM. Class II molecules with bound invariant chain, or CLIP, are transported into vesicles (the MIIC/CIIV), where the CLIP is removed by the action of DM. Antigenic peptides generated in the vesicles are then able to bind to the class II molecules. Another class II-like protein, called HLA-DO, may regulate the DM-catalyzed removal of CLIP. CIIV, class II vesicle; CLIP, class II-associated invariant chain peptide; ER, endoplasmic reticulum; Ii, invariant chain; MIIC, MHC class II compartment. Figure 6-12 Morphology of class II MHC-rich endosomal vesicles. A. Immunoelectron micrograph of a B lymphocyte that has internalized bovine serum albumin into early endosomes (labeled with 5-nm gold particles, arrow) and contains class II MHC molecules (labeled with 10-nm gold particles) in MIICs (arrowheads). The internalized albumin will reach the MIICs ultimately. B. Immunoelectron micrograph of a B cell showing location of class II MHC molecules and DM in MIICs (stars) and invariant chain concentrated in the Golgi (G) complex. In this example, there is virtually no invariant chain detected in the MIIC, presumably because it has been cleaved to generate CLIP. Figure 6-13 The class I MHC pathway of antigen presentation. The numbered stages in the processing of cytosolic proteins correspond to the stages described in the text. β2m, β2-microglobulin; ER, endoplasmic reticulum; TAP, transporter associated with antigen processing. Figure 6-14 Role of TAP in class I MHC-associated antigen presentation. In a cell line lacking functional TAP, class I molecules are not efficiently loaded with peptides and are degraded, mostly in the endoplasmic reticulum (ER). When a functional TAP gene is transfected into the cell line, normal assembly and expression of peptide-associated class I MHC molecules are restored. Note that the TAP dimer may be attached to class I molecules by a linker protein called tapasin, which is not shown in this and other illustrations. TAP, transporter associated with antigen processing. Figure 6-15 T cells survey APCs for foreign peptides. Antigen-presenting cells (APCs) present self peptides and foreign peptides associated with MHC molecules, and T cells respond to the foreign peptides. In response to infections, APCs also express costimulators (not shown) that activate T cells specific for the microbial antigens. Figure 6-17 Presentation of extracellular and cytosolic antigens to different subsets of T cells. A. Extracellular antigens are presented by macrophages or B lymphocytes to CD4+ helper T lymphocytes, which activate the macrophages or B cells and eliminate the extracellular antigens. B. B. Cytosolic antigens are presented by nucleated cells to CD8+ CTLs, which kill (lyse) the antigen- expressing cells. Figure 5-18 Immunodominance of peptides. Protein antigens are processed to generate multiple peptides; immunodominant peptides are the ones that bind best to the available class I and class II MHC molecules. The illustration shows an extracellular antigen generating a class II-binding peptide, but this also applies to peptides of cytosolic antigens that are presented by class I MHC molecules. • T cells recognize antigens only in the form of peptides displayed by the products of self MHC genes on the surface of APCs. CD4+ helper T lymphocytes recognize antigens in association with class II MHC gene products (class II MHC-restricted recognition), and CD8+ CTLs recognize antigens in association with class I gene products (class I MHC-restricted recognition). • Specialized APCs, such as dendritic cells, macrophages, and B lymphocytes, capture extracellular protein antigens, internalize and process them, and display class II-associated peptides to CD4+ T cells. Dendritic cells are the most efficient APCs for initiating primary responses by activating naive T cells, and macrophages and B lymphocytes present antigens to differentiated helper T cells in the effector phase of cell-mediated immunity and in humoral immune responses, respectively. All nucleated cells can present class I-associated peptides, derived from cytosolic proteins such as viral and tumor antigens, to CD8+ T cells. • Antigen processing is the conversion of native proteins into MHC-associated peptides. This process consists of the introduction of exogenous protein antigens into APCs or the synthesis of antigens in the cytosol, the proteolytic degradation of these proteins into peptides, the binding of peptides to MHC molecules, and the display of the peptide-MHC complexes on the APC surface for recognition by T cells. Antigen-processing pathways in APCs use basic cellular proteolytic mechanisms that also operate independently of the immune system. Both extracellular and intracellular proteins are sampled by these antigen-processing pathways, and peptides derived from both normal self proteins and foreign proteins are displayed by MHC molecules for surveillance by T lymphocytes. • For class II-associated antigen presentation, extracellular proteins are internalized into endosomes, where these proteins are

proteolytically cleaved by enzymes that function at acidic pH. Newly synthesized class II MHC molecules associated with the Ii are transported from the ER to the endosomal vesicles. Here the Ii is proteolytically cleaved, and a small peptide remnant of the Ii, called CLIP, is removed from the peptide-binding cleft of the MHC molecule by the DM molecules. The peptides that were generated from extracellular proteins then bind to the available cleft of the class II MHC molecule, and the trimeric complex (class II MHC α and β chains and peptide) moves to and is displayed on the surface of the cell. • For class I-associated antigen presentation, cytosolic proteins are proteolytically degraded in the proteasome, generating peptides with features that enable them to bind to class I molecules. These peptides are delivered from the cytoplasm to the ER by an ATP- dependent transporter called TAP. Newly synthesized class I MHC- β2-microglobulin dimers in the ER are attached to the TAP complex and receive peptides transported into the ER. Stable complexes of class I MHC molecules with bound peptides move out of the ER, through the Golgi complex, to the cell surface. • These pathways of MHC-restricted antigen presentation ensure that most of the body's cells are screened for the possible presence of foreign antigens. The pathways also ensure that proteins from extracellular microbes preferentially generate peptides bound to class II MHC molecules for recognition by CD4+ helper T cells, which activate effector mechanisms that eliminate extracellular antigens. Conversely, proteins synthesized by intracellular (cytosolic) microbes generate peptides bound to class I MHC molecules for recognition by CD8+ CTLs, which function to eradicate cells harboring intracellular infections. The immunogenicity of foreign protein antigens depends on the ability of antigen-processing pathways to generate peptides from these proteins that bind to self MHC molecules and the presence of antigen-specific T cells.