Immunologie II22. 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)

1.Central roles for T lymphocytes in all adaptive immune responses against proteinantigens:

• In cell-mediated immunity, CD4+ T cells activate macrophages to destroyphagocytosed 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 inductionphase and the effector phase of T cell responses are triggered by the specificrecognition 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 ofimmune responses to initiate these responses (professional APC’s), and someAPCs present antigens to differentiated T cells during the effector phase to triggerthe mechanisms that eliminate the antigens.

• Characteristics of the APCs that form and display these peptide-MHC complexesand how protein antigens are converted by APCs to peptides that associate withMHC molecules.

• Importance of MHC-restricted antigen presentation in immune responses.

Wesentliche Aspekte, die in dieser Doppelstunde besprochen werden

Figure 6 -1MHC restriction of cytolytic Tlymphocytes.Virus-specific cytolytic Tlymphocytes (CTLs) generated fromvirus-infected strain A mice kill onlysyngeneic (strain A) target cellsinfected with that virus. The CTLs donot kill uninfected strain A targets(which express self peptides but notviral peptides) or infected strain Btargets (which express differentMHC alleles than does strain A). Byuse of congenic mouse strains thatdiffer only at class I MHC loci, it hasbeen proved that recognition ofantigen by CD8+ CTLs is self class IMHC restricted.

Figure 6-2Antigen-presenting cells are required for T cell activation. Purified CD4+ T cells donot respond to a protein antigen by itself but do respond to the antigen in the presenceof an antigen-presenting cell (APC). The function of the APCs is to present a peptidederived from the antigen to the T cell. APCs also express costimulators that areimportant for T cell activation; these are not shown

Figure 6-3 Functions of different antigen-presenting cells. The three major types ofantigen-presenting cells for CD4+ T cells function to display antigens at different stagesand in different types of immune responses. Note that effector T cells activate macro-phages and B lymphocytes by production of cytokines and by expressing surfacemolecules

Figure 6-4Dendritic cells. A. Light micrograph of cultured dendritic cells derived from bone marrow precursors. B. Ascanning electron micrograph of a dendritic cell, showing the extensive membrane projections. C, D. Dendriticcells in the skin, illustrated schematically (C) and in a section of the skin stained with an antibody specific forLangerhans 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 antibodiesagainst B cells in follicles (green) and dendritic cells in the T cell zone (red) (F).

Dendritic cell subsets

Figure 6-5Routes of antigenentry.Microbial antigenscommonly enter throughthe skin andgastrointestinal andrespiratory tracts, wherethey are captured bydendritic cells andtransported to regionallymph nodes. Antigensthat enter the bloodstream are captured byantigen-presenting cellsin the spleen.

Figure 6-6Role of dendritic cells in antigencapture and presentation.Immature dendritic cells in the skin(Langerhans cells) capture antigensthat enter through the epidermis andtransport the antigens to regionallymph nodes. During this migration,the dendritic cells mature andbecome efficient antigen-presentingcells. The table summarizes someof the changes during dendritic cellmaturation that are important in thefunctions of these cells

GFP-MHC IIGFP-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 professionalantigen- presenting cells (APCs), particularly dendritic cells, and the antigens of theinfectious microbes are broken down and presented in association with the MHCmolecules of the APCs. T cells recognize the microbial antigens and costimulatorsexpressed on the APCs, and the T cells are activated. This example shows CD8+ T cellsrecognizing class I MHC-associated antigens; the same cross- presenting APC maydisplay 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 areprocessed and the peptides bind to class II MHC molecules. In the class I MHC pathway protein antigens in thecytosol are processed by proteasomes, and peptides are transported into the endoplasmic reticulum (ER), wherethey bind to class I MHC molecules. TAP, transporter associated with antigen processing.

Figure 6-8 Presentation of extracellular and cytosolic antigens. When a modelprotein ovalbumin is added as an extracellular antigen to an antigen-presenting cell thatexpresses both class I and class II MHC molecules, ovalbumin-derived peptides arepresented only in association with class II molecules (A). When ovalbumin issynthesized intracellularly as a result of transfection of its gene (B), or when it isintroduced 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 MHCmolecules. The measured response of class II-restricted helper T cells is cytokinesecretion, and the measured response of class I-restricted CTLs is killing of the antigen-presenting cells.

Figure 6-9The class II MHC pathway of antigen presentation.The numbered stages in processing of extracellular antigens correspond to thestages described in the text. APC, antigen-presenting cell; CLIP, class II-associatedinvariant 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 hoursor more after antigen internalization, it is capable of presenting antigen to T cells (A). Antigen is not processed or presented if APCsare fixed less than 3 hours after antigen uptake (B). Fixed APCs bind and present proteolytic fragments of antigens to specific Tcells (C). The artificial proteolysis therefore mimics physiologic antigen processing by APCs. Effective antigen presentation isassayed by measuring a T cell response, such as cytokine secretion. (Note that this type of experiment is done with populations ofantigen-specific T cells, such as T cell hybridomas, which respond to processed antigens on fixed APCs, but that normal T cellsrequire 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 6-11The 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 peptidesgenerated in the vesicles are then able to bind to the class II molecules. Another classII-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, endoplasmicreticulum; Ii, invariant chain; MIIC, MHC class II compartment.

Figure 6-12 Morphology of classII MHC-rich endosomal vesicles.A. Immunoelectron micrograph ofa B lymphocyte that hasinternalized bovine serum albumininto early endosomes (labeledwith 5-nm gold particles, arrow)and contains class II MHCmolecules (labeled with 10-nmgold particles) in MIICs(arrowheads). The internalizedalbumin will reach the MIICsultimately.B. Immunoelectron micrograph ofa B cell showing location of classII MHC molecules and DM inMIICs (stars) and invariant chainconcentrated in the Golgi (G)complex. In this example, there isvirtually no invariant chaindetected in the MIIC, presumablybecause it has been cleaved togenerate CLIP.

Figure 6-13 The class I MHC pathway of antigen presentation. The numbered stages in the processing ofcytosolic proteins correspond to the stages described in the text. β2m, β2-microglobulin; ER, endoplasmicreticulum; TAP, transporter associated with antigen processing.

Figure 6-14Role 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, normalassembly and expression of peptide-associated class I MHC molecules are restored. Note that the TAP dimermay be attached to class I molecules by a linker protein called tapasin, which is not shown in this and otherillustrations. 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, andT 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 bindbest to the available class I and class II MHC molecules. The illustration shows an extracellular antigen generatinga class II-binding peptide, but this also applies to peptides of cytosolic antigens that are presented by class I MHCmolecules.

• 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), andCD8+ 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 andprocess them, and display class II-associated peptides to CD4+ T cells. Dendritic cells are the most efficient APCs for initiatingprimary responses by activating naive T cells, and macrophages and B lymphocytes present antigens to differentiated helper Tcells in the effector phase of cell-mediated immunity and in humoral immune responses, respectively. All nucleated cells canpresent 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 ofexogenous protein antigens into APCs or the synthesis of antigens in the cytosol, the proteolytic degradation of these proteins intopeptides, the binding of peptides to MHC molecules, and the display of the peptide-MHC complexes on the APC surface forrecognition by T cells. Antigen-processing pathways in APCs use basic cellular proteolytic mechanisms that also operateindependently of the immune system. Both extracellular and intracellular proteins are sampled by these antigen-processingpathways, and peptides derived from both normal self proteins and foreign proteins are displayed by MHC molecules forsurveillance by T lymphocytes.

• For class II-associated antigen presentation, extracellular proteins are internalized into endosomes, where these proteins areproteolytically cleaved by enzymes that function at acidic pH. Newly synthesized class II MHC molecules associated with the Ii aretransported 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 weregenerated from extracellular proteins then bind to the available cleft of the class II MHC molecule, and the trimeric complex (classII 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 peptideswith 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 TAPcomplex and receive peptides transported into the ER. Stable complexes of class I MHC molecules with bound peptides move outof 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 presenceof foreign antigens. The pathways also ensure that proteins from extracellular microbes preferentially generate peptides bound toclass II MHC molecules for recognition by CD4+ helper T cells, which activate effector mechanisms that eliminate extracellularantigens. Conversely, proteins synthesized by intracellular (cytosolic) microbes generate peptides bound to class I MHCmolecules for recognition by CD8+ CTLs, which function to eradicate cells harboring intracellular infections. The immunogenicity offoreign protein antigens depends on the ability of antigen-processing pathways to generate peptides from these proteins that bindto self MHC molecules and the presence of antigen-specific T cells.