Antigen Presentation
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Questions and Answers

What is antigen processing and presentation?

The process by which antigen presenting cells digest antigens acquired from the inside or outside the cell and display the resulting antigenic fragments on cell surface MHC molecules for recognition by T lymphocytes

Which cells are primarily responsible for digesting antigens in the antigen processing and presentation process?

  • Natural killer cells
  • Neutrophils
  • T lymphocytes
  • Antigen presenting cells (correct)

    • Which of the following professional antigen presenting cells is NOT one of the three main types identified?

  • Eosinophils (correct)
  • Macrophages
  • Dendritic cells
  • B lymphocytes

    • What is the outcome of T lymphocytes recognizing antigenic fragments displayed on MHC molecules?

    <p>Activation to initiate a cellular immune response</p> Signup and view all the answers

    Antigen processing involves which of the following processes?

    <p>Digesting antigens acquired from inside or outside the cell</p> Signup and view all the answers

    What is the role of MHC molecules in antigen presentation?

    <p>Binding and displaying antigenic fragments for T lymphocyte recognition</p> Signup and view all the answers

    What are examples of non-professional antigen presenting cells?

    <p>Neutrophils, eosinophils, basophils, T cells, fibroblasts, NK cells smooth muscle cells, astrocytes, microglial cells etc.</p> Signup and view all the answers

    Non professional cells only present antigens when they become infected

    <p>True</p> Signup and view all the answers

    What unique function do dendritic cells perform that is crucial for T cell activation?

    <p>They present antigens through both MHC class I and MHC class II pathways.</p> Signup and view all the answers

    Which mechanism allows dendritic cells to present antigens from infected and tumor cells to CD8+ T cells?

    <p>Cross presentation.</p> Signup and view all the answers

    What property of dendritic cells significantly enhances their ability to stimulate T cell responses?

    <p>Their expression of high levels of costimulatory molecules.</p> Signup and view all the answers

    Which statement contradicts the conventional understanding of antigen presentation mechanisms?

    <p>Ingested antigens can be presented through MHC class I rather than MHC class II.</p> Signup and view all the answers

    What is the term used to describe the process by which dendritic cells present ingested antigens through MHC class I?

    <p>Cross priming.</p> Signup and view all the answers

    What type of antigens do dendritic cells primarily present via the MHC class 1 and MHC class 2 pathways?

    <p>Both intracellular and extracellular antigens</p> Signup and view all the answers

    In which location are dendritic cells predominantly found compared to other sites?

    <p>Epithelium</p> Signup and view all the answers

    Which statement accurately describes the characteristics of dendritic cells in relation to MHC expression?

    <p>They always express MHC-1 and MHC-2.</p> Signup and view all the answers

    What can be inferred about the role of dendritic cells in the immune system based on their locations?

    <p>They act as the first line of defense in tissues exposed to the external environment.</p> Signup and view all the answers

    Which of the following statements is true regarding their functionality in antigen presentation?

    <p>They can present antigens from both infected cells and tumor cells.</p> Signup and view all the answers

    What triggers macrophages to express MHC class 2 and present antigens?

    <p>Cytokines and damage-associated molecular patterns (DAMPs)</p> Signup and view all the answers

    In which location are macrophages most abundantly found?

    <p>Lymphoid tissue predominantly</p> Signup and view all the answers

    Which of the following statements about macrophages is accurate?

    <p>Macrophages present extracellular antigens through MHC class 2</p> Signup and view all the answers

    Which factor is NOT necessary for macrophages to present antigens to CD4+ T cells?

    <p>Presence of T cell receptors on macrophages</p> Signup and view all the answers

    What is the primary role of macrophages in the context of cell-mediated immunity?

    <p>To act as antigen-presenting cells for CD4+ T cells</p> Signup and view all the answers

    What role do B lymphocytes play in the activation of helper T-cells during the humoral immune response?

    <p>They present antigens to helper T-cells after activation.</p> Signup and view all the answers

    Which statement correctly describes the expression of MHC class II on B lymphocytes?

    <p>MHC class II is inducible and expressed upon activation.</p> Signup and view all the answers

    Where are B lymphocytes primarily located within the body?

    <p>In lymphoid tissue and blood.</p> Signup and view all the answers

    What type of antigens do B lymphocytes bind to during the humoral immune response?

    <p>Extracellular antigens that bind to specific Ig receptors.</p> Signup and view all the answers

    What is the sequence of event regarding B lymphocytes and helper T-cells in the immune response?

    <p>Activated B lymphocytes present antigens to helper T-cells after interacting with dendritic cells.</p> Signup and view all the answers

    What is the primary role of antigen presenting cells (APCs) in relation to T cells?

    <p>They convert protein antigens to peptides and display them for T cell recognition.</p> Signup and view all the answers

    Which statement correctly describes the process of co-stimulation provided by APCs?

    <p>Co-stimulation enhances T cell activation beyond antigen recognition alone.</p> Signup and view all the answers

    During antigen processing, which is a key step performed by APCs?

    <p>Conversion of protein antigens into peptide fragments.</p> Signup and view all the answers

    What distinguishes the functions of professional APCs from non-professional APCs?

    <p>Professional APCs consistently express MHC molecules and provide co-stimulation.</p> Signup and view all the answers

    Which of the following functions is NOT performed by antigen presenting cells?

    <p>Directly eliminating pathogens through phagocytosis.</p> Signup and view all the answers

    Antigen presentation begins with (…)

    <p>Antigen presentation begins with antigen processing.</p> Signup and view all the answers

    Which statement accurately describes the source of the peptides that are presented by MHC class I molecules?

    <p>Peptides come from protein antigens found in the cytosol.</p> Signup and view all the answers

    What distinguishes the antigen processing pathway that generates MHC class II-associated peptides from that of MHC class I-associated peptides?

    <p>MHC class II pathways process antigens originating from extracellular sources.</p> Signup and view all the answers

    Which of the following best characterizes the role of professional antigen presenting cells (APCs) in relation to MHC molecules?

    <p>APCs are responsible for loading peptides from both cellular and extracellular sources onto MHC molecules.</p> Signup and view all the answers

    What is the significance of acidic vesicular compartments in the context of antigen processing?

    <p>They facilitate the generation of peptides for MHC class II presentation.</p> Signup and view all the answers

    Which of the following peptides is least likely to be displayed by MHC class I molecules?

    <p>Peptides obtained from extracellular proteins after vesicular processing.</p> Signup and view all the answers

    What is the primary role of cathepsins in the processing of endocytosed antigens for MHC class II presentations?

    <p>To degrade proteins into peptides for presentation</p> Signup and view all the answers

    Which process is responsible for the uptake of extracellular proteins into the vesicular compartments of antigen presenting cells?

    <p>Receptor-mediated endocytosis</p> Signup and view all the answers

    What is the significance of the phagolysosome in antigen processing for MHC class II presentations?

    <p>To create an environment for peptide cleavage and degradation</p> Signup and view all the answers

    In the context of antigen processing, what is the expected length of peptides produced for MHC class II presentation?

    <p>14-20 amino acids</p> Signup and view all the answers

    Which component plays a critical role in the further degradation of proteins within endosomal or lysosomal vesicles during antigen processing?

    <p>Cathepsins</p> Signup and view all the answers

    What is the correct sequence of the first step involved in the biosynthesis of MHC class II molecules?

    <p>Synthesis of alpha and beta chains in the endoplasmic reticulum</p> Signup and view all the answers

    What happens to the invariant chain (Ii) during the processing of MHC class II molecules?

    <p>It dissociates in endosomes allowing peptide association</p> Signup and view all the answers

    Which cells primarily recognize the MHC class II-peptide complexes once expressed on the surface of antigen presenting cells?

    <p>CD4+ T cells</p> Signup and view all the answers

    Where is the invariant chain (Ii) specifically associated during the transport of MHC class II molecules?

    <p>In the endoplasmic reticulum during synthesis</p> Signup and view all the answers

    What outcome occurs after the stable association of peptides with MHC class II molecules?

    <p>The MHC class II-peptide complexes are expressed on the cell surface</p> Signup and view all the answers

    What role does the transporter associated with antigen presentation (TAP) play in the processing of cytosolic antigens for MHC class I presentation?

    <p>It mediates transport of peptides into the ER.</p> Signup and view all the answers

    Which step in the sequence of MHC class I antigen processing occurs after the proteolytic degradation of proteins?

    <p>Transport to the endoplasmic reticulum.</p> Signup and view all the answers

    What is the final outcome of the stable MHC class I-peptide complex in the immune response?

    <p>Recognition by CD8+ T cells.</p> Signup and view all the answers

    Which of the following correctly describes the origin of the proteins presented by MHC class I molecules?

    <p>Cytosolic proteins, potentially from viruses or intracellular microbes.</p> Signup and view all the answers

    What is the role of the Golgi apparatus in the process of MHC class I antigen presentation?

    <p>It delivers the stable MHC class I complex to the cell surface.</p> Signup and view all the answers

    What is the primary effect of microbial product detection on antigen presenting cells (APCs)?

    <p>Induces expression of MHC molecules</p> Signup and view all the answers

    What is the role of dendritic cells in the initiation of T cell responses?

    <p>They capture antigens and present them to naive T cells in lymph nodes.</p> Signup and view all the answers

    How do APCs respond to microbial products to enhance their function?

    <p>By activating toll-like receptors</p> Signup and view all the answers

    Which of the following describes a response of APCs upon the activation of toll-like receptors?

    <p>Induction of pro-inflammatory cytokines</p> Signup and view all the answers

    What is the significance of chemokine production by activated APCs?

    <p>It facilitates migration to the site of inflammation.</p> Signup and view all the answers

    What type of molecules are responsible for presenting lipid antigens to T-cells?

    <p>Non-polymorphic CD1 molecules</p> Signup and view all the answers

    Which type of T-cells are primarily activated by lipid antigens presented by CD1?

    <p>NKT-cells</p> Signup and view all the answers

    Where are CD1 molecules typically found?

    <p>On antigen presenting cells and some epithelial cells</p> Signup and view all the answers

    What distinguishes the T-cell activation by CD1 from that by MHC molecules?

    <p>CD1 presents lipid antigens to T-cells that aren't MHC restricted</p> Signup and view all the answers

    Which statement is accurate regarding the structure of CD1 molecules?

    <p>CD1 molecules are similar to MHC class I molecules in structure</p> Signup and view all the answers

    What types of T cells are activated by peptides derived from endogenously acquired proteins?

    <p>CD8+ T cells</p> Signup and view all the answers

    Which statement accurately reflects the nature of T cells in terms of antigen recognition?

    <p>T cells recognize only peptides presented on APCs with the correct MHC.</p> Signup and view all the answers

    What is the role of the CD3 complex in T cells?

    <p>It helps in the expression of TCR during T cell development.</p> Signup and view all the answers

    Which statement best describes the predominant type of TCR found in lymphoid tissues?

    <p>Alpha-beta TCR</p> Signup and view all the answers

    Which characteristic distinguishes alpha-beta TCR from gamma-delta TCR?

    <p>Alpha-beta TCR is MHC independent.</p> Signup and view all the answers

    What defines self MHC restriction in T cells?

    <p>T cells respond to peptides presented by their own MHC molecules.</p> Signup and view all the answers

    Which type of antigens are CD4+ T cells specifically designed to recognize?

    <p>Exogenous antigens from extracellular sources</p> Signup and view all the answers

    Which component of the TCR is crucial for recognizing specific amino acid sequences of peptides?

    <p>The anchor residues</p> Signup and view all the answers

    Which of the following are recognized by B cell receptors (BCRs)?

    <p>Exposed antigens such as proteins</p> Signup and view all the answers

    What is the consequence of T cells being stimulated without co-stimulatory signals?

    <p>Clonal anergy</p> Signup and view all the answers

    Which signal is primarily responsible for T cell activation?

    <p>Signal 1 generated by antigen-MHC binding</p> Signup and view all the answers

    Which of the following combinations properly represents the two signals required for T cell activation?

    <p>CD28-B7 interaction and antigen-MHC interaction</p> Signup and view all the answers

    What type of antigens can B cell receptors (BCRs) recognize?

    <p>Exposed polysaccharides and proteins</p> Signup and view all the answers

    What is the primary positive co-stimulatory signal provided by APCs to activate naive T cells?

    <p>APC CD90/CD86 and T cell CD28</p> Signup and view all the answers

    Which characteristic of dendritic cells enables them to effectively initiate primary T cell responses?

    <p>Their strategic location at common sites of microbial entry.</p> Signup and view all the answers

    What is the role of mature dendritic cells in relation to naive T cells?

    <p>They activate naive T cells through high levels of co-stimulatory molecules.</p> Signup and view all the answers

    What effect does the interaction of APC CD90/86 with T cell CTLA4 have on T cell activation?

    <p>It induces anergy in T cells.</p> Signup and view all the answers

    Which of the following properties distinguishes dendritic cells from other professional APCs?

    <p>Their preferential migration to T-cell rich zones of lymph nodes.</p> Signup and view all the answers

    Study Notes

    Antigen Processing And Presentation

    • Antigen presenting cells (APCs) process and present antigens to T lymphocytes.
    • APCs digest antigens acquired from inside or outside the cell.
    • APCs display resulting antigenic fragments on cell surface MHC molecules.
    • T lymphocytes recognize antigenic fragments presented on MHC molecules, initiating a cellular immune response.
    • The three professional APCs are dendritic cells, macrophages, and B lymphocytes.

    Dendritic Cells and T Cell Response

    • Dendritic cells (DCs) are crucial for initiating T cell responses.
    • They present antigens to naive CD4+ and CD8+ T cells.
    • DCs express high levels of costimulatory molecules which are required for T cell activation.
    • DCs can ingest infected and tumor cells.
    • This allows DCs to present antigens to CD8+ T cells, even if the antigen is not directly expressed by the DC itself.
    • This process, known as cross-presentation or cross-priming, is essential for generating CD8+ T cell responses against intracellular pathogens and tumors.
    • Cross-presentation allows DCs to present antigens through MHC class I, even though the antigen was initially ingested through the MHC class II pathway.

    Dendritic Cells

    • Dendritic cells always express MHC-2
    • Dendritic cells present intracellular and extracellular antigens
    • Dendritic cells present antigens via MHC class 1 and MHC class 2 pathways
    • Dendritic cells are located in the lymphoid tissue, connective tissue and epithelium -- with the highest concentration in the epithelial layer

    Macrophages in Cell-Mediated Immunity

    • Macrophages are key players in the effector phase of cell-mediated immunity.
    • They present antigens to differentiated CD4+ T cells, which are crucial for triggering an immune response.
    • Macrophages express MHC class 2, which is involved in antigen presentation.
    • Macrophage expression of MHC class 2 is low but can be induced by various stimuli, including:
      • Pathogen-associated molecular patterns (PAMPs)
      • Damage-associated molecular patterns (DAMPs)
      • Cytokines
    • Macrophages are considered extracellular antigen presenters.
    • They present antigens via MHC class 2.
    • Macrophages are found in various locations, including:
      • Lymphoid tissue
      • Connective tissue
      • Body cavities
    • The majority of macrophages are located in lymphoid tissue, highlighting their importance in immunity.

    B Lymphocyte Antigen Presentation

    • B lymphocytes present antigens to helper T-cells (CD4+) after being activated by dendritic cells.
    • This occurs during the humoral immune response.
    • B lymphocytes present extracellular antigens that bind to specific Ig receptors.
    • Presentation is facilitated by MHC II.
    • MHC II is always expressed on B lymphocytes but becomes inducible upon activation.
    • B lymphocytes are found in lymphoid tissue and blood.

    Antigen-Presenting Cells (APCs)

    • APCs are essential for T cell responses to protein antigens.
    • APCs capture, digest, and display antigens to T cells.
    • APCs convert protein antigens into peptides.
    • APCs display peptide-MHC complexes for recognition by T cells.
    • This process of converting proteins to peptides is called antigen processing.
    • APCs provide co-stimulatory signals that help activate T cells.

    Antigen Processing

    • Antigen processing converts protein antigens into peptides
    • MHC molecules are responsible for displaying these peptides to lymphocytes
    • Protein antigens in acidic vesicular compartments of antigen-presenting cells (APCs) generate MHC class 2-associated peptides
    • Protein antigens present in the cytosol generate MHC class 1-associated peptides

    Antigen Processing for MHC Class II Presentation

    • Antigen-presenting cells (APCs) internalize extracellular proteins through receptor-mediated endocytosis.
    • B cells use their B cell receptor (BCR) to internalize specific antigens.
    • Internalized proteins are processed within endosomal or lysosomal vesicles, which are acidic compartments containing proteolytic enzymes.
    • Endosomes and lysosomes combine to form phagolysosomes, further degrading proteins.
    • Proteins are degraded into peptides of 14-20 amino acids in length for MHC class II presentation.
    • Cathepsins are the primary enzymes responsible for protein degradation within these compartments.

    MHC Class II Antigen Processing - Step 3

    • MHC Class II alpha and beta chains are synthesized in the endoplasmic reticulum.
    • These chains are transported to the Golgi apparatus and then to endosomes.
    • An invariant chain (Ii) is associated with the MHC Class II molecules during transport.
    • In the endosomes, Ii dissociates, and processed peptides associate with the MHC Class II molecules.
    • The stable MHC Class II-peptide complexes are transported to the cell surface.
    • These complexes are then recognized by CD4+ T cells.

    Cytosolic Antigen Processing for MHC Class 1

    • Cytosolic proteins, including those from viruses, bacteria, and endocytosed antigens can be processed for MHC Class 1 presentation.

    • The proteasome, a large protein complex, degrades ubiquitinated proteins in the cytosol, generating peptides.

    • The transporter associated with antigen presentation (TAP) transports peptides from the cytosol to the endoplasmic reticulum (ER).

    • Within the ER, MHC Class 1 molecules assemble with peptides. The interaction of TAP with MHC Class 1 allows efficient peptide loading.

    • The MHC Class 1-peptide complex is transported through the Golgi apparatus to the cell surface.

    • The complex is recognized by CD8+ T cells, initiating an immune response.

    Antigen Presenting Cells and Microbial Products

    • Antigen presenting cells (APCs) enhance their presentation of antigens in the presence of microbial products.
    • APCs express Toll-like receptors (TLRs) that recognize and respond to microbial components.
    • TLR activation triggers a cascade of events that lead to:
      • Increased expression of major histocompatibility complex (MHC) molecules and co-stimulatory molecules, enhancing antigen presentation efficiency.
      • Production of cytokines that activate immune cells involved in the adaptive immune response.
      • Production of chemokines that recruit immune cells to the site of infection.

    Dendritic Cells as Primary APCs to Naive T Cells

    • Dendritic cells are the primary APCs responsible for presenting antigens to naive T cells.
    • Dendritic cells capture antigens in peripheral tissues, such as the skin or mucosal surfaces, and migrate to draining lymph nodes where they interact with T cells.
    • This antigen presentation to naive T cells initiates the adaptive immune response in peripheral lymphoid tissues.

    CD1 Antigen Presentation Pathway

    • CD1 molecules are a class 1-like non-polymorphic group of proteins
    • CD1 proteins are found on antigen presenting cells (APCs) and some epithelial cells
    • They function to present lipid antigens to T-cells
    • CD1-mediated antigen presentation is not MHC-restricted
    • NKT cells are an example of T-cells that recognize lipid antigens presented by CD1 molecules

    T Cell Responses

    • T cells are activated by the presentation of antigens by MHC class 1 or class 2 pathways.
    • CD4+ T cells are activated by exogenously acquired proteins presented by MHC class 2.
    • CD8+ T cells are activated by endogenously acquired proteins presented by MHC class 1.
    • The T cell receptor (TCR) recognizes antigens presented in the correct MHC context.
    • TCR is a member of the immunoglobulin superfamily and comes in two types: alpha-beta TCR and gamma-delta TCR.
    • Alpha-beta TCR is MHC dependent and found in lymphoid tissues.
    • Gamma-delta TCR is MHC independent and found on T lymphocytes at mucosal surfaces.
    • Each T cell expresses only one TCR specificity.
    • The CD3 complex is associated with the TCR and is composed of gamma, delta, epsilon, and Xi chains.
    • The CD3 complex is invariant and does not contribute to specificity.
    • The CD3 complex is necessary for TCR expression during T cell development.
    • The CD3 complex transmits signals to the nucleus after antigen interaction with the TCR.
    • T cells recognize peptides and are specific for amino acid sequences.
    • T cells recognize anchor residues in peptide sequences, and only recognize foreign peptides when presented in the MHC context.
    • Some T cells can recognize small chemicals called hapten conjugates.
    • T cells from one individual only recognize foreign peptides when they are bound to MHC molecules of that individual (self-MHC restriction).
    • CD4+ T cells are MHC class 2 restricted and recognize peptides from extracellular proteins.
    • CD8+ T cells are MHC class 1 restricted and recognize peptides from cytosolic proteins.

    B Cell Receptor (BCR)

    • BCR does not interact with MHC molecules.
    • BCR can recognize soluble and exposed antigens, including:
      • Peptides
      • Proteins
      • Nucleic acids
      • Polysaccharides
      • Lipids

    T Cell Activation

    • Co-stimulatory signals are necessary for complete T cell activation.
    • Signal 1: Antigen bound to MHC (specific signal)
    • Signal 2: Interaction between CD28 on T-cells and B7 (CD90/CD86) on antigen-presenting cells (APCs) (co-stimulatory signal)
    • Clonal anergy: Occurs when T cells are stimulated without co-stimulatory signals, resulting in a state of non-responsiveness.

    Antigen Presentation

    • T cell responses begin in peripheral lymphoid organs.
    • Dendritic cells are the primary antigen presenting cells (APCs) for naive T cells.
    • Dendritic cells are found in epithelia and tissues, capturing protein antigens and transporting them to draining lymph nodes.
    • Dendritic cells express receptors that capture microbes and respond to microbial signals.
    • Dendritic cells migrate to T-cell rich zones of lymph nodes through which naive T cells circulate.
    • Mature dendritic cells express high levels of co-stimulatory molecules required to activate naive T cells.

    Co-stimulation Regulation

    • Co-stimulatory molecules regulate T cell activation.
    • CD90/CD86 on APCs and CD28 on T cells lead to positive effects on T cell activation.
    • CD90/CD86 on APCs and CTLA4 on T cells lead to negative effects on T cell activation.

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