Immunology - OD 526 Medical Microbiology & Immunology I PDF

Welcome to Immunology OD 526: Medical Microbiology & Immunology I Educational Fair Use Notice Certain materials in this presentation are included under the educational fair use exemption of U.S. Copyright L...

Welcome to Immunology OD 526: Medical Microbiology & Immunology I Educational Fair Use Notice Certain materials in this presentation are included under the educational fair use exemption of U.S. Copyright Law (Title 17 U.S.C. Section 107). They have been prepared according to the multimedia fair use guidelines. Textbook materials are used with permission from John Wiley & Sons Ltd., Richard Coico, Elsevier Publishing, and Patrick Murray. Additional materials, such as content licensed through Creative Commons or Biorender images, have been attributed and appear in the References section of this presentation. Further use is prohibited.  Adaptive Immunity Part IV Welcome & Announceme nts  Who likes sushi? Generally speaking, how do you make it? OD 526  Sushi Preparation  Prepare the nori  Cut down the fish into smaller pieces and mix it with other ingredients and seasonings.  Spread the mixture on the nori  Roll the sushi OD 526  How are antigens prepared? Antigen processing and presentation: Antigen proteins are broken down into smaller peptide strands Peptide strands are loaded onto MHC molecules (Wrapped up in vesicles) MHC molecules on the cell surface display the peptide for T-cells to identify OD 526  Lesson Outcomes Identify the classes of Explain how MHC Discuss the variations major molecules bind to in MHC genes and histocompatibility peptides and create gene expression that complex (MHC) ligands that interact led to the polymorphic molecules and their with T-cells nature of the major role in presenting histocompatibility antigens to T-cells for complex recognition OD 526  MHC: Name, Role, and Structure OD 526  Major Histocompatibility Complex History Major histocompatibility comes from research into transplantations.  Mice experiments  Explored rules governing the acceptance or rejection of tissues  Discovered that histocompatibility was determined by more than one gene  changed to complex  The human MHC is known as HLA (human leukocyte antigen) OD 526  MHC: Antigen Presentation MHC Process: 1. Protein catabolized (processed) into peptides. 2. Selective binding of peptides to MHC inside the cell. 3. MHC + peptide migrate to the cell surface. 4. MHC + peptide recognized by TCR on T-cell surface. MHC Key Functions: OD 526  MHC: Antigen Presentation Three critical components of T-cell antigen recognition: OD 526 MHC Molecule Classes MHC Class I MHC Class II  Interact with CD8+ T-cells expressing CD8 co-receptor molecules  Interact with CD4+ T-cells  MHC class I is expressed on ALL expressing CD4 co-receptor nucleated cells (Not red blood molecules cells)  MHC class II only expressed on  Also interact with NK cells (KIR antigen-presenting cells and a receptor) few uncommon cell types OD 526  MHC Molecule Classes Many factors affect the expression of MHC class I and II molecules.  Cytokines enhance the expression of MHC molecules  Interferon (IFN) α, β, and γ upregulate MHC class I expression  Interferon (IFN) – γ upregulates MHC class II expression  Consequence of IFN – γ upregulation:  MHC class II expression induced on fibroblasts and endothelial cells (not normal expression) and increased on APCs  T-cell responses to infectious agents enhanced by MHC class I and II  Some viral infections and tumor development results in decreased expression of MHC molecules OD 526  MHC Class I & II Variability MHC class I and II molecules differ from individual to individual within a population.  Differences are genetically determined through alternative configurations of MHC genes MHC Molecular Variability Polymorphis Polygenicity m OD 526  MHC Class I & II Variability Polygenic The physical expression of a particular character is regulated ity by more than one gene Polygenic inheritance is a type of multifactorial inheritance influenced by a cumulative expression of multiple genes. MHC class I and II are coded for by multiple independent genes. HLA Complex:  Located on Chromosome 6  Site of MHC class I, II, and III genes OD 526  MHC Class I & II Variability Three independent genes code for MHC class I single-chain molecules:  HLA-A, HLA-B, HLA-C  Each HLA class I (MHC class I) molecule is expressed on every nucleated cell surface with a molecule called β2- microglobulin  (β2M gene is located outside of the HLA complex) Three different two-chain MHC class II molecules are coded on the HLA complex:  HLA-DP, HLA-DQ, HLA-DR  HLA class II (MHC class II) molecules OD 526 consist of an α and β strand  MHC Class I & II Variability HLA Class I HLA Class II Every cell has two sets of chromosomes (Maternal and Paternal):  Both maternal & paternal alleles are expressed  Result = every nucleated cell may express up to SIX D D different combinations of HLA A B C A B C DP DR DP DR Q Q class I molecules capable of binding peptides Possible Possible  Result = APCs may express D D A Combinations: A B B C C DP DPCombinations:DR DR up to SIX different Q Q combinations of HLA class II D D molecules capable of binding A B A C B C DP DP DR DR peptides Q Q OD 526  MHC Class I & II Variability Polymorph The existence of multiple alleles at a particular genetic locus results in variants of the gene product among different ism members of the species Multiple stable forms of each MHC gene exist in the population MHC is the body's (and population’s) most highly polymorphic gene system  Over 1,000 alleles of the MHC class I HLA-B and MHC class II HLA-DRB genes have been identified Extensive human MHC gene polymorphism makes it very unlikely that two random individuals will express identical sets of HLA class I or II molecules OD 526  MHC Class I Structure MHC Class I: A transmembrane glycoprotein expressed on the cell surface in non-covalent association with a small invariant (identical on all cells) polypeptide called β2 microglobulin.  β2M is encoded by a gene on a separate chromosome Referred to as the α or heavy chain, and comprises three extracellular Ig-like domains:  α1, α2, and α3  β2M has a structure homologous to a single Ig domain MHC class I plus β2M appear as a four-domain molecule:  α1 pairs with α2 at the top of the structure  α3 and β2M pair at the base of the molecule  CD8 co-receptor binds to the invariant α3 domain OD 526  MHC Class I Structure β2M and MHC class I are members of the Ig superfamily. Between α1 and α2, there is a deep cleft called the peptide-binding groove that can hold one 8-9 amino acid peptide chain. Every MHC class I molecule can bind to several different peptides, but only one at a time. OD 526  MHC Class I Structure The binding of peptide strands to MHC class I molecules is selective.  One MHC molecule will bind to certain peptides with high affinity  Anchor residues = closely related amino acids at certain positions in the active site  Preferentially bind to peptides of 8-9 amino acids in length  MHC class I molecules typically have two anchor residues  One MHC molecule can bind to many different sequenced peptides One to four amino acids on the peptide interface with the TCR.  A small number of peptide contacts are critical for TCR recognition OD 526  MHC Class II Structure MHC Class II:  A transmembrane glycoprotein expressed on the cell surface comprising two chains: α and β  Composed of variable (polymorphic) regions and invariable (non-polymorphic) regions  Expressed at the cell surface as a four-domain structure  α1 domain pairs with the β1 domain  α2 domain pairs with the β2 domain  CD4 co-receptor binds to the invariant β2 domain OD 526  MHC Class II Structure α and β chains have cytoplasmic tails and extracellular Ig-like domains.  Members of the Ig superfamily Contains a peptide-binding groove between the α1 and β1 domains.  Holds only one peptide that varies in length (12-17 linear amino acids) OD 526  MHC Class II Structure Each MHC class II molecule binds to peptides with specific anchor residues.  Typical MHC class II molecule has three (sometimes four) anchor residues  Due to structure, MHC class II molecules are also capable of binding a wide selection of peptides Between four and six amino acids in a peptide strand interface with the TCR. OD 526  MHC / T-Cell Coreceptor Binding MHC molecules can be divided into:  Polymorphic (Variable) Regions  Area in and around the peptide binding groove  Nonpolymorphic (Invariant) Regions  Area closest to the cell membrane The CD8 coreceptor on CD8+ T- cells binds to the α3 invariant domain region of all MHC class I molecules. The CD4 coreceptor on CD4+ T- cells binds to the β2 invariant OD 526 domain region of all MHC class II molecules.  MHC: Antigen Processing and Presentation OD 526  Exogenous Antigens With MHC Class II Complexes Exogenous antigens = Antigens from outside the host cell.  Taken inside the cell through endocytosis or phagocytosis  APCs take up exogenous antigens to prepare MHC class II molecules for presentation to T-cells Vesicles containing digestive enzymes break down antigens.  Proteases (cathepsins) and peptidases  Acid vesicles OD 526  Invariant Chain and CLIP Invariant Chain = Binds to the newly synthesized peptide-binding groove formed from joining the MHC class II α and β chains in the endoplasmic reticulum.  Acts like a placeholder, chaperone, or the lid on a container to prevent other molecules from binding CLIP:  Class II-Associated Invariant Polypeptide  Remnant of the invariant chain left in the peptide-binding groove after sequential degradation of the chaperone molecule  HLA-DM facilitates the removal of CLIP from the peptide-binding OD 526 groove  Exogenous Antigen Pathway Immunodominance (Selective binding):  MHC peptide binding is selective  Only those peptides that can induce a T-cell response in a particular person will be selected  Due to peptide-MHC binding selectivity, individuals who express different MHC molecules respond to different parts of the same protein OD 526  Endogenous Antigens With MHC Class I Complexes Endogenous antigens = Proteins synthesized inside a cell that are generally derived from pathogens.  Proteasomes (Enzymatic protein complex that degrades worn-out cellular components) in the cytoplasm encounter and engulf viral protein  The viral protein is broken down into peptides about 15 amino acids in length  Cytosolic enzymes (aminopeptidases) continue removing amino acids from the peptides or destroy them completely OD 526  TAP Peptide Transporter:  Two-chain peptide transporter associated with antigen processing  Takes peptides from proteasome and transfers them into the endoplasmic reticulum  Frequent target of viral inhibitors In the Endoplasmic Reticulum:  MHC class I and β2M chains are synthesized and joined  Chaperone molecules stabilize the MHC class I structure as the complex is transported through the cell  Only those peptides that bind to MHC class I OD 526 molecules trigger CD8+ T-cell responses  Cross-Presentation APCs (especially dendritic cells) use cross-presentation to take peptides derived from exogenous protein antigens and present them to CD8+ T-cells.  Cross-presentation plays an important role in activating CD8+ T-cells to respond to tissue cells infected by a virus that is not taken up by APCs  In rare circumstances, endogenous antigens may associate with MHC class II molecules through autophagy, an intracellular pathway in which proteins in the cytoplasm are transported to lysosomes for degradation. OD 526  Overall Exogenous & Endogenous Pathways  Proteins from bacteria, most viruses, allergens, and completely harmless antigens trigger CD4+ T-cell responses.  Only infectious pathogens (viruses) create epitopes via endogenous or cross- presentation pathways and are presented by MHC class I molecules that activate CD8+ T-cells.  Generally (but not always), infectious agents activate both CD4+ and CD8+ T-cells because of the action of APCs. OD 526  Other Antigens That Activate T-cells OD 526  Other Antigen T-cell Activation Responses Superantigens:  Predominantly exotoxins (some endotoxins) produced by pathogenic bacteria  Not processed by the body  Intact molecule binds to MHC class II and TCR β chain of CD4+ T-cell  Able to activate more than 10% of the total T-cell population  Diseases associated with superantigen exposure are partially due to hyperactivation of the immune system and subsequent release of high levels of cytokines. Lipids/Glycolipids:  Lipids and glycolipids are presented by a family of molecules known as CD1 (CD1a – CD1d)  Expressed by APCs like macrophages and dendritic cells OD 526  HLA Region Genes MHC genes are tightly linked and passed down as a unit.  Class I Region:  Polymorphic MHC class I genes (HLA-A, HLA- B, HLA-C)  Less polymorphic MHC class Ib genes (HLA- E, HLA-F, HLA-G)  Not well understood; HLA-E and HLA-F involved in NK cell presentation; HLA-G involved in placental regulation of fetus  MICA and MICB code for stress-induced products that interact with NK and γδ T-cells OD 526  HLA Region Genes MHC genes are tightly linked and passed down as a unit.  Class II Region:  Polymorphic MHC class II genes (HLA-DP, HLA-DQ, HLA-DR)  Each contains an A gene and a B gene, coding for the α and β chains  HLA-DPA codes for DPα; HLA-DPB codes for DPβ  Some people have more than one DRB gene (DRB1, DRB2, DRB3, DRB4)  Most individuals are heterozygous for their HLA genes  HLA-DM = Codes for the molecule involved in exogenous antigen processing in MHC class II pathway OD 526  MHC Gene Inheritance OD 526  Intermission: 10-Minute Break OD 526  Adaptive Immunity Part V Welcome & Announceme nts  “Two Person Rule” Dendritic Cell T- Cell OD 526  Lesson Outcomes Describe the Examine the effector Discuss the cytotoxic importance of the functions of CD4+ T- functions of CD8+ T- two-signal model for cells, including cells, including the the activation of T- activating release of cytotoxic cells. macrophages for granules that lead to phagocytosis, apoptosis of the promoting B-cell targeted cell. antibody production, and recruiting or OD 526 activating other  Two-Signal Model of T-Cell Activation OD 526 Second Signals At least TWO signals must be delivered to the antigen-specific T-cell for activation to occur: Involves Signal 1 Signal 2 binding of Involves co- the antigen stimulators peptide to (B7 Family – the TCR, CD28, etc.) which must and other be cell surface presented in molecules the expressed appropriate on APC and manner by T-cells APCs *(Co-stimulator = stimuli that enhances signal 1 when the signal is relatively weak.) In the absence of a second signal, the first signal alone turns OFF rather than on. OD 526  Dendritic Cells Plasmacytoid Dendritic Cells  Synthesize interferons (IFNs) α and β in early immune response  Major contributors to the innate Dendritic Cells: phase of the pathogenic response  Named for its star-shaped appearance  Principal APCs for initiating primary Myeloid Dendritic Cells responses in CD4+ and CD8+ T-cells  Induction role in T-cell responses (The  Express both MHC class I and II term “dendritic cell” refers to molecules myeloid subset)  Found in tissues (primary/secondary  Same hemopoietic precursor as lymphoid organs; close to pathogen monocytes and macrophages in the bone marrow entry sites) and circulation OD 526  Dendritic Cells Dendritic Cells:  Migrate from tissues to lymph nodes after encountering an antigen “Lymphoid “Migratory” Organ Resident” Tissue Lymphatic Vessel OD 526  Pattern Recognit ion Receptor s (PRRs) Not interacted with antigen Low levels of MHC class II OD 526  Mature Dendritic Cell Binds to a ligand expressed by HEVs One of the most important cytokines OD 526  Important Points: Without signals induced by pathogens, Adjuvant: immature dendritic cells Molecules that express low levels enhance the of co-stimulator activation of receptors and APCs by MHC class II inducing the molecules. expression of Antigens that do co-stimulatory not induce high receptors levels of co- stimulator function do not activate naïve T- cells. OD 526  CD4+ T-Cell Activation Peptide/MHC and TCR:  First signal for the activation of the CD4+ T-cell  TCR can detect a small number of foreign peptides (Very sensitive)  As low as four peptide molecules  Problem: Interaction has low affinity and interacting components dissociate rapidly  Must sustain contact for several hours to activate naïve CD4+ T-cell  Solution: Reaction must be stabilized to enhance affinity OD 526  CD4+ T-Cell Activation Co-Receptor:  CD4 co-receptor interacts with the nonpolymorphic region of an MHC class II molecule  β2 Domain of the MHC class II molecule  Greatly enhances the ability of T-cells to respond to antigens, lowering the stimulation threshold  CD4-MHC class II interaction increases response 100-fold!  CD4 plays an important role in T- cell signal transduction OD 526  CD4+ T-Cell Activation Co-Stimulator Pairs:  Deliver the second signals that enhance the first signal  (MHC class II + peptide with TCR)  CD28 (T-cells) / B7 Family (APCs)  Activates T-cells to synthesize cytokine interleukin-2 (IL-2) = T-cell growth factor  CTLA-4 (CD152; CD28 family) = When interacts with B7 molecules, it sends a negative signal to terminate the activation response  CD40 ligand (CD40L or CD154)  Interacts with CD40 on macrophages, dendritic cells, and B- cells  Enhances B7-CD28 co-stimulator interaction  Role in T-cell dependent class switching and somatic hypermutation of B-cells OD 526  CD4+ T-Cell Activation Adhesion Pairs:  Strengthen and stabilize the APC/T-cell interaction  Must maintain contact for several hours; Allows T-cell to “scan” the APC for the appropriate MHC class II + peptide  Key adhesion molecules expressed –  CD2 = Binds to LFA-3 (CD58) expressed on APCs (and many other cells)  Expressed almost exclusively on T-cells, both mature and immature, and NK cells  LFA-1 (CD11aCD18) = Member of the two-chain integrin family  Binds to ICAM-1 (CD54) expressed on endothelial cells, macrophages, and dendritic cells  ICAM-3 (CD50) = Binds to LFA-1 expressed on APC OD 526  CD4+ T-Cell Activation and Function OD 526  T-Cell Activation Intracellular Events Recognition of antigens at the cell surface triggers multiple intracellular cascades that transmit signals in an ordered manner from the cell’s surface to the nucleus.  Reorganizes the structure of the internal cytoskeleton and outer cell membrane  Some events take seconds to complete; others take minutes or hours OD 526  T-Cell Activation Intracellular Events Immunologic Synapse:  Area of contact between the MHC class II molecule and TCR  Required for sustained intracellular signaling (approx. 8 hours of contact)  Incorporates the MHC class II + peptide and TCR, CD4, pairs of co-stimulator and adhesion molecules  Synapse formation and development is dynamic (will change with time after contact) OD 526  T-Cell Activation Intracellular Events Assembly and Activation of Signaling Complexes at the Cell Membrane  Fyn (Cytoplasmic CD3 and Zeta chains)  Lck (Cytoplasmic CD4 regions)  Fyn and Lck are activated by membrane protein CD45 (not in pict.)  CD45 removes inhibitory phosphate groups  Fyn and Lck bind to ITAMs and phosphorylate them OD 526  T-Cell Activation Intracellular Events Assembly and Activation of Signaling Complexes at the Cell Membrane  ZAP-70 (Docks with phosphorylated ITAMS)  Activated ZAP-70 connects to LAT (Linker for Activation of T-cells)  SLP76 and Phospholipase Cγ1 (PLCγ1) activate the reorganization of actin molecules within the cell  Summary: a multiprotein signal transduction complex is assembled to activate transcription factors that enter the nucleus and alter gene expression OD 526  Response proliferation Activated CD4+ T-cell transcribes and translates IL-2Rα and IL-2 genes.  Leads to the development of high-affinity receptors on the cell membrane  IL-2 is a growth factor for T-cells and acts on any cell that expresses high-affinity IL-2 receptors  Result: rapid expansion of activated CD4+ T-cells (Causes swelling of the lymph node) OD 526  Response Termination Several pairs of molecules expressed on T-cell/APC surfaces convey negative signals to the activated T- cell.  Turns off activation response  Signals include:  Inactivation molecules  Inhibition of the cell cycle (mitosis)  Induction of T-cell exhaustion  CTLA-4/B7:  Initiated within 24-48 hours after initial activation  CTLA-4 outcompetes CD28 for B7 binding  ITIMs are recruited and activate phosphatases to remove phosphate groups from the signal transduction molecules  Programmed cell death – (PD)-1 (CD279) OD 526  CD4+ T-Cell Function Cytokine Synthesis:  Cytokines present during the activation of naïve CD4+ T-cells drive the differentiation of the CD4+ cell into subsets: TH1, TH2, TH17, and TREG  Each subset of CD4+ T-cell synthesizes a unique pattern of signature cytokines OD 526  CD4+ T-Cell Function TH1:  Develop in the presence of IL-12; synthesize IFN-γ and IL-2 OD 526  CD4+ T-Cell Function TH2:  Respond to parasitic worms and allergens  Synthesize IL-4, IL-5, and IL-13 OD 526  CD4+ T-Cell Function TH17:  Combination of transforming growth factor β (TGF-β), IL-21, and IL-23 stimulate TH17 development  Secrete family of IL-17 cytokines  Are proinflammatory OD 526  CD4+ T-Cell Function TREG: Inhibit or suppress the differentiation and function of the other subsets of CD4+ cells OD 526 OD 526 OD 526 OD 526  CD8+ T-Cell Activation and Function OD 526  CD8+ T-Cell Activation and Function CD8+ T-Cell:  Principal function – kill cells that have been infected with viruses and bacteria  Referred to as Cytotoxic T Lymphocyte (CTL)  Cell killed by CTL known as a target  Interact with MHC class I molecules  Follows similar activation and differentiation pathways as CD4+ OD 526 Granzymes OD 526  Function of Other Subsets of T-Cells Memory T-Cells NKT Cells Gamma-Delta T- Innate Lymphoid Cells Cells OD 526  LESSON SUMMARY  MHC molecules play a crucial role in the response of T-cells to antigens.  MHC molecules selectively bind peptides derived from protein antigens and present them to T-cells with the appropriate receptor. T-cell responses are MHC-restricted.  MHC is a complex set of genes inherited as a unit. Two major categories of cell surface transmembrane molecules exist MHC class I and MHC class II.  MHC class I is expressed on all nucleated cells in association with a small invariant peptide: β2 – macroglobulin. It interacts with the TCR of a CD8+ T- cell. (Class I restricted)  MHC class II is expressed only on cells that present antigens to T-cells, such as APCs. It interacts with the TCR of a CD4+ T-cell. (Class II restricted)  MHC molecule expression occurs in response to cytokines. This expression enhances T-cell responses directed at a pathogen. Viruses inhibit class I expression and attempt to subvert the T-cell response.  The same MHC class I and II molecules are expressed on all body cells, but different individuals express different versions of those molecules. This diversity arises because of polygenicity and polymorphism. OD 526  LESSON SUMMARY  Activation of naïve CD4+ or CD8+ T-cells requires two signals. (First signal: MHC molecule + peptide bonds with TCR; Second signal: Paired sets of co-stimulator molecules on APC and T-cell)  Dendritic cells are the principal APC to activate naïve T-cells.  Key APC/T-cell interactions are: MHC + peptide with TCR, co-receptor with MHC (MHC class 1 with CD8, MHC class II with CD4), co-stimulator pairs (B7/CD28, CD40/CD40L), and adhesion molecules.  Activation of CD4+ and CD8+ T-cells involves a cascade of events spreading from the contact area between the APC and T-cell (the immunologic synapse).  The most important genes transcribed and translated in an activated CD4+ T-cell are those coding for cytokines. (ex. IL-2 and cytokine receptors)  Four major subsets of cytokine-synthesizing CD4+ T-cells: T H1, TH2, TH17, and TREG.  CD8+ cells interact with and kill target cells infected by microorganisms. OD 526 Next Class: Immunogens and Antigens Part I Before our next session: Complete Pre-Lesson Reading #11 and Quiz on Canvas OD 526 References Coico R., Sunshine, G. Immunology: A Short Course. 7th Ed. Wiley/Blackwell; 2015:1-406. Mahmoudi M. Immunology Made Ridiculously Simple. MedMaster, Inc.; 2021:1-78. Todd I., Spickett G., Fairclough L. Lecture Notes: Immunology. 7th Ed. Wiley Professional, Reference & Trade; 2016:1-230. Available from: vbk://9781118451632 Hato T, Dagher PC. How the Innate Immune System Senses Trouble and Causes Trouble. CJASN Aug. 2015. 10 (8): 1459-1469; DOI: 10.2215/CJN.04680514 https://cjasn.asnjournals.org/content/10/8/1459 Kindt TJ., Osborne BA., Goldsby RA. Kuby Immunology. 6th Ed. W. H. Freeman and Company; 2007: 1-554. Images used in this presentation are from Microsoft PowerPoint stock images or under license from Shutterstock.com.