Brown University BIOL 0380 Fall 2015 Ecology and Evolution of Infectious Disease A Necessarily Incomplete Overview of the Immune System Friday, September 25 th, 2015 Richard Bungiro, Ph.D. Richard_Bungiro@Brown.edu
Pathogens are all around us Parasite: Protozoa 2
The Immune System Has the ability to resist/resolve diseases caused by a great diversity of infectious agents: Viruses -> polio, smallpox, influenza, measles, HIV Bacteria -> tuberculosis, tetanus, pertussis Fungi -> thrush, ringworm Parasites: Protozoan -> malaria, leishmaniasis Helminths (worms) -> tapeworms, flukes, roundworms There is compelling evidence that the immune system may also protect against certain types of cancer Operates at two basic levels to prevent infection and (when infection occurs) restrict progression to disease & death Innate immunity Adaptive (acquired) immunity 3
Immunity Influences the Outcome of Infectious Disease EXPOSURE STOP NOT INFECTED INFECTED NO DISEASE RESOLUTION OF INFECTION PROTECTIVE IMMUNITY STOP DISEASE STOP CHRONIC INFECTION STOP DEATH 4
Innate Immunity Basic resistance mechanisms that an individual is born with (requires no prior experience) Employs anatomical barriers, phagocytic cells, soluble mediators, and inflammatory responses First line of defense in vertebrates, and the only form of defense in other multicellular organisms Acts quickly (minutes to hours) Fixed specificity that broadly recognizes molecular patterns shared by various classes of microorganisms Is not adaptive (does not improve with experience) Limits infections and generates signals that activate and enhance the adaptive immune response 5
Skin and Epithelial Barriers to Infection 6
The Inflammatory Response 7
Elimination of Bacteria by Phagocytosis Phagocytosis may be enhanced by opsonins, which are various immune factors that bind to microorganisms and are in turn bound by receptors on phagocytes 8
Innate Antiviral Molecules Such As TRIM5α, Tetherin and APOBEC May Target HIV ivanovlab.uthscsa.edu/images/trim5alpha.jpg www.nature.com/nature/journal/v451/n7177/ images/nature06364-f1.2.jpg www.nature.com/nri/journal/v4/n11/fig_tab/ nri1489_f1.html 9
Pattern Recognition Receptors If a pathogen gains entry, its Pathogen-Associated Molecular Patterns (PAMPs) interact with Pattern Recognition Receptors (PRRs) of the host PAMPs are not found in the host and are broad features of pathogens that are essential for survival, reproduction, etc. Immune cells employ membrane-bound PRRs to bind PAMPs, leading to increased phagocytic activity, cytokine production and antigen processing/presentation Soluble PRRs in the blood and other fluids bind to microorganisms and activate various immune effector functions Although PRRs are a feature of the innate immune system, their engagement by pathogens generates signals that influence the development of adaptive immune responses 1 0
Pattern Recognition: A Cartoon Example Daffy and Donald each possess recognizable duck patterns 11
Pattern Recognition: An Actual Example Each has unmethylatyed CpG in its DNA Each has LPS in cell wall Escherichia coli Salmonella typhimurium 1 2
Pattern Recognition Receptors 1 3
TLRs Recognize a Wide Range of Microbial Patterns 1 4
If so many effective innate mechanisms exist, why do we need anything else? Infectious organisms are smart They have much shorter generation times (i.e. rapid replication) Their genomes are more plastic (changeable) than ours Many have evolved ways to circumvent our innate defenses Our adaptive immune response helps to level the playing field The cells that mediate acquired immunity are capable of rapid clonal expansion The genes which encode receptors of the adaptive system(antigen recognition molecules; ARMs) are also plastic Adaptive responses can counteract many of the evasion strategies that microbes deploy 1 5
Adaptive (Acquired) Immunity Enlisted if innate immunity fails to eliminate an invading pathogen Specifically recognizes and selectively eliminates pathogens by targeting their antigens Requires several days to a week for optimal induction the first time a particular pathogen is encountered, but is generally faster, stronger better in later encounters with that pathogen Mediated by lymphocytes (B and T cells) and antigen-presenting cells (macrophages, dendritic cells and B cells) Functions through humoral (antibody mediated) and cellular mechanisms 1 6
Characteristics of the Adaptive Immune Response SPECIFICITY Capacity to distinguish among various molecules (antigens) produced by pathogens Mediated by antigen recognition molecules (ARMs; Immunoglobulins and T Cell Receptors) DIVERSITY Capacity to react with an almost limitless variety of antigens Millions of different Igs and TCRs can be produced from a relatively small amount of DNA 1 7
Characteristics of the Adaptive Immune Response MEMORY Ability to remember a previous encounter with a pathogen The secondary response is induced more quickly and is considerably more vigorous than the original ( primary ) response SELF/NONSELF RECOGNITION Ability to respond to and eliminate foreign pathogens without bringing harm to one s own tissues 1 8
Memory & Specificity in Adaptive Immunity 2 Anti-A Response 1 Anti-A Response Ag B 1 Anti-B Response Note that the secondary (memory) anti-a response is specific to Antigen A 1 9
Antigens T epitope B epitope Macromolecules produced by pathogens that are recognized and responded to by the immune system - Proteins - Nucleoproteins - Glycoproteins - Hapten-protein complexes - Lipoproteins - Polysaccharides Contain antigenic determinants (epitopes) B lymphocytes generally recognize the three dimensional (3D) conformations of antigens T lymphocytes require protein Ags to be processed into linear peptide fragments for presentation by antigen presenting cells 2 0
HIV HPV Influenza Hepatitis A 2 1
(helper) (cytotoxic) Antigen Recognition Molecules Mediate Adaptive Immune Responses Antibodies (Abs) Also known as immunoglobulins (Igs) Produced by B lymphocytes (B cells) May be surface bound or secreted Recognize native antigens T Cell Receptors (TCRs) Produced by helper and cytotoxic T lymphocytes (T cells) Are surface bound, not secreted Recognize processed antigens presented on specialized molecules known as MHC 2 2
Generation of an Adaptive Immune Response Occurs Through 3 Steps 1) Hematopoiesis Process by which immune (white) cells and red blood cells are generated The bone marrow is the source of all blood cells 370 billion white cells must be produced per day by an average human to maintain steady-state levels 2) Induction (Activation) Process by which immune cells are activated by foreign antigen and selectively expanded 3) Antigen Elimination Process by which foreign antigen (and the pathogen that produced it) is removed from the body through various effector mechanisms 2 3
Hematopoiesis Lymphoid progenitor cell Y II Natural Killer Cells B-cell T-cell Lymphocytes rearrange DNA to generate ARMs Multipotential Hematopoietic Stem Cell (MHSC) in bone marrow (self-renewing) Myeloid progenitor cell Macrophages, Dendritic Cells Granulocytes RBCs, Platelets 2 4
Morphology of Blood Cells Blood Cell Counts: (per mm 3 ) Red Blood Cells (RBCs or erythrocytes) 5.0 x 10 6 Platelets -> 2.5 x 10 5 White Blood Cells (WBCs or leukocytes) 7.3 x 10 3 total: 50-70% neutrophils 20-40% lymphocytes 1-6% monocytes 1-3% eosinophils <1% basophils 2 5
What Do They Really Look Like? Red Blood Cells Platelets Lymphocyte Monocyte Neutrophil Eosinophil Basophil Granulocytes 2 6
The Lymphoid System Consists of primary and secondary lymphoid organs Bone Marrow Thymus Spleen Lymph Nodes Mucosal- Associated Lymphoid Tissue (MALT) Lymphatic Vessels Sites where immune cells are generated Sites where immune cells are activated 2 7
Lymphocyte Development & Migration BONE MARROW HSC prob preb B LYMPH NODES prot BLOOD SPLEEN THYMUS prot pret T MALT Primary Lymphoid Organs Secondary Lymphoid Organs 2 8
B Lymphocytes Prior to maturity, B cells rearrange their antibody genes Mature naive B cells express surface Ig (IgM & IgD) Upon specifically binding antigens, B cells internalize, process, and present peptides to T cells using MHC Upon interaction with a T cell specific for the processed antigen, B cells become activated and differentiate: Plasma cells: secrete antibody, generally short-lived Memory cells: long-lived, allow secondary response The B-T interaction is critical for optimal Ab production Following activation, B cells can undergo class switching so that IgG, IgA, and IgE may be produced Activated B cells also undergo affinity maturation of their antibodies (improves Ag-binding) 2 9
T Lymphocytes Prior to maturity, T cells rearrange their TCR genes in the thymus and express both CD4 and CD8 T cells then undergo selection, commit to CD4 or CD8 expression, and leave the thymus Upon interaction with antigen presenting cells presenting specific processed antigen, T cells become activated and differentiate: Effector cells: mediate Ag clearance, generally short-lived Memory cells: long-lived, facilitate secondary responses Major T cell effector subsets T helper (Th) cells -> generally CD4 + ; help for antibody, activate macrophages and promote T cytotoxic responses T cytotoxic cells (CTLs) -> generally CD8 + ; kill foreign, virus-infected or tumor cells 3 0
The B-T Interaction Is Critical for the Activation of B Cells 3 1
Clonal Expansion of Activated B Cells 3 2
Antibodies Also known as immunoglobulins (Igs) or gamma globulins Produced by B cells May be surface bound or secreted Recognize native (unprocessed) antigens Ag Ag Consist of heavy (H) and light (L) chains with variable and constant regions; basic formula H 2 L 2 H and L chains encoded by separate genes Ig genes must rearrange before functional antibodies can be produced Five major classes: IgM, IgG, IgA, IgE, IgD 3 3
Gene Rearrangement Generates Antibody Diversity 3 4
The Five Major Classes of Antibody IgG IgG1, IgG2, IgG3, IgG4 Most abundant class in blood Activates complement and binds phagocyte receptors Crosses the placenta IgA IgA1, IgA2 Secreted at mucosal surfaces as a dimer Found in breast milk, tears, saliva IgM First isotype produced Excellent complement activator Secreted as a pentamer IgE Mediates allergies Defense against worms IgD May mediate respiratory immunity 3 5
T Cell Receptors (TCRs) Produced by T lymphocytes Surface bound, not secreted Recognize processed peptide antigens presented on MHC molecules Heterodimer of ab or gd chains Like antibodies, each chain has a variable and constant region Also undergo gene rearrangement prior to expression, generating great diversity 3 6
Major Histocompatibility Complex (MHC) Molecules Also known as the Human Leukocyte Antigens (HLAs) Surface bound, not secreted Present processed protein antigens to T cells Class I MHC -> produced by almost all nucleated cells, present endogenous Ags (synthesized inside the cell) Class II MHC -> produced by professional antigen presenting cells (DCs, MFs, B cells), present exogenous Ags (acquired from outside the cell) Peptide 3 7
The Role of MHC Molecules in Antigen Recognition by T Cells 3 8
The Role of MHC Molecules in Antigen Recognition by T Cells 3 9
Polymorphic Amino Acids in the Peptide-Binding Cleft of MHC I These domains make up peptide-binding region 4 0
MHC and Immune Responsiveness There are multiple genes that encode MHC, and each gene is highly polymorphic (i.e. many alleles exist in the population) Experiments using synthetic peptides have demonstrated that adaptive immune responses are heavily influenced by the efficiency of antigenic peptide binding by MHC, allowing for T cell help to B cells IMPORTANT POINT: Even if you have a normal number of B and T cells that could respond to a given Ag, if peptides from this Ag cannot bind your MHC you are unlikely to respond efficiently Thus MHC alleles can have a major effect on the outcome of infectious disease at both the individual and population level 4 1
Antigen Elimination (Effector) Phase: Antibody-Dependent Neutralization Prevent pathogen (or toxin) from binding Examples: tetanus toxin, polio virus Agglutination Bacteria or viruses may be physically clumped together and immobilized Opsonization Coating of Ags or target cells with Abs and complement proteins makes them more likely to be engulfed by phagocytic cells 4 2
Antigen Elimination (Effector) Phase: Antibody-Dependent Complement-dependent lysis May act against cells or enveloped viruses Assembly of complement components into a Membrane Attack Complex (MAC) Antibody-Dependent Cell-mediated Cytotoxicity (ADCC) Targeting of effector cells to foreign cell by receptors for the Fc portion of bound antibodies 4 3
Neutralization of Intestinal Pathogens by Secretory IgA in the Gut MILK X GUT Intestinal epithelium 4 4
The Role of Antibody and Complement in Opsonization Virus Virus + Ab Virus + Ab & Complement 4 5
Complement-Mediated Lysis The Membrane Attack Complex Live cells Lysed cells 4 6
Antibody Dependent Cell- Mediated Cytotoxicity (ADCC) Cytotoxic Cell Granule Contents Cell Lysis Fc receptor Target Cell 4 7
Types of Effector T Cells Antigen Recognition Cell Types Peptide + MHC Class II CD4 + T Cells Peptide + MHC class I CD8 + T Cells T H 1 T H 2 Cytotoxic T Lymphocytes (CTLs) Role in Host Defense Mac Help (DTH) CTL Help B Cell Help Elimination of altered self cells (i.e. virus infected cells, tumor cells) 4 8
Induction of Target Cell Apoptosis by CTLs Perforin/Granzyme pathway: 24-48 hours after activation, CD8 + T cells produce granules containing: - Perforin - a pore forming protein - Granzymes - proteolytic enzymes found in the granules which induce apoptosis Fas/FasL pathway: - Fas ligand (FasL) is found on the cell surface of the CTL - Fas ligand binds to Fas expressed on target cells and Fas signaling induces apoptosis in that cell 4 9
Two Pathways of Target-Cell Apoptosis Induction by CTLs Perforin pores in the surface of a target cell 5 0
Summary of Adaptive Immune Responses T epitope B epitope MHC II TCR Helper T cell Activated Th cell ANTIGEN Antigen Presenting Cell (APC) B Cell IL-4 Peptide * Activated Th cells IL-2 * Viral Peptide * IFN-γ Activated Macrophage (DTH) TCR MHC I Plasma Cell (effector B) Secreted Antibodies Killer T cell * Killing Virus-infected Cell 5 1
Immune Responses Against Viruses Innate mechanisms - Alternative complement pathway for free virus - NK cells or IFN-a/b for infected cells Antibodies & Complement - Abs block target cell binding (neutralization) - C'-mediated lysis of enveloped viruses (via MAC) - FcR & C3R promote phagocytosis - ADCC of virally infected host cells Th1 polarization: cytokines like IL-2, IFN-g and TNF promote an antiviral state, activate CTLs and macrophages CTLs: recognize virus-infected host cells (MHC-I+peptides) & lyse them 5 2
Cellular Activation in Response to Viral Infection 5 3
Evasion of Host Defenses by Viruses Inhibition of IFN-a/b action - Hepatitis C inhibits translation of ISG mrnas Inhibition of antigen presentation - Herpes simplex virus (HSV) inhibits TAP Interference with MHC expression - Adenovirus and cytomegalovirus (CMV) -> MHC I - CMV, measles, HIV -> MHC II Interference with complement activation - Vaccinia -> secretes inhibitor of C4b - HSV -> binds and inactivates C3b Antigenic variation affecting both B & T-cell epitopes - Influenza -> antigenic drift and shift - Rhinovirus, HIV -> error-prone polymerase General immune suppression - Mumps, measles, EBV, CMV, HIV -> infect immune cells - EBV -> IL-10 analogue downregulates Th1 responses 5 4
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