Medical Microbiology & Immunology Guri Tzivion, PhD Extension 506 MICR 600: Fall 2015 Windsor University School of Medicine

Questions on innate immunity?

Two kinds of the immune system:  Innate immunity  Adaptive immunity

The various types of immune cells

- Monocyte/Macrophage - Dendritic cell (DC) - Polymorphonuclear granulocytes (PMN): Neutrophil, Eosinophil, Basophil - Mast cell Classes of innate immune cells

1.Cells of a tissue injured by physical damage or bacteria release chemical signals such as histamine and prostaglandin 2.In response to the signals, nearby capillaries dilate and became more permeable. Fluid and clotting elements move from the blood to the site and clotting begins. 3.Chemokines released by cells attract phagocytic cells from the blood 4.Phagocytic cells absorbe pathogens and cell debris promoting tissue healing

Pathogen PHAGOCYTIC CELL Vacuole Lysosome (contains degrading enzymes)Phagocytosis

Inflammatory Responses  The inflammatory response, such as pain and swelling, is brought about by molecules released upon injury or infection  Mast cells release histamine, which triggers blood vessels to dilate and become more permeable  Activated macrophages and neutrophils release cytokines, signaling molecules that enhance the immune response and induce fever

Inflammatory Responses  Inflammation can be either local or systemic (throughout the body)  Fever is a systemic inflammatory response triggered by pyrogens released by macrophages and by toxins from pathogens  Septic shock is a life-threatening condition caused by an overwhelming or unbalanced inflammatory response

MICR 600 MDIII Immunology Class 2 Adaptive Immune Responses Antigens & Antibodies Cell-mediated Immunity

Pathogens (such as bacteria, fungi, and viruses) INNATE IMMUNITY (all animals) Rapid response Recognition of traits shared by a broad range of pathogens using a small set of receptors Recognition of traits specific to particular pathogens using a large array of receptors Slower response Barrier defenses: Skin Mucousal membranes Secretions Internal defenses: Phagocytic cells Natural killer cells Antimicrobial proteins Inflammatory response Humoral response: Antibodies defend against infection in body fluids Cell-mediated response: Cytotoxic cells defend against infection via cells ADAPTIVE IMMUNITY (vertebrates only) The Immune Response

 Adaptive immunity, or acquired immunity, develops after exposure to agents such as microbes, toxins, or other foreign substances  It involves a very specific response to the pathogens involving surface receptors and antibodies  The adaptive response relies on two types of lymphocytes (white blood cells): T and B cells  T cells mature in the thymus above the heart and B cells mature in bone marrow and spleen

Thymu s Peyer’s patches (small intestine) Appendix (cecum) Adenoid Tonsils Lymphatic vessels Spleen Lymph nodes Lymph node Blood capillary Interstitial fluid Tissue cells Lymphatic vessel Masses of defensive cells

 Antigen: any substance that can elicit a response from a B or T cell. Has a specific receptor on a corresponding B or T cell.  Exposure to a pathogen will activate a specific set of B and T cells that express surface receptors specific for antigens present on the pathogen  The accessible part of an antigen that binds to the antigen receptor is called an epitope

Antigen receptors Mature B cellMature T cell T cell and B cell antigen receptors

Antigen Recognition by B Cells and Antibodies  The B cell antigen receptors are a Y-shaped molecules with two heavy chains and two light chains  The constant regions of the chains vary little among B cells, whereas the variable regions differ greatly  The variable regions provide antigen specificity

Cytoplasm of B cell Antigen- binding site B cell antigen receptor B cell Light chain Disulfide bridge Antigen- binding site Variable regions Constant regions Transmembrane region Heavy chains Plasma membrane CC C C V V V V

 Binding of a B cell antigen receptor to an antigen is an early step in B cell activation  This gives rise to cells that secrete a soluble form of the protein called an antibody or immunoglobulin (Ig)  Secreted antibodies are similar to the B cell receptors but lack the transmembrane regions that anchor the receptors in the plasma membrane

Antibody Antigen receptor B cell Antigen Epitope Pathogen (a) B cell antigen receptors and antibodies Activation of B cells by pathogens and antibody production

Antibody C Antibody B Antibody A Antigen B cell antigen receptor specificity Epitope

 Each T cell receptor consists of two different polypeptide chains  The tips of the chains form a variable (V) region while the rest form the constant (C) region  T cell and B cell antigen receptors are functionally different Antigen Recognition by T Cells

T cell antigen receptor T cell Cytoplasm of T cell Plasma membrane Disulfide bridge Antigen- binding site Variable regions Constant regions Transmembrane region V V C C

 T cells bind to antigen fragments displayed or presented on host cells: antigen- presenting cells (APC)  These antigen fragments are bound to cell- surface proteins called major histocompatibility complex (MHC) molecules  MHC molecules are host proteins that display the antigen fragments on the cell surface

 In infected cells, MHC molecules bind and transport antigen fragments to the cell surface, a process called antigen presentation  T cells can then bind both the antigen fragment and the MHC molecule  This interaction is necessary for the T cell to get activated and participate in the adaptive immune response

Displayed antigen fragment MHC molecule Antigen fragment Pathogen Host cell (macrophage or dendritic cell) T cell T cell antigen receptor Antigen recognition by T cells

A closer look at antigen presentation Antigen fragment MHC molecule Host cell Top view

B Cell and T Cell Development  The adaptive immune system has four major characteristics:  Diversity of lymphocytes and receptors  Self-tolerance; lack of reactivity against “self” molecules  B and T cells proliferate after activation  Immunological memory

Generation of B and T Cell Diversity  By combining variable elements, the immune system assembles a diverse variety of antigen receptors  The immunoglobulin (Ig) gene encodes one chain of the B cell receptor, however, many different chains can be produced from the same gene by rearrangement of the DNA

DNA of undifferentiated B cell DNA of differentiated B cell Recombination deletes DNA between randomly selected V segment and J segment Functional gene Transcription RNA processing Translation pre-mRNA mRNA Light-chain polypeptide Antigen receptor B cell Variable region Constant region Poly-A tail Cap V 39 J5J5 J5J5 V 37 V 38 V 39 V 37 V 38 V 39 V 40 J5J5 J5J5 J4J4 J3J3 J2J2 J1J1 Intron V C C C C C C C C C V V V V Hyper Variable Region

Origins of Self-Tolerance  Antigen receptors are generated by random rearrangement of DNA, each cell will code for a single specific receptor.  As lymphocytes mature in the bone marrow and the thymus, they are tested for self-reactivity  B and T cells that express receptors specific for self antigens molecules are destroyed by programmed cell death (apoptosis)  Some cells that escape this process can later cause various autoimmune diseases

Proliferation of B and T Cells  Organisms initially express only few lymphocytes with a given antigen receptor for any particular epitope (happens during embryonic development and soon after birth)  When a mature T or B cell is exposed to a matching antigen in lymph nodes or peripheral tissues, it induces several processes that activate the lymphocytes and initiates their proliferation, resulting in an antigen-specific expansion of lymphocytes in a process defined as clonal expansion or selection

Two types of cells/clones are produced during clonal expansion: 1. Short-lived activated effector cells that act immediately against the antigen and/or cells that express the antigen. 2. long-lived memory cells that can rapidly give rise to effector cells if the same antigen is encountered in the future. Effector and Memory lymphocytes

Antigen Antigen receptor Antibody Plasma cells effector B cells Memory B cells B cells differing in antigen specificity Clonal expansion

 Immunological memory is responsible for long-term protection against diseases. Can be induced by prior infection or by vaccination  The first exposure to a specific antigen represents the primary immune response, this is a slow process, can take up to two weeks  During this time, selected B and T cells give rise to their effector and memory cells (memory cells stay in the body for years, while most effector cells die at the end of the infection)  In the secondary immune response, memory cells facilitate a faster (few days) and more efficient response Immunological Memory

Primary immune response to antigen A produces antibodies to A. Secondary immune response to antigen A produces antibodies to A; primary immune response to antigen B produces antibodies to B. Exposure to antigen A Exposure to antigens A and B Time (days) Antibody concentration (arbitrary units) Antibodies to A Antibodies to B

Humoral and cellular adaptive immunity  Acquired immunity has two branches: the humoral immune response and the cell-mediated immune response  In the humoral immune response, antibodies help neutralize or eliminate toxins and pathogens in the blood and lymph (the complement system will be discussed at a later class)  In the cell-mediated immune response, specialized T cells destroy pathogens and affected host cells  There are also antibody-mediated cellular responses, where antibody-covered pathogens are targeted by immune cells such as macrophages and dendritic cells

Helper and cytotoxic T cells  T cells are divided into two major classes: helper T cells and cytotoxic T cells  Helper T cells participate both in the humoral and the cell-mediated immune responses  Signals from helper T cells initiate on one hand production of antibodies that neutralize pathogens and on the other, activate T cells that kill infected cells (cytotoxic T cells)

 Cytotoxic T cells are the effector cells in the cell- mediated immune response  Cytotoxic T cells recognize fragments of foreign proteins complexed with MHC molecules present on infected cells  Activated cytotoxic T cells secrete proteins that disrupt the membrane of target cells and trigger apoptosis

 Class II MHC molecules coordinate antigen processing and presentation on antigen presenting cells and facilitate interaction with the T cell antigen receptors  Antigen receptors on the surface of helper T cells bind to antigens and the class II MHC molecule; then signals are exchanged between the helper and cytotoxic T cells, resulting in their activation  After activation, helper T cells proliferate, forming a specific T helper clone, which then activates the appropriate B cells

Pathogen 3 12 Antigen-presenting cell Antigen fragment Class II MHC molecule Antigen receptor Accessory protein Helper T cell B cell Cytokines Activated helper T cell Memory B cells Plasma cells Secreted antibodies Activation of B cells by helper T cells

 Activation of the humoral immune response involves B cells and helper T cells as well as proteins on the surface of pathogens  In response to cytokines from helper T cells and an antigen, B cells proliferate and differentiate into memory B cells and antibody-secreting effector cells called plasma cells  The humoral response is characterized by secretion of antibodies by B cells

Select antibody functions  Antibodies do not kill pathogens; instead they mark pathogens for destruction by macrophages and netrophils: opsonization (also complement activation)  Antibodies can also bind to viral surface proteins and prevent infection of a host cell ( neutralization )  Antibodies can also bind to various toxins released by pathogens and prevent ( masking ) them from entering body cells (also can induce aggregation and clearing of toxins form the body)

OpsonizationNeutralization Antibody Virus Bacterium Macrophage Activation of complement system and pore formation Complement proteins Formation of membrane attack complex Flow of water and ions Pore Antigen Foreign cell

 B cells can express five different forms (or classes) of immunoglobulin (Ig) with similar antigen-binding specificity but different heavy chain C regions  IgD: Membrane bound  IgM: First soluble class produced  IgG: Second soluble class; most abundant  IgA and IgE: Remaining soluble classes

Antigen- presenting cell Pathogen Antigen fragment Class II MHC molecule Accessory protein Antigen receptor Helper T cell Cytokines Humoral immunity Cell- mediated immunity B cell Cytotoxic T cell Helper T cells promote both humarol and cell-mediate immune response

Cytotoxic T cell 3 12 Accessory protein Class I MHC molecule Infected cell Antigen receptor Antigen fragment Perforin Pore Released cytotoxic T cell Dying infected cell Cytotoxic T cell-mediated killing of infected cells

Active and Passive Immunity  Active immunity develops naturally when memory cells form clones in response to an infection  It can be also induced by immunization (vaccination)  In immunization, a non-pathogenic form of a microbe or parts of microbe are used to elicit an immune response and generation of immunological memory  Passive immunity provides immediate, short-term protection. It is conferred naturally when IgG crosses the placenta from a mother to the fetus or when IgA passes in breast milk  It can be conferred artificially by injecting antibodies to non-immunized individuals (snake poisoning)

Humoral (antibody-mediated) immune response Cell-mediated immune response Antigen (1st exposure) Engulfed by Antigen- presenting cell Helper T cellB cell Cytotoxic T cell Key Stimulates Gives rise to

Helper T cell Memory helper T cells Antigen (2nd exposure) B cell Plasma cells Secreted antibodies Defend against extracellular pathogens Memory B cells Memory cytotoxic T cells Active cytotoxic T cells Defend against intracellular pathogens and cancer Cytotoxic T cell