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General Microbiology II

Concepts of Immunology

Innate Host Defenses

Innate host defense factors are non-specific immune (resistance) factors that are present before infection ... they work all the time and effective against many different kinds of microbes

• External resistance factors - those which act on body surfaces
  • physical - factors that depend on structural integrity for function
    • barriers - skin, mucus, nasal hair
    • environmental conditions - dryness
    • flushing action - blinking of eyelid, peristalsis, urination
  • chemical - activities carried out by molecules
    • salts - secreted by sweat glands in skin
    • acids - low pH (HCl in stomach; lactic and other skin acids produced by normal microbiota)
    • lipids - fatty acids secreted by sebaceous glands (sebum) and by gall bladder (bile)
    • enzymes - lysozyme (secretions), digestive enzymes (stomach, small intestine)
  • cellular - activities carried out by cells
    • normal microbiota compete with pathogens for attachment sites and nutrients as well as producing factors toxic to pathogens
    • phagocytes (macrophages and polymorphonuclear leukocytes (PMNs) are cells that engulf and destroy microbes on mucous membranes by a process known as phagocytosis
• Internal resistance factors - those which act within tissues
  • physical - barriers such as connective tissue, blood-brain barrier
  • chemical - activities carried out by molecules
    • enzymes - lysozyme and other enzymes in blood and tissue fluids
    • complement - can be nonspecifically activated by bacterial and fungal polysaccharides or lipopolysaccharides present on pathogens, including bacteria, fungi, etc.
      • C3 convertase generation and action
        • C3 is cleaved by proteases (or esterases) present in blood and tissue fluids to generate fragments designated as C3a and C3b
        • C3 possesses a labile thiolester group is exposed via cleavage to C3b, allowing this component to form a covalent (ester or amide) bond with free hydroxyl or amino groups on microbial surfaces
        • C3b binds factor B in the presence of Mg ions, to form a C3b,B complex
        • factor D cleaves factor B to generate factor Bb, which is part of the C3b,Bb complex on the microbial surface
        • Properdin binds to this complex to generate stable C3 convertase (C3b,Bb,P), which cleaves C3 to form fragments designated as C3a and C3b
      • C5 convertase generation and action
        • generated by binding of multiple C3b molecules to C3b,Bb complexes on the microbial surface, thus forming C3b_n,Bb,P complexes
        • activity - cleaves C5 into C5a and C5b
      • membrane attack complex (MAC) generation and action
        • generated in three steps:
          • C5b,6,7 complex, which binds to the microbial membrane via hydrophobic tails that are unfurled by C6 and C7 as a result of allosteric conformational changes that occur when C6 and C7 bind sequentially to C5
          • C8 binds to the membrane-bound C5b,6,7 complex
          • C9 binds to the membrane-bound C5b,6,7,8 complex
        • lyses (disrupts) enveloped viruses and Gram-negative bacteria
      • other activities mediated by complement:
        • C3a and C5a trigger inflammation
        • opsonization - enhancement of phagocytosis by promoting binding of particles, such as pathogens, to phagocytes via C3b receptors (C3bR)
  • cellular - activities carried out by cells
    • pattern recognition receptors (PRRs)
      • phagocytes (and some other cells, such as epithelial cells) utilize pattern recognition receptors (PRR) to recognize (bind) conserved molecular moieties frequently referred to as pathogen-associated molecular patterns (PAMPs) that are found in bacteria, fungi and viruses
      • PRR binding of these conserved molecular moieties (ligands) activates immune system cells to produce cytokines as well as upregulating certain other factors/functions that play roles in host defense against pathogens
      • PRRs include membrane-associated toll-like receptors (TLRs) and cytoplasmic receptors such as nod1 and nod2
        • TLRs are cell-associated receptors that detect pathogen-associated molecular patterns (PAMPs) found in bacteria, fungi and viruses
        • TLR binding of the appropriate ligand (see table) induces intracellular signaling:
          
  • interferons - production is initiated by Pattern Recognition Receptor (PRR) binding of conserved microbial motifs (especially dsRNA)
    • interferon alpha and interferon beta are produced by all cells when they are infected with (most) viruses; they help diminish viral replication
    • interferon gamma is produced by type 1 T helper cell (Th1 cells) and acts to stimulate macrophage activity in addition to helping diminish viral replication
  • inflammation - "early-warning" system
    • histamine released from mast cells at sites of tissue damage
    • increases permeability of capillaries at these sites, causing accumulation of complement, enzymes and phagocytes at sites of infection
    • phagocytes eliminate pathogens and healing is initiated
    • cardinal signs of inflammation - erythema (redness); edema (swelling); fever (heat); pain (soreness)
  • phagocytosis - process by which phagocytes (macrophages and polymorphonuclear leukocytes, aka PMNs) engulf and destroy microbes
    • adhesion (attachment) - phagocyte and pathogen bind to each other
    • ingestion (engulfment) - pseudopodia surround the pathogen, then fuse to form a vacuole called a phagosome
    • killing and digestion - inside phagolysosomes, which are formed by fusion of lysosomes with phagosomes:
      • hydrogen peroxide and related compounds kill microbes
      • enzymes (proteases, polysaccharidases, lipases, nucleases) digest microbes
    • egestion - release of non-digestible material by a process that is essentially the reverse of ingestion

    Phagocytosis and Bacterial Pathogens Animation
    From: ASM MicrobeLibrary, Author: Thomas M. Terry, University of Connecticut

Adaptive host defenses ... How do our bodies prevent infectious diseases?

Adaptive host defense factors are immune factors that are triggered by antigens, substances produced by microbes during infection, and are specific for those pathogens to which one is exposed

• Antibody-mediated immunity (AMI)
  • antibody responses
    • antibody is produced by B cells (lymphocytes) stimulated by T cells in response to antigens made by infectious agents
      • Phagocytes called antigen presenting cells (APCs) engulf, process and present fragments of antigen molecules on their surfaces
      • Lymphocytes called T helper (Th) cells recognize (using antigen receptors on their surfaces) antigen fragments, then proliferate and differentiate to form a large population of cytokine-producing Th cells
      • Lymphocytes called B cells bind antigen (using antigen receptors on their surfaces) and bind cytokines (using cytokine receptors on their surfaces), then proliferate and differentiate to form a large population of antibody-producing plasma cells
    • antibody binds specifically to the antigen that induced its formation
  • mechanisms of action (ways in which AMI functions)
    • neutralization - inhibition of toxin function, viral infectivity, microbe attachment due to antibody binding to surface antigens of pathogens
    • antibody/complement-mediated lysis - complement is activated by binding to IgG or IgM antibody molecules that have bound to antigens, and causes lysis of microbes via formation of a membrane attack complex (MAC)
      • C3 convertase generation and action
        • generation
          • antibody binds to the surface of a microbe (two adjacent IgG molecules, or a single IgM molecule)
          • C1 (complement component 1 - composed of a Ca ion- stabilized complex of C1q, C1r and C1s) binds two antibody Fc regions
          • C1s is activated by C1r during this process . . . the resulting enzyme is called C1 esterase
          • C1 esterase cleaves C4 to form C4a and C4b
          • C4b binds to C1 esterase and to adjacent cell surfaces (especially membranes)
          • C2 is cleaved by C1,4b complex to form C2a and C2b
          • C2a binds the C1,4b complex, thus forming the C1,4b,2a complex
        • action
          • C1,4b,2a is the C3 convertase of this "classical" pathway of complement activation (C2a possesses the enzymatic activity)
          • cleaves C3 into C3a and C3b
            • C3a induces histamine release (mast cell and basophil degranulation) thus inducing/enhancing inflammation
            • C3b mediates opsonization and induces alternate pathway activation
      • C5 convertase generation and action
        • generation - binding of C3b molecules to C1,4b,2a (either on the microbial surface, or free in solution) generates C1,4b,2a,3b complexes
        • action - C1,4b,2a,3b cleaves C5 into C5a and C5b
          • C5a
            • induces histamine release (mast cell and basophil degranulation) thus inducing/enhancing inflammation
            • mediates phagocyte chemotaxis
          • C5b initiates formation of MAC
      • membrane attack complex (MAC) generation and action
        • generated in three steps:
          • C5b,6,7 complex, which binds to the microbial membrane via hydrophobic tails that are unfurled by C6 and C7 as a result of allosteric conformational changes that occur when C6 and C7 bind sequentially to C5b
          • C8 binds to the membrane-bound C5b,6,7 complex
          • C9 binds to the membrane-bound C5b,6,7,8 complex
        • lyses (disrupts) enveloped viruses and Gram-negative bacteria
    • opsonization - antibody and complement both enhance phagocytosis by promoting binding of particles, including pathogens, to phagocytes via C3b receptors (C3bR)
• Cell-mediated immunity (CMI)
  • cellular responses
    • cytotoxic T lymphocytes (CTLs) are activated by T helper cells in the presence of viral antigens on virus-infected cells
      • Phagocytes called antigen presenting cells (APCs)engulf, process and "present" fragments of antigen molecules on their surfaces
      • Lymphocytes called T helper (Th) cells cells recognize (using antigen receptors on their surfaces) antigen fragments, then proliferate and differentiate to form a large population of cytokine-producing Th cells
      • Lymphocytes called Tc cells (CTL precursor cells) bind antigen (using antigen receptors on their surfaces) and bind Th cell-produced cytokines (using cytokine receptors on their surfaces), then proliferate and differentiate to form a large population of CTLs
    • macrophages are also activated by T helper cells
      • APCs engulf, process and "present" fragments of antigen molecules on their surfaces
      • Th cells recognize (using antigen receptors on their surfaces) antigen fragments, then proliferate and differentiate to form a large population of cytokine-producing Th cells
      • macrophages bind Th cell-produced cytokines (using cytokine receptors on their surfaces), then differentiate to form activated macrophages
  • mechanisms of action (ways in which CMI functions)
    • CTLs kill "target" cells by inducing them to destroy themselves via programmed cell death (apoptosis) ... apoptosis video
    • activated macrophages engulf, kill and digest intracellular bacteria and fungi much more efficiently than normal macrophages do, partially because they starve these microbes (by destroying nutrients the microbes need to grow)

Vaccines

Killed or attenuated (weakened) preparations containing microbial antigens that are used to stimulate immune responses without causing disease are called vaccines

• Types of vaccines
  • toxoid vaccines - a chemically inactivated toxin that is used to induce production of neutralizing antibody
  • killed vaccines - chemically inactivated microbe used to induce antibody production
    • whole microbe vaccines - include the entire microbe
    • split microbe vaccines - include only the antigens needed to induce antibody responses
  • live attenuated vaccines - living, but less virulent, microbe causes a transient infection that leads to generation of both AMI and CMI
  • recombinant vaccines
    • vector vaccines are those generated by incorporating antigen genes isolated from pathogens into non-harmful bacteria or viruses
    • food vaccines are those generated by incorporating genes for microbial antigens into foods such as bananas or tomatoes
  • DNA vaccines - injection of DNA containing genes that code for microbial antigens can lead to transient expression of those genes, resulting in immunization of the host
• Childhood immunizations (vaccinations)
  • Hepatitis B - vaccinate within 2 months after birth; boost 1 month later; boost again 2 months after the second dose (and 4 months after the first dose); if not vaccinated earlier, assess immunity, then vaccinate as needed at 11-12 years of age (combined vaccine against both hepatitis A and B)
  • DTaP (diphtheria, pertussis, tetanus) - vaccinate with DTaP vaccine at 2, 4, 6,15-18 months, then boost at age 4-6 years; Td vaccine (no pertussis component) is used as a booster at age 11-16, then every 7-10 years after that (combined vaccine ... DTP + HiB)
  • Hepatitis A - vaccinate at 2-12 years of age in areas where high risk of infection occurs; especially recommended for children living in communities with high rates of hepatitis A . Vaccination helps prevent carrier state as well as outright disease. (combined vaccine against both hepatitis A and B)
  • Hib (Haemophilus influenza b) - vaccinate at 2, 4, 6, and 12-15 months (combined vaccine ... DTP + HiB)
  • Poliomyelitis - injectable polio vaccine (IPV) at 2, 4 and 6-18 months; boost at 4-6 years (the oral polio vaccine (OPV) is no longer recommended in the US because it causes the very small number of cases (~4 cases from 1997-1998) of polio in this country
  • MMR (measles, mumps, rubella) - vaccinate at 12-15 months; boost at 4-6 years of age; if not vaccinated earlier, assess immunity, then vaccinate as needed at11-12 years of age; populations that don't get vaccine have Congenital Rubella Syndrome
  • Varicella (chickenpox) - vaccinate at 12-18 months; if not vaccinated earlier, assess immunity, then vaccinate as needed at 11-12 years of age (or up to 18)
• Other immunizations (vaccinations)
  • Influenza vaccines must be developed each year and are administered mainly to people over the age of 65 due to their high risk of dying from this disease or its complications; there has recently been interest in vaccinating children in addition to the "usual" over-65 group
  • Menomune induces immunity against major strains of Neisseria meningitidis, the major cause of meningitis in college-age people; currently recommended for first-year college students, especially those living in dorms
  • Prevnar is a new vaccine that induces immunity against major strains of Streptococcus pneumoniae, the major cause of meningitis in infants; this bacterium currently accounts for ~24 million pediatrician visits every year, because it also accounts for 30-50% of otitis media (middle ear infection) cases; the vaccine will be administered at 2, 4 and 6 months of age
  • RotaShield vaccine aimed at preventing diarrheal disease (rotavirus causes at least 50,000 hospital admissions and 20-40 infant deaths each year in the US)
  • Smallpox vaccine, used to eradicate smallpox worldwide in 1977, is no longer used outside of research environments
  • Anthrax vaccine