Infection and Immunity Lecture Notes PDF

BIOL341/982 Infection and Immunity PATHOGENIC MECHANISMS AND EVASION STRATEGIES Martina Sanderson-Smith [email protected] 1. Damage to host caused by viruses 2. Bacterial Exotoxins 3. Bacterial Endotoxins 1 ...

BIOL341/982 Infection and Immunity PATHOGENIC MECHANISMS AND EVASION STRATEGIES Martina Sanderson-Smith [email protected] 1. Damage to host caused by viruses 2. Bacterial Exotoxins 3. Bacterial Endotoxins 1  DAMAGE TO THE HOST BY VIRUSES Viruses are able to damage the host by the following mechanisms: Diverting the cells metabolic processes and stopping the synthesis of proteins or other macromolecules Causing the release of lysosomal enzymes into the cytoplasm of the cell Inserting viral proteins into the membrane of the cell during the release phase, resulting in the rupture of the membrane and cell lysis 2  DAMAGE TO THE HOST BY VIRUSES - CMV Cytomegalovirus (CMV) - double-stranded DNA genome Infects most humans after birth without serious consequences 70-90% of the worlds population carry the CMV genome integrated into their own genome Infection is life long, mostly in a latent state Infected tissues contain enlarged cells May produce symptoms similar to mononucleosis (glandular fever) CMV can cause disease in the following groups: Infants infected before, during, or shortly after birth Transplant recipients (solid organ or bone marrow/stem cell) People with severe immune suppression such as AIDS CMV can be transmitted by close contact, saliva, blood, breast milk and also sexually transmitted 3  DAMAGE TO THE HOST BY VIRUSES - CMV CMV inhibit antigen presentation to cytotoxic T cells (CTLs) CMV express a MHC class I homolog (UL18), which binds to the MHC class I inhibitory receptor of natural killer (NK) cells, and prevents cell lysis. MHC class I homolog LIR-1 MHC class I UL18 TCR X NK cell CTL CMV infected cell Kim (2008) PLOS Path 4(8) Maffei (2008) J Immunol 180: 969–979 4  BACTERIAL TOXINS Toxins are substances produced by some bacteria that interfere with the normal functioning of host cells and tissues The ability of pathogens to produce toxins is referred to as toxigenicity Toxins are produced for different functions: Increase vascular permeability – Bacillus anthracis (anthrax) Damage respiratory mucosal cells – Bordetella pertussis (whooping cough) Lysis of red and white blood cells – Streptococcus pyogenes Inhibits protein synthesis in cells – Pseudomonas aeruginosa Blocks acetylcholine release at neuromuscular junction – Clostridium botulinum (botulism) 5  BACTERIAL TOXINS Bacterial toxins are classically divided into broad groups: EXOTOXINS: Are toxins secreted into the environment in which the bacteria are growing (e.g. host tissues, soil, water) Cytolytic toxins – kills host cells in general/affects their function Neurotoxins – interferes with nerve cell function Enterotoxins – affects cells lining the gastrointestinal tract ENDOTOXINS: Are toxins that form part of the cell wall of bacteria They are not actively secreted but are released when the bacterial cell is damaged or lysed 6 EXOTOXIN vs. ENDOTOXIN 7  EXOTOXINS Cytolytic toxins damage cell membranes → lysis → death e.g. Phospholipases – hydrolyse phospholipids in cell membranes § Clostridium perfringens alpha-toxin (lecithinase) phospholipase 8  EXOTOXINS Cytolytic toxins damage cell membranes → lysis → death Eg. Pore-forming toxins S. aureus alpha-toxin protein inserts into membrane → pores or channels S. pyogenes – streptolysin-O → pores or channels 9  BACTERIAL TOXINS Toxins with an intracellular site of action Bind to cell membrane receptor → transport across CM → modify or inactivate intracellular target AB toxins (2 distinct types of subunit) - B subunit binds host cells and delivers A subunit into host cells 10  EXOTOXINS: Shiga Toxin Has a compound AB structure Secreted from Shigella dysenteriae STEC & EHEC secrete a shiga-like toxin Schematic (A) and crystallographic structure (B) of Shiga toxin 11  A bad day at the office: Shiga Toxin Shigellosis Two-stage disease Early stage: Watery diarrhea attributed to the enterotoxic activity of Shiga toxin following ingestion and noninvasive colonization, multiplication, and production of enterotoxin in the small intestine Second stage: Adherence to and tissue invasion of large intestine with typical symptoms of dysentery. Cytotoxic activity of Shiga toxin increases severity of pathogenesis of Shigella 12  EXOTOXINS ADP ribosylating toxins: Toxin Producer Target Structure Exotoxin A P. aeruginosa EF2 AB Adenylate cyclase Pertussis toxin B. pertussis regulatory G proteins AB5 Adenylate cyclase HLT ETEC regulatory G proteins AB5 Adenylate cyclase CT V. cholerae regulatory G proteins AB5 13  EXOTOXINS: Cholera Toxin Enterotoxins are exotoxins that cause secretion of fluids within intestine → diarrhoea Cholera toxin Vibrio cholerae – transmitted via contaminated water Acute watery diarrhoea, epidemic, pandemic High mortality rate due to massive fluid loss Treatment by rehydration therapy (rice-water) Antibiotics shorten course of disease and decrease severity of symptoms - not necessary AB5 structure B B B A B B 14  EXOTOXINS: Cholera Toxin Cholera toxin responsible for most of pathology: B - pentameric ring, binds to ganglioside receptor on intestinal cells A = A1 + A2 disulphide bonded A1 enters cell; ADP-ribosylates Gs → permanently stimulate adenylate cyclase  cAMP (cyclic adenosine monophosphate) → affect activity of ion channels  secretion of Cl- by intestinal crypt cells ↓ absorption of Na+ net flow of ions & H20 into intestinal lumen profuse watery diarrhoea up to 20L/day www.itb.uni-stuttgart.de 15 16 CHOLERA TOXIN MECHANISM OF ACTION [cAMP] increases in intestinal epithelial cells. Constant electrolyte and water secretion into intestinal lumen results in dehydration. Das et al (2018) Plos Pathogens 17  EXOTOXINS: Neurotoxins Neurotoxins are exotoxins that have their effect on the nervous system The botulinum toxin (most powerful neurotoxin known) Acts on peripheral nerve endings – neuromuscular junctions Binds to presynaptic terminal membranes and inhibits the release of acetylcholine – preventing transmission of impulses from the nerve cell to the muscle cell Reduces muscle contraction - results in flaccid paralysis 1.2 ng is lethal for a mouse Cosmetic Botox = 4U/100 ul injection (approx. 0.2 ng) 18  EXOTOXINS: Botulinum Toxin Mode of action of botulinum toxin: The flaccid paralysis in botulism can lead to death due to dysfunction of the respiratory muscles and respiratory failure 19  BOTULISM Home-canned vegetables are the most common cause of botulism outbreaks in the United States. In 2018, 232 laboratory-confirmed botulism cases were reported to the CDC These outbreaks often occur because home canners did not follow canning instructions, did not use pressure cookers, ignored signs of food spoilage, and were unaware of the risk of botulism from improperly preserving vegetables. http://www.cdc.gov/features/homecanning/ 20  EXOTOXINS: Superantigens Superantigens are secreted exotoxins that can activate CD4+ T cells. CD4+ T cell 21 Remember this? Group A streptococci N produce super-antigens CD4: that non-specifically cross- binds to APC link MHC class II proteins class II MHC and TCR, in order to disrupt the immune class II peptide response. This leads to MHC antigen massive T cell over proliferation and inflammatory cytokine N TCR production – lead to organ superantigen TH cell failure and shock. Nature Medicine 8: 1398-1404 22  TOXOIDS and ANTI-TOXINS Exotoxins are proteins – usually antigenic; that are capable of stimulating the production of antitoxins by the host immune system Antitoxin – is a specific antibody produced against a toxin – able to bind to the toxin and neutralise it – preventing it from binding to its target cell or tissue Exotoxins are therefore effective immunogens – cannot be used in their natural form in vaccines Exotoxins can be modified by heat or chemicals, such as formaldehyde or phenol – removes their toxicity but maintains ability to elicit immune response Toxins modified in this manner – Toxoids 23 BACTERIAL EXOTOXINS SUMMARY 24  BACTERIAL TOXINS Bacterial toxins are classically divided into broad groups: EXOTOXINS: Are toxins secreted into the environment in which the bacteria are growing (e.g. host tissues, soil, water) Cytolytic toxins – kills host cells in general/affects their function Neurotoxins – interferes with nerve cell function Enterotoxins – affects cells lining the gastrointestinal tract ENDOTOXINS: Are toxins that form part of the cell wall of bacteria They are not actively secreted but are released when the bacterial cell is damaged or lysed 25  ENDOTOXINS § Composed of lipid polysaccharide complexes (LPS) Mainly cell associated but some shed (lysis) → cell free form Less toxic, but more heat stable than exotoxins Lipid A – toxic moiety Effects on host inflammation, tissue damage (localised infections) fever, widespread tissue damage, septic shock (systemic infections) → death (20-50%) Mechanism of action – over stimulation of immune system during systemic infection 26 ENDOTOXINS 27  ENDOTOXINS Gram negative LPS from outer membrane Typically composed of 1. A lipid portion (lipid A) – inserted into the cell wall responsible for much of the toxic activity 2. A conserved core polysaccharide 3. A highly variable O-polysaccharide, responsible for the serologic diversity which is the feature of organisms such as Salmonella and Shigella 28 ENDOTOXINS disseminated intravascular coagulation 29  BACTERIAL TOXINS A comparison of Exotoxins and Endotoxins Characteristics Exotoxins Endotoxins Source Gram positive and Gram negative bacteria Gram negative bacteria Location in bacterium Cytoplasm Cell wall Chemical composition Protein Lipid-polysaccharide-peptide Antibodies elicited Yes No Conversion to toxoid Possible Not possible Liberation of toxin On production by the cell On disintegration of the cell Representative effects Interfere with synaptic activity Increase body temperature Interupt protein synthesis Increase haemorrhaging Increase capillary permeability Increase swelling in tissues Increase water elimination Induce vomiting, diarrhoea 30  KNOWLEDGE LEVEL REQUIRED FOR BIOL341 1. Damage to host by viruses Mechanisms/examples 2. Bacterial Exotoxins Definition Mode of action/examples 3. Bacterial Endotoxins Definition Mode of action/examples 31