Lecture 28 – Host-Pathogen Interactions

Toxins and Evasins

This lecture explores the mechanisms by which microbial pathogens interact with their hosts, focusing on the roles of toxins and evasins. Understanding these interactions is crucial for grasping how infectious diseases develop and how the immune system responds.

Learning Objectives

• Explain how LPS endotoxemia leads to septic shock.

• Describe methods for detecting LPS in medical preparations.

• Compare endotoxins and typical bacterial exotoxins.

• Discuss challenges in developing vaccines against endotoxin.

• Describe the mechanism of action of bacterial A-B toxins.

• Differentiate between types of A-B toxins (e.g., AB5 or AB2).

• Explain how anthrax toxin kills intoxicated cells (lethal factor, edema factor).

• Describe types of cell-damaging toxins (e.g., hemolysins, phospholipases).

• Differentiate between alpha and beta hemolysins.

• Explain how phospholipases kill cells and allow bacteria to escape phagosomes.

• Describe how hyaluronidases allow bacteria to spread into deep tissues.

• Explain the mode of action of superantigen toxins.

• Give examples of infections where inflammation causes main damage.

• Describe induced autoimmunity (immune mimicry) and give examples.

• Explain viral and fungal evasion of the immune system.

• Describe protozoal avoidance of the immune system.

Vocabulary

• Endotoxin: Lipopolysaccharide (LPS) component of Gram-negative bacteria, toxic to mammals.

• Exotoxin: Protein toxins secreted by bacteria, often with specific targets.

• Amoebocyte lysate (LAL) assay: Test for detecting LPS contamination.

• Anthrax toxin: Composed of lethal factor, edema factor, and protective antigen.

• Phospholipase: Enzyme that hydrolyzes phospholipids, damaging cell membranes.

• Hemolysin: Toxin that lyses red blood cells.

• Hyaluronidase: Enzyme that degrades connective tissue, aiding bacterial spread.

• Superantigen toxin: Toxin that hyperactivates T cells by crosslinking TCR and MHC-II.

• Necrotizing fasciitis: Severe tissue infection, often toxin-mediated.

• Induced autoimmunity: Immune response against self, e.g., acute glomerulonephritis, rheumatic fever.

• Streptococcal M protein: Bacterial protein involved in immune mimicry.

• Interferon repressor: Viral protein that inhibits host interferon response.

• p53 inhibitor: Viral protein that blocks apoptosis.

• MHC-I analog: Viral protein mimicking host MHC-I to evade immunity.

• Syncytium: Multinucleated cell formed by fusion, often in viral infections.

• Keratinase: Fungal enzyme that degrades keratin in skin.

Endotoxin

Structure and Mechanism

Endotoxins are lipopolysaccharides (LPS) found in the outer membrane of Gram-negative bacteria. The toxic component is lipid A.

• LPS is recognized by serum protein LBP, which binds LPS from lysed bacteria.

• LBP-LPS complex binds to phagocyte surface proteins CD14 and TLR4.

• This induces secretion of cytokines (e.g., TNF-α, IL-1, IL-6, IL-8) from monocytes.

• Cytokines increase synthesis of white blood cells and permeability of capillaries.

• Systemic infection can result in excessive blood loss to tissues, hypovolemia, organ failure, disseminated intravascular coagulation (DIC), and death (septic shock).

Detection and Clinical Relevance

• LPS contamination in medical fluids is detected using the LAL assay, which is highly sensitive to picogram quantities of LPS.

• New methods for LPS detection involve observing LPS aggregation in lipid droplets under a microscope.

Recent Advances

• Some native flora have LPS lacking certain phosphate groups, making them more resistant to antimicrobial peptides.

• Therapeutic derivatives of LPS (e.g., with fewer acyl chains) may activate immune pathways without excessive inflammation.

• Vaccines using modified LPS (with only 3 acyl chains) can induce protective antibodies without causing excessive fever.

Exotoxins

Comparison with Endotoxins

A-B Toxins

• Composed of two parts: A (active) and B (binding).

• B subunit binds to host cell receptor; A subunit exerts toxic effect.

• Examples: Cholera toxin, Anthrax toxin (lethal factor, edema factor, protective antigen).

Entry of Toxin Components

• Bacillus anthracis (anthrax) toxin has multiple components that enter cells via receptor-mediated endocytosis.

• Lethal factor and edema factor disrupt cellular signaling, leading to cell death and edema.

Cell-Damaging Toxins

Hemolysins (Cytolysins)

• Detected by hemolysis on blood agar.

• Alpha hemolysin (e.g., from Staphylococcus aureus) forms pores in cell membranes, similar to the membrane attack complex (MAC) of the immune system.

• Phospholipases remove phospholipid head groups, damaging membranes and allowing bacteria to escape phagosomes.

Tissue-Damaging Toxins

• Exfoliative toxin: Causes sloughing of skin layers.

• Hyaluronidase: Destroys connective tissue between cells, aiding spread into deep tissues.

Superantigen Toxins

Superantigens crosslink T cell receptors (TCR) to MHC-II molecules on antigen-presenting cells without the need for antigen specificity, causing massive T cell activation.

• Results in excessive cytokine release, hypovolemia, DIC, shock, and death.

• Example: Toxic Shock Syndrome Toxin (TSST) from Staphylococcus aureus.

Immune-Mediated Damage

Inflammation-Induced Tissue Damage

• Phagocytes recruited by inflammation can damage host tissue.

• Examples: Bacterial meningitis (Neisseria meningitidis), pelvic inflammatory disease (PID) caused by Neisseria gonorrhoeae or Chlamydia.

Induced Autoimmunity (Immune Mimicry)

• Immune complexes can deposit in glomeruli, causing acute glomerulonephritis.

• Antibodies against Streptococcal M protein can cross-react with heart muscle, leading to rheumatic heart disease (acute rheumatic fever).

• Symptoms: Fever, malaise, skin rash, joint pain, carditis.

Viral Evasion of Immunity

• Viruses may produce interferon repressors to block host antiviral responses.

• p53 inhibitors prevent apoptosis of infected cells (e.g., papillomavirus E6 protein).

• Some viruses coat themselves with host molecules (LPS, peptidoglycan) to evade detection.

• Proteins that prevent or reverse MHC-I antigen presentation (e.g., cytomegalovirus UL18) help viruses avoid cytotoxic T cell killing.

• Formation of syncytia (multinucleated cells) allows cell-to-cell spread and evasion of antibodies (e.g., respiratory syncytial virus).

Fungal and Protozoal Evasion

Fungal Infections

• Fungal toxins: Aflatoxin (carcinogenic), ergot poisoning.

• Dermatophytes produce keratinase, invading superficial skin (e.g., ringworm, athlete's foot).

• Serious fungal infections often result from inhaling spores of dimorphic fungi (mold at cool temp, yeast at warm temp).

Protozoal Evasion

• Plasmodium (malaria) lives inside red blood cells, avoiding humoral and cellular immunity (no MHC on RBCs).

• Trypanosomes vary their surface antigens to evade immune detection.

• Schistosomes express surface receptors for host proteins, camouflaging themselves from the immune system.

Key Equations

• Cytokine induction by LPS:

• Superantigen activation:

Summary Table: Pathogen Evasion Strategies

Additional info: Some recent advances in LPS modification and detection are under investigation for improved vaccine and diagnostic development. The immune system's response to pathogens can sometimes cause more damage than the pathogen itself, as seen in autoimmune phenomena and excessive inflammation.