Immunity to Bacterial Infections

IMMUNITY TO BACTERIAL INFECTIONS Bacterial Diseases • Many Infectious agents and many diseases • Bacteria can Infect any part of the body • Cause disease due to – Growth of the microbe in a tissue – Produce Bacterial factors that are harmful to host – Elicite an inflammatory response that causes damage • but also leads to aquired immunity Gram Positive Bacteria Clostridium species Gram Negative Bacteria E. coli Staph aureus Moraxella species Steps of Bacterial Infection 1. Attachment of bacterium to host tissue Preferential infection of some bacterial species • Persistence and growth called colonization 2. Invasion into deeper host tissues and production of toxins • Results in host cell and tissue injury 3. Inflammation at site of invasion • • • Initiated by antibody binding to bacterium Initiated by complement activation at bacterial surface Initiated by wound healing mechanisms • Fig 18.2 Attachment of bacterium to host tissue All can activate complement pathways that alters vascular permeability and activates local macrophage (M!) and neutrophils (PMNs) Invasion into deeper host tissues and production of toxins Inflammation at site of invasion Factors that promote colonization, entry, and progression to disease • Bacterial Aliginate – Promotes adherence to host tissue • Bacterial LPS and pilus – Promote persistence by resisting complement and phagocytosis • Bacterial Type III secretion system – Deliver enzymes and toxins that • injure host tissues or • diminish host immune responses • Includes signaling proteins, proteases, and superantigens Factors that promote colonization, entry, and progression to disease Innate Immune Effectors and Bacterial Infection • Major mediators of resistance to bacterial infections • High numbers of bacteria on skin and mucous membranes • Usually do not cause disease • Commensal organisms • Physical and physiologic barriers prevent entry or destroy quickly after entry Innate Immune Effectors and Bacterial Infection Immune System Function and Resistance to Bacterial Infection • Major mediators of resistance to extracellular bacterial infections is ANTIBODIES • Provide protective immunity by Killing live organisms through – Opsonophagocytosis = promotion of complement mediated phagocytosis – Neutralizing bacterial toxins to prevent damage and intense damaging inflammatory responses Immunity to extracellular bacteria by antibodies: gram negative bacteria Immunity to extracellular bacteria by antibodies: gram positive bacteria • Antibodies and complement result in opsonization via FcReceptors (FcR) or Complement Receptors (CR) on M! and PMN • Antibodies and complement result in opsonization via FcReceptors (FcR) or Complement Receptors (CR) on M! and PMN • Antibody can activate classical complement pathway resulting membrane attack complex (MAC) and opsonization via CR • Antibody cannot activate classical complement pathway resulting membrane attack complex (MAC) because of the thick cell wall, but can do opsonization via CR • Antibodies can also trigger antibody-dependent cellmediated cytotoxicity (ADCC) by PMN with FcR and CR release proteases, mucleases, lipases, and ROIs • Antibodies can also trigger antibody-dependent cellmediated cytotoxicity (ADCC) by PMN with FcR and CR release proteases, mucleases, lipases, and ROIs Immunity to intracellular bacteria by cell mediated immunity (CMI) • Bacterial antigens present in the cytoplasm of infected host cell Immunity to extracellular bacteria by antibodies – Processed via endogenous pathway and presented to CD8+ T cells (cytotoxic T lymphocytes or CTLs) on MHC class I • Bacterial antigens present in the endosomes of infected host cell – Processed via the exogenous pathway and presented to CD4+ helper T cells (TH) on MHC class II • Some TH cells (TH1) can initiate a DTH response – Activates M! and kill bacteria and host cell via proteases and ROI • Bacterial antigens present on the surface of the infected host cell can be bound by antibodies and targeted for killing by NK cells using ADCC Bacterial Virulence Factors: How they Thwart Host Immunity Mechanisms of Immunity to intracellular bacteria by CMI • Virulence factors are anything that bacteria use to cause disease • Examples: – Proteins, glycolipids, carbohydrates, teichoic acids, peptidoglycan, metabolites, enzymes, toxins • Impede the function of lymphocytes and granulocytes – Proteins, enzymes, and toxins • Overstimulate inflammation so normal defenses do not work – Glycolipids like LPS, cell wall structures like teichoic acids ad peptidoglycan, also enterotoxins and bacterial superantigens • Prevent phagocytosis – Carbohydrates like LPS side chains and capsular polysaccharides • Evade Complement mediated killing – Lots of ways.. Bacteria Evade Complement mediated killing Bacteria Evade Complement mediated killing • Prevent activated complement components from attaching to the bacterial cell wall or outer membrane – Presence of capsule or calyx outside bacterial cell • prevent CR on phagocytes from binding activated complement on bacterial surface – Long or bulky surface components • Divert activation of MAC from bacterial membrane – Expression of certain surface proteins • Cleave, Inactivate, or disassemble activated complement components – Expression of enzymes • Prevent membrane insertion of complement complex – Outer membrane Resistance to insertion – Secretion of inhibitors Bacterial Virulence Factors continued… • Biofilms • Quorum Sensing • Bacterial Pathogenesis through these mechanisms Bacterial Virulence Factors continued… • Biofilms – An organized growth of many microbial cells in a microcolony – Establishment of cell-to-cell communication network using soluble factors • Communication occurs by Quorum Sensing – The time when the bacteria has reached a critical mass where the cells in the biofilm can detect and respond to each other • Concentration of signaling molecules is high enough to bind to receptors and initiate gene transcription of virulence factors • = Bacterial Pathogenesis • Example: – Dental plaque and the pathology of cavities Biofilm Maturation Pathway Microbial Virulence Factors continued • Yersina pestis • Causative agent of “the bubonic plague” or Black Death Examples of Quorum sensing molecules Mechanism of quorum sensing • 13th and 14th centuries devastated European populations • Estimated 33% death of population R = transcription regulator I = biosynthetic enzyme HSL = quorum sensing molecule • Yop proteins block – inflammatory response – and promote apoptosis of M! ABCDEF = virulence factors Yop proteins block inflammatory response and promote apoptosis of M! Type III Secretion System and molecular factors involved in pathogenesis of Yersina