Principles of Science: Infection & Immunity - Principles of Innate Immunity

Physical and Biochemical Barriers to Infection

• Physical barriers:

  • Skin: Acts as a barrier. Composed of multiple layers of tightly packed epithelial cells, creating a physical impediment to pathogens. The outermost layer, the stratum corneum, is keratinized, adding to its protective nature.

  • Mucous membranes: Line the respiratory, gastrointestinal, and urogenital tracts. They employ mechanisms like the mucociliary escalator in the respiratory tract and peristalsis in the gastrointestinal tract to trap and expel pathogens. Mucus contains antimicrobial substances like lysozyme and lactoferrin.

• Biochemical barriers:

  • Fatty acids in sweat & sebum: Create an inhospitable environment for pathogens due to their low pH and disruption of microbial membranes. Sebaceous glands secrete sebum, which coats the skin, providing an additional layer of protection.

  • Lysozyme in secretions: Breaks down bacterial cell walls by cleaving the peptidoglycan layer. Found in tears, saliva, and mucus.

  • Acid in the stomach: Kills ingested pathogens due to its low pH (around 1.5 to 3.5). Parietal cells in the stomach lining secrete hydrochloric acid (HCl).

  • Commensal microflora: Compete with pathogens for nutrients and attachment sites, preventing colonization. They also produce antimicrobial substances like bacteriocins.

Defenses Provided by Cells in Tissues

• When pathogens penetrate the barriers, they face defenses by cells in the tissues, including macrophages, dendritic cells, and mast cells. These cells recognize and respond to pathogens that breach the physical and biochemical barriers.

• The foreign organism must be detected and destroyed. Detection involves pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs).

Innate Immunity as an Early Warning System

• Innate immunity uses pattern-recognition receptors (PRRs) to detect microbial components that are intrinsically foreign, providing an immediate response to infection. This system does not require prior exposure to the pathogen.

• These components are called pathogen-associated molecular patterns (PAMPs). Examples include lipopolysaccharide, peptidoglycans, and mannose sugars found in bacterial cell walls.

Pathogen-Associated Molecular Patterns (PAMPs)

• Bacterial PAMPs are detected by the immune system. Gram-positive bacteria have peptidoglycan and teichoic acid in their cell walls. Peptidoglycan is a major component of the bacterial cell wall, while teichoic acid is specific to Gram-positive bacteria. Gram-negative bacteria have lipopolysaccharides (LPS) in their outer membrane, which is a potent activator of the immune system.

Pattern Recognition Receptors (PRRs)

• Types of PRRs:

  • Toll-like receptors (TLRs): Located on cell surfaces and endosomes; recognize a variety of PAMPs.

  • NOD receptors: Cytoplasmic receptors that recognize intracellular pathogens.

  • Complement C3 protein: Part of the complement system, enhancing opsonization and phagocytosis.

  • Acute phase proteins: Produced by the liver in response to inflammation; enhance phagocytosis and complement activation.

Toll-like Receptors (TLRs)

• Examples:

  • TLR-2: Recognizes peptidoglycan from Gram-positive bacteria, activating inflammatory responses.

  • TLR-4: Recognizes LPS from Gram-negative bacteria, leading to the production of cytokines and chemokines.

  • TLR-3: Recognizes viral double-stranded RNA (dsRNA) in endosomes, triggering interferon production.

  • TLR-7: Recognizes single-stranded RNA (ssRNA) in endosomes, inducing interferon production.

Innate Immunity to Viral Infection

• Viruses are detected by:

  • The presence of double-stranded RNA (dsRNA) produced during replication, indicating viral replication within the cell.

  • The presence of DNA in the cytoplasm, which is unusual and signals viral or microbial infection.

• Cells respond by producing interferons, which are cytokines that induce an antiviral state in neighboring cells.

Detection of Viral dsRNA or Cytoplasmic DNA

• PRRs like TLR-3 and RIG-I detect viral components. TLR-3 is located in endosomes, while RIG-I is a cytoplasmic receptor.

• This leads to the synthesis and secretion of Type I Interferons (Interferon α and Interferon β), which have antiviral properties.

• IRF3 and IRF7 are transcription factors involved in the signaling pathway, leading to interferon gene expression.

Functions of Type 1 Interferons

• Resistance to viral replication:

  • Increased degradation of viral mRNA, reducing the amount of viral proteins produced.

  • Inhibition of viral protein synthesis, further limiting viral replication.

  • Increased antigen presentation of viral antigens, enhancing adaptive immune responses.

How Infected Cells Alert Neighboring Cells

• Infected cells produce interferon, which signals neighboring uninfected cells to destroy RNA and reduce protein synthesis (paracrine effect), thereby limiting the spread of the virus.

• Interferon also signals neighboring infected cells to undergo apoptosis and activates immune cells like natural killer (NK) cells.

Innate Immunity to Viruses

• Viruses can infect any nucleated cell.

• All nucleated cells can respond to viral infection by producing type 1 interferons. This is a critical early defense mechanism against viral infections.

• Interferon omega can be used to treat persistent viral infections of cats, such as FeLV/FIV, by boosting the antiviral response.

Natural Killer (NK) Cells

• Also known as large granular lymphocytes. NK cells are a type of cytotoxic lymphocyte critical to the innate immune system.

• Recognize decreased levels of MHC molecules on host cells, which can occur during viral protein synthesis, signaling that the cell is infected.

NK Cells Monitoring MHC Molecules

• Healthy cells express MHC molecules on their surface, presenting self-antigens to immune cells.

• Infected cells may have blocked MHC production by the virus to evade detection by the adaptive immune system.

Action of NK Cells

• NK cells leave healthy cells alone but kill infected cells that have reduced MHC expression. This prevents the virus from replicating and spreading.

• The activated NK cell releases toxic granules containing perforin and granzymes, killing the infected cell before viral replication is complete. Perforin creates pores in the target cell membrane, while granzymes induce apoptosis.

Summary of Innate Immunity to Viruses

• Innate immunity to viruses is relatively weak but crucial for early control of the infection.

• Interferon is produced, which is a natural antiviral molecule, inducing an antiviral state in neighboring cells.

• NK cells can identify and kill some virus-infected cells, limiting the spread of the virus.

• Innate immunity keeps you alive until your adaptive immunity develops, providing a more targeted and effective response.

Innate Immunity to Bacterial Infection: Phagocytosis

• Cellular mechanisms:

  • Recognition of pathogen (membrane, vesicular, and cytoplasmic PRRs). PRRs detect PAMPs on bacteria, initiating an immune response.

  • Response: Phagocytosis & inflammation. Phagocytes engulf and destroy bacteria, while inflammation helps to recruit more immune cells to the site of infection.

• Humoral mechanisms:

  • Recognition of pathogen (soluble PRRs). Soluble PRRs like complement proteins recognize bacteria in the blood.

  • Response: Killing of foreign organism, enhanced phagocytosis, and inflammation. Complement activation leads to bacterial lysis, opsonization, and inflammation.

PRRs for Bacteria

• TLR-2: Peptidoglycan

• TLR-4: Lipopolysaccharide

• TLR-5: Flagellin

• TLR-9: Prokaryotic DNA

• NOD2: Muramyl dipeptide

NOD2 Receptor and Crohn's Disease

• A defect in the NOD2 receptor is responsible for Crohn's disease in humans and possibly inflammatory bowel disease or anal furunculosis in German Shepherd Dogs. NOD2 mutations impair the ability to recognize bacterial components, leading to chronic inflammation in the gut.

Phagocytosis and Inflammatory Response

• Recognition of bacteria by macrophage TLRs leads to phagocytosis and an inflammatory response. TLR activation triggers the release of cytokines and chemokines.

• Phagocytosis is performed by neutrophils and macrophages, which engulf and destroy bacteria.

• Bacteria are endocytosed, killed, and digested within phagolysosomes.

The Respiratory Burst

• Enhanced cellular aerobic metabolism used by phagocytes to kill bacteria.

• Production of reactive oxygen intermediates (ROI):

  • Superoxide anion $(O_2^-)$

  • Hydroxyl radicals $$(OH