Innate Immunity Mechanisms

• Barriers to infection are present in the first immuno week session.

Physical Barriers Against Microbial Infection

• Tight junctions between epithelial cells prevent microbial passage.
• The stratum corneum, or outer layer of the skin, removes adherent microbes through flaking.
• Mucus captures pathogens as well as foreign agents and prevents microbial attachment to host cells.
• Mucus is removed by sneezing, coughing, peristalsis, vomiting, diarrhea, and ciliary action.
• Mucus is produced by goblet cells or mucosal tissues.
• Epithelial layers produce protective substances,
• Acidic pH as provided by the HCl in the stomach degrades microbial structures.
• Enzymes and binding proteins break down microbes.
• Antimicrobial peptides (AMPs) directly lyse microbes.

Chemical Barriers Against Microbial Infection

• Defensins penetrate microbial membranes and disrupt their integrity to form pores that lead to osmotic lysis.
• Defensins are constitutively secreted at mucosal surfaces to maintain levels of normal microbiota, thus reducing overgrowth.
• Defensins function poorly in physiological conditions but operate best in sweat, tears, the gut lumen, and phagosomes.
• Paneth cells, epithelial cells, and neutrophils produce defensins.
• Upon binding, pentraxins allow phagocytes to engulf and digest the attached pathogen with phagocytosis
• IL-6 is the primary cytokine driving pentraxin production.
• Liver output of pentraxins is significantly increased when hepatocytes are stimulated by IL-1, IL-6, and TNF-alpha, and locally in tissues by infiltrating immune cells upon innate receptor activation.
• Lysozyme is found in saliva, tears, mucus, plasma, and tissue fluids.
• Lysozyme hydrolyzes the 1,4 beta linkages between NAG and NAM, increasing permeability, causing the bacterial cell wall to collapse and the bacteria to burst.
• Phospholipase A2 is found in body secretions.
• Phospholipase A2 penetrates the bacterial cell wall and hydrolyzes membrane phospholipids to produce free fatty acids and lysophospholipids.
• Cathelicidins are released by skin and mucosal epithelial cells.
• Cathelicidins damage bacteria, fungi, viruses, and parasites, creating disruption in the cell membrane.
• The active human cathelicidin is LL-37, which is 37 residues in length.
• LL-37 is released in the skin and mucosa to disrupt microbe membranes through micelle formation.
• Fatty acids are found in perspiration and sebaceous secretions.
• Fatty acids impact the generation of peptidoglycan of Gram-positive and Gram-negative bacteria, limiting growth and disrupting the cell membrane, like LL-37.
• Fatty acids can block horizontal transfer of genetic information between bacteria known as conjugation.
• Lactoferrin is found in body secretions and tissue fluid.
• Lactoferrin sequesters and transports iron in body fluids such as tears, saliva, and milk.
• Lactoferrin binds to Gram-positive bacteria, interrupting peptidoglycan synthesis, and Gram-negative bacteria, disrupting the outer membrane cell walls.
• Transferrin is found in plasma.
• Transferrin is a main protein that sequesters and transports iron in the blood.

Mechanisms of Action of Physical and Chemical Barriers

• Chemical barriers include the complement system and cytokines.
• The three functions of the complement system and cytokines are microbial lysis, inflammation, and enhanced phagocytosis.
• Complement proteins coat the bacterial surface and extracellular viral particles to make them more easily phagocytosed, especially encapsulated bacteria.
• Cytokines are proteins produced in response to an antigen.
• Interleukins are cytokines that operate between white blood cells, like IFNs.
• Macrophage activation by microbe encounter leads to production of IL-12.
• NK cells are activated by IL-12.
• Activated NK cells release IFN-gamma.
• IFN increases toxin production by macrophages to aid in the destruction of phagocytosed microbes.

Acute Phase Proteins

• Positive APPs increase certain plasma proteins associated with immune reactivity.
• Negative APPs decrease certain plasma proteins involved in transport.
• Ferritin sequesters free iron in cells
• Ferritin levels in the plasma reflect the amount of stored iron, indicating iron deficiency.
• Fibrinogen is a blood coagulation factor.
• Hepcidin prevents ferritin from releasing iron.
• Serum amyloid A has a chemotaxis of immune cells.
• Albumin maintains osmotic pressure of blood.
• Transferrin transports iron in the blood and sequesters free iron from microbes.
• Acute phase proteins in immunity operate to limit resources to microbial species and promote immune capture and the destruction of invaders.

Role of Normal Flora in Infection Prevention

• Commensal bacteria outcompete pathogens and stimulate microbial surveillance by immunity.
• Aspiring pathogens must compete with commensal bacteria for nutrients and space.
• Overuse or chronic use of antibiotics can destroy normal flora, allowing opportunistic pathogens a chance to attach and colonize, which can lead to infection.
• Before birth, mammalian babies have no commensal microbes.
• The highest density of bacteria is present in the large intestine.
• A westernized diet that is high in saturated fats and sugars, low in fiber, aging, and a reduction in physical activity reduces microbiome diversity.
• Increased fiber intake results in more microbial diversity and less inflammation.

Receptors for Microbial Detection

Innate Receptors and Cellular Distribution

• Pathogen-associated molecular patterns (PAMPS) are highly conserved structures of microbial species that change little over time.
• Damage-associated molecular patterns (DAMPS)
• Pattern Recognition Receptors (PRRs).
• Activation of PRRs leads to cytokine production and secretion, inflammation, and immune cell recruitment.
• Binding to PAMP promotes the uptake of microbes by phagocytes and cytokine production.

Types of PRRs

• Endocytic PRRs promote receptor-mediated phagocytosis in macrophages and neutrophils.
• C-type lectin receptors are mannose receptors that recognize terminal sugars on the microbe surface, including glucans on fungi.
• Scavenger receptors can bind to mediate the uptake of bacteria expression lipoproteins.
• Formyl Peptide Receptors (FPR) bind to fMET residues, which are amino acids coded by the AUG start codon, as Fmet attached peptides are released upon degradation of bacteria or host cells.
• Opsonin receptors are produced from the activation of the complement system to produce opsonins that can be recognized by complement protein receptors (CPRs) on phagocytes, thus enhancing phagocytosis.
• Signaling PRRs triggers cytokine production on the cell surface, in endosomes, and in the cytoplasm.
• Toll-like receptors (TLRs) are found on the cell surface for extracellular bacteria and on endosomal membranes for intracellular microbes.
• MyD88 recruitment to TIR
• Mast cell activation by TLR ligation causes endothelial cell contraction, increasing vascular permeability
• TLR-4 recognizes bacterial LPS.
• TLR-7 detects single-stranded RNA from viruses.
• TLR-9 detects unmenthylated CpG DNA from DNA viruses.
• NOD-like Receptors (NLRs) are cytoplasmic receptors that detect bacterial cell wall components.
• NOD1 detects degraded peptidoglycan of Gram- bacteria.
• NOD2 detects degraded peptidoglycan of other bacteria.
• Phosphorylated IKK attaches to NFkB with IkB, NFkB translocates to the nucleus, and induces cytokine production, phagocytosis, and defensin synthesis, but IkB degradation is required.
• RIG-I-like receptors (RLRs) detect viral RNA in the cytoplasm.
• Viral RNA binds to RIG-I and MDA-5, and through CARDS interacts with MAVS.
• This leads to phosphorylation of IRF3 which activate dimers transcription factor for type 1 IFNs.
• Autocrine as well as paracrine type I IFN signaling induces the antiviral state.
• The phosphorylation and inactivation of elF2 inhibits viral protein synthesis

Cytokines Released Upon Pathogen Recognition

• Pro-inflammatory cytokines include TNF-alpha, IL-1, and IL-6.
• Initial sources of pro-inflammatory cytokines are tissue cells, tissue-resident mast cells, and macrophages
• Lead to leukocyte attachment and migration into tissues, such as the capillary endothelium.
• IL-6 results to fever, muscle aches, and joint aches in the brain, as well as acute phase protein release from the liver and complement activation.
• IL-6 increases the output of neutrophils from the bone marrow as well as increased glucose metabolism and local heat in adipocytes and monocytes.
• Vasodilation, increased vascular permeability, and upregulation of adhesion molecules occur on endothelia
• Trigger fever and drive inflammation
• Inflammasomes are released by PRR ligation and induces IL-1 production from epithelial and T cells, macrophages, neutrophils, and monocytes.
• IL-1B stimulates an increase in prostaglandin E2 production, increasing cAMP which causes fever.
• IL-1B binding to its receptor elevates prostaglandin levels in the hypothalamus.
• IL-12 is produced early in response.
• First cells activated by IL-12 are natural killer (NK) cells, which destroy infected host cells.
• Type 1 interferons (IFN-alpha, IFN-beta) are antiviral cytokines but do not directly induce fever.
• Chemokines such as IL-8 cause neutrophil attachment as well as migration into tissue.
• Activates intracellular signaling, enhancing the affinity of integrins for ICAMs and strengthening adhesion and increasing integrin binding.

Innate Cytokines and Chemokines in Early Immune Responses

• Neutrophils are the first immune cells to infiltrate tissues.
• TNF-alpha and IL-1 drive inflammation and vascular permeability and also induce adhesion molecule expression.
• IL-6 triggers an acute-phase response.
• IL-8 attracts neutrophils to infection sites.
• The best host defense against viral infections is Type 1 IFNs, which induce an antiviral state, inhibiting replication
• IL-12 activates NK cells to kill infected cells.

Steps in the Migration of Immune Cells to Sites of Microbial Colonization

• Rolling:
  • IL-1 and TNF-a induce selectin expression on endothelial cells.
  • Selectin ligands on leukocytes bind weakly to selectins, which causes rolling.
• Adhesion:
  • TNF-a induces ICAM-1 and ICAM-2 expression on endothelia.
  • Neutrophils express integrins that can bind to ICAMs for firm adhesion.
• Chemotaxis:
  • IL-8 binds to neutrophil receptors, increasing integrin affinity.
  • Neutrophils follow the chemokine gradient.
• Extravasation:
  • TNF-a weakens tight junctions.
  • Neutrophils squeeze through endothelial layers into tissues.

Explore the multifaceted mechanisms of innate immunity, including pathogen attachment prevention, antimicrobial peptides like defensins and LL-37, and the roles of lysozyme and Phospholipase A2. Learn about the complement system, cytokine stimulation, and physical barriers against infection.