Innate Immunity

• The lecture covers innate immunity, emphasizing its characteristics and components.
• Prof. Alex Vargas S. is the instructor for the Veterinary Immunology Chair.

Characteristics of Innate Immunity

• Specificity is based on Pattern Recognition Receptors (PRRs).
• Diversity is limited compared to adaptive immunity.
• There is no immunological memory.

Components of Innate Immunity

• Bactericidal mechanisms are involved.
• Resistance systems such as skin, mucous membranes, and the ciliary apparatus are essential.
• Cells, complement, phagocytosis, and inflammation are all part of it.
• PRRs are key components.

Resistance Systems

• Skin, mucous membranes, and the ciliary system are involved.
• Epithelial barriers offer physical protection against infection.
• Antimicrobial peptides destroy pathogens.
• Intraepithelial lymphocytes recognize and eliminate pathogens.
• Endogenous microflora is present in mucosal immunity.
• Peristalsis and secretions are defense mechanisms.
• Resident leukocytes are present.
• Mucociliary system, surfactants, IgA, defensins, lysozyme, nasal filter, and alveolar macrophages are essential for resistance.

Bactericidal Mechanisms

• Phagosomes digest bacteria.
• Defensins, lysozyme, lactoferrin, MPO, and lysosomal proteins are involved.
• Reactive oxygen species (ROS) participate.
• Oxygen-dependent mechanisms involve superoxide anion, hydroxyl radical, hydrogen peroxide, and hypochlorite anion.
• Nitrogen-dependent mechanisms involve nitric oxide, nitrogen dioxide, and nitrous acid.
• Independent of O2: lysozyme destroys bacterial cell walls.
• Lactoferrin sequesters iron and defensins are antimicrobial peptides.
• TNF-α activates inflammation, C-reactive protein activates the complement system.
• Protein BPI destroys Gram-negative bacteria.

Antimicrobial Peptides

• Cathelicidins are produced by neutrophils and epithelial barriers, and are toxic to microorganisms.
• Cathelicidins can bind to LPS and neutralize it.
• Defensins are cationic peptides that create pores in bacterial and fungal membranes.
• Three types of defensins that have been described are: α, β y θ.

Damage-Associated Molecular Patterns (DAMPs)

• DAMPs are endogenous molecules released by damaged or dying cells.
• DAMPs are released due to infections, burns, chemical toxins, trauma, or oxygen deprivation.
• Apoptotic cells generally do not release DAMPs.
• Extracellular DAMPs include collagen-derived peptides, fibrinogen, hyaluronic acid, heparan sulfate, laminin, elastin, and fibronectin.
• Intracellular DAMPs include HMGB-1, chromatin, heat shock proteins, defensins, cathelicidins, lactoferrin, uric acid, galectins, adenosine, N-formyl peptides, and S100 proteins.
• HMGB-1 affects endothelium, macrophages, neutrophils, epithelium, and dendritic cells.
• The effects of HMGB-1 include inflammation, tissue damage, and shock.

Pathogen-Associated Molecular Patterns (PAMPs)

• PAMPs are microbial structures shared by microorganisms that stimulate the innate immune system.
• Viruses, Gram-positive/negative bacteria, and other microorganisms express PAMPs, such as nucleic acids (DNA, unmethylated CpG sequences), lipids, and complex carbohydrates like LPS.

How the Immune System Recognizes Pathogens

• The immune system has specific receptors distributed in a non-clonal manner that detect conserved molecules in microorganisms.
• These receptors are called Pattern Recognition Receptors (PRRs).

Pattern Recognition Receptors (PRRs)

• PRRs are receptors that recognize molecular patterns.
• Macrophages, neutrophils, B lymphocytes, and dendritic cells all have them.
• PRRs can be soluble and bind pathogens in the blood, like C1q, MBL, ficolins, pentraxins etc.
• PRRs can be transmembrane or intracellular

Types of PRRs

• Toll-like receptors (TLRs): They recognize a broad spectrum of pathogens and are expressed by dendritic cells, macrophages, mast cells, epithelial cells, and eosinophils.
• There are at least 14 different TLRs known, localized on cell surfaces and on endosomes.
• TLR ligands include bacterial lipopeptides (TLR1), bacterial peptidoglycans and lipoteichoic acid (TLR2), viral dsRNA (TLR3), LPS and viral proteins (TLR4), bacterial flagellin (TLR5), RNA viral (TLR7/8) and CpG DNA bacterial (TLR9).
• NOD-like receptors (NLRs): Cytosolic proteins that recognize bacterial products like peptidoglycan fragments.
• NLRs are found in epithelial cells, macrophages, and dendritic cells.
• RIG-1-like receptors (RLRs): Cytoplasmic receptors that recognize viral RNA.
• RLR activation leads to the transcription of type I interferons.
• RIG-I, MDA5, and LGP2 have been identified.
• Lectin-type receptors exist as soluble (blood, extracellular fluids) or transmembrane proteins, that bind to carbohydrates.
• Mannose-binding lectin (MBL) is a protein that recognizes carbohydrates like mannose, fucose, and N-acetylglucosamine, activating the lectin pathway.
• There are three families of lectins involved in innate immunity: P-type (pentraxins), S-type, and C-type (colectins).
• C-type lectin receptors (CLRs) are distributed in endothelial cells, macrophages, and dendritic cells, functioning against fungi.

Proteins

• SP-A and SP-D are collectins that act as soluble pattern recognition molecules secreted by the respiratory epithelium and lungs.
• SP-A and SP-D act as opsonins, and inhibit pathogens from invading tissues.
• Ficolins are pattern recognition molecules that bind acetylated carbohydrates and can activate the complement system .
• There are 3 types: L, M, and H.
• C1q directly binds LPS in bacterial cell walls.
• C1q binds antibodies that have opsonized pathogens.
• C1q binds bacteria opsonized by C-reactive protein and activates the classical complement system.
• Pentraxins are soluble PRRs that form multimers of 5 subunits.
• Short pentraxins are C-reactive protein (CRP) and serum amyloid P (SAP); long pentraxins are PTX-3.
• CRP, SAP, and PTX3 bind C1q to activate/regulate the complement system.
• C-reactive protein binds to phosphocholine in bacterial and mycotic lipopolysaccharides, acting as an opsonin.
• CRP bound to bacterial surfaces can be recognized by C1q activating the complement system.
• DNA sensors that activate antimicrobial responses such as the production of type I interferons.
• Examples are cGAS, interferon gamma inducible 16(IFI16), absent in melanoma 2 (AIM2), and DNA-dependent interferon regulator.
• cGAS, IFI16, AIM2, and DAI converge on STING, involved in production of type I interferons.
• STING is a transmembrane protein which is indirectly activated by cytosolic DNA.

N-formyl metionil (FPR)

• Receptors for N-formyl metionil are found on neutrophils and macrophages and bind bacterial peptides containing N-formylmetionil, a residue present in prokaryotes, but not in eukaryotes.
• FPR (neutrophils) and FPRL-1 (macrophages) are examples of cells with these receptors .
• Scavenger receptors (SR) are distributed on macrophages to recognize modified LDL, microbial ligands such as bacteria, viruses, fungi, or endogenous ligands and mediates phagocytosis.

Cells of the Innate Immune System

• Monocytes, macrophages, dendritic cells, mast cells and NK cells
• Neutrophils, eosinophils, and basophils are innate immune system cells.
• Innate lymphoid cells are family of lymphocytes which respond rapidly to tissular damage by releasing cytokines.
• ILCs lack specific receptors, but detect stimuli in the microenvironment such as released cytokines, nutrients and microbial components.
• The three groups of ILCs are ILC1/NK, ILC2, and ILC3/LTi.
• Macrophages, mast cells, dentritic cells, fibroblast and leucocytes are included.
• Mononuclear phagocytic cells include monocytes, macrophages, microglia, histiocytes, and Kupffer cells.

Macrophages

• Macrophages include surface receptors like CD16, CD32, CD64, CD35, CD11b/18, CD71, CD25, and CD40.
• Tissue-resident macrophages detect pathogens, sending signals to recruit circulating monocytes.
• Macrophages can be inflammatory (M1) or regulate inflammation (M2 as a result of secreted cytokines and phagocytosis.

Macrophage Activation

• TLR ligands activate M1 macrophages to produce ROS, NO, lysosomal enzymes, thus internalize pathogens.
• IL-4 and IL-13 activate M2 macrophages to produce the anti-inflammatory cytokines and modulate fibrosis.