Innate Immunity

Innate Immunity is mediated by resistance factors that are present before infection and nonspecific - i.e., work all the time and are effective against many different kinds of microbes

External resistance factors - those which act on body surfaces

physical
- barriers - skin, mucus, nasal hair
- environmental conditions - dryness
- flushing action - blinking of eyelid, peristalsis, urination
chemical - activities carried out by molecules
- salts - secreted by sweat glands in skin
- acids - low pH (HCl in stomach; lactic and other skin acids produced by normal microbiota)
- lipids - fatty acids secreted by sebaceous glands and by gall bladder
- enzymes - lysozyme (secretions), digestive enzymes (stomach, small intestine)
cellular - activities carried out by cells
- normal microbiota compete with pathogens for attachment sites and nutrients as well as producing factors toxic to pathogens
- phagocytes - macrophages and polymorphonuclear leukocytes (PMN) are leukocytes that engulf and destroy particles on mucous membranes by a process known as phagocytosis

Internal resistance factors - those which act within tissues

physical - barriers such as connective tissue, blood-brain barrier (image ... more info)
chemical - activities carried out by molecules
- enzymes - lysozyme and other enzymes in blood and tissue fluids
- complement - a naturally-occurring, self-regulating system consisting of about 30 proteins (e.g., components C1-C9; factors B, D, H, I) that can be nonspecifically activated by polysaccharides or lipopolysaccharides present on pathogens, including bacteria, fungi, etc.
  - C3 convertase
    - C3 is cleaved by proteases (or esterases) present in blood and tissue fluids to generate fragments designated as C3a and C3b
    - C3 possesses a labile thiolester group is exposed via cleavage to C3b, allowing this component to form a covalent (ester or amide) bond with free hydroxyl or amino groups on microbial surfaces
    - C3b binds factor B in the presence of Mg ions, to form a C3b,B complex
    - factor D cleaves factor B to generate factor Bb, which is part of the C3b,Bb complex on the microbial surface
    - Properdin binds to this complex to generate stable C3 convertase (C3b,Bb,P), which cleaves C3 to form fragments designated as C3a and C3b
  - C5 convertase
    - generated by binding of multiple C3b molecules to C3b,Bb complexes on the microbial surface, thus forming C3b_n,Bb,P complexes
    - activity - cleaves C5 into C5a and C5b
  - membrane attack complex (MAC)
    - generated in three steps:
      - C5b,6,7 complex, which binds to the microbial membrane via hydrophobic tails that are unfurled by C6 and C7 as a result of allosteric conformational changes that occur when C6 and C7 bind sequentially to C5
      - C8 binds to the membrane-bound C5b,6,7 complex
      - C9 binds to the membrane-bound C5b,6,7,8 complex
    - lyses (disrupts) enveloped viruses and Gram-negative bacteria
  - other activities mediated by complement
    - C3a and C5a trigger inflammation
    - enhances phagocytosis (opsonization) by promoting binding of particles, such as pathogens, to phagocytes via C3b receptors (C3bR)
  - negative regulation is mediated by a number of components, the most important of which is factor I, also known as C3b inactivator (C3b Ina)
    - factor H binds to neuraminic (sialic) acid residues on cell surface polysaccharides, then substitutes itself for factor Bb in C3b,Bb complexes
    - factor I then cleaves C3b, forming iC3b

cellular - activities carried out by cells

Pattern Recognition Receptors (PRRs)
- phagocytes (and some other cells, such as epithelial cells) utilize pattern recognition receptors (PRR) to recognize (bind) conserved molecular moieties frequently referred to as pathogen-associated molecular patterns (PAMPs) that are found in bacteria, fungi and viruses
- PRR binding of these conserved molecular moieties (ligands) activates immune system cells to produce cytokines as well as upregulating certain other factors/functions that play roles in host defense against pathogens
- PRRs include membrane-associated toll-like receptors (TLRs) and cytoplasmic receptors such as nod1 and nod2
  - TLRs are cell-associated receptors that detect pathogen-associated molecular patterns (PAMPs) found in bacteria, fungi and viruses
  - TLR binding of the appropriate ligand (see table) induces intracellular signaling:

TLR	Ligand
TLR 1	unknown*
TLR 2	peptidoglycan
TLR 3	dsRNA
TLR 4	LPS
TLR 5	flagellin
TLR 6	lipoproteins**
TLR 7	imidazoquinolines*
TLR 8	imidazoquinolines*
TLR 9	CpG motifs*
TLR 10	unknown*

* Natural ligands for TLR1, TLR7, TLR8 and TLR10 are not known, but TLR7 and TLR8 bind several imidazoquinoline antiviral drugs, including: Bropirimine (2-amino-5-bromo-6-phenyl-4-pyrimidinone); Imiquimod (1-(2-methylpropyl)-1H-imidazo> 4,5-c@quinolin-4-amine); Loxoribine (7-Allyl-8-oxoguanosine); Resiquimod (R-848;4- amino-2-ethoxymethyl-a,a-dimethyl-1H-imidazol[4,5-c]quinoline-1-ethanol)
** TLR6 (in association with TLR2) binds myoplasma lipoproteins
*** TLR9 binds bacterial DNA containing unmethylated CpG motifs

most commonly, TLRs utilize the MyD88-dependent interleukin 1 receptor (IL-1R)-TLR signaling pathway, resulting in activation of NF-kB
NF-kB is a transcription factor that plays a role in upregulation of a number of genes that encode cytokines including those encoding proinflammatory cytokines such as tumor necrosis factor-a (TNFa) and the regulatory cytokine IL-12

nod1 and nod2 (nod = nucleotide-binding oligomerization domain containing)
- nod1 mediates NF-kB responsiveness to Gram-negative peptidoglycans (diaminopimelate-containing N-acetylglucosamine–N-acetylmuramic acid (GlcNAc-MurNAc) tripeptide motifs) (DATP) but not Gram-positive peptidoglycans (induces IL-8) via an MyD88-independent pathway
- nod2 binds muramyl dipeptide (MDP), which is a peptidoglycan constituent of both Gram positive and Gram negative bacteria

interferons
- interferon alpha and interferon beta are produced by many types of cells (both immune and nonimmune) when they are infected with (most) viruses; they help diminish viral replication
- interferon gamma is produced by type 1 T helper cell (Th1 cells) and acts to stimulate macrophage activity in addition to helping diminish viral replication (not considered part of innate immunity)
inflammation - "early-warning" system whose cardinal signs include erythema (redness); edema (swelling); fever (heat); pain (soreness)

mast cells and basophils are well-known mediators of acute inflammation
- histamine released from mast cells at sites of tissue damage
- histamine (and other mediators) increases permeability of capillaries at these sites
- capillaries become "leaky" causing accumulation of complement, enzymes and phagocytes at sites of infection
- pathogens eliminated (phagocytosis, etc.) and healing initiated
activated complement components also induce acute inflammation
- C3a and C5a both trigger histamine release from mast cells and basophils
- C5a triggers increased vascular permeability by direct action on capillary endothelial cells
- C5a induces chemotaxis of phagocytes towards sites of infection
macrophages and neutrophils (eosinophils, too?) are now becoming known as mediators of acute inflammation
- IL-1, IL-6, IL-8, TNF-alpha are secreted by these cells when they are stimulated by a range of factors, including LPS
- these proinflammatory cytokines trigger increased vascular permeability and its sequalae to induce and maintain acute inflammation

phagocytosis process by which macrophages, dendritic cells, and polymorphonuclear leukocytes (PMN) engulf and destroy particles (animation)
- adhesion (attachment)
  - binding of phagocyte and pathogen using noncovalent interactions
  - opsonization is enhancement of phagocytosis due to better binding mediated by complement or antibody receptors on the phagocytes' surfaces
- ingestion (engulfment)
  - pseudopodia surround the pathogen, then fuse to form a vacuole called a phagosome
  - phagosomes are converted into phagolysosomes by fusion with lysosomes (vacuoles containing a broad spectrum of digestive enzymes) which release their contents into the newly-forming phagolysosomes
- digestion/killing occurs inside phagolysosomes
  - killing
    - oxygen-dependent
      - reactive oxygen intermediates - superoxide anion, hydrogen peroxide and hydroxyl radical - kill microbes directly by oxidizing their nucleic acid and proteins
      - myeloperoxidase is activated by hydrogen peroxide to generate hypochlorous acid, which transfers Chloride ions to various functional groups on amino acid sidechains of proteins (especially to the -OH of tyrosine)
    - oxygen-independent
      - defensins - antibiotic-like peptides made by phagocytes, especially PMNs
      - digestive enzymes present in the phagolysosomes damage microbes
  - digestion - enzymes present in the phagolysosomes digest the dead microbes
- egestion - the phagocyte ejects any material that could not be digested by a process that is essentially the reverse of ingestion
NK cells
- use perforins and granzymes to kill host cells that have modified by infection or oncogenesis
- discriminate between self, modified-self and non-self cells by triggering:
  - inhibitory pathways when they come into contact with self or certain modified-self cells
  - activation pathways when they come into contact with non-self or certain modified-self cells

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Innate Immunity

Innate Immunity is mediated by resistance factors that are present before infection and nonspecific - i.e., work all the time and are effective against many different kinds of microbes