03 - Innate Immunity

phases of immune response

• immediate innate response: 0-4 hours

  • antimicrobial peptides → disrupt microbial membranes

  • complement → opsonize and lyse microbes

  • resident macrophages → phagocytose pathogens, release cytokines

  • dendritic cells → sample antigens, prepare adaptive immunity

• induced innate response: 4 hours - 4 days

  • cytokine release → induce vascular permeability, raise temperature

  • chemokines → recruit neutrophils and monocytes

  • fever → restrict microbial survival

  • inflammation → stimulate dendritic cell migration to lymph nodes

• adaptive immune response: >4 days

  • occurs only if innate immune response fails

  • T cell and B cell activation

barriers of innate immunity

• mechanical barriers → tight junctions, mucus, cilia, fluid flow

• chemical barriers → low pH, lysozymes, antimicrobial peptides

• microbiological barriers → normal microbiota

  • antibiotics will cause loss of microbiota → C. difficile, Candida

antimicrobial peptides (AMPs)

amphipathic proteins that insert into membranes, forming pores and causing cell death (e.g. defensins)

• constitutively expressed or inducible

• produced by neutrophils, keratinocytes, paneth cells

• can be downregulated by cytokines

  • IL4 → decreased defensins → eczema → S. aureus

complement

set of proteins found in blood and lymph

• opsonization → C3b

• inflammation → C3a, C5a

• cell lysis → MAC: C5b - C9

complement system pathways

• alternative → first-activated

  • environment at pathogen surface alters C3 conformation to resemble that of activated C3b

• lectin pathway → second activated

  • mannose-binding lectin binds to pathogen surface

  • activates complement cascade to produce C3b

• classical pathway → last activated

  • C-reactive protein or antibody binds specific antigens on pathogen surface

  • activates complement cascade to produce C3b

macrophages

mature form of monocytes

• found in tissues

• long-lived

• produce cytokines

• initiate inflammation

neutrophils

most abundant WBC

• short-lived

• attracted by cytokines produced by macrophages

• forms pus

• kill using reactive oxygen species

• undergoes apoptosis or NETosis (neutrophil extracellular traps)

innate immune receptors

pattern recognition receptors (PRRs) recognize pathogen-associated molecular patterns (PAMPs)

• binding facilitates phagocytosis, production of antimicrobial products, cytokine release and inflammation, and expression of co-stimulatory molecules

production of anti-microbial products

binding immune receptors to PAMPs can initiate production of antimicrobial products

• acidification → pH of 3.5-4.0 is bacteriostatic or bacteriocidal

• respiratory burst → generates reactive oxygen / nitrogen species

• antimicrobial peptides → defensins

• enzymes → lysozyme, acid hydrolase

pathogen associated molecular patterns (PAMPs)

danger signals that trigger innate immune response

• lipopolysaccharides (LPS)

• peptidoglycan

• teichoic acid

• flagellin

• dsRNA

• CpG DNA (unmethylated)\

• zymosan (fungi)

toll-like receptors

family of transmembrane proteins on macrophages that act as PRRs in innate immunity

• sense danger, trigger signal transduction, and induce inflammatory cytokine and interferon release

• surface TLRs → sense bacterial products

• endosomal TLRs → sense viral nucleic acids

• TLR4 → detects LPS from gram-negative bacteria

• TLR9 → detects unmethylated CpG DNA

• TLR3 → detects dsRNA from viruses

bacterial lipopolysaccharide is recognized by

TLR4, MD2, and CD14 complex

signal transduction for cytokine release

NFκB is the transcription factor that turns on inflammatory genes

• TLR engagement → bacterial LPS is recognized by complex of TLR4, MD2 and CD14

• IKK activation → inside the cell, MyD88 binds TLR4 to activate IRAK4 for phosphorylating TRAF6; leads to IKK activation

• IκB degradation → IKK phosphorylates IκB, inducing its degradation and allowing passage of NFκB into nucleus

  • IκB → inhibitor of NFκB

  • IKK → inhibitor of NFκB kinase

• NFκB enters nucleus → NFκB induces transcription of cytokine genes for release via ER

• steroids upregulate IκB, reducing NFκB and reducing inflammation

intracellular innate receptors

• bacteria → NOD-like receptors (NLRs)

  • detect bacterial cell wall fragments in cytoplasm

  • NOD1/2 bind fragments and dimerize

  • RIPK2 recruited to activate NFκB

• viruses → RIG-I-like receptors (RLRs)

  • detect viral DNA that should not be in cytoplasm

  • virus infects cell, and its RNA accumulates in cytosol

  • RIG-1 / MDA5 bind viral RNA to activate adaptor proteins

  • activates interferons and NFκB

cytokines

substances released by macrophages for inflammatory response at site of infection

• TNF-⍺ → induces vessel permeability

• IL-1β → induces fever, activate endothelial cells to express adhesion molecules

• IL-6 → trigger acute phase by inducing CRP and MBL production

• CXCL8 (IL-8) → recruits neutrophils

• CCL2 → recruits monocytes

• IL-12 → activates NK cells to secrete cytokines

NLRP3 inflammasome

multi-protein complex of innate immunity that senses pathogens and cellular damage, triggering inflammation and producing cytokines

• activated by ATP and damage-associated molecular patterns (DAMPs)

• activates caspase-1

• converts pro-IL-1β and pro-IL-18 into IL-1β and IL-18

• positive feedback loop → cytokine storm

  • IL-5 induces proinflammatory pathways

acute phase proteins

IL-6 induces hepatocytes to synthesize acute-phase proteins

• C-reactive protein (CRP) → opsonizes microbes, activates complement

  • activates classical pathway

  • serum indicator of systemic inflammation

• mannose-binding lectin (MBL) → binds mannose residues on pathogens, activates complement

interferons ⍺ and β

produced by virus-infected cells and plasmacytoid dendritic cells

• inhibit viral replication by shutting down protein synthesis

• upregulate MHC1 to enhance CD8+ recognition

• activate NK cells to kill virus-infected cells

• induce antiviral genes in neighboring cells

• activate dendritic cells and macrophages

• induce chemokines to recruit lymphocytes

NK cells

innate cytotoxic T cells without T cell receptors that kill stressed, infected, and transformed cells

• multiple receptors → balance of activating vs inhibitory signals

• ligands → glycoproteins in cells undergoing stress

• activated by IFN-⍺, IFN-β and IL-12

• produces IFN-ɣ to stimulate T cells and macrophages

STAT1 / STAT2 activation

type I interferons activate STAT1 and STAT2 to induce resistance against viral infection

• increase MHCI expression

• activate NK cells

• induce chemokines to recruit additional immune cells