Innate Immunity - Langer 3/31/16 Flashcards
Elie Metchnikoff
noticed that marine invertebrates displayed mobile ameboid cells that surrounded the site of a “poke”
found that these mobile ameboid cells could actually ingest things too → phagocytosis
innate immunity: functions
- initial response to microbes to prevent, control, eliminate infection
- stimulates subsequent adaptive immune response
* can influence or tailor it to specific type of microbe - can recog some pdts of damaged/dead host cells and eliminate them + initiate tissue repair
* often able to do this without inflammatory response
properties of innate immunity
- always functional, immediately available
- responds to common features of classes of microbes
- key molecules are encoded in genome (NOT PDTS OF ANTIGEN-SPECIFIC genetic rearrangements)
- stimulates adaptive immunity
- DOES NOT retain memory of antigens/pathogens encountered previously
properties of adaptive immunity
- requires exposure to antigen/pathogen; time lapse between exposure and immune response
- highly specific response (vs. general response of innate)
- generates memory based on past infection
- focuses and potentiates innate response
pathgen recognition in innate immunity
- players and what they recognize
- location
- effect of recognition
pattern recognition receptors (PRRs) recognize molecules on pathogens that have specific molecular patterns not seen in healthy hosts
- PAMPs : pathogen-assoc mol patterns - on pathogen/produced by pathogen
- DAMPs : danger/damage-assoc mol patterns - “unmasked” or unusual host components
PRRs locations:
- on cell membrane to sense PAMPs outside cell
- on endosomal membrane to sense PAMPs brought in by endocytosis
- in cytoplasm to sense PAMPs in cyto
binding of PRRs can…
- directly affect pathogen
- trigger cascade of molecular, cellular, global responses in host
- acute inflammation
- antiviral response
origin of innate immunity
long evolutionary need for organisms to find protective mechanisms against common classes of pathogens (bacteria, viruses, fungi, parasites) → evolution of molecules that recognize and protect hosts
- now, manifest as hard-wired molecules expressed in cells and protective responses that form rapid response to pathogens
functional outcomes of innate immune responses
1. acute inflammation (usually against microbes)
- against microbes; also in resp to damaged/dead cells, accumulation of abnormal substances in cells or tissues
- critical for would healing BUT chronic is bad/pathological
2. antiviral response
3. possible initiation of longer-term, specific adaptive immune response
- generation of antigen-specific T and B cells and antibodies
- whether or not occurs depends on pathogen, how much pathogen was encountered, and how quickly initial infection was resolved
PAMPs and DAMPs
what are they?
allow host organisms to recognize microbes and damaged/unusual self cells
PAMPs : pathogen-associated mol patterns
- unique molecules often essential for microbial survival
ex. single, double stranged RNA (viruses); pilin, flagellin protein (bacteria); cell wall lipids (bacteria); carbohydrates (fungi, bacteria)
DAMPs: damage-associated mol patterns
- produced by host cell damage (not as a result of apop)
ex. HSPs, monosodium urate crystals, HMGB1 (nuclear protein)
cell-associated PRRs
greatest variety expressed by phagocytes (macrophages, neutrophils) and dendritic cells
- epi and endo cells might also express
binding → activates signal transduction pathways leading to anti-microbial and pro-inflammatory responses
include. ..
1. Toll-like receptors
2. cytoplasmic PRRs
Toll-like receptors
TLRs
cell surface TLRs recognize various PAMPs from bacteria or fungi
- part of intact pathogen
- shed by pathogen
- released following phagocytosis by macrophages, killing by neutrophils
endosomal TLRs recognize patterns of endocytosed pathogens (ex. viral RNA, unmethylated CpG dinucleotide motif of bacterial DNA)
binding → signal transduction cascade resulting in activation of transc factors in nucleus → key for inflammatory and antiviral responses
- infl response? NF-kappaB
→ production and secretion of “pro-inflammatory” cytokines: TNF alpha, IL-1, interleukin 6
- antiviral response? Interferon Response Factor family (INF)
→ production and secretion of Type I interferons (IFN-alpha, IFN-beta) - crucial to early antiviral defense
cytoplasmic PRRs
- 5 groups (descriptions, basic mechanisms of action)
imp bc some pathogens/viruses have parts of their life cycle take place in cytoplasm or escape from phagocytic vesicles and hang in cytoplasm (bacteria, parasites)
NOD-like receptors (NLRs) : immune infl, epithelial barrier cells
- binding of NLRs to PAPMPs or DAMPs → intracellular signaling cascade
key: activation of NF-kB (tf for infl genes), activation of infl cytokine IL-1
cytoplasmic sensors for RNA and DNA
- RIG-I RNA sensor senses viral RNA (doesn’t bind cellular mRNA bc of the 5’ cap!)
- DNA sensors may recognize bacterial DNA or host DNA showing stress or damage signs
bacterial carbohydrate receptors (C lectin family)
- lectins are proteins that bind carbs (C = Ca-dependent)
- aid in phagocytosis of microbes and can also stimulate signaling pathways
scavenger receptors
- cell surface receptors with broad specificity (ex. oxidized lipoproteins); mediate phagocytosis
FMLP (f-Met-Leo-Phe) receptor
- responds to N-term of bacterial proteins (fMet)
- unlike other receptors, these are Gprotein-coupled receptors
- activation of cell and cytoskeletal/cell surface changes that facilitate chemotais towards bacterial infection and entry into infected tissue
NOD-like receptors (NLRs)
major family of cytoplasmic PRRs occuring in immune, infl, epithelial barrier cells
binding of NLRs to PAMPs or DAMPs → intracellular signaling cascade
key: activation of NF-kB (tf for infl genes), activation of infl cytokine IL-1
*misreg of NLRs might be involved in pathogen of gut diseases (IBD)
*activation of NLRs and infl cytokines might be related to other infl disease (gout, pseudogout, lung disease from silica/asbestos exposure
cytoplasmic sensors for RNA and DNA
RIG-I RNA sensor senses viral RNA (doesn’t bind cellular mRNA bc of the 5’ cap!)
DNA sensors may recognize bacterial DNA or host DNA showing stress or damage signs
bacterial carbohydrate receptors
C lectin family
- lectins are proteins that bind carbs (C = Ca-dependent)
- aid in phagocytosis of microbes and can also stimulate signaling pathways
scavenger receptors
- cell surface receptors with broad specificity (ex. oxidized lipoproteins); mediate phagocytosis
FMLP (f-Met-Leo-Phe) receptor
- responds to N-term of bacterial proteins (fMet)
- unlike other receptors, these are Gprotein-coupled receptors
- activation of cell and cytoskeletal/cell surface changes that facilitate chemotais towards bacterial infection and entry into infected tissue
cell types involved in innate immunity
- phagocytes (macrophages, neutrophils)
- dendritic cells
- NK dells
- mast cells
role of epithelial barrier
first line of defense
skin and mucosa of respiratory, GI, GU tracts = main sites of interaction with microbes
- play part of physical barriers to infection, are immunologically active, incorp several surveillance/effector cells, produce secretions/antimicrobial molecules
- defensins have direct antimicrobial toxicity, activate cells involved in the infl response
- cathelicidins have microbicidal activity, immune activation fx
- intraepithelial T lymphocytes : T lymphocytes typically associated with adaptive response, however, some have fixed/ltd antigen specificity to recognize common PAMPS
phagocytes
- first line of defense after physical barriers
- engulf, kill/process microbes
two primary types:
- macrophages
- neutrophils
macrophages
type of phagocyte
arise from circulating monocytes that migrate into tissues and differentiate → residential macrophages
alveolar macrophages (lung), microglia (CNS), Kupffer cells (liver), osteoclasts (bone) : all phagocytic, some have tissue-specific fx
primary fx: surveillance
activation via…
- binding of microbes to TLRs or other macrophage cel-surface PRRs
- signaling from other activated immune cells (ex. cytokine IFN-gamma from NK cells)
effects of activation:
- more efficient phagocytosis (expression of addt’l cell-surface PRRs)
- killing molecules/mechs upreg’d to better knock out phagocytized microbes (ROS and NO producing mechs in cells)
- production of “pro-infl” cytokins (TNF, IL1, IL6) upreg’d → activate other immune cells, change vascular permeability, initiate/propagate infl response
*macrophages also have roles in adaptive immunity
neutrophils
type of phagocyte
aka polymorphonuclear leukocytes (PMNs)
- make up 60% of white cells in blood; number goes up during infection
- first cells to infiltrate areas of bacterial and fungal infection
- dominant cells in tissue with acute infl
- phagocytose bacteria in circulation/in tissues
- short half-life (do not provide long-lasting defense)
natural killer cells
NK cells
originate from same lymphocyte lineage as T cells and B cells, but have distinguishing features/fx
- large cells with numerous cytoplasmic granules containing cell-killing substances
- perforin: facilitates entry of other NK cell contents into target cells
- granzymes: enzymes that enter the cytoplasm of target cells and initate signaling cascades → apoptosis
- also binding of antigen Fas to NK cell Fas ligand
*growth/activation can be stimulated by cytokines (IL12, IFN 1) from other cells or from environment
main fx
- kill virus-infected cells in early stage of viral infection
- kill cells with intracellular microbes
- release IFN-gamma → acivates macrophages for more efficient phagocytosis
how do NK cells know which cells to kill?
integration of cytoplasmic signaling pathways triggered by inhibitory and activating receptors
- inhibitory receptors recog Class I MHC mols (normally on surface of healthy nucleated cells)
- when viruses or stress hit, MHC mols involved in antigen presentation can be downreg’d → fewer inhibitory receptors engaged…
- activating receptors recognize diverse ligands
dendritic cells
- two types
- functions and how they carry them out
form a surveillance and antigen-capture network in epithelia and subepithelial tissues in several organs
1. “conventional DCs” (majority, incl Langerhans cells in skin)
fx: recognize and/or capture antigen (microbes or free antigen) via array of TLRs and other PRRs
once recog/capture antigen, 2 major effects (which can vary depending on antigen):
- produce pro-infl cytokins (TNF, IL1, IL6, IL12)
- leave resident tissue, travel to lymph node, mature and present antigen to naive T cells → crank up antigen-specific response by T cells
- link between innate and adaptive immunity!
2. plasmacytoid dendritic cell (pDC) (rarer, v critical)
high levels of TLR 3, 7, 9 iin endosome which allows them to response to ss/dsRNA and DNA (viruses) with high sensitivity
fx: recog RNA/DNA to start high level production of antiviral-family of type 1 IFN (IFN-alpha, IFN-beta), which (contrast to IFN-gamma)…
- have direct anti-viral effects → stimulate genes in nearby/distal cells to make them more virus-resistant
- modulate cellular reactions in other immune cells
- link between innate and adaptive immunity!
mast cells
primarily associated with immunity to multicellular parasite and allergy and asthma…but prob also have a role in innate immunity
- present in skin and mucosal epi, often adj to microvasculature and nerves
- numerous cytoplasmic granules (infl mediators like histamine, vasoactive amines, proteolytic enzymes)
- produce lipid mediators (prostaglandins) and cytokines (TNF)
- contain number of PRRs (most TLRs)
location of mast cells and presence of key PRRs suggests imp role in innate immunity - as yet not well understood
soluble recognition and effector molecules of innate immunity
found in serum and tissues, help tag and destroy pathogens via 2 main fx:
1. opsonization : act as opsonins - tag microbes to enhance recognition by cellular receptors on macrophages, neutrophils, dendritic cells
2. initiate or promote infl responses : attract phagocytes and/or participate in direct killing of pathgens
complement system
- basics
- 3 pathways
- 3 outcomes
best known, most imp soluble recog/effector molecules comprising approx 20 proteins, mostly produced in liver
- “complement” serum antibodies and help them kill cells under approp conditions
- activated via 3 pathways - classical, alternative, lectin - differ in early events, but all converge at critical C3 protein
- classical involves pathogen-specific antibodies (adaptive)
- alt/lectin do not depend on patho-specific antibodies - fully active in immunologically naive host (innate)
efficient/potent system with 3 primary outcomes:
- opsonize microbes for phagocytosis and killing
- mobilize and recruit phagocytes to sites of infection
- directly kill pathogens
alternative pathway (complement)
complement C3 is present in high conc in serum and tissue
- alt pathway is initiated by spontaneous hydrolysis of C3 → C3b and C3a
- C3b is highly reactive, binds to bacterial or fungal polysacchs or proteins
- if no targets nearby, reactive group on C3b hydrolyzes → C3b inactivated
- if targets nearby, C3b binds to cells → binds serum protein Factor B, which is cleaved by plasma protease to form Bb → bacteria bound to C3bBb complex
- C3bBb is a potent protease (aka C3 convertase) → further cleaves C3 → C3b
major consequences:
- pathogens are tagged with multiple C3b → targets for phagocytes which have C3b complement receptors
- complement pathway can run to finish → final recruitment of membrane attack complex (C5b, 6, 7, 8, 9) which can lyse some cells
- cleavage events release smaller fragments C3a and C5a → key roles in inflammation
- mobilize and recruit phagocytes, promote inflammation via binding to specific neutrophil C3a/C5a receptors (in GPCR family) → chemotaxis of neutrophils to infected site
- bind to receptors on mast cells →release of inflammatory mediators (incl histamine, leukotrienes) - critical effects in acute infl
- additional direct effects on cells (ex. sm muscle cells)
C3a, C5a aka anaphylatoxins bc can cause anaphylaxis
lectin pathway (complement)
- hinges on fact that carbs on surface of healthy human cells are diff from those on pathogens
- carbs are recognized by collectins, family of lectins incl MBL, pulmonary surfactant proteins A and D (SP-A, SP-D)
ex. mannose-binding lectin/mannan-binding lectin (MBL) recognizes terminal mannose/fructose residues on bacteria/fungi glycolipids or glycoproteins (not found on human cells)
* once bound, MBL recruits MBL-assoc serine proteases (MASPs) → cleave early proteins in complement pathway (C4, C2) → form a C3 convertase → cleaves C3 into C3a and C3b
primary outcomes:
- opsonization by C3b for binding to phagocytes
- potential assempty of membrane attack complex → cell lysis
- generation of C3a and C5a → infl response
classical pathway (complement)
relies on antibody for pathogen recog → involves both complement (innate) and antibodies (from B cells, adaptive)
generally only engaged on repeat/prolonged encounter with pathogen, once speicifc antibodies have been produced
- antibody-antigen complexes are bound by complement C1 to form protease → activates components C2 and C4 to form C3 convertase → cleaves C3 to C3a and C3b
- also, C4b,2a,3b complex acts as C5 convertase → cleaves C5, whose products can go on to bind and form membrane attack complex
antibody can also serve as an opsonin (independent of C1)
important aux features of complement system
- self cell protection
- immune complexes
self-cell protection
- host cells are protected from complement reactions via regulatory proteins at several point in pathway
ex. decay accelerating factor/DAF, CD59/protectin, protein I (inhibitor of C3b in serum/tissue)
immune complexes
- plays role in clearing antigen-ab complexes (immune complexes) from body: RBCs have complement receptors and bind immune complexes → phagocytosed and destroyed by macrophages in liver/spleen
- if trapped in lymph nodes…stimulated immne resp!
-
immune complexes lodged in tissue create damage via complement activation (major target: kidney, which filters complexes from blood)
- autoimmune diseases lead to continuous formation of immune complexes → kidney damage
pentraxins
soluble recog/effector molecule of innate immunity
- incl some proteins produced during infl (acute-phase proteins, acute-phase reactants)
ex. C-reactive protein, serum amyloid protein → can activate complement C1, feeding complement pathway - sometimes serve as key biomarkers for infl
cytokines
proteins that act as signals within imune system (and during devpt, etc)
major pro-inflammatory cytokines: TNF alpha, IL1 beta, IL6
viral infection cytokines: Type 1 interferons (most common: IFN-alpha, IFN-beta
major outcome of innate immune response:
1. inflammatory response
- antiviral response
response to bacteria, physical objects, fungi, parasites
includes…
- recruitment of leukocytes
- phagocytosis and killing of microbes by phagocytes
- stimulating elements of adaptive immunity to produce stronger response and initiate immunological memory
major outcome of innate immune response:
- inflammatory response
2. antiviral response
response to viruses
dependent on production of Type 1 IFN (IFN alpha and IFN beta)
***IFN-gamma is NOT A TYPE 1 IFN!!! doesnt have a role in antiviral response. involved elsewhere***
- nucleated mammalian cells can respond via PRRs
- plasmacytoid dendritic cells (pDCs) [high levels of endosomal TLR7, TLR9] are sensitive, specialized to respond to viruses → pump out Type 1 IFNs, which in turn…
- bind to neighboring cells (paracrine) to induce synthesis of several proteins → confer braod viral infection resistance (“antiviral state”)
* can extend thorugh circ as endocrine effect - secreted IFN can feed back to infected cell’s IFN receptor → induce apoptosis
- Type 1 IFN stimulates cells’ innate and adaptive immune systems to change in a way that confers strong short-/long-term antiviral response
- increased class I MHC/HLA presentation
- maturation of dendritic cells and upreg of class II MHC/HLA
- stimulation of B cell class-switching → production of higher affinity/more effective antibodies
