Innate Immunity Early Defense Against Infection

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Chapter 2

• Innate Immunity Early Defense Against Infection

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Innate Immunity

• Always present, ready to recognize and eliminate
  microbes and host cells damaged by microbes
• Doesnt react against nonmicrobial substances
• Powerful early defense mechanism can control
  infection before adaptive immunity can be
  activated
• Cross-talk within innate and adaptive immunity

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Innate vs. Adaptive Immunity
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Recognition of Microbes

• Cells of innate immunity recognize structures
  that are shared by various classes of microbes
  and are not present on host cells
• if bind to host cell, other molecules will
  prevent activation
• Phagocytes recognize lipopolysaccharide (LPS or
  endotoxin) on microbes
• have receptors for LPS
• Recognize molecular patterns with pattern
  recognition receptors
• terminal mannose structures which are different
  than mammalian cells
• dsRNA of viruses and unmethylated CpG nucleotides
  of bacteria
• Recognize things on microbes that make them
  infective and able to survive
• Adaptive immune system recognizes Ag whether
  microbial or nonmicrobial very specific

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Recognition of Microbes

• Receptors of innate immune system are encoded in
  the germline
• not by somatic recombination as are T and B cell
  receptors (Chapter 4)
• Innate recognizes lt1000 microbial patterns thru
  the many different receptors on each cell, need
  to start over at every exposure no memory
• Adaptive immunity recognizes over 109 Ag, can get
  stronger with each exposure because of memory and
  adaptive changes (Chapter 7)
• Innate immunity doesnt react with the host

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Components of Innate

• Epithelia which act as barrier to infection
• Cells in circulation and tissues
• Several plasma proteins

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EpithelialBarrier

• Skin, GI tract, respiratory tract continuous
  epithelia
• Act as physical and chemical barrier
• Epithelial cells produce peptide antibiotic that
  kill bacteria
• Also have intraepithelial lymphocytes (T-cell)
  not as diverse as adaptive immune T-cells (??
  chains in receptor)
• B-1 cells in peritoneal cavity limited
  diversity make Ab (IgM) that recognize
  carbohydrates of bacteria
• Chapters 4 and 5

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Cells of the Innate Immunity

• Phagocytes
• Macrophage/monocytes
• Neutrophils (polymorphonuclear cell, PMN)
• Recruited to site of infection and ingest and
  kill bacteria

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Neutrophils

• Also called polymorphonuclear or PMNs
• Most abundant and increases when infection is
  present
• A cytokine called colony-stimulating factor is
  secreted by many cells in response to infection,
  acts in BM
• First on site of bacterial and fungal infection
• Eats bacteria in circulation and enters
  extravascular tissues
• Die after a few hours

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Monocytes
Macrophage

• Less abundant than PMN
• Ingests microbes in blood enter extravascular
  tissue to become macrophage
• 2 stages of same cell line
• Also resident macrophages in connective
  tissue/all organs functions like monocyte

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Movement of WBC

• Neutrophils and macrophages leave by binding to
  endothelial adhesion molecules and in response to
  chemoattractants produced on the encounter with
  microbes
• soluble protein called cytokines are released by
  resident macrophages
• Tumor necrosis factor (TNF) and interleukin-1
  (IL-1) act on endothelium of small vessels,
  stimulate the expression of integrins
  E-selectin and P-selectin on epithelial cells
  which bind to carbohydrates
• Neutrophil and monocytes bind weakly to
  selectins, but when activated, in the presence of
  TNF and IL-1 the affinity increases, stops
  movement, spread out and migrate thru endothelial
  wall

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Extravasation
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Chemokines

• Chemoattractant cytokines
• Stimulate the increase in affinity of integrins
  on WBC for their ligands on the endothelium which
  are influenced by TNF and IL-1
• When attached tightly, the cell undergoes
  cytoskeletal changes and migrate thru the wall
  and follow the gradient to the infection site

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Inflammation

• Allows for the accumulation of leukocytes, causes
  vascular dilation and increased vascular
  permeability
• This helps to get the cells to the area of where
  the microbe is located

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Recognition Receptors

• Neutrophils and macrophages have several
  different types of receptor to recognize
  structures and patterns
• Toll-like receptors (similar to receptor in fruit
  flies) have many kinds different microbe
  recognition
• binding of TLR causes expression of NF-?B
  (nuclear factor transcription factor) causing
  cytokine production
• receptors for mannose, N-formylmethionine
  containing peptides
• Macrophages have cytokine receptors INF?
  (interferon) from innate and adaptive immune
  response activates microbiocidal function of
  phagocytes
• Also respond to complement proteins or Ab on
  surface of bacteria called opsonization

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Receptors and Responses
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Phagocyte Killing

• Phagocyte will ingest the microbe and form a
  phagosome around microbe, fuse with the lysosome
  to form the phagolysosome
• Phagocytosis activates several enzymes such as
• phagocyte oxidase converts O2 to superoxide
  anion and free radicals (called reactive oxygen
  intermediates, ROI) which are toxic to microbes
• inducible nitric oxide synthase which coonverts
  Arg to NO which is microbiocidal
• lysosomal proteases that breakdown microbial
  proteins
• Doesnt harm the phagocyte, but if released into
  the tissues, it will cause tissue damage
• why inflammation causes tissue damage

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Killing of Microbes
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Immune Deficiency

• Chronic granulomatous disease
• phagocyte oxidase deficiency
• cant clear intracellular infections so they are
  walled off into a coat of macrophage and
  lymphocytes called a granuloma

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Macrophage Function

• Produce cytokines
• Secrete growth factors and enzymes to remodel
  damaged tissue
• Stimulate T cells and respond to products of
  T-cell (Chapter 6)

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Natural Killer Cell

• Respond to intracellular microbes by killing the
  infected cell and producing the macrophage
  activating cytokine INF?
• 10 total lymphs
• Contain cytoplasmic granules and surface markers
  but no immunoglobulins or T-cell receptors
• Recognize altered host cells
• usually virally infected host cells or phagocytes
  with viruses or intracellular bacteria

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NK Cell vs Lymphocyte
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NK Cells
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NK and Host Cells

• Can recognize normal host cells but have
  inhibitory receptors to prevent activation of NK
  cell
• Inhibitory receptors are specific for MHC I
  expressed on all nucleated cells

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Major Inhibitory Receptor Families

• Killer-cell immunoglobulin-like receptor (KIR)
• CD94 and lectin subunit NKG2
• Both have cytoplasmic domain immunorecptor
  tyrosine-based inhibitory motifs (ITIM)
• become phosphorylated on a Tyr when bind MHC-I
  activates Tyr phosphatase to remove PO4 block NK
  activation when the receptor is activated

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Functions of NK Cells

• Viruses that prevent MHC-I expression cant get
  killed by cytotoxic T-cells but without MHC-I the
  inhibitor is inactive and host cell is killed by
  NK cell (Chapter 6)
• IL-12 (macrophage derived) enhance NK cell
  activity, causes secretion of IFN? to make
  macrophage better
• NK cells have Fc receptor (part of Ab) of IgG
• part of cell-mediated humoral immunity (Chapter 8)

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Inhibitory Receptor Response
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Activation of NK Cells

• Activation causes release of proteins in NK cells
  cytoplasmic granules
• create holes in plasma membrane
• activate enzymes involved in apoptosis after
  entering the cell (Chapter 6)
• kills reservoirs of infectious organisms
• Secrete cytokine IFN? to make macrophage better
  at removing intracellular microbes

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Complement System (Chapter 8)

• Circulating and membrane proteins used in the
  defense of microbes
• Series of proteolytic enzymes that are cleaved to
  the active form
• Complement fixation is a cascade event

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3 Pathways

• Alternative pathway triggered by some
  complement proteins being activated on microbe
  surfaces host cells have inhibitory proteins to
  prevent activation Innate immune system
• Classical pathway triggered by Ab binding to
  microbe or other Ag humoral arm of Adaptive
  immune system
• Lectin pathway when plasma protein mannose
  binding lectin protein recognizes mannose on
  microbe proteins initiates the classical pathway
  but from different starting point innate system
  because no Ab involved

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Complement

• Activated complement proteins cleave other
  complement proteins
• All pathways initiate differently but end up in a
  common pathway
• central component is C3 which is cleaved to C3b
  and C3a
• C3b binds microbe and initiates downstream events
• perform the same effector function

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3 Functions

• C3b coats microbes which lead to phagocytosis by
  recognition of C3b receptors on macrophage
• Some of the remaining cleavage products serve as
  powerful chemoattractants to neutrophils and
  monocytes promote inflammation at site of C
  fixation
• Make multimeric protein complexes in microbe
  membrane that leads to death
• osmotic lysis or apoptotic death

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Cytokines of Innate Immunity

• Macrophages and other cells secrete cytokines
  that mediate cellular reactions of innate
  immunity
• Communication between leukocytes leukocytes and
  other cells

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Cytokines

• Interleukins made by leukocytes to act on
  other leukocytes, definition is too narrow as we
  are finding other sources and activities
• Innate immunity cytokines principle source is
  macrophage after recognition of microbes
• stimulated by binding of LPS to receptor and TLRs
• Adaptive immunity cytokines come from the
  T-helper cells (Chapter 5)
• Produced in very small amounts, bind high
  affinity receptors on target cells
• Function as autocrine (on cell that made it) or
  paracrine (on nearby cells)

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Functions of Cytokines

• Serve various functions in host defense
• TNF, IL-1 and chemokines recruit blood
  neutrophils and monocytes to sites of infection
• TNF in high concentrations cause thrombosis,
  decrease blood pressure by decreasing myocardial
  contractility and vascular dilatation
• see in septic shock with gram negative bacteria
  caused by TNF in response to LPS, disseminated
  intravascular coagulation and metabolic disorders
• Marcophage produce IL-12 in response to LPS and
  phagocytosed microbes activates NK cells
• IFN? from the NK cells to activate macrophages
• IFN? is part of both systems also from T cells
• Type 1 IFNs are produced in macrophages,
  fibroblasts and infected cells that prevent viral
  replication used to treat some viral infections
  such as hepatitis (IFN?)

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? Must Know
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Other Plasma Proteins

• Plasma mannose-binding lectin (MBL) recognizes
  microbial carbohydrates, coat microbes for
  phagocytosis (activate complement)
• C-Reactive Protein (CRP) binds
  phosphorylcholine of microbes for phagocytosis by
  macrophages with CRP receptors
• increases rapidly after infection
• protective phase is called Acute Phase Response

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Innate Responses

• Different innate responses to different microbes
• extracellular bacteria and fungi
• phagocytosis
• complement and/or
• acute phase proteins
• intracellular bacteria and viruses
• phagocytosis and NK cells with communication
  using cytokines

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Evasion of Innate Immunity by Microbes

• Intracellular bacteria that infect phagocytes are
  usually resistant to destruction because in the
  cytoplasm
• L. monocytogenes evades ROI and NO
• Mycobacterium have a lipid that inhibits fusion
  of vesicles containing bacteria to the lysosomes
• Use cell mediated and humoral immunity to handle
  these infections
• Other bacterial cell walls inhibit complement
  proteins

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Mechanisms of Envasion
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Innate Stimulating Adaptive

• Innate system generates second signals with Ag
  to activate B and T cells (Chapters 5 and 7)
• 2nd signal system ensures response to infectious
  agents and not to non-infectious substances
• Adjuvants are used with Ag to develop immune
  response, used in vaccination, usually products
  of microbes
• induce adaptive response to Ag without microbe
  present

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2nd Signals for Phagocytosed or Intracellular
Microbes

• Microbes or INF? from NK cells stimulate
  dendritic cells and macrophages, 2 types of 2nd
  signals to activate T cells
• express co-stimulatory molecules on the surface
  of dendritic cell or macrophage, these will bind
  to receptors on naïve T cells for Ag yield T
  cell activation
• secrete IL-12 stimulating the differentiation of
  naïve T cells into effector cells (cell-mediated
  immunity)

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2nd Signal for Blood-Bourne Pathogens

• Activates the alternative pathway of complement
  fixation
• C3d attaches to microbe, Ag receptor on B cell
  recognizes Ag and C3d on microbe is recognized
  with C3d receptor on B cell, makes an Ab
  producing B cell
• complement is 2nd signal of humoral immunity

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Innate Stimulating Adaptive
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T-cell or B-cell Response??

• Second signals determine which arm of the
  adaptive immune system is activated
• intracellular/phagocytosed T-cell
• blood borne B cell