Exam 1: Innate Immunity & Phagocytes Flashcards
1
Q
Innate Immune System
Definition
A
Set of host defense mechanisms that are always in place to provide early protection against microbial infections.
2
Q
Innate Immunity
Functions
A
- Controlling infection and in some cases eliminating microbial pathogens prior to any symptom onset
- Facilitating the initiation and development of pathogen-specific adaptive immune responses
- Cooperating with adaptive immune defenses to effectively eliminate microbial pathogens
- Removes damaged tissues and promote repair
3
Q
External Defenses
A
Physical and Mechanical Barriers:
- Stratified squamous epithelium
- Mucus layer
- Mucocilliary escalator
- Peristalsis
- Normal Flora
Chemical and Biochemical Barriers:
- Sweat and sebaceous secretions
- Lactic acid
- Fatty acids
- Waxes
- Alcohols
- Gastric acid
- Digestive enzymes
- Bile salts
- Lysozyme
- Lactoferrin
- Transferrin
4
Q
Internal Defenses
A
Cellular Components:
- Neutrophils
- Monocytes/macrophages
- Natural killer cells
Soluble Components:
- Complement system
- Cytokines
- Chemokines
- Acute phase proteins
5
Q
Innate Immunity
Recognition
A
- Recognize repeating patterns of molecular structure that are common to certain classes of pathogens ⇒ PAMPs
- Recognize tissue damage associated molecules ⇒ DAMPs
- Structures are not expressed by host cells and signal the presence of non-self or foreign antigens or injury
- Shows coarse specificity
6
Q
Pathogen Associated Molecular Patterns
(PAMPs)
A
- Conserved molecular structures common to classes of pathogens
- Often part of essential structures with limited variability
- Recognized by cells of the innate immune system
7
Q
Damage Associated Molecular Pattern Molecules
(DAMPs)
A
-
Molecules generated or released following tissue damage
- Induced by microbial infection
- Induced during a non-infectious inflammatory response where cells are damaged or stressed
- Trauma
- Burns
- Chemical toxic exposure
- Ischemia/reperfusion injury
- Released during necrotic death but not apoptotic death
- Recognized by the innate immune system
8
Q
Pattern Recognition Receptors
(PRR)
A
Innate immune system receptors for PAMPs and DAMPs
- Germ-line encoded (no somatic recombination)
- Limited repertoire compared to T/B cell receptors
- Are not clonally distributed
- Present on all cells of the same lineage
- I.E. all macrophages have a certain type regardless of location
- Families of PRR’s exist to respond to specific treats i.e. extracellular, cytosolic, and endosomal classes
-
Cell-associated PRR:
- Toll-like receptors
- Scavenger receptors
-
Soluble recognition molecules:
- Collectins
- Collagen-containing carbohydrate binding proteins
- Complement
- Collectins
-
Cell-associated PRR:
9
Q
Toll-Like Receptors
A
- Prototypical PRR
- On cytoplasmic and endosomal membranes
- After binding ligand will dimerize to transduce a signal
- Have relatively conserved cytoplasmic tails
- Activate common adaptor proteins
10
Q
Toll-like Receptor
Activation
A
Functions through two common adaptor proteins:
-
MyD88
- Activates NF-𝛋B transcription factors
- Turns on genes associated with:
- Proinflammatory response
- Cytokine response
- Stimulation of adaptive immunity
-
TRIF
- Activates IRF transcription factors
- Turns on Type 1 Interferon genes
- Important in viral infections
11
Q
Activated PRR
Functions
A
- Phagocytosis and killing of the organism
- Recruitement of immune cells to the site of infection
- Production of effector molecules that:
- Limit pathogen growth
- Recruit and activate additional immune cells to the site of infection (T/B cells)
- Influence the development of the adaptive immune response
- Tissue repair and remodeling
12
Q
Innate Immunity
Defense Against Extracellular Bacteria
A
- Phagocytes
- IFN-γ
- Complement
- Inflammation
13
Q
Neutrophils
(PMNs)
A
- Short-lived cell
- Predominant WBC of peripheral blood
- Contains numerous granules which fall into two categories:
- Primary (Azurophilic) granules: contain myeloperoxidase and cationic proteins
- Secondary (specific) granules: contain lysozyme and lactoferrin
- First cells to arrive at an inflammatory focus (~6-12 hrs)
- Major phagocytic cell
-
Major defense against pyogenic bacteria:
- Staphylococcus
- Streptococcus
- Neisseria
14
Q
Mononuclear Phagocytes
A
Monocytes/Macrophages
- Large, long-lived cells
- Monocytes in blood ⇒ macrophages in tissue
- Assume different names depending on which tissue they reside in
- Resident macrophages found at sites of filtration where microorganisms can be removed
- Circulating monocytes move from blood into tissue in response to infection and inflammation
- Arrives after neutrophils (~12-24 h)
- Can become “activated” ⇒ potent effector cells
15
Q
Resident Macrophages
A
- Clean up and maintain homeostasis of tissues
- Filter fluids to remove microorganisms
- Functions as gaurds in tissues
16
Q
M1 Macrophages
A
“Classically activated or Inflammatory Macrophages”
- Recruited from the blood into tissue in response to infection and inflammation
- Usually arrive later than neutrophils
- Development induced by:
- IFN-γ
- TNF
- LPS
- Activation results in:
-
Increased production of mediators
- ROI
- RNI
- Proinflammatory cytokines
- Upregulation of MHC II to promote T-cell activation
-
Increased production of mediators
17
Q
M2 Macrophages
A
“Alternatively-activated macrophages”
- Development induced by IL-4 and IL-10
- Not actively antimicrobial
- Secretes:
- TGF-β
- IL-10
- Other growth factors
- Functions:
- Downregulates the inflammatory response
- Promotes wound repair and fibrosis
18
Q
Dendritic Cells
A
- Located in peripheral tissues
- Phagocytic capabilities
- Antigen presenting cells
- Helps to activate T-cells and initiate the adaptive immune response
19
Q
Phagocyte Activation
A
Mediated by receptors
20
Q
Phagocyte
Attachment
A
- Dependent on cell-surface receptors
- Mediate attachment of organism to phagocytes
- Examples:
- MBP : collectin receptors
- Scavenger receptors
- IgG : Fc receptors (FcR)
- Receptors for complement components
-
C3b: deposited on bacterial cell surfaces via alternative pathway
- Binds to CR-1 on phagocytes
- Mac1 receptors can bind iC3b or C4b
-
C3b: deposited on bacterial cell surfaces via alternative pathway
21
Q
Phagocytosis
A
- Attach via cell-surface receptors
- Engulfs organism into a phagosome
- Becomes increasingly acidic
- Fuses with cytoplasmic granules to form phagolysosome
- Granule contents discharged around ingested microbe
- Subsequent intracellular response includes:
- Oxygen-independent mechanisms
- Oxygen-dependent pathways
22
Q
Oxygen-Dependent
Antimicrobial Mechanisms
A
Occurs in macrophages and neutrophils:
- Respiratory Burst ⇒ vigorous burst of oxygen consumption following activation
-
NADPH oxidase ⇒ reactive oxygen intermediates (ROI)
- Superoxide anion (O2-)
- Hydrogen peroxide (H2O2)
- Singlet oxygen (O2)
- Hydroxyl radicals (OH-)
-
ROI are potent anti-microbials
- Causes local damage to host
- System is down-regulated quickly
Neutrophils only:
- Neutrophils also have myeloperoxidase
- Hydrogen peroxide + Cl- ⇒ OCl- or hypochlorite anion
23
Q
Reactive Nitrogen Intermediates
(RNI)
A
- Occurs as part of the respiratory burst
- Nitric oxide (NO) produced from arginine and oxygen by nitric oxide synthase
- NO reacts with oxygen radical to produce peroxynitrite and reactive nitrogen intermediates (RNI)
- Peroxynitrite causes damage to cell walls and viral capsules
24
Q
Oxygen-Independent
Antimicrobial Mechanisms
A
-
Defensins
- Damage bacterial plasma membranes
-
Cationic proteins
- Insert into plasma membrane and cause cell lysis
-
Cathepsin G
- Serine protease that breaks down peptides
-
Lysozyme
- Disrupts PG in bacterial cell walls
-
Bactericidal permeability increasing (BPI) proteins
- Forms pores in bacterial outer membrane
- Proteolytic enzymes
- Lactoferrins
- DNAses
- Acid hydrolases
25
LPS Effects
Surface receptors on phagocytes binds microbial components on _gram negative bacteria_:
1. **Lipopolysaccharide (LPS)** binds serum **LPS-binding protein** ⇒ LPS+LPS-BP complex
2. **LPS+LPS-BP complex** binds the **CD14** receptors on surface of monocytes/macrophages
3. LPS-CD14 receptor complex interacts with cell-surface **Toll-like receptor-4 (_TLR-4_)** leading to cell activation
4. Stimulates cytokine synthesis and release:
* TNF-α
* IL-1
* IL-6
* IL-8
* IL-12
5. Cytokines important for:
* Recruitment of effector cells
* Lymphocyte activation & differentiation
26
Interferon-γ
**Type II Interferon**
* **Secreted by _NK cells_ early** ⇒ innate immune response
* **Secreted by _T cells_ late** ⇒ adaptive immune response
* **Effects:**
* **Recruitment and activation of macrophages**
* Phagocytize and kill microbial pathogens
* Secrete additional cytokines, chemokines, and antimicrobial products
* **Stimulation of adaptive immune responses**
* **Influence the nature of that response**
* Example:
* Tuberulosis lives within modified vacuoles of macrophages
* Macrophages can be induced by IFN-γ to kill intracellular pathogens
27
Defense Against Viruses
1. **Production of cytokines**
* IFN-α and IFN-β by viral infected cells
* TNF-α and IL-12 by macrophages
2. **NK cell activation**
* Killing of virally infected cells
3. **Development of the adaptive immune response** with virus specific:
* T-cells ⇒ kill virally infected cells
* B-cells ⇒ secrete antibodies that can block viral entry into host cells
28
Type I Interferons
Basics
* Large family of structurally related cytokines
* Mediate the early innate immune response to viral infections
* Able to interfere with viral replication
* Most significant are **IFN-α** and **IFN-β**
29
Type I IFN
Induction
* Viral nucleic aids bind to intracellular receptors linked to the production of transcription factors
* TLR-3
* RIG-1
* **Toll-like receptors** can be found on endosomal membranes that recognize dsRNA and ssRNA
* **Cytoplasmic sensors** recognize viral RNA
* Stimulation of either causes activation of interferon regulatory transcription factors (IRFs)
* Within several hours of a viral infection, host cells begin to produce and secrete IFN-α and/or IFN-β
30
Type I IFN
Effector Functions
1. **_Transcription Regulation_**
* All Type I IFN's bind to the same cell surface receptors
* Signals host cell to _activate or increase synthesis of a large set of proteins_
* Net result is an _increased resistance to viral replication_ in all cells
* Stimulation of _effector mechanisms to kill virally infected cells_
2. **_Anti-viral Functions_**
* IFN α/β induced proteins can _directly inhibit_ one or more steps in the _viral life cycle_ (entry, transcription, translation, assembly, release)
* _Promotes viral genome degradation_ inside host cell
* Activates **oligoadenylate synthetase**
* Polymerizes ATP into 2',5' linked oligomers
* Activates **endoribonuclease** (RNase) to degrade viral RNA
* _Inhibits viral protein synthesis_
* Activates **P1 kinase** (ser/thr kinase)
* Phosphorylates eIF-2
* Leads to inhibition of all protein synthesis
3. **_Immuno-regulatory functions_**
* Increases MHC expression, viral antigen processing, and presentation to virus-specific T cells
* Helps to initiate the adaptive immune response
* Activates NK cells to kill virus infected host cells
31
NK cells
Basics
* Large granular lymphocytes
* Distinct from T cells and B cells
* Found in blood and spleen
* Migrates into infected tissues in response to inflammatory cytokines
* After recognition, able to kill various targets without need for additional activation
* Function can be enhanced by:
* **IFN α/β** initially produced in response to a viral infection
* Cytokines produced by macrophages early in the course of many infections:
* **Interleukin-12 (IL-12)**
* **Tumor necrosis factor-α (TNF-α)**
32
NK Cells
Recognition Methods
_NK cells do not express an antigen specific receptor:_
1. Express a set of **activating and inhibitory receptors**
* Cooperate to allow the recognition of many virally infected host cells and tumor cells
* Example:
* Viruses down-regulate MHC I receptors to evade T-cells
* Lack of MHC class I causes activation of NK cells
2. Express an **IgG binding FC receptor (CD16)** on surface
* Facilitate _Antibody Dependent Cellular Cytotoxicity_ (ADCC)
* IgG binds to surface of virally infected cells
* Facilitates NK cell recognition
33
NK Cell
Cytotoxicity
Important in the early control of viral infections and mediate the killing of virus infected cells.
1. Activation
* Activating & Inhibitory Receptor imbalance
* CD16
2. Cytoplasmic granules are discharged onto the surface of virally infected cells
3. **Perforin** ⇒ creates a pore in the plasma membrane of infected cells
4. Allows entry of **granzymes** ⇒ activates apoptotic cascade
5. Leads to host cell death
Also able to kill through the **Fas:FasL pathway**.
(Discussed later)
34
NK Cell
Cytokine Production
Activated NK cells secrete cytokines including:
* **Interferon-γ**
* Important in the recruitment and activation of macrophages
* Helps shape the cytokine profile secreted by TH cells
35
Innate & Adaptive Immune System
Interactions
Cooperation between adaptive and innate immune systems can enhance the effectiveness of the overall response.
* **Antibody-dependent cellular cytotoxicity (ADCC)**
* NK cells bind Fc portion of IgG
* Facilitates NK cell identification and killing of targets prior to the release of cytotoxic mediators
* **Opsonization**
* IgG or complement components (C3b) coat surface of pathogens
* Phagocytes express Fc receptors for IgG and C3b receptors
* Opsonized antigen phagocytized more readily
* **Classical pathway of complement activation**
* Mediated by IgM and IgG
