Innate Immunity -M261 Spring 2005May 6, 2005 Kathleen A. Kelly

Reading:

Immunobiology (6th Edition) Janeway, Travers, Walpert & Capra Chapter 2 (p. 37-100), and Chapter 6 (p. 209-212)

Fundamental Immunology (5th Edition) Lippincott, Williams & Wilkins Chapter 17 (p.497-517)

Janeway, CA, et al. Innate Immune Recognition. Annu. Rev. Immunol. 20:197-252, 2002

Innate Immunity -M261 Spring 2004Kathleen A. Kelly

Innate immunity predates development of adaptive immunity

Does not produce protective immunity● No memory response● Prerequisite for developing adaptive immunity

Non-antigen-specific immunity● Found in plants, invertebrates and vertebrates

Innate Immunity1. Provides a barrier to prevent the spread of

infection● Mechanical (tight junctions, movement)● Chemical (fatty acids, enzymes, pH, antimicrobial

peptides)● Microbiological (normal flora)

● Mucosal surfaceso Nasopharyngeal, Oral, Respiratory, Intestinal tractUrogenital tract

● Skin (epithelial cells)o Wounds, burns, insect bites

Innate Immunity2. Identifies and eliminates pathogens

● Non-adaptive recognition systems● Activates molecules that target the microbe and aid

in it’s identification.o These factors may be expressed at the surface or

within cells, released from immune cells or are secreted and present within circulatory system

Innate Immunity3. Initiates an inflammatory response

● Reaction to injury or infectiono Trauma to tissues or cellso Presence of foreign matter (self vs. non-self)o Infectious agents (viruses, bacteria, fungi)

● Delivers effector molecules & immune cells to the site of infection

● Componentso Leukocytes & secreted factorso Blood vesselso Plasma proteins

Innate Immunity4. Provides signals to activate and regulate the

type of adaptive immune response generated● Stimulation of co-stimulatory molecules

o B7 family (CD80/86, PD-L, ICOSL)o TNFR family (OX40L)

● Induction of a cytokine/chemokine responseo Cytokines: IL-12, IL-23, IL-4o Chemokines: CXCR1, CXCR2, CCL20

• a variety and depends on stimulus

The Phases of Immunity

Identification of Microbes

Recognition● Receptors – Pattern Recognition Receptors

(PRRs)o Fixed in the genome, ie gene

rearrangement is not needed

● Distributiono Non-clonal, ie all cells of a class are

identical

Differentiation● Pathogen vs. Commensal

Identification of MicrobesPRR

● Recognize conserved molecular patterns on microbes called microbe associated molecular patterns (MAMPs) which are not present on the host

o Not limited to pathogens● Identify a class of microbes

o LPS, LTA, peptidoglycan, lipoarabinomannan, dsRNA, mannans, b-glycans

● MAMPs are often essential for microbe survivalAction Time

● Immediate activation of effectors● Delays need for adaptive immunity

Pattern Recognition Receptors (PRRs)

Three broad classes of PRRs based on expression profile, localization, function● 1) PRRs that signal an infection

o Include the Toll Receptor Family

o Expressed external or internally

o Activation of “pro-inflammatory” signaling pathways NFB and MAP kinase signaling pathways

• Antimicrobial peptides (Defensins) / lysozyme,• Inflammatory cytokines (TNF, IL-8, IL-1)

o Regulate activation of adaptive immune response• co-stimulatory molecules

● 2) Phagocytic (endocytic) PRRso Expressed on the surface of phagocytic cells

(MQs, PMNs, DCs)

o Mediate uptake of microbe into phagocytes

● 3) Secreted PRRso Secreted by MQs, epithelial cells, liver

o Activate C’, opsonize microbial cells, function as accessory proteins for MAMP recognition

Pattern Recognition Receptors (PRRs)

Toll-like Receptor Family

Curr. Opin. Hematology 9:2-10, 2002

PPR receptor● Found both on the

surface and within cells

● First discovered in Drosophila

● Currently 13 receptors

o 1-9 mouse & human

o 10 human

o 11-13 mouse

Toll-like Receptor family

Intracellular PRRs:Present in the Cytosol of Host Cells

1. Protein kinase receptor (PKR)● Activated upon binding to dsRNA (viruses)

o Blocks viral & cellular protein synthesis (eIF2)

o Activates NFB, MAP kinase STATs & IRF signaling pathways

o Induces apoptosis & IFN production of infected cells

2. 2’-5’ Oligoadenylate Synthase & RNaseL ● Family of IFN-inducible enzymes

o dsRNA activates OAS

o RNaseL degrades viral and host RNA

o Induces apoptosis

Intracellular PRRs:Present in the Cytosol of Host Cells

3. NOD proteins or nucleotide-binding oligomerization domain● Recognize intracellular peptidoglycan-derived

MAMPs and transduce signals ● three distinct functional domains

o carboxy-terminal ligand-recognition domain (LRD)

o centrally located NOD

o amino-terminal effector-binding domain (EBD) CARD domains in mammals Interacts and activates RIP2 inducing NFB and MAP-

kinase pathways

Structure of NOD Proteins

Inohara N, & Nunez G. Nat Rev Immunol. 2003 3:371

NOD Proteins

Inohara N, & Nunez G. Nat Rev Immunol. 2003 3:371

Phagocytic (endocytic) PRRsBind Carbohydrates 1. Macrophage Mannose Receptor (C-type lectin)

● Type 1 transmembrane receptor● Recognizes patterns of mannose residues in a certain spatial

orientation unique to microbes (CRD)● Only found on macrophages (not monocytes or PMNs)

2. Glucan Receptor (Dectin-1)● Type 2 transmembrane receptor● Recognizes -1,3 & -1,6 linked glycans● Present on all phagocytes

Phagocytic (endocytic) PRRs: Cont.

3. Scavenger Receptors● Recognize charged ligands

o Polyanionic ligands (ds-RNA, LPS, LTA)o Acetylated low-density lipoproteins (LDL)

● Found on all phagocytes● MARCO (macrophage receptor with collagenous

struction)o binds bacterial cell walls but not yeast

● Phagocytose apoptotic cellso new factor MFG-E8 (released from activated

macrophages and binds to apoptotic cells via phosphatidylserine)

Secreted PRRs activate the Complement (C’) System

Complement system is activated by innate immunity Recognition by Complement receptors (CR)

o CR1, CR2, CR3, CR4, C5a, C3a

Comprised of plasma proteins that when activated forms a triggered enzyme cascade

● Zymogens – activated by the cleavage of other proteaseso Precursor enzymes

Function● Facilitates the uptake & destruction of pathogens by phagocytes● Induces an inflammatory responses

b

C4b+

C2b

C3b+

Bb

Activation of C’ System

Secreted Pattern Recognition Molecules

Acute Phase Proteins

Activation of ComplementOpsonization of microbial cellsPrimarily produced by the liver but can be produced by phagocytes

Secreted Pattern Recognition Molecules

1. Collectins● Recognizes microbial carbohydrates (CRD domain)● Effector function mediated by collagenous domain ● Mannan-binding lectin (MBL)

o Recognizes patterns of mannose & fucose residues in a certain spatial orientation unique to microbes

o Initiates the lectin pathway of C’ cleaving C2 & C4

o Can function as an opsonin Binds a receptor on phagocytes (C1qRp)

● Surfactant proteins (SP-A / SP-D)o lung

Collectins

• Structure is conserved and similar to other proteins with similar function:

o Some Complement proteins & Mannose Binding Protein

o Binds to bacteria, fungi & viruses

• Function by binding microbes and are important for mediating phagocytosis of alveolar macrophages

Microbe

C-type Lectin domain

Collagen helix

-coiled helix

Secreted Pattern Recognition Molecules – Cont.2. Pentraxin

● Members includeo Serum amyloid protein (SAP)

o C-reactive protein (CRP)● Recognize phosphorylcholines on microbes ● Functions as an opsonins ● Binds to C1q & activate classical C’ pathway

Secreted Pattern Recognition Molecules – Cont.

3. Lipid Transferases● LPS binding protein (LBP)

o Opsonin● Bactericidal permeability increasing protein (BPI)

o Bactericidal protein 4. Peptidoglycan recognition proteins (PGRS)

● Recognizes peptidoglycans in evolutionarily distant organisms● 4 human PGRS● Function is unknown

o One has bactericidal effectso Triggers a serine protease cascade in insects

? Complement cascade ?

Inflammatory Response

Inflammatory Response

Leukocyte Adhesion

Naïve and Memory T Cells Travel in Different Paths

Naïve (have not seen their antigen) T cells travel in the blood and lymphatics

Memory T cells (have been activated by their antigen) can also travel through tissues

Lymphocyte Trafficking Patternsof Naïve T Cells

PeripheralBlood

Peripherallymphnodes

Lymphaticsystem

HEVGlyCAM-1

CD62L:selectin

ChemokinesCCL21 / SLCCCL19 / ELC (MIP-3)

CCR7:chemokine receptor

ICAM-1

Lymphocyte Trafficking Patternsof Naïve T Cells

PeripheralBlood

Peripherallymphnodes

Lymphaticsystem

HEVGlyCAM-1

CD62L:selectin

ChemokinesCCL21 / SLCCCL19 / ELC (MIP-3)

CCR7:chemokine receptor

ICAM-1

Lymphocyte Trafficking Patterns of Effector/Memory T Cells

Lymphaticsystem

Any TissuePeripherallymphnodes

Inflammation

PSGL-1:selectin41:Integrin

CXCR3: chemokine receptor

ChemokinesCXCL9 / MIGCXCL10 / IP-10CXCL11 / I-TAC

HEVCD44ICAM-1

Endothelial Cell

1. Tethering 3. Firm adhesion2. Triggering 4. Diapedesis

Lymphocyte

PathogensMacrophage

Cytokines

Chemokines

Stromal cells

Blood Vessel

Steps in Lymphocyte Trafficking

PhagocytosisPhagocytosis

● Definition: uptake of large particles (>0.5 m)● Actin-dependent, clathrin-independent● High rate & efficiency of internalization

Professional phagocytic cells● Macrophages● Neutrophils

These cells have phagocytic receptorso External receptors

FcR, CR3, Mannose receptoro Internal receptors

TLRs

Macrophages(MQ)

Blood - Called monocytes (1-6% WBC)Tissues - Called macrophages

● mature form of monocytes● normally found in tissues such as gastrointestinal

tract, lung, liver and spleen

Functions:● Phagocytose and kills after bactericidal mechanisms

are activated (T cells)● Produce cytokines/chemokines (initiates inflammation)● Is an antigen presenting cell (co-stim. Molecules)

Neutrophils(PMN)

Present in blood (55-60% of WBC)Not normally present in tissuesShort lifespan - 12 hoursFunctions:

● First at the site of infection/injury

Ingest and kill microbes after bactericidal mechanisms are activated (binding to pathogen)

Phagocytosis (MQ & PMN)

Active process initiated by binding to pathogenPathogen is surrounded and then internalized

Signaling Interactions during Phagocytosis

Ann. Rev. Immunol. 20:825-852, 2002

Killing MechanismsPhagosome - membrane bounded vesicle that

becomes acidifiedLysozome - granules that contain products that

damage or kill pathogens● Enzymes

o Lysozyme - dissolves cell walls of some bacteria

o Acid hydrolases - digests bacteria● Proteins

o Lactoferrin - binds Fe++ needed for bacterial growth

o Vitamin B12-binding protein● Peptides

o Defensins and cationic proteins - direct antimicrobials

Killing Mechanisms - cont.Respiratory Burst

● Activated following phagocytosis● Stimulated by PRR● Requires increased oxygen consumption ● Produces substances that are directly toxic to the

bacteriao Oxygen-derived products

O2-, H2O2 & Myeloperoxidase

o Nitrogen-derived products NO (nitrogen oxide) Produced by inducible NO synthase (iNOS) enzyme Enzyme is induced by cytokines (LT, TNF)

NADPH Oxidase Mitochondrial-independent respiratory burst

P47phox & p67phoxnormally resides in the cytoplasma.

P47phox becomes hyperhposphorylatedfollowing phagocytosis and binds to p67phox.

These components move to the membrane and bind the NADPH complex resulting in an active complex.

Enzyme Reactions of Respiratory Burst

Respiratory BurstNADPH NADP+ Superoxide

+ dismutase

2 O2 2 O- H2O2

Myeloperoxidase

● Enzyme which is stored in primary granules of PMN & MQ and uses the products of the respiratory burst.

● H2O2 + C1-Chloramines

Professional APC

Regulation of Adaptive Response

Veterinary Immunology & Immunopathology 91: 1, 2003

T cells Recirculate to “Find” Antigen-loaded Dendritic cells

GerminalCenter

EfferentlymphaticsAfferent

lymphatics

ParacorticalArea

FollicularArea

HEV

Mucosal Immunity – Reading Assignment

Immunobiology (6th Edition) Janeway, Travers, Walpert and Capra Chapter 10 (p. 432-445).

Neutra, MR et al Antigen sampling across epithelial barriers and induction of mucosal immune responses. Annu. Rev. Immunol. 14:275-300, 1996

Wright, JR. Immunoregulatory Functions of Surfactant Proteins. Nature Review Immunol. 5:58-68, 2005.

Cheroutre, H. Start at the beginning: new perspectives on the biology of mucosal T cells. Annu. Rev. Immunol. 22:217-46, 2004.

Weiner, H. Oral tolerance: immune mechanism and the generation of Th3-type TGF-beta-secreting regulatory cells. Microbes & Infection 3:947-954, 2001.

May 10, Spring 2004