Transcript Tan3

How is the Immune Response Initiated?
What are the ligands and receptors?
• Infectious-Nonself Model
– Charles Janeway Jr.
– Cold Spring Harbor Symp. Quant. Biol. (1989)
• Danger Model
– Polly Metzinger
– Ann. Rev. Immunol. (1994)
• Guard Model
– Jeffrey Dangl & Jonathan Jones
– Nature (2001)
Infectious-Nonself Model
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Detection of conserved molecular patterns
(pathogen-associated molecular patterns,
PAMPs) by pattern recognition receptors
(PRR).
Upon recognition of PAMPs by …
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Membrane-associated PRR: signaling
pathways that induce antimicrobial effectors
and inflammation is activated.
Soluble PRRs: pathogens bound and flagged
for destruction by phagocytosis or the
complement system.
Identification of PRR in Drosophila
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Drosophila produces 7 structurally diverse, cationic and predominantly membrane active
AMPs.
Activity Spectra:
– Fungi: Drosomycins, Metchnikowin
– Gram-positive bacteria: Defensin
– Gram-negative bacteria: Attacins, Cecropins, Drosocin, Diptericins
The promoter regions of these AMP genes contain nucleotide motifs similar to mammalian
binding sites for NF-kB/Rel proteins.
The Drosophila NF-kB/Rel ortholog is Dorsal, had been genetically identified as a regulator
of dorsoventral patterning in the early embryo.
In 1996, Hoffmann’s group reported that loss-of-function mutations in the Toll receptor
compromised the survival of flies faced with fungal infection and the challenge-dependent
transcription of the antifungal peptide Drosomycin.
Identification of PRR in mammals
• In 1994, the first mammalian TLR, TLR1 was cloned and was
identified as a homolog of the Drosophila Toll [DNA Res. 1, 27].
It was designated as TIL (Toll/IL-1 receptor-like) by Taguchi et al
in 1996 [Genomics 32, 486] and was suspected to have a
developmental function as the immune function of Toll had not
been demonstrated.
• In 1997, Janeway and Medzhitov successful cloned a human
homologue of Drosophila Toll and showed this Toll-like receptor
(TLR) activate NF-kB [Nature 388, 394].
• The immune relevance of TLR was shown in 1998 when Bruce
Buetler and colleagues showed that defective LPS signaling in
C3H/HeJ and C57BL/10ScCr mice was a consequence to
mutation in Tlr4 [Science 282, 2085].
• Akira’s group, by means of gene targeting, determined the main
microbial specificity of TLR2, TLR6 and TLR9.
Human Toll-like Receptors
Members of the interleukin-1
receptors (IL-1Rs) family. They
share a conserved cytoplasmic
region known as the Toll/IL-1R
(TIR) domain. But the
extracellular portion of the TLRs
contains a leucine-rich repeat
(LRR) motif whereas that of the
IL-1Rs contains three
immunoglobulin domains.
LRR domains may be directly
involved in the recognition of
PAMPs.
TLR7 & TLR9 are in intracellular
compartments.
Individual TLR can interact with
several structurally unrelated
ligands of exogenous and
endogenous origin.
TLR
Ligands (origin)
TLR1
Tri-acyl lipopeptides (bacteria, mycobacteria); Soluble factors (Neisseria
meningitides)
TLR2
Lipoprotein/lipopeptides (a variety of pathogens); Peptidoglycan & Lipoteichoic
acid (Gram + bacteria); Lipoarabinomannan (mycobacteria); A phenol-soluble
modulin (Staph. epidermidis); Glycoinositolphospholipids (Tryp. cruzi);
Glycolipids (Trep. maltophilum); Porins (Neisseria); Zymosan (fungi); Atypical
LPS (Leptospira interrogans, Porphyromonas gingivalis)
HSP70 (host)
TLR3
Double-stranded RNA (virus)
TLR4
LPS (Gram-negative bacteria); Fusion protein (RSV); Envelope proteins
(MMTV); HSP60 (Chlamydia pneumoniae); Taxol (plant);
HSP60; HSP70 (host); Type III repeat extra domain A of fibronectin;
Oligosaccharides of hyaluronic acid; Polysaccharide fragments of heparan
sulfate; Fibrinogen (host)
TLR5
Flagellin (bacteria)
TLR6
Di-acyl lipopeptides (mycoplasma)
TLR7
Imidazoquinoline; Loxoribine, Bropirimine (synthetic compounds)
TLR8
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TLR9
CpG DNA (bacteria)
TLR10 ?
Lipid A structure
Effect of Lipid A Structure on TLR-mediated Signaling
TLR Signal
Transduction pathway
All TLR signal through
MyD88.
Individual TLR also
induce pathogen-specific
immune response.
TLR2 & TLR4 also signal
through TIRAP/Mal.
TLR3 (?TLR4) also signal
through TRIF.
Induction of IFNa/b
expression through IRF3
by TLR3 & TLR4 is
MyD88-ndependent.
Defense Pathways in Drosophila
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What features distinguish Gram +ve and Gram -ve Peptidoglycan (PGN)?
Gram-positive
Gram-negative
Recognition of PGN in Drosophila and mammals
NOD Proteins as PRR
The Danger Model
• The immune system is activated in response to
substances that cause damage, rather than those
that are simply foreign. These alarm/danger
signals are molecules or molecular structures,
released or produced by cells undergoing stress or
abnormal death. These signals are perceived by
resting APCs.
• The alarm signals can be either endogenous or
exogenous, intracellular or extracellular/secreted.
They may function as primal initiators or simply
give positive-feedback to enhance or modify an
ongoing response.
Empirical Support for the Danger Model
• Resting DCs are activated by cells killed by acute necrotic death
but no by cells dying of physiological apoptotic death [Nat. Med. 5,
1249 (99); J. Exp. Med. 191,423 (00)]. Signals produced by
necrotic cells are heat shock proteins gp96 and hsp70 [Int.
Immunol. 12, 1539 (00)].
• Crystalline, but not soluble, uric acid can activate DCs [Nature 425,
516 (03)].
Gene-for-Gene Hypothesis by H.H. Flor
Disease resistance in plants requires
two complementary genes: an
avirulence (Avr) gene in the pathogen
and a matching, resistance (R) gene in
the host.
Suggests a receptor-ligand model in
which R-protein-mediated recognition of
the pathogen-derived Avr products
leads to hypersensitive response (HR)
in plant.
This hypothesis has led to the identification
of many R-Avr protein pairs from plants and
pathogens. However, with the exception of
the in vitro interaction between Pi-ta (a rice R
protein) and AvrPita from the fungal
pathogen Magnaporthe grisea, no direct
interactions between Avr and R protein has
been demonstrated.
Identification of R genes in Plants
• Prior to results obtained in animals, in 1993, Greg Martin cloned
the Pto gene in tomato confers resistance to races of
Pseudomonas syringae pv. tomato that carry the avirulence
gene avrPto.
• In 1994, the Ausubel and Staskawicz groups clones the
resistance gene RPS2 from Arabidopsis [Science 265, 1856 &
Cell 78, 1089]. In the same year, Barbara Baker’s group cloned
the N gene from tobacco [Cell, 78, 1101].
• The N and RPS2 proteins, like Toll and TLRs has the LRR and
TIR domains. They define a large group of cytoplasmic R
proteins called the NBS-LRRs. They contain a central nucleotide
binding site and a C-terminal LRRs called the NBS-LRR.
• A third class of R genes are membrane-anchored glycoprotein
with extracytoplasmic LRRs.
Plant, Drosophila and Mammalian PRRs
Recognition of AvrPto by tomato NB-LRR
protein (Prf) is mediated by Pto
The Guard Model
R proteins associate physically and specifically with cellular targets (or
“guardee”) of bacterial type III effectors (Avr). The interaction between
guardee and Avr is recognized by the R protein, which is thus activated to
initiate disease resistance. Guardees are likely to be plant defense
components or host proteins whose function is modified to nourish the
extracellular bacterial pathogen. In the absence of a specific R protein, the
host target is not guarded from the virulence function of Avr, and disease
ensues.
Evidence Supporting the Guard Hypothesis
Cell 108, 743 (02); 112, 369 (03); 112, 379 (03)
AvrRpt2 is a Cysteine Protease [Mol. Micro. 49,1537 (03)]