Biol 316

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Biol 316 THE IMMUNE SYSTEMS of INVERTEBRATES
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DO ALL ANIMALS NEED HOST DEFENCE SYSTEMS?

• all multicellular individuals are clones!!!!
  derived from mitotic division of fertilized egg
  or bud

Fig 1. All metazoans are clonal individuals
(http//worms.zoology.wisc.edu/urchins)
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• multicellularity is a major benefit because it
  allows cellular specialisation

• but evolutionary benefits occur only if the
  clonality/integrity of an organism is defended

• this predicts that all metazoans have host
  defence systems to prevent harmful stow-aways

Fig 2. Sir Frank MacFarlane Burnet, Nobel Prize
in Medicine, 1960 http//www.austehc.unimelb.edu.a
u/guides/burn/gifs/burnetportrait.jpg

• this concept is the basis of Burnetts theory of
  self vs non-self i.e. all animals have host
  defence systems that rely on the capacity to
  distinguish between self and non-self

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CORE DEFENSIVE PARADIGMS

• all host defence systems must fulfil 3 key
  activities predicted by Burnet

• recognition capacity to identify threat whilst
  discriminating between self and not self
• induction ability to activate defense responses
  after recognition of threat
• effect killing or incapacitation of threat once
  defensive systems have been activated

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immuno- competent cells
effector activity
non-self
recognition
Fig 3. The central paradigm of defence
induction
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• RECOGNITION

• in gnathostomes (jawed vertebrates),
  hypervariable receptors provide global defence

• vertebrates rely on immunoglobulin (Ig)
  superfamily members (antibodies and T-cell
  receptors) to detect non-self

highly specific recognition systems that
identify extraordinarily precise antigens
discriminative recognition
Fig 4. The Ig domain
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DISCRIMINATIVE RECOGNITION

• effectiveness of Ig and TCR based on their
  extreme structural diversity

bugs
B-cells
Y
V
B
U
B
U
C
B
Fig 5. Discrimminative recognition
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Ig DOMAINS IN INVERTEBRATES

• both antibodies and TCR have evolved from
  ancient immunoglobulin superfamily (IgSF) domains

• IgSF domains are found throughout the animal
  kingdom

• BUT, no hypervariable IgSF receptors have been
  implicated in the defence of animals other than
  vertebrates

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Fig 6. Phylogeny of the metazoans
Nature, 439, 965-968
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INVERTEBRATES RELY ON PATTERN RECOGNITION
alternative to discriminative recognition

• pattern recognition receptors (PRR) identify
  repeating
• molecular patterns that
• are common to large
• groups of potential
• pathogens
• Pathogen-associated
• molecular patterns
• PAMPs

microbes
PAMP
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Fig 8. Examples of PAMPs
mycoplasma some Gram positive bacteria
i. phosphorylcholines on bacteria
other Gram positive bacteria
ii. surface carbohydrates e.g. peptidoglycan on
bacteria and b-1,3 glucan on fungi, carragenans
on marine algae
fungi, algae
Gram negative bacteria
iii. lipopolysaccharides on Gram negative
bacteria
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HOW ARE MOLECULAR PATTERNS RECOGNIZED?

• PAMPs identified by pattern recognition
  receptors (PRR)
• classic example
• LECTINS carbohydrate recognition
• proteins that can detect surface carbohydrates on
  potential pathogens

Fig 9. Recognition of carbohydrates by lectins
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COLLECTINS IN TUNICATES

• collectins common form of defensive lectin
  designed to detect broad array of microbes
  (bacteria, fungi, viruses, algae)

• collectins are composite proteins containing 4
  different domains,
• each of which contributes to function

Fig 9.8a Pyura stolonifera




20 aa
170aa
40aa
120aa
neck
collagen
tail
CRD
Fig 10. Domain structure of collectins. Comp.
Biochem. Physiol. B. 125 279-289
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2. INDUCTION

• recognition of non-self leads to the induction
  of defensive systems
• can result from immediate cellular responses
  like exocytosis or longer term gene induction

Fig 11. Defensive proteins in the secretory
vesicles of tunicate hemocytes
Fig 12. Induction of collectin expression in
tunicate hemocytes at various times after the
injection of the yeast cell wall antigen,
zymosan.
Dev. Comp. Immunol. 27 3-9
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• gene induction can be exploited to tailor
  responses toward particular types of infection

Fig 13. The antimicrobial peptides of Drosophila
and their targets. Hultmark, Current Opinions in
Immunology. 1512-19, 2003
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• antimicrobial peptide genes in Drosophila are
  selectively induced to tailor responses to
  specific microbes

Fig 14. Signalling pathways involved in
antimicrobial protein induction. Hultmark,
Current Opinions in Immunology. 1512-19, 2003
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3. EFFECTOR SYSTEMS

• invertebrate hemocytes (or coelomocytes) can
  secrete a
• range of
• defensive
• molecules
• with
• different
• defensive
• activities

Fig 15. Common effector systems in invertebrates.
Nappi and Ottaviani. BioEssays 22469-480,
(2000) John Wiley Sons, Inc.
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i. Opsonisation by TEP proteins

• thiolester bearing (TEP) proteins may be key
  elements of defense in many invertebrates
• thiolester group allows TEPs to covalently bind
  onto the surface of microbes
• TEPs can be activated by collectins
• TEPs found so far in insects, sea urchins,
  tunicates

thiolester bond
Fig 15. Opsonic phagoytosis by sea urchin
coelomocytes J Exp Biol. 2072147-55 2004
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ii. Phenoloxidase

• multifunctional enzyme system involved in
  pigmentation, schleritisation and defence
• identified in tunicates, crustaceans insects
  probably ubiquitous

proPO
tyrosine
b-1,3glucan binding protein
PO
melanin
intermediates
L-dopa
anti-bacterial, anti-fungal
Fig 16. The phenoloxidase cascade
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iii. Antimicrobial peptides (AMPs)

• small proteins with direct antimicrobial
  activities
• appear to be ubiquitous, also common in
  gnathostomes
• 4 generic families based on sequence similarities

Fig 17. Families of antimicrobial peptides.
Nappi and Ottaviani. BioEssays 22469-480,
(2000) John Wiley Sons, Inc.
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• AMPs often operate by lysing microbial surfaces

Fig 18. Mode of action of anitmicrobial
peptides. Oren and Shai. Biopolymers (Peptide
Science), Vol. 47, 451463 (1998)
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• there are often more than one AMP with closely
  related sequences in a single species

- this suggests that sequence diversity might
target AMPs toward particular species of pathogen
or prevent pathogen escape mechanisms
Fig 19. Sequence alignments of antimicrobial
peptides from Xenopus
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• the selective toxicity of different amino acid
  sequences has been confirmed by generating
  synthetic peptides and testing their effects on a
  range of pathogens

Fig 20. Antimicrobial activities of synthetic
AMPs against a range of bacteria
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Defense and diversity

• in some species that need for diversity may have
  generated hypervariable gene systems

• in the purple sea urchin, Strongylocentrotus
  purpratus, gt70 of transcripts encode a group of
  antimicrobial proteins called purpuratins
  Physiol. Genomics 2233-47, 2005.

• it appears that thousand of different variants
  of these proteins can be produced by a single sea
  urchin

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• this diversity of purpuratins is generated by
  domain shuffling, sequence repeats and point
  mutations
• if this proves to be true, it will represent one
  of the most variable genetic system ever
  discovered

B.
A.
NDSSEEDGRHHLHHDRHHAHHGHH -E-------P-P--HG--R--R--
-Y-------P-P--H---G--R-- -Y-------P-P--H---G-----
-E-ND-G--P-PR-HGR-HQ-H-R -E--D-G--P-PR-HGR-HQ-H-R
-E-ND-G--P-PR-HGR-HQ-H-R -E-ND----P-PS-HVR-HQ-H-R
-E-------P-PR-HGR-HQ-H-R -E-------P-P--H---G--HR-
-Y-------P-P--H---G--R-- -Y------GP-P--H---G-----
-ERN-----P-P--HG--G-Q--- -ERN-----P-P--HG--G----R
Fig 20. Generation of diversity among purpuratins
by A. domain shuffling and B. single nucleotide
substitutions. Physiol. Genomics 2233-47, 2005.
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Fig 21. Fibrinogen-Related Peptides (FREPs) from
Biomphalaria glabrata.
Zhang et al (2003), Immunogenetics 53, 684-694.
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Fig 21. Chitin-binding proteins (VCBPs) from
Branchiostoma floridae.
Current Biol., 14, R465-R466 (2004)
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Fig 22. DSCAMs in insects
Science, 309 ,1874-8 (2005)
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Fig 23. Throughout the metazoans, a huge number
of different receptors and effector molecules are
used to effect innate immunity, and these
typically vary between the major phylogenetic
groups.
Nature Reviews Immunology 5, 866-879 (2005)
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Fig. 24. A simplified phylogenetic tree depicting
the general relationships of the major bilaterian
phyla and chordate subphyla, highlighting select
species that use different somatic mechanisms of
immune receptor diversification
J. P. Rast et al., Science 314, 952 -956
(2006)
Published by AAAS
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Perspectives

• We are still scratching the surface of
  invertebrate immune responses
• Much more knowledge will be gained in the future
  by shotgun analytical approached like EST or
  microarray analyses
• Innate immune components can be structurally
  diverse.
• May provide a reasonable basis for invertebrates
  to fight off potential infections, despite not
  possessing a vertebrate-like adaptive immune
  system.
• Many of the components for the evolution of the
  vertebrate adaptive immune system may have been
  present in their invertebrate ancestors.