Supplement bij veertiende jaargang, april 2006 - NVMM

01.01
Innate immunity of plants against fungi; arms race or
balancing selection
P.J.G.M. de Wit, M. Bolton, O. Boras, S. Gabriëls, J. van
’t Klooster, I. Stulemeijer, J. Vossen, P. van Esse. E. Fradin,
U. Ellendorff, I. Stergiopoulos, M. Joosten, B. Thomma.
Wageningen University, Laboratory of Phytopathology,
Wageningen
Avr genes are supposed to have virulence functions in the
absence of the corresponding resistance (R) gene. We have
cloned four Avr and four Ecp genes of the tomato pathogen
Cladosporium fulvum that all encode cysteine-rich peptides
secreted by the fungus during infection of tomato leaves.
Recognition of Avr and Ecp proteins is mediated by Cf
proteins and leads to an innate immune or hypersensitive
response (HR), co-ordinated death of a few host cells at
the site of penetration by the pathogen. C. fulvum avoids
recognition by its host by various mechanisms including:
loss of Avr genes or point mutations, frame shift mutations
or transposon insertions in Avr genes. Avrs are supposed to
interact with a virulence target in the host that is sensed by
Cf proteins that subsequently trigger an HR. Although all
Avr and Ecp proteins are supposed to represent virulence
functions, deletion of single genes do not significantly
reduce virulence of the fungus. For two Avr proteins we
have indications for their biological function. Avr4 is a
chitin-binding protein that protects the fungus against
basic plant chitinases. Avr4 proteins encoded by virulent
alleles in strains of C. fulvum are no longer recognised by
Cf-4 plants, but still bind to chitin, suggesting that chitinbinding
by Avr4 could represent a defensive virulence
function. The Avr2 peptide is secreted by C. fulvum into
the apoplast of tomato leaves and, in the presence of the
tomato extracellular, membrane-anchored Cf2 protein,
triggers the HR that also requires the extracellular tomato
cysteine protease Rcr3. Avr2 binds and inhibits Rcr3, and
the Rcr3-Avr2 complex is subsequently recognized by the
Cf-2 protein.
01.02
Innate immunity of insects
J.M. Reichhart
Strasbourg, France
Drosophila mounts a potent host defence when challenged
by various microorganisms. Molecular and genetic analyses
of this defence have now provided a global picture of
the mechanisms by which this insect senses infection,
discriminates between various classes of microorganisms
A B S T R A C T S
Ned Tijdschr Med Microbiol 2006; 4:Supplement
S 5
and induces the production of effector molecules, among
which antimicrobial peptides are prominent. A major
result in these studies was the discovery that most of the
genes involved in the Drosophila host defence are similar
to genes implicated in the mammalian innate immune
response. Recent progress in research on Drosophila
immune defence and the newly discovered similarities or
differences between Drosophila defence mechanisms and
mammalian innate immunity will be discussed.
01.03
Poxvirus immune evasion strategies are linked to host
tropism
G. McFadden
Robarts Research Institute, London, Canada
Despite the eradication of smallpox as an extant human
disease a quarter of a century ago, there remains
considerable fear that variola virus, or other related
pathogenic poxviruses like monkeypox, could emerge
and spread in the human population again. Although
remarkable advances have been made in our understanding
in the molecular events of poxvirus infections, we are still
mostly ignorant about why most poxvirus infections of
vertebrate hosts usually exhibit strict species specificity, or
how zoonotic poxvirus infections occur when poxviruses
occasionally leap into novel host species. Unlike many
other viruses, poxvirus tropism appears to be regulated
not at the level of specific host receptors, but rather at intracellular
events downstream of virus binding and entry.
This seminar summarizes our current understanding of
poxvirus tropism and host range, with specific emphasis on
the prospects for exploiting host-restricted poxvirus vectors
for vaccines or gene therapy and developing host-targeted
oncolytic viral therapies for human cancers. Our lab has
studied one particular poxvirus, myxoma virus, which
exhibits strict species specificity for the rabbit. Targeted
knockout analysis of specific myxoma virus genes has
revealed new clues about the viral and host determinants
of tropism and host range.
01.04
Bacterial innate immune evasion
J.A.G. van Strijp
UMC Utrecht, Eijkman-Winkler Institute, Dept. of Experimental
Microbiology, Utrecht
Upon entering the human body, bacteria are confronted
with the sophisticated innate defense mechanisms of the