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Leukocyte Biology Section, Biomedical Sciences Division, Imperial College School of Medicine, South Kensington, London, United Kingdom; and
Institute of Pharmaceutical Sciences, University of Nottingham, Nottingham, United Kingdom
Eotaxin is a potent eosinophil chemoattractant that acts
selectively through CCR3, which is expressed on eosinophils, basophils,
mast cells, and Th2-type T cells. This arm of the immune system is
believed to have evolved to control helminthic parasites. We
hypothesized that helminths may employ mechanisms to inhibit eosinophil
recruitment, to prolong worm survival in the host. We observed that the
excretory/secretory products of the hookworm Necator
americanus inhibited eosinophil recruitment in vivo in response
to eotaxin, but not leukotriene B4, a phenomenon that could
be prevented by the addition of protease inhibitors. Using Western
blotting, N. americanus supernatant was shown to cause
rapid proteolysis of eotaxin, but not IL-8 or eotaxin-2. N.
americanus homogenate was fractionated by gel filtration
chromatography, and a FACS-based bioassay measured the ability of each
fraction to inhibit the activity of a variety of chemokines. This
resulted in two peaks of eotaxin-degrading activity, corresponding to
15 and 50 kDa molecular mass. This activity was specific for
eotaxin, as responses to other agonists tested were unaffected.
Proteolysis of eotaxin was prevented by EDTA and phenanthroline,
indicating that metalloprotease activity was involved. Production of
enzymes inactivating eotaxin may be a strategy employed by helminths to
prevent recruitment and activation of eosinophils at the site of
infection. As such this represents a novel mechanism of regulation of
chemokine function in vivo. The existence of CCR3 ligands other than
eotaxin (e.g., eotaxin-2) may reflect the evolution of host counter
measures to parasite defense systems.
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