Biomedical Engineering Reference
In-Depth Information
HcPep1 is a component of the highly host-protective integral membrane gly-
coprotein complex from H. contortus, H-gal-GP, which reduces faecal egg
counts in sheep by 90% and worm burdens by 70% when used as a vaccine. 28
The fraction of H-gal-GP containing HcPEP1 and another pepsin-like aspartic
protease (HcPEP2) is protective, resulting in significantly reduced worm and
egg counts following vaccination. 29
8.2.2 Cysteine Proteases
Parasite cysteine proteases have structural and biochemical properties that are
distinct from their host orthologues, including pH optima, alterations in pep-
tide loops or domain extensions, diverse substrate specificities and cellular
locations. These features, as well as their central roles in parasitism, make these
proteases attractive targets for intervention strategies. 30 The most widely
reported class of cysteine proteases that helminths express is clan CA. The clan
CA proteases are further divided into two families: C1, which comprises
cathepsins B and L-like proteases, and C2, which comprises calpain-like
proteases. 31
Numerous hookworm cysteine proteases have been reported to play known
or putative roles in digestion of the blood meal due to their expression in gut
tissue. 5,32 Indeed, a number of these proteases have been expressed in recom-
binant form and degrade Hb: Ac-CP-2, a cathepsin-B-like cysteine protease
from A. caninum, cleaved intact Hb 33 and Na-CP-3, a N. americanus cathepsin
B-like protease, hydrolysed globin fragments after initial cleavage of the Hb
tetramer by Na-APR-1. 12
Vaccination of dogs with Ac-CP-2 resulted in a decrease in parasite faecal egg
counts, and the number of female hookworms present in the intestine was
significantly reduced compared to control dogs. 33 Like vaccination with APR-
1, anti-Ac-CP-2 antibodies bound to the gut of hookworms from vaccinated
dogs (Figure 8.2), suggesting that these antibodies were ingested by the para-
sites with their blood meal, and IgG from vaccinated dogs decreased the pro-
teolytic activity of the recombinant protein against a peptide substrate by 73%,
which implies that neutralizing antibodies were induced by vaccination. 33
Vaccination with Na-CP-2 and subsequent challenge with N. americanus in
hamsters also showed a significant reduction in worm burden. 20
It is estimated that potentially 16% of the H. contortus transcriptome
encodes cysteine protease genes, 34 many of which are homologous to the
intestinal cysteine proteases of N. americanus. 32 These molecules have been
demonstrated to inhibit blood clot formation and to degrade Hb, fibrinogen,
collagen, and IgG, suggesting a role for the enzymes in attachment, blood
feeding, and immune evasion by the adult worm. 32 A cysteine protease enriched
fraction (thiol sepharose binding fraction (TSBP)), prepared from membrane
extracts of adult H. contortus, localized to the microvillar surface of intestinal
cells of the worm, 8 and lambs immunized with TSBP were protected against a
single challenge infection with H. contortus, with reductions in daily faecal egg
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