Further immunization and biochemical studies with a protective antigen complex from the microvillar membrane of the intestine of Haemonchus contortus

Immunization of sheep with a protective antigen complex from the intestinal cells of Haemonchus contortus in Freund's adjuvant stimulated individually variable antibody responses but still conferred significant protection aga inst parasite infection. Correlation between antibody concentration and deg ree of protection was suggestive of antibody being the effector mechanism. The antigen is known as Haemonchus galactose-containing glycoprotein comple x (H-gal-GP) because it binds to lectins with a specificity for N-acetyl-ga lactosamine. Polypeptide composition analysis by polyacrylamide gel electro phoresis indicated an apparent molecular weight of about 1000 kDa and SDS g els revealed four major polypeptides, containing between 2 and 5 disulphide linked subunits, nearly all being glycosylated. N-terminal amino acid sequ ence was obtained from 12 subunits, ten showing homologies with cDNAs from Haemonchus encoding either pepsin, metalloprotease or cysteine protease-lik e enzymes, pH optima, inhibitor and various substrate studies confirmed tha t the native complex possessed proteolytic activities in agreement with the sequence data. Although the cDNAs predicted water soluble enzymes, little of the complex was solubilized from worm membranes without the use of a det ergent, such as Triton X-100. It is hypothesized that H-gal-GP is a gut mem brane associated multiprotease complex which Is involved in the digestion o f the blood meal and which can be neutralized by specific antibodies with d rastic consequences for the parasite.