Antibodies to surface epitopes of the carbohydrate larval antigen CarLA are associated with passive protection in strongylid nematode challenge infections.

Sheep were immunized by multiple truncated infections with the gastrointestinal nematodes Trichostrongylus colubriformis, Haemonchus contortus and Teladorsagia circumcincta. Three infections with T. colubriformis of 14 days plus five booster doses of L3 stimulated highly effective protection against challenge (99%). Three infections of 14 days plus three booster doses with H. contortus also resulted in significant protection against challenge infection (87%), but the same procedure was not effective for T. circumcincta. Antibodies derived from gastrointestinal mucus of these immunized sheep were tested for their ability to reduce worm burden following injection of antibody-coated exsheathed larvae into the abomasum (H. contortus and T. circumcincta) or duodenum (T. colubriformis) of nematode-naïve sheep in a passive immunity test. The IgG fraction from the mucus of immunized sheep reduced worm burdens by 62%, 76% and 91% in three tests with T. colubriformis but was not effective for either of the abomasal dwelling nematodes H. contortus and T. circumcincta. Antibodies in immune mucus predominantly recognized two larval surface antigens on immunoblots of L3 extract, a high MW surface glycoprotein and the carbohydrate larval antigen (CarLA). Antibodies raised against purified T. colubriformis glycoprotein Tc-120 and CarLA were tested in the passive immunity model and it was found that only the antibody against CarLA resulted in a significant reduction of infection (87%). The protective anti-CarLA antibodies strongly recognized the surface of living T. colubriformis L3. Antibodies from abomasal mucus of sheep immunized by H. contortus and T. circumcincta infections reacted weakly with CarLA and the larval surface and did not reduce worm counts in a passive immunity test. The results provide further evidence that the larval surface carbohydrate antigen CarLA has potential as a mucosal immunogen for a strongylid nematode vaccine.