Induction of protective immunity in cattle against infection with Fasciola hepatica by vaccination with cathepsin L proteinases and with hemoglobin

Two cathepsin L proteinases, cathepsin L1 and cathepsin L2, secreted by liver flukes may be involved in tissue penetration, nutrition, and protection from immune attack. To ascertain the immunoprophylactic potential of these proteinases, and of another molecule, liver fluke hemoglobin (Hb), we performed vaccine trials in cattle. In the first vaccine trial various doses of cathepsin LI were tested. The mean protection level obtained was 53.7%. In a second vaccine trial cathepsin L1 and Hb elicited 42.5 and 43.8% protection levels, respectively, while a combination of the two molecules induced a significantly higher level of protection (51.9%). Cathepsin L2 was not examined alone; however, vaccination of cattle with a combination of cathepsin L2 and Hb elicited the highest level of protection (72.4%). The animals that received cathepsin L1-Hb or cathepsin L2-Hb shelved reduced liver damage as assessed by serum glutamic dehydrogenase and gamma-glutamyl transferase levels. Furthermore, a reduced viability was observed for fluke eggs recovered from all vaccine groups. This anti-embryonation effect of vaccination was particularly evident in the group that received cathepsin L2-Hb where >98% of the eggs recovered did not embryonate to miracidia. Although all vaccine preparations induced high antibody titers which were boosted following the challenge infection, there was no correlation between antibody titers and protection. The results of these trials demonstrate that cathepsin Ls and Hb could form the basis of a molecular vaccine that would not only reduce parasite burden but would also prevent transmission of liver fluke disease.