Vaccine testing of a recombinant activation-associated secreted protein (ASP1) from Ostertagia ostertagi

Previous vaccination trials against the economically important cattle parasite Ostertagia ostertagi have indicated the protective capacity of activation-associated secreted proteins (ASPs). The further development of these antigens into a commercial vaccine will require their recombinant expression. The aim of the current study was to clone and express Oo-asp1 in a baculovirus expression system and to evaluate the protective capacity of the recombinant protein against an O. ostertagi challenge infection in cattle. The full coding sequence of Oo-asp1 was cloned in a baculovirus expression vector in frame with a carboxy-terminal Histidine tag and recombinant virus was used to infect an insect cell culture. Western blot analysis with anti-His and anti-Oo-ASP1 antibodies showed the production of recombinant Oo-ASP1. The cell pellet containing the recombinant was subsequently used to immunize seven calves three times intramuscularly with QuilA as adjuvant. Control animals were solely injected with the QuilA adjuvant. The challenge infection with O. ostertagi consisted of 30,000 L3 larvae per animal given over 30 days (1000 larvae/day, 5 days/week) and started the same day as the final immunization. Immunization with the recombinant Oo-ASP1 did not result in any level of protection against the challenge infection. There was no reduction in faecal egg output or in worm burdens. Moreover, Western blot analyses and ELISA indicated that, although the animals raised an antibody response against the recombinant Oo-ASP1, there was hardly a response against the native Oo-ASP1, suggesting that the baculovirus expressed recombinant was wrongly folded or lacked essential secondary modifications. Further analysis of the structure of the native ASPs and their glycosylations is being done.