An antibody derivative expressed from viral vectors passively immunizes pigs against transmissible gastroenteritis virus infection when supplied orally in crude plant extracts.

To investigate the potential of antibody derivatives to provide passive protection against enteric infections when supplied orally in crude plant extracts, we have expressed a small immune protein (SIP) in plants using two different plant virus vectors based on potato virus X (PVX) and cowpea mosaic virus (CPMV). The epsilonSIP molecule consisted of a single-chain antibody (scFv) specific for the porcine coronavirus transmissible gastroenteritis virus (TGEV) linked to the epsilon-CH4 domain from human immunoglobulin E (IgE). In some constructs, the sequence encoding the epsilonSIP molecule was flanked by the leader peptide from the original murine antibody at its N-terminus and an endoplasmic reticulum retention signal (HDEL) at its C-terminus to allow the expressed protein to be directed to, and retained within, the endoplasmic reticulum. Western blot analysis of samples from Nicotiana clevelandii or cowpea tissue infected with constructs revealed the presence of SIP molecules which retained their ability to dimerize. The analysis of crude plant extracts revealed that the plant-expressed epsilonSIP molecules could bind to and neutralize TGEV in tissue culture, the levels of binding and neutralization reflecting the level of expression. Oral administration of crude extracts from SIP-expressing plant tissue to 2-day-old piglets demonstrated that the extracts which showed the highest levels of in vitro neutralization could also provide in vivo protection against challenge with TGEV.

Immunization of pregnant gilts with PRCV induces lactogenic immunity for protection of nursing piglets from challenge with TGEV.

The level of passive protection against transmissible gastroenteritis virus (TGEV) was evaluated by experimentally infecting 12 pregnant gilts with different doses of porcine respiratory coronavirus (PRCV) and challenging their litters at 4 days of age. An overall survival rate of 70% was found for piglets nursing the 12 PRCV-infected gilts, compared to a 16% survival rate for piglets of nine uninfected control gilts. Six of the PRCV-infected gilts had adequate levels of immunity to resist infection with TGEV following the challenge of their litters. These six completely immuned gilts also solidly protected their litters from TGEV as shown by a 96% piglet survival rate through weaning at 3 weeks of age. The results suggest that respiratory infection with PRCV induces a substantial degree of protective lactogenic immunity against TGEV.

A double-protease-resistant variant of transmissible gastroenteritis virus and its ability to induce lactogenic immunity.

A virus resistant to 2 major intestinal proteases (trypsin and alpha-chymotrypsin) was derived from the attenuated Purdue strain of transmissible gastroenteritis virus. Its enzymatic stability was confirmed, in vitro, by exposure to proteolytic enzymes and to porcine intestinal fluids. Vaccination of 5 seronegative pregnant sows with the variant virus by a series of 2 oral and 1 IM inoculations resulted in high titers of neutralizing antibody in serum and colostrum. The mean antibody titer in milk whey decreased 44-fold within 1 week after parturition. At 3 days of age, the 40 pigs delivered by these sows were challenge exposed orally with virulent transmissible gastroenteritis virus. Pigs nursing the 5 vaccinated sows underwent a relatively mild clinical course of illness. The average mortality of these 40 pigs was 33%. Thirty-six pigs which had been raised by 4 nonvaccinated sows had a more severe illness, greater daily weight loss, and higher mortality (92%).

Passive immunity in transmissible gastroenteritis of swine: intramuscular injection of pregnant swine with a modified live-virus vaccine.

Nine pregnant swine were intramuscularly injected with a modified live-virus transmissible gastroenteritis (TGE) vaccine at approximately 6 and 2 weeks before parturition. The TGE antibody levels in milk whey rapidly decreased within a few days after parturition, and the antibodies were mainly, if not solely, of the immunoglobulin G (IgG) class. Nursing newborn pigs were challenge exposed with virulent TGE virus when 2 to 4 days old. All pigs in 8 of the 9 litters became affected with typical signs of TGE. Of the pigs nursing vaccinated sows, 55% died, and of the 9 vaccinated sows, 8 became affected after challenge exposure of their litters. Subsequently, the antibody titers in serum and milk markedly increased in all 9 sows, with the antibody in milk being predominantly of the immunoglobulin A (IgA) class. Serum samples of prenursing newborn pigs from vaccinated sows did not contain detectable TGE antibodies. The capability of the vaccinated sows to provide only limited passive immunity to their suckling pigs was considered to be due mainly to the low titer and class (IgG) of TGE antibody existing in the milk of the sows at the time the newborn pigs were challenge exposed.