Oral administration of cholera toxin-Sendai virus conjugate potentiates gut and respiratory immunity against Sendai virus

Successful oral immunization to prevent infectious diseases in the gastrointestinal tract as well as distant mucosal tissues may depend on the effectiveness of an Ag to induce gut immune responses. We and others have previously reported that cholera toxin possesses strong adjuvant effects on the gut immune response to co-administered Ag. To explore further adjuvant effects of cholera toxin, the holotoxin or its B subunit was chemically cross-linked to Sendai virus. The resulting conjugates, which were not infectious, were evaluated for their capacity to induce gut immune responses against Sendai virus after oral administration to mice. Conjugating cholera toxin to virus significantly enhanced the adjuvant activity of cholera toxin compared to simple mixing. Cholera toxin B subunit, however, did not show an adjuvant effect either by itself or conjugated with the virus. Oral administration of the Sendai virus-cholera toxin conjugate was also able to prime for protective anti-viral responses in the respiratory tract. Mice that were orally immunized with the conjugate and intra-nasally boosted with inactivated virus alone showed virus-specific IgA titers in nasal secretions that correlated with protection against direct nasal challenge with live Sendai virus. For comparison, s.c. immunization was also studied. Systemic immunization with the virus-cholera toxin conjugate induced virus-specific antibody responses in serum as well as in the respiratory tract but failed to protect the upper respiratory tract against virus challenge. Systemic immunization plus an intra-nasal boost did, however, confer a variable degree of protection to the upper respiratory tract, which correlated primarily with bronchoalveolar lavage (lung) antibody titers.