Induction of systemic immune responses in sheep by topical application of cholera toxin to skin

Cholera (and related) toxins (CT) when applied topically on unbroken skin induce systemic immune responses in mice, a procedure called transcutaneous immunization (TCI). The current study examined the capacity for TCI to induce systemic immune responses in sheep. Three groups (n=5 per group) were immunized at day 0 (priming) and day 28 (boosting) with 250 μg of CT in water by TCI, with 25 μg of CT in alum by intramuscular injection, or not immunized. Serum samples were taken at days 0, 28, 42, 56 and 70 after immunization for measurement of CT-specific IgG as well as CT-specific IgG1, IgG2, IgA and IgM antibodies by ELISA. After immunization, IgG, IgG1 and IgG2 antibody in immunized groups were significantly higher than in the control group, and boosting further increased these titres. IgG, IgG1 and IgG2 in the injection group were significantly higher than in the TCI group. There was a preponderance of IgG1 antibody, relative to IgG2, in both immunized groups. CT-specific IgA and IgM were detected in both immunized groups. Lymphocyte proliferation to CT was measured at day 90. A CT-specific lymphocyte proliferative response (stimulation index>2) was detected in all sheep from the injection group, in two sheep from the TCI group and in none of the controls. Results demonstrated that TCI induces primary and secondary antibody responses and specific proliferative responses to CT in sheep.