Abstract

Vaccine antigen-specific antibody-secreting cell (ASC) responses in peripheral blood of healthy adult volunteers were studied after oral immunization with a prototype enterotoxigenic Escherichia coli (ETEC) vaccine by means of the enzyme-linked immunospot technique. Three doses of vaccine consisting of formalin-killed ETEC bacteria expressing fimbrial colonization factor antigens I and II (CFA/I and CFA/II) in combination with purified cholera toxin B subunit (CTB) were given 2 weeks apart. The ASC responses were detected 7 days after each immunization. Immunoglobulin A (IgA) was the predominant isotype produced by CFA/I- as well as CFA/II-specific ASCs. Moderate CFA/I- and CFA/II-specific IgM-secreting ASC (IgM-ASC) responses were also seen, whereas IgG-ASC responses to either of the CFAs were negligible. The ASC responses to CTB, on the other hand, comprised both IgA- and IgG-ASCs, with few if any specific IgM-ASCs. Almost 90% of the volunteers developed CFA-specific ASC responses after vaccination. Maximal CFA-specific ASC responses were usually observed after a single dose or two doses of vaccine. A third dose of vaccine did not result in increased but rather resulted in decreased magnitudes of CFA-specific ASC responses. Furthermore, it was found that CTB did not function as a mucosal adjuvant, since CFA-specific ASC responses were not enhanced by the simultaneous administration of CTB. These results suggest that two oral doses of ETEC vaccine induce a strong mucosal immune response, as reflected by the presence of large numbers of antigen-specific mucosal B cell immunoblasts in the blood.