Enhancement of intranasal vaccination in mice with deglycosylated chain A ricin by LTR72, a novel mucosal adjuvant

Intranasal (i.n.) vaccination with two suboptimal doses of 8 μg of deglycosylated chain A ricin (DGCA) stimulated low anti-ricin ELISA IgG and neutralizing antibody responses and the vaccine was only marginally protective against a lethal ricin toxin aerosol challenge. However, in the presence of 4, 2, or 1 μg of the mucosal adjuvant LTR72, a mutant of the heat-labile enterotoxin of Escherichia coli, the low antibody response and protection were substantially enhanced. In comparison to the vaccination with DGCA alone, vaccination with DGCA in the presence of three dose levels of LTR72, the anti-ricin ELISA serum IgG geometric mean titer (GMT) was increased, respectively, 191-, 572-, and 51-fold for IgG; 91-, 93-, and 60-fold for IgG1; nine-, six-, and two-fold for IgG2a; zero-, two-, and zero-fold for IgA. The three dose levels of the adjuvant enhanced the anti-ricin ELISA immunoglobulin GMTs in the lung lavage 4-, 14-, and 7-fold for IgG; two-, five-, and six-fold for IgG1; two-, six-, and two-fold IgG2a; and zero-, three-, and zero-fold for IgA, respectively. Compared to GMT obtained with the aqueous vaccine (1:2), the 10% serum neutralizing antibody GMT for the three dose levels was enhanced 25-, 60-, and 62-fold, respectively while the 50% neutralizing antibody GMT was enhanced more than 3-, 19- and 10-fold. Only 20% of the mice immunized with DGCA survived the lethal whole body aerosol challenge with 5–10 LD 50 ricin toxin, while in the presence of 4, 2, and 1 μg LTR72, 100, 100 and 90% of the vaccinated mice survived, respectively. Safety of administration of two doses of LTR72 is indicated by the absence of histopathological changes in every organ including the lung and the CNS of the mice during the vaccination and during 57 days of the study. In the nasal passages of the mice in the absence of DGCA, LTR72 caused a transient inflammation for less than 7 weeks without permanent epithelial changes. Administration of the adjuvant in the presence of DGCA did not cause additional changes. Compared to the surviving mice vaccinated with DGCA alone, administration of the mucosal adjuvant with DGCA in spite of the better efficacy did not attenuate the lung injury at a single time point (16 days post-challenge). In mice treated with high(er) dose of vaccine, histological examinations during longer observation period rather than at one time point could reveal a different pattern.