Enhancement of intranasal vaccination with recombinant chain A ricin vaccine (rRV) in mice by the mucosal adjuvants LTK63 and LTR72

Abstract

Intranasal (i.n.) vaccination of mice with three doses of 40 μg of rRV stimulated low anti-ricin ELISA and neutralizing antibody responses, which were only marginally protective against aerosol-delivered 5–10 LD 50 of ricin toxin. To enhance the protection, and to reduce the lung injury of vaccinated mice that survived ricin toxin challenge, the mucosal adjuvant LTK63 or LTR72, two mutants of Escherichia coli LT enterotoxin adjuvant was administered with rRV. The safety of intranasally administered LTR63 was assessed as well. With 4, 2, or l μg of LTR63, the anti-ricin ELISA serum immunoglobulin geometric mean titer (GMT) increased up to 147-, 356-, 493-, and 17-fold for IgG, IgG1, IgG2a, and IgA, respectively. The comparable increases for GMTs of IgG and IgG1 in the presence LTR72 were up to 147-, and 617-fold, respectively. All three dose levels of LTK63 enhanced the ELISA GMTs in the lung lavage up to 192-, 22-, 4-, and 5-fold for IgG, IgG1, IgG2a, and IgA, respectively. Compared to GMT of rRV alone, the serum-neutralizing antibody GMTs for the three dose levels were enhanced up to 11-fold with LTK63. LTK63 augmented the ricin-related lymphoproliferative response of the cultured spleen lymphocytes and of the isolated CD4+ T lymphocytes. In the cultured lymphocytes, LTK63 stimulated predominantly TH1 cytokines. While only 10% of the mice that were vaccinated with rRV survived lethal challenge, in the presence of LTK63 or LTR72, the respective survival rates were augmented to 100%. Compared to the surviving mice vaccinated with rRV alone, the vaccine with LTK63 or LTR72 did not attenuate the extent of the ricin-related lung injury at a single or two time-points, respectively. Safety of LTK63 administration was indicated by the absence of histopathological changes in every organ, including the lungs and in the central nervous systems (CNS) of the mice during the entire 92 days of the study. In the nasal passages of the mice that received LTK63, a transient inflammation occurred without permanent epithelial changes. Administration of three dose levels of the adjuvant in the presence of rRV caused no additional changes. LTK63 and LTR72 both were very effective and safe mucosal adjuvants at all three dose levels employed in these studies. Both significantly enhanced the protection of a marginally effective dose of rRV against aerosol-delivered ricin challenge. LTK63 stimulated cytokines, which could be surrogate markers of efficacy, with human relevance potential. In spite of the better efficacy, rRV with LTK63, or with LTR72, failed to reduce the ricin-related lung injury. Most likely, a larger than suboptimal dose could resolve the lung injury of the vaccinated mice in the presence of a larger dose of the mucosal adjuvant.