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Infection and Immunity, August 2008, p. 3640-3650, Vol. 76, No. 8
0019-9567/08/$08.00+0     doi:10.1128/IAI.00050-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Effective Plague Vaccination via Oral Delivery of Plant Cells Expressing F1-V Antigens in Chloroplasts

Department of Molecular Biology and Microbiology, College of Medicine, University of Central Florida, Biomolecular Science Building #20, Room 336, 4000 Central Florida Blvd., Orlando, Florida 32816-2364,¹ Bacteriology Division, United States Army Medical Research Institute of Infectious Diseases (USAMRIID), 1425 Porter Street, Fort Detrick, Frederick, Maryland 21702²

Received 14 January 2008/ Returned for modification 10 March 2008/ Accepted 19 May 2008

The chloroplast bioreactor is an alternative to fermentation-based systems for production of vaccine antigens and biopharmaceuticals. We report here expression of the plague F1-V fusion antigen in chloroplasts. Site-specific transgene integration and homoplasmy were confirmed by PCR and Southern blotting. Mature leaves showed the highest level of transgene expression on the third day of continuous illumination, with a maximum level of 14.8% of the total soluble protein. Swiss Webster mice were primed with adjuvant-containing subcutaneous (s.c.) doses of F1-V and then boosted with either adjuvanted s.c. doses (s.c. F1-V mice) or unadjuvanted oral doses (oral F1-V mice). Oral F1-V mice had higher prechallenge serum immunoglobulin G1 (IgG1) titers than s.c. F1-V mice. The corresponding serum levels of antigen-specific IgG2a and IgA were 2 and 3 orders of magnitude lower, respectively. After vaccination, mice were exposed to an inhaled dose of 1.02 x 10⁶ CFU of aerosolized Yersinia pestis CO92 (50% lethal dose, 6.8 x 10⁴ CFU). All control animals died within 3 days. F1-V given s.c. (with adjuvant) protected 33% of the immunized mice, while 88% of the oral F1-V mice survived aerosolized Y. pestis challenge. A comparison of splenic Y. pestis CFU counts showed that there was a 7- to 10-log reduction in the mean bacterial burden in survivors. Taken together, these data indicate that oral booster doses effectively elicit protective immune responses in vivo. In addition, this is the first report of a plant-derived oral vaccine that protected animals from live Y. pestis challenge, bringing the likelihood of lower-cost vaccines closer to reality.

Published ahead of print on 27 May 2008.

Editor: R. P. Morrison