Immunogenicity of recombinant F4 (K88) fimbrial adhesin FaeG expressed in tobacco chloroplast

To test the possibility of producing the novel vaccine in plants against diarrhea normally found in neonatal and newly weaned piglets, the faeG gene, encoding a major F4ac fimbrial subunit protein, was introduced into the tobacco chloroplast genome. After two rounds of selection under spectinomycin, we obtained the transgenic plants nearly homoplasmic. RNA gel blot analysis indicated that faeG and the antibiotic selective gene aminoglycoside 3' adenylyltransferase (aadA) were highly transcribed as a dicistron, while the translational level of recombinant FaeG in transplastomic tobacco was about 0.15% of total soluble protein. The immunogenicity of recombinant FaeG produced in tobacco chloroplasts was confirmed by the observation that FaeG-specific antibodies were elicited in mice immunized with total soluble protein of transgenic plants, as well as the result that mouse sera stimulated by chloroplast-derived recombinant FaeG could neutralize F4ac enterotoxigenic Escherichia coli (ETEC) in vivo. This study provides a new alternative for producing the ETEC vaccine using the chloroplast expression system.