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Infection and Immunity, June 2004, p. 3228-3236, Vol. 72, No. 6
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.6.3228-3236.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Recombinant Streptococcus equi Proteins Protect Mice in Challenge Experiments and Induce Immune Response in Horses

Margareta Flock,¹ Karin Jacobsson,² Lars Frykberg,² Timothy R. Hirst,^3, Anders Franklin,⁴ Bengt Guss,² and Jan-Ingmar Flock^1*

Department of Laboratory Medicine, Karolinska Institutet, Stockholm,¹ Department of Microbiology, Swedish University of Agricultural Sciences,² National Veterinary Institute, Uppsala, Sweden,⁴ Department of Pathology and Microbiology, University of Bristol, Bristol, United Kingdom³

Received 16 January 2004/ Returned for modification 9 February 2004/ Accepted 15 February 2004

Horses that have undergone infection caused by Streptococcus equi subspecies equi (strangles) were found to have significantly increased serum antibody titers against three previously characterized proteins, FNZ (cell surface-bound fibronectin binding protein), SFS (secreted fibronectin binding protein), and EAG (₂-macroglobulin, albumin, and immunoglobulin G [IgG] binding protein) from S. equi. To assess the protective efficacy of vaccination with these three proteins, a mouse model of equine strangles was utilized. Parts of the three recombinant proteins were used to immunize mice, either subcutaneously or intranasally, prior to nasal challenge with S. equi subsp. equi. The adjuvant used was EtxB, a recombinant form of the B subunit of Escherichia coli heat-labile enterotoxin. It was shown that nasal colonization of S. equi subsp. equi and weight loss due to infection were significantly reduced after vaccination compared with a mock-vaccinated control group. This effect was more pronounced after intranasal vaccination than after subcutaneous vaccination; nearly complete eradication of nasal colonization was obtained after intranasal vaccination (P < 0.001). When the same antigens were administered both intranasally and subcutaneously to healthy horses, significant mucosal IgA and serum IgG antibody responses against FNZ and EAG were obtained. The antibody response was enhanced when EtxB was used as an adjuvant. No adverse effects of the antigens or EtxB were observed. Thus, FNZ and EAG in conjunction with EtxB are promising candidates for an efficacious and safe vaccine against strangles.

Editor: J. T. Barbieri

Present address: The University of Sydney, Sydney, Australia.

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