Protection against Streptococcus equi infection by monoclonal antibodies against an M-like protein

We have developed an in vivo passive transfer assay using mice to identify monoclonal antibodies (mAbs) which offer protection against Streptococcus equi infection. The assay was developed using serum antibodies collected from horses convalescing from strangles. In this study, we show that a preparation of M-like protein, acid-extracted from S. equi, affords 80% protection to mice immunized with it. A number of mouse mAbs directed against a preparation of M-like protein were then assessed for their ability to passively protect mice against challenge with a lethal dose of the bacteria. Two mAbs, 1D10 and 2A6, were shown to be highly protective. It was also demonstrated, by means of a competitive enzyme immunoassay, that these mAbs recognized different epitopes in the preparation. Examination of a dose-response curve for mAbs 1D10 and 2A6 revealed that optimal levels of protection were achieved using 1 mg of either 1D10 or 2A6, or 0.5 mg 1D10 and 0.5 mg 2A6 given together. Immunological reactivity of these mAbs with a preparation of M-like protein showed that the antigens they recognized were comparable in size to some of the antigens recognized by convalescent horse serum antibodies. The role of immunoglobulin isotype in conferring protection is discussed.