Epitope mosaic on the surface proteins of orthopoxviruses.

Epitopes on the surface components of orthopoxviruses were analyzed with monoclonal antibodies (MAbs) against monkeypox and vaccinia viruses by enzyme-linked immunosorbent assay (ELISA), Western blotting (WB), radioimmunoprecipitation (RIP), and competitive binding inhibition assay (CBIA). When compared by ELISA, three vaccinia virus strains exhibited a similar reactivity to 99 tested MAbs despite their remote passage history. All five isolates of monkeypox virus closely resembled one another, irrespective of the host species (human, monkey, squirrel) from which they were isolated. Taterapox virus reacted similar to vaccinia virus against 97 of the 99 tested MAbs, and reacted with 2 MAbs which were cross-reactive with monkeypox and mousepox. Mousepox and cowpox viruses reacted with these MAbs in a species-specific manner: MAbs reactive to cowpox virus distinctly differ from those reactive to mousepox virus. Of the 99 tested MAbs, 32 reacted with all the 11 tested orthopoxviruses, indicating that the corresponding epitopes existed in all the viruses. Fifty-four MAbs reacted with two or more virus species and were classified as partially common MAbs. Eight MAbs were apparently type-specific for monkeypox, and five were specific for vaccinia and taterapox viruses. No strain-specific epitope was detected. Sera of monkeypox-infected patients, when analyzed by CBIA, interfered with the binding of monkeypox-specific MAb H12C1 but not of vaccinia-specific MAb G6C6. Sera of monkeypox-infected patients who had been vaccinated competed against both MAbs, demonstrating the original antigenic sin phenomenon. The two MAbs could distinguish between the sera of monkeypox patients and those of vaccinated persons. However, the serum of a smallpox patient was competitive against these apparently vaccinia- or monkeypox-specific MAbs. Three of the eight monkeypox-specific epitopes were recognized by the above CBIA test, which suggests that they also exist in smallpox virus. The mosaic-like combination of common epitopes and the small number of type-specific epitopes manifested the antigenic characteristics of orthopox viruses. The species boundary was obscured due to the partially common epitopes, but the total composition of epitopes was stable enough to maintain the antigenic species-specificity. The mutual relationship of the orthopoxviruses was visualized in a three-dimensional network.