Neutralizing monoclonal antibodies (MAbs) were produced in BALB/c mice immunized with live modified vaccinia virus Ankara or infected with sublethal doses of the neurovirulent vaccinia virus strain Munich 1. The immunization scheme proved to be important for obtaining MAbs of different specificity. The MAbs could be classified into three epitope groups (1 A, 1 B and 2). Immunogold electron microscopy demonstrated that the epitopes were localized on the virus surface. In immunoblotting, MAbs were reactive with polypeptides of 14K, 16K and 30K. Purified MAbs binding to the epitopes 1 A and 2 showed a 50% reduction of 100 p.f.u./0.05 ml vaccinia virus M1 with respectively 3.9 and 5.9 ng of immunoglobulin/0.05 ml. MAbs binding to the epitope 1 B neutralized the virus at a concentration of 250 ng/0.05 ml. In intraperitoneal challenge experiments, MAbs binding to the epitopes 1 A and 2 protected mice against 4 LD of vaccinia virus M1, but not against local lesions by subcutaneous application. MAbs against epitope 1 B had no protective effect . The three epitopes were present in 14 of 16 orthopoxviruses tested but with quantitative differences. Maximal binding ( ) and the antibody concentration at half-maximal binding ( ) which were calculated as for Michaelis-Menten kinetics from regression analysis of the ELISA data and the MAb concentration giving 50% plaque reduction were the basis for the evaluation. In monkeypox virus Kopenhagen the epitopes 1 A and 1 B were absent. MAbs binding to epitope 2 reacted just as well as with vaccinia viruses. Ectromelia virus lacked all the epitopes.


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