Studies on cross-reactive monoclonal antibodies to sindbis and semliki forest virus El glycoproteins

Mouse MAbs were made against Sindbis (SIN) and Semliki Forest viruses (SFV), members of two different alphavirus subgroups. Some MAbs were found to react with the El glycoprotein of both viruses using the immunoblot technique. These MAbs can lyse SIN- or SFV-infected L_{929>/sub> cells in the antibody-dependent, complement-mediated cytotoxicity assay to high titer. Of four selected MAbs studied in depth, one is IgM, two are IgG2a, and one is IgG2b, and none neutralize either virus. The IgG MAbs were found to protect mice from lethal SFV infection, but the IgM MAb did not. Digestion of SFV virions with either chymotrypsin, Staphylococcal V8 protease or cyanogen bromide treatment generated some fragments, but no fragments smaller than Mr = 19,000 reacted with the MAbs. These data, suggesting that the protective epitope(s) defined by these MAbs is (are) probably dependent on conformation, are discussed.}