Neutralizing and non-neutralizing monoclonal antibodies to the E2 glycoprotein of Semliki Forest virus can protect mice from lethal encephalitis

Two monoclonal antibodies (UM 4.2 and UM 5.1) directed against the glycoprotein E2 of Semliki Forest virus (SFV) are described; both belong to the IgG2a isotype but are of different idiotype. Analysis employing isoelectric focusing resulted in different focusing patterns for both monoclonals (UM 4.2, pI 8; UM 5.1, pI 7.2). They further differed in their ability to neutralize virus. The UM 4.2 antibodies were inactive in neutralization, while the UM 5.1 antibodies exceeded conventional mouse hyperimmune serum in this respect. Both monoclonal antibodies, however, were able to protect mice passively from a lethal infection with SFV. Based on the amount of protein, the UM 5.1 antibodies were 100-fold more effective than the UM 4.2 antibodies in mouse protection tests.