In vitro mechanisms of monoclonal antibody neutralization of alphaviruses.

We have previously identified at least eight epitopes on the E2 glycoprotein of Venezuelan equine encephalomyelitis (VEE) virus vaccine strain TC-83 by using monoclonal antibodies (MAbs). Several of these antibodies identified a critical neutralization (N) domain in competitive binding assays. Passive transfer of these MAbs protected animals from a lethal virus challenge. Using radioactive, purified virus as a marker, we have demonstrated that antibody-mediated virus N, preattachment, can be effected by one of three mechanisms. Interaction of antibody can block virus attachment to susceptible Vero or human embryonic lung cells. The MAbs that were most efficient at blocking attachment were those that defined epitopes spatially proximal to the E2c epitope. The E2c MAbs were, however, the most efficient antibodies for neutralizing virus postattachment. Other E2 MAbs were unable to efficiently block virus attachment to cells; however, resulting replication as monitored by plaque assay or intracellular viral RNA synthesis could not be detected. One novel MAb that defined the E2f epitope appeared to enhance virus attachment to Vero cells, but not BHK-21 or LLC-MK2 cells, by stabilizing virus-cell interaction. This antibody did, however, efficiently neutralize virus infectivity. Once virus had attached to cells, the ability of most MAbs to neutralize infectivity was diminished, except for E2c MAbs. On a molar basis antibody Fab fragments were less efficient than intact antibody at blocking virus attachment.