418 Defining Mechanisms of Antibody-mediated Neutralization of SIV K. S. Cole*1, J. Steckbeck1, K. Miller2, I. Orlov2, J. Bruno2, J. Werkman1, N. Polansky1, B. Puffer3, R. Doms3, R. Montelaro1 1Univ of Pittsburgh, PA; 2BIACORE, Inc, Piscataway, NJ; and 3Univ of Pennsylvania, Philadelphia
Background: The ability of neutralizing antibodies to block infection from both parenteral and mucosal infections in passive protection experiments has been demonstrated. However, the mechanisms of this neutralization process remain unclear. One hypothesis is that neutralization sensitivity is related to the density of envelope spikes on the virion, requiring a threshold level of antibody binding. Alternately, neutralization has been associated with differences in the qualitative antibody binding properties. Our goal in this study was to define the mechanism(s) by which rhesus monoclonal antibodies (mAbs) inhibited virus infection at the early events of virus binding, fusion, and internalization.
Methods: We utilized a panel of rhesus mAbs derived from monkeys infected with attenuated SIV that represent at least 9 binding domains (linear and conformational) on SIV gp120, including 3 major in vitro neutralization domains. Rhesus mAbs were analyzed for their ability to inhibit virus internalization as measured by DNA PCR amplification of the LTR region within 4 hrs after infection. To determine whether neutralization activity correlated with antibody binding, quantitative and qualitative properties of antibody binding to SIV env were characterized using real-time biosensor analyses. Finally, the ability of mAbs to inhibit cell-to-cell fusion was determined.
Results: Neutralizing mAbs recognizing more than 1 binding domain on SIV gp120 effectively inhibited SIV entry into permissive cells. No single property of antibody binding to SIV env correlated with the mAb-mediated neutralization of SIV in vitro. Interestingly, 1 group of rhesus mAbs recognizing closely related conformational epitopes in the C-terminal half of SIV gp120 consistently inhibited cell-mediated fusion in a CD4-independent manner.
Conclusions: No single mechanism of antibody-mediated neutralization of SIV was identified. Our data suggest that multiple mechanisms at the levels of virus binding and fusion are involved, and that specific mechanisms may to be related to antibody binding domains on SIV env. Identification of antibody-mediated neutralization mechanisms associated with protection from virus challenge in vivo will be critical for the design of an effective AIDS vaccine.