Background:  Neutralizing antibodies to HIV-1 inhibit infection by physically blocking the interaction of surface envelope proteins with target cell receptors or by preventing viral fusion. However, non-neutralizing antibodies against HIV predominate after vaccination or infection. The value of non-neutralizing antibodies in preventing infection by opsonisation, antibody-directed cell-mediated cytotoxicity (ADCC) and activation of the complement (C’) cascade leading to lysis of virions or infected cells warrants further investigation as primary transmitting HIV-1 strains and plasma HIV-1 are sensitive to C’-mediated lysis.    

Methods:  In concert with our effort to identify and deliver HIV envelope immunogens that elicit a strong neutralizing antibody response in small animal models, we have developed a pseudoviral, enhanced green fluorescent protein (EGFP) reporter-based neutralizing antibody assay. A panel of pseudoviruses were generated by co-transfection of an env-deleted proviral DNA with the egfp gene fused into the nef frame and exchangeable viral envelope protein expression vectors. Pseudovirus infection of target cells generated EGFP that was quantified by FACS analysis.

Results:  We used pseudovirus to measure susceptibility to C’ and mimic plasma-derived virus for the detection of non-neutralising, yet virus-inactivating, antibodies. We incorporated a source of active human C’ to trigger C’-mediated virus neutralisation. We also inactivated pseudovirion-associated complement inhibitory proteins (CIP) by various means, such as phospholipase C treatment to remove the CIP, CD55, and CD59. Upon disablement of the C’ inhibitory activity of CD55 by antibody, we observed virus neutralization, whereas an alternate CD55-specific antibody that does not inhibit CD55 activity, unexpectedly, led to an enhancement of virus infection. We also observed enhancement of infection contributed by non-C’ serum factor(s).

Conclusions:  Assays using pseudovirus-reporters demonstrated both beneficial and undesirable effects of non-neutralizing antibodies, such as enhancement of infection. Our data highlights the importance of the alternate protective functions of antibodies, especially in light of the extreme difficulty in eliciting useful neutralising antibody responses with current experimental vaccines. We are currently investigating the evolution of these antibody activities during acute HIV-1 infection.