Background:  Epidemiological data prove the beneficial effect of GB virus C (GBV-C) co-infection in HIV patients. Hereby not only GBV-C viremia, but also GBV-C antibodies directed against the envelope protein E2, decelerates progression of HIV to AIDS. Here we investigated the underlying mechanism in more detail.

Methods:  Infection assays as well as immunoprecipitations using HIV-1 and -2, SIV, MLV, Ad5, Ad12, and YFV isolates were performed to determine the neutralization capacity of purified human and murine anti-E2 antibodies. A membrane lipid screening was performed to narrow down the putative epitope of the HIV neutralizing anti-E2 antibodies.

Results:  Whereas no significant inhibitory effect on MLV, YFV, Ad5, and Ad12 could be observed, anti-E2 antibodies neutralize HIV-1 and -2 as well as SIV. Performing 2 different neutralization assays using clinical HIV isolates on PBMC and HIV-derived pseudotypes on TZMbl cells, the IC50 ranged between 0.01 and 6 µg/mL and were in the same range or even lower than for the HIV-1 neutralizing antibodies, such as 2F5 and 2G12. Hereby the HIV-inhibitory anti-E2 effect could be blocked by preincubation with recombinant GBV-C E2 protein, but not with HIV gp120. Furthermore, anti-E2 antibodies precipitate HIV-particles independent of the presence of viral envelope glycoproteins suggesting that an E2-like epitope is present on the surface of HIV particles that is not part of the gp120/gp41. Although FACS analysis reveal cell binding by HIV neutralizing anti-E2 antibodies, a protein-protein Blast search indicated no significant homologies with HIV-1 or cellular proteins. Therefore a membrane lipid screening was performed. Exclusively HIV neutralizing anti-E2 antibodies bind to Phospholipids like PI(4)P. Furthermore, preincubation of anti-E2 antibodies with PI(4)P, as well as preincubation of HIV with anti-PI(4)P antibodies, blocks the inhibitory effect. Additional preincubation of HIV with anti-PI(4)P antibodies leads also to a decrease in HIV precipitation by anti-E2 antibodies.

Conclusions:  Here, we demonstrate that anti-E2 antibodies neutralize HIV-1 and 2, as well as SIV by targeting Phospholipids within the retroviral lipid bilayer. In a comparable manner, these lipid structures are recognized by the broad HIV neutralizing antibodies (BNAb) 2F5, 4E10, and Z13e1. Therefore, we demonstrate for the first time that such lipid targeted BNAb can be elicited by a heterologous viral glycoprotein, outlining new promising HIV vaccination strategies.