Antibody Function during Persistent HIV Infection: the Effect of Virus Inoculum and Fc-Fc Receptor Interactions

Donald Forthal, Gary Landucci, Tran Phan. University of California, Irvine College of Medicine (USA).

Background: If antibody plays a role in setting or maintaining the virological set point during persistent HIV infection, it must do so in the presence of large amounts of virus. We therefore explored the mechanisms by which antibodies neutralize high inocula of HIV. Material and Methods: A primary R5 HIV-1,  at inocula ranging from 100 to 4,500 TCID50, was incubated with pooled IgG (0.5 mg/ml) from HIV-infected patients or controls and added to target cells consisting of either 5 x 104 PHA-stimulated CD4+ lymphocytes or PHA-stimulated, non-adherent PBMCs (containing about 5 x 104 CD4+ cells) from the same donor. The inoculum and antibody were removed 4 days post-infection (p.i.), cultures were washed, and supernatant p24 was sampled on day 7 p.i. Results: At 100 TCID50, virus inhibition by antibody was similar whether target cells for virus growth were purified CD4+ cells or PBMCs. However, at higher inocula, antibody was better able to inhibit virus on PBMCs than on CD4+ cells. The difference was greatest at the highest inoculum (4,500 TCID50),  where antibody inhibited replication by an average of 91% on PBMCs and by only 33% on CD4+ cells (p = 0.0003). The enhanced inhibition on PBMCs was abrogated when F(ab)’ 2 was used. Furthermore, adding NK cells, which express Fc receptors (FcRs), to CD4+ cells completely restored the ability of whole IgG to inhibit virus at a level comparable to that observed with PBMCs.  At 7 days p.i. on PBMCs, supernatant MIP-1α levels were on average twice as high with patient IgG than with control IgG (p = 0.05);  on the other hand, MIP-1α levels were very low in supernatants from CD4 cells and did not differ between patient and control IgG (p = 0.8). Conclusion: 1) For high inocula of virus, antibody inhibitory activity is greatly enhanced with PBMC target cells compared with purified CD4+ target cells; 2) since the addition of NK cells to CD4+ cells restores activity, and the removal of Fc from antibody reduces activity on PBMCs, Fc-FcR interactions are likely required for enhanced inhibition and; 3) MIP-1α, triggered by crosslinking of FcR on NK cells present in the PBMCs, may contribute to the enhanced inhibition. These results imply that antibodies inhibit HIV in vivo in an FcR-dependent manner when large quantities of virus are present.  Since the effector functions of FcR-bearing cells are depressed during HIV infection, therapy aimed at augmenting such functions may improve antibody control of persistent viremia.