Background
HIV-1 replicates in different body compartments and sub-compartments characterized by distinct compositions of small solutes and proteins. It is often assumed that virus proteins maintain the same conformation, function and sensitivity to inhibitors in different molecular environments, in vivo and in vitro. The protein albumin shows remarkable variability in concentration between the blood and brain compartments – 35-50 mg/mL in blood versus 0.2 mg/mL in cerebrospinal fluid (CSF) and interstitial fluid of the brain. Given such differences, and the promiscuous interactions of albumin with other molecules, we examined the effects of this protein on conformation of Env and HIV-1 sensitivity to antibodies.
Methods
Pseudoviruses and replicative HIV-1 that contain the Env of strain AD8 were produced and tested for their sensitivity to antibodies in whole CSF and serum from HIV-negative donors. To determine the isolated effects of albumin, viruses were also tested in media containing CSF- or serum-like levels of this protein, and in albumin-supplemented CSF. Antibodies that target the major neutralizing and non-neutralizing epitopes of gp120 and gp41 were tested. Conformation of Env was determined by binding of the antibodies to the membrane-bound form of Env expressed on HOS cells. Structural stability of Env was analyzed in a ligand-independent manner by virus sensitivity to cold inactivation in the different fluids.
Results
The interaction between HIV-1 and specific antibodies was markedly affected by the nature of the fluid. In low-albumin fluids, including CSF, Env exhibited a unique open-at-the-base conformation, with increased exposure of epitopes at the gp120-gp41 interface and membrane-proximal external region. Epitopes in the gp120 ectodomain were unaffected by composition of the fluids. Neutralization assays of HIV-1 in the different synthetic and whole biological fluids confirmed that albumin reduces exposure of epitopes at the trimer base. Interestingly, sensitivity of HIV-1 to cold inactivation was increased in the high-albumin fluids, indicating a more closed but structurally unstable form of this protein.
Conclusions
These studies suggest that Env assumes unique conformations in different body compartments. Such changes result in distinct exposure profiles of epitopes that are considered as targets for therapeutics and vaccine development. These understandings may contribute to tailoring therapeutics to the conformational properties of HIV-1 proteins in each compartment.