Contribution of Intrinsic Reactivity of the HIV-1 Envelope Glycoproteins to CD4-Independent Infection and Global Inhibitor Sensitivity

Abstract

Human immunodeficiency virus (HIV-1) enters cells following sequential activation of the high-potential-energy viral envelope glycoprotein trimer by target cell CD4 and coreceptor. HIV-1 variants differ in their requirements for CD4; viruses that can infect coreceptor-expressing cells that lack CD4 have been generated in the laboratory. These CD4-independent HIV-1 variants are sensitive to neutralization by multiple antibodies that recognize different envelope glycoprotein epitopes. The mechanisms underlying CD4 independence, global sensitivity to neutralization and the association between them are still unclear. By studying HIV-1 variants that differ in requirements for CD4, we investigated the contribution of CD4 binding to virus entry. CD4 engagement exposes the coreceptor-binding site and increases the “intrinsic reactivity” of the envelope glycoproteins; intrinsic reactivity describes the propensity of the envelope glycoproteins to negotiate transitions to lower-energy states upon stimulation. Coreceptor-binding site exposure and increased intrinsic reactivity promote formation/exposure of the HR1 coiled coil on the gp41 transmembrane glycoprotein and allow virus entry upon coreceptor binding. Intrinsic reactivity also dictates the global sensitivity of HIV-1 to perturbations such as exposure to cold and the binding of antibodies and small molecules. Accordingly, CD4 independence of HIV-1 was accompanied by increased susceptibility to inactivation by these factors. We investigated the role of intrinsic reactivity in determining the sensitivity of primary HIV-1 isolates to inhibition. Relative to the more common neutralization-resistant (“Tier 2-like”) viruses, globally sensitive (“Tier 1”) viruses exhibited increased intrinsic reactivity, i.e., were inactivated more efficiently by cold exposure or by a given level of antibody binding to the envelope glycoprotein trimer. Virus sensitivity to neutralization was dictated both by the efficiency of inhibitor/antibody binding to the envelope glycoprotein trimer and by envelope glycoprotein reactivity to the inhibitor/antibody binding event. Quantitative differences in intrinsic reactivity contribute to HIV-1 strain variability in global susceptibility to neutralization and explain the long-observed relationship between increased inhibitor sensitivity and decreased entry requirements for target cell CD4.

Phenotypic Correlates of HIV-1 Macrophage Tropism

“…Differences in Env protein stability affect the conformational changes required for entry (104), and differences in stability can be probed by sensitivity to inactivation at different temperatures. Env proteins display variation in sensitivity to heat (75,104,105) and cold (105)(106)(107). Heat lability is a feature of the open conformation of tissue culture-adapted viruses but not of comparatively stable primary isolates (75,104).…”

Trimeric, Coiled-coil Extension on Peptide Fusion Inhibitor of HIV-1 Influences Selection of Resistance Pathways

“…Perhaps the gp120 mutations that are associated with each pathway modulate gp120 interactions with receptors so that they are coordinated with changes in gp41 refolding due to the coiled-coil and bundle-enhancing resistance mutations. In this regard, the mutations identified in these resistance studies may be altering the threshold of Env for triggering or enhancing its intrinsic reactivity (45) in response to receptor activation.…”

CD4 mimetics sensitize HIV-1-infected cells to ADCC

“…To ensure that sensitization of HIV-1-infected cells by CD4 mimetics was also observed when using full-length clinically relevant primary HIV-1 isolates, we infected primary CD4 T cells with extensively characterized infectious molecular clones (IMCs) constructed from two transmitted/founder (T/F) and their corresponding 6-mo consensus sequences (36)(37)(38)(39). Primary viruses are known to exhibit low Env reactivity and, as such, have little or no intrinsic exposure of CD4i epitopes (40). JP-III-48 and DMJ-I-228 CD4 mimetics were able to significantly enhance recognition of cells infected with the four primary viruses by HIV-1 + sera (Fig.…”