Relationships between CD4 Independence, Neutralization Sensitivity, and Exposure of a CD4-Induced Epitope in a Human Immunodeficiency Virus Type 1 Envelope Protein

Abstract

A CD4-independent version of the X4 human immunodeficiency virus type 1 (HIV-1) HXBc2 envelope (Env) protein, termed 8x, mediates infection of CD4-negative, CXCR4-positive cells, binds directly to CXCR4 in the absence of CD4 due to constitutive exposure of a conserved coreceptor binding site in the gp120 subunit, and is more sensitive to antibody-mediated neutralization. To study the relationships between CD4 independence, neutralization sensitivity, and exposure of CD4-induced epitopes associated with the coreceptor binding site, we generated a large panel of Env mutants and chimeras between 8x and its CD4-dependent parent, HXBc2. We found that a frameshift mutation just proximal to the gp41 cytoplasmic domain in 8x Env was necessary but not sufficient for CD4 independence and led to increased exposure of the coreceptor binding site. In the presence of this altered cytoplasmic domain, single amino acid changes in either the 8x V3 (V320I) or V4/C4 (N386K) regions imparted CD4 independence, with other changes playing a modulatory role. The N386K mutation resulted in loss of an N-linked glycosylation site, but additional mutagenesis showed that it was the presence of a lysine rather than loss of the glycosylation site that contributed to CD4 independence. However, loss of the glycosylation site alone was sufficient to render Env neutralization sensitive, providing additional evidence that carbohydrate structures shield important neutralization determinants. Exposure of the CD4-induced epitope recognized by monoclonal antibody 17b and which overlaps the coreceptor binding site was highly sensitive to an R298K mutation at the base of the V3 loop and was often but not always associated with CD4 independence. Finally, while not all neutralization-sensitive Envs were CD4 independent, all CD4-independent Envs exhibited enhanced sensitivity to neutralization by HIV-1-positive human sera, indicating that the humoral immune response can exert strong selective pressure against the CD4-independent phenotype in vivo. Whether this can be used to advantage in designing more effective immunogens remains to be seen.

Cell Tropism of Simian Immunodeficiency Virus in Culture Is Not Predictive of in Vivo Tropism or Pathogenesis

“…19 An additional consequence of the amino acid changes that confer macrophage tropism is that the virus is rendered much more susceptible to antibody neutralization. 23,25,32,47,48 This is reflected in the longer survival and lower viral set points in animals infected with SIVmac239/316 32 as compared to SIVmac239 but similar peak viral loads. This suggests that macrophage tropic viruses such as SIVmac239/316 are relatively easy to control in the absence of a more virulent virus such as SIVmac239 and that viral replication may be insufficient to cause severe giant cell disease.…”

Aiming to induce broadly reactive neutralizing antibody responses with HIV-1 vaccine candidates

“…Therefore, in order to be neutralizing, these antibodies must bind to susceptible virus prior to virion engagement of the cell surface. Only a small number of TCLA CXCR4 HIV-1 strains and CD4-independent strains on which the coreceptor binding domain of gp120 is constitutively exposed are susceptible to CD4i antibodies [111][112][113].…”

“…Therefore, in order to be neutralizing, these antibodies must bind to susceptible virus prior to virion engagement of the cell surface. Only a small number of TCLA CXCR4 HIV-1 strains and CD4-independent strains on which the coreceptor binding domain of gp120 is constitutively exposed are susceptible to CD4i antibodies [111][112][113].There are several epitopes of gp41 that are conserved targets for broadly neutralizing antigp41 antibodies. These sites are located in the membrane proximal external region (MPER) of gp41 (FIGURE 3), and are defined by the whole Ig mAbs 2F5 and 4E10 and the Fab phage display selected molecule (Z13) [114,115].…”

An amphipathic sequence in the cytoplasmic tail of HIV-1 Env alters cell tropism and modulates viral receptor specificity

“…The results are shown with the standard deviation (n = 3). 2000); (ii) changes in phenotypic characteristics of the SU are affected by CT length (Vzorov et al, 2005); (iii) the length and sequence of the CT domain can impact viral biological properties including trimer stability, syncytia formation activity and infectivity (Bonavia et al, 2005;Edwards et al, 2001;Taylor et al, 2008;Vzorov and Compans, 2011;Wyss et al, 2005). In this study we show that the tropism of FL 92UG046 Env changes from a CD4-high phenotype to a CD8-tropic phenotype when a truncated form of Env is used to initiate an infection.…”

“…HIV-1 (KB-1) carrying an 18aa Env CT was isolated from a Japanese male hemophiliac (Shimizu et al, 1992). HIV-1 isolates that show CD4 independence bear extensive truncations in their gp41 CTs, located prior to the membrane-interactive regions (Edwards et al, 2001(Edwards et al, , 2002. SIV with a truncated Env-CT17 exhibits a defect in early steps of replication in non-dividing cells, and demonstrates an extended range of cells susceptible to infection (Vzorov et al, 2007).…”