Abstract
HIV-1 is spread by cell-free virions and by cell-cell viral transfer. We asked whether the structure and function of a broad neutralizing antibody (bNAb) epitope, the membrane-proximal ectodomain region (MPER) of the viral gp41 transmembrane glycoprotein, differ in cell-free and cell-cell-transmitted viruses and whether this difference could be related to Ab neutralization sensitivity. Whereas cell-free viruses bearing W666A and I675A substitutions in the MPER lacked infectivity, cell-associated mutant viruses were able to initiate robust spreading infection. Infectivity was restored to cell-free viruses by additional substitutions in the cytoplasmic tail (CT) of gp41 known to disrupt interactions with the viral matrix protein. We observed contrasting effects on cell-free virus infectivity when W666A was introduced to two transmitted/founder isolates, but both mutants could still mediate cell-cell spread. Domain swapping indicated that the disparate W666A phenotypes of the cell-free transmitted/founder viruses are controlled by sequences in variable regions 1, 2, and 4 of gp120. The sequential passaging of an MPER mutant (W672A) in peripheral blood mononuclear cells enabled selection of viral revertants with loss-of-glycan suppressor mutations in variable region 1, suggesting a functional interaction between variable region 1 and the MPER. An MPER-directed bNAb neutralized cell-free virus but not cell-cell viral spread. Our results suggest that the MPER of cell-cell-transmitted virions has a malleable structure that tolerates mutagenic disruption but is not accessible to bNAbs. In cell-free virions, interactions mediated by the CT impose an alternative MPER structure that is less tolerant of mutagenic alteration and is efficiently targeted by bNAbs.
Highlights
HIV-1 is spread by cell-free virions and by cell– cell viral transfer
To examine the role of the membraneproximal ectodomain region (MPER) in cell-free infection and cell– cell viral spread, an assay was developed to compare the spread of two well-characterized HIV-1AD8 MPER mutants, W666A and I675A (Fig. 1, A and B; Fig. S1A) [52], by cell-free and cell-to-cell modes
Mutations in the MPER that inhibited cell-free virus infectivity were tolerated in virions transmitted via the cell-to-cell route and in cell– cell fusion mediated by the envelope glycoprotein (Env) glycoproteins expressed at the cell surface
Summary
Envelope glycoprotein; MPER, membrane proximal ectodomain region; PBMC, peripheral blood mononuclear cell; bNAb, broad neutralizing antibody; CT, cytoplasmic tail; MA, matrix protein; T/F, transmitted/founder; VS, virological synapse; MDM, monocytederived macrophage; RT, reverse transcriptase; WLKD, W596L/K601D; RLU, relative light units; LLP, lentiviral lytic helical peptide; PHA, phytohemagglutinin; SC45, SC45.4B5.2631; PRB958, PRB958_06.TB1.4305; PDB, Protein Data Bank. VS-mediated HIV-1 transmission between CD4ϩ T cells and between HIV-1–infected MDMs and uninfected CD4ϩ T cells is less sensitive to neutralization by bNAbs, when compared with cell-free virus infections, indicating that this mode of spread may represent an obstacle to successful vaccine development and neutralizing antibody therapy [19, 33,34,35,36] These differences between cell-to-cell and cell-free virus transmission can be explained in part by a higher local multiplicity of infection at the VS, it is plausible that cell-free and cell-associated viruses possess structural differences that confer distinct functional advantages to the two viral forms. Our data indicate that the MPER adopts distinct structural/functional forms during cell-free and cell-to-cell viral spread, with the latter form being unavailable for interaction with bNAbs
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