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The HIV Env N283 Genetic Variant Is Associated with Brain Infection and Dementia
Rebecca Dunfee*1,2, E Thomas1,2, P Gorry1,2, J Taylor3, K Kunstman3, J Bell4, S Wolinsky3, and D Gabuzda1,2
1Dana-Farber Cancer Inst, Boston, MA, US; 2Harvard Med Sch, Boston, MA, US; 3Northwestern Univ Med Sch, Chicago, IL, US; and 4Univ of Edinburgh, Scotland
Background: Macrophages and microglia,
which express low levels of CD4 and CCR5, are targets for HIV infection in the
central nervous system. Macrophage-tropic strains with reduced dependence on
CD4 are frequent in the brain of patients with HIV-associated dementia (HAD),
but mechanisms underlying the neurotropism of these
strains are unknown.
Methods: Env cloned from primary brain isolates UK1br
and UK7br were analyzed to identify amino acid changes associated with the
ability to use low CD4. Mutagenesis was used to investigate the role of
specific amino acids in reduced CD4 dependence and virus replication in monocyte-derived macrophages (MDM) or microglia.
Recombinant gp120 were analyzed for CD4 binding affinity by BIACORE.
Results: UK1br and UK7br Env
with reduced CD4 dependence had asparagines, rather than the consensus threonine, at position 283 in the C2 region of gp120. N283T
mutations reduced the ability of these Env to use low
CD4 for fusion and entry, whereas reciprocal mutations in other Env had opposite effects. Viruses with the parental or
N283T mutant UK1br Env replicated similarly in
peripheral blood mononuclear cells (PBMC). In contrast, the N283T mutant virus
replicated at lower levels and induced fewer syncytia
in MDM and microglia compared to the parental virus.
The crystal structure of the gp120-CD4 complex shows that T283 contacts CD4 at
position Q40 via a hydrogen bond. Energy minimization of the crystal structure
increases the distance between Env and CD4, reducing
the potential for T283, but not N283, to form this hydrogen bond. In entry
assays, UK1br N283T had reduced ability to use low CD4 compared to the parental
virus, but showed no difference when target cells expressed a Q40A mutant CD4.
BIACORE analysis of soluble gp120 demonstrated that UK1br N283T Env had a significantly higher Kd for sCD4 compared to the parental Env. In 5 published studies with matched brain- and
lymphoid-derived Env from 31 patients, N283 was
detected at a 3.5-fold higher frequency in brain-derived Env
(0.36; n = 351) than lymphoid-derived
Env (0.10; n
= 305). Furthermore, N283 occurred at a 5-fold higher frequency in
brain-derived Env from HAD patients (0.49; n = 232) compared with nondemented patients (0.09; n = 119).
Conclusions: The HIV Env N283 genetic variant is
associated with brain infection and HAD. N283 enhances macrophage and microglia tropism by increasing gp120-CD4 affinity through
contact with Q40 of CD4. These results lead to a better understanding of
mechanisms that underlie HIV neurotropism.
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