Background:  HIV infects central nervous system macrophages and microglia, causing HIV-associated dementia (HAD) and related neurological disorders in approximately 10 to 20% of AIDS patients. Genetic and phenotypic characteristics of HIV that underlie neurotropism are unclear.

Methods:  Full-length HIV env genes were amplified from autopsy brain and lymphoid tissue from 4 patients with HAD. Sequence signatures were identified using VESPA. Point mutants of amino acid 308 were made in brain Env from 2 patients and analyzed in fusion and single cycle infection assays. Blocking experiments were performed with compounds and antibodies targeting CD4, CCR5 or HIV gp120.

Results:  We amplified 55 functional HIV Env from brain and lymphoid tissue from 4 patients. Env that mediated fusion or infection with cells expressing low CD4 and CCR5 were more frequent in brain (n = 30 of 35) compared with lymphoid tissue (n = 4 of 20). Sequence alignments and VESPA-identified residue 308 in the V3 loop of gp120 as a brain signature position. This position was identified previously as a cerebrospinal fluid (CSF) signature and implicated in escape from small molecule CCR5 inhibitors. The clade B consensus at this position is a histidine. However, a proline at position 308 occurred with a frequency of 0.77 in Env amplified from brain compared with 0.2 in Env from lymphoid tissue. In published sets of matched brain and blood or lymphoid sequences from patients that had Pro at position 308 in any Env sequence (n = 12), Pro occurred at a frequency of 0.5 in brain (n = 132) and 0.38 in blood/lymphoid (n = 118) Env. In 23,000 clade B V3 sequences in the Los Alamos database, His is found at position 308 in 53% and Pro in 14%. Mutagenesis was used to generate point mutants at position 308 in brain Env from 2 patients. No differences were observed in the ability of Env containing H vs P at position 308 to use low CD4 and CCR5 for fusion and infection. Additionally, no differences were found in sensitivity to inhibition by sCD4, RANTES, TAK779, or the monoclonal antibodies 2D7, 17b, and b12. Experiments were performed to investigate the influence of p308 on macrophage and microglia tropism.

Conclusions:  Analysis of HIV Env from brain and lymphoid tissue from 4 patients with HAD identified P308 as a brain signature, which may reflect genetic adaptation to replication in macrophages/microglia or reduced immune selection pressures within the central nervous system.