Background:  Approximately 10 to 20% of AIDS patients have HIV-associated dementia (HAD) and related neurological disorders. Genetic and phenotypic characteristics of HIV that underlie neurotropism are unclear. Previous studies described HIV envelopes from brain that fuse with cells expressing low amounts of CD4 and CCR5, but whether Env with reduced CD4/CCR5 dependence are more frequent in brain compared to other tissues is unknown.

Methods:  Full-length env genes were amplified by limiting dilution PCR from autopsy brain and lymphoid tissue from 4 patients with HAD. Functional Env clones were identified by a rapid screen using GFP pseudoviruses. Full-length functional Env sequences were compared within a patient and across the entire data set using VESPA to identify any brain-specific signature sequences. Fusion and infectivity assays were performed with cells expressing different amounts of CD4 and CCR5.

Results:  From 4 AIDS patients, 10 tissue samples yielded 55 functional Env clones (n = 35 brain and 20 lymphoid). Phylogenetic analysis demonstrated distinct compartmentalization of brain vs lymphoid sequences within each patient. Certain amino acids were more frequent in brain compared with lymphoid sequences including an asparagine at position 283 in the C2 region, which can influence CD4 dependence, and a proline at position 308 in the V3 loop, a position that has been shown to influence CCR5 usage and susceptibility to CCR5 small molecule inhibitors. The H308P change occurred with a frequency of 0.771 in brain Env compared with 0.19 in Env from lymphoid tissue. The frequency and location of N-linked glycosylation sites within gp160 or specific variable loops did not differ significantly between brain and lymphoid sequences. Fusion and infection assays using cells expressing different amounts of CD4 and CCR5 demonstrated that Env used low amounts of CD4 and CCR5 were more frequent in brain (n = 30/35) compared to lymphoid tissue (n = 4/20).

Conclusions:  These results suggest that HIV in the brain may have adapted to use low levels of CD4 and CCR5 for fusion and virus entry to enable efficient infection of macrophages and microglia. These findings have relevance for understanding mechanisms of HIV neurotropism as well as the development of coreceptor inhibitors.

Keywords: Neurotropism; Envelope; CD4