Determinants Flanking the CD4 Binding Loop Modulate Macrophage Tropism of Human Immunodeficiency Virus Type 1 R5 Envelopes

Program in Molecular Medicine and Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA.
Journal of Virology (Impact Factor: 4.44). 02/2009; 83(6):2575-83. DOI: 10.1128/JVI.02133-08
Source: PubMed


Human immunodeficiency virus type 1 R5 viruses vary extensively in phenotype. Thus, R5 envelopes (env) in the brain tissue of individuals with neurological complications are frequently highly macrophage-tropic. Macrophage tropism correlates with the capacity of the envelope to exploit low CD4 levels for infection. In addition, the presence of an asparagine at residue 283 within the CD4 binding site has been associated with brain-derived envelopes, increased env-CD4 affinity, and enhanced macrophage tropism. Here, we identify additional envelope determinants of R5 macrophage tropism. We compared highly macrophage-tropic (B33) and non-macrophage-tropic (LN40) envelopes from brain and lymph node specimens of one individual. We first examined the role of residue 283 in macrophage tropism. Introduction of N283 into LN40 (T283N) conferred efficient macrophage infectivity. In contrast, substitution of N283 for the more conserved threonine in B33 had little effect on macrophage infection. Thus, B33 carried determinants for macrophage tropism that were independent of N283. We prepared chimeric B33/LN40 envelopes and used site-directed mutagenesis to identify additional determinants. The determinants of macrophage tropism that were identified included residues on the CD4 binding loop flanks that were proximal to CD4 contact residues and residues in the V3 loop. The same residues affected sensitivity to CD4-immunoglobulin G inhibition, consistent with an altered env-CD4 affinity. We predict that these determinants alter exposure of CD4 contact residues. Moreover, the CD4 binding loop flanks are variable and may contribute to a general mechanism for protecting proximal CD4 contact residues from neutralizing antibodies. Our results have relevance for env-based vaccines that will need to expose critical CD4 contact residues to the immune system.

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    • "This dogma has been challenged by work demonstrating that R5 tropic envelopes are diverse in their ability to replicate in monocyte derived macrophages [8]. More specifically, envelope glycoproteins (Envs) from viruses isolated from central nervous system often confer efficient macrophage replication upon pseudotyping viruses as compared to Envs from blood or lymph nodes in patients with neurocognitive disease [9,10], and since then a diverse range of envelope determinants have been implicated in this phenotype, often associated with the CD4 binding site [11-13]. "
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    • "It appears that our CD4-independent variants exist constitutively in or spontaneously acquire the CD4-bound conformation, even in the absence of CD4 [9]. Therefore, loss of coreceptor use plasticity may be specific to Envs that have a pre-formed or spontaneously exposed coreceptor binding site [5-8] rather than Envs that scavenge low levels of CD4 on the cell surface [3,4,31-34]. Taken together, these data suggest that diverse mechanisms exist by which virus can expand its host range into CD4-low or negative cells, which may have different consequences for coreceptor interactions. "
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    • "However, unlike CD4 usage, no alteration in CCR5 usage of LT5 Envs was found suggesting that coreceptor binding sites exposure on gp120 post CD4 binding was similar but it substantially varied when gp120 was not engaged by CD4, evident from 17b MAb binding to Env trimers expressed on 293T cells. Our study also highlighted presence of a rare substitution in the V3 loop (I315F) in the LT5.J4b Env (but absent in other LT5 Envs) which was previously shown to modulate susceptibility of HIV-1 Envs to anti-V3 MAbs [42], [44]. Interestingly, the determinants in Env found to modulate the sensitivity of HIV-1 to IgG1b12 or sCD4 did not show any impact on susceptibility to the VRC01 MAb; a broad and a very potent CD4bs neutralizing MAb [16]. "
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