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- Open Access
Modulation of HIV-1 macrophage-tropism among R5 envelopes occurs before detection of neutralizing antibodies
© Richards et al; licensee BioMed Central Ltd. 2010
- Received: 16 March 2010
- Accepted: 27 May 2010
- Published: 27 May 2010
HIV-1 R5 viruses vary widely in their capacity to infect primary macrophages. R5 macrophage-tropism is associated with an increased envelope:CD4 affinity that partly results from an increased exposure of CD4 contact residues on gp120 and allows the use of low levels of CD4 for infection. The selective pressures in vivo that modulate R5 macrophage-tropism are not understood. It is possible that different R5 variants adapt for replication in either T-cells (high CD4) or in macrophages (low CD4). However, other selective pressures in vivo (e.g. neutralizing antibodies) may also impact R5 tropism. Here, we measured macrophage infectivity conferred by gp120 sequences amplified sequentially from subjects in London followed from the acute stage of infection. We report wide variation in the capacity of these envelopes to confer macrophage infection in the complete absence of both autologous and heterologous neutralizing antibodies. Our data show that the variation in macrophage tropism observed at early times cannot have been influenced by neutralizing antibodies.
- Macrophage Infectivity
- Primary Macrophage
- Viral Clone
- Gp120 Sequence
- Macrophage Tropism
HIV-1 R5 viruses that use CCR5 (R5) as a coreceptor are preferentially transmitted. Although such viruses are often termed macrophage-tropic or M-tropic , we and others have described a wide variation in their capacity to infect primary macrophages [2–7]. In particular, we showed that R5 envelopes amplified from brain tissue of subjects with neurological complications were frequently highly macrophage-tropic (mac-tropic), while many of those from immune tissue, blood, and semen infected macrophages inefficiently . The capacity of R5 envelopes to confer infection of macrophages correlated with their sensitivity to inhibitors that blocked envelope: CD4 interactions, but not with those targeting envelope:CCR5 interactions or gp41 conformational changes . These observations indicated that R5 mac-tropism was modulated by changes in the affinity of envelope for CD4. R5 mac-tropism also correlated with the capacity of envelopes to infect cells via low levels of CD4 [2, 3, 9, 10] consistent with infection of macrophages that express substantially lower amounts of CD4 compared to T-cells [11–13]. In agreement with these observations, we and others have identified gp120 determinants within or proximal to the CD4 binding site (CD4bs) that modulate R5 mac-tropism [11–16].
The selective pressures in vivo that drive changes in the mac-tropism of R5 envelopes are poorly understood. It is possible that the different R5 mac-tropic phenotypes result from adaptation for replication in T-cells (high CD4) or in macrophages (low CD4). However, R5 mac-tropism forms a spectrum rather than two separate phenotypes. Thus, other selective pressures in vivo need to be considered including neutralizing antibodies (nabs). We previously reported a trend where mac-tropic R5 envelopes were more sensitive to the CD4bs monoclonal antibody, b12, while Dunfee et al. reported a significant correlation for envelopes derived from brain and lymph node tissue . Thus, the presence of CD4bs antibodies in vivo may select for variants where the CD4bs is protected from neutralization. This possibility was supported by our identification of determinants on the flanks of the CD4 binding loop that modulate mac-tropism and affect b12 sensitivity [14, 18]. Such determinants presumably affect the exposure of proximal CD4 contact residues on the CD4 binding loop, which is likely the first contact for CD4 . The protection of these CD4 contact residues from antibodies may compromise the affinity of envelope for CD4 and in turn restrict tropism to cells expressing higher levels of CD4 (e.g. T-cells). In support of this hypothesis, Ryzhova et al. reported that the presence of nabs in the CSF correlated with the absence of M-tropic SIVs in rhesus macaques . The predominance of highly mac-tropic envelopes in brain tissue could also reflect adaptation in an immuno-privileged site where antibodies are excluded by the blood brain barrier and usually reach only low concentrations [21, 22]. However, brain macrophages and microglia are the predominant targets for HIV-1 in the brain, and the presence of highly mac-tropic variants there may simply reflect an adaptation for infection of these low CD4 cell types.
Here, we have investigated mac-tropism of gp120 sequences amplified sequentially from subjects in London followed from the acute stage of infection. We report wide variation in the capacity of 'early' envelopes to confer macrophage infection in the complete absence of nabs.
Subject Details: Viral Load, CD4 counts and envelope PCR
env PCR e
Envelope residues potentially involved in modulating macrophage-tropism
Days post onset of symptoms
Potential amino acid tropism determinants
No. amino acid changes in gp120
Amino acid changes
HIV-1 R5 envelopes that vary dramatically in their capacities to infect macrophages were detected early after infection and long before nabs develop. For the three subjects investigated here, envelopes that varied in macrophage infection were detected at 28, 17 and 12 days respectively after the onset of acute stage symptoms. However, we were unable to detect autologous or heterologous nabs until many weeks or months later. The identification of macrophage-tropic variants early in disease seems at variance with the study by Salazar-Gonzalez et al.  who reported weak macrophage-tropism among 'so called' founder strains. However, such strains are believed to represent transmitted viruses rather than those from acute phase plasma studied here. Our study could be consistent with Isaacman-Beck's who showed a range of macrophage infectivity among the clade C envelopes, amplified from near acute phase plasma, although these authors did not investigate the envelopes' temporal relationship with nabs.
Our study shows that the selective pressures that confer variation in R5 mac-tropism in the early stages of infection do not involve nabs and thus remain unclear. However, there remains the possibility that ADCC and or complement mediated antibody functions play a role. In addition, when nabs do arise, they are likely to act as a selective pressure that impacts on tropism. This could be particularly true for heterologous nabs that target the CD4bs thus favouring non-mac-tropic variants that protect this site.
In summary, we show that variation in macrophage tropism observed at early times in HIV-1 infection has not been influenced by neutralizing antibodies.
Our work was supported by NIH grants R01 HD049273 and P01 AI082274. We also wish to acknowledge the University of Massachusetts Center for AIDS Research (CFAR), the NIH AIDS Research and Reference Reagent Program and the Centre for AIDS Reagents, NIBSC, UK, for services and reagents. We thank the Ian Williams and his colleagues at the UCL Centre for Sexual Health & HIV Research (London) along with the Jenner cohort of seroconverters for the HIV-1 molecular clones used here.
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