Evolution of the human immunodeficiency virus type 2 envelope in the first years of infection is associated with the dynamics of the neutralizing antibody response
© Rocha et al.; licensee BioMed Central Ltd. 2013
Received: 2 April 2013
Accepted: 15 September 2013
Published: 24 October 2013
Differently from HIV-1, HIV-2 disease progression usually takes decades without antiretroviral therapy and the majority of HIV-2 infected individuals survive as elite controllers with normal CD4+ T cell counts and low or undetectable plasma viral load. Neutralizing antibodies (Nabs) are thought to play a central role in HIV-2 evolution and pathogenesis. However, the dynamic of the Nab response and resulting HIV-2 escape during acute infection and their impact in HIV-2 evolution and disease progression remain largely unknown. Our objective was to characterize the Nab response and the molecular and phenotypic evolution of HIV-2 in association with Nab escape in the first years of infection in two children infected at birth.
CD4+ T cells decreased from about 50% to below 30% in both children in the first five years of infection and the infecting R5 viruses were replaced by X4 viruses within the same period. With antiretroviral therapy, viral load in child 1 decreased to undetectable levels and CD4+ T cells recovered to normal levels, which have been sustained at least until the age of 12. In contrast, viral load increased in child 2 and she progressed to AIDS and death at age 9. Beginning in the first year of life, child 1 raised high titers of antibodies that neutralized primary R5 isolates more effectively than X4 isolates, both autologous and heterologous. Child 2 raised a weak X4-specific Nab response that decreased sharply as disease progressed. Rate of evolution, nucleotide and amino acid diversity, and positive selection, were significantly higher in the envelope of child 1 compared to child 2. Rates of R5-to-X4 tropism switch, of V1 and V3 sequence diversification, and of convergence of V3 to a β-hairpin structure were related with rate of escape from the neutralizing antibodies.
Our data suggests that the molecular and phenotypic evolution of the human immunodeficiency virus type 2 envelope are related with the dynamics of the neutralizing antibody response providing further support for a model in which Nabs play an important role in HIV-2 pathogenesis.
Infection with human immunodeficiency virus type 2 (HIV-2) affects 1-2 million individuals mostly living in West Africa, India and Europe [1, 2]. Eight different HIV-2 groups named A through H have been reported but only viruses from groups A and B are known to cause human epidemics [3, 4]. Among those, viruses from group A are responsible for the vast majority of HIV-2 infections worldwide.
Even though HIV-1 and HIV-2 are closely related viruses and share a high degree of similarity, infections by these viruses lead to very different immunological and clinical outcomes. HIV-2 infection eventually leads to CD4 depletion, AIDS and death [5–7]. However, differently from HIV-1, HIV-2 disease progression usually takes decades without antiretroviral therapy and the majority of HIV-2 infected individuals survive as elite controllers with normal CD4+ T cell counts and low or undetectable plasma viral load [8–16]. Understanding of the factors involved in the effective control of viral replication and disease progression in HIV-2 infected individuals might prove crucial to devise the best strategy to prevent and treat HIV-1.
Enhanced immune control could explain the mild outcome of most HIV-2 infections. Unlike HIV-1 infected patients, most HIV-2 patients in chronic stage produce potent and broad neutralizing antibodies [17–21]. Recent evidence has shown that the viruses isolated from HIV-2 infected patients with advanced disease are characterized by increased resistance to entry inhibitors, including the CCR5-antagonist maraviroc  and neutralizing antibodies , and by a remarkably high evolutionary rate [24, 25]. These results suggest that neutralizing antibodies play a central role in HIV-2 evolution and pathogenesis. However, in contrast to HIV-1, still nothing is known about the neutralizing antibody response and the molecular and phenotypic features of HIV-2 in acute/early infection because HIV-2 patients are usually diagnosed many years after seroconversion.
Most neutralizing epitopes in the HIV-2 envelope glycoprotein complex are located in the surface gp125 glycoprotein. They have been identified in V1, V2, V3, V4 and C5 regions, and in the CD4-binding site [19, 21, 23, 26–29]. These epitopes are well exposed in the envelope complex of CCR5-using isolates that are usually highly sensitive to antibody neutralization [21, 27]. However, X4 isolates that emerge in late stage infection in some HIV-2 patients when C2V3C3-specific neutralizing antibodies wane are highly resistant to antibody neutralization . The V3 loop sequence, size and conformation of the X4 isolates are markedly different from those of R5-neutralization sensitive isolates supporting a direct role of this region in escape from neutralization and a direct role of the neutralizing antibodies in shaping the evolution of V3 in progressive HIV-2 infection. The neutralizing domains expressed in the envelope glycoproteins in acute/early infection and the role of the neutralizing antibodies and neutralization escape in shaping the evolution of the HIV-2 envelope in this period remains to be determined.
Perinatal transmission of HIV-2 is a rare event that in Europe has only been documented in Portugal [30–34] and France . Vertical transmission cases constitute a unique opportunity to study the phenotypic and molecular evolution of HIV-2 Env in acute and early infection as well as the role of Nabs in this process. Our objective was to characterize the evolution of the Nab response in two children infected with HIV-2 at birth in association with the molecular and phenotypic evolution of the virus. We show that broad and potent Nabs can be elicited very early after infection and that HIV-2 Env evolves at a very high rate in the first years of infection, this rate being directly associated to the potency of the Nab response. R5-to-X4 tropism change, increased diversity in V1 and V3, and selected changes in V3 conformation were associated with escape from antibody neutralization. The data suggests that the rapid molecular and phenotypic evolution of the HIV-2 envelope in the first years of infection is related with the selective pressure imposed by the neutralizing antibodies.
Clinical and virological progression is very fast in the first years of infection
Child 1 infection was diagnosed with HIV-2 infection by PCR and virus isolation in the first month of life in 1998. To confirm the vertical transmission event and characterize the initial infecting virus population, 8 clonal full-length env gene sequences were obtained from samples collected in 1998, 2000 and 2003 (in total 24 env sequences) and from his mother (mother 1-PTHCC20) in 2000 and 2003 (16 env gene sequences). We were unable to obtain 1998 samples from the mother.
Child 2 infection was diagnosed in 1992 at day 39 after birth by PCR and virus isolation, and vertical transmission was confirmed by phylogenetic analysis of partial env sequences from the mother and the child [31, 32]. Eight new clonal full-length env sequences were obtained from samples collected in 1992, 1997 and 2001 (in total 24 env sequences).
Virological and immunological characterization of the patients
Viral load (RNA copies/ml plasma)
Number of CD4+T cells/μl (%)
d4T + 3TC + LPV/RTV
d4T + 3TC + LPV/RTV
3TC + ABC + LPV/RTV
AZT + 3TC
Child 2 was born with encephalopathy (CDC clinical stage C1) but with normal CD4+ T cell percentage (52%) and undetectable viral load [31, 32] (Table 1). The initial infecting virus was CCR5-tropic, as determined by our V3 loop sequence analysis, but induced syncytia formation in peripheral blood mononuclear cells [31, 32].
At age 5, CD4 percentage decreased to 25% and the virus changed to CXCR4-tropic, as determined by V3 loop sequence analysis. AZT therapy (1992 up to 1997) and AZT + 3TC therapy (in 2001) did not prevent increase in viral load and further CD4+ T cell decline and the child died of AIDS at age 9.
Potent neutralizing antibodies are produced since the first year of infection especially against R5 isolates
In child 2 we could only analyse the evolution of Nab response against the autologous X4-isolate (SC01) from age 5 onwards. Comparing Nab response at age 5 in both patients (the only age-matched data point), we found that it was significantly weaker in child 2 than in child 1 [median (range) of reciprocal log10 IC50 neutralization titer of child 2 against SC01 was 3.5 (3.4-3.6), and those of child 1 against autologous CT00 and CT03 isolates were 5.3 (5.0-5.5) and 4.2 (4.0-4.4), respectively, P < 0.001] (Figure 2). Moreover, in contrast to child 1, Nab titer decreased steadily with infection time as viral load increased and disease progressed to AIDS and death at age 9 (Figure 2 and Table 1). Considering all time points together, average Nab titers were lower than those of child 1 against the age-matched X4 isolate CT03 [median (range) of reciprocal log10 IC50 neutralization titer against isolates SC01 and CT03 were 3.3 (3.0-3.5) and 4.1 (3.2-4.4), respectively, P = 0.057] and against the R5 isolate CT00 [median (range) of reciprocal log10 IC50 neutralization titer against isolates SC01 and CT00 were 3.3 (3.0-3.5) and 4.6 (3.7-5.4), respectively, P = 0.0106].
HIV-negative plasmas failed to neutralize HIV-2 strains and HIV-2 plasmas failed to neutralize HIV-1SG3.1 or viruses pseudotyped with VSV envelope indicating the absence of nonspecific inhibitory activities in these samples.
Overall, the results obtained with child 1 demonstrate that potent neutralizing antibodies (autologous and heterologous) can be elicited very rapidly after HIV-2 vertical infection. Nabs are highly effective against the transmitted R5 isolates but seem to rapidly select for X4 isolates that escape neutralization. In the absence of effective antiretroviral therapy, as was the case of child 2, increased replication of the Nab-resistant X4 isolates likely contributed to rapid CD4+ T cell depletion and progression to AIDS.
HIV-2 Env evolution in acute/early and late infection
Nucleotide diversity and divergence rates in the env gene and C2V3C3 region
Evolutionary ratec(95% HPD)
0.0141 (0.0075, 0.0211)
0.0142 (0.0082, 0.0208)
0.0073 (0.0036, 0.0115)
0.0105 (0.0053, 0.0174)
The evolutionary rate of env was significantly higher in child 1 than in child 2 (0.0141 vs 0.0073 substitutions/site/year, posterior probability (PP) value <5%) (Table 2). When focusing on the C2V3C3 region, the evolutionary rates were not significantly different (0.0142 vs 0.0105 substitutions/site/year, PP=20%). There was a trend towards positive selection in child 1 (non-synonymous rate: env, 0.0143 and V3, 0.0152; synonymous rate: env, 0.0137 and V3, 0.0124 substitutions/site/year) and purifying selection in child 2 (non-synonymous rate: env, 0.0069 and V3, 0.0092; synonymous rate: env, 0.0082 and V3, 0.0132 substitutions/site/year) in both env and V3.
Evolution of amino acid diversity in variable Env regions in the first five years of infection
Sum of entropy
year of birth
In V3, mutations occurred almost exclusively within previously described neutralizing epitopes , and at residues 18, 19 and 27 that have been associated with R5 and X4 tropism [36–38]. One amino acid insertion occurred in the same position in both children and involved a hydrophobic residue (V in child 1; I in child 2). This type of insertion has also been associated to R5-to-X4 tropism switch [36, 37].
Positive selective pressure on the Env glycoproteins in both children over the course of infection
Codons under selective pressure (location)1
Year of birth
Year of birth
5, 7 (in SP)
178 (V2), 255, 259 (C2), 320 (V3), 459 (V5), 467, 471 (C5)
552 (HR1), 672, 673 (MPER)
In all, these results show that HIV-2 env can evolve and diversify very rapidly in the first years of infection. The positive correlation between the rate of Env evolution, in terms of nucleotide divergence from the initial virus, nucleotide diversity, amino acid diversity, and positive selection, and the rate of Nab response and escape indicates that Nabs likely have a major impact on HIV-2 Env evolution in the first years of infection.
Tropism and susceptibility to antibody neutralization are closely associated with V3 structure
There is limited knowledge on the natural history of HIV-2 infection and on the molecular and phenotypic evolutionary dynamics of HIV-2 because no study has investigated the full course of infection from the time of seroconversion. The current study is the first characterization of the Nab response and molecular and phenotypic evolution of HIV-2 followed from acute infection to late stage infection. Our studies were based on two children infected by vertical transmission and spanned the first 12 years of infection in one case and the complete infection period in the other (9 years). We show that a potent Nab response is raised very early after infection and that the rate and pattern of molecular and phenotypic evolution of the HIV-2 Env are closely associated to the rate of Nab escape.
Child 2 was born severely ill despite the normal CD4+ T cell percentage [31, 32] whereas child 1 was born asymptomatic and with normal CD4 levels. Despite the contrasting clinical conditions at birth, major CD4 depletion and disease progression occurred in both children in the first 5 years of infection. This fast disease course is typical of HIV-1 infected children in the absence of antiretroviral treatment  but is highly unusual in HIV-2 infected individuals [10, 34, 41, 42]. Both patients were infected with R5 strains but transitions to X4 tropism occurred rapidly, being detected after only 5 years of infection. This is the first time that a full R5-to-X4 tropism switch is observed in HIV-2 infected patients and it was unexpected to find it in paediatric patients. Like in some adult HIV-2 patients with advanced disease [23, 37, 43], the emergence of X4 viruses in our patients was associated with high viral load, marked CD4 depletion and disease progression. Hence, the rapid disease course in the two infants may have been determined by the early emergence of X4 isolates.
At birth, HIV-2 nucleotide diversity in child 1 was 2-fold higher than in child 2 both in env and C2V3C3. Nucleotide diversity in child 1 was also 2-fold higher than in HIV-1 infected children in the first weeks after birth [44–46] and in HIV-1 adult patients during seroconversion . Envelope diversity also increased more significantly, both at the nucleotide and amino acid levels, in child 1 than in child 2 in the first 5 years of infection. Consistently, the evolutionary rate of the env gene in child 1 was almost two times higher than child 2 and similar to that found in chronic HIV-2 infected patients under ART (0.0102 substitutions/site/year) . These results reveal a remarkably high rate of molecular evolution of the HIV-2 envelope in child 1 during the first 5 years of infection and a moderate rate of evolution in child 2. The evolutionary rate in child 2 was similar to that in HIV-1 patients who, when untreated, have a disease progression generally similar to that of child 2. Thus, the higher rate in child 1 is consistent with the better immune control.
Previously, we have shown that production of gp36-specific and gp125-specific antibodies occurred during the first year of age in child 1 and that, at age 2, levels of binding antibodies to these glycoproteins were already similar to those found in HIV-2 infected patients in the chronic stage of disease . In child 2, although gp36-specific antibodies were produced to near normal levels, there was a remarkably weak production of gp125-specific binding antibodies. Consistently, in this work we found that child 1 produced a much stronger Nab response than child 2. In child 1 autologous neutralizing antibodies appeared within the first year of infection, increased over time to levels that were similar to chronically infected patients [18, 19, 21], and were sustained until at least the age of 12. Moreover, child 1 also produced a potent Nab response against several heterologous virus isolates. As for child 2, the autologous Nab titer was significantly lower compared to child 1 at age 5 and decreased continuously to very low titers following the rapid decline of CD4 cells and progression to AIDS and death at age 9. Similar findings have been reported in late stage disease adult patients in whom Nab titers decrease in direct association with CD4+ T cell depletion . Overall, these results suggest that a potent Nab response during the acute/early phase of infection might contribute to control HIV-2 disease progression. Further investigation of the relationship between Nab response and disease progression in the acute and early stages of HIV-2 infection of adult patients is required as this might provide new insights into the benign course of most HIV-2 infections.
The differences in Nab response between the two children also correlate well with the magnitude and rate of envelope evolution in the infants which suggest a close association between the neutralizing antibody response and the evolution of the HIV-2 envelope in these patients. Several lines of evidence further suggest that escape from Nab response is a major determinant of the evolution of the HIV-2 envelope in these infants, especially in child 1. First, complete replacement of virus quasispecies was noted in phylogenetic analysis of env sequences produced at the different time points which is compatible with a situation of ongoing viral escape from antibody neutralization . Second, amino acid diversity increased significantly with infection time, especially in V1 and V3 which are major neutralizing domains in HIV-2. This is a major HIV escape mechanism as a single polymorphism can alter epitope sequence and/or conformation and reduce recognition and/or binding affinity by neutralizing antibodies [39, 48–52]. Third, increase in dN/dS ratio and positive selection in the envelope were closely related to rate of Nab escape in child 1 [39, 51]. Finally, the similar gain of secondary structure in V3 in both patients fully correlated with the rate of escape from antibody neutralization. This has been recently associated to HIV-2 resistance to antibody neutralization in chronic HIV-2 infected patients .
Nabs were significantly more potent against R5 isolates than against X4 isolates (autologous and heterologous) confirming the inherent resistance of X4 viruses to antibody neutralization [23, 53]. More importantly, increase in Nab resistance in child 1 preceded the emergence of X4 variant suggesting that tropism switch may have been driven by the neutralizing antibodies. Given the immunodominance of the V3 region in HIV-2 infected patients , the location of two of the three amino acid residues that are associated to R5 and X4 tropism (positions 18 and 19) [36–38] within the first neutralizing epitope in V3  and the major difference in V3 conformation of R5 and X4 strains , the close association between HIV-2 susceptibility to antibody neutralization and tropism seen in these infants is not surprising.
The main limitations of this study are the small number of patients and the inexistence of viral isolates from all time points in both children. However, worldwide it has been impossible to find individuals acutely infected with HIV-2. Moreover, due to the low or absent viral load it is often impossible to isolate virus from HIV-2 infected patients. Notwithstanding these limitations, we believe that our results are a major contribution to our understanding of the natural history of HIV-2 infection and of the role of the immune system in controlling and shaping HIV-2 evolution.
In conclusion, we show that a potent Nab response is elicited very early after HIV-2 infection and that the HIV-2 envelope evolves at a high rate in the first years of infection, this rate being directly correlated to the potency of the Nab response. R5-to-X4 tropism switch, increased nucleotide and amino acid diversity in V1 and V3, and convergence of V3 to a β-hairpin structure were closely associated with escape from the Nab response suggesting that Nabs have a major impact on the rapid molecular and phenotypic evolution of the viral envelope in acute and early in HIV-2 infection. Our studies provide further support to a model of HIV-2 pathogenesis in which Nabs play a central role.
Study subjects and ethics
Two children infected by vertical transmission were studied. Blood samples were collected from child 1 (patient PT.HDE.CT), 39 days after birth in 1998, in 1999, 2000, 2001, 2003, 2006 and 2010, and from child 2 (patient PT.HDE.SC), 27 days after birth in 1992, in 1997 and 2001. Clinical and immunological characteristics of the patients are shown in Table 1. Child 1 started ART (stavudine + lamivudine + lopinavir/ritonavir) in November 2003. Presently, the child is taking lamivudine + abacavir + lopinavir/ritonavir; his viral load is undetectable and he is clinically and immunologically stable. Child 2 started ART with zidovudine immediately after birth and lamivudine was added in 2001. In 2001, viral load increased slightly and CD4+ T cells decreased sharply leading to the child’s death. Ethical approval was obtained from the Ethics Committee of Hospital Curry Cabral and written informed consent was obtained from the children's parents before entry into the study.
HIV-2 env gene PCR amplification, cloning and sequencing
Chromossomal DNA was extracted from infected PBMC’s using Wizard Genomic DNA Purification Kit (Promega) according to the manufacturer recommendations. A 2600 bp fragment encompassing the entire env gene was amplified by nested Polymerase Chain Reaction (PCR) using the Expand Long Template PCR Systemkit (Roche) and newly designed primers (Additional file 2: Table S2). The PCR protocol consisted of denaturation at 95°C for 5 min, 35 amplification cycles of 15 sec at 94°C, 30 sec at 59°C and 3 min at 68°C with 5 sec increments and a final elongation step at 68°C for 30 min. 5 μl of PCR product was used as the template for nested PCR. The amplification profile of the nested PCR was identical to the first PCR, except for annealing temperature and extension time (61°C and 2 min respectively). PCR amplicons were purified with a JETQUICK Gel Extraction Spin Kit (Genomed). For each sample, PCR products were cloned into the pcDNA3.1/V5-His-TOPO vector (Invitrogen), using the TOPO TA Expression Kit (Invitrogen) according to the manufacturer’s instructions. At least eight clones from each patient/year were sequenced using the BigDye Terminator V3.1 Cycle sequencing Kit (Applied Biosystems); sequencing primers are displayed in Additional file 2: Table S2. Sequencing was performed on an ABI 3100–Avant Genetic Analyzer (Applied Biosystems).
Clustal × 2.1  software was used to construct alignments of HIV-2 env sequences. Reference HIV-2 sequences were obtained from the Los Alamos National Laboratory HIV sequence database . Maximum likelihood phylogenetic analyses were performed using the best-fit model of molecular evolution estimated in PAUP by Modeltest using likelihood ratio tests . The chosen model was GTR + I + G. Tree searches were conducted in PAUP using nearest-neighbour interchange (NNI) and tree-bisection plus reconnection (TBR) heuristic search strategies , and bootstrap resampling with 1000 replicates . The genetic distances between sequences were calculated by averaging pairwise tree distances using all sequences obtained for each patient at each time point, as previously described . Putative recombinants were identified using the Phi-statistic  available in SplitsTree version 4.10  by performing 10 randomized reductions of putative recombinants. Putative recombinant sequences were removed before doing the evolutionary rate analyses. These were: 00PTHDECT_9, 00PTHDECT_16, 00PTHDECT_22, 00PTHDECT_6, 00PTHDECT_24, 00PTHDECT_19, 00PTHDECT_8, 00PTHDECT_12, 03PTHDECT_17, 03PTHDECT_33, 03PTHDECT_21, 01PTHDESC_13, 01PTHDESC_6 and 01PTHDESC_14.
Selective pressure on the HIV-2 Env was examined with the DATAMONKEY web-server , after removing all positions containing gaps and missing data from the dataset. All estimations were performed using the MG94 codon substitution model  crossed with the nucleotide substitution model GTR, previously selected with Modeltest (see above). The single-likelihood ancestor counting (SLAC) method was used to infer the ratio of nonsynonymous to synonymous nucleotide substitutions (dN/dS) averaged over all codon positions of the alignment. To identify individual codons under selective pressure, site-specific dN/dS rates were estimated by the relaxed-effects likelihood (REL) method, with a cut-off value for the Bayes factor of 50 .
The Bayesian program BEAST was used to estimate the nucleotide evolutionary rates . The SRD06 model  of substitution was used and two different clock models were used, relaxed lognormal and strict clock. A constant parametric demographic model as well as the non-parametric Skyline plot with 3 groups was tested. The MCMC chains were chosen so that the effective sample size for all parameters exceeded 300 and convergence was assessed by inspecting the traces in the program Tracer . Appropriate demographic and molecular clock models were chosen by examining the marginal posterior distributions of relevant parameters.
Potential N-linked glycosylation sites were identified using the N-Glycosite software  and the entropy at each position in protein alignment was measured with Shannon’s entropy , both available at the Los Alamos National Laboratory HIV sequence database .
Virus isolation and tropism characterization
Primary virus isolates were obtained from both patients using the co-cultivation method as described previously . Viral tropism (CCR5 and/or CXCR4 usage) was determined in TZM-bl cells in the presence of CCR5 or CXCR4 antagonists as described previously . Tropism was also determined genetically using the V3 loop clonal sequences and the algorithm described by Visseuxet al which is based in the sequence, size and charge of the V3 loop.
The neutralizing activity present in patients serum was analyzed against autologous and heterologous primary virus isolates using a luciferase reporter gene assay in TZM-bl cells, as described previously [2, 53, 71]. Briefly, the cells [10,000 cells in 100 μl of complete growth medium (GM) that consists of DMEM supplemented with 10% heat-inactivated fetal bovine serum (FBS)], were added to each well of 96-well flat-bottom culture plates (Nunc) and allowed to adhere overnight. One hundred μl of each virus (corresponding to 200 TCID50) were incubated for 1 h at 37°C with 2-fold serial dilutions of heat-inactivated patients sera in a total volume of 200 μl of GM containing DEAE-Dextran (20 μg/ml). The lowest serum dilution used in the assays was 1:80. Forty-eight hours later, plates were analyzed for luciferase activity on a luminometer (TECAN) using the One-Glow Luciferase Assay System (Promega, Madison, WI). Medium only control wells were measured as background, and virus-only control wells were included as 100% infection. Neutralization titers were expressed as the reciprocal of the plasma dilution that inhibited virus infection by 50% (IC50). IC50 was estimated by the sigmoidal dose–response (variable slope) equation in Prism version 5.0 . Nonspecific inhibition was assessed by testing all HIV-2 isolates against HIV-negative plasma and all plasma s∆env as backbone).
Structural models of the C2-V3-C3 domain in gp125 were produced with SWISS-MODEL homology modelling server in automated mode, using PDB file 2BF1 (SIV) as template [73, 74]. Accelrys Discovery Studio 2.1 (Accelrys Inc., San Diego, USA, 2008) was used to produce three dimensional images of the obtained models and perform the secondary structure analysis of the V3 loop.
Statistical analysis was performed with GraphPad Prism 5.0  with a level of significance of 5%. F test was used to compare best fit values of IC50 slopes obtained with CT00 and CT03 isolates from child 1. Non parametric Mann Whitney test was used to compare autologous Nab responses (mean IC50s) between child 1 and child 2. To compare evolutionary rates we computed the posterior probability (PP) that one rate exceeded the other and the probability was determined numerically by randomly sampling from the empirical posterior distributions . Kruskal-Wallis test was used to compare mean Shannon’s entropies between variable Env regions of both patients.
GenBank accession numbers
Full-length envelope sequences generated in this study are available from GenBank under the following accession numbers: GU983917-GU983940 and JX219591-JX219614.
This work was supported by grants PTDC/SAU-FAR/115290/2009 and PTDC/SAU-EPI/122400/2010 from Fundação para a Ciência e Tecnologia (FCT) (http://www.fct.pt), Portugal, a NIH grant (R01AI087520), and by Collaborative HIV and Anti-HIV Drug Resistance Network (CHAIN), from the European Union. Cheila Rocha, Rita Calado, Pedro Borrego and Inês Bártolo were supported by PhD scholarships from Fundação para a Ciência e Tecnologia, Portugal. Helena Skar was supported by a postdoctoral fellowship from the Swedish Research Council (623-2011-1100). The following reagents were obtained through the AIDS Research and Reference Reagent Program, Division of AIDS, NIAID, NIH: TZM-bl from Dr. John C. Kappes, Dr. Xiaoyun Wu and Tranzyme Inc.
- De Silva TI, Cotten M, Rowland-Jones SL: HIV-2: the forgotten AIDS virus. Trends Microbiol. 2008, 16: 588-595. 10.1016/j.tim.2008.09.003.View ArticlePubMedGoogle Scholar
- Gottlieb GS, Eholie SP, Nkengasong JN, Jallow S, Rowland-Jones S, Whittle HC, Sow PS: A call for randomized controlled trials of antiretroviral therapy for HIV-2 infection in West Africa. AIDS. 2008, 22: 2069-2072. 10.1097/QAD.0b013e32830edd44. discussion 2073-2064PubMed CentralView ArticlePubMedGoogle Scholar
- Yamaguchi J, Devare SG, Brennan CA: Identification of a new HIV-2 subtype based on phylogenetic analysis of full-length genomic sequence. AIDS Res Hum Retroviruses. 2000, 16: 925-930. 10.1089/08892220050042864.View ArticlePubMedGoogle Scholar
- Damond F, Worobey M, Campa P, Farfara I, Colin G, Matheron S, Brun-Vezinet F, Robertson DL, Simon F: Identification of a highly divergent HIV type 2 and proposal for a change in HIV type 2 classification. AIDS Res Hum Retroviruses. 2004, 20: 666-672. 10.1089/0889222041217392.View ArticlePubMedGoogle Scholar
- van der Loeff MF S, Jaffar S, Aveika AA, Sabally S, Corrah T, Harding E, Alabi A, Bayang A, Ariyoshi K, Whittle HC: Mortality of HIV-1, HIV-2 and HIV-1/HIV-2 dually infected patients in a clinic-based cohort in The Gambia. AIDS. 2002, 16: 1775-1783. 10.1097/00002030-200209060-00010.View ArticleGoogle Scholar
- Campbell-Yesufu OT, Gandhi RT: Update on human immunodeficiency virus (HIV)-2 infection. Clin Infect Dis. 2011, 52: 780-787. 10.1093/cid/ciq248.PubMed CentralView ArticlePubMedGoogle Scholar
- Matheron S, Pueyo S, Damond F, Simon F, Lepretre A, Campa P, Salamon R, Chene G, Brun-Vezinet F: Factors associated with clinical progression in HIV-2 infected-patients: the French ANRS cohort. AIDS. 2003, 17: 2593-2601. 10.1097/00002030-200312050-00006.View ArticlePubMedGoogle Scholar
- van der Loeff MF, Larke N, Kaye S, Berry N, Ariyoshi K, Alabi A, Van Tienen C, Leligdowicz A, Sarge-Njie R, Da Silva Z, et al: Undetectable plasma viral load predicts normal survival in HIV-2-infected people in a West African village. Retrovirology. 2010, 7: 46-10.1186/1742-4690-7-46.PubMed CentralView ArticlePubMedGoogle Scholar
- Berry N, Ariyoshi K, Jaffar S, Sabally S, Corrah T, Tedder R, Whittle H: Low peripheral blood viral HIV-2 RNA in individuals with high CD4 percentage differentiates HIV-2 from HIV-1 infection. J Hum Virol. 1998, 1: 457-468.PubMedGoogle Scholar
- Jaffar S, Wilkins A, Ngom PT, Sabally S, Corrah T, Bangali JE, Rolfe M, Whittle HC: Rate of decline of percentage CD4+ cells is faster in HIV-1 than in HIV-2 infection. J Acquir Immune Defic Syndr Hum Retrovirol. 1997, 16: 327-332. 10.1097/00042560-199712150-00003.View ArticlePubMedGoogle Scholar
- Lisse IM, Poulsen AG, Aaby P, Knudsen K, Dias F: Serial CD4 and CD8 T-lymphocyte counts and associated mortality in an HIV-2-infected population in Guinea-Bissau. J Acquir Immune Defic Syndr Hum Retrovirol. 1996, 13: 355-362. 10.1097/00042560-199612010-00009.View ArticlePubMedGoogle Scholar
- Popper SJ, Sarr AD, Gueye-Ndiaye A, Mboup S, Essex ME, Kanki PJ: Low plasma human immunodeficiency virus type 2 viral load is independent of proviral load: low virus production in vivo. J Virol. 2000, 74: 1554-1557. 10.1128/JVI.74.3.1554-1557.2000.PubMed CentralView ArticlePubMedGoogle Scholar
- Andersson S, Norrgren H, Da Silva Z, Biague A, Bamba S, Kwok S, Christopherson C, Biberfeld G, Albert J: Plasma viral load in HIV-1 and HIV-2 singly and dually infected individuals in Guinea-Bissau, West Africa: significantly lower plasma virus set point in HIV-2 infection than in HIV-1 infection. Arch Intern Med. 2000, 160: 3286-3293. 10.1001/archinte.160.21.3286.View ArticlePubMedGoogle Scholar
- Marlink R, Kanki P, Thior I, Travers K, Eisen G, Siby T, Traore I, Hsieh CC, Dia MC, Gueye EH, et al: Reduced rate of disease development after HIV-2 infection as compared to HIV-1. Science. 1994, 265: 1587-1590. 10.1126/science.7915856.View ArticlePubMedGoogle Scholar
- Drylewicz J, Matheron S, Lazaro E, Damond F, Bonnet F, Simon F, Dabis F, Brun-Vezinet F, Chene G, Thiebaut R: Comparison of viro-immunological marker changes between HIV-1 and HIV-2-infected patients in France. AIDS. 2008, 22: 457-468. 10.1097/QAD.0b013e3282f4ddfc.PubMed CentralView ArticlePubMedGoogle Scholar
- Marcelino JM, Nilsson C, Barroso H, Gomes P, Borrego P, Maltez F, Rosado L, Doroana M, Antunes F, Taveira N: Envelope-specific antibody response in HIV-2 infection: C2V3C3-specific IgG response is associated with disease progression. AIDS. 2008, 22: 2257-2265. 10.1097/QAD.0b013e3283155546.View ArticlePubMedGoogle Scholar
- Rodriguez SK, Sarr AD, MacNeil A, Thakore-Meloni S, Gueye-Ndiaye A, Traore I, Dia MC, Mboup S, Kanki PJ: Comparison of heterologous neutralizing antibody responses of human immunodeficiency virus type 1 (HIV-1)- and HIV-2-infected Senegalese patients: distinct patterns of breadth and magnitude distinguish HIV-1 and HIV-2 infections. J Virol. 2007, 81: 5331-5338. 10.1128/JVI.02789-06.PubMed CentralView ArticlePubMedGoogle Scholar
- Ozkaya Sahin G, Holmgren B, Da Silva Z, Nielsen J, Nowroozalizadeh S, Esbjornsson J, Mansson F, Andersson S, Norrgren H, Aaby P, et al: Potent intratype neutralizing activity distinguishes human immunodeficiency virus type 2 (HIV-2) from HIV-1. J Virol. 2012, 86: 961-971. 10.1128/JVI.06315-11.PubMed CentralView ArticlePubMedGoogle Scholar
- De Silva TI, Aasa-Chapman M, Cotten M, Hue S, Robinson J, Bibollet-Ruche F, Sarge-Njie R, Berry N, Jaye A, Aaby P, et al: Potent autologous and heterologous neutralizing antibody responses occur in HIV-2 infection across a broad range of infection outcomes. J Virol. 2012, 86: 930-946. 10.1128/JVI.06126-11.PubMed CentralView ArticlePubMedGoogle Scholar
- Bjorling E, Scarlatti G, Von Gegerfelt A, Albert J, Biberfeld G, Chiodi F, Norrby E, Fenyo EM: Autologous neutralizing antibodies prevail in HIV-2 but not in HIV-1 infection. Virology. 1993, 193: 528-530. 10.1006/viro.1993.1160.View ArticlePubMedGoogle Scholar
- Kong R, Li H, Bibollet-Ruche F, Decker JM, Zheng NN, Gottlieb GS, Kiviat NB, Sow PS, Georgiev I, Hahn BH, et al: Broad and potent neutralizing antibody responses elicited in natural HIV-2 infection. J Virol. 2012, 86: 947-960. 10.1128/JVI.06155-11.PubMed CentralView ArticlePubMedGoogle Scholar
- Borrego P, Calado R, Marcelino JM, Bartolo I, Rocha C, Cavaco-Silva P, Doroana M, Antunes F, Maltez F, Caixas U, et al: Baseline susceptibility of primary HIV-2 to entry inhibitors. Antivir Ther. 2012, 17 (3): 565-570. 10.3851/IMP1996.View ArticlePubMedGoogle Scholar
- Marcelino JM, Borrego P, Nilsson C, Familia C, Barroso H, Maltez F, Doroana M, Antunes F, Quintas A, Taveira N: Resistance to antibody neutralization in HIV-2 infection occurs in late stage disease and is associated with X4 tropism. AIDS. 2012, 26: 2275-2284. 10.1097/QAD.0b013e328359a89d.View ArticlePubMedGoogle Scholar
- Skar H, Borrego P, Wallstrom TC, Mild M, Marcelino JM, Barroso H, Taveira N, Leitner T, Albert J: HIV-2 genetic evolution in patients with advanced disease is faster than that in matched HIV-1 patients. J Virol. 2010, 84: 7412-7415. 10.1128/JVI.02548-09.PubMed CentralView ArticlePubMedGoogle Scholar
- Borrego P, Marcelino JM, Rocha C, Doroana M, Antunes F, Maltez F, Gomes P, Novo C, Barroso H, Taveira N: The role of the humoral immune response in the molecular evolution of the envelope C2, V3 and C3 regions in chronically HIV-2 infected patients. Retrovirology. 2008, 5: 78-10.1186/1742-4690-5-78.PubMed CentralView ArticlePubMedGoogle Scholar
- Bjorling E, Chiodi F, Utter G, Norrby E: Two neutralizing domains in the V3 region in the envelope glycoprotein gp125 of HIV type 2. J Immunol. 1994, 152: 1952-1959.PubMedGoogle Scholar
- Kong R, Li H, Georgiev I, Changela A, Bibollet-Ruche F, Decker JM, Rowland-Jones SL, Jaye A, Guan Y, Lewis GK, et al: Epitope mapping of broadly neutralizing HIV-2 human monoclonal antibodies. J Virol. 2012, 86: 12115-12128. 10.1128/JVI.01632-12.PubMed CentralView ArticlePubMedGoogle Scholar
- McKnight A, Shotton C, Cordell J, Jones I, Simmons G, Clapham PR: Location, exposure, and conservation of neutralizing and nonneutralizing epitopes on human immunodeficiency virus type 2 SU glycoprotein. J Virol. 1996, 70: 4598-4606.PubMed CentralPubMedGoogle Scholar
- Skott P, Achour A, Norin M, Thorstensson R, Bjorling E: Characterization of neutralizing sites in the second variable and fourth variable region in gp125 and a conserved region in gp36 of human immunodeficiency virus type 2. Viral Immunol. 1999, 12: 79-88. 10.1089/vim.1999.12.79.View ArticlePubMedGoogle Scholar
- Barroso H, Araujo F, Gomes MH, Mota-Miranda A, Taveira N: Phylogenetic demonstration of two cases of perinatal human immunodeficiency virus type 2 infection diagnosed in adulthood. AIDS Res Hum Retroviruses. 2004, 20: 1373-1376. 10.1089/aid.2004.20.1373.View ArticlePubMedGoogle Scholar
- Cavaco-Silva P, Taveira NC, Lourenco MH, Santos Ferreira MO, Daniels RS: Vertical transmission of HIV-2. Lancet. 1997, 349: 177-178. 10.1016/S0140-6736(05)60978-6.View ArticlePubMedGoogle Scholar
- Cavaco-Silva P, Taveira NC, Rosado L, Lourenco MH, Moniz-Pereira J, Douglas NW, Daniels RS, Santos-Ferreira MO: Virological and molecular demonstration of human immunodeficiency virus type 2 vertical transmission. J Virol. 1998, 72: 3418-3422.PubMed CentralPubMedGoogle Scholar
- Padua E, Almeida C, Nunes B, Cortes Martins H, Castela J, Neves C, Paixao MT: Assessment of mother-to-child HIV-1 and HIV-2 transmission: an AIDS reference laboratory collaborative study. HIV Med. 2009, 10: 182-190. 10.1111/j.1468-1293.2008.00669.x.View ArticlePubMedGoogle Scholar
- Mota-Miranda A, Gomes H, Lima-Alves C, Araujo F, Cunha-Ribeiro LM, Taveira N: Perinatally acquired HIV-2 infection diagnosed at 15 and 24 years of age. AIDS. 2001, 15: 2460-2461. 10.1097/00002030-200112070-00018.View ArticlePubMedGoogle Scholar
- Burgard M, Jasseron C, Matheron S, Damond F, Hamrene K, Blanche S, Faye A, Rouzioux C, Warszawski J, Mandelbro L: Mother-to-child transmission of HIV-2 infection from 1986 to 2007 in the ANRS French perinatal cohort EPF-CO1. Clin Infect Dis. 2010, 51: 833-843. 10.1086/656284.View ArticlePubMedGoogle Scholar
- Visseaux B, Hurtado-Nedelec M, Charpentier C, Collin G, Storto A, Matheron S, Larrouy L, Damond F, Brun-Vezinet F, Descamps D: Molecular determinants of HIV-2 R5-X4 tropism in the V3 loop: development of a new genotypic tool. J Infect Dis. 2012, 205: 111-120. 10.1093/infdis/jir698.View ArticlePubMedGoogle Scholar
- Shi Y, Brandin E, Vincic E, Jansson M, Blaxhult A, Gyllensten K, Moberg L, Brostrom C, Fenyo EM, Albert J: Evolution of human immunodeficiency virus type 2 coreceptor usage, autologous neutralization, envelope sequence and glycosylation. J Gen Virol. 2005, 86: 3385-3396. 10.1099/vir.0.81259-0.View ArticlePubMedGoogle Scholar
- Isaka Y, Sato A, Miki S, Kawauchi S, Sakaida H, Hori T, Uchiyama T, Adachi A, Hayami M, Fujiwara T, Yoshie O: Small amino acid changes in the V3 loop of human immunodeficiency virus type 2 determines the coreceptor usage for CXCR4 and CCR5. Virology. 1999, 264: 237-243. 10.1006/viro.1999.0006.View ArticlePubMedGoogle Scholar
- Bunnik EM, Pisas L, Van Nuenen AC, Schuitemaker H: Autologous neutralizing humoral immunity and evolution of the viral envelope in the course of subtype B human immunodeficiency virus type 1 infection. J Virol. 2008, 82: 7932-7941. 10.1128/JVI.00757-08.PubMed CentralView ArticlePubMedGoogle Scholar
- Rey MA, Girard PM, Harzic M, Madjar JJ, Brun-Vezinet F, Saimot AG: HIV-1 and HIV-2 double infection in French homosexual male with AIDS-related complex (Paris, 1985). Lancet. 1987, 1: 388-389.View ArticlePubMedGoogle Scholar
- Matheron S, Mendoza-Sassi G, Simon F, Olivares R, Coulaud JP, Brun-Vezinet F: HIV-1 and HIV-2 AIDS in African patients living in Paris. AIDS. 1997, 11: 934-936.PubMedGoogle Scholar
- Norrgren H, Da Silva Z, Biague A, Andersson S, Biberfeld G: Clinical progression in early and late stages of disease in a cohort of individuals infected with human immunodeficiency virus-2 in Guinea-Bissau. Scand J Infect Dis. 2003, 35: 265-272. 10.1080/00365540310000210.View ArticlePubMedGoogle Scholar
- Blaak H, van der Ende ME, Boers PH, Schuitemaker H, Osterhaus AD: In vitro replication capacity of HIV-2 variants from long-term aviremic individuals. Virology. 2006, 353: 144-154. 10.1016/j.virol.2006.05.029.View ArticlePubMedGoogle Scholar
- Wu X, Parast AB, Richardson BA, Nduati R, John-Stewart G, Mbori-Ngacha D, Rainwater SM, Overbaugh J: Neutralization escape variants of human immunodeficiency virus type 1 are transmitted from mother to infant. J Virol. 2006, 80: 835-844. 10.1128/JVI.80.2.835-844.2006.PubMed CentralView ArticlePubMedGoogle Scholar
- Hoffmann FG, He X, West JT, Lemey P, Kankasa C, Wood C: Genetic variation in mother-child acute seroconverter pairs from Zambia. AIDS. 2008, 22: 817-824. 10.1097/QAD.0b013e3282f486af.View ArticlePubMedGoogle Scholar
- Zhang H, Tully DC, Hoffmann FG, He J, Kankasa C, Wood C: Restricted genetic diversity of HIV-1 subtype C envelope glycoprotein from perinatally infected Zambian infants. PLoS One. 2010, 5: e9294-10.1371/journal.pone.0009294.PubMed CentralView ArticlePubMedGoogle Scholar
- Salazar-Gonzalez JF, Bailes E, Pham KT, Salazar MG, Guffey MB, Keele BF, Derdeyn CA, Farmer P, Hunter E, Allen S, et al: Deciphering human immunodeficiency virus type 1 transmission and early envelope diversification by single-genome amplification and sequencing. J Virol. 2008, 82: 3952-3970. 10.1128/JVI.02660-07.PubMed CentralView ArticlePubMedGoogle Scholar
- Zhang H, Hoffmann F, He J, He X, Kankasa C, Ruprecht R, West JT, Orti G, Wood C: Evolution of subtype C HIV-1 Env in a slowly progressing Zambian infant. Retrovirology. 2005, 2: 67-10.1186/1742-4690-2-67.PubMed CentralView ArticlePubMedGoogle Scholar
- Zhang H, Hoffmann F, He J, He X, Kankasa C, West JT, Mitchell CD, Ruprecht RM, Orti G, Wood C: Characterization of HIV-1 subtype C envelope glycoproteins from perinatally infected children with different courses of disease. Retrovirology. 2006, 3: 73-10.1186/1742-4690-3-73.PubMed CentralView ArticlePubMedGoogle Scholar
- Doria-Rose NA, Georgiev I, O’Dell S, Chuang GY, Staupe RP, McLellan JS, Gorman J, Pancera M, Bonsignori M, Haynes BF, et al: A short segment of the HIV-1 gp120 V1/V2 region is a major determinant of resistance to V1/V2 neutralizing antibodies. J Virol. 2012, 86: 8319-8323. 10.1128/JVI.00696-12.PubMed CentralView ArticlePubMedGoogle Scholar
- Frost SD, Wrin T, Smith DM, Kosakovsky Pond SL, Liu Y, Paxinos E, Chappey C, Galovich J, Beauchaine J, Petropoulos CJ, et al: Neutralizing antibody responses drive the evolution of human immunodeficiency virus type 1 envelope during recent HIV infection. Proc Natl Acad Sci U S A. 2005, 102: 18514-18519. 10.1073/pnas.0504658102.PubMed CentralView ArticlePubMedGoogle Scholar
- Rong R, Li B, Lynch RM, Haaland RE, Murphy MK, Mulenga J, Allen SA, Pinter A, Shaw GM, Hunter E, et al: Escape from autologous neutralizing antibodies in acute/early subtype C HIV-1 infection requires multiple pathways. PLoS Pathog. 2009, 5: e1000594-10.1371/journal.ppat.1000594.PubMed CentralView ArticlePubMedGoogle Scholar
- Marcelino JM, Borrego P, Rocha C, Barroso H, Quintas A, Novo C, Taveira N: Potent and broadly reactive HIV-2 neutralizing antibodies elicited by a vaccinia virus vector prime-C2V3C3 polypeptide boost immunization strategy. J Virol. 2010, 84: 12429-12436. 10.1128/JVI.01102-10.PubMed CentralView ArticlePubMedGoogle Scholar
- Soares R, Foxall R, Albuquerque A, Cortesao C, Garcia M, Victorino RM, Sousa AE: Increased frequency of circulating CCR5+ CD4+ T cells in human immunodeficiency virus type 2 infection. J Virol. 2006, 80: 12425-12429. 10.1128/JVI.01557-06.PubMed CentralView ArticlePubMedGoogle Scholar
- Larkin MA, Blackshields G, Brown NP, Chenna R, McGettigan PA, McWilliam H, Valentin F, Wallace IM, Wilm A, Lopez R, et al: Clustal W and clustal X version 2.0. Bioinformatics. 2007, 23: 2947-2948. 10.1093/bioinformatics/btm404.View ArticlePubMedGoogle Scholar
- SequenceDatabase: Los Alamos sequence database. 2009, Los Alamos, New Mexico: Los Alamos National Laboratory, http://www.hiv.lanl.gov/,Google Scholar
- Posada D, Crandall KA: MODELTEST: testing the model of DNA substitution. Bioinformatics. 1998, 14: 817-818. 10.1093/bioinformatics/14.9.817.View ArticlePubMedGoogle Scholar
- Swofford DL, Waddell PJ, Huelsenbeck JP, Foster PG, Lewis PO, Rogers JS: Bias in phylogenetic estimation and its relevance to the choice between parsimony and likelihood methods. Syst Biol. 2001, 50: 525-539.View ArticlePubMedGoogle Scholar
- Felsenstein J: Confidence limits on phylogenies: an approach using the bootstrap. Evolution. 1985, 39: 783-791. 10.2307/2408678.View ArticleGoogle Scholar
- Lee HY, Perelson AS, Park SC, Leitner T: Dynamic correlation between intrahost HIV-1 quasispecies evolution and disease progression. PLoS Comput Biol. 2008, 4: e1000240-10.1371/journal.pcbi.1000240.PubMed CentralView ArticlePubMedGoogle Scholar
- Bruen TC, Philippe H, Bryant D: A simple and robust statistical test for detecting the presence of recombination. Genetics. 2006, 172: 2665-2681.PubMed CentralView ArticlePubMedGoogle Scholar
- Huson DH, Bryant D: Application of phylogenetic networks in evolutionary studies. Mol Biol Evol. 2006, 23: 254-267.View ArticlePubMedGoogle Scholar
- Pond SL, Frost SD: Datamonkey: rapid detection of selective pressure on individual sites of codon alignments. Bioinformatics. 2005, 21: 2531-2533. 10.1093/bioinformatics/bti320.View ArticlePubMedGoogle Scholar
- Muse SV, Gaut BS: A likelihood approach for comparing synonymous and nonsynonymous nucleotide substitution rates, with application to the chloroplast genome. Mol Biol Evol. 1994, 11: 715-724.PubMedGoogle Scholar
- Kosakovsky Pond SL, Frost SD: Not so different after all: a comparison of methods for detecting amino acid sites under selection. Mol Biol Evol. 2005, 22: 1208-1222. 10.1093/molbev/msi105.View ArticlePubMedGoogle Scholar
- Drummond AJ, Rambaut A: BEAST: Bayesian evolutionary analysis by sampling trees. BMC Evol Biol. 2007, 7: 214-10.1186/1471-2148-7-214.PubMed CentralView ArticlePubMedGoogle Scholar
- Shapiro B, Rambaut A, Drummond AJ: Choosing appropriate substitution models for the phylogenetic analysis of protein-coding sequences. Mol Biol Evol. 2006, 23: 7-9.View ArticlePubMedGoogle Scholar
- Rambaut A, Drummond AJ: Tracer v1.4. 2007, Available from http://beast.bio.ed.ac.uk/TracerGoogle Scholar
- Zhang M, Gaschen B, Blay W, Foley B, Haigwood N, Kuiken C, Korber B: Tracking global patterns of N-linked glycosylation site variation in highly variable viral glycoproteins: HIV, SIV, and HCV envelopes and influenza hemagglutinin. Glycobiology. 2004, 14: 1229-1246. 10.1093/glycob/cwh106.View ArticlePubMedGoogle Scholar
- Korber BT, Kunstman KJ, Patterson BK, Furtado M, McEvilly MM, Levy R, Wolinsky SM: Genetic differences between blood-and brain-derived viral sequences from human immunodeficiency virus type 1-infected patients: evidence of conserved elements in the V3 region of the envelope protein of brain-derived sequences. J Virol. 1994, 68: 7467-7481.PubMed CentralPubMedGoogle Scholar
- Wei X, Decker JM, Wang S, Hui H, Kappes JC, Wu X, Salazar-Gonzalez JF, Salazar MG, Kilby JM, Saag MS, et al: Antibody neutralization and escape by HIV-1. Nature. 2003, 422: 307-312. 10.1038/nature01470.View ArticlePubMedGoogle Scholar
- GraphPad: Graph pad software Inc.: Prism 5, version 5.04. 2010Google Scholar
- Arnold K, Bordoli L, Kopp J, Schwede T: The SWISS-MODEL workspace: a web-based environment for protein structure homology modelling. Bioinformatics. 2006, 22: 195-201. 10.1093/bioinformatics/bti770.View ArticlePubMedGoogle Scholar
- Schwede T, Kopp J, Guex N, Peitsch MC: SWISS-MODEL: an automated protein homology-modeling server. Nucleic Acids Res. 2003, 31: 3381-3385. 10.1093/nar/gkg520.PubMed CentralView ArticlePubMedGoogle Scholar
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