The role of HIV replicative fitness in perinatal transmission of HIV

Perinatal transmission of Human immunodeficiency virus(HIV),also called mother-to-child transmission(MTCT),accounts for 90% of infections in infants worldwide and occurs in 30%-45% of children born to untreated HIV-1 infected mothers.Among HIV-1 infected mothers,some viruses are transmitted from mothers to their infants while others are not.The relationship between virologic properties and the pathogenesis caused by HIV-1 remains unclear.Previous studies have demonstrated that one obvious source of selective pressure in the perinatal transmission of HIV-1 is maternal neutralizing antibodies.Recent studies have shown that viruses which are successfully transmitted to the child have growth advantages over those not transmitted,when those two viruses are grown together.Furthermore,the higher fitness is determined by the gp120 protein of the virus envelope.This suggests that the selective transmission of viruses with higher fitness occurred exclusively,regardless of transmission routes.There are many factors contributing to the selective transmission and HIV replicative fitness is an important one that should not be neglected.This review summarizes current knowledge of the role of HIV replicative fitness in HIV MTCT transmission and the determinants of viral fitness upon MTCT.     

The human immunodeficiency virus type 1 envelope confers higher rates of replicative fitness to perinatally transmitted viruses than to nontransmitted viruses

Selection of a minor viral genotype during perinatal transmission of human Immunodeficiency virus type 1 (HIV-1) has been observed, but there is a lack of information on the correlation of the restrictive transmission with biological properties of the virus, such as replicative fitness. Recombinant viruses expressing the enhanced green fluorescent protein or the Discosoma sp. red fluorescent (DsRed2) protein carrying the V1 to V5 regions of env from seven mother-infant pairs (MIPs) infected by subtype C HIV-1 were constructed, and competition assays were carried out to compare the fitness between the transmitted and nontransmitted viruses. Flow cytometry was used to quantify the frequency of infected cells, and the replicative fitness was determined based on a calculation that takes into account replication of competing viruses in a single infection versus dual infections. Transmitted viruses from five MIPs with the mothers chronically infected showed a restrictive env genotype, and all the recombinant viruses carrying the infants' Env had higher replicative fitness than those carrying the Env from the mothers. This growth fitness is lineage specific and can be observed only within the same MIP. In contrast, in two MIPs where the mothers had undergone recent acute infection, the viral Env sequences were similar between the mothers and infants and showed no further restriction in quasispecies during perinatal transmission. The recombinant viruses carrying the Env from the infants' viruses also showed replication fitness similar to those carrying the mothers' Env proteins. Our results suggest that newly transmitted viruses from chronically infected mothers have been selected to have higher replicative fitness to favor transmission, and this advantage is conferred by the V1 to V5 region of Env of the transmitted viruses. This finding has important implications for vaccine design or development of strategies to prevent HIV-1 transmission.     

The Role of HIV Replicative Fitness in Perinatal Transmission of HIV

Perinatal transmission of Human immunodeficiency virus(HIV),also called mother-to-child transmission(MTCT),accounts for 90% of infections in infants worldwide and occurs in 30%-45% of children born to untreated HIV-1 infected mothers.Among HIV-1 infected mothers,some viruses are transmitted from mothers to their infants while others are not.The relationship between virologic properties and the pathogenesis caused by HIV-1 remains unclear.Previous studies have demonstrated that one obvious source of selectiv...     

Cross-talk between virus and host innate immunity in pediatric HIV-1 infection and disease progression

Variability in the susceptibility to HIV-1 infection and disease progression depends on both virus and host determinants. Some exposed individuals remain HIV-1-uninfected and HIV-1-infected subjects develop disease at varying intervals with a small percentage remaining long-term non-progressors. As innate immunity is the earliest response to microbial entry and injury, host factors that impact innate immunity may play a role in viral infectivity and pathogenesis. In the pediatric population the interactions between the virus and the host may be of particular relevance due to the still developing adaptive immune system. Data indicate that genetic variants of defensins and Toll-Like Receptors (TLRs), key elements of innate immunity, play a role in mother-to-child transmission (MTCT) of HIV-1, and in the outcome of pediatric HIV-1 disease. Although the mechanisms by which these genetic variants influence HIV-1 interactions with the host are still largely unknown, defensins and TLRs, along with their link with regulatory T cells (Tregs), may play a critical role in the onset and persistence of immune activation, a hallmark of HIV-1 disease.     

The genetic bottleneck in vertical transmission of subtype C HIV-1 is not driven by selection of especially neutralization-resistant virus from the maternal viral population

Subtype C human immunodeficiency virus type 1 (HIV-1C) continues to cause the majority of new cases of mother-to-child transmission (MTCT), and yet there are limited data on HIV-1C transmission. We amplified env from plasma RNA for 19 HIV-1C MTCT pairs, 10 transmitting in utero (IU) and 9 transmitting intrapartum (IP). There was a strong genetic bottleneck between all mother-infant pairs, with a majority of transmission events involving the transmission of a single virus. env genes of viruses transmitted to infants IP, but not IU, encoded Env proteins that were shorter and had fewer putative N-linked glycosylation sites in the V1-V5 region than matched maternal sequences. Viruses pseudotyped with env clones representative of each maternal and infant population were tested for neutralization sensitivity. The 50% inhibitory concentration of autologous serum was similar against both transmitted (infant) and nontransmitted (maternal) viruses in a paired analysis. Mother and infant Env proteins were also similar in sensitivity to soluble CD4, to a panel of monoclonal antibodies, and to heterologous HIV-1C sera. In addition, there was no difference in the breadth or potency of neutralizing antibodies between sera from 50 nontransmitting and 23 IU and 23 IP transmitting HIV-1C-infected women against four Env proteins from heterologous viruses. Thus, while a strong genetic bottleneck was detected during MCTC, with viruses of shorter and fewer glycosylation sites in env present in IP transmission, our data do not support this bottleneck being driven by selective resistance to antibodies.     

Cell-Free (RNA) and Cell-Associated (DNA) HIV-1 and Postnatal Transmission through Breastfeeding

Introduction

Transmission through breastfeeding remains important for mother-to-child transmission (MTCT) in resource-limited settings. We quantify the relationship between cell-free (RNA) and cell-associated (DNA) shedding of HIV-1 virus in breastmilk and the risk of postnatal HIV-1 transmission in the first 6 months postpartum.

Materials and Methods

Thirty-six HIV-positive mothers who transmitted HIV-1 by breastfeeding were matched to 36 non-transmitting HIV-1 infected mothers in a case-control study nested in a cohort of HIV-infected women. RNA and DNA were quantified in the same breastmilk sample taken at 6 weeks and 6 months. Cox regression analysis assessed the association between cell-free and cell-associated virus levels and risk of postnatal HIV-1 transmission.

Results

There were higher median levels of cell-free than cell-associated HIV-1 virus (per ml) in breastmilk at 6 weeks and 6 months. Multivariably, adjusting for antenatal CD4 count and maternal plasma viral load, at 6 weeks, each 10-fold increase in cell-free or cell-associated levels (per ml) was significantly associated with HIV-1 transmission but stronger for cell-associated than cell-free levels [2.47 (95% CI 1.33–4.59) vs. aHR 1.52 (95% CI, 1.17–1.96), respectively]. At 6 months, cell-free and cell-associated levels (per ml) in breastmilk remained significantly associated with HIV-1 transmission but was stronger for cell-free than cell-associated levels [aHR 2.53 (95% CI 1.64–3.92) vs. 1.73 (95% CI 0.94–3.19), respectively].

Conclusions

The findings suggest that cell-associated virus level (per ml) is more important for early postpartum HIV-1 transmission (at 6 weeks) than cell-free virus. As cell-associated virus levels have been consistently detected in breastmilk despite antiretroviral therapy, this highlights a potential challenge for resource-limited settings to achieve the UNAIDS goal for 2015 of eliminating vertical transmission. More studies would further knowledge on mechanisms of HIV-1 transmission and help develop more effective drugs during lactation.     

Different regions of HIV-1 subtype C env are associated with placental localization and in utero mother-to-child transmission

HIV infections are initiated by a limited number of variants that diverge into a diverse quasispecies swarm. During in utero mother-to-child transmission (IU MTCT), transmitted viral variants must pass through multiple unique environments, and our previously published data suggest a nonstochastic model of transmission. As an alternative to a stochastic model of viral transmission, we hypothesize that viral selection in the placental environment influences the character of the viral quasispecies when HIV-1 is transmitted in utero. To test this hypothesis, we used single-template amplification to isolate HIV-1 envelope gene (env) sequences from both peripheral plasma and the placentas of eight nontransmitting (NT) and nine IU-transmitting participants. Statistically significant compartmentalization between peripheral and placental HIV-1 env was detected in one of the eight NT cases and six of the nine IU MTCT cases. In addition, viral sequences isolated from IU MTCT placental tissue showed variation in env V1 loop lengths compared to matched maternal sequences, while NT placental env sequences did not. Finally, comparison of env sequences from NT and IU MTCT participants indicated statistically significant differences in Kyte-Doolittle hydropathy in the signal peptide, C2, V3, and C3 regions. Our working hypothesis is that the hydropathy differences in Env associated with IU MTCT alter viral cellular tropism or affinity, allowing HIV-1 to efficiently infect placentally localized cells.     

Maternal-fetal transmission of human immunodeficiency virus

The World Health Organization estimates that by year 2000, 10 million children will be infected with human immunodeficiency virus type 1 (HIV-1) at birth and will subsequently develop AIDS. Perinatally acquired infections account for the majority of all HIV-1 cases in children, with an estimated mother-to-infant transmission rate of more than 30%. It is not clear why more than half of the children born to HIV-1-infected mothers are uninfected. Maternal transmission of HIV-1 occurs at three levels: prepartum, intrapartum, and postpartum. Several maternal parameters including advanced clinical stages of the mother, low CD4+ lymphocyte counts, maternal immune response to HIV-1, recent infection, high level of circulating HIV-1, and maternal disease progression have been implicated in an increased risk of mother-to-infant transmission of HIV-1. Viral factors influencing mother-to-infant transmission are not known. Furthermore, several other factors such as acute infection during pregnancy, presence of other sexually transmitted diseases (STD) or other chronic infections, vaginal bleeding, disruption of placental integrity, premature rupture of membrane (PROM), and preterm PROM have been associated with mother-to-infant transmission of HIV-1. In addition, tobacco and cigarette smoking during pregnancy have been shown to triple the rate of maternal transmission of HIV-1. The AIDS Clinical Trial Group (ACTG) suggested that zidovudine (ZDV) can reduce the rate of mother-to-infant transmission of HIV-1 if administered to HIV-1-infected pregnant women with CD4 counts greater than 200. Moreover, this study failed to take into consideration several factors that may influence maternal transmission of HIV-1. However, the molecular mechanisms involved in mother-to-infant transmission of HIV-1 are not understood, which makes it more difficult to define strategies for effective treatment and prevention of HIV-1 infection in children. Several groups are engaged in the understanding of the molecular and biological properties of HIV-1 influencing mother-to-infant transmission. Results from my and several other laboratories suggest that the minor genotypes, subtypes, or variants of HIV-1 found in a genetically heterogeneous virus population of infected mothers are transmitted to their infants. The minor HIV-1 genotype predominates initially as a homogeneous population in the infant and then becomes diverse as the infant matures. Furthermore, transmission of a major or multiple HIV-1 genotypes from mother to infant has been reported. Taken together, these results strongly suggest that there are differences among the molecular and biological properties of the maternal variants that are transmitted to the infants and the maternal variants that are not transmitted to the infants. The understanding of the molecular and biological properties of the transmitted viruses will enable researchers to target a particular subtype in the mothers that is transmitted to the infants.     

Initial B-cell responses to transmitted human immunodeficiency virus type 1: virion-binding immunoglobulin M (IgM) and IgG antibodies followed by plasma anti-gp41 antibodies with ineffective control of initial viremia

A window of opportunity for immune responses to extinguish human immunodeficiency virus type 1 (HIV-1) exists from the moment of transmission through establishment of the latent pool of HIV-1-infected cells. A critical time to study the initial immune responses to the transmitted/founder virus is the eclipse phase of HIV-1 infection (time from transmission to the first appearance of plasma virus), but, to date, this period has been logistically difficult to analyze. To probe B-cell responses immediately following HIV-1 transmission, we have determined envelope-specific antibody responses to autologous and consensus Envs in plasma donors from the United States for whom frequent plasma samples were available at time points immediately before, during, and after HIV-1 plasma viral load (VL) ramp-up in acute infection, and we have modeled the antibody effect on the kinetics of plasma viremia. The first detectable B-cell response was in the form of immune complexes 8 days after plasma virus detection, whereas the first free plasma anti-HIV-1 antibody was to gp41 and appeared 13 days after the appearance of plasma virus. In contrast, envelope gp120-specific antibodies were delayed an additional 14 days. Mathematical modeling of the earliest viral dynamics was performed to determine the impact of antibody on HIV replication in vivo as assessed by plasma VL. Including the initial anti-gp41 immunoglobulin G (IgG), IgM, or both responses in the model did not significantly impact the early dynamics of plasma VL. These results demonstrate that the first IgM and IgG antibodies induced by transmitted HIV-1 are capable of binding virions but have little impact on acute-phase viremia at the timing and magnitude that they occur in natural infection.     

Mother-to-Child HIV-1 Transmission Events Are Differentially Impacted by Breast Milk and Its Components from HIV-1-Infected Women

Breast milk is a vehicle of infection and source of protection in post-natal mother-to-child HIV-1 transmission (MTCT). Understanding the mechanism by which breast milk limits vertical transmission will provide critical insight into the design of preventive and therapeutic approaches to interrupt HIV-1 mucosal transmission. However, characterization of the inhibitory activity of breast milk in human intestinal mucosa, the portal of entry in postnatal MTCT, has been constrained by the limited availability of primary mucosal target cells and tissues to recapitulate mucosal transmission ex vivo. Here, we characterized the impact of skimmed breast milk, breast milk antibodies (Igs) and non-Ig components from HIV-1-infected Ugandan women on the major events of HIV-1 mucosal transmission using primary human intestinal cells and tissues. HIV-1-specific IgG antibodies and non-Ig components in breast milk inhibited the uptake of Ugandan HIV-1 isolates by primary human intestinal epithelial cells, viral replication in and transport of HIV-1- bearing dendritic cells through the human intestinal mucosa. Breast milk HIV-1-specific IgG and IgA, as well as innate factors, blocked the uptake and transport of HIV-1 through intestinal mucosa. Thus, breast milk components have distinct and complementary effects in reducing HIV-1 uptake, transport through and replication in the intestinal mucosa and, therefore, likely contribute to preventing postnatal HIV-1 transmission. Our data suggests that a successful preventive or therapeutic approach would require multiple immune factors acting at multiple steps in the HIV-1 mucosal transmission process.     

Uterine epithelial cell regulation of DC-SIGN expression inhibits transmitted/founder HIV-1 trans infection by immature dendritic cells

Background

Sexual transmission accounts for the majority of HIV-1 infections. In over 75% of cases, infection is initiated by a single variant (transmitted/founder virus). However, the determinants of virus selection during transmission are unknown. Host cell-cell interactions in the mucosa may be critical in regulating susceptibility to infection. We hypothesized in this study that specific immune modulators secreted by uterine epithelial cells modulate susceptibility of dendritic cells (DC) to infection with HIV-1.

Methodology/Principal Findings

Here we report that uterine epithelial cell secretions (i.e. conditioned medium, CM) decreased DC-SIGN expression on immature dendritic cells via a transforming growth factor beta (TGF-β) mechanism. Further, CM inhibited dendritic cell-mediated trans infection of HIV-1 expressing envelope proteins of prototypic reference. Similarly, CM inhibited trans infection of HIV-1 constructs expressing envelopes of transmitted/founder viruses, variants that are selected during sexual transmission. In contrast, whereas recombinant TGF- β1 inhibited trans infection of prototypic reference HIV-1 by dendritic cells, TGF-β1 had a minimal effect on trans infection of transmitted/founder variants irrespective of the reporter system used to measure trans infection.

Conclusions/Significance

Our results provide the first direct evidence for uterine epithelial cell regulation of dendritic cell transmission of infection with reference and transmitted/founder HIV-1 variants. These findings have immediate implications for designing strategies to prevent sexual transmission of HIV-1.     

HIV-1 with multiple CCR5/CXCR4 chimeric receptor use is predictive of immunological failure in infected children

Background

HIV-1 R5 viruses are characterized by a large phenotypic variation, that is reflected by the mode of coreceptor use. The ability of R5 HIV-1 to infect target cells expressing chimeric receptors between CCR5 and CXCR4 (R5^broad viruses), was shown to correlate with disease stage in HIV-1 infected adults. Here, we ask the question whether phenotypic variation of R5 viruses could play a role also in mother-to-child transmission (MTCT) of HIV-1 and pediatric disease progression.

Methodology/Principal Findings

Viral isolates obtained from a total of 59 HIV-1 seropositive women (24 transmitting and 35 non transmitting) and 28 infected newborn children, were used to infect U87.CD4 cells expressing wild type or six different CCR5/CXCR4 chimeric receptors. HIV-1 isolates obtained from newborn infants had predominantly R5^narrow phenotype (n = 20), but R5^broad and R5X4 viruses were also found in seven and one case, respectively. The presence of R5^broad and R5X4 phenotypes correlated significantly with a severe decline of the CD4+ T cells (CDC stage 3) or death within 2 years of age. Forty-three percent of the maternal R5 isolates displayed an R5^broad phenotype, however, the presence of the R5^broad virus was not predictive for MTCT of HIV-1. Of interest, while only 1 of 5 mothers with an R5X4 virus transmitted the dualtropic virus, 5 of 6 mothers carrying R5^broad viruses transmitted viruses with a similar broad chimeric coreceptor usage. Thus, the maternal R5^broad phenotype was largely preserved during transmission and could be predictive of the phenotype of the newborn''s viral variant.

Conclusions/Significance

Our results show that R5^broad viruses are not hampered in transmission. When transmitted, immunological failure occurs earlier than in children infected with HIV-1 of R5^narrow phenotype. We believe that this finding is of utmost relevance for therapeutic interventions in pediatric HIV-1 infection.     

Adaptation to human populations is revealed by within-host polymorphisms in HIV-1 and hepatitis C virus

CD8(+) cytotoxic T-lymphocytes (CTLs) perform a critical role in the immune control of viral infections, including those caused by human immunodeficiency virus type 1 (HIV-1) and hepatitis C virus (HCV). As a result, genetic variation at CTL epitopes is strongly influenced by host-specific selection for either escape from the immune response, or reversion due to the replicative costs of escape mutations in the absence of CTL recognition. Under strong CTL-mediated selection, codon positions within epitopes may immediately "toggle" in response to each host, such that genetic variation in the circulating virus population is shaped by rapid adaptation to immune variation in the host population. However, this hypothesis neglects the substantial genetic variation that accumulates in virus populations within hosts. Here, we evaluate this quantity for a large number of HIV-1- (n > or = 3,000) and HCV-infected patients (n > or = 2,600) by screening bulk RT-PCR sequences for sequencing "mixtures" (i.e., ambiguous nucleotides), which act as site-specific markers of genetic variation within each host. We find that nonsynonymous mixtures are abundant and significantly associated with codon positions under host-specific CTL selection, which should deplete within-host variation by driving the fixation of the favored variant. Using a simple model, we demonstrate that this apparently contradictory outcome can be explained by the transmission of unfavorable variants to new hosts before they are removed by selection, which occurs more frequently when selection and transmission occur on similar time scales. Consequently, the circulating virus population is shaped by the transmission rate and the disparity in selection intensities for escape or reversion as much as it is shaped by the immune diversity of the host population, with potentially serious implications for vaccine design.     

Heterosexual Transmission of Subtype C HIV-1 Selects Consensus-Like Variants without Increased Replicative Capacity or Interferon-α Resistance

Heterosexual transmission of HIV-1 is characterized by a genetic bottleneck that selects a single viral variant, the transmitted/founder (TF), during most transmission events. To assess viral characteristics influencing HIV-1 transmission, we sequenced 167 near full-length viral genomes and generated 40 infectious molecular clones (IMC) including TF variants and multiple non-transmitted (NT) HIV-1 subtype C variants from six linked heterosexual transmission pairs near the time of transmission. Consensus-like genomes sensitive to donor antibodies were selected for during transmission in these six transmission pairs. However, TF variants did not demonstrate increased viral fitness in terms of particle infectivity or viral replicative capacity in activated peripheral blood mononuclear cells (PBMC) and monocyte-derived dendritic cells (MDDC). In addition, resistance of the TF variant to the antiviral effects of interferon-α (IFN-α) was not significantly different from that of non-transmitted variants from the same transmission pair. Thus neither in vitro viral replicative capacity nor IFN-α resistance discriminated the transmission potential of viruses in the quasispecies of these chronically infected individuals. However, our findings support the hypothesis that within-host evolution of HIV-1 in response to adaptive immune responses reduces viral transmission potential.     

Transmitted Virus Fitness and Host T Cell Responses Collectively Define Divergent Infection Outcomes in Two HIV-1 Recipients

Control of virus replication in HIV-1 infection is critical to delaying disease progression. While cellular immune responses are a key determinant of control, relatively little is known about the contribution of the infecting virus to this process. To gain insight into this interplay between virus and host in viral control, we conducted a detailed analysis of two heterosexual HIV-1 subtype A transmission pairs in which female recipients sharing three HLA class I alleles exhibited contrasting clinical outcomes: R880F controlled virus replication while R463F experienced high viral loads and rapid disease progression. Near full-length single genome amplification defined the infecting transmitted/founder (T/F) virus proteome and subsequent sequence evolution over the first year of infection for both acutely infected recipients. T/F virus replicative capacities were compared in vitro, while the development of the earliest cellular immune response was defined using autologous virus sequence-based peptides. The R880F T/F virus replicated significantly slower in vitro than that transmitted to R463F. While neutralizing antibody responses were similar in both subjects, during acute infection R880F mounted a broad T cell response, the most dominant components of which targeted epitopes from which escape was limited. In contrast, the primary HIV-specific T cell response in R463F was focused on just two epitopes, one of which rapidly escaped. This comprehensive study highlights both the importance of the contribution of the lower replication capacity of the transmitted/founder virus and an associated induction of a broad primary HIV-specific T cell response, which was not undermined by rapid epitope escape, to long-term viral control in HIV-1 infection. It underscores the importance of the earliest CD8 T cell response targeting regions of the virus proteome that cannot mutate without a high fitness cost, further emphasizing the need for vaccines that elicit a breadth of T cell responses to conserved viral epitopes.     

Design and Development of a Multiplex Real-Time PCR Assay for Detection of HIV-1 and HCV Using Molecular Beacons

At least 10 million individuals worldwide are co-infected with immunodeficiency virus type 1 (HIV-1) and hepatitis C virus (HCV). These two viruses are transmitted most primarily by exposure to infected blood or blood products. Various nucleic acid assays have been developed for diagnostics and therapeutic monitoring of infections. In the present study, a multiplex real-time PCR assay for simultaneous detection of HCV and HIV-1 using molecular beacons were designed and validated. A well-conserved region in the HIV-1 pol gene and 5′NCR of HCV genome were used for primers and molecular beacon design. The analysis of scalar concentrations of the samples indicated that this multiplex procedure detects at least 1,000 copies/ml of HIV-1 and 100 copies/ml of HCV with linear reference curve (R ² > 0.94). The results demonstrate that a specificity of 100 % and sensitivity of 96 % can be achieved. The analytical sensitivity study with BLAST software demonstrated that the primers do not attach to any other sequences except for that of HIV-1 or HCV. The primers and molecular beacon probes only detected HIV-1 and all major variants of HCV. This assay may represent an alternative rapid and relatively inexpensive screening method for detection of HIV-1/HCV co-infection especially in blood screening.     

Evidence of Long-Lived Founder Virus in Mother-to-Child HIV Transmission

Exposure of the infant’s gut to cell-associated and cell-free HIV-1 trafficking in breast milk (BM) remains a primary cause of mother-to-child transmission (MTCT). The mammary gland represents a unique environment for HIV-1 replication and host-virus interplay. We aimed to explore the origin of the virus transmitted during breastfeeding, and the link with quasi-species found in acellular and cellular fractions of breast-milk (BM) and in maternal plasma. The C2–V5 region of the env gene was amplified, cloned and sequenced from the RNA and DNA of BM, the RNA from the mother’s plasma (PLA) and the DNA from infant’s dried blood spot (DBS) in 11 post-natal mother-infant pairs. Sequences were assembled in Geneious, aligned in ClustalX, manually edited in SeAL and phylogenetic reconstruction was undertaken in PhyML and MrBayes. We estimated the timing of transmission (ETT) and reconstructed the time for the most recent common ancestor (TMRCA) of the infant in BEAST. Transmission of single quasi-species was observed in 9 of 11 cases. Phylogenetic analysis illustrated a BM transmission event by cell-free virus in 4 cases, and by cell-associated virus in 2 cases but could not be identified in the remaining 5 cases. Molecular clock estimates, of the infant ETT and TMRCA, corresponded well with the timing of transmission estimated by sequential infant DNA PCR in 10 of 11 children. The TMRCA of BM variants were estimated to emerge during gestation in 8 cases. We hypothesize that in the remaining cases, the breast was seeded with a long-lived lineage latently infecting resting T-cells. Our analysis illustrated the role of DNA and RNA virus in MTCT. We postulate that DNA archived viruses stem from latently infected quiescent T-cells within breast tissue and MTCT can be expected to continue, albeit at low levels, should interventions not effectively target these cells.     

Passive sexual transmission of human immunodeficiency virus type 1 variants and adaptation in new hosts

Human immunodeficiency virus type 1 (HIV-1) genetic diversity is a major obstacle for the design of a successful vaccine. Certain viral polymorphisms encode human leukocyte antigen (HLA)-associated immune escape, potentially overcoming limited vaccine protection. Although transmission of immune escape variants has been reported, the overall extent to which this phenomenon occurs in populations and the degree to which it contributes to HIV-1 viral evolution are unknown. Selection on the HIV-1 env gene at transmission favors neutralization-sensitive variants, but it is not known to what degree selection acts on the internal HIV-1 proteins to restrict or enhance the transmission of immune escape variants. Studies have suggested that HLA class I may determine susceptibility to HIV-1 infection, but a definitive role for HLA at transmission remains unproven. Comparing populations of acute seroconverters and chronically infected patients, we found no evidence of selection acting to restrict transmission of HIV-1 variants. We found that statistical associations previously reported in chronic infection between viral polymorphisms and HLA class I alleles are not present in acute infection, suggesting that the majority of viral polymorphisms in these patients are the result of transmission rather than de novo adaptation. Using four episodes of HIV-1 transmission in which the donors and recipients were both sampled very close to the time of infection we found that, despite a transmission bottleneck, genetic variants of HIV-1 infection are transmitted in a frequency-dependent manner. As HIV-1 infections are seeded by unique donor-adapted viral variants, each episode is a highly individual antigenic challenge. Host-specific, idiosyncratic HIV-1 antigenic diversity will seriously tax the efficacy of immunization based on consensus sequences.