Vertical HIV-1 transmission: importance of neutralizing antibody titer and specificity

Mutations that confer resistance to broadly-neutralizing antibodies define HIV-1 variants of transmitting mothers from that of non-transmitting mothers

Despite considerable reduction of mother-to-child transmission (MTCT) of HIV through use of maternal and infant antiretroviral therapy (ART), over 150,000 infants continue to become infected with HIV annually, falling far short of the World Health Organization goal of reaching <20,000 annual pediatric HIV cases worldwide by 2020. Prior to the widespread use of ART in the setting of pregnancy, over half of infants born to HIV-infected mothers were protected against HIV acquisition. Yet, the role of maternal immune factors in this protection against vertical transmission is still unclear, hampering the development of synergistic strategies to further reduce MTCT. It has been established that infant transmitted/founder (T/F) viruses are often resistant to maternal plasma, yet it is unknown if the neutralization resistance profile of circulating viruses predicts the maternal risk of transmission to her infant. In this study, we amplified HIV-1 envelope genes (env) by single genome amplification and produced representative Env variants from plasma of 19 non-transmitting mothers from the U.S. Women Infant Transmission Study (WITS), enrolled in the pre-ART era. Maternal HIV Env variants from non-transmitting mothers had similar sensitivity to autologous plasma as observed for non-transmitting variants from transmitting mothers. In contrast, infant variants were on average 30% less sensitive to paired plasma neutralization compared to non-transmitted maternal variants from both transmitting and non-transmitting mothers (p = 0.015). Importantly, a signature sequence analysis revealed that motifs enriched in env sequences from transmitting mothers were associated with broadly neutralizing antibody (bnAb) resistance. Altogether, our findings suggest that circulating maternal virus resistance to bnAb-mediated neutralization, but not autologous plasma neutralization, near the time of delivery, predicts increased MTCT risk. These results caution that enhancement of maternal plasma neutralization through passive or active vaccination during pregnancy may potentially drive the evolution of variants fit for vertical transmission.

Despite widespread, effective use of ART among HIV infected pregnant women, new pediatric HIV infections increase by about 150,000 every year. Thus, alternative strategies will be required to reduce MTCT and eliminate pediatric HIV infections. Interestingly, in the absence of ART, less than half of HIV-infected pregnant women will transmit HIV, suggesting natural immune protection of infants from virus acquisition. To understand the impact of maternal plasma autologous virus neutralization responses on MTCT, we compared the plasma and bnAb neutralization sensitivity of the circulating viral population present at the time of delivery in untreated, HIV-infected transmitting and non-transmitting mothers. While there was no significant difference in the ability of transmitting and non-transmitting women to neutralize their own circulating virus strains, specific genetic motifs enriched in variants from transmitting mothers were associated with resistance to bnAbs, suggesting that acquired bnAb resistance is a common feature of vertically-transmitted variants. This work suggests that enhancement of plasma neutralization responses in HIV-infected mothers through passive or active vaccination could further drive selection of variants that could be vertically transmitted, and cautions the use of passive bnAbs for HIV-1 prophylaxis or therapy during pregnancy.

Infant transmitted/founder HIV-1 viruses from peripartum transmission are neutralization resistant to paired maternal plasma

Despite extensive genetic diversity of HIV-1 in chronic infection, a single or few maternal virus variants become the founders of an infant’s infection. These transmitted/founder (T/F) variants are of particular interest, as a maternal or infant HIV vaccine should raise envelope (Env) specific IgG responses capable of blocking this group of viruses. However, the maternal or infant factors that contribute to selection of infant T/F viruses are not well understood. In this study, we amplified HIV-1 env genes by single genome amplification from 16 mother-infant transmitting pairs from the U.S. pre-antiretroviral era Women Infant Transmission Study (WITS). Infant T/F and representative maternal non-transmitted Env variants from plasma were identified and used to generate pseudoviruses for paired maternal plasma neutralization sensitivity analysis. Eighteen out of 21 (85%) infant T/F Env pseudoviruses were neutralization resistant to paired maternal plasma. Yet, all infant T/F viruses were neutralization sensitive to a panel of HIV-1 broadly neutralizing antibodies and variably sensitive to heterologous plasma neutralizing antibodies. Also, these infant T/F pseudoviruses were overall more neutralization resistant to paired maternal plasma in comparison to pseudoviruses from maternal non-transmitted variants (p = 0.012). Altogether, our findings suggest that autologous neutralization of circulating viruses by maternal plasma antibodies select for neutralization-resistant viruses that initiate peripartum transmission, raising the speculation that enhancement of this response at the end of pregnancy could further reduce infant HIV-1 infection risk.

Mother to child transmission (MTCT) of HIV-1 can occur during pregnancy (in utero), at the time of delivery (peripartum) or by breastfeeding (postpartum). With the availability of anti-retroviral therapy (ART), rate of MTCT of HIV-1 have been significantly lowered. However, significant implementation challenges remain in resource-poor areas, making it difficult to eliminate pediatric HIV. An improved understanding of the viral population (escape variants from autologous neutralizing antibodies) that lead to infection of infants at time of transmission will help in designing immune interventions to reduce perinatal HIV-1 transmission. Here, we selected 16 HIV-1-infected mother-infant pairs from WITS cohort (from pre anti-retroviral era), where infants became infected peripartum. HIV-1 env gene sequences were obtained by the single genome amplification (SGA) method. The sensitivity of these infant Env pseudoviruses against paired maternal plasma and a panel of broadly neutralizing monoclonal antibodies (bNAbs) was analyzed. We demonstrated that the infant T/F viruses were more resistant against maternal plasma than non-transmitted maternal variants, but sensitive to most (bNAbs). Signature sequence analysis of infant T/F and non-transmitted maternal variants revealed the potential importance of V3 and MPER region for resistance against paired maternal plasma. These findings provide insights for the design of maternal immunization strategies to enhance neutralizing antibodies that target V3 region of autologous virus populations, which could work synergistically with maternal ARVs to further reduce the rate of peripartum HIV-1 transmission.

The Role of Immune Responses in HIV Mother-to-Child Transmission

HIV mother-to-child transmission (MTCT) represents a success story in the HIV/AIDS field given the significant reduction in number of transmission events with the scale-up of antiretroviral treatment and other prevention methods. Nevertheless, MTCT still occurs and better understanding of the basic biology and immunology of transmission will aid in future prevention and treatment efforts. MTCT is a unique setting given that the transmission pair is known and the infant receives passively transferred HIV-specific antibodies from the mother while in utero. Thus, infant exposure to HIV occurs in the face of HIV-specific antibodies, especially during delivery and breastfeeding. This review highlights the immune correlates of protection in HIV MTCT including humoral (neutralizing antibodies, antibody-dependent cellular cytotoxicity, and binding epitopes), cellular, and innate immune factors. We further discuss the future implications of this research as it pertains to opportunities for passive and active vaccination with the ultimate goal of eliminating HIV MTCT.

Maternal Neutralization-Resistant Virus Variants Do Not Predict Infant HIV Infection Risk

Mother-to-child transmission (MTCT) of HIV provides a setting for studying immune correlates of protection. Neutralizing antibodies (NAbs) are suggested to contribute to a viral bottleneck during MTCT, but their role in blocking transmission is unclear, as studies comparing the NAb sensitivities of maternal viruses have yielded disparate results. We sought to determine whether transmitting mothers differ from nontransmitting mothers in the ability to neutralize individual autologous virus variants present at transmission. Ten transmitting and 10 nontransmitting HIV-infected mothers at high risk of MTCT were included in this study. Full-length HIV envelope genes (n = 100) were cloned from peripheral blood mononuclear cells obtained near transmission from transmitting mothers and at similar time points from nontransmitting mothers. Envelope clones were tested as pseudoviruses against contemporaneous, autologous maternal plasma in neutralization assays. The association between transmission and the log₂ 50% inhibitory concentration (IC₅₀) for multiple virus variants per mother was estimated by using logistic regression with clustered standard errors. t tests were used to compare proportions of neutralization-resistant viruses. Overall, transmitting mothers had a median IC₅₀ of 317 (interquartile range [IQR], 202 to 521), and nontransmitting mothers had a median IC₅₀ of 243 (IQR, 95 to 594). Transmission risk was not significantly associated with autologous NAb activity (odds ratio, 1.25; P = 0.3). Compared to nontransmitting mothers, transmitting mothers had similar numbers of or fewer neutralization-resistant virus variants, depending on the IC₅₀ neutralization resistance cutoff. In conclusion, HIV-infected mothers harbor mostly neutralization-sensitive viruses, although resistant variants were detected in both transmitting and nontransmitting mothers. These results suggest that MTCT during the breastfeeding period is not driven solely by the presence of maternal neutralization escape variants.

There are limited data demonstrating whether NAbs can prevent HIV transmission and infection in humans, and for this reason, NAbs have been studied in MTCT, where maternal antibodies are present at the time of transmission. Results of these studies have varied, perhaps because of differences in methods. Importantly, studies often used cultured viruses and samples from time points outside the window of transmission, which could confound findings. Here, we considered the role of maternal NAbs against individual maternal virus variants near the time of transmission. We found no evidence that NAbs are associated with protection from infection. In fact, depending on the cutoff used to define neutralization resistance, we found evidence that nontransmitting mothers have more neutralization-resistant virus variants. These results suggest that lack of virus transmission in the early breastfeeding period is not simply due to an absence of maternal neutralization escape variants and likely includes multiple factors.

The role of neutralizing antibodies in prevention of HIV-1 infection: what can we learn from the mother-to-child transmission context?

In most viral infections, protection through existing vaccines is linked to the presence of vaccine-induced neutralizing antibodies (NAbs). However, more than 30 years after the identification of AIDS, the design of an immunogen able to induce antibodies that would neutralize the highly diverse HIV-1 variants remains one of the most puzzling challenges of the human microbiology. The role of antibodies in protection against HIV-1 can be studied in a natural situation that is the mother-to-child transmission (MTCT) context. Indeed, at least at the end of pregnancy, maternal antibodies of the IgG class are passively transferred to the fetus protecting the neonate from new infections during the first weeks or months of life. During the last few years, strong data, presented in this review, have suggested that some NAbs might confer protection toward neonatal HIV-1 infection. In cases of transmission, it has been shown that the viral population that is transmitted from the mother to the infant is usually homogeneous, genetically restricted and resistant to the maternal HIV-1-specific antibodies. Although the breath of neutralization was not associated with protection, it has not been excluded that NAbs toward specific HIV-1 strains might be associated with a lower rate of MTCT. A better identification of the antibody specificities that could mediate protection toward MTCT of HIV-1 would provide important insights into the antibody responses that would be useful for vaccine development. The most convincing data suggesting that NAbs migh confer protection against HIV-1 infection have been obtained by experiments of passive immunization of newborn macaques with the first generation of human monoclonal broadly neutralizing antibodies (HuMoNAbs). However, these studies, which included only a few selected subtype B challenge viruses, provide data limited to protection against a very restricted number of isolates and therefore have limitations in addressing the hypervariability of HIV-1. The recent identification of highly potent second-generation cross-clade HuMoNAbs provides a new opportunity to evaluate the efficacy of passive immunization to prevent MTCT of HIV-1.

Short communication: neutralizing antibodies in HIV-1-infected Brazilian individuals

Tests for the detection of the humoral immune response to HIV-1 have to be standardized and established, demanding regional efforts. For this purpose the neutralizing antibody (NAb) assay for HIV-1 in TZM-bl cells was introduced in Brazil. Twenty plasma samples from HIV-1-infected individuals were assayed: 10 progressors and 10 long-term nonprogressors. These were tested against eight env-pseudotyped viruses (psVs) in the TZM-bl NAb assay and against HIV-1 strain HTLV/IIIB (HIV-1 IIIB) in primary lymphocytes. Forty-four percent of the samples showed neutralizing titers for psVs and 55% for HIV-1 IIIB. Plasma from progressors showed a broader neutralization and a higher potency. The introduction of these reference reagents encourages the participation of Brazil in future comparative assessments of anti-HIV-1 antibodies.

The breadth and titer of maternal HIV-1-specific heterologous neutralizing antibodies are not associated with a lower rate of mother-to-child transmission of HIV-1

It has been hypothesized that neutralizing antibodies (NAbs) should have broad specificity to be effective in protection against diverse HIV-1 variants. The mother-to-child transmission model of HIV-1 provides the opportunity to examine whether the breadth of maternal NAbs is associated with protection of infants from infection. Samples were obtained at delivery from 57 transmitting mothers (T) matched with 57 nontransmitting mothers (NT) enrolled in the multicenter French perinatal cohort (ANRS EPF CO1) between 1990 and 1996. Sixty-eight (59.6%) and 46 (40.4%) women were infected by B and non-B viruses, respectively. Neutralization assays were carried out with TZM-bl cells, using a panel of 10 primary isolates of 6 clades (A, B, C, F, CRF01_AE, and CRF02_AG), selected for their moderate or low sensitivity to neutralization. Neutralization breadths were not statistically different between T and NT mothers. However, a few statistically significant differences were observed, with higher frequencies or titers of NAbs toward several individual strains for NT mothers when the clade B-infected or non-clade B-infected mothers were analyzed separately. Our study confirms that the breadth of maternal NAbs is not associated with protection of infants from infection.

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.

The role of antibodies in HIV vaccines

Licensed vaccines against viral diseases generate antibodies that neutralize the infecting virus and protect against infection or disease. Similarly, an effective vaccine against HIV-1 will likely need to induce antibodies that prevent initial infection of host cells or that limit early events of viral dissemination. Such antibodies must target the surface envelope glycoproteins of HIV-1, which are highly variable in sequence and structure. The first subunit vaccines to enter clinical trails were safe and immunogenic but unable to elicit antibodies that neutralized most circulating strains of HIV-1. However, potent virus neutralizing antibodies (NAbs) can develop during the course of HIV-1 infection, and this is the type of antibody response that researchers seek to generate with a vaccine. Thus, current vaccine design efforts have focused on a more detailed understanding of these broadly neutralizing antibodies and their epitopes to inform the design of improved vaccines.

Mother to child transmission of HIV-1 in a Thai population: role of virus characteristics and maternal humoral immune response

The objective of this study was to investigate factors influencing mother to child transmission of HIV-1 in Thailand, where HIV-1 CRF01_AE, the major subtype in Southeast Asia, predominates. Samples from 84 HIV-1 infected, anti-retroviral treatment-naïve, non-breast feeding mothers, 28 who transmitted HIV-1 to their babies (transmitters) and 56 who did not (non-transmitters), were studied for maternal humoral immune response and virus characteristics. Maternal humoral immune response was measured by lymphocyte phenotyping; neutralizing antibodies to laboratory HIV-1 MN strain and two clinical isolates; peptide binding antibody to gp41 and V3 from strains CRF01_AE, B, and MN; autologous antibodies; and quasispecies diversity. Virus characteristics studied were viral load, co-receptor usage, and viral replication capacity. No significant difference between transmitters and non-transmitters was found for any parameter of maternal humoral immune response. However, viral load and viral replication capacity were significantly higher in transmitters versus non-transmitters and were not correlated with each other. This suggests that viral replication capacity may be a transmission factor independent of viral load, which is already well established as a risk factor for transmission of HIV-1. All except four viral isolates used the CCR5 co-receptor. This is one of few studies of vertical transmission in a population where HIV-1 CRF01_AE predominates. The data suggest that in this population the maternal humoral immune response was not important in preventing transmission at parturition, but that virus characteristics were key factors, and that viral replication capacity may contribute to birth-associated mother to child transmission of HIV-1.

Maternal neutralizing antibodies against a CRF01_AE primary isolate are associated with a low rate of intrapartum HIV-1 transmission

Mother-to-child transmission (MTCT) of HIV-1 provides a model for studying the role of passively acquired antibodies in preventing HIV infection. We determined the titers of neutralizing antibodies (NAbs) against six primary isolates of clades B and CRF01_AE in sera from 45 transmitting and 45 nontransmitting mothers matched for the main independent factors associated with MTCT in Thailand. A lower risk of MTCT, particularly for intrapartum transmission, was associated only with higher NAb titers against the CRF01_AE strain, MBA. The envelope glycoprotein of this strain showed an unusually long V2 domain of 63 amino acids, encoding six potential N-linked glycosylation sites. We provided experimental data indicating that the extended V2 domain contributed to the higher level of resistance to neutralization by mothers' sera in this strain. Taken together the data suggest that some primary isolates with specific properties may be useful indicators for identifying protective antibodies.

Immunogenicity comparison between codon optimized HIV-1 CRF BC_07 gp140 and gp145 vaccines

To develop an effective vaccine against the most prevalent HIV strain "B'/C recombinant" in China, we compared the immunogenicity of B'/C-derived gp140 and gp145. The codon optimized gp140 and gp145 env gene derived from CN54, an ancestor-like B'/C recombinant strain, were synthesized and cloned into a plasmid as DNA vaccines, designated as pDRVISV140 and pDRVISV145, respectively. BALB/c mice were inoculated three times at week 0, 2, and 4 and sacrificed at week 7. Both T cell immunity and humoral immunity were determined. The mock vector pDRVISV1.0 carrying no HIV immunogen was included as control. Our data showed that B'/C recombinant-derived gp145 mounted stronger T cell and broader linear antibody but less binding antibody immune responses than gp140 did. Though both gp145 and gp140 raised neutralization antibodies against laboratory-adapted strain SF33, both failed to neutralize B' or B'/C clade primary strains. Overall, this is the first time the immunogenicity of B'/C recombinant-derived gp140 and gp145 was examined and compared; our data prefer B'/C-derived gp145 to gp140 as an HIV vaccine immunogen. The failure to induce neutralization antibodies against primary isolates indicates that it is insufficient to enhance the immunogenicity of conserved epitopes by simply employing gp145 or gp140; strategies to enhance antibody responses against conserved epitopes should be explored further.

HIV-1 binding and neutralizing antibodies of injecting drug users

Previous studies have demonstrated a stronger seroreactivity against some synthetic peptides responsible for inducing neutralizing antibodies in injecting drug users (IDU) compared to that of individuals sexually infected with HIV-1 (S), but the effectiveness in terms of the neutralizing ability of these antibodies has not been evaluated. Our objective was to study the humoral immune response of IDU by determining the specificity of their antibodies and the presence of neutralizing antibodies. The neutralization capacity against the HIV-1 isolate MN (genotype B), the primary HIV-1 isolate 95BRRJ021 (genotype F), and the seroreactivity with peptides known to induce neutralizing antibodies, from the V2 and V3 loops of different HIV-1 subtypes, were analyzed. Seroreactivity indicates that IDU plasma are more likely to recognize a broader range of peptides than S plasma, with significantly higher titers, especially of V3 peptides. Similar neutralization frequencies of the MN isolate were observed in plasma of the IDU (16/47) and S (20/60) groups in the 1:10 dilution. The neutralization of the 95BRRJ021 isolate was more frequently observed for plasma from the S group (15/23) than from the IDU group (15/47, P = 0.0108). No correlation between neutralization and seroreactivity with the peptides tested was observed. These results suggest that an important factor responsible for the extensive and broad humoral immune response observed in IDU is their infection route. There was very little difference in neutralizing antibody response between the IDU and S groups despite their differences in seroreactivity and health status.

Adaptive evolution in perinatal HIV-1

The immune-viral dynamics of the transmission of HIV-1 from mother to child are poorly understood, despite 20 years of research. Here we review evidence that the maternal immune response against HIV-1 can select forms of the virus that evade immunity and when transmitted have negative consequences in the child. Moreover, recent studies indicate that when wild-type virus is transmitted, an early immune response in the child can lead to the selection of viral escape forms in the first few months of life. These data suggest that adaptive immune surveillance in both mother and child contributes to the pathogenesis of early perinatal HIV-1. These observations augment our general understanding of the processes that determine the evolution of HIV-1 as it passes from one host to another.

Human immunodeficiency virus type 1 neutralization by plasma from B or F genotype infected individuals

Anti-human immunodeficiency virus type 1 (HIV-1) "binding antibodies" (antibodies capable of binding to synthetic peptides or proteins) occur throughout HIV-1 infection, are high-titered and highly cross-reactive, as confirmed in this study by analyzing plasma from B and F genotype HIV-1 infected individuals. Plasma from individuals infected with clade F HIV-1 displayed the most frequent cross-reactivity, in high titers, while Bbr plasma showed much higher specificity. Similarly, neutralization of a reference HIV-1 isolate (HIV-1 MN) was more frequently observed by plasma from F than B genotype infected individuals. No significant difference was seen in neutralization susceptibility of primary B, Bbr or F clade HIV-1 by plasma from individuals infected with the classical B (GPGR) or F HIV-1, but Bbr (GWGR) plasma were less likely to neutralize the F genotype primary HIV-1 isolates. The data indicate that both B and F genotype derived vaccines would be equally effective against B and F HIV-1 infection, with a slightly more probable effectiveness for F than B genotype. Although the Bbr variant appears to induce a much more specific humoral immune response, the susceptibility in neutralizing the Brazilian HIV-1 B genotype Bbr variant is similar to that observed with the classical B genotype HIV-1.

Neutralization titres and vertical HIV-1 transmission

Replication of the human immunodeficiency virus type 1 (HIV-1) isolate MN in CEM cells was less neutralized by the plasma from the mothers of infected children (MIC) in comparison with the plasma from the mothers of uninfected children (MUC). Significantly higher neutralization titres were observed for the sera from MUCs compared with MICs, and only the sera from MUC showed 100% neutralization of the HIV-1 MN strain. We suggest that a simple neutralization assay as described here could be useful in prognostic analyses.