Gp120 Epitopes Research Articles

A gp41 MPER-specific Llama VHH Requires a Hydrophobic CDR3 for Neutralization but not for Antigen Recognition

The membrane proximal external region (MPER) of the HIV-1 glycoprotein gp41 is targeted by the broadly neutralizing antibodies 2F5 and 4E10. To date, no immunization regimen in animals or humans has produced HIV-1 neutralizing MPER-specific antibodies. We immunized llamas with gp41-MPER proteoliposomes and selected a MPER-specific single chain antibody (VHH), 2H10, whose epitope overlaps with that of mAb 2F5. Bi-2H10, a bivalent form of 2H10, which displayed an approximately 20-fold increased affinity compared to the monovalent 2H10, neutralized various sensitive and resistant HIV-1 strains, as well as SHIV strains in TZM-bl cells. X-ray and NMR analyses combined with mutagenesis and modeling revealed that 2H10 recognizes its gp41 epitope in a helical conformation. Notably, tryptophan 100 at the tip of the long CDR3 is not required for gp41 interaction but essential for neutralization. Thus bi-2H10 is an anti-MPER antibody generated by immunization that requires hydrophobic CDR3 determinants in addition to epitope recognition for neutralization similar to the mode of neutralization employed by mAbs 2F5 and 4E10.

Cross-neutralizing activity of human anti-V3 monoclonal antibodies derived from non-B clade HIV-1 infected individuals

One approach to the development of an HIV vaccine is to design a protein template which can present gp120 epitopes inducing cross-neutralizing antibodies. To select a V3 sequence for immunogen design, we compared the neutralizing activities of 18 anti-V3 monoclonal antibodies (mAbs) derived from Cameroonian and Indian individuals infected with clade AG and C, respectively. It was found that V3 mAbs from the Cameroonian patients were significantly more cross-neutralizing than those from India. Interestingly, superior neutralizing activity of Cameroonian mAbs was also observed among the nine VH5-51/VL lambda genes encoding V3 mAbs which mediate a similar mode of recognition. This correlated with higher relative binding affinity to a variety of gp120s and increased mutation rates in V3 mAbs from Cameroon. These results suggest that clade C V3 is probably weakly immunogenic and that the V3 sequence of CRF02_AG viruses can serve as a plausible template for vaccine immunogen design.

Expression and immunological characterization of Cardamom mosaic virus coat protein displaying HIV gp41 epitopes

The coat protein of cardamom mosaic virus (CdMV), a member of the genus Macluravirus, assembles into virus-like particles when expressed in an Escherichia coli expression system. The N and C-termini of the coat protein were engineered with the Kennedy peptide and the 2F5 and 4E10 epitopes of gp41 of HIV. The chimeric proteins reacted with sera from HIV positive persons and also stimulated secretion of cytokines by peripheral blood mononuclear cells from these persons. Thus, a system based on the coat protein of CdMV can be used to display HIV-1 antigens.

Identification of autoantigens recognized by the 2F5 and 4E10 broadly neutralizing HIV-1 antibodies

Many human monoclonal antibodies that neutralize multiple clades of HIV-1 are polyreactive and bind avidly to mammalian autoantigens. Indeed, the generation of neutralizing antibodies to the 2F5 and 4E10 epitopes of HIV-1 gp41 in man may be proscribed by immune tolerance because mice expressing the V(H) and V(L) regions of 2F5 have a block in B cell development that is characteristic of central tolerance. This developmental blockade implies the presence of tolerizing autoantigens that are mimicked by the membrane-proximal external region of HIV-1 gp41. We identify human kynureninase (KYNU) and splicing factor 3b subunit 3 (SF3B3) as the primary conserved, vertebrate self-antigens recognized by the 2F5 and 4E10 antibodies, respectively. 2F5 binds the H4 domain of KYNU which contains the complete 2F5 linear epitope (ELDKWA). 4E10 recognizes an epitope of SF3B3 that is strongly dependent on hydrophobic interactions. Opossums carry a rare KYNU H4 domain that abolishes 2F5 binding, but they retain the SF3B3 4E10 epitope. Immunization of opossums with HIV-1 gp140 induced extraordinary titers of serum antibody to the 2F5 ELDKWA epitope but little or nothing to the 4E10 determinant. Identification of structural motifs shared by vertebrates and HIV-1 provides direct evidence that immunological tolerance can impair humoral responses to HIV-1.

Prime-boost immunization of rabbits with HIV-1 gp120 elicits potent neutralization activity against a primary viral isolate.

Development of a vaccine for HIV-1 requires a detailed understanding of the neutralizing antibody responses that can be experimentally elicited to difficult-to-neutralize primary isolates. Rabbits were immunized with the gp120 subunit of HIV-1 JR-CSF envelope (Env) using a DNA-prime protein-boost regimen. We analyzed five sera that showed potent autologous neutralizing activity (IC50s at ∼103 to 104 serum dilution) against pseudoviruses containing Env from the primary isolate JR-CSF but not from the related isolate JR-FL. Pseudoviruses were created by exchanging each variable and constant domain of JR-CSF gp120 with that of JR-FL or with mutations in putative N-glycosylation sites. The sera contained different neutralizing activities dependent on C3 and V5, C3 and V4, or V4 regions located on the glycan-rich outer domain of gp120. All sera showed enhanced neutralizing activity toward an Env variant that lacked a glycosylation site in V4. The JR-CSF gp120 epitopes recognized by the sera are generally distinct from those of several well characterized mAbs (targeting conserved sites on Env) or other type-specific responses (targeting V1, V2, or V3 variable regions). The activity of one serum requires specific glycans that are also important for 2G12 neutralization and this serum blocked the binding of 2G12 to gp120. Our findings show that different fine specificities can achieve potent neutralization of HIV-1, yet this strong activity does not result in improved breadth.

Macrophage-tropic HIV-1 variants from brain demonstrate alterations in the way gp120 engages both CD4 and CCR5

BR-derived HIV-1 strains have an exceptional ability to enter macrophages via mechanisms involving their gp120 Env that remain incompletely understood. Here, we used cell-based affinity-profiling methods and mathematical modeling to generate quantitative VERSA metrics that simultaneously measure Env-CD4 and Env-CCR5 interactions. These metrics were analyzed to distinguish the phenotypes of M-tropic and non-M-tropic CCR5-using HIV-1 variants derived from autopsy BRs and LNs, respectively. We show that highly M-tropic Env variants derived from brain can be defined by two distinct and simultaneously occurring phenotypes. First, BR-derived Envs demonstrated an enhanced ability to interact with CD4 compared with LN-derived Envs, permitting entry into cells expressing scant levels of CD4. Second, BR-derived Envs displayed an altered mechanism of engagement between CD4-bound gp120 and CCR5 occurring in tandem. With the use of epitope mapping, mutagenesis, and structural studies, we show that this altered mechanism is characterized by increased exposure of CD4-induced epitopes in gp120 and by a more critical interaction between BR-derived Envs and the CCR5 N-terminus, which was associated with the predicted presence of additional atomic contacts formed at the gp120-CCR5 N-terminus interface. Our results suggest that BR-derived HIV-1 variants with highly efficient macrophage entry adopt conformations in gp120 that simultaneously alter the way in which the Env interacts with CD4 and CCR5.

Neutralization of porcine endogenous retrovirus by antibodies against the membrane-proximal external region of the transmembrane envelope protein

Immunization of different species including goats, rats, hamsters and guinea pigs with the recombinant ectodomain of the transmembrane envelope (TM) protein p15E of porcine endogenous retrovirus (PERV) has been shown to result in the production of virus-neutralizing antibodies. The sera recognize two groups of epitopes, one located in the fusion peptide-proximal region (FPPR) and the second in the membrane-proximal external region (MPER) of p15E. Most interestingly, the epitopes in the MPER are similar to epitopes in the TM protein gp41 of human immunodeficiency virus type 1 (HIV-1) recognized by mAbs 2F5 and 4E10, which broadly neutralize HIV-1. To study which epitope and which antibody population are involved in the process of neutralization of PERV, this study generated a new antiserum in a goat using an elongated ectodomain of p15E. The immune serum neutralized PERV at a higher titre and recognized broader epitopes in the FPPR and MPER of p15E. For the first time, antibody subpopulations were isolated from this serum using affinity chromatography with immobilized proteins and peptides corresponding to the FPPR and MPER of p15E. Only the affinity-purified antibodies specifically binding the MPER neutralized PERV, indicating that, as in the case of HIV-1, the MPER is an important target of neutralizing activity.

Molecular Anatomy and Number of Antigen Specific CD8 T Cells Required to Cause Type 1 Diabetes

We quantified CD8 T cells needed to cause type 1 diabetes and studied the anatomy of the CD8 T cell/beta (β) cell interaction at the immunologic synapse. We used a transgenic model, in situ tetramer staining to distinguish antigen specific CD8 T cells from total T cells infiltrating islets and a variety of viral mutants selected for functional deletion(s) of various CD8 T cell epitopes. Twenty percent of CD8 T cells in the spleen were specific for all immunodominant and subdominant viral glycoprotein (GP) epitopes. CTLs to the immunodominant LCMV GP33-41 epitope accounted for 63% of the total (12.5% of tetramers). In situ hybridization analysis demonstrated only 1 to 2% of total infiltrating CD8 T cells were specific for GP33 CD8 T cell epitope, yet diabetes occurred in 94% of mice. The immunologic synapse between GP33 CD8 CTL and β cell contained LFA-1 and perforin. Silencing both immunodominant epitopes (GP33, GP276–286) in the infecting virus led to a four-fold reduction in viral specific CD8 CTL responses, negligible lymphocyte infiltration into islets and absence of diabetes.

Rational Design of Membrane Proximal External Region Lipopeptides Containing Chemical Modifications for HIV-1 Vaccination

The inability to generate broadly neutralizing antibody (bnAb) responses to the membrane proximal external region (MPER) of HIV-1 gp41 using current vaccine strategies has hampered efforts to prevent the spread of HIV. To address this challenge, we investigated a novel hypothesis to help improve the anti-MPER antibody response. Guided by structural insights and the unique lipid reactivity of anti-MPER bnAbs, we considered whether amino acid side chain modifications that emulate hydrophilic phospholipid head groups could contribute to the generation of 2F5-like or 4E10-like neutralizing anti-MPER antibodies. To test this hypothesis, we generated a series of chemically modified MPER immunogens through derivatization of amino acid side chains with phosphate or nitrate groups. We evaluated the binding affinity of the chemically modified peptides to their cognate monoclonal antibodies, 2F5 and 4E10, using surface plasmon resonance. The modifications had little effect on binding to the antibodies and did not influence epitope secondary structure when presented in liposomes. We selected five of the chemically modified sequences to immunize rabbits and found that an immunogen containing both the 2F5 and 4E10 epitopes and a phosphorylated threonine at T676 elicited the highest anti-peptide IgG titers, although the high antipeptide titers did not confer higher neutralizing activity. These data indicate that side chain modifications adjacent to known neutralizing antibody epitopes are capable of eliciting antibody responses to the MPER but that these chemically modified gp41 epitopes do not induce neutralizing antibodies.

A gp41 MPER-specific llama VHH requires a hydrophobic CDR3 determinant for neutralization but not for antigen recognition

The membrane proximal external region (MPER) of the HIV-1 glycoprotein gp41 is targeted by the broadly neutralizing antibodies 2F5 and 4E10. To date, no immunization regimen in animals or humans has produced HIV-1 neutralizing MPER-specific antibodies. We immunized llamas with gp41-MPER proteoliposomes and selected a MPER-specific single chain antibody (VHH), 2H10, whose epitope overlaps with that of mAb 2F5. Bi-2H10, a bivalent form of 2H10, which displayed an approximately 20-fold increased affinity compared to the monovalent 2H10, neutralized various sensitive and resistant HIV-1 strains, as well as SHIV strains in TZM-bl cells. X-ray and NMR analyses combined with mutagenesis and modeling revealed that 2H10 recognizes its gp41 epitope in a helical conformation. Notably, tryptophan 100 at the tip of the long CDR3 is not required for gp41 interaction but essential for neutralization. Thus bi-2H10 is an anti-MPER antibody generated by immunization that requires hydrophobic CDR3 determinants in addition to epitope recognition for neutralization similar to the mode of neutralization employed by mAbs 2F5 and 4E10.

Anti-gp41 antibodies inhibit infection and transcytosis of HIV-1 infectious molecular clones expressing transmitted/founder envelopes

Background Prophylactic vaccine strategies against HIV-1 must effectively prevent virus transmission, infection and cellto-cell spread during the earliest stages of acute infection. Since the genital mucosa is the primary site of entry, mucosal defense is critical for early control of infection. Recent identification of transmitted/founder (T/F) HIV-1 genomes has demonstrated a consistent genetic bottleneck during mucosal transmission and suggests that T/F viruses may exhibit distinct phenotypes. HIV-1 Env gp41-specific responses are among the first to be generated in natural HIV infection. Methods Previously, we developed chimeric virus-like particle (VLP) immunogens that elicit potent systemic and mucosal antibodies against two highly conserved regions of gp41, by employing an optimized immunization strategy. The ELDKWA and QARVLAVERY epitopes are found with the membrane proximal external region (MPER) and the coiled coil region of gp41, respectively. Importantly, the epitope-specific IgG and IgA fractions derived from immunized mice were shown to be effective in neutralizing and preventing transcytosis of HIV in vitro. In particular, the QARVLAVERY epitope is remarkably conserved and induced unusually high and early levels of anti-QARV IgA, making it an attractive candidate for generation of broadly reactive mucosal antibodies. Results In this study, we assessed the effectiveness of mucosal and systemic mouse antibodies elicited against these gp41 epitopes to inhibit T/F Env function. For this, we employed recombinant infectious molecular clones (Env-IMC) of HIV-1 that encode mucosally transmitted/ founder env genes. Our results show that the gp41-specific IgG and IgA fractions effectively prevented the infection of TZM-bl cells and inhibited HIV transcytosis in an assay measuring the passage of infectious virus across an epithelial monolayer. Interestingly, the T/F Env-IMC tested were more sensitive to the antibodies than the R5 lab-adapted strains included as controls.

Analysis of gp41 epitopes in model viral membranes to study HIV-1 neutralization

Background The membrane proximal external region (MPER) of the HIV surface glycoprotein gp41 is the binding site for several potent broadly neutralizing antibodies. These antibodies recognize highly conserved linear peptide epitopes and antigens derived from this region have the potential to be important components of a vaccine directed against HIV-1. Highly conserved peptide epitopes for these antibodies are sterically obscured from the immune system by an intimate association with the viral membrane and by the presence of a large trimeric glycoprotein spike. As a result, previous attempts to immunize with linear peptides have failed to elicit broadly neutralizing antibodies.

Antigen processing sites in gp120 are conserved across HIV virus clades

Background A puzzling observation in HIV vaccine research is the fact that recombinant gp120 is able to adsorb bNAbs from HIV+ patient sera, but unable to elicit such bNAbs. To account for its poor immunogenicity, we wondered if the epitopes recognized by bNAbs might be proteolyzed in vivo. Cathepsins L, S, and D are the major proteases responsible for antigen processing and presentation. Previously, we defined the cathepsin cleavage sites on MN rgp120 and found that they co-localized with epitopes recognized by bNAbs. Although examination of gp120 sequences suggested that these sites were conserved, the recognition motifs for cathepsins are poorly defined and it was important to verify their presence by protease digestion studies. Methods Purified gp120s from three clades of HIV were digested with cathepsins L, S, and D. These included envelope proteins from the from the 108060 (clade B), A244 (CRF01_AE) and 97001 (clade C) isolates. N-terminal sequencing was used to identify the cathepsin cleavage sites. Results When combined with the previous MN-rgp120 results, we found that 6 out of 10 cathepsin cleavage sites were conserved in four viruses from three different clades of HIV. We found that polymorphisms that inactivate cleavage sites often result in the formation of an alternate nearby site. Although cleavage is an ordered processing beginning in the V3 domain, the cleavage sites in this domain are more polymorphic than other sites. Conclusion Our results suggest that cathepsin cleavage sites are highly conserved in gp120 and co-localize to regions recognized by bNAbs. These results are consistent with our hypothesis that the poor immunogenicity of gp120 epitopes results from protease digestion in vivo. Current studies are in progress to determine whether inactivation of these sites or enzymes may provide a new approach to improving the immunogenicity of epitopes recognized by bNAbs.

Glycoform and net charge heterogeneity in gp120 immunogens used in HIV vaccine trials.

BackgroundThe RV144 clinical trial showed for the first time that vaccination could provide modest but significant protection from HIV-1 infection. To understand the protective response, and to improve upon the vaccine's efficacy, it is important to define the structure of the immunogens used in the prime/boost regimen. Here we examined the heterogeneity in net charge, attributable to glycoform variation, of the gp120 immunogens contained in the AIDSVAX B/E vaccine.Methodology/Principal FindingsIsoelectric focusing and glycosidase digestion were used to assess variation in net charge of the gp120s contained in the AIDSVAX B/E vaccine used in the RV144 trial. We observed 16 variants of MN-rgp120 and 24 variants of A244-rgp120. Glycoform variation in gp120 produced in Chinese hamster ovary cells was compared to glycoform variation in gp120 produced in the 293F human embryonic kidney cell line, often used for neutralization assays. We found that gp120 variants produced in CHO cells were distinctly more acidic than gp120 variants produced in 293 cells. The effect of glycoform heterogeneity on antigenicity was assessed using monoclonal antibodies. The broadly neutralizing PG9 MAb bound to A244-rgp120, but not to MN-rgp120, whether produced in CHO or in 293. However, PG9 was able to bind with high affinity to MN-rgp120 and A244-rgp120 produced in 293 cells deficient in N-acetylglucosaminyltransferase I.Conclusions/SignificanceMN- and A244-rgp120 used in the RV144 trial exhibited extensive heterogeneity in net charge due to variation in sialic acid-containing glycoforms. These differences were cell line-dependent, affected the antigenicity of recombinant envelope proteins, and may affect assays used to measure neutralization. These studies, together with recent reports documenting broadly neutralizing antibodies directed against carbohydrate epitopes of gp120, suggest that glycoform variation is a key variable to be considered in the production and evaluation of subunit vaccines designed to prevent HIV infection.

The LCMV gp33-specific memory T cell repertoire narrows with age

BackgroundThe memory response to LCMV in mice persists for months to years with only a small decrease in the number of epitope specific CD8 T cells. This long persistence is associated with resistance to lethal LCMV disease. In contrast to studies focused on the number and surface phenotype of the memory cells, relatively little attention has been paid to the diversity of TCR usage in these cells. CD8+ T cell responses with only a few clones of identical specificity are believed to be relatively ineffective, presumably due to the relative ease of virus escape. Thus, a broad polyclonal response is associated with an effective anti-viral CD8+ T cell response.ResultsIn this paper we show that the primary CD8+ T cell response to the LCMV gp33-41 epitope is extremely diverse. Over time while the response remains robust in terms of the number of gp33-tetramer+ T cells, the diversity of the response becomes less so. Strikingly, by 26 months after infection the response is dominated by a small number TCRβ sequences. In addition, it is of note the gp33 specific CD8+ T cells sorted by high and low tetramer binding populations 15 and 22 months after infection. High and low tetramer binding cells had equivalent diversity and were dominated by a small number of clones regardless of the time tested. A similar restricted distribution was seen in NP396 specific CD8+ T cells 26 months after infection. The identical TCRVβ sequences were found in both the tetramerhi and tetramerlo binding populations. Finally, we saw no evidence of public clones in the gp33-specific response. No CDR3 sequences were found in more than one mouse.ConclusionsThese data show that following LCMV infection the CD8+ gp33-specific CD8 T cell response becomes highly restricted with enormous narrowing of the diversity. This narrowing of the repertoire could contribute to the progressively ineffective immune response seen in aging.

Protective Capacity of Virus-Specific T Cell Receptor-Transduced CD8 T Cells In Vivo

The transfer of T cell receptor (TCR) genes by viral vectors represents a promising technique to generate antigen-specific T cells for adoptive immunotherapy. TCR-transduced T cells specific for infectious pathogens have been described, but their protective function in vivo has not yet been examined. Here, we demonstrate that CD8 T cells transduced with the P14 TCR specific for the gp33 epitope of lymphocytic choriomeningitis virus exhibit protective activities in both viral and bacterial infection models in mice.

Kynureninase is the conserved self-antigen mimicked by the 2F5 Neutralizing Epitope of HIV-1 gp41 (105.35)

Abstract Accumulating evidence suggests that the generation of neutralizing HIV-1 antibody may be proscribed by immunological tolerance. A significant majority of monoclonal human antibodies, including 2F5, that neutralize multiple clades of HIV-1 are polyreactive and bind avidly to unknown components of mammalian cells. Knock-in mice expressing the VH and VL regions of 2F5 are characterized by a central defect in B-cell development that indicates the presence of a tolerizing autoantigen(s) that is mimicked by the membrane-proximal, 2F5 epitope of HIV-1 gp41. If immunological tolerance does indeed prevent regular expression of 2F5-like neutralizing HIV-1 antibody, then discrete autoantigens that structurally mimic the HIV-1 2F5 epitope must be present in those mammalian species unable to make 2F5-like antibody. We now demonstrate that the 2F5 antibody and its unmutated ancestor avidly bind human kynureninase (KYNU), a conserved enzyme of tryptophan metabolism. The H4 domain of most mammalian KYNU contains the complete 2F5 epitope (ELDKWA) of gp41; a H4 mutation of human KYNU to that (ELEKWA) of laboratory opossums resulted in a fully active enzyme completely unreactive with the 2F5 antibody. Immunization of opossums with HIV-1 gp140 induced extraordinary titers of ELDKWA antibody but little or no response to adjacent gp41 epitopes. Identification of an epitope shared by most mammals and HIV-1 provides direct evidence that immunological tolerance can impair humoral responses to HIV-1.

Pre-Clinical Evaluation of a 213Bi-Labeled 2556 Antibody to HIV-1 gp41 Glycoprotein in HIV-1 Mouse Models as a Reagent for HIV Eradication

BackgroundAny strategy for curing HIV infection must include a method to eliminate viral-infected cells. Based on our earlier proof-of-principle results targeting HIV-1 infected cells with radiolabeled antibody (mAb) to gp41 viral antigen, we embarked on identifying a suitable candidate mAb for preclinical development.Methodology/Principal FindingsAmong the several human mAbs to gp41 tested, mAb 2556 was found to have high affinity, reactivity with multimeric forms of gp41 present on both the surface of virus particles and cells expressing HIV-1 Env, and recognition of a highly conserved epitope of gp41 shared by all HIV-1 subtypes. Also, mAb 2556 was the best in competition with HIV-1+ serum antibodies, which is an extremely important consideration for efficacy in the treatment of HIV patients. When radiolabeled with alpha-emitting radionuclide 213-Bismuth (213Bi) - 213Bi-2556 efficiently and specifically killed ACH-2 human lymphocytes chronically infected with HIV-1, and HIV-1 infected human peripheral blood mononuclear cells (hPBMCs). The number of binding sites for 213Bi-2556 on the surface of the infected cells was >106. The in vivo experiments were performed in two HIV-1 mouse models – splenic and intraperitoneal. In both models, the decrease in HIV-1 infected hPBMCs from the spleens and peritoneum, respectively, was dose-dependent with the most pronounced killing of hPBMCs observed in the 100 µCi 213Bi-2556 group (P = 0.01). Measurement of the blood platelet counts and gross pathology of the treated mice demonstrated the lack of toxicity for 213Bi-2556.Conclusions/SignificanceWe describe the preclinical development of a novel radiolabeled mAb reagent that could potentially be part of an HIV eradication strategy that is ready for translation into the clinic as the next step in its development. As viral antigens are very different from “self” human antigens - this approach promises high selectivity, increased efficacy and low toxicity, especially in comparison to immunotoxins.

Diversification of Specificity after Maturation of the Antibody Response to the HIV gp41 Epitope ELDKWA

During maturing antibody responses the increase in affinity for target antigens is achieved by genetic diversification of antibody genes followed by selection for improved binding. The effect this process has on the specificity of antibody for variants of the antigen is not well-defined, despite the potential role of antibody diversification in generating enhanced protection against pathogen escape mutants, or novel specificities after vaccination. To investigate this, a library of single amino-acid substitution epitope variants has been screened with serum obtained at different time-points after immunization of mice with the HIV gp41 peptide epitope ELDKWA. The serum IgG response is shown to mature and increase affinity for ELDKWA, and the titre and affinity of IgG against most epitope variants tested increases. Furthermore there is a bias towards high affinity serum IgG binding to variant epitopes with conservative substitutions, although underlying this trend there is also significant binding to many epitopes with non-conservative substitutions. Thus, maturation of the antibody response to a single epitope results in a broadening of the high-affinity response toward variant epitopes. This implies that many pathogen epitope escape variants that could manifest as single amino-acid substitutions would not emerge by escaping immune surveillance.

Passively Transmitted gp41 Antibodies in Babies Born from HIV-1 Subtype C-Seropositive Women: Correlation between Fine Specificity and Protection

HIV-exposed, uninfected (EUN) babies born to HIV-infected mothers are examples of natural resistance to HIV infection. In this study, we evaluated the titer and neutralizing potential of gp41-specific maternal antibodies and their correlation with HIV transmission in HIV-infected mother-child pairs. Specific gp41-binding and -neutralizing antibodies were determined in a cohort of 74 first-time mother-child pairs, of whom 40 mothers were infected with HIV subtype C. Within the infected mother cohort, 16 babies were born infected and 24 were PCR negative and uninfected at birth (i.e., exposed but uninfected). Thirty-four HIV-uninfected and HIV-unexposed mother-child pairs were included as controls. All HIV-positive mothers and their newborns showed high IgG titers to linear epitopes within the HR1 region and to the membrane-proximal (MPER) domain of gp41; most sera also recognized the disulfide loop immunodominant epitope (IDE). Antibody titers to the gp41 epitopes were significantly lower in nontransmitting mothers (P < 0.01) and in the EUN babies (P < 0.005) than in HIV-positive mother-child pairs. Three domains of gp41, HR1, IDE, and MPER, elicited antibodies that were effectively transmitted to EUN babies. Moreover, in EUN babies, epitopes overlapping the 2F5 epitope (ELDKWAS), but not the 4E10 epitope, were neutralization targets in two out of four viruses tested. Our findings highlight important epitopes in gp41 that appear to be associated with exposure without infection and would be important to consider for vaccine design.