Neutralizing antibody responses against autologous and heterologous viruses in acute versus chronic human immunodeficiency virus (HIV) infection: evidence for a constraint on the ability of HIV to completely evade neutralizing antibody responses

Acute human immunodeficiency virus (HIV) infection is associated with the rapid development of neutralization escape mutations. The degree to which viral evolution persists in chronic infection has not been well characterized, nor is it clear if all patients develop high-level neutralization antibody escape. We therefore measured neutralizing antibody responses against autologous and heterologous viruses in a cohort of acutely and chronically infected subjects (n = 65). Neutralizing antibody responses against both autologous virus and heterologous viruses were lower among individuals with acute infection than among those with chronic infection. Among chronically infected individuals, there was a negative correlation between the level of neutralizing antibodies against autologous virus and the level of viremia. In contrast, there was a positive correlation between the level of neutralizing antibodies against a panel of heterologous viruses and the level of viremia. Viral evolution, as defined by the presence of higher neutralizing titers directed against earlier viruses than against contemporaneous viruses, was evident for subjects with recent infection but absent for those with chronic infection. In summary, neutralizing antibody responses against contemporaneous autologous viruses are absent in early HIV infection but can be detected at low levels in chronic infection, particularly among those controlling HIV in the absence of therapy. HIV replication either directly or indirectly drives the production of increasing levels of antibodies that cross-neutralize heterologous primary isolates. Collectively, these observations indicate that although HIV continuously drives the production of neutralizing antibodies, there may be limits to the capacity of the virus to evolve continuously in response to these antibodies. These observations also suggest that the neutralizing antibody response may contribute to the long-term control of HIV in some patients while protecting against HIV superinfection in most patients.     

Neutralizing antibodies associated with viremia control in a subset of individuals after treatment of acute human immunodeficiency virus type 1 infection

Immediate treatment of acute human immunodeficiency virus type 1 (HIV-1) infection has been associated with subsequent control of viremia in a subset of patients after therapy cessation, but the immune responses contributing to control have not been fully defined. Here we examined neutralizing antibodies as a correlate of viremia control following treatment interruption in HIV-1-infected individuals in whom highly active antiretriviral therapy (HAART) was initiated during early seroconversion and who remained on therapy for 1 to 3 years. Immediately following treatment interruption, neutralizing antibodies were undetectable with T-cell-line adapted strains and the autologous primary HIV-1 isolate in seven of nine subjects. Env- and Gag-specific antibodies as measured by enzyme-linked immunosorbent assay were also low or undetectable at this time. Despite this apparent poor maturation of the virus-specific B-cell response during HAART, autologous neutralizing antibodies emerged rapidly and correlated with a spontaneous downregulation in rebound viremia following treatment interruption in three subjects. Control of rebound viremia was seen in other subjects in the absence of detectable neutralizing antibodies. The results indicate that virus-specific B-cell priming occurs despite the early institution of HAART, allowing rapid secondary neutralizing-antibody production following treatment interruption in a subset of individuals. Since early HAART limits viral diversification, we hypothesize that potent neutralizing-antibody responses to autologous virus are able to mature and that in some persons these responses contribute to the control of plasma viremia after treatment cessation.     

Effect of humoral immune responses on controlling viremia during primary infection of rhesus monkeys with simian immunodeficiency virus

Cellular immune responses mediated by CD8+ lymphocytes exert efficient control of virus replication during primary simian immunodeficiency virus (SIV) infection. However, the role that antibodies may play in the early control of virus replication remains unclear. To evaluate how antibody responses may affect virus replication during primary SIVmac infection, we depleted rhesus monkeys of B cells with anti-CD20 antibody. In normal rhesus monkeys immunized with tetanus toxoid, anti-CD20 treatment and resulting depletion of B cells inhibited the generation of antitetanus antibodies, while tetanus-specific T-cell responses were preserved. During the first 4 weeks after inoculation with SIVmac251, development of SIV-specific neutralizing antibody was delayed, and titers were significantly lower in B-cell-depleted monkeys than control-antibody-treated monkeys. Despite the lower neutralizing antibody titers, the levels of plasma SIV RNA and the linear slope of the decline seen in B-cell-depleted monkeys did not differ from that observed in monkeys treated with control antibody. However, beginning at day 28 after SIV infection, the B-cell-depleted monkeys showed a significant inverse correlation between neutralizing antibody titers and plasma virus level. These results suggest that the rapid decline of peak viremia that typically occurs during the first 3 weeks of infection was not significantly affected by SIV-specific antibodies. However, the inverse correlation between neutralizing antibodies and plasma virus level during the postacute phases of infection suggests that humoral immune responses may contribute to the control of SIV replication.     

Autologous and heterologous neutralizing antibody responses following initial seroconversion in human immunodeficiency virus type 1-infected individuals.

In the course of human immunodeficiency virus type 1 (HIV-1) infection, patients develop a strong and persistent immune response characterized by the production of HIV-specific antibodies. The aim of our study was to analyze the appearance of autologous and heterologous neutralizing antibodies in the sera of HIV-infected individuals. For this purpose, primary strains have been isolated from 18 HIV-1-infected subjects prior to seroconversion (in one case) or within 1 to 8 months after seroconversion. Sera, collected at the same time as the virus was isolated and at various times after isolation, have been analyzed for their ability to neutralize the autologous primary strains isolated early after infection, heterologous primary isolates, and cell-line adapted strains. Our neutralization assay, which combines serial dilutions of virus and serial dilutions of sera, is based on the determination of the serum dilution at which a fixed reduction in virus titer (90%) occurs. We have shown that (i) we could not detect autologous neutralizing antibodies in sera collected at the same time as we isolated viruses; (ii) we detected neutralizing antibodies against the autologous strains about 1 year after seroconversion, occasionally after 8 months, but sera were not always available to exclude the presence of neutralizing antibodies at earlier times; (iii) after 1 year, the neutralization response was highly specific to virus present during the early phase of HIV infection; and (iv) heterologous neutralization of primary isolates was detected later (after about 2 years). These results reveal the enormous diversity of neutralization determinants on primary isolates as well as a temporal evolution of the humoral response generating cross-reactive neutralizing antibodies.     

Neutralizing antibody responses in acute human immunodeficiency virus type 1 subtype C infection

The study of the evolution and specificities of neutralizing antibodies during the course of human immunodeficiency virus type 1 (HIV-1) infection may be important in the discovery of possible targets for vaccine design. In this study, we assessed the autologous and heterologous neutralization responses of 14 HIV-1 subtype C-infected individuals, using envelope clones obtained within the first 2 months postinfection. Our data show that potent but relatively strain-specific neutralizing antibodies develop within 3 to 12 months of HIV-1 infection. The magnitude of this response was associated with shorter V1-to-V5 envelope lengths and fewer glycosylation sites, particularly in the V1-V2 region. Anti-MPER antibodies were detected in 4 of 14 individuals within a year of infection, while antibodies to CD4-induced (CD4i) epitopes developed to high titers in 12 participants, in most cases before the development of autologous neutralizing antibodies. However, neither anti-MPER nor anti-CD4i antibody specificity conferred neutralization breadth. These data provide insights into the kinetics, potency, breadth, and epitope specificity of neutralizing antibody responses in acute HIV-1 subtype C infection.     

Antibody responses induced by SHIV infection are more focused than those induced by soluble native HIV-1 envelope trimers in non-human primates

The development of an effective human immunodeficiency virus (HIV-1) vaccine is a high global health priority. Soluble native-like HIV-1 envelope glycoprotein trimers (Env), including those based on the SOSIP design, have shown promise as vaccine candidates by inducing neutralizing antibody responses against the autologous virus in animal models. However, to overcome HIV-1’s extreme diversity a vaccine needs to induce broadly neutralizing antibodies (bNAbs). Such bNAbs can protect non-human primates (NHPs) and humans from infection. The prototypic BG505 SOSIP.664 immunogen is based on the BG505 env sequence isolated from an HIV-1-infected infant from Kenya who developed a bNAb response. Studying bNAb development during natural HIV-1 infection can inform vaccine design, however, it is unclear to what extent vaccine-induced antibody responses to Env are comparable to those induced by natural infection. Here, we compared Env antibody responses in BG505 SOSIP-immunized NHPs with those in BG505 SHIV-infected NHPs, by analyzing monoclonal antibodies (mAbs). We observed three major differences between BG505 SOSIP immunization and BG505 SHIV infection. First, SHIV infection resulted in more clonal expansion and less antibody diversity compared to SOSIP immunization, likely because of higher and/or prolonged antigenic stimulation and increased antigen diversity during infection. Second, while we retrieved comparatively fewer neutralizing mAbs (NAbs) from SOSIP-immunized animals, these NAbs targeted more diverse epitopes compared to NAbs from SHIV-infected animals. However, none of the NAbs, either elicited by vaccination or infection, showed any breadth. Finally, SOSIP immunization elicited antibodies against the base of the trimer, while infection did not, consistent with the base being placed onto the virus membrane in the latter setting. Together these data provide new insights into the antibody response against BG505 Env during infection and immunization and limitations that need to be overcome to induce better responses after vaccination.     

Autologous antibody response against the principal neutralizing domain of human immunodeficiency virus type 1 isolated from infected humans.

High titers of neutralizing antibodies in human immunodeficiency virus type 1 (HIV-1) infection are directed primarily against the third hypervariable domain (V3) of the virion envelope glycoprotein gp120. This region has been designated the principal neutralizing domain of HIV-1. Because the frequency and significance of autologous V3 antibodies in natural infection are not fully clarified, we have cloned, sequenced, and expressed the V3 domain from virus of HIV-1-infected patients to test the autologous and heterologous V3 antibody response. The resulting recombinant Escherichia coli V3 fusion proteins reacted strongly with both autologous and heterologous patient antibodies in Western blots. Thirty-one different V3 fragments were cloned from 24 hemophiliac patients with different immunological and clinical statuses. Antibody reactivity against the autologous V3 fusion proteins was detected in all serum samples except one; moreover, all serum samples contained antibody reactivity against a vast majority of heterologous fusion proteins despite significant amino acid variability in V3. The results suggest that V3 antibodies are highly prevalent; further, we find no association between the stage of the HIV-1 infection and the presence of V3 antibodies.     

Factors associated with the development of cross-reactive neutralizing antibodies during human immunodeficiency virus type 1 infection

The characterization of the cross-reactive, or heterologous, neutralizing antibody responses developed during human immunodeficiency virus type 1 (HIV-1) infection and the identification of factors associated with their generation are relevant to the development of an HIV vaccine. We report that in healthy HIV-positive, antiretroviral-naïve subjects, the breadth of plasma heterologous neutralizing antibody responses correlates with the time since infection, plasma viremia levels, and the binding avidity of anti-Env antibodies. Anti-CD4-binding site antibodies are responsible for the exceptionally broad cross-neutralizing antibody responses recorded only in rare plasma samples. However, in most cases examined, antibodies to the variable regions and to the CD4-binding site of Env modestly contributed in defining the overall breadth of these responses. Plasmas with broad cross-neutralizing antibody responses were identified that targeted the gp120 subunit, but their precise epitopes mapped outside the variable regions and the CD4-binding site. Finally, although several plasmas were identified with cross-neutralizing antibody responses that were not directed against gp120, only one plasma with a moderate breadth of heterologous neutralizing antibody responses contained cross-reactive neutralizing antibodies against the 4E10 epitope, which is within the gp41 transmembrane subunit. Overall, our study indicates that more than one pathway leads to the development of broad cross-reactive neutralizing antibodies during HIV infection and that the virus continuously escapes their action.     

Macaques infected with a CCR5-tropic simian/human immunodeficiency virus (SHIV) develop broadly reactive anti-HIV neutralizing antibodies

The development of anti-human immunodeficiency virus (anti-HIV) neutralizing antibodies and the evolution of the viral envelope glycoprotein were monitored in rhesus macaques infected with a CCR5-tropic simian/human immunodeficiency virus (SHIV), SHIVSF162P4. Homologous neutralizing antibodies developed within the first month of infection in the majority of animals, and their titers were independent of the extent and duration of viral replication during chronic infection. The appearance of homologous neutralizing antibody responses was preceded by the appearance of amino acid changes in specific variable and conserved regions of gp120. Amino acid changes first appeared in the V1, V2, C2, and V3 regions and subsequently in the C3, V4, and V5 regions. Heterologous neutralizing antibody responses developed over time only in animals with sustained plasma viremia. Within 2 years postinfection the breadth of these responses was as broad as that observed in certain patients infected with HIV type 1 (HIV-1) for over a decade. Despite the development of broad anti-HIV-1 neutralizing antibody responses, viral replication persisted in these animals due to viral escape. Our studies indicate that cross-reactive neutralizing antibodies are elicited in a subset of SHIVSF162P4 infected macaques and that their development requires continuous viral replication for extended periods of time. More importantly, their late appearance does not prevent progression to disease. The availability of an animal model where cross-reactive anti-HIV neutralizing antibodies are developed may facilitate the identification of virologic and immunologic factors conducive to the development of such antibodies.     

In vitro inhibition of HIV-1 replication in autologous CD4~+ T cells indicates viral containment by multifactorial mechanisms

HIV-1-specific cytotoxic T lymphocytes(CTLs) and neutralizing antibodies(NAbs) are present during chronic infection, but the relative contributions of these effector mechanisms to viral containment remain unclear. Here, using an in vitro model involving autologous CD4+ T cells,primary HIV-1 isolates, HIV-1-specific CTLs, and neutralizing monoclonal antibodies, we show that b12, a potent and broadly neutralizing monoclonal antibody to HIV-1, was able to block viral infection when preincubated with virus prior to infection, but was much less effective than CTLs at limiting virus replication when added to infected cell cultures. However, the same neutralizing antibody was able to contain viruses by antibody-dependent cell-mediated virus inhibition in vitro,which was mediated by natural killer cells(NKs) and dependent on an Fc-Fc receptor interaction.Meanwhile, bulk CTLs from HIV-1 controllers were more effective in suppression of virus replication than those from progressors. These findings indicate that control of HIV-1 replication in activated CD4~+ T cells is ineffectively mediated by neutralizing antibodies alone, but that both CTLs and antibody-dependent NK-mediated immune mechanisms contribute to viral containment. Our study systemically compared three major players in controlling HIV-1 infection, CTLs, NAbs, and NKs, in an autologous system and highlighted the multifactorial mechanisms for viral containment and vaccine success.     

Vaccine-elicited V3 loop-specific antibodies in rhesus monkeys and control of a simian-human immunodeficiency virus expressing a primary patient human immunodeficiency virus type 1 isolate envelope

Vaccine-elicited antibodies specific for the third hypervariable domain of the surface gp120 of human immunodeficiency virus type 1 (HIV-1) (V3 loop) were assessed for their contribution to protection against infection in the simian-human immunodeficiency virus (SHIV)/rhesus monkey model. Peptide vaccine-elicited anti-V3 loop antibody responses were examined for their ability to contain replication of SHIV-89.6, a nonpathogenic SHIV expressing a primary patient isolate HIV-1 envelope, as well as SHIV-89.6P, a pathogenic variant of that virus. Low-titer neutralizing antibodies to SHIV-89.6 that provided partial protection against viremia following SHIV-89.6 infection were generated. A similarly low-titer neutralizing antibody response to SHIV-89.6P that did not contain viremia after infection with SHIV-89.6P was generated, but a trend toward protection against CD4+ T-lymphocyte loss was seen in these infected monkeys. These observations suggest that the V3 loop on some primary patient HIV-1 isolates may be a partially effective target for neutralizing antibodies induced by peptide immunogens.     

Human immunodeficiency virus (HIV)-positive sera obtained shortly after seroconversion neutralize autologous HIV type 1 isolates on primary macrophages but not on lymphocytes

The aim of this study was to analyze the role of humoral immunity in early human immunodeficiency virus (HIV) infection. As neutralizing activities in HIV-positive sera are rarely detectable earlier than 9 to 12 months after infection using primary lymphocytes as target cells in neutralization assays, humoral immunity is generally thought not to contribute significantly to early virus control in the patients. Besides lymphocytes, cells of the monocyte/macrophage lineage are known to be important target cells for HIV in vivo during the establishment of the infection. Therefore, we studied the neutralization of early primary HIV isolates by autologous serum samples using primary macrophages as target cells in the neutralization assays. We analyzed neutralizing activities against the autologous HIV-1 isolates in 10 patients' sera taken shortly after seroconversion, both on primary macrophages and, for comparison, on lymphocytes. Viruses were isolated and expanded in primary mixed cultures containing macrophages and lymphocytes in order to avoid selection for one particular cell type. All viruses replicated to different degrees in macrophages and lymphocytes; nine had a nonsyncytium-inducing phenotype, and one was syncytium inducing. The detection of neutralizing antibodies in acute primary HIV infection depended on the target cells used. Confirming previous studies, we did not find neutralizing activities on lymphocytes at this early time point. In contrast, neutralizing activities were detectable in the same sera if primary macrophages were used as target cells. Differences in neutralizing activities on macrophages and lymphocytes were not due to different virus variants being present in the different cell systems, as gp120 sequences derived from both cell types were homogeneous. Neutralization activities on macrophages did not correlate with the amount of beta-chemokines in the sera. As affinity-purified immunoglobulin G preparations from an early patient serum also exhibited neutralization of the autologous virus isolate on primary macrophages, but not on lymphocytes, neutralization is very likely due to antibodies against viral epitopes necessary for infection of macrophages but not for infection of lymphocytes. Our data suggest that, along with cell-mediated immunity, humoral immunity may contribute to the reduction of primary viremia in the patient. This was further supported by a certain association between neutralizing antibody titers on macrophages and viral load in the patients.     

Autologous neutralizing humoral immunity and evolution of the viral envelope in the course of subtype B human immunodeficiency virus type 1 infection

Most human immunodeficiency virus type 1 (HIV-1)-infected individuals develop an HIV-specific neutralizing antibody (NAb) response that selects for escape variants of the virus. Here, we studied autologous NAb responses in five typical CCR5-using progressors in relation to viral NAb escape and molecular changes in the viral envelope (Env) in the period from seroconversion until after AIDS diagnosis. In sera from three patients, high-titer neutralizing activity was observed against the earliest autologous virus variants, followed by declining humoral immune responses against subsequent viral escape variants. Autologous neutralizing activity was undetectable in sera from two patients. Patients with high-titer neutralizing activity in serum showed the strongest positive selection pressure on Env early in infection. In the initial phase of infection, gp160 length and the number of potential N-linked glycosylation sites (PNGS) increased in viruses from all patients. Over the course of infection, positive selection pressure declined as the NAb response subsided, coinciding with reversions of changes in gp160 length and the number of PNGS. A number of identical amino acid changes were observed over the course of infection in the viral quasispecies of different patients. Our results indicate that although neutralizing autologous humoral immunity may have a limited effect on the disease course, it is an important selection pressure in virus evolution early in infection, while declining HIV-specific humoral immunity in later stages may coincide with reversion of NAb-driven changes in Env.     

In vivo efficacy of human immunodeficiency virus neutralizing antibodies: estimates for protective titers

The definition of plasma neutralizing antibody titers capable of controlling human immunodeficiency virus (HIV) infection in vivo is considered a critical step in vaccine development. Here we provide estimates for effective neutralization titers by assessing samples from a recent passive immunization trial with the neutralizing monoclonal antibodies (MAbs) 2G12, 2F5, and 4E10 using an analytic strategy that dissects the contributions of these MAbs to the total neutralization activity in patient plasma. Assessment of neutralization activities for six responding patients with partial or complete control of viremia during the MAb treatment and for the eight nonresponding patients revealed a significant difference between these groups: Among responders, MAb-mediated activity exceeded the autologous neutralization response by 1 to 2 log units (median difference, 43.3-fold), while in the nonresponder group, the autologous activity prevailed (median difference, 0.63-fold). In order to reach a 50% proportion of the responders in our study cohort, MAb neutralizing titers higher than 1:200 were required based on this analysis. The disease stage appears to have a significant impact on the quantities needed, since titers above 1:1,000 were needed to reach the same effect in chronic infection. Although our analysis is based on very small sample numbers and thus cannot be conclusive, our data provide a first estimate on how in vitro-measured neutralizing antibody activity can relate to in vivo efficacy in controlling HIV infection and may therefore provide valuable information for vaccine development. Interestingly, lower neutralizing antibody levels showed an effect in acute compared to chronic infection, suggesting that in early disease stages, therapeutic vaccination may show promise. Equally, this raises hopes that a preventive vaccine could become effective at comparatively lower neutralizing antibody titers.     

Primary virus envelope cross-reactivity of the broadening neutralizing antibody response during early chronic human immunodeficiency virus type 1 infection

To test the hypothesis that changing neutralizing antibody responses against human immunodeficiency virus type 1 (HIV-1) during chronic infection were a response to emergence of neutralization escape mutants, we cloned expressed and characterized envelope clones from patients in the Multicenter AIDS Cohort Study (MACS). Pseudotyped HIV-1 envelope clones obtained from differing time points were assessed for sensitivity to neutralization by using sera from different times from the same and different patients. Clones from early and late time points during chronic infection had similar neutralization sensitivity, and neutralizing antibody responses cross-reacted with early, late, and heterologous envelopes. The potential for broadly effective HIV-1 immunization is supported.     

Neutralizing antibody responses to HIV-1 infection

Neutralizing antibodies represent an important component of immune control in many viral infections. In HIV-1 infection, almost all individuals develop antibodies capable of neutralizing autologous viruses in vitro; however, the role of these antibodies in vivo still remains unclear. Their absence during the acute phase of infection, when the viral levels are brought under control, suggests they play a minor role in immune control and that cellular immune responses are more critical during this time. However, during chronic infection these antibodies may be important in preventing cell-to-cell spread and they still represent our best hope of providing sterilizing immunity (i.e., prevention of infection) by vaccination. Significant advances over the last few years in understanding the structure of the envelope glycoproteins have renewed interest in the role of neutralizing antibodies and the possibility that immunogens capable of stimulating a neutralizing antibody response can be developed.     

Protection of rhesus monkeys against infection with minimally pathogenic simian-human immunodeficiency virus: correlations with neutralizing antibodies and cytotoxic T cells

We studied the capacity of active immunization of rhesus monkeys with HIV-1 envelope protein (Env) to induce primary virus cross-reactive neutralizing antibodies to prevent infection following intravenous challenge with simian-human immunodeficiency virus (SHIV). Monkeys were immunized with the human immunodeficiency type 1 (HIV-1) strain R2 Env. Initially, the Env was expressed in vivo by an alphavirus replicon particle system, and then it was administered as soluble oligomeric gp140. Concurrently, groups of monkeys received expression vectors that encoded either simian immunodeficiency virus (SIV) gag/pol genes or no SIV genes in vivo to test the additional protective benefit of concurrent induction of virus-specific cell-mediated immune (CMI) responses. Groups of control monkeys received either the gag/pol regimen or sham immunizations. The antibodies induced by the Env immunization regimen neutralized diverse primary HIV-1 strains. Similarly, potent CMI responses were induced by the gag/pol regimen, as measured by gamma interferon enzyme-linked immunospot assays. Differences in the responses among groups of monkeys strongly suggested that there was interference between the Env and gag/pol immunization regimens. Complete protection of some of the monkeys against infection after intravenous challenge with the partially pathogenic SHIV(DH12R (Clone 7)) was associated independently with both neutralizing antibody and CMI responses. Protection was associated with SHIV(DH12 (Clone 7)) serum neutralizing antibody titers of > or =1:80 or with cellular immune responses corresponding to >2,000 spot forming cells per 10(6) peripheral blood mononuclear cells. Immunization was also associated with a reduction in the magnitude and duration of virus load. Induction of cross-reactive, primary HIV-1-neutralizing antibodies is feasible and, when potent, may result in complete protection against infection with a heterologous challenge virus strain.     

Antibody-dependent cell-mediated viral inhibition emerges after simian immunodeficiency virus SIVmac251 infection of rhesus monkeys coincident with gp140-binding antibodies and is effective against neutralization-resistant viruses

Antibody-dependent cell-mediated viral inhibition (ADCVI) is an attractive target for vaccination because it takes advantage of both the anamnestic properties of an adaptive immune response and the rapid early response characteristics of an innate immune response. Effective utilization of ADCVI in vaccine strategies will depend on an understanding of the natural history of ADCVI during acute and chronic human immunodeficiency virus type 1 (HIV-1) infection. We used the simian immunodeficiency virus (SIV)-infected rhesus monkey as a model to study the kinetics of ADCVI in early infection, the durability of ADCVI through the course of infection, and the effectiveness of ADCVI against viruses with envelope mutations that are known to confer escape from antibody neutralization. We demonstrate the development of ADCVI, capable of inhibiting viral replication 100-fold, within 3 weeks of infection, preceding the development of a comparable-titer neutralizing antibody response by weeks to months. The emergence of ADCVI was temporally associated with the emergence of gp140-binding antibodies, and in most animals, ADCVI persisted through the course of infection. Highly evolved viral envelopes from viruses isolated at late time points following infection that were resistant to plasma neutralization remained susceptible to ADCVI, suggesting that the epitope determinants of neutralization escape are not shared by antibodies that mediate ADCVI. These findings suggest that despite the ability of SIV to mutate and adapt to multiple immunologic pressures during the course of infection, SIV envelope may not escape the binding of autologous antibodies that mediate ADCVI.     

The c3-v4 region is a major target of autologous neutralizing antibodies in human immunodeficiency virus type 1 subtype C infection

The early autologous neutralizing antibody response in human immunodeficiency virus type 1 (HIV-1) subtype C infections is often characterized by high titers, but the response is type specific with little to no cross-neutralizing activity. The specificities of these early neutralizing antibodies are not known; however, the type specificity suggests that they may target the variable regions of the envelope. Here, we show that cross-reactive anti-V3 antibodies developed within 3 to 12 weeks in six individuals but did not mediate autologous neutralization. Using a series of chimeric viruses, we found that antibodies directed at the V1V2, V4, and V5 regions contributed to autologous neutralization in some individuals, with V1V2 playing a more substantial role. However, these antibodies did not account for the total neutralizing capacity of these sera against the early autologous virus. Antibodies directed against the C3-V4 region were involved in autologous neutralization in all four sera studied. In two sera, transfer of the C3-V4 region rendered the chimera as sensitive to antibody neutralization as the parental virus. Although the C3 region, which contains the highly variable α2-helix was not a direct target in most cases, it contributed to the formation of neutralization epitopes as substitution of this region resulted in neutralization resistance. These data suggest that the C3 and V4 regions combine to form important structural motifs and that epitopes in this region are major targets of the early autologous neutralizing response in HIV-1 subtype C infection.