Immunotherapy during the acute SHIV infection of macaques confers long-term suppression of viremia.

We report that combination bNAb immunotherapy initiated on day 3 post-infection (PI) maintained durable CD8+ T cell-mediated suppression of SHIVAD8 viremia and preinoculation levels of CD4+ T cells in 9 of 13 treated monkeys during nearly 6 yr of observation, as assessed by successive CD8+ T cell-depletion experiments. In an extension of that study, two treatment interventions (bNAbs alone or cART plus bNAbs) beginning on week 2 PI were conducted and conferred controller status to 7 of 12 monkeys that was also dependent on control mediated by CD8+ cells. However, the median time to suppression of plasma viremia following intervention on week 2 was markedly delayed (85 wk) compared with combination bNAb immunotherapy initiated on day 3 (39 wk). In both cases, the principal correlate of virus control was the induction of CD8+ T cellular immunity.

Prevention and treatment of SHIVAD8 infection in rhesus macaques by a potent d-peptide HIV entry inhibitor.

Cholesterol-PIE12-trimer (CPT31) is a potent d-peptide HIV entry inhibitor that targets the highly conserved gp41 N-peptide pocket region. CPT31 exhibited strong inhibitory breadth against diverse panels of primary virus isolates. In a simian-HIV chimeric virus AD8 (SHIVAD8) macaque model, CPT31 prevented infection from a single high-dose rectal challenge. In chronically infected animals, CPT31 monotherapy rapidly reduced viral load by ∼2 logs before rebound occurred due to the emergence of drug resistance. In chronically infected animals with viremia initially controlled by combination antiretroviral therapy (cART), CPT31 monotherapy prevented viral rebound after discontinuation of cART. These data establish CPT31 as a promising candidate for HIV prevention and treatment.

Early antibody therapy can induce long-lasting immunity to SHIV.

Highly potent and broadly neutralizing anti-HIV-1 antibodies (bNAbs) have been used to prevent and treat lentivirus infections in humanized mice, macaques, and humans. In immunotherapy experiments, administration of bNAbs to chronically infected animals transiently suppresses virus replication, which invariably returns to pre-treatment levels and results in progression to clinical disease. Here we show that early administration of bNAbs in a macaque simian/human immunodeficiency virus (SHIV) model is associated with very low levels of persistent viraemia, which leads to the establishment of T-cell immunity and resultant long-term infection control. Animals challenged with SHIVAD8-EO by mucosal or intravenous routes received a single 2-week course of two potent passively transferred bNAbs (3BNC117 and 10-1074 (refs 13, 14)). Viraemia remained undetectable for 56-177 days, depending on bNAb half-life in vivo. Moreover, in the 13 treated monkeys, plasma virus loads subsequently declined to undetectable levels in 6 controller macaques. Four additional animals maintained their counts of T cells carrying the CD4 antigen (CD4+) and very low levels of viraemia persisted for over 2 years. The frequency of cells carrying replication-competent virus was less than 1 per 106 circulating CD4+ T cells in the six controller macaques. Infusion of a T-cell-depleting anti-CD8ß monoclonal antibody to the controller animals led to a specific decline in levels of CD8+ T cells and the rapid reappearance of plasma viraemia. In contrast, macaques treated for 15 weeks with combination anti-retroviral therapy, beginning on day 3 after infection, experienced sustained rebound plasma viraemia when treatment was interrupted. Our results show that passive immunotherapy during acute SHIV infection differs from combination anti-retroviral therapy in that it facilitates the emergence of potent CD8+ T-cell immunity able to durably suppress virus replication.

A single injection of anti-HIV-1 antibodies protects against repeated SHIV challenges.

Despite the success of potent anti-retroviral drugs in controlling human immunodeficiency virus type 1 (HIV-1) infection, little progress has been made in generating an effective HIV-1 vaccine. Although passive transfer of anti-HIV-1 broadly neutralizing antibodies can protect mice or macaques against a single high-dose challenge with HIV or simian/human (SIV/HIV) chimaeric viruses (SHIVs) respectively, the long-term efficacy of a passive antibody transfer approach for HIV-1 has not been examined. Here we show, on the basis of the relatively long-term protection conferred by hepatitis A immune globulin, the efficacy of a single injection (20 mg kg(-1)) of four anti-HIV-1-neutralizing monoclonal antibodies (VRC01, VRC01-LS, 3BNC117, and 10-1074 (refs 9 - 12)) in blocking repeated weekly low-dose virus challenges of the clade B SHIVAD8. Compared with control animals, which required two to six challenges (median = 3) for infection, a single broadly neutralizing antibody infusion prevented virus acquisition for up to 23 weekly challenges. This effect depended on antibody potency and half-life. The highest levels of plasma-neutralizing activity and, correspondingly, the longest protection were found in monkeys administered the more potent antibodies 3BNC117 and 10-1074 (median = 13 and 12.5 weeks, respectively). VRC01, which showed lower plasma-neutralizing activity, protected for a shorter time (median = 8 weeks). The introduction of a mutation that extends antibody half-life into the crystallizable fragment (Fc) domain of VRC01 increased median protection from 8 to 14.5 weeks. If administered to populations at high risk of HIV-1 transmission, such an immunoprophylaxis regimen could have a major impact on virus transmission.

Emergence of gp120 V3 variants confers neutralization resistance in an R5 simian-human immunodeficiency virus-infected macaque elite neutralizer that targets the N332 glycan of the human immunodeficiency virus type 1 envelope glycoprotein.

Neutralization-resistant simian-human immunodeficiency virus AD8 (SHIVAD8) variants that emerged in an infected macaque elite neutralizer targeting the human immunodeficiency virus type 1 (HIV-1) gp120 N332 glycan acquired substitutions of critical amino acids in the V3 region rather than losing the N332 glycosylation site. One of these resistant variants, carrying the full complement of gp120 V3 changes, was also resistant to the potent anti-HIV-1 monoclonal neutralizing antibodies PGT121 and 10-1074, both of which are also dependent on the presence of the gp120 N332 glycan.

Most rhesus macaques infected with the CCR5-tropic SHIV(AD8) generate cross-reactive antibodies that neutralize multiple HIV-1 strains.

The induction of broadly reacting neutralizing antibodies has been a major goal of HIV vaccine research. Characterization of a pathogenic CCR5 (R5)-tropic SIV/HIV chimeric virus (SHIV) molecular clone (SHIV(AD8-EO)) revealed that eight of eight infected animals developed cross-reactive neutralizing antibodies (NAbs) directed against an envelope glycoprotein derived from the heterologous HIV-1(DH12) strain. A panel of plasmas, collected from monkeys inoculated with either molecularly cloned or uncloned SHIV(AD8) stocks, exhibited cross-neutralization against multiple tier 1 and tier 2 HIV-1 clade B isolates. One SHIV(AD8)-infected animal also developed NAbs against clades A and C HIV-1 strains. In this particular infected macaque, the cross-reacting anti-HIV-1 NAbs produced between weeks 7 and 13 were directed against a neutralization-sensitive virus strain, whereas neutralizing activities emerging at weeks 41-51 targeted more neutralization-resistant HIV-1 isolates. These results indicate that the SHIV(AD8) macaque model represents a potentially valuable experimental system for investigating B-cell maturation and the induction of cross-reactive NAbs directed against multiple HIV-1 strains.

Pathogenicity and mucosal transmissibility of the R5-tropic simian/human immunodeficiency virus SHIV(AD8) in rhesus macaques: implications for use in vaccine studies.

There is an urgent need to develop new pathogenic R5 simian/human immunodeficiency viruses (SHIVs) for the evaluation of candidate anti-HIV vaccines in nonhuman primates. Here, we characterize swarm SHIV(AD8) stocks, prepared from three infected rhesus macaques with documented immunodeficiency at the time of euthanasia, for their capacity to establish durable infections in macaques following inoculation by the intravenous (i.v.) or intrarectal (i.r.) route. All three viral stocks (SHIV(AD8-CE8J), SHIV(AD8-CK15), and SHIV(AD8-CL98)) exhibited robust replication in vivo and caused marked depletion of CD4(+) T cells affecting both memory and naïve CD4(+) T lymphocyte subsets following administration by either route. Eleven of 22 macaques inoculated with the new SHIV(AD8) stocks were euthanized with clinical symptoms of immunodeficiency and evidence of opportunistic infections (Pneumocystis, Candida, and Mycobacterium). A single but unique founder virus, also present in the SHIV(AD8-CE8J) swarm stock, was transmitted to two animals following a single i.r. inoculation of approximately 3 50% animal infectious doses, which is close to the threshold required to establish infection in all exposed animals. Because the three new SHIV(AD8) viruses are mucosally transmissible, exhibited tier 2 sensitivity to anti-HIV-1 neutralizing antibodies, deplete CD4(+) T lymphocytes in vivo, and induce AIDS in macaques, they are eminently suitable as challenge viruses in vaccine experiments.

Rapid development of glycan-specific, broad, and potent anti-HIV-1 gp120 neutralizing antibodies in an R5 SIV/HIV chimeric virus infected macaque.

It is widely believed that the induction of a broadly neutralizing antibody (bNAb) response will be a critical component of a successful vaccine against HIV. A significant fraction of HIV-infected individuals mount bNAb responses, providing support for the notion that such responses could be elicited through vaccination. Infection of macaques with simian immunodeficiency virus (SIV) or SIV/HIV chimeric virus (SHIV) has been widely used to model aspects of HIV infection, but to date, only limited bNAb responses have been described. Here, we screened plasma from 14 R5-tropic SHIV-infected macaques for broadly neutralizing activity and identified a macaque with highly potent cross-clade plasma NAb response. Longitudinal studies showed that the development of broad and autologous NAb responses occurred coincidentally in this animal. Serum-mapping studies, using pseudovirus point mutants and antigen adsorption assays, indicated that the plasma bNAbs are specific for epitopes that include carbohydrates and are critically dependent on the glycan at position 332 of Env gp120. The results described herein provide insight into the development and evolution of a broad response, suggest that certain bNAb specificities may be more rapidly induced by immunization than others, and provide a potential model for the facile study of the development of bNAb responses.

Recombination-mediated changes in coreceptor usage confer an augmented pathogenic phenotype in a nonhuman primate model of HIV-1-induced AIDS.

Evolution of the env gene in transmitted R5-tropic human immunodeficiency virus type 1 (HIV-1) strains is the most widely accepted mechanism driving coreceptor switching. In some infected individuals, however, a shift in coreceptor utilization can occur as a result of the reemergence of a cotransmitted, but rapidly controlled, X4 virus. The latter possibility was studied by dually infecting rhesus macaques with X4 and R5 chimeric simian simian/human immunodeficiency viruses (SHIVs) and monitoring the replication status of each virus using specific primer pairs. In one of the infected monkeys, both SHIVs were potently suppressed by week 12 postinoculation, but a burst of viremia at week 51 was accompanied by an unrelenting loss of total CD4+ T cells and the development of clinical disease. PCR analyses of plasma viral RNA indicated an env gene segment containing the V3 region from the inoculated X4 SHIV had been transferred into the genetic background of the input R5 SHIV by intergenomic recombination, creating an X4 virus with novel replicative, serological, and pathogenic properties. These results indicate that the effects of retrovirus recombination in vivo can be functionally profound and may even occur when one of the recombination participants is undetectable in the circulation as cell-free virus.

Generation of the pathogenic R5-tropic simian/human immunodeficiency virus SHIVAD8 by serial passaging in rhesus macaques.

A new pathogenic R5-tropic simian/human immunodeficiency virus (SHIV) was generated following serial passaging in rhesus macaques. All 13 animals inoculated with SHIV(AD8) passaged lineages experienced marked depletions of CD4(+) T cells. Ten of these infected monkeys became normal progressors (NPs) and had gradual losses of both memory and naïve CD4(+) T lymphocytes, generated antiviral CD4(+) and CD8(+) T cell responses, and sustained chronic immune activation while maintaining variable levels of plasma viremia (10(2) to 10(5) RNA copies/ml for up to 3 years postinfection [p.i.]). To date, five NPs developed AIDS associated with opportunistic infections caused by Pneumocystis carinii, Mycobacterium avium, and Campylobacter coli that required euthanasia between weeks 100 and 199 p.i. Three other NPs have experienced marked depletions of circulating CD4(+) T lymphocytes (92 to 154 cells/microl) following 1 to 2 years of infection. When tested for coreceptor usage, the viruses isolated from four NPs at the time of their euthanasia remained R5 tropic. Three of the 13 SHIV(AD8)-inoculated macaques experienced a rapid-progressor syndrome characterized by sustained plasma viremia of >1 x 10(7) RNA copies/ml and rapid irreversible loss of memory CD4(+) T cells that required euthanasia between weeks 19 and 23 postinfection. The sustained viremia, associated depletion of CD4(+) T lymphocytes, and induction of AIDS make the SHIV(AD8) lineage of viruses a potentially valuable reagent for vaccine studies.

High frequencies of resting CD4+ T cells containing integrated viral DNA are found in rhesus macaques during acute lentivirus infections.

We and others have reported that the vast majority of virus-producing CD4(+) T cells during the acute infection of rhesus macaques with simian immunodeficiency virus (SIV) or CXCR4 (X4)-using simian/human immunodeficiency viruses (SHIVs) exhibited a nonactivated phenotype. These findings have been extended to show that resting CD4(+) T lymphocytes collected from SIV- or X4-SHIV-infected animals during the first 10 days of infection continue to release virus ex vivo. Furthermore, we observed high frequencies of integrated viral DNA (up to 5.1 x 10(4) DNA copies per 10(5) cells) in circulating resting CD4(+) T cells during the first 10 days of the infection. Integration of SIV DNA was detected only in memory CD4(+) T cells and SHIVs preferentially integrated into resting naïve CD4(+) T cells. Taken together, these results show that during the acute infection large numbers of resting CD4(+) T cells carry integrated nonhuman primate lentiviral DNA and are the major source of progeny virions irrespective of coreceptor usage. Prompt and sustained interventions are therefore required to block the rapid systemic dissemination of virus and prevent an otherwise fatal clinical outcome.

CD8+ and CD20+ lymphocytes cooperate to control acute simian immunodeficiency virus/human immunodeficiency virus chimeric virus infections in rhesus monkeys: modulation by major histocompatibility complex genotype.

We have previously described two isogenic molecularly cloned simian immunodeficiency virus/human immunodeficiency virus chimeric viruses (SHIVs) that differ from one another by 9 amino acids and direct distinct clinical outcomes in inoculated rhesus monkeys. SHIV(DH12R-Clone 7), like other highly pathogenic CXCR4-tropic SHIVs, induces rapid and complete depletions of CD4+ T lymphocytes and immunodeficiency in infected animals. In contrast, macaques inoculated with SHIV(DH12R-Clone 8) experience only partial and transient losses of CD4+ T cells, show prompt control of their viremia, and remain healthy for periods of time extending for up to 4 years. The contributions of CD8+ and CD20+ lymphocytes in suppressing the replication of the attenuated SHIV(DH12R-Clone 8) and maintaining a prolonged asymptomatic clinical course was assessed by treating animals with monoclonal antibodies that deplete each lymphocyte subset at the time of virus inoculation. The absence of either CD8+ or CD20+ cells during the SHIV(DH12R-Clone 8) acute infection resulted in the rapid, complete, and irreversible loss of CD4+ T cells; sustained high levels of postpeak plasma viremia; and symptomatic disease in Mamu-A*01-negative Indian rhesus monkeys. In Mamu-A*01-positive animals, however, the aggressive, highly pathogenic phenotype was observed only in macaques depleted of CD8+ cells; SHIV(DH12R-Clone 8) was effectively controlled in Mamu-A*01-positive monkeys in the absence of B lymphocytes. Taken together, these results indicate that both CD8+ and CD20+ B cells contribute to the control of primate lentiviral infection in Mamu-A*01-negative macaques. Furthermore, the major histocompatibility complex genotype of an infected animal, as exemplified by the Mamu-A*01 allele in this study, has the additional capacity to shift the balance of the composite immune response.

Induction of disease by a molecularly cloned highly pathogenic simian immunodeficiency virus/human immunodeficiency virus chimera is multigenic.

One of three full-length infectious molecular clones of SHIV(DH12R), designated SHIV(DH12R-CL-7) and obtained from productively infected rhesus monkey peripheral blood mononuclear cells, directed rapid and irreversible loss of CD4+ T cells within 3 weeks of its inoculation into Indian rhesus monkeys. Induction of complete CD4+ T-cell depletion by SHIV(DH12R-CL-7) was found to be dependent on inoculum size. The acquisition of this pathogenic phenotype was accompanied by the introduction of 42 amino acid substitutions into multiple genes of parental nonpathogenic SHIV(DH12). Transfer of the entire SHIV(DH12R-CL-7) env gene into the genetic background of nonpathogenic SHIV(DH12) failed to confer the rapid CD4+ T-lymphocyte-depleting syndrome; similarly, the substitution of gag plus pol sequences from SIV(smE543) for analogous SIV(mac239) genes in SHIV(DH12R-CL-7) attenuated the pathogenic phenotype. Amino acid changes affecting multiple viral genes are necessary, but insufficient by themselves, to confer the prototypically rapid and irreversible CD4+ T-cell-depleting phenotype exhibited by molecularly cloned SHIV(DH12R-CL-7).

Macrophage-tropic simian/human immunodeficiency virus chimeras use CXCR4, not CCR5, for infections of rhesus macaque peripheral blood mononuclear cells and alveolar macrophages.

After the nearly complete and irreversible depletion of CD4(+) T lymphocytes induced by highly pathogenic simian/human immunodeficiency virus chimeric viruses (SHIVs) during infections of rhesus monkeys, tissue macrophages are able to sustain high levels (>10(6) viral RNA copies/ml) of plasma viremia for several months. We recently reported that the virus present in the plasma during the late macrophage phase of infection had acquired changes that specifically targeted the V2 region of gp120 (H. Imamichi et al., Proc. Natl. Acad. Sci. USA 99:13813-13818, 2002); some of these SHIV variants were macrophage-tropic (M-tropic). Those findings have been extended by examining the tropic properties, coreceptor usage, and gp120 structure of five independent SHIVs recovered directly from lymph nodes of late-stage animals. All of these tissue-derived SHIV isolates were able to infect alveolar macrophages. These M-tropic SHIVs used CXCR4, not CCR5, for infections of rhesus monkey PBMC and primary alveolar macrophages. Because the starting highly pathogenic T-tropic SHIV inoculum also utilized CXCR4, these results indicate that the acquisition of M-tropism in the SHIV-macaque system is not accompanied by a change in coreceptor usage. Compared to the initial T-tropic SHIV inoculum, tissue-derived M-tropic SHIVs from individual infected animals carry gp120s containing similar changes (specific amino acid deletions, substitutions, and loss of N-linked glycosylation sites), primarily within the V1 and/or V2 regions of gp120.

Transfer of neutralizing IgG to macaques 6 h but not 24 h after SHIV infection confers sterilizing protection: implications for HIV-1 vaccine development.

Passive transfer of high-titered antiviral neutralizing IgG, known to confer sterilizing immunity in pig-tailed monkeys, has been used to determine how soon after virus exposure neutralizing antibodies (NAbs) must be present to block a simian immunodeficiency virus (SIV)/HIV chimeric virus infection. Sterilizing protection was achieved in three of four macaques receiving neutralizing IgG 6 h after intravenous SIV/HIV chimeric virus inoculation as monitored by PCR analyses of and attempted virus isolations from plasma, peripheral blood mononuclear cell, and lymph node specimens. In the fourth animal, the production of progeny virus was suppressed for >4 weeks. A delay in transferring NAbs until 24 h after virus challenge resulted in infection in two of two monkeys. These results suggest that even if a vaccine capable of eliciting broadly reactive NAbs against primary HIV-1 were at hand, the Abs generated must remain at, or rapidly achieve, high levels within a relatively short period after exposure to virus to prevent the establishment of a primate lentivirus infection.

Early control of highly pathogenic simian immunodeficiency virus/human immunodeficiency virus chimeric virus infections in rhesus monkeys usually results in long-lasting asymptomatic clinical outcomes.

In contrast to simian immunodeficiency viruses (SIVs), which induce immunodeficiency over a 1- to 2-year period, highly pathogenic simian-human immunodeficiency viruses (SHIVs) cause an irreversible and systemic depletion of CD4(+) T lymphocytes in macaque monkeys within weeks of inoculation. Nonetheless, the seemingly more aggressive SHIVs have proven to be easier to control by the same vaccine regimens which fail to contain SIV. Because early events during in vivo infections may determine both the pathogenic consequences of the challenge virus and its sensitivity to interventions that prevent disease, we have evaluated the effects of inoculum size and a potent antiretroviral drug on the development of disease in monkeys infected with SHIV(DH12R). The results obtained show that in a majority of inoculated animals, suppression of SHIV replication during the first 2 weeks of infection, which prevents complete loss of CD4(+) T cells, leads to very low to undetectable postpeak viremia and an asymptomatic clinical course for periods up to 4 years.

Determination of a statistically valid neutralization titer in plasma that confers protection against simian-human immunodeficiency virus challenge following passive transfer of high-titered neutralizing antibodies.

We previously reported that high-titered neutralizing antibodies directed against the human immunodeficiency virus type 1 (HIV-1) envelope can block the establishment of a simian immunodeficiency virus (SIV)/HIV chimeric virus (SHIV) infection in two monkeys following passive transfer (R. Shibata et al., Nat. Med. 5:204-210, 1999). In the present study, increasing amounts of neutralizing immunoglobulin G (IgG) were administered to 15 pig-tailed macaques in order to obtain a statistically valid protective neutralization endpoint titer in plasma. Using an in vitro assay which measures complete neutralization of the challenge SHIV, we correlated the titers of neutralizing antibodies in plasma at the time of virus inoculation (which ranged from 1:3 to 1:123) with the establishment of infection in virus-challenged animals. Ten of 15 monkeys in the present experiment were virus free as a result of neutralizing IgG administration as monitored by DNA PCR (peripheral blood mononuclear cells and lymph node cells), RNA PCR (plasma), virus isolation, and the transfer of lymph node cell suspensions (10(8) cells) plus 8 ml of whole blood from protected animals to naïve macaques. The titer of neutralizing antibodies in the plasma calculated to protect 99% of virus-challenged monkeys was 1:38.