A CCR5-tropic simian-HIV molecular clone capable of inducing AIDS in rhesus macaques

Increased, Durable B-Cell and ADCC Responses Associated with T-Helper Cell Responses to HIV-1 Envelope in Macaques Vaccinated with gp140 Occluded at the CD4 Receptor Binding Site

Strategies are needed to improve the immunogenicity of HIV-1 envelope (Env) antigens (Ag) for more long-lived, efficacious HIV-1 vaccine-induced B-cell responses. HIV-1 Env gp140 (native or uncleaved molecules) or gp120 monomeric proteins elicit relatively poor B-cell responses which are short-lived. We hypothesized that Env engagement of the CD4 receptor on T-helper cells results in anergic effects on T-cell recruitment and consequently a lack of strong, robust, and durable B-memory responses. To test this hypothesis, we occluded the CD4 binding site (CD4bs) of gp140 by stable cross-linking with a 3-kDa CD4 miniprotein mimetic, serving to block ligation of gp140 on CD4⁺ T cells while preserving CD4-inducible (CDi) neutralizing epitopes targeted by antibody-dependent cellular cytotoxicity (ADCC) effector responses. Importantly, immunization of rhesus macaques consistently gave superior B-cell (P < 0.001) response kinetics and superior ADCC (P < 0.014) in a group receiving the CD4bs-occluded vaccine compared to those of animals immunized with gp140. Of the cytokines examined, Ag-specific interleukin-4 (IL-4) T-helper enzyme-linked immunosorbent spot (ELISpot) assays of the CD4bs-occluded group increased earlier (P = 0.025) during the inductive phase. Importantly, CD4bs-occluded gp140 antigen induced superior B-cell and ADCC responses, and the elevated B-cell responses proved to be remarkably durable, lasting more than 60 weeks postimmunization.

IMPORTANCE Attempts to develop HIV vaccines capable of inducing potent and durable B-cell responses have been unsuccessful until now. Antigen-specific B-cell development and affinity maturation occurs in germinal centers in lymphoid follicles through a critical interaction between B cells and T follicular helper cells. The HIV envelope binds the CD4 receptor on T cells as soluble shed antigen or as antigen-antibody complexes, causing impairment in the activation of these specialized CD4-positive T cells. We proposed that CD4-binding impairment is partly responsible for the relatively poor B-cell responses to HIV envelope-based vaccines. To test this hypothesis, we blocked the CD4 binding site of the envelope antigen and compared it to currently used unblocked envelope protein. We found superior and durable B-cell responses in macaques vaccinated with an occluded CD4 binding site on the HIV envelope antigen, demonstrating a potentially important new direction in future design of new HIV vaccines.

The Envelope Gene of Transmitted HIV-1 Resists a Late Interferon Gamma-Induced Block

Type I interferon (IFN) signaling engenders an antiviral state that likely plays an important role in constraining HIV-1 transmission and contributes to defining subsequent AIDS pathogenesis. Type II IFN (IFN-γ) also induces an antiviral state but is often primarily considered to be an immunomodulatory cytokine. We report that IFN-γ stimulation can induce an antiviral state that can be both distinct from that of type I interferon and can potently inhibit HIV-1 in primary CD4⁺ T cells and a number of human cell lines. Strikingly, we find that transmitted/founder (TF) HIV-1 viruses can resist a late block that is induced by type II IFN, and the use of chimeric IFN-γ-sensitive/resistant viruses indicates that interferon resistance maps to the env gene. Simultaneously, in vitro evolution also revealed that just a single amino acid substitution in the envelope can confer substantial resistance to IFN-mediated inhibition. Thus, the env gene of transmitted HIV-1 confers resistance to a late block that is phenotypically distinct from blocks previously described to be resisted by env and is therefore mediated by unknown IFN-γ-stimulated factor(s) in human CD4⁺ T cells and cell lines. This important unidentified block could play a key role in constraining HIV-1 transmission.

IMPORTANCE The human immune system can hinder invading pathogens through interferon (IFN) signaling. One consequence of this signaling is that cells enter an antiviral state, increasing the levels of hundreds of defenses that can inhibit the replication and spread of viruses. The majority of HIV-1 infections result from a single virus particle (the transmitted/founder) that makes it past these defenses and colonizes the host. Thus, the founder virus is hypothesized to be a relatively interferon-resistant entity. Here, we show that certain HIV-1 envelope genes have the unanticipated ability to resist specific human defenses mediated by different types of interferons. Strikingly, the envelope gene from a founder HIV-1 virus is far better at evading these defenses than the corresponding gene from a common HIV-1 lab strain. Thus, these defenses could play a role in constraining the transmission of HIV-1 and may select for transmitted viruses that are resistant to this IFN-mediated inhibition.

Genetic Dissection of the Host Tropism of Human-Tropic Pathogens

Infectious diseases are the second leading cause of death worldwide. Although the host multitropism of some pathogens has rendered their manipulation possible in animal models, the human-restricted tropism of numerous viruses, bacteria, fungi, and parasites has seriously hampered our understanding of these pathogens. Hence, uncovering the genetic basis underlying the narrow tropism of such pathogens is critical for understanding their mechanisms of infection and pathogenesis. Moreover, such genetic dissection is essential for the generation of permissive animal models that can serve as critical tools for the development of therapeutics or vaccines against challenging human pathogens. In this review, we describe different experimental approaches utilized to uncover the genetic foundation regulating pathogen host tropism as well as their relevance for studying the tropism of several important human pathogens. Finally, we discuss the current and future uses of this knowledge for generating genetically modified animal models permissive for these pathogens.

Development of a novel rhesus macaque model with an infectious R5 simian-human immunodeficiency virus encoding HIV-1 CRF08_BC env

BACKGROUND

The CRF08_BC strain is one of the most predominant circulating Human immunodeficiency virus type 1 (HIV-1) strains in the Chinese pandemic. A simian-human immunodeficiency virus (SHIV) encoding HIV-1 CRF08_BC env is highly desirable to evaluate candidate AIDS vaccines in non-human primates.

METHODS

SHIV-KBQJ-12, which carries the envelope glycoprotein from QJ001, an infectious molecular clone of HIV-1 CRF08_BC, was generated. The replication capacity of SHIV-KBQJ-12 was determined both in human and rhesus macaque (Macaca mulatta) peripheral blood mononuclear cells (PBMCs) and in Chinese rhesus macaques.

RESULTS

SHIV-KBQJ-12 replicated efficiently in human and macaque PBMCs and displayed a preference for CCR5 as an entry coreceptor. Productive infection of two macaques by intravenous inoculation with SHIV-KBQJ-12 was confirmed.

CONCLUSIONS

SHIV-KBQJ-12 is an R5-tropic chimeric virus that can establish productive infection both in vitro and in vivo in Chinese rhesus macaques and will be useful to assess candidate HIV-1 CRF08_BC vaccines in China.

In vitro neutralization of low dose inocula at physiological concentrations of a monoclonal antibody which protects macaques against SHIV challenge

BACKGROUND

Passive transfer of antibodies can be protective in the simian human immunodeficiency virus (SHIV)--rhesus macaque challenge model. The human monoclonal antibody IgG1 b12 neutralizes human immunodeficiency type 1 (HIV-1) in vitro and protects against challenge by SHIV. Our hypothesis is that neutralizing antibodies can only completely inactivate a relatively small number of infectious virus.

METHODS AND FINDINGS

We have used GHOST cell assays to quantify individual infectious events with HIV-1SF162 and its SHIV derivatives: the relatively neutralization sensitive SHIV(SF162P4) isolate and the more resistant SHIV(SF162P3). A plot of the number of fluorescent GHOST cells with increasing HIV-1SF162 dose is not linear. It is likely that with high-dose inocula, infection with multiple virus produces additive fluorescence in individual cells. In studies of the neutralization kinetics of IgG1 b12 against these isolates, events during the absorption phase of the assay, as well as the incubation phase, determine the level of neutralization. It is possible that complete inactivation of a virus is limited to the time it is exposed on the cell surface. Assays can be modified so that neutralization of these very low doses of virus can be quantified. A higher concentration of antibody is required to neutralize the same dose of resistant SHIV(SF162P3) than the sensitive SHIV(SF162P4). In the absence of selection during passage, the density of the CCR5 co-receptor on the GHOST cell surface is reduced. Changes in the CD4 : CCR5 density ratio influence neutralization.

CONCLUSIONS

Low concentrations of IgG1 b12 completely inactivate small doses of the neutralization resistant SHIV(SF162P3). Assays need to be modified to quantify this effect. Results from modified assays may predict protection following repeated low-dose shiv challenges in rhesus macaques. It should be possible to induce this level of antibody by vaccination so that modified assays could predict the outcome of human trials.

Mucosal transmissibility, disease induction and coreceptor switching of R5 SHIVSF162P3N molecular clones in rhesus macaques

Background

Mucosally transmissible and pathogenic CCR5 (R5)-tropic simian-human immunodeficiency virus (SHIV) molecular clones are useful reagents to identity neutralization escape in HIV-1 vaccine experiments and to study the envelope evolutionary process and mechanistic basis for coreceptor switch during the course of natural infection.

Results

We observed progression to AIDS in rhesus macaques infected intrarectally with molecular clones of the pathogenic R5 SHIV_SF162P3N isolate. Expansion to CXCR4 usage was documented in one diseased macaque that mounted a neutralizing antibody response and in another that failed to do so, with the latter displaying a rapid progressor phenotype. V3 loop envelop glycoprotein gp120 sequence changes that are predictive of a CXCR4 (X4)-using phenotype in HIV-1 subtype B primary isolates, specifically basic amino acid substations at positions 11 (S11R), 24 (G24R) and 25 (D25K) of the loop were detected in the two infected macaques. Functional assays showed that envelopes with V3 S11R or D25K mutation were dual-tropic, infecting CD4+ target cells that expressed either the CCR5 or CXCR4 coreceptor. And, consistent with findings of coreceptor switching in macaques infected with the pathogenic isolate, CXCR4-using variant was first detected in the lymph node of the chronically infected rhesus monkey several weeks prior to its presence in peripheral blood. Moreover, X4 emergence in this macaque coincided with persistent peripheral CD4+ T cell loss and a decline in neutralizing antibody titer that are suggestive of immune deterioration, with macrophages as the major virus-producing cells at the end-stage of disease.

Conclusions

The data showed that molecular clones derived from the R5 SHIV_SF162P3N isolate are mucosally transmissible and induced disease in a manner similar to that observed in HIV-1 infected individuals, providing a relevant and useful animal infection model for in-depth analyses of host selection pressures and the env evolutionary changes that influence disease outcome, coreceptor switching and vaccine escape.

Animal models in virus research: their utility and limitations

Viral diseases are important threats to public health worldwide. With the number of emerging viral diseases increasing the last decades, there is a growing need for appropriate animal models for virus studies. The relevance of animal models can be limited in terms of mimicking human pathophysiology. In this review, we discuss the utility of animal models for studies of influenza A viruses, HIV and SARS-CoV in light of viral emergence, assessment of infection and transmission risks, and regulatory decision making. We address their relevance and limitations. The susceptibility, immune responses, pathogenesis, and pharmacokinetics may differ between the various animal models. These complexities may thwart translating results from animal experiments to the humans. Within these constraints, animal models are very informative for studying virus immunopathology and transmission modes and for translation of virus research into clinical benefit. Insight in the limitations of the various models may facilitate further improvements of the models.

Protection against rectal transmission of an emtricitabine-resistant simian/human immunodeficiency virus SHIV162p3M184V mutant by intermittent prophylaxis with Truvada

Daily preexposure prophylaxis (PrEP) with Truvada (emtricitabine [FTC] and tenofovir disoproxil fumarate [TDF]) is a novel HIV prevention strategy recently found to reduce HIV incidence among men who have sex with men. We used a macaque model of HIV transmission to investigate if Truvada maintains prophylactic efficacy against an FTC-resistant isolate containing the M184V mutation. Five macaques received a dose of Truvada 3 days before exposing them rectally to the simian/human immunodeficiency virus mutant SHIV162p3(M184V), followed by a second dose 2 h after exposure. Five untreated animals were used as controls. Virus exposures were done weekly for up to 14 weeks. Despite the high (>100-fold) level of FTC resistance conferred by M184V, all five treated animals were protected from infection, while the five untreated macaques were infected (P = 0.0008). Our results show that Truvada maintains high prophylactic efficacy against an FTC-resistant isolate. Increased susceptibility to tenofovir due to M184V and other factors, including residual antiviral activity by FTC and/or reduced virus fitness due to M184V, may all have contributed to the observed protection.

Generation and mucosal transmissibility of emtricitabine- and tenofovir-resistant SHIV162P3 mutants in macaques

Transmission of drug-resistant HIV has been widely documented. We generated tenofovir (TFV)- and emtricitabine (FTC)-resistant SHIV162P3 mutants that can be used to investigate the transmission efficiency of drug-resistant viruses and their impact on the efficacy of pre-exposure prophylaxis. Both SHIV162p3(M184V) and SHIV162p3(K65R) replicated in vitro at high titers. Drug resistance profiles were similar to those seen in HIV. Virus infectivity to virion particle ratios were 4- and 10-fold lower in SHIV162p3(M184V) and SHIV162p3(K65R), compared to a concurrently generated WT SHIV162p3, respectively. Mucosal transmissibility studies using a repeat low-dose macaque model of rectal and vaginal transmission showed that both mutants were able to efficiently infect macaques only after the dose was increased to adjust for fitness reductions due to K65R and M184V. Our results in limited number of macaques suggest that the reduction in fitness due to M184V and K65R decreases virus transmissibility, and identify in vitro infectivity parameters that associate with mucosal transmissibility.

Immunization with wild-type or CD4-binding-defective HIV-1 Env trimers reduces viremia equivalently following heterologous challenge with simian-human immunodeficiency virus

We recently reported that rhesus macaques inoculated with CD4-binding-competent and CD4-binding-defective soluble YU2-derived HIV-1 envelope glycoprotein (Env) trimers in adjuvant generate comparable levels of Env-specific binding antibodies (Abs) and T cell responses. We also showed that Abs directed against the Env coreceptor binding site (CoRbs) were elicited only in animals immunized with CD4-binding-competent trimers and not in animals immunized with CD4-binding-defective trimers, indicating that a direct interaction between Env and CD4 occurs in vivo. To investigate both the overall consequences of in vivo Env-CD4 interactions and the elicitation of CoRbs-directed Abs for protection against heterologous simian-human immunodeficiency virus (SHIV) challenge, we exposed rhesus macaques immunized with CD4-binding-competent and CD4-binding-defective trimers to the CCR5-tropic SHIV-SF162P4 challenge virus. Compared to unvaccinated controls, all vaccinated animals displayed improved control of plasma viremia, independent of the presence or absence of CoRbs-directed Abs prior to challenge. Immunization resulted in plasma responses that neutralized the heterologous SHIV challenge stock in vitro, with similar neutralizing Ab titers elicited by the CD4-binding-competent and CD4-binding-defective trimers. The neutralizing responses against both the SHIV-SF162P4 stock and a recombinant virus pseudotyped with a cloned SHIV-SF162P4-derived Env were significantly boosted by the SHIV challenge. Collectively, these results suggest that the capacity of soluble Env trimers to interact with primate CD4 in vivo and to stimulate the production of moderate titers of CoRbs-directed Abs did not influence the magnitude of the neutralizing Ab recall response after viral challenge or the subsequent control of viremia in this heterologous SHIV challenge model.

"Resistance" to PSC-RANTES revisited: two mutations in human immunodeficiency virus type 1 HIV-1 SF162 or simian-human immunodeficiency virus SHIV SF162-p3 do not confer resistance

Resistance of human immunodeficiency virus type 1 (HIV-1) to small-molecule CCR5 inhibitors is well demonstrated, but resistance to macromolecular CCR5 inhibitors (e.g., PSC-RANTES) that act by both CCR5 internalization and receptor blockade had not been reported until recently (3). The report of a single simian-human immunodeficiency virus SHIV(SF162-p3) variant with one V3 and one gp41 sequence change in gp160 that conferred both altered replicative fitness and resistance to PSC-RANTES was therefore surprising. We introduced the same two mutations into both the parental HIV-1(SF162) and the macaque-adapted SHIV(SF162-p3) and found minor differences in entry fitness but no changes in sensitivity to inhibition by either PSC-RANTES or the small-molecule allosteric inhibitor TAK-779. We attribute the earlier finding to confounding fitness effects with inhibitor sensitivity.

Cellular gene expression profiles in rhesus macaques challenged mucosally with a pathogenic R5 tropic simian human immunodeficiency virus isolate

Insights into the host factors that contribute to an effective antiviral immune response may be obtained by examining global gene expression in simian human immunodeficiency virus (SHIV)-infected nonhuman primates that exhibit different virological outcomes. Immune responses and gene expression profiles in peripheral blood mononuclear cells (PBMCs) were compared between animals that controlled or did not control viremia after infection. Rectal inoculation of eight rhesus macaques with R5-tropic SHIV(SF162P3) resulted in a high level of plasma viremia during the acute phase of infection. The viremia was controlled to below levels of detection in six of these animals at the set point (controllers), whereas two animals had persistent viremia throughout the 140 wk that the animals were monitored (non-controllers). CD4(+) T-cell counts declined slightly in both controllers and non-controllers in the acute phase of infection, but CD4(+) T-cell counts continued to decline only in the non-controllers. Neutralizing antibodies to the challenge virus were variable and could not account for the control of viremia. However, analysis of the cellular gene expression profiles in the PBMCs from both groups of animals revealed distinctive gene expression patterns between controllers and non-controllers. Using the paired LPE test, 59 genes with p values <0.01 were identified and specific differences in the gene expression profiles in PBMCs from controllers versus non-controllers were detected.

Antiretroviral pre-exposure prophylaxis prevents vaginal transmission of HIV-1 in humanized BLT mice

BACKGROUND

Worldwide, vaginal transmission now accounts for more than half of newly acquired HIV-1 infections. Despite the urgency to develop and implement novel approaches capable of preventing HIV transmission, this process has been hindered by the lack of adequate small animal models for preclinical efficacy and safety testing. Given the importance of this route of transmission, we investigated the susceptibility of humanized mice to intravaginal HIV-1 infection.

METHODS AND FINDINGS

We show that the female reproductive tract of humanized bone marrow-liver-thymus (BLT) mice is reconstituted with human CD4+ T and other relevant human cells, rendering these humanized mice susceptible to intravaginal infection by HIV-1. Effects of HIV-1 infection include CD4+ T cell depletion in gut-associated lymphoid tissue (GALT) that closely mimics what is observed in HIV-1-infected humans. We also show that pre-exposure prophylaxis with antiretroviral drugs is a highly effective method for preventing vaginal HIV-1 transmission. Whereas 88% (7/8) of BLT mice inoculated vaginally with HIV-1 became infected, none of the animals (0/5) given pre-exposure prophylaxis of emtricitabine (FTC)/tenofovir disoproxil fumarate (TDF) showed evidence of infection (Chi square = 7.5, df = 1, p = 0.006).

CONCLUSIONS

The fact that humanized BLT mice are susceptible to intravaginal infection makes this system an excellent candidate for preclinical evaluation of both microbicides and pre-exposure prophylactic regimens. The utility of humanized mice to study intravaginal HIV-1 transmission is particularly highlighted by the demonstration that pre-exposure prophylaxis can prevent intravaginal HIV-1 transmission in the BLT mouse model.

Heterologous prime/boost immunization of rhesus monkeys by using diverse poxvirus vectors

As the diversity of potential immunogens increases within certain classes of vectors, the possibility has arisen of employing heterologous prime/boost immunizations using diverse members of the same family of vectors. The present study was initiated to explore the use of divergent pox vectors in a prime/boost regimen to elicit high-frequency cellular immune responses to human immunodeficiency virus type 1 envelope and simian immunodeficiency virus gag in rhesus monkeys. We demonstrated that monkeys vaccinated with a recombinant modified vaccinia virus Ankara (rMVA) prime/recombinant fowlpox virus (rFPV) boost regimen and monkeys vaccinated with a recombinant vaccinia virus prime/rFPV boost regimen developed comparable cellular immune responses that were greater in magnitude than those elicited by a homologous prime/boost with rMVA. Nevertheless, comparable magnitude recall cellular immune responses were observed in monkeys vaccinated with heterologous and homologous recombinant poxvirus following challenge with the CXCR4-tropic SHIV-89.6P. Consistent with this finding, comparable levels of containment of viral replication and CD4(+) T-lymphocyte preservation were seen in these groups of recombinant poxvirus-vaccinated monkeys. This study supports further exploration of combining recombinant vectors of the same family in prime/boost immunization strategies to optimize vaccine-elicited cellular immune responses.

Molecularly cloned SHIV-1157ipd3N4: a highly replication- competent, mucosally transmissible R5 simian-human immunodeficiency virus encoding HIV clade C Env

Human immunodeficiency virus type 1 (HIV-1) clade C causes >50% of all HIV infections worldwide, and an estimated 90% of all transmissions occur mucosally with R5 strains. A pathogenic R5 simian-human immunodeficiency virus (SHIV) encoding HIV clade C env is highly desirable to evaluate candidate AIDS vaccines in nonhuman primates. To this end, we generated SHIV-1157i, a molecular clone from a Zambian infant isolate that carries HIV clade C env. SHIV-1157i was adapted by serial passage in five monkeys, three of which developed peripheral CD4(+) T-cell depletion. After the first inoculated monkey developed AIDS at week 137 postinoculation, transfer of its infected blood to a naïve animal induced memory T-cell depletion and thrombocytopenia within 3 months in the recipient. In parallel, genomic DNA from the blood donor was amplified to generate the late proviral clone SHIV-1157ipd3. To increase the replicative capacity of SHIV-1157ipd3, an extra NF-kappaB binding site was engineered into its 3' long terminal repeat, giving rise to SHIV-1157ipd3N4. This virus was exclusively R5 tropic and replicated more potently in rhesus peripheral blood mononuclear cells than SHIV-1157ipd3 in the presence of tumor necrosis factor alpha. Rhesus macaques of Indian and Chinese origin were next inoculated intrarectally with SHIV-1157ipd3N4; this virus replicated vigorously in both sets of monkeys. We conclude that SHIV-1157ipd3N4 is a highly replication-competent, mucosally transmissible R5 SHIV that represents a valuable tool to test candidate AIDS vaccines targeting HIV-1 clade C Env.