Vaccination of macaques with long-standing SIVmac251 infection lowers the viral set point after cessation of antiretroviral therapy

Effects of therapeutic vaccination on the control of SIV in rhesus macaques with variable responsiveness to antiretroviral drugs

A therapeutic vaccine that induces lasting control of HIV infection could eliminate the need for lifelong adherence to antiretroviral therapy. This study investigated a therapeutic DNA vaccine delivered with a single adjuvant or a novel combination of adjuvants to augment T cell immunity in the blood and gut-associated lymphoid tissue in SIV-infected rhesus macaques. Animals that received DNA vaccines expressing SIV proteins, combined with plasmids expressing adjuvants designed to increase peripheral and mucosal T cell responses, including the catalytic subunit of the E. coli heat-labile enterotoxin, IL-12, IL-33, retinaldehyde dehydrogenase 2, soluble PD-1 and soluble CD80, were compared to mock-vaccinated controls. Following treatment interruption, macaques exhibited variable levels of viral rebound, with four animals from the vaccinated groups and one animal from the control group controlling virus at median levels of 10³ RNA copies/ml or lower (controllers) and nine animals, among all groups, exhibiting immediate viral rebound and median viral loads greater than 10³ RNA copies/ml (non-controllers). Although there was no significant difference between the vaccinated and control groups in protection from viral rebound, the variable virological outcomes during treatment interruption enabled an examination of immune correlates of viral replication in controllers versus non-controllers regardless of vaccination status. Lower viral burden in controllers correlated with increased polyfunctional SIV-specific CD8⁺ T cells in mesenteric lymph nodes and blood prior to and during treatment interruption. Notably, higher frequencies of colonic CD4⁺ T cells and lower Th17/Treg ratios prior to infection in controllers correlated with improved responses to ART and control of viral rebound. These results indicate that mucosal immune responses, present prior to infection, can influence efficacy of antiretroviral therapy and the outcome of immunotherapeutic vaccination, suggesting that therapies capable of modulating host mucosal responses may be needed to achieve HIV cure.

Impact of analytical treatment interruption on the central nervous system in a simian-HIV model

OBJECTIVE(S)

Analytical treatment interruption (ATI) studies are often used to evaluate potential HIV cure strategies. This study was conducted to determine the impact of ATI on simian-HIV (SHIV) infection in the central nervous system.

DESIGN

Animal study.

METHODS

Nine rhesus macaques were inoculated with SHIV-1157ipd3N4. Antiretroviral therapy (ART) was administered from week 2 to 18. At week 18, four animals were euthanized (no-ATI-group) and five underwent ATI (ATI-group) and were euthanized at 12 weeks post viral rebound. Plasma and cerebrospinal fluid (CSF) SHIV-RNA, markers of inflammation and brain CD3+, CD68+/CD163+ and RNA+ cells were measured.

RESULTS

All nine animals were SHIV-infected, with median pre-ART plasma and CSF SHIV-RNA of 6.2 and 3.6 log10copies/ml. Plasma and CSF IL-15, monocyte chemoattractant protein-1, IFN-γ-induced protein-10 and neopterin increased postinfection. ART initiation was associated with rapid and complete suppression of plasma viremia and reductions in plasma and CSF IL-15, IFN-γ-induced protein-10, neopterin and CSF monocyte chemoattractant protein-1. Median time to plasma viral rebound was 21 days post-ATI. At 12 weeks postrebound, CSF SHIV-RNA was undetectable and no increases in plasma and CSF markers of inflammation were found. Higher numbers of CD3+ and CD68+/CD163+ cells were seen in the brains of 3/5 and 1/5 animals, respectively, in the ATI-group when compared with no-ATI-group. SHIV-RNA+ cells were not identified in the brain in either group post-ATI.

CONCLUSION

ATI in macaques that initiated ART during early SHIV-1157ipd3N4 infection was associated with mild, localized T-cell infiltrate in the brain without detectable SHIV-RNA in the brain or CSF, or elevation in CSF soluble markers of inflammation.

IL-2 Restores T-Cell Dysfunction Induced by Persistent Mycobacterium tuberculosis Antigen Stimulation

Tuberculosis (TB) is a chronic disease mainly caused by Mycobacterium tuberculosis. The function of T cells usually decreased and even exhausted in severe TB such as multiple drug resistant TB (MDR-TB), which might lead to the failure of treatment in return. The mechanism of T cell dysfunction in TB is still not clear. In this study we set up a mouse model of T cell dysfunction by persistent M. tuberculosis antigen stimulation and investigated the therapeutic role of interleukin 2 (IL-2) in it. C57BL/6 mice were primed with Mycobacterium bovis Bacillus Calmette-Guérin (BCG) and boosted repeatedly with a combination of M. tuberculosis fusion proteins Mtb10.4-HspX (MH) plus ESAT6-Ag85B-MPT64 _<190-198>-Mtb8.4-Rv2626c (LT70) or MH plus ESAT6 and CFP10 with adjuvant of N, N'-dimethyl-N, N'-dioctadecylammonium bromide (DDA) plus polyinosinic-polycytidylic acid (Poly I:C). Following persistent antigen stimulation, the mice were treated with IL-2 and the therapeutic effects were analyzed. The results showed that compared with the mice that received transient antigen stimulation (boost twice), persistent antigen stimulation (boost more than 10 times) resulted in decrease of antigen specific IFN-γ and IL-2 production, reduction of memory CD8⁺ T cells, over-expression of immune checkpoint programmed cell death protein 1 (PD-1), and impaired the protective immunity against bacterial challenge. Treating the T cell functionally exhausted mice with IL-2 restored antigen-specific T cell responses and protective efficacy. In conclusion, persistent stimulation with M. tuberculosis antigens induced T cell dysfunction, which could be restored by complement of IL-2.

Negative modulation of suppressive HIV-specific regulatory T cells by IL-2 adjuvanted therapeutic vaccine

The potential benefit in using IL-2 in immunotherapy for cancer and autoimmunity has been linked to the modulation of immune responses, which partly relies on a direct effect on Tregs populations. Here, we revisited the role of IL-2 in HIV infection and investigated whether its use as an adjuvant with therapeutic vaccination, impacts on HIV-specific responses. Antiretroviral therapy treated-patients were randomized to receive 4 boosts of vaccination (ALVACHIV/Lipo-6T, weeks 0/4/8/12) followed by 3 cycles of IL-2 (weeks 16/24/32) before treatment interruption (TI) at week40. IL-2 administration increased significantly HIV-specific CD4⁺CD25⁺CD134⁺ T-cell responses, which inversely correlated with viral load after TI (r = -0.7, p <0.007) in the vaccine/IL-2 group. IL-2 increased global CD25⁺CD127^lowFoxP3⁺Tregs (p <0.05) while it decreased HIV- but not CMV- specific CD39⁺FoxP3⁺CD25⁺CD134⁺Tregs (p <0.05). HIV-specific Tregs were inversely correlated with IFN-γ producing specific-effectors (p = 0.03) and positively correlated with viral load (r = 0.7, p = 0.01), revealing their undesired presence during chronic infection. Global Tregs, but not HIV-specific Tregs, inversely correlated with a decrease in exhausted PD1⁺CD95⁺ T-cells (p = 0.001). Altogether, our results underline the negative impact of HIV-specific Tregs on HIV-specific effectors and reveal the beneficial use of IL-2 as an adjuvant as its administration increases global Tregs that impact on T-cell exhaustion and decreases HIV-specific CD39⁺Tregs by shifting the balance towards effectors.

Interleukin-2 (IL-2) has been used in immunotherapy for cancer and autoimmunity and its beneficial effect has been linked to the modulation of immune responses, which partly relies on a direct effect on Tregs populations. In this study, we assessed the role of IL-2 in HIV infection and investigated whether its use as an adjuvant with therapeutic vaccination, impacts on HIV-specific responses. We show that IL-2 administration increased HIV-specific CD4⁺CD25⁺CD134⁺ T-cell responses which inversely correlated with viral load after treatment interruption in the vaccine/IL-2 group. We also show that IL-2 increased global CD25⁺CD127^lowFoxP3⁺Tregs while it decreased HIV- but not CMV- specific CD39⁺FoxP3⁺CD25⁺CD134⁺Tregs. Moreover, we show that HIV-specific Tregs were inversely correlated with IFN-γ-producing specific-effectors and positively correlated with viral load. Moreover, we show that global Tregs, but not HIV-specific Tregs, inversely correlated with a decrease in exhausted PD1⁺CD95⁺ T-cells. Altogether, our results underline the negative impact of HIV-specific Tregs on HIV-specific effectors and reveal the beneficial use of IL-2 as an adjuvant as its administration increases global Tregs that impact on T-cell exhaustion and decreases HIV-specific CD39⁺Tregs by shifting the balance towards effectors.

Intact dendritic cell pathogen-recognition receptor functions associate with chronic hepatitis C treatment-induced viral clearance

Although studies have addressed the exhaustion of the host's immune response to HCV and its role in treatment, there is little information about the possible contribution of innate immunity to treatment-induced clearance. We hypothesized that because intact myeloid dendritic cell (MDC) pathogen sensing functions are associated with improved HCV-specific CD8⁺ T cell functionality in some chronically infected patients, it might enhance HCV clearance rate under standard interferon therapy. To investigate this hypothesis, TLR-induced MDC activation and HCV-specific CD8⁺ T cell response quality were monitored longitudinally at the single-cell level using polychromatic flow cytometry in chronically infected patients undergoing interferon therapy. We correlated the immunological, biochemical and virological data with response to treatment. We demonstrate that the clinical efficacy of interferon-induced viral clearance is influenced by the extent to which HCV inhibits MDC functions before treatment, rather than solely on a breakdown of the extrinsic T cell immunosuppressive environment. Thus, viral inhibition of MDC functions before treatment emerges as a co-determining factor in the clinical efficacy of interferon therapy during chronic HCV infection.

Immune interventions in HIV infection

Immune-based therapy (IBT) interventions have found a window of opportunity within some limitations of the otherwise successful combined antiretroviral therapy (cART). Two major paradigms drove immunotherapeutic research to combat human immunodeficiency virus (HIV) infection. First, IBTs were proposed either to help restore CD4(+) T-cell counts in cases of therapeutic failures with cytokines, interleukin-2 (IL-2) or IL-7, or to better control HIV and disease progression during treatment interruptions with anti-HIV therapeutic candidate vaccines. The most widely used candidates were HIV-recombinant live vector-based alone or combined with other vaccine compounds and dendritic cell (DC) therapies. A more recent and current paradigm aims at achieving HIV cure by combining IBT with cART using either cytokines to reactivate virus production in latently infected cells and/or therapeutic immunization to boost HIV-specific immunity in a 'shock and kill' strategy. This review summarizes the rationale, hopes, and mechanisms of successes and failures of these cytokine-based and vaccine-based immune interventions. Results from these first series of IBTs have been so far somewhat disappointing in terms of clinical relevance, but have provided lessons that are discussed in light of the future combined strategies to be developed toward an HIV cure.

Enhancing virus-specific immunity in vivo by combining therapeutic vaccination and PD-L1 blockade in chronic hepadnaviral infection

Hepatitis B virus (HBV) persistence is facilitated by exhaustion of CD8 T cells that express the inhibitory receptor programmed cell death-1 (PD-1). Improvement of the HBV-specific T cell function has been obtained in vitro by inhibiting the PD-1/PD-ligand 1 (PD-L1) interaction. In this study, we examined whether in vivo blockade of the PD-1 pathway enhances virus-specific T cell immunity and leads to the resolution of chronic hepadnaviral infection in the woodchuck model. The woodchuck PD-1 was first cloned, characterized, and its expression patterns on T cells from woodchucks with acute or chronic woodchuck hepatitis virus (WHV) infection were investigated. Woodchucks chronically infected with WHV received a combination therapy with nucleoside analogue entecavir (ETV), therapeutic DNA vaccination and woodchuck PD-L1 antibody treatment. The gain of T cell function and the suppression of WHV replication by this therapy were evaluated. We could show that PD-1 expression on CD8 T cells was correlated with WHV viral loads during WHV infection. ETV treatment significantly decreased PD-1 expression on CD8 T cells in chronic carriers. In vivo blockade of PD-1/PD-L1 pathway on CD8 T cells, in combination with ETV treatment and DNA vaccination, potently enhanced the function of virus-specific T cells. Moreover, the combination therapy potently suppressed WHV replication, leading to sustained immunological control of viral infection, anti-WHs antibody development and complete viral clearance in some woodchucks. Our results provide a new approach to improve T cell function in chronic hepatitis B infection, which may be used to design new immunotherapeutic strategies in patients.

Chronic hepatitis B virus (HBV) infection is still one of the major public health problems. Two billion people worldwide have been infected with HBV, of whom more than 360 million developed chronic infection. Every year, approximately one million of these individuals will die from HBV-associated liver diseases such as cirrhosis and hepatocellular carcinoma (HCC). Treatment of chronic hepatitis B remains a clinical challenge, and alternative strategies to treat chronic HBV infection are urgently needed. Here, we designed a new combination strategy to enhance the patient's own antiviral immune response and to achieve long-term viral suppression. The therapeutic effect of our combination therapy strategy for chronic hepadnaviral infection was tested in the woodchuck model. We demonstrated that our novel combination therapy could elicit potent antiviral immune response and achieved a strong antiviral effect, leading to sustained immunological control of chronic hepadnaviral infection and complete viral clearance in treated woodchucks. The results of this study may have an impact on clinical trials of the immunotherapy in chronically HBV-infected patients.

PD-L1 blockade synergizes with IL-2 therapy in reinvigorating exhausted T cells

The inhibitory receptor programmed cell death 1 (PD-1) plays a major role in functional exhaustion of T cells during chronic infections and cancer, and recent clinical data suggest that blockade of the PD-1 pathway is an effective immunotherapy in treating certain cancers. Thus, it is important to define combinatorial approaches that increase the efficacy of PD-1 blockade. To address this issue, we examined the effect of IL-2 and PD-1 ligand 1 (PD-L1) blockade in the mouse model of chronic lymphocytic choriomeningitis virus (LCMV) infection. We found that low-dose IL-2 administration alone enhanced CD8+ T cell responses in chronically infected mice. IL-2 treatment also decreased inhibitory receptor levels on virus-specific CD8+ T cells and increased expression of CD127 and CD44, resulting in a phenotype resembling that of memory T cells. Surprisingly, IL-2 therapy had only a minimal effect on reducing viral load. However, combining IL-2 treatment with blockade of the PD-1 inhibitory pathway had striking synergistic effects in enhancing virus-specific CD8+ T cell responses and decreasing viral load. Interestingly, this reduction in viral load occurred despite increased numbers of Tregs. These results suggest that combined IL-2 therapy and PD-L1 blockade merits consideration as a regimen for treating human chronic infections and cancer.

Prolonged tenofovir treatment of macaques infected with K65R reverse transcriptase mutants of SIV results in the development of antiviral immune responses that control virus replication after drug withdrawal

BACKGROUND

We reported previously that while prolonged tenofovir monotherapy of macaques infected with virulent simian immunodeficiency virus (SIV) resulted invariably in the emergence of viral mutants with reduced in vitro drug susceptibility and a K65R mutation in reverse transcriptase, some animals controlled virus replication for years. Transient CD8+ cell depletion or short-term tenofovir interruption within 1 to 5 years of treatment demonstrated that a combination of CD8+ cell-mediated immune responses and continued tenofovir therapy was required for sustained suppression of viremia. We report here follow-up data on 5 such animals that received tenofovir for 8 to 14 years.

RESULTS

Although one animal had a gradual increase in viremia from 3 years onwards, the other 4 tenofovir-treated animals maintained undetectable viremia with occasional viral blips (≤ 300 RNA copies/ml plasma). When tenofovir was withdrawn after 8 to 10 years from three animals with undetectable viremia, the pattern of occasional episodes of low viremia (≤ 3600 RNA/ml plasma) continued throughout the 10-month follow-up period. These animals had low virus levels in lymphoid tissues, and evidence of multiple SIV-specific immune responses.

CONCLUSION

Under certain conditions (i.e., prolonged antiviral therapy initiated early after infection; viral mutants with reduced drug susceptibility) a virus-host balance characterized by strong immunologic control of virus replication can be achieved. Although further research is needed to translate these findings into clinical applications, these observations provide hope for a functional cure of HIV infection via immunotherapeutic strategies that boost antiviral immunity and reduce the need for continuous antiretroviral therapy.

Therapeutic DNA vaccine induces broad T cell responses in the gut and sustained protection from viral rebound and AIDS in SIV-infected rhesus macaques

Immunotherapies that induce durable immune control of chronic HIV infection may eliminate the need for life-long dependence on drugs. We investigated a DNA vaccine formulated with a novel genetic adjuvant that stimulates immune responses in the blood and gut for the ability to improve therapy in rhesus macaques chronically infected with SIV. Using the SIV-macaque model for AIDS, we show that epidermal co-delivery of plasmids expressing SIV Gag, RT, Nef and Env, and the mucosal adjuvant, heat-labile E. coli enterotoxin (LT), during antiretroviral therapy (ART) induced a substantial 2-4-log fold reduction in mean virus burden in both the gut and blood when compared to unvaccinated controls and provided durable protection from viral rebound and disease progression after the drug was discontinued. This effect was associated with significant increases in IFN-γ T cell responses in both the blood and gut and SIV-specific CD8+ T cells with dual TNF-α and cytolytic effector functions in the blood. Importantly, a broader specificity in the T cell response seen in the gut, but not the blood, significantly correlated with a reduction in virus production in mucosal tissues and a lower virus burden in plasma. We conclude that immunizing with vaccines that induce immune responses in mucosal gut tissue could reduce residual viral reservoirs during drug therapy and improve long-term treatment of HIV infection in humans.

The use of nonhuman primate models of HIV infection for the evaluation of antiviral strategies

Several nonhuman primate models are used in HIV/AIDS research. In contrast to natural host models, infection of macaques with virulent simian immunodeficiency virus (SIV) isolates results in a disease (simian AIDS) that closely resembles HIV infection and AIDS. Although there is no perfect animal model, and each of the available models has its limitations, a carefully designed study allows experimental approaches that are not feasible in humans, but that can provide better insights in disease pathogenesis and proof-of-concept of novel intervention strategies. In the early years of the HIV pandemic, nonhuman primate models played a minor role in the development of antiviral strategies. Since then, a better understanding of the disease and the development of better compounds and assays to monitor antiviral effects have increased the usefulness and relevance of these animal models in the preclinical development of HIV vaccines, microbicides, and antiretroviral drugs. Several strategies that were first discovered to have efficacy in nonhuman primate models are now increasingly used in humans. Recent trends include the use of nonhuman primate models to explore strategies that could reduce viral reservoirs and, ultimately, attempt to cure infection. Ongoing comparison of results obtained in nonhuman primate models with those observed in human studies will lead to further validation and improvement of these animal models so they can continue to advance our scientific knowledge and guide clinical trials.

SIV antigen immunization induces transient antigen-specific T cell responses and selectively activates viral replication in draining lymph nodes in retroviral suppressed rhesus macaques

Background

HIV infection causes a qualitative and quantitative loss of CD4⁺ T cell immunity. The institution of anti-retroviral therapy (ART) restores CD4⁺ T cell responses to many pathogens, but HIV-specific responses remain deficient. Similarly, therapeutic immunization with HIV antigens of chronically infected, ART treated subjects results in poor induction of HIV-specific CD4 responses. In this study, we used a macaque model of ART treatment during chronic infection to study the virologic consequences of SIV antigen stimulation in lymph nodes early after immunization. Rhesus CMV (RhCMV) seropositive, Mamu A*01 positive rhesus macaques were chronically infected with SIVmac251 and treated with ART. The immune and viral responses to SIV gag and RhCMV pp65 antigen immunization in draining lymph nodes and peripheral blood were analyzed. Animals were immunized on contralateral sides with SIV gag and RhCMV pp65 encoding plasmids, which allowed lymph nodes draining each antigen to be obtained at the same time from the same animal for direct comparison.

Results

We observed that both SIV and RhCMV immunizations stimulated transient antigen-specific T cell responses in draining lymph nodes. The RhCMV-specific responses were potent and sustained (50 days post-immunization) in the periphery, while the SIV-specific responses were transient and extinguished quickly. The SIV antigen stimulation selectively induced transient SIV replication in draining lymph nodes.

Conclusions

The data are consistent with a model whereby viral replication in response to SIV antigen stimulation limits the generation of SIV antigen-specific responses and suggests a potential mechanism for the early loss and poor HIV-specific CD4⁺ T cell response observed in HIV-infected individuals.

Limited transplantation of antigen-expressing hematopoietic stem cells induces long-lasting cytotoxic T cell responses

Harnessing the ability of cytotoxic T lymphocytes (CTLs) to recognize and eradicate tumor or pathogen-infected cells is a critical goal of modern immune-based therapies. Although multiple immunization strategies efficiently induce high levels of antigen-specific CTLs, the initial increase is typically followed by a rapid contraction phase resulting in a sharp decline in the frequency of functional CTLs. We describe a novel approach to immunotherapy based on a transplantation of low numbers of antigen-expressing hematopoietic stem cells (HSCs) following nonmyeloablative or partially myeloablative conditioning. Continuous antigen presentation by a limited number of differentiated transgenic hematopoietic cells results in an induction and prolonged maintenance of fully functional effector T cell responses in a mouse model. Recipient animals display high levels of antigen-specific CTLs four months following transplantation in contrast to dendritic cell-immunized animals in which the response typically declines at 4–6 weeks post-immunization. Majority of HSC-induced antigen-specific CD8⁺ T cells display central memory phenotype, efficiently kill target cells in vivo, and protect recipients against tumor growth in a preventive setting. Furthermore, we confirm previously published observation that high level engraftment of antigen-expressing HSCs following myeloablative conditioning results in tolerance and an absence of specific cytotoxic activity in vivo. In conclusion, the data presented here supports potential application of immunization by limited transplantation of antigen-expressing HSCs for the prevention and treatment of cancer and therapeutic immunization of chronic infectious diseases such as HIV-1/AIDS.

Improved innate and adaptive immunostimulation by genetically modified HIV-1 protein expressing NYVAC vectors

Attenuated poxviruses are safe and capable of expressing foreign antigens. Poxviruses are applied in veterinary vaccination and explored as candidate vaccines for humans. However, poxviruses express multiple genes encoding proteins that interfere with components of the innate and adaptive immune response. This manuscript describes two strategies aimed to improve the immunogenicity of the highly attenuated, host-range restricted poxvirus NYVAC: deletion of the viral gene encoding type-I interferon-binding protein and development of attenuated replication-competent NYVAC. We evaluated these newly generated NYVAC mutants, encoding HIV-1 env, gag, pol and nef, for their ability to stimulate HIV-specific CD8 T-cell responses in vitro from blood mononuclear cells of HIV-infected subjects. The new vectors were evaluated and compared to the parental NYVAC vector in dendritic cells (DCs), RNA expression arrays, HIV gag expression and cross-presentation assays in vitro. Deletion of type-I interferon-binding protein enhanced expression of interferon and interferon-induced genes in DCs, and increased maturation of infected DCs. Restoration of replication competence induced activation of pathways involving antigen processing and presentation. Also, replication-competent NYVAC showed increased Gag expression in infected cells, permitting enhanced cross-presentation to HIV-specific CD8 T cells and proliferation of HIV-specific memory CD8 T-cells in vitro. The recombinant NYVAC combining both modifications induced interferon-induced genes and genes involved in antigen processing and presentation, as well as increased Gag expression. This combined replication-competent NYVAC is a promising candidate for the next generation of HIV vaccines.

A tonsillar PolyICLC/AT-2 SIV therapeutic vaccine maintains low viremia following antiretroviral therapy cessation

BACKGROUND

HIV-infected individuals rely on antiretroviral therapy (ART) to control viral replication. Despite abundant demonstrable benefits, the multiple limitations of ART point to the potential advantages of therapeutic vaccination approaches that could provide sustained host control of viral replication after discontinuation of ART. We provide evidence from a non-human primate model that a therapeutic vaccine applied to the tonsils can maintain low viral loads after cessation of ART.

METHODOLOGY/PRINCIPAL FINDINGS

Animals received 40 weeks of ART initiated 9 weeks after rectal SIVmac239 infection. During ART, animals were vaccinated (or not) with AT-2 inactivated SIVmac239 using CpG-C ISS-ODN (C274) or polyICLC as adjuvants. PolyICLC/AT-2 SIV vaccinated animals maintained viral loads <3×10(3) copies/ml for up to 16 weeks post-ART, whereas the C274/AT-2 SIV vaccinated and non-vaccinated animals' viremia ranged between 1×10(4)-4×10(5) copies/ml (p<0.03). Neutralizing Ab activity in plasma was increased by polyICLC/AT-2 tonsillar vaccination under ART, compared to controls (p<0.03). Subsequent vaccination of all animals with polyICLC/AT-2 SIV in the absence of ART did not alter viral loads. Other immune parameters measured in blood and tissues were comparable between groups.

CONCLUSIONS/SIGNIFICANCE

These results provide support for the potential benefit of mucosally delivered vaccines in therapeutic immunization strategies for control of AIDS virus infection.

Repeated DNA therapeutic vaccination of chronically SIV-infected macaques provides additional virological benefit

We have previously reported that therapeutic immunization by intramuscular injection of optimized plasmid DNAs encoding SIV antigens effectively induces immune responses able to reduce viremia in antiretroviral therapy (ART)-treated SIVmac251-infected Indian rhesus macaques. We subjected such therapeutically immunized macaques to a second round of therapeutic vaccination using a combination of plasmids expressing SIV genes and the IL-15/IL-15 receptor alpha as molecular adjuvant, which were delivered by the more efficacious in vivo constant-current electroporation. A very strong induction of antigen-specific responses to Gag, Env, Nef, and Pol, during ART (1.2-1.6% of SIV-specific T cells in the circulating T lymphocytes) was obtained with the improved vaccination method. Immunological responses were characterized by the production of IFN-gamma, IL-2, and TNF-alpha either alone, or in combination as double or triple cytokine positive multifunctional T cells. A significant induction of CD4(+) T cell responses, mainly targeting Gag, Nef, and Pol, as well as of CD8(+) T cells, mainly targeting Env, was found in both T cells with central memory and effector memory markers. After release from ART, the animals showed a virological benefit with a further approximately 1 log reduction in viremia. Vaccination with plasmid DNAs has several advantages over other vaccine modalities, including the possibility for repeated administration, and was shown to induce potent, efficacious, and long-lasting recall immune responses. Therefore, these data support the concept of adding DNA vaccination to the HAART regimen to boost the HIV-specific immune responses.

Memory T cells in Rhesus macaques

The Rhesus macaque (Macaca mulatta) is one of the best studied species of Old World monkeys. DNA sequencing of the entire Rhesus macaque genome, completed in 2007, has demonstrated that humans and macaques share about 93% of their nucleotide sequence. Rhesus macaques have been widely used for medical research including drug testing, neurology, behavioral and cognitive science, reproduction, xenotransplantation and genetics. Because of the Rhesus macaque's sensitivity to bacteria, parasites and viruses that cause similar disease in humans, these animals represent an excellent model to study infectious diseases. The recent pandemic of HIV and the discovery of SIV, a lentivirus genetically related to HIV Type 1 that causes AIDS in Rhesus macaques, have prompted the development of reagents that can be used to study innate and adaptive immune responses in macaques at the single cell level. This review will focus on the distribution of memory cells in the different immunologic compartments of Rhesus macaques. In addition, the strategies available to manipulate memory cells in Rhesus macaques to understand their trafficking and function will be discussed. Emphasis is placed on studies of memory cells in macaques infected with SIV because many studies are available. Lastly, we highlight the usefulness of the Rhesus macaque model in studies related to the aging of the immune system.

HIV-1 Tat-based vaccines: an overview and perspectives in the field of HIV/AIDS vaccine development

The HIV epidemic continues to represent one of the major problems worldwide, particularly in the Asia and Sub-Saharan regions of the world, with social and economical devastating effects. Although antiretroviral drugs have had a dramatically beneficial impact on HIV-infected individuals that have access to treatment, it has had a negligible impact on the global epidemic. Hence, the inexorable spreading of the HIV pandemic and the increasing deaths from AIDS, especially in developing countries, underscore the urgency for an effective vaccine against HIV/AIDS. However, the generation of such a vaccine has turned out to be extremely challenging. Here we provide an overview on the rationale for the use of non-structural HIV proteins, such as the Tat protein, alone or in combination with other HIV early and late structural HIV antigens, as novel, promising preventative and therapeutic HIV/AIDS vaccine strategies.

Interleukin-10 responses to therapeutic vaccination during highly active antiretroviral therapy and after analytical therapy interruption

We investigated whether therapeutic vaccination in highly active antiretroviral therapy (HAART)-treated patients with a modified vaccinia virus Ankara-vectored HIV-1 vaccine, with or without therapy interruption, induced the production of interleukin (IL)-10. Plasma IL-10 levels were not significantly increased postvaccination, but increased in parallel with viraemia in patients who interrupted therapy. Surprisingly, IL-10 blockade augmented HIV-specific T cell proliferative responses in HAART-suppressed patients but had no effect once virological control was lost. Modulation of IL-10 might enhance vaccine-induced immune responses.

Evaluation of antiretrovirals in animal models of HIV infection

Animal models of HIV infection have played an important role in the development of antiretroviral drugs. Although each animal model has its limitations and never completely mimics HIV infection of humans, a carefully designed study allows experimental approaches that are not feasible in humans, but that can help to better understand disease pathogenesis and to provide proof-of-concept of novel intervention strategies. While rodent and feline models are useful for initial screening, further testing is best done in non-human primate models, such as simian immunodeficiency virus (SIV) infection of macaques, because they share more similarities with HIV infection of humans. In the early years of the HIV pandemic, non-human primate models played a relatively minor role in the antiretroviral drug development process. Since then, a better understanding of the disease and the development of better drugs and assays to monitor antiviral efficacy have increased the usefulness of the animal models. In particular, non-human primate models have provided proof-of-concept for (i) the benefits of chemoprophylaxis and early treatment, (ii) the preclinical efficacy of novel drugs such as tenofovir, (iii) the virulence and clinical significance of drug-resistant viral mutants, and (iv) the role of antiviral immune responses during drug therapy. Ongoing comparison of results obtained in animal models with those observed in human studies will further validate and improve these animal models so they can continue to help advance our scientific knowledge and to guide clinical trials. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

A model for testing the immunogenicity of simian immunodeficiency virus and simian-human immunodeficiency virus vaccine candidates in mice

HIV-1 Gag protein represents a promising target of cellular immunity-based vaccines due to its immunogenicity and high conservation among diverse viral subtypes. Development of novel and effective Gag-targeted vaccine candidates inducing CD8(+) and CD4(+) T cell responses requires large scale pre-clinical testing in a small animal model. In this report, the MHC class I and II-restricted epitopes in the simian immunodeficiency virus (SIV) Gag protein recognized in C57Bl/6 and Balb/c mice were determined and characterized. In addition, using the newly defined epitopes, the relationship is described between the amount of plasmid DNA, volume of inoculate, and the extent of ensuing immune responses following intramuscular DNA immunization.

Effect of therapeutic immunization using Ad5/35 and MVA vectors on SIV infection of rhesus monkeys undergoing antiretroviral therapy

Antiretroviral therapy (ART) effectively slows the progression of AIDS. However, drug resistance and/or toxicity can limit the utility of ART in many patients. In this study, we assessed whether a viral vector-based vaccine can be used as a therapeutic vaccine in simian immunodeficiency virus (SIV)-infected monkeys. The effect of vaccinating SIVmac239-infected rhesus monkeys with an SIV gag and gp120-expressing adenovirus (Ad) vector vaccine and a modified vaccinia Ankara (MVA) vaccine was explored while being treated with ART. Rhesus monkeys were intravenously infected with 10 and 1000 TCID(50) (50% tissue culture infectious dose) of SIVmac239. Two months after SIV infection, the monkeys received a 4-month treatment with ART. Some of the monkeys were immunized with adenovirus-based vaccine and MVA-based vaccine with 2 months interval during ART. Viral load, CD4 count and SIV-specific immune responses were observed for 7 months after interruption of ART. The vaccinated animals had higher (i) CD4 counts, (ii) SIV-specific cell-mediated immune responses and (iii) anti-SIV-neutralizing antibody (Ab) titers than monkeys treated with ART alone. More importantly, the vaccination significantly reduced the SIV RNA load from animals infected with a low dose of SIV (10 TCID(50)). The anti-SIV cell-mediated and humoral responses induced by the vaccination was inversely correlated with a reduction in SIV viral load and positively correlated with an increase in CD4(+) T cell counts. These results suggest that vaccination can improve antiviral cell-mediated and humoral immunity, which may contribute to controlling viral replication.

Reduced protection from simian immunodeficiency virus SIVmac251 infection afforded by memory CD8+ T cells induced by vaccination during CD4+ T-cell deficiency

Adaptive CD4(+) and CD8(+) T-cell responses have been associated with control of human immunodeficiency virus/simian immunodeficiency virus (HIV/SIV) replication. Here, we have designed a study with Indian rhesus macaques to more directly assess the role of CD8 SIV-specific responses in control of viral replication. Macaques were immunized with a DNA prime-modified vaccinia virus Ankara (MVA)-SIV boost regimen under normal conditions or under conditions of antibody-induced CD4(+) T-cell deficiency. Depletion of CD4(+) cells was performed in the immunized macaques at the peak of SIV-specific CD4(+) T-cell responses following the DNA prime dose. A group of naïve macaques was also treated with the anti-CD4 depleting antibody as a control, and an additional group of macaques immunized under normal conditions was depleted of CD8(+) T cells prior to challenge exposure to SIV(mac251). Analysis of the quality and quantity of vaccine-induced CD8(+) T cells demonstrated that SIV-specific CD8(+) T cells generated under conditions of CD4(+) T-cell deficiency expressed low levels of Bcl-2 and interleukin-2 (IL-2), and plasma virus levels increased over time. Depletion of CD8(+) T cells prior to challenge exposure abrogated vaccine-induced protection as previously shown. These data support the notion that adaptive CD4(+) T cells are critical for the generation of effective CD8(+) T-cell responses to SIV that, in turn, contribute to protection from AIDS. Importantly, they also suggest that long-term protection from disease will be afforded only by T-cell vaccines for HIV that provide a balanced induction of CD4(+) and CD8(+) T-cell responses and protect against early depletion of CD4(+) T cells postinfection.

Greater viral rebound and reduced time to resume antiretroviral therapy after therapeutic immunization with the ALVAC-HIV vaccine (vCP1452)

OBJECTIVE

Evaluate immunogenicity and clinical efficacy of two immunization strategies with the ALVAC-HIV-recombinant canarypox vaccine (vCP1452) in treated HIV-infected patients.

DESIGN

Randomized, double-blind, placebo-controlled, phase II study of vCP1452 immunization in chronically HIV-infected patients on therapy with CD4 T-cell count more than 350 cells/microl, CD4 nadir less than 400 cells/microl and pHIV-RNA less than 400 copies/ml. Patients were equally randomized to four injections at weeks 0, 4, 8, 20; three injections at weeks 4, 8, 20; and placebo. The primary endpoint was vaccine immunogenicity at week 24 measured by enzyme-linked immunospot-interferon-gamma against the HIV-gag-reverse transcriptase-nef vaccine sequences. Secondary endpoints included time to treatment resumption and viral quantitation following treatment interruption at week 24. Criteria to resume therapy included CD4 T-cell count decline less than 250 cells/microl or 50% decrease from baseline or pHIV-RNA more than 50,000 copies/ml.

RESULTS

Sixty-five patients enrolled. Changes from baseline in HIV-specific T cells in the four injection arms (+480 spot-forming cells/M-peripheral blood mononuclear cell) were significant compared to placebo (+8; P = 0.014), but not in the three injection arms (+322). The week 36 pHIV-RNA (log10 copies/ml) after treatment interruption was higher in the four (4.71; P = 0.023) and three (4.82; P = 0.009) injection arms compared to placebo (4.40). Percentages of patients reaching treatment resumption criteria by week 48 were 74, 55 and 23% in the three respective arms (P = 0.013). Two independent factors influenced time to therapy resumption: immunization (hazards ratio = 2.7, P = 0.048 for three injections; hazards ratio = 4.1, P = 0.003 for four injections) and CD4 nadir (hazards ratio = 0.4, P = 0.002).

CONCLUSIONS

Significant immunogenicity was induced by vCP1452; however, this strategy was independently associated with a shorter time to resume therapy and higher viral rebound.

Manipulating both the inhibitory and stimulatory immune system towards the success of therapeutic vaccination against chronic viral infections

SUMMARY

One potentially promising strategy to control chronic infections such as human immunodeficiency virus, hepatitis B virus, and hepatitis C virus is therapeutic vaccination, which aims to reduce persisting virus by stimulating a patient's own antiviral immune responses. However, this approach has fallen short of expectations, because antiviral T cells generated during chronic infections often become functionally exhausted and thus do not respond properly to therapeutic vaccination. Therefore, it is necessary to develop a therapeutic vaccine strategy to more effectively boost endogenous T-cell responses to control persistent viral infections. Studies to elucidate the cause of impaired T-cell function have pointed to sustained inhibitory receptor signaling through T-cell expression of programmed death 1 (PD-1). Recently, another inhibitory molecule, cytotoxic T lymphocyte antigen 4 (CTLA-4), and also an immunosuppressive cytokine, interleukin 10 (IL-10), have been reported to be potential factors of establishing immune suppression and viral persistence. Blocking these negative signaling pathways could restore the host immune system, enabling it to respond to further stimulation. Indeed, combining therapeutic vaccination along with the blockade of inhibitory signals could synergistically enhance functional CD8(+) T-cell responses and improve viral control in chronically infected mice, providing a promising strategy for the treatment of chronic viral infections. Furthermore, not only the ablation of negative signals but also the addition of stimulatory signals, such as interleukin 2 (IL-2), might prove to be a potentially promising strategy to augment the efficacy of therapeutic vaccination against chronic viral infections.

Immune activation driven by CTLA-4 blockade augments viral replication at mucosal sites in simian immunodeficiency virus infection

The importance of chronic immune activation in progression to AIDS has been inferred by correlative studies in HIV-infected individuals and in nonhuman primate models of SIV infection. Using the SIV(mac251) macaque model, we directly address the impact of immune activation by inhibiting CTLA-4, an immunoregulatory molecule expressed on activated T cells and a subset of regulatory T cells. We found that CTLA-4 blockade significantly increased T cell activation and viral replication in primary SIV(mac251) infection, particularly at mucosal sites, and increased IDO expression and activity. Accordingly, protracted treatment with anti-CTLA-4 Ab of macaques chronically infected with SIV(mac251) decreased responsiveness to antiretroviral therapy and abrogated the ability of therapeutic T cell vaccines to decrease viral set point. These data provide the first direct evidence that immune activation drives viral replication, and suggest caution in the use of therapeutic approaches for HIV infection in vivo that increase CD4(+) T cell proliferation.

Enhancing therapeutic vaccination by blocking PD-1-mediated inhibitory signals during chronic infection

Therapeutic vaccination is a potentially promising strategy to enhance T cell immunity and viral control in chronically infected individuals. However, therapeutic vaccination approaches have fallen short of expectations, and effective boosting of antiviral T cell responses has not always been observed. One of the principal reasons for the limited success of therapeutic vaccination is that virus-specific T cells become functionally exhausted during chronic infections. We now provide a novel strategy for enhancing the efficacy of therapeutic vaccines. In this study, we show that blocking programmed death (PD)-1/PD-L1 inhibitory signals on exhausted CD8(+) T cells, in combination with therapeutic vaccination, synergistically enhances functional CD8(+) T cell responses and improves viral control in mice chronically infected with lymphocytic choriomeningitis virus. This combinatorial therapeutic vaccination was effective even in the absence of CD4(+) T cell help. Thus, our study defines a potent new approach to augment the efficacy of therapeutic vaccination by blocking negative signals. Such an approach may have broad applications in developing treatment strategies for chronic infections in general, and perhaps also for tumors.

Regulatory T-cell markers, indoleamine 2,3-dioxygenase, and virus levels in spleen and gut during progressive simian immunodeficiency virus infection

High levels of viral replication occur in gut-associated lymphoid tissue (GALT) and other lymphoid tissues (LT) since the early phase of human/simian immunodeficiency virus (HIV/SIV) infection. Regulatory T cells (T(reg)), a subset of immunosuppressive T cells expressing CTLA-4 and the FoxP3 transcription factor, accumulate in LT during HIV/SIV infection. Here we show that FoxP3 and CTLA-4 mRNA are increased in leukocytes from the spleens, lymph nodes (LN), and mucosal sites of chronically SIV-infected macaques with high viremia (SIV(HI)) compared to animals with low viremia (SIV(LO)). FoxP3 and CTLA-4 correlated with SIV RNA levels in tissues; SIV virus levels in the spleen, inguinal LN, mesenteric LN, colon, and jejunum directly correlated with the plasma virus level. Importantly, CTLA-4 and FoxP3 mRNA were predominantly increased in the CD25(-) subpopulation of leukocytes from SIV(HI), further challenging the classical definition of T(reg) as CD4(+) CD25(+) T cells. Similar to CTLA-4 and FoxP3, expression of indoleamine 2,3-dioxygenase (IDO), an immunosuppressive enzyme induced by T(reg) in antigen-presenting cells, was increased in the spleens, mesenteric LN, colons, and jejuna from SIV(HI) compared to SIV(LO) and directly correlated to SIV RNA in the same tissues. Accordingly, plasma kynurenine/tryptophan, a marker for IDO enzymatic activity, was significantly higher in SIV(HI) compared to SIV(LO) and correlated with plasma viral levels. Increased T(reg) and IDO in LT of SIV-infected macaques may be the consequence of increased tissue inflammation and/or may favor virus replication during the chronic phase of SIV infection.

SIV-specific CD8+ T cells express high levels of PD1 and cytokines but have impaired proliferative capacity in acute and chronic SIVmac251 infection

Programmed death-1 (PD-1) is a critical mediator of virus-specific CD8+ T-cell exhaustion. Here, we examined the expression of PD-1 on simian immunodeficiency virus (SIV)-specific CD8+ T cells and its possible involvement in regulation of cytokine production, proliferation, and survival of these cells. The majority of SIV-specific CD8+ T cells expressed a PD-1(high) phenotype, independent of their differentiation status, in all tissues tested. PD-1 expression gradually declined on CD8+ T cells specific for SIV-derived epitopes that had undergone mutational escape, indicating that antigen-specific TCR stimulation is the primary determinant of PD-1 expression. SIV-specific PD-1(high)CD8+ T cells produced IFN-gamma, TNF-alpha, and IL-2 under cognate peptide stimulation. While CD8+ T cells that proliferated in response to antigen had a PD-1(high) phenotype, it was determined that there was a reduced proliferative capacity of PD-1(high) compared with PD-1(low) SIV-specific CD8+ T cells. PD-1(high) SIV-specific CD8+ T cells were highly susceptible to cell death leading to loss of such cells after in vitro stimulation. Thus, PD-1 is a negative regulator of SIV-specific CD8+ T cells, operating predominantly through the induction of cell death. Manipulation of the interaction of PD-1 with its ligands could thus potentially restore the CD8+ T-cell responses in SIV infection.

Therapeutic immunization with Modified Vaccinia Virus Ankara (MVA) vaccines in SIV-infected rhesus monkeys undergoing antiretroviral therapy

BACKGROUND

The long-term benefits of highly active antiretroviral therapy in HIV-infected patients are limited by emergence of drug-resistant variants and side effects. Therefore, we studied the concept of therapeutic immunization in 18 rhesus monkeys infected with a highly pathogenic simian immunodeficiency virus (SIV) swarm.

METHODS

Monkeys were treated with the reverse transcriptase inhibitor (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for 19 weeks starting 10 days after infection. After suppression of viremia, one group of monkeys was immunized with recombinant modified vaccinia virus Ankara (MVA) vectors expressing gag-pol and env. A second group received MVA vectors expressing the regulatory genes tat, rev and nef, while a third group was not immunized.

RESULTS

Immunization with gag-pol and env expressing MVA enhanced SIV antibody titers. Following discontinuation of PMPA treatment, a rebound in viral load was observed. However, in three of six monkeys immunized with MVA gag-pol and MVA env, and two of six monkeys immunized MVA expressing regulatory genes set point RNA levels were below or close to a threshold level of 10(4) RNA copies/ml, while only one of six unvaccinated monkeys maintained such low RNA levels.

CONCLUSIONS

Although a subset of animals seem to benefit from therapeutic immunization with MVA vectors, the difference in set point RNA levels between the groups did not reach statistical significance.

Interleukin-15 but not interleukin-7 abrogates vaccine-induced decrease in virus level in simian immunodeficiency virus mac251-infected macaques

The loss of CD4(+) T cells and the impairment of CD8(+) T cell function in HIV infection suggest that pharmacological treatment with IL-7 and IL-15, cytokines that increase the homeostatic proliferation of T cells and improve effector function, may be beneficial. However, these cytokines could also have a detrimental effect in HIV-1-infected individuals, because both cytokines increase HIV replication in vitro. We assessed the impact of IL-7 and IL-15 treatment on viral replication and the immunogenicity of live poxvirus vaccines in SIV(mac251)-infected macaques (Macaca mulatta). Neither cytokine augmented the frequency of vaccine-expanded CD4(+) or CD8(+) memory T cells, clonal recruitment to the SIV-specific CD8(+) T cell pool, or CD8(+) T cell function. Vaccination alone transiently decreased the viral set point following antiretroviral therapy suspension. IL-15 induced massive proliferation of CD4(+) effector T cells and abrogated the ability of vaccination to decrease set point viremia. In contrast, IL-7 neither augmented nor decreased the vaccine effect and was associated with a decrease in TGF-beta expression. These results underscore the importance of testing immunomodulatory approaches in vivo to assess potential risks and benefits for HIV-1-infected individuals.

Problems and emerging approaches in HIV/AIDS vaccine development

According to recent estimates, 39.5 million people have been infected with HIV and 2.9 million have already died. The effect of HIV infection on individuals and communities is socially and economically devastating. Although antiretroviral drugs have had a dramatically beneficial impact on HIV-infected individuals who have access to treatment, it has had a negligible impact on the global epidemic. Therefore, the need for an efficacious HIV/AIDS vaccine remains the highest priority of the world HIV/AIDS agenda. The generation of a vaccine against HIV/AIDS has turned out to be extremely challenging, as indicated by > 20 years of unsuccessful attempts. This review discusses the major challenges in the field and key experimental evidence providing a rationale for the use of non-structural HIV proteins, such as Rev, Tat and Nef, either in the native form or expressed by viral vectors such as a replicating adeno-vector. These non-structural proteins alone or in combination with modified structural HIV-1 Env proteins represent a novel strategy for both preventative and therapeutic HIV/AIDS vaccine development.

Decreased number of CD4+ and CD8+ T cells that express the interleukin-7 receptor in blood and tissues of SIV-infected macaques

Acute HIV/SIV (human/simian immunodeficiency virus) infection results in severe CD4(+) T cell depletion in lymphoid compartments. During the chronic phase of infection, CD4(+) T cell numbers rebound in blood but remain low in the gut-associated lymphoid tissue (GALT), even when viral replication is suppressed by antiretroviral therapy (ART). Thus, strategies to repopulate lymphoid compartments may ameliorate the clinical outcome of HIV/SIV infection. Interleukin (IL)-7 is a key cytokine for the maintenance of homeostatic proliferation of T cells. In HIV/SIV infection, IL-7 expression is increased, likely to compensate for T cell loss, suggesting that supraphysiological administration of IL-7 could provide additional benefit. However, the ability of T cells to respond to IL-7 is dependent on the level of expression of the IL-7 receptor (IL-7R) in T cells in various body compartments. In here, we investigated the proportion of IL-7R(+) T cells in blood, spleen, gut, and genitourinary tract of healthy and SIV-infected macaques with various degrees of CD4(+) T cell depletion. We found that the percentage of T cells expressing IL-7R was significantly lower in both CD4(+) and CD8(+) T cell subsets in SIV-infected macaques than in healthy animals and this decrease directly correlated with the CD4(+) T cell number. Importantly, the proportion of CD4(+) and CD8(+) T cells expressing IL-7R in blood paralleled that found in tissues. IL-7R(+) T cells within the SIV-specific CD8(+) T cells varied and were lowest in most tissues of viremic macaques, likely reflecting continuous antigen stimulation of effector cells.

CTLA-4 blockade decreases TGF-beta, IDO, and viral RNA expression in tissues of SIVmac251-infected macaques

Regulatory T (T(reg)) cells are a subset of CD25(+)CD4(+) T cells that constitutively express high levels of cytotoxic T lymphocyte antigen-4 (CTLA-4) and suppress T-cell activation and effector functions. T(reg) cells are increased in tissues of individuals infected with HIV-1 and macaques infected with simian immunodeficiency virus (SIV(mac251)). In HIV-1 infection, T(reg) cells could exert contrasting effects: they may limit viral replication by decreasing immune activation, or they may increase viral replication by suppressing virusspecific immune response. Thus, the outcome of blocking T(reg) function in HIV/SIV should be empirically tested. Here, we demonstrate that CD25(+) T cells inhibit virus-specific T-cell responses in cultured T cells from blood and lymph nodes of SIV-infected macaques. We investigated the impact of CTLA-4 blockade using the anti-CTLA-4 human antibody MDX-010 in SIV-infected macaques treated with antiretroviral therapy (ART). CTLA-4 blockade decreased expression of the tryptophan-depleting enzyme IDO and the level of the suppressive cytokine transforming growth factor-beta (TGF-beta) in tissues. CTLA-4 blockade was associated with decreased viral RNA levels in lymph nodes and an increase in the effector function of both SIV-specific CD4(+) and CD8(+) T cells. Therefore, blunting T(reg) function in macaques infected with SIV did not have detrimental virologic effects and may provide a valuable approach to complement ART and therapeutic vaccination in the treatment of HIV-1 infection.

Long-lasting decrease in viremia in macaques chronically infected with simian immunodeficiency virus SIVmac251 after therapeutic DNA immunization

Rhesus macaques chronically infected with highly pathogenic simian immunodeficiency virus (SIV) SIVmac251 were treated with antiretroviral drugs and vaccinated with combinations of DNA vectors expressing SIV antigens. Vaccination during therapy increased cellular immune responses. After the animals were released from therapy, the virus levels of 12 immunized animals were significantly lower (P = 0.001) compared to those of 11 animals treated with only antiretroviral drugs. Vaccinated animals showed a persistent increase in immune responses, thus indicating both a virological and an immunological benefit following DNA therapeutic vaccination. Several animals show a long-lasting decrease in viremia, suggesting that therapeutic vaccination may provide an additional benefit to antiretroviral therapy.

Therapeutic immunization for HIV

Vaccines have entered into human clinical trials against infectious diseases and as therapies against cancer. The HIV virus establishes a latent infection at a very early stage and the T cell memory of the infected patient is rapidly destroyed. However, results of immunotherapy after DNA and protein immunization show that vaccine-induced immune responses might be present for a long period of time. Patients subjected to therapeutic immunization appear to do well, and to have a small immunological advantage, which, however, will have to be improved. The vaccine therapy should start early, while adequate reservoirs of appropriate T helper cells are available and still inducible. The DNA vaccines induce a relatively long-lived immunological memory, and gene-based immunization is effective in inducing cytotoxic CD8(+) T cells and CD4+ helper cells. Protein vaccines, on the other hand, primarily give T cell help. It thus appears that DNA and protein approaches to HIV immunization complement each other. A surprisingly broad reactivity to peptides from different subtypes of HIV was identified in individuals infected with several subtypes of HIV.

Immunisation with recombinant modified vaccinia virus Ankara expressing HIV-1 gag in HIV-1-infected subjects stimulates broad functional CD4+ T cell responses

Virus-specific CD4+ T cells with IL-2-secreting and/or proliferative capacity are detected readily in HIV-1-infected long-term nonprogressors and rarely in persons with untreated progressive infection. The contribution of these cells to viraemia control is uncertain, but this question might be addressed in clinical therapeutic vaccination studies. However, the quality of T helper responses induced by currently available HIV-1 vaccine candidates has not been explored in depth. We determined the effect of vaccination with modified vaccinia virus Ankara (MVA) expressing HIV-1 gag p24/p17 (MVA.HIVA) on HIV-1-specific CD4+ T cell responses in 16 chronically infected, highly active antiretroviral therapy (HAART)-treated subjects using CD8-depleted IFN-gamma ELISPOT assays, intracellular cytokine staining assays for IL-2 and IFN-gamma, and a CFSE-based proliferation assay. Gag-specific CD4+ T cell responses were significantly increased in magnitude and breadth after vaccination and targeted both known and new epitopes, several of which were also recognised by healthy HIV-uninfected volunteers immunised with the same vaccines. The frequencies of CD4+ T cells expressing IL-2 or IFN-gamma, alone or simultaneously, were also augmented. These findings indicate that functional virus-specific T helper cells can be boosted by vaccination in chronic HIV-1 infection. Further evaluation of their role in viraemia control is warranted.

Structured treatment interruptions with tenofovir monotherapy for simian immunodeficiency virus-infected newborn macaques

We demonstrated previously that prolonged tenofovir treatment of infant macaques, starting early during infection with virulent simian immunodeficiency virus (SIVmac251), can lead to persistently low or undetectable viremia even after the emergence of mutants with reduced in vitro susceptibility to tenofovir as a result of a K65R mutation in reverse transcriptase; this control of viremia was demonstrated to be mediated by the generation of effective antiviral immune responses. To determine whether structured treatment interruptions (STI) can induce similar immunologic control of viremia, eight newborn macaques were infected with highly virulent SIVmac251 and started on a tenofovir STI regimen 5 days later. Treatment was withdrawn permanently at 33 weeks of age. All animals receiving STI fared much better than 22 untreated SIVmac251-infected infant macaques. However, there was a high variability among animals in the viral RNA set point after complete drug withdrawal, and none of the animals was able to achieve long-term immunologic suppression of viremia to persistently low levels. Early immunologic and viral markers in blood (including the detection of the K65R mutation) were not predictive of the viral RNA set point after drug withdrawal. These results, which reflect the complex interactions between drug resistance mutations, viral virulence, and drug- and immune-mediated inhibition of virus replication, highlight the difficulties associated with trying to develop STI regimens with predictable efficacy for clinical practice.

Therapeutic immunization strategies for the control of HIV-1

Highly active antiretroviral therapy (HAART) is currently the only means to halt or prevent progression to AIDS. However, lack of access to medications for the vast majority of HIV-1-infected individuals in immediate need, together with the requirement for lifelong adherence and potential for serious toxicity, are significant limitations which have yet to be overcome. Augmentation of HIV-specific immunity by therapeutic vaccination is being explored as a possible alternative to continuous HAART. A few candidate HIV-1 vaccines have entered clinical trials involving an assessment of viremia control during an analytic therapy interruption, but even the most promising of these achieve a short-lived suppression of HIV-1 without HAART. Nevertheless, these studies are guiding the development of better immune augmentation strategies, which could extend the time off therapy and will also contribute to a better understanding of the immune correlates of protection against AIDS. The status of therapeutic vaccines that are currently undergoing preclinical and clinical evaluation is reviewed.

HIV infection: first battle decides the war

The traditional view of HIV-1 infection characterized by the slow decline of CD4+ T cells has radically changed in light of recent observations in rhesus macaques and humans of rapid and extensive infection and removal of memory CD4+ T cells in mucosal tissues within the first three weeks of infection. This initial strike to the immune system seems to be the distinguishing feature of HIV-1 pathogenesis and its extent sets the overall course of the ensuing infection. Qualitatively different mechanisms of CD4+ T-cell depletion prevail during the acute, chronic and advanced phases of infection depending on the availability of the target-cell population and competence of the immune system. The elimination of CD4+ T cells in mucosal lymphoid tissues results in the absence of important regulatory and effector functions that these cells normally perform in controlling immune responses to environmental antigens and pathogens. Ablation of acute HIV-1 viremia limits the initial damage to the CD4+ T-cell compartment and helps to establish a state of equilibrium between the replicating virus, the availability of the target-cell population and the immune control characteristic of long-term non-progression.

Expansion and diversification of virus-specific T cells following immunization of human immunodeficiency virus type 1 (HIV-1)-infected individuals with a recombinant modified vaccinia virus Ankara/HIV-1 Gag vaccine

Affordable therapeutic strategies that induce sustained control of human immunodeficiency virus type 1 (HIV-1) replication and are tailored to the developing world are urgently needed. Since CD8(+) and CD4(+) T cells are crucial to HIV-1 control, stimulation of potent cellular responses by therapeutic vaccination might be exploited to reduce antiretroviral drug exposure. However, therapeutic vaccines tested to date have shown modest immunogenicity. In this study, we performed a comprehensive analysis of the changes in virus-specific CD8(+) and CD4(+) T-cell responses occurring after vaccination of 16 HIV-1-infected individuals with a recombinant modified vaccinia virus Ankara-vectored vaccine expressing the consensus HIV-1 clade A Gag p24/p17 sequences and multiple CD8(+) T-cell epitopes during highly active antiretroviral therapy. We observed significant amplification and broadening of CD8(+) and CD4(+) gamma interferon responses to vaccine-derived epitopes in the vaccinees, without rebound viremia, but not in two unvaccinated controls followed simultaneously. Vaccine-driven CD8(+) T-cell expansions were also detected by tetramer reactivity, predominantly in the CD45RA(-) CCR7(+) or CD45RA(-) CCR7(-) compartments, and persisted for at least 1 year. Expansion was associated with a marked but transient up-regulation of CD38 and perforin within days of vaccination. Gag-specific CD8(+) and CD4(+) T-cell proliferation also increased postvaccination. These data suggest that immunization with MVA.HIVA is a feasible strategy to enhance potentially protective T-cell responses in individuals with chronic HIV-1 infection.

DNA immunization in combination with effective antiretroviral drug therapy controls viral rebound and prevents simian AIDS after treatment is discontinued

DNA immunization in conjunction with antiretroviral therapy was evaluated in SIV-infected rhesus macaques treated with [R]-9-[2-phosphonylmethoxypropyl]adenine (PMPA). Macaques were immunized monthly with DNA vaccines expressing either SIV gag/tat or SIV gag/tat and 19 CD8+ T cell epitopes during 7 months of therapy. Half the animals from each group were additionally immunized before infection. Only 60% of the animals (4 controls, 20 vaccinated) responded to PMPA (ART responders). All 4 ART responder controls demonstrated viral rebound or CD4 decline after PMPA was withdrawn. In contrast, 17 of 20 vaccinated ART responders contained viral rebound for over 7 months after PMPA was withdrawn. Viral control correlated with stable CD4 counts, higher lymphoproliferation and an increase in the magnitude and breadth of the CD8+ T cell response. Immunizing before infection or with multi-epitopes enhanced these effects. These results demonstrate that DNA immunization during antiretroviral therapy may be an effective strategy to treat HIV infection.

HIV vaccines

A prophylactic vaccine for HIV-1 is badly needed. Despite 20 years of effort, it is still a long way off. However, considerable progress has been made in understanding the problem. The virus envelope has evolved to evade neutralizing antibodies in an extraordinary way, yet a vaccine that can stimulate such antibodies remains the best hope. Anti-HIV-1 T cell responses are evaded by continuous mutation of the virus. Vaccine strategies that concentrate on stimulating T cell immunity will at best generate broadly reactive and persisting T cell responses that can suppress virus without preventing infection, limiting or preventing the damage the virus causes. The SIV macaque models give encouragement that this is possible, but they need further understanding. Therapeutic vaccination should also be considered.

Therapeutic vaccination against HIV: current progress and future possibilities

Although effective in reducing mortality, current antiretroviral therapy for HIV infection involves complex and expensive drug regimens that are toxic and difficult to take. Eradication of HIV reservoirs is not possible with existing therapies. The concept of therapeutic vaccination has been investigated to increase the potency and breadth of anti-HIV immune responses in order to delay or reduce antiretroviral therapy use. A variety of approaches targeted to both cell- and antibody-mediated immunity have been developed, including whole inactivated HIV-1, protein subunits and synthetic peptides, DNA vaccines and a number of viral vectors expressing HIV-1. These investigations have occurred in the absence of a clear understanding of disease pathogenesis or the correlates of protective immunity. At this time, there is no licensed therapeutic vaccine for any viral disease, including HIV; however, this review will consider recent progress in the field and summarize the challenges faced in the development of a therapeutic HIV vaccine.

Vaccine-induced CD8+ central memory T cells in protection from simian AIDS

Critical to the development of an effective HIV vaccine is the identification of adaptive immune responses that prevent infection or disease. In this study we demonstrate in a relevant nonhuman primate model of AIDS that the magnitude of vaccine-induced virus-specific CD8(+) central memory T cells (T(CM)), but not that of CD8(+) effector memory T cells, inversely correlates with the level of SIVmac251 replication, suggesting their pivotal role in the control of viral replication. We propose that effective preventive or therapeutic T cell vaccines for HIV-1 should induce long-term protective central memory T cells.

DNA vaccines: progress and challenges

In the years following the publication of the initial in vivo demonstration of the ability of plasmid DNA to generate protective immune responses, DNA vaccines have entered into a variety of human clinical trials for vaccines against various infectious diseases and for therapies against cancer, and are in development for therapies against autoimmune diseases and allergy. They also have become a widely used laboratory tool for a variety of applications ranging from proteomics to understanding Ag presentation and cross-priming. Despite their rapid and widespread development and the commonplace usage of the term "DNA vaccines," however, the disappointing potency of the DNA vaccines in humans underscores the challenges encountered in the efforts to translate efficacy in preclinical models into clinical realities. This review will provide a brief background of DNA vaccines including the insights gained about the varied immunological mechanisms that play a role in their ability to generate immune responses.

Identification of NKG2A and NKp80 as specific natural killer cell markers in rhesus and pigtailed monkeys

Investigations of natural killer (NK) cells in simian models of disease have been hampered by a lack of appropriate phenotypic markers and by an inadequate understanding of the regulation of NK cell activities. In the present study, a panel of monoclonal antibodies (mAbs) specific for various human NK receptors was screened for cross-reactivity with NK cells from rhesus macaques and pigtailed macaques. Flow cytometric analyses using anti-human NKG2A and anti-human NKp80 mAbs individually, and particularly in combination with anti-CD16 mAb, allowed for the identification of the entire NK cell population in both species. NK cells in monkeys were generally identified by negative selection of peripheral blood mononuclear cells (PBMCs) for the absence of T-cell, B-cell, and monocyte markers. mAb-mediated ligation of NKp80 induced NK cell cytotoxicity, while in the case of NKG2A it displayed a clear capability to inhibit the lysis of target cells by NK cells from macaques, as well as from humans. This new phenotypic and functional characterization of NKG2A and NKp80 in rhesus and pigtailed macaque NK cells provides a new approach in the analysis of their innate immune system.