Modified vaccinia Ankara expressing HIVA antigen stimulates HIV-1-specific CD8 T cells in ELISpot assays of HIV-1 exposed infants

Clinical trials and recent progress in HIV vaccine development

The greatest obstacle for scientists is to develop an effective HIV vaccine. An effective vaccine represents the last hope for halting the unstoppable global spread of HIV and its catastrophic clinical consequences. Creating this vaccine has been challenging due to the virus's extensive genetic variability and the unique role of cytotoxic T lymphocytes (CTL) in containing it. Innovative methods to stimulate CTL have demonstrated significant therapeutic advantages in nonhuman primate model systems, unlike traditional vaccination techniques that are not expected to provide safe and efficient protection against HIV. Human clinical trials are currently evaluating these vaccination strategies, which involve plasmid DNA and live recombinant vectors. This review article covers the existing vaccines and ongoing trial vaccines. It also explores the different approaches used in developing HIV vaccines, including their molecular mechanisms, target site effectiveness, and potential side effects.

HIV-1 Vaccine Trials: Evolving Concepts and Designs

An effective prophylactic HIV-1 vaccine is needed to eradicate the HIV/AIDS pandemic but designing such a vaccine is a challenge. Despite many advances in vaccine technology and approaches to generate both humoral and cellular immune responses, major phase-II and -III vaccine trials against HIV/AIDS have resulted in only moderate successes. The modest achievement of the phase-III RV144 prime-boost trial in Thailand re-emphasized the importance of generating robust humoral and cellular responses against HIV. While antibody-directed approaches are being pursued by some groups, others are attempting to develop vaccines targeting cell-mediated immunity, since evidence show CTLs to be important for the control of HIV replication. Phase-I and -IIa multi-epitope vaccine trials have already been conducted with vaccine immunogens consisting of known CTL epitopes conserved across HIV subtypes, but have so far fallen short of inducing robust and consistent anti-HIV CTL responses. The concepts leading to the development of T-cell epitope-based vaccines, the outcomes of related clinical vaccine trials and efforts to enhance the immunogenicity of cell-mediated approaches are summarized in this review. Moreover, we describe a novel approach based on the identification of SIV and FIV antigens which contain conserved HIV-specific T-cell epitopes and represent an alternative method for developing an effective HIV vaccine against global HIV isolates.

Novel recombinant Mycobacterium bovis BCG, ovine atadenovirus, and modified vaccinia virus Ankara vaccines combine to induce robust human immunodeficiency virus-specific CD4 and CD8 T-cell responses in rhesus macaques

Mycobacterium bovis bacillus Calmette-Guérin (BCG), which elicits a degree of protective immunity against tuberculosis, is the most widely used vaccine in the world. Due to its persistence and immunogenicity, BCG has been proposed as a vector for vaccines against other infections, including HIV-1. BCG has a very good safety record, although it can cause disseminated disease in immunocompromised individuals. Here, we constructed a recombinant BCG vector expressing HIV-1 clade A-derived immunogen HIVA using the recently described safer and more immunogenic BCG strain AERAS-401 as the parental mycobacterium. Using routine ex vivo T-cell assays, BCG.HIVA(401) as a stand-alone vaccine induced undetectable and weak CD8 T-cell responses in BALB/c mice and rhesus macaques, respectively. However, when BCG.HIVA(401) was used as a priming component in heterologous vaccination regimens together with recombinant modified vaccinia virus Ankara-vectored MVA.HIVA and ovine atadenovirus-vectored OAdV.HIVA vaccines, robust HIV-1-specific T-cell responses were elicited. These high-frequency T-cell responses were broadly directed and capable of proliferation in response to recall antigen. Furthermore, multiple antigen-specific T-cell clonotypes were efficiently recruited into the memory pool. These desirable features are thought to be associated with good control of HIV-1 infection. In addition, strong and persistent T-cell responses specific for the BCG-derived purified protein derivative (PPD) antigen were induced. This work is the first demonstration of immunogenicity for two novel vaccine vectors and the corresponding candidate HIV-1 vaccines BCG.HIVA(401) and OAdV.HIVA in nonhuman primates. These results strongly support their further exploration.

The challenge of assessing infant vaccine responses in resource-poor settings

Newborns and infants are highly susceptible to infectious diseases, resulting in high mortality and morbidity, particularly in resource-poor settings. Many vaccines require several booster doses, resulting in an extensive vaccine schedule, and yet there is still inadequate protection from some of these diseases. This is partly due to the immaturity of the neonate and infant immune system. Little is known about the specific modifications to immunological assessment protocols in early life but increasing knowledge of infant immunology has helped provide better recommendations for assessing these responses. Since most new vaccines will eventually be deployed in low-income settings such as Africa, the logistics and resources of assessing immunity in such settings also need to be understood. In this article, we will review immunity to vaccines in early life, discuss the many challenges associated with assessing immunogenicity and provide practical tips.

Ovine atadenovirus, a novel and highly immunogenic vector in prime-boost studies of a candidate HIV-1 vaccine

Ovine adenovirus type 7 (OAdV) is the prototype member of the genus Atadenovirus. No immunity to the virus has so far been detected in human sera. We describe the construction and evaluation of a candidate HIV-1 vaccine based on OAdV and its utilisation alone and in combination with plasmid-, human adenovirus type 5 (HAdV5; a Mastadenovirus)-, and modified vaccinia Ankara (MVA)-vectored vaccines. All vectors expressed HIVA, an immunogen consisting of HIV-1 clade A consensus Gag-derived protein coupled to a T cell polyepitope. OAdV.HIVA was genetically stable, grew well and expressed high levels of protein from the Rous sarcoma virus promoter. OAdV.HIVA was highly immunogenic in mice and efficiently primed and boosted HIV-1-specific T cell responses together with heterologous HIVA-expressing vectors. There were significant differences between OAdV and HAdV5 vectors in priming of naïve CD8(+) T cell responses to HIVA and in the persistence of MHC class I-restricted epitope presentation in the local draining lymph nodes. OAdV.HIVA primed T cells more rapidly but was less persistent than AdV5.HIVA and thus induced a qualitatively distinct T cell response. Nevertheless, both vectors primed a response in mice that reduced viral titres in a surrogate challenge model by three to four orders of magnitude. Thus, OAdV is a novel, underexplored vaccine vector with potential for further development for HIV-1 and other vaccines. The data are discussed in the context of the latest HIV-1 vaccine developments.

HV-1-specific cytotoxic T lymphocytes and breast milk HIV-1 transmission

BACKGROUND

Breast-feeding by infants exposed to human immunodeficiency virus type 1 (HIV-1) provides an opportunity to assess the role played by repeated HIV-1 exposure in eliciting HIV-1-specific immunity and in defining whether immune responses correlate with protection from infection.

METHODS

Breast-feeding infants born to HIV-1-seropositive women were assessed for HLA-selected HIV-1 peptide-specific cytotoxic T lymphocyte interferon (IFN)-gamma responses by means of enzyme-linked immunospot (ELISpot) assays at 1, 3, 6, 9, and 12 months of age. Responses were deemed to be positive when they reached > or = 50 HIV-1-specific sfu/1 x 10(6) peripheral blood mononuclear cells (PBMCs) and were at least twice those of negative controls.

RESULTS

A total of 807 ELISpot assays were performed for 217 infants who remained uninfected with HIV-1 at approximately 12 months of age; 101 infants (47%) had at least 1 positive ELISpot result (median, 78-170 sfu/1 x 10(6) PBMCs). The prevalence and magnitude of responses increased with age (P = .01 and P = .007, respectively); the median log(10) value for HIV-1-specific IFN-gamma responses increased by 1.0 sfu/1 x 10(6) PBMCs/month (P < .001) between 1 and 12 months of age. Of 141 HIV-1-uninfected infants with 1-month ELISpot results, 10 (7%) acquired HIV-1 infection (0/16 with positive vs. 10/125 [8%] with negative ELISpot results; P = .6). Higher values for log(10) HIV-1-specific spot-forming units at 1 month of age were associated with a decreased risk of HIV-1 infection, adjusted for maternal HIV-1 RNA level (adjusted hazard ratio, 0.09 [95% confidence interval, 0.01-0.72]).

CONCLUSION

. Breast-feeding HIV-1-exposed uninfected infants frequently had HIV-1-specific IFN-gamma responses. Greater early HIV-1-specific IFN-gamma responses were associated with decreased HIV-1 acquisition.

The magnitude of interferon-gamma responses to human cytomegalovirus is predictive for HIV-1 disease progression

BACKGROUND

Human cytomegalovirus (HCMV) infection has been strongly associated to HIV-1 progression. We have investigated whether the magnitude of the overall peripheral blood mononuclear cell responses to HCMV stimulation correlated with HIV-1 progression.

METHODS

Blood samples were collected from 75 HIV-1-positive individuals on highly active antiretroviral therapy with CD4 count>500 cells per cubic millimeter and undetectable HIV RNA just before interrupting treatment. Specific interferon-gamma (IFN-gamma) HCMV cell responses were measured by an enzyme-linked immunospot (ELISPOT) assay. The results were analyzed by Kaplan-Meier survival curves, contingency tests, and the Cox proportional hazard models to evaluate the predictive value of peripheral blood responses to HCMV and the length of time that patients were off treatment.

RESULTS

Patients were stratified into those with weak (<500 spot-forming units) or strong (>500 spot-forming units) IFN-gamma responses to HCMV. During the 3-year follow-up, 51% of patients with strong responses remained untreated compared with 14% of patients with weak HCMV responses (P=0.0015). Length of time without therapy was also longer in patients with stronger responses (hazard ratio=2.08; P=0.001). HCMV responses were still predictive of restarting therapy after adjusting for the CD4 nadir counts.

CONCLUSION

Specific IFN-gamma responses to HCMV may be employed as a predictive useful marker for the evolution of HIV-1 infection.

Secretion of interferon-gamma by human macrophages demonstrated at the single-cell level after costimulation with interleukin (IL)-12 plus IL-18

The interferon (IFN)-gamma component of the immune response plays an essential role in combating infectious and non-infectious diseases. Induction of IFN-gamma secretion by human T and natural killer (NK) cells through synergistic costimulation with interleukin (IL)-12 and IL-18 in the adaptive immune responses against pathogens is well established, but induction of similar activity in macrophages is still controversial, with doubts largely focusing on contamination of macrophages with NK or T cells in the relevant experiments. The possible contribution of macrophages to the IFN response is, however, an important factor relevant to the pathogenesis of many diseases. To resolve this issue, we analysed the production of IFN-gamma at the single-cell level by immunohistochemistry and by enzyme-linked immunosorbent spot (ELISPOT) analysis and unequivocally demonstrated that human macrophages derived from monocytes in vitro through stimulation with a combination of IL-12 and IL-18 or with macrophage colony-stimulating factor (M-CSF) were able to produce IFN-gamma when further stimulated with a combination of IL-12 and IL-18. In addition, naturally activated alveolar macrophages immediately secreted IFN-gamma upon treatment with IL-12 and IL-18. Therefore, human macrophages in addition to lymphoid cells contribute to the IFN-gamma response, providing another link between the innate and acquired immune responses.

Construction, characterization, and immunogenicity of a multigene modified vaccinia Ankara (MVA) vaccine based on HIV type 1 subtype C

Candidate vaccines composed of a DNA construct to prime the immune system, followed by modified vaccinia Ankara (MVA) containing matching genes as a booster vaccination, have produced encouraging immune responses in human volunteers. This study presents the detailed construction and characterization of a recombinant MVA that will be tested in combination with a DNA vaccine in Phase I clinical trials in South Africa and the United States. To match recently transmitted viruses in the southern African region and to maximize epitope coverage, the vaccines were constructed to contain five HIV-1 subtype C genes, namely gag, reverse transcriptase, tat, and nef (grttn), expressed as a polyprotein, and a truncated env (gp150). An initial recombinant MVA construct containing wild-type env was found to be genetically unstable, and thus a human codon-optimized gene was used. Grttn and gp150 were inserted into two different sites in MVA yielding a double recombinant, SAAVI MVA-C. The recombinant MVA was shown to be genetically stable and high level expression of the transgenes was observed. Env retained infectivity in a functional infectivity assay despite a point mutation that arose during virus generation. Mice inoculated with SAAVI MVA-C at various doses developed high levels of Gag, RT, and Env-specific CD8(+) and CD4(+) T cells, and some of these responses could be boosted by a second inoculation. An accompanying paper describes the immunogenicity of SAAVI MVA-C when given in combination with SAAVI DNA-C.

Design and pre-clinical evaluation of a universal HIV-1 vaccine

Background

One of the big roadblocks in development of HIV-1/AIDS vaccines is the enormous diversity of HIV-1, which could limit the value of any HIV-1 vaccine candidate currently under test.

Methodology and Findings

To address the HIV-1 variation, we designed a novel T cell immunogen, designated HIV_CONSV, by assembling the 14 most conserved regions of the HIV-1 proteome into one chimaeric protein. Each segment is a consensus sequence from one of the four major HIV-1 clades A, B, C and D, which alternate to ensure equal clade coverage. The gene coding for the HIV_CONSV protein was inserted into the three most studied vaccine vectors, plasmid DNA, human adenovirus serotype 5 and modified vaccine virus Ankara (MVA), and induced HIV-1-specific T cell responses in mice. We also demonstrated that these conserved regions prime CD8⁺ and CD4⁺ T cell to highly conserved epitopes in humans and that these epitopes, although usually subdominant, generate memory T cells in patients during natural HIV-1 infection.

Significance

Therefore, this vaccine approach provides an attractive and testable alternative for overcoming the HIV-1 variability, while focusing T cell responses on regions of the virus that are less likely to mutate and escape. Furthermore, this approach has merit in the simplicity of design and delivery, requiring only a single immunogen to provide extensive coverage of global HIV-1 population diversity.

Vaccine platform for prevention of tuberculosis and mother-to-child transmission of human immunodeficiency virus type 1 through breastfeeding

Most children in Africa receive their vaccine against tuberculosis at birth. Those infants born to human immunodeficiency virus type 1 (HIV-1)-positive mothers are at high risk of acquiring HIV-1 infection through breastfeeding in the first weeks of their lives. Thus, the development of a vaccine which would protect newborns against both of these major global killers is a logical yet highly scientifically, ethically, and practically challenging aim. Here, a recombinant lysine auxotroph of Mycobacterium bovis bacillus Calmette-Guérin (BCG), a BCG strain that is safer than those currently used and expresses an African HIV-1 clade-derived immunogen, was generated and shown to be stable and to induce durable, high-quality HIV-1-specific CD4(+)- and CD8(+)-T-cell responses. Furthermore, when the recombinant BCG vaccine was used in a priming-boosting regimen with heterologous components, the HIV-1-specific responses provided protection against surrogate virus challenge, and the recombinant BCG vaccine alone protected against aerosol challenge with M. tuberculosis. Thus, inserting an HIV-1-derived immunogen into the scheduled BCG vaccine delivered at or soon after birth may prime HIV-1-specific responses, which can be boosted by natural exposure to HIV-1 in the breast milk and/or by a heterologous vaccine such as recombinant modified vaccinia virus Ankara delivering the same immunogen, and decrease mother-to-child transmission of HIV-1 during breastfeeding.

Detection of HIV vaccine-induced cell-mediated immunity in HIV-seronegative clinical trial participants using an optimized and validated enzyme-linked immunospot assay

An effective vaccine for HIV is likely to require induction of T-cell-mediated immune responses, and the interferon-gamma (IFNgamma) enzyme-linked immunospot (ELISPOT) assay has become the most commonly used assay for measuring these responses in vaccine trials. We optimized and validated the HIV ELISPOT assay using an empirical method to establish positivity criteria that results in a < or =1% false-positive rate. Using this assay, we detected a broad range of HIV-specific ELISPOT responses to peptide pools of overlapping 20mers, 15mers, or 9mers in study volunteers receiving DNA- or adenovirus vector-based HIV vaccines and in HIV-seropositive donors. We found that 15mers generally had higher response magnitudes than 20mers and lower false-positive rates than 9mers. These studies show that our validated ELISPOT assay using 15mer peptide pools and the positivity criteria of > or =55 spots per 10(6) cells and > or =4-fold over mock (negative control) is a sensitive and specific assay for the detection of HIV vaccine-induced cell-mediated immunity.

Recent advances in delivery systems for anti-HIV1 therapy

In the last years, different non-biological and biological carrier systems have been developed for anti-HIV1 therapy. Liposomes are excellent potential anti-HIV1 carriers that have been tested with drugs, antisense oligonucleotides, ribozymes and therapeutic genes. Nanoparticles and low-density lipoproteins (LDLs) are cell-specific transporters of drugs against macrophage-specific infections such as HIV1. Through a process of protein transduction, cell-permeable peptides of natural origin or designed artificially allow the delivery of drugs and genetic material inside the cell. Erythrocyte ghosts and bacterial ghosts are a promising delivery system for therapeutic peptides and HIV vaccines. Of interest are the advances made in the field of HIV gene therapy by the use of autologous haematopoietic stem cells and viral vectors for HIV vaccines. Although important milestones have been reached in the development of carrier systems for the treatment of HIV, especially in the field of gene therapy, further clinical trials are required so that the efficiency and safety of these new systems can be guaranteed in HIV patients.

Clinical experience with plasmid DNA- and modified vaccinia virus Ankara-vectored human immunodeficiency virus type 1 clade A vaccine focusing on T-cell induction

Candidate human immunodeficiency virus type 1 (HIV-1) vaccines focusing on T-cell induction, constructed as pTHr.HIVA DNA and modified vaccinia virus Ankara (MVA).HIVA, were delivered in a heterologous prime-boost regimen. The vaccines were tested in several hundred healthy or HIV-1-infected volunteers in Europe and Africa. Whilst larger trials of hundreds of volunteers suggested induction of HIV-1-specific T-cell responses in <15 % of healthy vaccinees, a series of small, rapid trials in 12-24 volunteers at a time with a more in-depth analysis of vaccine-elicited T-cell responses proved to be highly informative and provided more encouraging results. These trials demonstrated that the pTHr.HIVA vaccine alone primed consistently weak and mainly CD4(+), but also CD8(+) T-cell responses, and the MVA.HIVA vaccine delivered a consistent boost to both CD4(+) and CD8(+) T cells, which was particularly strong in HIV-1-infected patients. Thus, whilst the search is on for ways to enhance T-cell priming, MVA is a useful boosting vector for human subunit genetic vaccines.

Naive CD4(+) T cells and recent thymic emigrant levels in treated individuals with HIV: clinical relevance

An increase in the levels of naive T cells after the administration of HAART is an indicator of the quality of immune reconstitution. We investigated whether levels of naive CD4 T cells (CD4(+)/CD45RA(+)) and of recent thymic emigrants (RTEs; CD4(+)/CD45RA(+)/CD31(+)) achieved in chronically treated HIV-infected patients could predict the length of time patients could interrupt antiretroviral treatment before their CD4 counts reached values < or =350 cells/mm(3) or HIV-1 RNA levels increased to > or =100,000 copies/ml (Tibet cohort). Serial measurements revealed that the level of naive CD4 T cells among patients was extremely variable (5-95%), but the values for each patient remained stable throughout the study. We then focused on those patients who showed percentages of naive CD4 T cells above 60% or below 30%. The levels of naive T cells, RTEs, mononuclear cells containing T cell receptor excision circles (TRECs), and the strength of CD4 helper responses to HIV p24 antigen during treatment failed to predict the duration of treatment interruptions. In contrast, the median survival time of patients with a CD4 nadir > or =350 cells/mm(3) was 2-fold higher than that of patients with a CD4 nadir <350. However, the probability of restarting therapy in these two groups of patients was independent of the levels of naive T cells, RTEs, or TRECs.

Statistical positivity criteria for the analysis of ELISpot assay data in HIV-1 vaccine trials

The enzyme-linked immunospot (ELISpot) assay is one of the leading technologies used to measure cellular responses in many settings including HIV vaccine clinical trials. HIV-1-specific effector T lymphocyte responses are considered necessary in the control of infection. Accurate measurement and summary of cellular immune responses are critical to HIV research. ELISpot assay readout is often dichotomized into positive/negative responses according to some pre-specified criteria. Given the increase in the number of replicates used for each stimulation with current assay configurations in the HIV Vaccine Trials Network (HVTN) laboratories, a new approach is now possible. We propose an objective criteria based on a hypothesis-driven method that controls the false positive rate while maintaining sensitivity to each of the antigen-specific responses under study. The new approach is compared to other commonly employed criteria using real and simulated data.

Helminth infection suppresses T-cell immune response to HIV-DNA-based vaccine in mice

A number of HIV-1 vaccines are in various phases of clinical trials and many more are in the developmental pipeline. Vaccines are especially needed for developing countries where morbidity and mortality due to HIV/AIDS is most severe, the prevalence of HIV infection is highest, and its incidence is often still rising dramatically. Individuals living in these regions are often infected with one or more helminth parasites which systemically bias the immune system towards Th2-type as well as drive immune anergy. The goal of this study was to develop a multi-T-cell epitope DNA-based vaccine for HIV-1 subtype C and to determine the impact of helminth infection on the immune response to this vaccine. We found that vaccination of naïve mice with the multi-epitope vaccine, designated TD158, induced a strong HIV-1C-specific T-cell immune response, and that the addition of the Igkappa leader sequence to the TD158 vaccine construct significantly increased the frequencies of IFN-gamma secreting CD8+ T cells. However, the TD158 vaccine specific response of mice infected with the human helminth Schistosoma mansoni was significantly suppressed. The impact of schistosome infection on suppressing the virus-specific immune response was the same whether mice were vaccinated with the TD158 vaccine or with the Igkappa enhanced TD158. The results of this study suggest that helminth infection may pose a serious problem for vaccination with the DNA-based HIV-1 vaccine in developing country populations, and that the prevalence of helminth infections in the vaccine cohorts should be taken into account for HIV-1 vaccine trial design.