Vasee S. Moorthy

Enhanced T-cell immunogenicity of plasmid DNA vaccines boosted by recombinant modified vaccinia virus Ankara in humans

In animals, effective immune responses against malignancies and against several infectious pathogens, including malaria, are mediated by T cells. Here we show that a heterologous prime-boost vaccination regime of DNA either intramuscularly or epidermally, followed by intradermal recombinant modified vaccinia virus Ankara (MVA), induces high frequencies of interferon (IFN)-γ-secreting, antigen-specific T-cell responses in humans to a pre-erythrocytic malaria antigen, thrombospondin-related adhesion protein (TRAP). These responses are five- to tenfold higher than the T-cell responses induced by the DNA vaccine or recombinant MVA vaccine alone, and produce partial protection manifest as delayed parasitemia after sporozoite challenge with a different strain of Plasmodium falciparum. Such heterologous prime-boost immunization approaches may provide a basis for preventative and therapeutic vaccination in humans.

Phase 1 Trials of rVSV Ebola Vaccine in Africa and Europe.

BACKGROUND The replication-competent recombinant vesicular stomatitis virus (rVSV)-based vaccine expressing a Zaire ebolavirus (ZEBOV) glycoprotein was selected for rapid safety and immunogenicity testing before its use in West Africa. METHODS We performed three open-label, dose-escalation phase 1 trials and one randomized, double-blind, controlled phase 1 trial to assess the safety, side-effect profile, and immunogenicity of rVSV-ZEBOV at various doses in 158 healthy adults in Europe and Africa. All participants were injected with doses of vaccine ranging from 300,000 to 50 million plaque-forming units (PFU) or placebo. RESULTS No serious vaccine-related adverse events were reported. Mild-to-moderate early-onset reactogenicity was frequent but transient (median, 1 day). Fever was observed in up to 30% of vaccinees. Vaccine viremia was detected within 3 days in 123 of the 130 participants (95%) receiving 3 million PFU or more; rVSV was not detected in saliva or urine. In the second week after injection, arthritis affecting one to four joints developed in 11 of 51 participants (22%) in Geneva, with pain lasting a median of 8 days (interquartile range, 4 to 87); 2 self-limited cases occurred in 60 participants (3%) in Hamburg, Germany, and Kilifi, Kenya. The virus was identified in one synovial-fluid aspirate and in skin vesicles of 2 other vaccinees, showing peripheral viral replication in the second week after immunization. ZEBOV-glycoprotein-specific antibody responses were detected in all the participants, with similar glycoprotein-binding antibody titers but significantly higher neutralizing antibody titers at higher doses. Glycoprotein-binding antibody titers were sustained through 180 days in all participants. CONCLUSIONS In these studies, rVSV-ZEBOV was reactogenic but immunogenic after a single dose and warrants further evaluation for safety and efficacy. (Funded by the Wellcome Trust and others; ClinicalTrials.gov numbers, NCT02283099, NCT02287480, and NCT02296983; Pan African Clinical Trials Registry number, PACTR201411000919191.).

Malaria vaccine developments

Large gains in the reduction of malaria mortality in the early 20th century were lost in subsequent decades. Malaria now kills 2-3 million people yearly. Implementation of malaria control technologies such as insecticide-treated bednets and chemotherapy could reduce mortality substantially, but an effective malaria vaccine is also needed. Advances in vaccine technology and immunology are being used to develop malaria subunit vaccines. Novel approaches that might yield effective vaccines for other diseases are being evaluated first in malaria. We describe progress in malaria vaccine development in the past 5 years: reasons for cautious optimism, the type of vaccine that might realistically be expected, and how the process could be hastened. Although exact predictions are not possible, if sufficient funding were mobilised, a deployable, effective malaria vaccine is a realistic medium-term to long-term goal.

Experimental human challenge infections can accelerate clinical malaria vaccine development

Malaria is one of the most frequently occurring infectious diseases worldwide, with almost 1 million deaths and an estimated 243 million clinical cases annually. Several candidate malaria vaccines have reached Phase IIb clinical trials, but results have often been disappointing. As an alternative to these Phase IIb field trials, the efficacy of candidate malaria vaccines can first be assessed through the deliberate exposure of participants to the bites of infectious mosquitoes (sporozoite challenge) or to an inoculum of blood-stage parasites (blood-stage challenge). With an increasing number of malaria vaccine candidates being developed, should human malaria challenge models be more widely used to reduce cost and time investments? This article reviews previous experience with both the sporozoite and blood-stage human malaria challenge models and provides future perspectives for these models in malaria vaccine development.

A Randomised, Double-Blind, Controlled Vaccine Efficacy Trial of DNA/MVA ME-TRAP Against Malaria Infection in Gambian Adults

Background Many malaria vaccines are currently in development, although very few have been evaluated for efficacy in the field. Plasmodium falciparum multiple epitope (ME)– thrombospondin-related adhesion protein (TRAP) candidate vaccines are designed to potently induce effector T cells and so are a departure from earlier malaria vaccines evaluated in the field in terms of their mechanism of action. ME-TRAP vaccines encode a polyepitope string and the TRAP sporozoite antigen. Two vaccine vectors encoding ME-TRAP, plasmid DNA and modified vaccinia virus Ankara (MVA), when used sequentially in a prime-boost immunisation regime, induce high frequencies of effector T cells and partial protection, manifest as delay in time to parasitaemia, in a clinical challenge model. Methods and Findings A total of 372 Gambian men aged 15–45 y were randomised to receive either DNA ME-TRAP followed by MVA ME-TRAP or rabies vaccine (control). Of these men, 296 received three doses of vaccine timed to coincide with the beginning of the transmission season (141 in the DNA/MVA group and 155 in the rabies group) and were followed up. Volunteers were given sulphadoxine/pyrimethamine 2 wk before the final vaccination. Blood smears were collected weekly for 11 wk and whenever a volunteer developed symptoms compatible with malaria during the transmission season. The primary endpoint was time to first infection with asexual P. falciparum. Analysis was per protocol. DNA ME-TRAP and MVA ME-TRAP were safe and well-tolerated. Effector T cell responses to a non-vaccine strain of TRAP were 50-fold higher postvaccination in the malaria vaccine group than in the rabies vaccine group. Vaccine efficacy, adjusted for confounding factors, was 10.3% (95% confidence interval, −22% to +34%; p = 0.49). Incidence of malaria infection decreased with increasing age and was associated with ethnicity. Conclusions DNA/MVA heterologous prime-boost vaccination is safe and highly immunogenic for effector T cell induction in a malaria-endemic area. But despite having produced a substantial reduction in liver-stage parasites in challenge studies of non-immune volunteers, this first generation T cell–inducing vaccine was ineffective at reducing the natural infection rate in semi-immune African adults.

The effect of dose on the safety and immunogenicity of the VSV Ebola candidate vaccine: a randomised double-blind, placebo-controlled phase 1/2 trial

BACKGROUND Safe and effective vaccines against Ebola could prevent or control outbreaks. The safe use of replication-competent vaccines requires a careful dose-selection process. We report the first safety and immunogenicity results in volunteers receiving 3 × 10(5) plaque-forming units (pfu) of the recombinant vesicular stomatitis virus-based candidate vaccine expressing the Zaire Ebola virus glycoprotein (rVSV-ZEBOV; low-dose vaccinees) compared with 59 volunteers who had received 1 ×10(7) pfu (n=35) or 5 × 10(7) pfu (n=16) of rVSV-ZEBOV (high-dose vaccinees) or placebo (n=8) before a safety-driven study hold. METHODS The Geneva rVSV-ZEBOV study, an investigator-initiated phase 1/2, dose-finding, placebo-controlled, double-blind trial conducted at the University Hospitals of Geneva, Switzerland, enrolled non-pregnant, immunocompetent, and otherwise healthy adults aged 18-65 years. Participants from the low-dose group with no plans to deploy to Ebola-aff5cted regions (non-deployable) were randomised 9:1 in a double-blind fashion using randomly permuted blocks of varying sizes to a single injection of 3 × 10(5) pfu or placebo, whereas deployable participants received single-injection 3 × 10(5) pfu open-label. Primary safety and immunogenicity outcomes were the incidence of adverse events within 14 days of vaccination and day-28 antibody titres, respectively, analysed by intention to treat. After viral oligoarthritis was observed in 11 of the first 51 vaccinees (22%) receiving 10(7) or 5 × 10(7) pfu, 56 participants were given a lower dose (3 × 10(5) pfu, n=51) or placebo (n=5) to assess the effect of dose reduction on safety and immunogenicity. This trial is ongoing with a follow-up period of 12 months; all reported results are from interim databases. This study is registered with ClinicalTrials.gov, number NCT02287480. FINDINGS Between Jan 5 and Jan 26, 2015, 43 non-deployable participants received low-dose rVSV-ZEBOV (3 × 10(5) pfu) or placebo in a double-blind fashion, whereas 13 deployable participants received 3 × 10(5) pfu open-label. Altogether, in the low-dose group, 51 participants received rVSV-ZEBOV and five received placebo. No serious adverse events occurred. At 3 × 10(5) pfu, early-onset reactogenicity remained frequent (45 [88%] of 51 compared with 50 [98%] of 51 high dose and two [15%] of 13 placebo recipients), but mild. Objective fever was present in one (2%) of 51 low-dose versus 13 (25%) of 51 high-dose vaccinees receiving at least 1 ×10(7) pfu (p<0·0001). Subjective fever (p<0·0001), myalgia (p=0·036), and chills (p=0·026) were significantly reduced and their time of onset delayed, reflecting significantly lower viraemia (p<0·0001) and blood monocyte-activation patterns (p=0·0233). Although seropositivity rates remained similarly high (48 [94%] of 51), day-28 EBOV-glycoprotein-binding and neutralising antibody titres were lower in low-dose versus high-dose vaccinees (geometric mean titres 344·5 [95% CI 229·7-516·4] vs 1064·2 [757·6-1495·1]; p<0·0001; and 35·1 [24·7-50·7] vs 127·0 [86·0-187·6]; p<0·0001, respectively). Furthermore, oligoarthritis again occurred on day 10 (median; IQR 9-14) in 13 (25%) of 51 low-dose vaccinees, with maculopapular, vesicular dermatitis, or both in seven (54%) of 13; arthritis was associated with increasing age in low-dose but not high-dose vaccinees. Two vaccinees presented with purpura of the lower legs; histological findings indicated cutaneous vasculitis. The presence of rVSV in synovial fluid and skin lesions confirmed causality. INTERPRETATION Reducing the dose of rVSV-ZEBOV improved its early tolerability but lowered antibody responses and did not prevent vaccine-induced arthritis, dermatitis, or vasculitis. Like its efficacy, the safety of rVSV-ZEBOV requires further definition in the target populations of Africa. FUNDING Wellcome Trust through WHO.

Durable Human Memory T Cells Quantifiable by Cultured Enzyme-Linked Immunospot Assays Are Induced by Heterologous Prime Boost Immunization and Correlate with Protection against Malaria

Immunological memory is a required component of protective antimalarial responses raised by T cell-inducing vaccines. The magnitude of ex vivo IFN-γ T cell responses is widely used to identify immunogenic vaccines although this response usually wanes and may disappear within weeks. However, protection in the field is likely to depend on durable central memory T cells that are not detected by this assay. To identify longer-lived memory T cells, PBMC from malaria-naive vaccinated volunteers who had received prime boost vaccinations with a combination of DNA and/or viral vectors encoding the multiepitope string-thrombospondin-related adhesion protein Ag were cultured in vitro with Ag for 10 days before the ELISPOT assay. Ex vivo T cell responses peaked at 7 days after the final immunization and declined substantially over 6 mo, but responses identified after T cell culture increased over the 6-mo period after the final immunization. Moreover, individual cultured ELISPOT responses at the day of challenge time point correlated significantly with degree of protection against malaria sporozoite challenge, whereas ex vivo responses did not, despite a correlation between the peak ex vivo response and magnitude of memory responses 6 mo later. This cultured assay identifies long-lasting protective T cell responses and therefore offers an attractive option for assessments of vaccine immunogenicity.

Phase 1 Evaluation of 3 Highly Immunogenic Prime-Boost Regimens, Including a 12-Month Reboosting Vaccination, for Malaria Vaccination in Gambian Men

Successful vaccination against intracellular pathogens, including liver-stage Plasmodium falciparum, will require induction of strong antigen-specific T lymphocyte responses. The multiple epitope (ME)-thrombospondin-related adhesion protein (TRAP) construct includes CD8(+) and CD4(+) T cell epitopes from pre-erythrocytic P. falciparum antigens fused in-frame to the entire pre-erythrocytic antigen TRAP. Three carriers for this construct--plasmid DNA and 2 recombinant nonreplicating poxviruses (modified vaccinia virus Ankara [MVA] and fowlpox strain 9 [FP9])--were administered at 3-week intervals in a heterologous prime-boost combination to 29 Gambian men aged 18-45 years. Doses of DNA ME-TRAP, MVA ME-TRAP, and FP9 ME-TRAP were 2 mg and 1.5x10(8) and 1x10(8) plaque-forming units, respectively. DNA ME-TRAP was injected intramuscularly; MVA ME-TRAP and FP9 ME-TRAP were injected intradermally. There were no clinically relevant laboratory abnormalities and no severe or serious adverse events related to vaccination. DNA/MVA and FP9/MVA regimens were the most potent inducers of circulating effector T cells seen to date in sub-Saharan Africa. Twelve months after the final vaccination, a single booster vaccination expanded the effector T cell pool to a similar or higher magnitude than that after the primary vaccinations. These results highlight optimized combination regimens with general relevance to the development of vaccines targeting intracellular pathogens.

Use of ChAd3-EBO-Z Ebola virus vaccine in Malian and US adults, and boosting of Malian adults with MVA-BN-Filo: a phase 1, single-blind, randomised trial, a phase 1b, open-label and double-blind, dose-escalation trial, and a nested, randomised, double-blind, placebo-controlled trial

Summary Background The 2014 west African Zaire Ebola virus epidemic prompted worldwide partners to accelerate clinical development of replication-defective chimpanzee adenovirus 3 vector vaccine expressing Zaire Ebola virus glycoprotein (ChAd3-EBO-Z). We aimed to investigate the safety, tolerability, and immunogenicity of ChAd3-EBO-Z in Malian and US adults, and assess the effect of boosting of Malians with modified vaccinia Ankara expressing Zaire Ebola virus glycoprotein and other filovirus antigens (MVA-BN-Filo). Methods In the phase 1, single-blind, randomised trial of ChAd3-EBO-Z in the USA, we recruited adults aged 18–65 years from the University of Maryland medical community and the Baltimore community. In the phase 1b, open-label and double-blind, dose-escalation trial of ChAd3-EBO-Z in Mali, we recruited adults 18–50 years of age from six hospitals and health centres in Bamako (Mali), some of whom were also eligible for a nested, randomised, double-blind, placebo-controlled trial of MVA-BN-Filo. For randomised segments of the Malian trial and for the US trial, we randomly allocated participants (1:1; block size of six [Malian] or four [US]; ARB produced computer-generated randomisation lists; clinical staff did randomisation) to different single doses of intramuscular immunisation with ChAd3-EBO-Z: Malians received 1 × 1010 viral particle units (pu), 2·5 × 1010 pu, 5 × 1010 pu, or 1 × 1011 pu; US participants received 1 × 1010 pu or 1 × 1011 pu. We randomly allocated Malians in the nested trial (1:1) to receive a single dose of 2 × 108 plaque-forming units of MVA-BN-Filo or saline placebo. In the double-blind segments of the Malian trial, investigators, clinical staff, participants, and immunology laboratory staff were masked, but the study pharmacist (MK), vaccine administrator, and study statistician (ARB) were unmasked. In the US trial, investigators were not masked, but participants were. Analyses were per protocol. The primary outcome was safety, measured with occurrence of adverse events for 7 days after vaccination. Both trials are registered with ClinicalTrials.gov, numbers NCT02231866 (US) and NCT02267109 (Malian). Findings Between Oct 8, 2014, and Feb 16, 2015, we randomly allocated 91 participants in Mali (ten [11%] to 1 × 1010 pu, 35 [38%] to 2·5 × 1010 pu, 35 [38%] to 5 × 1010 pu, and 11 [12%] to 1 × 1011 pu) and 20 in the USA (ten [50%] to 1 × 1010 pu and ten [50%] to 1 × 1011 pu), and boosted 52 Malians with MVA-BN-Filo (27 [52%]) or saline (25 [48%]). We identified no safety concerns with either vaccine: seven (8%) of 91 participants in Mali (five [5%] received 5 × 1010 and two [2%] received 1 × 1011 pu) and four (20%) of 20 in the USA (all received 1 × 1011 pu) given ChAd3-EBO-Z had fever lasting for less than 24 h, and 15 (56%) of 27 Malians boosted with MVA-BN-Filo had injection-site pain or tenderness. Interpretation 1 × 1011 pu single-dose ChAd3-EBO-Z could suffice for phase 3 efficacy trials of ring-vaccination containment needing short-term, high-level protection to interrupt transmission. MVA-BN-Filo boosting, although a complex regimen, could confer long-lived protection if needed (eg, for health-care workers). Funding Wellcome Trust, Medical Research Council UK, Department for International Development UK, National Cancer Institute, Frederick National Laboratory for Cancer Research, Federal Funds from National Institute of Allergy and Infectious Diseases.

Immunological mechanisms underlying protection mediated by RTS,S: a review of the available data

The RTS,S/AS candidate malaria vaccine has demonstrated efficacy against a variety of endpoints in Phase IIa and Phase IIb trials over more than a decade. A multi-country phase III trial of RTS,S/AS01 is now underway with submission as early as 2012, if vaccine safety and efficacy are confirmed. The immunologic basis for how the vaccine protects against both infection and disease remains uncertain. It is, therefore, timely to review the information currently available about the vaccine with regard to how it impacts the human-Plasmodium falciparum host-pathogen relationship. In this article, what is known about mechanisms involved in partial protection against malaria induced by RTS,S is reviewed.