IMRAS-A clinical trial of mosquito-bite immunization with live, radiation-attenuated P. falciparum sporozoites: Impact of immunization parameters on protective efficacy and generation of a repository of immunologic reagents

Background

Immunization with radiation-attenuated sporozoites (RAS) by mosquito bite provides >90% sterile protection against Plasmodium falciparum (Pf) malaria in humans. RAS invade hepatocytes but do not replicate. CD8+ T cells recognizing parasite-derived peptides on the surface of infected hepatocytes are likely the primary protective mechanism. We conducted a randomized clinical trial of RAS immunization to assess safety, to achieve 50% vaccine efficacy (VE) against controlled human malaria infection (CHMI), and to generate reagents from protected and non-protected subjects for future identification of protective immune mechanisms and antigens.

Methods

Two cohorts (Cohort 1 and Cohort 2) of healthy, malaria-naïve, non-pregnant adults age 18–50 received five monthly immunizations with infected (true-immunized, n = 21) or non-infected (mock-immunized, n = 5) mosquito bites and underwent homologous CHMI at 3 weeks. Immunization parameters were selected for 50% protection based on prior clinical data. Leukapheresis was done to collect plasma and peripheral blood mononuclear cells.

Results

Adverse event rates were similar in true- and mock-immunized subjects. Two true- and two mock-immunized subjects developed large local reactions likely caused by mosquito salivary gland antigens. In Cohort 1, 11 subjects received 810–1235 infected bites; 6/11 (55%) were protected against CHMI vs. 0/3 mock-immunized and 0/6 infectivity controls (VE 55%). In Cohort 2, 10 subjects received 839–1131 infected bites with a higher first dose and a reduced fifth dose; 9/10 (90%) were protected vs. 0/2 mock-immunized and 0/6 controls (VE 90%). Three/3 (100%) protected subjects administered three booster immunizations were protected against repeat CHMI vs. 0/6 controls (VE 100%). Cohort 2 uniquely showed a significant rise in IFN-γ responses after the third and fifth immunizations and higher antibody responses to CSP.

Conclusions

PfRAS were generally safe and well tolerated. Cohort 2 had a higher first dose, reduced final dose, higher antibody responses to CSP and significant rise of IFN-γ responses after the third and fifth immunizations. Whether any of these factors contributed to increased protection in Cohort 2 requires further investigation. A cryobank of sera and cells from protected and non-protected individuals was generated for future immunological studies and antigen discovery.

Trial registration

ClinicalTrials.gov NCT01994525.

Controlled Human Malaria Infection at the Walter Reed Army Institute of Research: The Past, Present, and Future From an Entomological Perspective

Thirty years ago, the Entomology Branch at the Walter Reed Army Institute of Research (WRAIR) performed the first controlled human malaria infection, in which lab-reared mosquitoes were infected with lab-cultured malaria parasites and allowed to feed on human volunteers. The development of this model was a turning point for pre-erythrocytic malaria vaccine research and, through decades of refinement, has supported 30 years of efficacy testing of a suite of antimalarial vaccines and drugs. In this article, we present a historical overview of the research that enabled the first challenge to occur and the modifications made to the challenge over time, a summary of the 104 challenges performed by WRAIR from the first into 2015, and a prospective look at what the next generation of challenges might entail.

Using infective mosquitoes to challenge monkeys with Plasmodium knowlesi in malaria vaccine studies

BACKGROUND

When rhesus monkeys (Macaca mulatta) are used to test malaria vaccines, animals are often challenged by the intravenous injection of sporozoites. However, natural exposure to malaria comes via mosquito bite, and antibodies can neutralize sporozoites as they traverse the skin. Thus, intravenous injection may not fairly assess humoral immunity from anti-sporozoite malaria vaccines. To better assess malaria vaccines in rhesus, a method to challenge large numbers of monkeys by mosquito bite was developed.

METHODS

Several species and strains of mosquitoes were tested for their ability to produce Plasmodium knowlesi sporozoites. Donor monkey parasitaemia effects on oocyst and sporozoite numbers and mosquito mortality were documented. Methylparaben added to mosquito feed was tested to improve mosquito survival. To determine the number of bites needed to infect a monkey, animals were exposed to various numbers of P. knowlesi-infected mosquitoes. Finally, P. knowlesi-infected mosquitoes were used to challenge 17 monkeys in a malaria vaccine trial, and the effect of number of infectious bites on monkey parasitaemia was documented.

RESULTS

Anopheles dirus, Anopheles crascens, and Anopheles dirus X (a cross between the two species) produced large numbers of P. knowlesi sporozoites. Mosquito survival to day 14, when sporozoites fill the salivary glands, averaged only 32% when donor monkeys had a parasitaemia above 2%. However, when donor monkey parasitaemia was below 2%, mosquitoes survived twice as well and contained ample sporozoites in their salivary glands. Adding methylparaben to sugar solutions did not improve survival of infected mosquitoes. Plasmodium knowlesi was very infectious, with all monkeys developing blood stage infections if one or more infected mosquitoes successfully fed. There was also a dose-response, with monkeys that received higher numbers of infected mosquito bites developing malaria sooner.

CONCLUSIONS

Anopheles dirus, An. crascens and a cross between these two species all were excellent vectors for P. knowlesi. High donor monkey parasitaemia was associated with poor mosquito survival. A single infected mosquito bite is likely sufficient to infect a monkey with P. knowlesi. It is possible to efficiently challenge large groups of monkeys by mosquito bite, which will be useful for P. knowlesi vaccine studies.

Mosquito bisection as a variable in estimates of PCR-derived malaria sporozoite rates

Background

Highly sensitive polymerase chain reaction (PCR) methods offer an alternative to the light microscopy examination of mosquito salivary glands for the determination of malaria sporozoite rates in wild caught female Anopheles. Removal of mosquito abdomens is assumed to eliminate false positives caused by malaria oocyst DNA in the midgut. This assumption has not been tested with current gold standard PCR assays, and for the variety of conditions that specimens could encounter in the laboratory and field.

Methods

Laboratory Anopheles stephensi were used that had been infected with Plasmodium falciparum 6–7 days and 14 days post infection (p.i.), when oocysts only and oocysts + sporozoites, respectively, are developed. Mosquitoes were killed and immediately frozen, air dried before being frozen, or stored under humid conditions overnight before being frozen, to simulate a range of conditions in the field. Additionally, abdomens were removed anterior to, at, or posterior to the junction of the abdomen and thorax, and both portions were processed using a standard nested PCR of the small sub-unit nuclear ribosomal genes (ssrDNA) with products visualized on agarose gels.

Results

Overall, 4.1 % (4/97) of head + thorax samples that were 6–7 days p.i. gave apparent false positives for sporozoites, compared to 9.3 % (9/97) that were positive for abdomens. No positives (0/52) were obtained when similar specimens were bisected anterior to the junction of the thorax and abdomen, compared to 21.2 % (11/52) that were positive for posterior portions. Multiple bands were noted for positives from the ‘Frozen’ treatment and the rate of false negatives due to DNA degradation appears higher under the ‘Humid’ treatment. Reproducibility of results for the ‘Frozen’ treatment was 90 %.

Conclusions

Despite the importance of specimen condition and the bisection step in determining sporozoite rates, little attention has been paid to them in the literature. Recommendations from this study are that: 1) care needs to be taken to reduce DNA degradation in the field; 2) mosquito abdomens be separated anterior to the junction of the thorax and abdomen; and 3) DNA sequencing of a subsample of positive results should be undertaken if possible.

Randomized, double-blind, phase 2a trial of falciparum malaria vaccines RTS,S/AS01B and RTS,S/AS02A in malaria-naive adults: safety, efficacy, and immunologic associates of protection

BACKGROUND

To further increase the efficacy of malaria vaccine RTS,S/AS02A, we tested the RTS,S antigen formulated using the AS01B Adjuvant System (GlaxoSmithKline Biologicals).

METHODS

In a double-blind, randomized trial, 102 healthy volunteers were evenly allocated to receive RTS,S/AS01B or RTS,S/AS02A vaccine at months 0, 1, and 2 of the study, followed by malaria challenge. Protected vaccine recipients were rechallenged 5 months later.

RESULTS

RTS,S/AS01B and RTS,S/AS02A were well tolerated and were safe. The efficacy of RTS,S/AS01B and RTS,S/AS02A was 50% (95% confidence interval [CI], 32.9%-67.1%) and 32% (95% CI, 17.6%-47.6%), respectively. At the time of initial challenge, the RTS,S/AS01B group had greater circumsporozoite protein (CSP)-specific immune responses, including higher immunoglobulin (Ig) G titers, higher numbers of CSP-specific CD4(+) T cells expressing 2 activation markers (interleukin-2, interferon [IFN]-gamma, tumor necrosis factor-alpha, or CD40L), and more ex vivo IFN-gamma enzyme-linked immunospots (ELISPOTs) than did the RTS,S/AS02A group. Protected vaccine recipients had a higher CSP-specific IgG titer (geometric mean titer, 188 vs 73 mug/mL; P < .001), higher numbers of CSP-specific CD4(+) T cells per 10(6) CD4(+) T cells (median, 963 vs 308 CSP-specific CD4(+) T cells/10(6) CD4(+) T cells; P < .001), and higher numbers of ex vivo IFN-gamma ELISPOTs (mean, 212 vs 96 spots/million cells; P < .001). At rechallenge, 4 of 9 vaccine recipients in each group were still completely protected.

CONCLUSIONS

The RTS,S/AS01B malaria vaccine warrants comparative field trials with RTS,S/AS02A to determine the best formulation for the protection of children and infants. The association between complete protection and immune responses is a potential tool for further optimization of protection. Trial registration. ClinicalTrials.gov identifier NCT00075049.

Recombinant Liver Stage Antigen-1 (LSA-1) formulated with AS01 or AS02 is safe, elicits high titer antibody and induces IFN-gamma/IL-2 CD4+ T cells but does not protect against experimental Plasmodium falciparum infection

Plasmodium falciparum Liver Stage Antigen 1 (LSA-1) is a pre-erythrocytic stage antigen. Our LSA-1 vaccine candidate is a recombinant protein with full-length C- and N-terminal flanking domains and two of the 17 amino acid repeats from the central repeat region termed "LSA-NRC." We describe the first Phase I/II study of this recombinant LSA-NRC protein formulated with either the AS01 or AS02 adjuvant system. We conducted an open-label Phase I/II study. Thirty-six healthy malaria-naïve adults received one of four formulations by intra-deltoid injection on a 0 and 1 month schedule; low dose (LD) LSA-NRC/AS01:10microg LSA-NRC/0.5ml AS01 (n=5), high dose (HD) LSA-NRC/AS01: 50microg LSA-NRC/0.5ml AS01 (n=13); LD LSA-NRC/AS02: 10microg LSA-NRC/0.5ml AS02 (n=5) and HD LSA-NRC/AS02: 50microg LSA-NRC/0.5ml AS02 (n=13). Two weeks post-second immunization, the high dose vaccinees and 6 non-immunized infectivity controls underwent experimental malaria sporozoite challenge. The vaccines showed a reassuring safety profile but were moderately reactogenic. There were no serious adverse events. All subjects seroconverted after the first immunization. Following the second immunization, LSA-1-specific CD4+ T cells producing two cytokines (IL-2 and IFN-gamma) were found by intra-cellular staining in all subjects in the LD LSA-NRC/AS01B group and in 3 of 5 subjects in the LD LSA-NRC/AS02 group. In contrast, the HD LSA-NRC/AS01 and HD LSA-NRC/AS02 group subjects had fewer LSA-1-specific CD4+ T cells, and minimal to no IFN-gamma responses. There was no increase in LSA-1-specific CD8+ T cells found in any group. Per protocol, 22 high dose vaccinees, but no low dose vaccinees, underwent P. falciparum homologous malaria challenge (3D7 clone). All vaccinees became parasitemic and there was no delay in their pre-patent period versus controls (p=0.95). LSA-NRC/AS01 and LSA-NRC/AS02 elicited antigen-specific antibody and CD4+ T cell responses, but elicited no protective immunity. Although the optimal antigen dose of LSA-NRC may not have been selected for the challenge portion of the protocol, further vaccine development based upon LSA-1 should not be excluded and should include alternative vaccine platforms able to elicit additional effector mechanisms such as CD8+ T cells.

Phase 1/2a study of the malaria vaccine candidate apical membrane antigen-1 (AMA-1) administered in adjuvant system AS01B or AS02A

Background

This Phase 1/2a study evaluated the safety, immunogenicity, and efficacy of an experimental malaria vaccine comprised of the recombinant Plasmodium falciparum protein apical membrane antigen-1 (AMA-1) representing the 3D7 allele formulated with either the AS01B or AS02A Adjuvant Systems.

Methodology/Principal Findings

After a preliminary safety evaluation of low dose AMA-1/AS01B (10 µg/0.5 mL) in 5 adults, 30 malaria-naïve adults were randomly allocated to receive full dose (50 µg/0.5 mL) of AMA-1/AS01B (n = 15) or AMA-1/AS02A (n = 15), followed by a malaria challenge. All vaccinations were administered intramuscularly on a 0-, 1-, 2-month schedule. All volunteers experienced transient injection site erythema, swelling and pain. Two weeks post-third vaccination, anti-AMA-1 Geometric Mean Antibody Concentrations (GMCs) with 95% Confidence Intervals (CIs) were high: low dose AMA-1/AS01B 196 µg/mL (103–371 µg/mL), full dose AMA-1/AS01B 279 µg/mL (210–369 µg/mL) and full dose AMA-1/AS02A 216 µg/mL (169–276 µg/mL) with no significant difference among the 3 groups. The three vaccine formulations elicited equivalent functional antibody responses, as measured by growth inhibition assay (GIA), against homologous but not against heterologous (FVO) parasites as well as demonstrable interferon-gamma (IFN-γ) responses. To assess efficacy, volunteers were challenged with P. falciparum-infected mosquitoes, and all became parasitemic, with no significant difference in the prepatent period by either light microscopy or quantitative polymerase chain reaction (qPCR). However, a small but significant reduction of parasitemia in the AMA-1/AS02A group was seen with a statistical model employing qPCR measurements.

Significance

All three vaccine formulations were found to be safe and highly immunogenic. These immune responses did not translate into significant vaccine efficacy in malaria-naïve adults employing a primary sporozoite challenge model, but encouragingly, estimation of parasite growth rates from qPCR data may suggest a partial biological effect of the vaccine. Further evaluation of the immunogenicity and efficacy of the AMA-1/AS02A formulation is ongoing in a malaria-experienced pediatric population in Mali.

Trial Registration

www.clinicaltrials.govNCT00385047

Protection of humans against malaria by immunization with radiation-attenuated Plasmodium falciparum sporozoites

During 1989-1999, 11 volunteers were immunized by the bites of 1001-2927 irradiated mosquitoes harboring infectious sporozoites of Plasmodium falciparum (Pf) strain NF54 or clone 3D7/NF54. Ten volunteers were first challenged by the bites of Pf-infected mosquitoes 2-9 weeks after the last immunization, and all were protected. A volunteer challenged 10 weeks after the last immunization was not protected. Five previously protected volunteers were rechallenged 23-42 weeks after a secondary immunization, and 4 were protected. Two volunteers were protected when rechallenged with a heterologous Pf strain (7G8). In total, there was protection in 24 of 26 challenges. These results expand published findings demonstrating that immunization by exposure to thousands of mosquitoes carrying radiation-attenuated Pf sporozoites is safe and well tolerated and elicits strain-transcendent protective immunity that persists for at least 42 weeks.

Causal prophylactic efficacy of atovaquone-proguanil (Malarone) in a human challenge model

Plasmodia infect the liver for about 7 days before subsequently infecting the blood. Present prophylaxis against Plasmodium falciparum malaria employs agents that primarily kill blood stages and must be continued for 28 days after the last exposure. Atovaquone-proguanil (Malarone) is a new antimalarial agent that is licensed in 35 countries as treatment against blood-stage infection, but its components (atovaquone and proguanil) have separately been shown to be active also against liver stages. To determine whether atovaquone-proguanil is sufficiently active against liver stages to be discontinued 7 days after exposure, we challenged 16 volunteers with P. falciparum via infected mosquitoes. Twelve volunteers received atovaquone-proguanil (1 tablet daily) on the day prior to challenge, on the day of challenge, and for the next 6 days; 4 volunteers received matching placebo. All placebo volunteers demonstrated parasitaemia and malarial symptoms beginning on days 11-12 after challenge. No atovaquone-proguanil volunteer acquired malaria. Atovaquone-proguanil is the first licensed antimalarial agent that kills P. falciparum in the liver and that may be discontinued 7 days after the last exposure.

Long-term efficacy and immune responses following immunization with the RTS,S malaria vaccine

The malaria sporozoite vaccine candidate RTS,S, formulated with an oil-in-water emulsion plus the immunostimulants monophosphoryl lipid A and the saponin derivative QS21 (vaccine 3), recently showed superior efficacy over two other experimental formulations. Immunized volunteers were followed to determine the duration of protective immune responses. Antibody levels decreased to between one-third and one-half of peak values 6 months after the last dose of vaccine. T cell proliferation and interferon-gamma production in vitro were observed in response to RTS,S or hepatitis B surface antigen. Seven previously protected volunteers received sporozoite challenge, and 2 remained protected (1/1 for vaccine 1, 0/1 for vaccine 2, and 1/5 for vaccine 3). The prepatent period was 10.8 days for the control group and 13.2 days for the vaccinees (P < .01). Immune responses did not correlate with protection. Further optimization in vaccine composition and/or immunization schedule will be required to induce longer-lasting protective immunity.