Live Attenuated Malaria Vaccine Designed to Protect Through Hepatic CD8+ T Cell Immunity

Malaria induces anemia through CD8+ T cell-dependent parasite clearance and erythrocyte removal in the spleen

Severe malarial anemia (SMA) in semi-immune individuals eliminates both infected and uninfected erythrocytes and is a frequent fatal complication. It is proportional not to circulating parasitemia but total parasite mass (sequestered) in the organs. Thus, immune responses that clear parasites in organs may trigger changes leading to anemia. Here, we use an outbred-rat model where increasing parasite removal in the spleen escalated uninfected-erythrocyte removal. Splenic parasite clearance was associated with activated CD8(+) T cells, immunodepletion of which prevented parasite clearance. CD8(+) T cell repletion and concomitant reduction of the parasite load was associated with exacerbated (40 to 60%) hemoglobin loss and changes in properties of uninfected erythrocytes. Together, these data suggest that CD8(+) T cell-dependent parasite clearance causes erythrocyte removal in the spleen and thus anemia. In children infected with the human malaria parasite Plasmodium falciparum, elevation of parasite biomass (not the number of circulating parasites) increased the odds ratio for SMA by 3.5-fold (95% confidence intervals [CI95%], 1.8- to 7.5-fold). CD8(+) T cell expansion/activation independently increased the odds ratio by 2.4-fold (CI95%, 1.0- to 5.7-fold). Concomitant increases in both conferred a 7-fold (CI95%, 1.9- to 27.4-fold)-greater risk for SMA. Together, these data suggest that CD8(+)-dependent parasite clearance may predispose individuals to uninfected-erythrocyte loss and SMA, thus informing severe disease diagnosis and strategies for vaccine development.

IMPORTANCE

Malaria is a major global health problem. Severe malaria anemia (SMA) is a complex disease associated with partial immunity. Rapid hemoglobin reductions of 20 to 50% are commonly observed and must be rescued by transfusion (which can carry a risk of HIV acquisition). The causes and risk factors of SMA remain poorly understood. Recent studies suggest that SMA is linked to parasite biomass sequestered in organs. This led us to investigate whether immune mechanisms that clear parasites in organs trigger anemia. In rats, erythropoiesis is largely restricted to the bone marrow, and critical aspects of the spleen expected to be important in anemia are similar to those in humans. Therefore, using a rat model, we show that severe anemia is caused through CD8(+) T cell-dependent parasite clearance and erythrocyte removal in the spleen. CD8 activation may also be a new risk factor for SMA in African children.

Flow Cytometry-Based Assessment of Antibody Function Against Malaria Pre-erythrocytic Infection

The development of new interventional strategies against pre-erythrocytic malaria is hampered by the lack of standardized approaches to assess inhibition of sporozoite infection of hepatocytes. The following methodology, based on flow cytometry, can be used to quantitatively assess P. falciparum sporozoite infection in vitro in medium throughput. In addition to assessing the efficacy of antibodies, this assay has a wide variety of applications for investigating basic science questions about the malaria liver stage. This approach is easily applied in a variety of laboratory settings, assesses the functionality of antibody responses against malaria sporozoites, and can be adapted for the limited quantities of sample which are typically available from clinical investigations.

Measurement of the T Cell Response to Preerythrocytic Vaccination in Mice

Whole attenuated parasite vaccines designed to elicit immunity against the clinically silent preerythrocytic stage of Plasmodium infection represent the most efficacious experimental platforms currently in clinical trial. Studies in rodents and humans show that T cells mediate vaccine-induced protection. Thus, determining the quantitative and qualitative properties of these T cells remains a major research focus. Most rodent models of preerythrocytic anti-Plasmodium vaccination focus on circumsporozoite-specific CD8 T cell responses in BALB/c mice. However, CD4 T cells and non-circumsporozoite-specific CD8 T cells also significantly contribute to protection. Here we describe alternative approaches that enable detection and functional characterization of total CD8 and CD4 T cell responses induced by preerythrocytic vaccination in mice. These flow cytometry-based approaches rely on monitoring the modulation of expressed integrins and co-receptors on the surface of T cells in vaccinated mice. The approaches enable direct determination of the magnitude, kinetics, distribution, phenotype, and functional features of T cell responses induced by infection or whole-parasite vaccination using any mouse-parasite species combination.

Measurement of Antibody-Mediated Reduction of Plasmodium yoelii Liver Burden by Bioluminescent Imaging

Antibodies against the infectious sporozoite stage of malaria have been shown to be effective in preventing infection of the liver and in mitigating the ensuing blood stage. However, only a handful of antibody targets have been vetted and shown to be successful in mediating in vivo protection. Even more limited are the means with which to measure how effectively antibodies can reduce the number of parasites establishing infection in the liver. Traditionally, only qPCR of infected mouse livers could accurately measure liver parasite burden. However, this procedure requires sacrifice of the animal and precludes monitoring of the ensuing blood stage infection. Herein we describe a method of accurately assessing antibody-mediated reduction of parasite liver burden by combining passive or active immunization of mice and mosquito bite challenge with luciferase-expressing transgenic P. yoelii parasites. This method is rapid, reliable and allows for observation of blood stage disease in the same animal. This model will prove integral in screening the efficacy of novel antibody targets as the search for a more effective malaria vaccine continues.

IFNγ Responses to Pre-erythrocytic and Blood-stage Malaria Antigens Exhibit Differential Associations With Past Exposure and Subsequent Protection

BACKGROUND

The malaria-specific T-cell response is believed to be important for protective immunity. Antimalarial chemoprevention may affect this response by altering exposure to malaria antigens.

METHODS

We performed interferon γ (IFNγ) ELISpot assays to assess the cellular immune response to blood-stage and pre-erythrocytic antigens longitudinally from 1 to 3 years of age in 196 children enrolled in a randomized trial of antimalarial chemoprevention in Tororo, Uganda, an area of high transmission intensity.

RESULTS

IFNγ responses to blood-stage antigens, particularly MSP1, were frequently detected, strongly associated with recent malaria exposure, and lower in those adherent to chemoprevention compared to nonadherent children and those randomized to no chemoprevention. IFNγ responses to pre-erythrocytic antigens were infrequent and similar between children randomized to chemoprevention or no chemoprevention. Responses to blood-stage antigens were not associated with subsequent protection from malaria (aHR 0.96, P = .83), but responses to pre-erythrocytic antigens were associated with protection after adjusting for prior malaria exposure (aHR 0.52, P = .009).

CONCLUSIONS

In this high transmission setting, IFNγ responses to blood-stage antigens were common and associated with recent exposure to malaria but not protection from subsequent malaria. Responses to pre-erythrocytic antigens were uncommon, not associated with exposure but were associated with protection from subsequent malaria.

A genetically attenuated malaria vaccine candidate based on P. falciparum b9/slarp gene-deficient sporozoites

A highly efficacious pre-erythrocytic stage vaccine would be an important tool for the control and elimination of malaria but is currently unavailable. High-level protection in humans can be achieved by experimental immunization with Plasmodium falciparum sporozoites attenuated by radiation or under anti-malarial drug coverage. Immunization with genetically attenuated parasites (GAP) would be an attractive alternative approach. In this study, we present data on safety and protective efficacy using sporozoites with deletions of two genes, that is the newly identified b9 and slarp, which govern independent and critical processes for successful liver-stage development. In the rodent malaria model, PbΔb9ΔslarpGAP was completely attenuated showing no breakthrough infections while efficiently inducing high-level protection. The human PfΔb9ΔslarpGAP generated without drug resistance markers were infective to human hepatocytes in vitro and to humanized mice engrafted with human hepatocytes in vivo but completely aborted development after infection. These findings support the clinical development of a PfΔb9ΔslarpSPZ vaccine.

DOI:http://dx.doi.org/10.7554/eLife.03582.001

Vaccines commonly contain a weakened or dead version of a disease-causing microorganism, or its toxins, or surface proteins. These prime the immune system to rapidly recognize, respond to, and eliminate the actual infectious pathogen if later encountered.

While vaccines are currently available to help prevent a large number of diseases, vaccines for many deadly diseases, including malaria, do not yet exist. Malaria is caused by a group of parasites called Plasmodium, which are transferred to humans by mosquitoes. While measures to control mosquito populations and prevent mosquito bites have helped to reduce the incidence of malaria in some countries, the number of people—and especially children—that die of malaria every year remains very high.

When a mosquito carrying Plasmodium in its salivary glands bites a human, the parasite is injected into the human's bloodstream and travels to the liver. The parasite reproduces in the liver cells until there are so many of them that the cells rupture, and the parasites are released back into the bloodstream. Any mosquito that then feeds on the blood of the infected individual may also suck up the parasite. The parasite then goes through a further stage of development in the mosquito, eventually migrating to the salivary glands, from where the parasite can be transmitted into a new human host.

Recent work in rodents suggests that genetically altered or weakened Plasmodium falciparum sporozoites—the form of the parasite found in mosquito saliva—could be used to vaccinate humans against malaria caused by this parasite species. Now, van Schaijk, Ploemen et al. evaluate whether a safe and effective vaccine could be made from sporozoites that lack two genes, called b9 and slarp, which are critical for the parasites to develop inside liver cells. When mice were injected with the modified sporozoites, their immune cells were able to detect the parasites and respond against them. The mice subsequently did not develop malaria when they were infected with normal, unmodified parasites. Furthermore, none of the mice contracted malaria from the modified sporozoites.

The modified sporozoites behaved similarly in human liver cells: after invading these cells, the parasites were unable to develop. Clinical testing and further development are now needed to see if a successful malaria vaccine can be made from these sporozoites.

DOI:http://dx.doi.org/10.7554/eLife.03582.002

Malaria immunity in man and mosquito: insights into unsolved mysteries of a deadly infectious disease

Malaria is a mosquito-borne disease caused by parasites of the obligate intracellular Apicomplexa phylum the most deadly of which, Plasmodium falciparum, prevails in Africa. Malaria imposes a huge health burden on the world's most vulnerable populations, claiming the lives of nearly one million children and pregnant women each year. Although there is keen interest in eradicating malaria, we do not yet have the necessary tools to meet this challenge, including an effective malaria vaccine and adequate vector control strategies. Here we review what is known about the mechanisms at play in immune resistance to malaria in both the human and mosquito hosts at each step in the parasite's complex life cycle with a view toward developing the tools that will contribute to the prevention of disease and death and, ultimately, to the goal of malaria eradication. In so doing, we hope to inspire immunologists to participate in defeating this devastating disease.

APC licensing and CD4+T cell help in liver-stage malaria

Malaria parasites spend a critical phase of their life cycle inside hepatocytes, in an environment with complex and distinctive immunological features. Here I will discuss how the immunological features of the liver and the adaptations of malaria parasites interact, resulting in defective CD8+ T cell immunity. These processes are explored with a focus on the mechanism by which CD4+ T cells deliver help to CD8+ T cells, and specifically through their interaction with antigen-presenting cells (APCs), resulting in “licensing” of the APCs and enhanced capacity to optimally activate CD8+ T cells. Synthesis of the available evidence supports a model in which the parasite-mediated manipulation of programmed cell death in infected hepatocytes impairs the capacity of the liver’s immune system to successfully license APCs and fully activate T cell immunity.

Plasmodium berghei circumsporozoite protein encapsulated in oligomannose-coated liposomes confers protection against sporozoite infection in mice

Background

The design and development of an effective malaria vaccine against the pre-erythrocytic and erythrocytic-stages of infection present a great challenge.

Methods

In the present study, protective efficacy of oligomannose-coated liposome (OML)-entrapped merozoite and sporozoite antigens against Plasmodium berghei challenge infection in BALB/c mice was evaluated.

Results

Subcutaneous immunization with truncated merozoite surface protein 1 entrapped with OML (OML-PbMSP1) prolonged survival, but failed to protect the mice from erythrocytic-stage infection, despite the antigen-specific antibody responses induced by the immunization regimen. In contrast, immunization with circumsporozoite protein entrapped with OML (OML-PbCSP) elicited antigen-specific humoral and cellular responses, which correlated with substantial protection against sporozoite challenge infections.

Conclusions

The current results represent the use of an oligomannose-coated liposome-based vaccine against pre-erythrocytic and erythrocytic stages malaria infection. This approach may offer a new vaccination strategy against malaria infection.

Evaluation of the efficacy of ChAd63-MVA vectored vaccines expressing circumsporozoite protein and ME-TRAP against controlled human malaria infection in malaria-naive individuals

Background. Circumsporozoite protein (CS) is the antigenic target for RTS,S, the most advanced malaria vaccine to date. Heterologous prime-boost with the viral vectors simian adenovirus 63 (ChAd63)-modified vaccinia virus Ankara (MVA) is the most potent inducer of T-cells in humans, demonstrating significant efficacy when expressing the preerythrocytic antigen insert multiple epitope–thrombospondin-related adhesion protein (ME-TRAP). We hypothesized that ChAd63-MVA containing CS may result in a significant clinical protective efficacy.

Methods. We conducted an open-label, 2-site, partially randomized Plasmodium falciparum sporozoite controlled human malaria infection (CHMI) study to compare the clinical efficacy of ChAd63-MVA CS with ChAd63-MVA ME-TRAP.

Results. One of 15 vaccinees (7%) receiving ChAd63-MVA CS and 2 of 15 (13%) receiving ChAd63-MVA ME-TRAP achieved sterile protection after CHMI. Three of 15 vaccinees (20%) receiving ChAd63-MVA CS and 5 of 15 (33%) receiving ChAd63-MVA ME-TRAP demonstrated a delay in time to treatment, compared with unvaccinated controls. In quantitative polymerase chain reaction analyses, ChAd63-MVA CS was estimated to reduce the liver parasite burden by 69%–79%, compared with 79%–84% for ChAd63-MVA ME-TRAP.

Conclusions. ChAd63-MVA CS does reduce the liver parasite burden, but ChAd63-MVA ME-TRAP remains the most promising antigenic insert for a vectored liver-stage vaccine. Detailed analyses of parasite kinetics may allow detection of smaller but biologically important differences in vaccine efficacy that can influence future vaccine development.

Clinical Trials Registration. NCT01623557.

Immunization of mice with live-attenuated late liver stage-arresting Plasmodium yoelii parasites generates protective antibody responses to preerythrocytic stages of malaria

Understanding protective immunity to malaria is essential for the design of an effective vaccine to prevent the large number of infections and deaths caused by this parasitic disease. To date, whole-parasite immunization with attenuated parasites is the most effective method to confer sterile protection against malaria infection in clinical trials. Mouse model studies have highlighted the essential role that CD8(+) T cells play in protection against preerythrocytic stages of malaria; however, there is mounting evidence that antibodies are also important in these stages. Here, we show that experimental immunization of mice with Plasmodium yoelii fabb/f(-) (Pyfabb/f(-)), a genetically attenuated rodent malaria parasite that arrests late in the liver stage, induced functional antibodies that inhibited hepatocyte invasion in vitro and reduced liver-stage burden in vivo. These antibodies were sufficient to induce sterile protection from challenge by P. yoelii sporozoites in the absence of T cells in 50% of mice when sporozoites were administered by mosquito bite but not when they were administered by intravenous injection. Moreover, among mice challenged by mosquito bite, a higher proportion of BALB/c mice than C57BL/6 mice developed sterile protection (62.5% and 37.5%, respectively). Analysis of the antibody isotypes induced by immunization with Pyfabb/f(-) showed that, overall, BALB/c mice developed an IgG1-biased response, whereas C57BL/6 mice developed an IgG2b/c-biased response. Our data demonstrate for the first time that antibodies induced by experimental immunization of mice with a genetically attenuated rodent parasite play a protective role during the preerythrocytic stages of malaria. Furthermore, they highlight the importance of considering both the route of challenge and the genetic background of the mouse strains used when interpreting vaccine efficacy studies in animal models of malaria infection.

Sterile immunity to malaria after DNA prime/adenovirus boost immunization is associated with effector memory CD8+T cells targeting AMA1 class I epitopes

Background

Fifteen volunteers were immunized with three doses of plasmid DNA encoding P. falciparum circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1) and boosted with human adenovirus-5 (Ad) expressing the same antigens (DNA/Ad). Four volunteers (27%) demonstrated sterile immunity to controlled human malaria infection and, overall, protection was statistically significantly associated with ELISpot and CD8+ T cell IFN-γ activities to AMA1 but not CSP. DNA priming was required for protection, as 18 additional subjects immunized with Ad alone (AdCA) did not develop sterile protection.

Methodology/Principal Findings

We sought to identify correlates of protection, recognizing that DNA-priming may induce different responses than AdCA alone. Among protected volunteers, two and three had higher ELISpot and CD8+ T cell IFN-γ responses to CSP and AMA1, respectively, than non-protected volunteers. Unexpectedly, non-protected volunteers in the AdCA trial showed ELISpot and CD8+ T cell IFN-γ responses to AMA1 equal to or higher than the protected volunteers. T cell functionality assessed by intracellular cytokine staining for IFN-γ, TNF-α and IL-2 likewise did not distinguish protected from non-protected volunteers across both trials. However, three of the four protected volunteers showed higher effector to central memory CD8+ T cell ratios to AMA1, and one of these to CSP, than non-protected volunteers for both antigens. These responses were focused on discrete regions of CSP and AMA1. Class I epitopes restricted by A*03 or B*58 supertypes within these regions of AMA1 strongly recalled responses in three of four protected volunteers. We hypothesize that vaccine-induced effector memory CD8+ T cells recognizing a single class I epitope can confer sterile immunity to P. falciparum in humans.

Conclusions/Significance

We suggest that better understanding of which epitopes within malaria antigens can confer sterile immunity and design of vaccine approaches that elicit responses to these epitopes will increase the potency of next generation gene-based vaccines.

Tissue signatures influence the activation of intrahepatic CD8(+) T cells against malaria sporozoites

Plasmodium sporozoites and liver stages express antigens that are targeted to the MHC-Class I antigen-processing pathway. After the introduction of Plasmodium sporozoites by Anopheles mosquitoes, bone marrow-derived dendritic cells in skin-draining lymph nodes are the first cells to cross-present parasite antigens and elicit specific CD8⁺ T cells. One of these antigens is the immunodominant circumsporozoite protein (CSP). The CD8⁺ T cell-mediated protective immune response against CSP is dependent on the interleukin loop involving IL-4 receptor expression on CD8⁺ cells and IL-4 secretion by CD4⁺ T cell helpers. In a few days, these CD8⁺ T cells re-circulate to secondary lymphoid organs and the liver. In the liver, the hepatic sinusoids are enriched with cells, such as dendritic, sinusoidal endothelial and Kupffer cells, that are able to cross-present MHC class I antigens to intrahepatic CD8⁺ T cells. Specific CD8⁺ T cells actively find infected hepatocytes and target intra-cellular parasites through mechanisms that are both interferon-γ-dependent and -independent. Immunity is mediated by CD8⁺ T effector or effector-memory cells and, when present in high numbers, these cells can provide sterilizing immunity. Human vaccination trials with recombinant formulations or attenuated sporozoites have yet to achieve the high numbers of specific effector T cells that are required for sterilizing immunity. In spite of the limited number of specific CD8⁺ T cells, attenuated sporozoites provided multiple times by the endovenous route provided a high degree of protective immunity. These observations highlight that CD8⁺ T cells may be useful for improving antibody-mediated protective immunity to pre-erythrocytic stages of malaria parasites.

Circumsporozoite protein-specific K(d)-restricted CD8+ T cells mediate protective antimalaria immunity in sporozoite-immunized MHC-I-K(d) transgenic mice

Although the roles of CD8+ T cells and a major preerythrocytic antigen, the circumsporozoite (CS) protein, in contributing protective antimalaria immunity induced by radiation-attenuated sporozoites, have been shown by a number of studies, the extent to which these players contribute to antimalaria immunity is still unknown. To address this question, we have generated C57BL/6 (B6) transgenic (Tg) mice, expressing K^d molecules under the MHC-I promoter, called MHC-I-K^d-Tg mice. In this study, we first determined that a single immunizing dose of IrPySpz induced a significant level of antimalaria protective immunity in MHC-I-K^d-Tg mice but not in B6 mice. Then, by depleting various T-cell subsets in vivo, we determined that CD8+ T cells are the main mediator of the protective immunity induced by IrPySpz. Furthermore, when we immunized (MHC-I-K^d-Tg × CS-Tg) F1 mice with IrPySpz after crossing MHC-I-K^d-Tg mice with PyCS-transgenic mice (CS-Tg), which are unable to mount PyCS-specific immunity, we found that IrPySpz immunization failed to induce protective antimalaria immunity in (MHC-I-K^d-Tg × CS-Tg) F1 mice, thus indicating the absence of PyCS antigen-dependent immunity in these mice. These results indicate that protective antimalaria immunity induced by IrPySpz in MHC-I-K^d-Tg mice is mediated by CS protein-specific, K^d-restricted CD8+ T cells.

Protozoan parasites and type I interferons: a cold case reopened

Protozoan parasites, such as Plasmodium, Toxoplasma, Cryptosporidium, trypanosomes, and Leishmania, are a major cause of disease in both humans and other animals, highlighting the need to understand the full spectrum of strategies used by the host immune system to sense and respond to parasite infection. Although type II interferon (IFN-γ) has long been recognized as an essential antiparasite immune effector, much less is known about the role of type I interferons (IFN-α and -β) in host defense, particularly in vivo. Recent studies are reviewed which collectively highlight that type I IFN can be induced in response to parasite infection and influence the outcome of infection.

Controlled human malaria infection of Tanzanians by intradermal injection of aseptic, purified, cryopreserved Plasmodium falciparum sporozoites

Controlled human malaria infection (CHMI) by mosquito bite has been used to assess anti-malaria interventions in > 1,500 volunteers since development of methods for infecting mosquitoes by feeding on Plasmodium falciparum (Pf) gametocyte cultures. Such CHMIs have never been used in Africa. Aseptic, purified, cryopreserved Pf sporozoites, PfSPZ Challenge, were used to infect Dutch volunteers by intradermal injection. We conducted a double-blind, placebo-controlled trial to assess safety and infectivity of PfSPZ Challenge in adult male Tanzanians. Volunteers were injected intradermally with 10,000 (N = 12) or 25,000 (N = 12) PfSPZ or normal saline (N = 6). PfSPZ Challenge was well tolerated and safe. Eleven of 12 and 10 of 11 subjects, who received 10,000 and 25,000 PfSPZ respectively, developed parasitemia. In 10,000 versus 25,000 PfSPZ groups geometric mean days from injection to Pf positivity by thick blood film was 15.4 versus 13.5 (P = 0.023). Alpha-thalassemia heterozygosity had no apparent effect on infectivity. PfSPZ Challenge was safe, well tolerated, and infectious.

Discovery of novel cross-protective Rickettsia prowazekii T-cell antigens using a combined reverse vaccinology and in vivo screening approach

Rickettsial agents are some of the most lethal pathogens known to man. Among them, Rickettsia prowazekii is a select agent with potential use for bioterrorism; yet, there is no anti-Rickettsia vaccine commercially available. Owing to the obligate intracellular lifestyle of rickettsiae, CD8(+) T cells are indispensable for protective cellular immunity. Furthermore, T cells can mediate cross-protective immunity between different pathogenic Rickettsia, a finding consistent with the remarkable similarity among rickettsial genomes. However, Rickettsia T cell antigens remain unidentified. In the present study, we report an algorithm that allowed us to identify and validate four novel R. prowazekii vaccine antigen candidates recognized by CD8(+) T cells from a set of twelve in silico-defined protein targets. Our results highlight the importance of combining proteasome-processing as well as MHC class-I-binding predictions. The novel rickettsial vaccine candidate antigens, RP778, RP739, RP598, and RP403, protected mice against a lethal challenge with Rickettsia typhi, which is indicative of cross-protective immunity within the typhus group rickettsiae. Together, our findings validate a reverse vaccinology approach as a viable strategy to identify protective rickettsial antigens and highlight the feasibility of a subunit vaccine that triggers T-cell-mediated cross-protection among diverse rickettsiae.

Local immune response to injection of Plasmodium sporozoites into the skin

Malarial infection is initiated when the sporozoite form of the Plasmodium parasite is inoculated into the skin by a mosquito. Sporozoites invade hepatocytes in the liver and develop into the erythrocyte-infecting form of the parasite, the cause of clinical blood infection. Protection against parasite development in the liver can be induced by injection of live attenuated parasites that do not develop in the liver and thus do not cause blood infection. Radiation-attenuated sporozoites (RAS) and genetically attenuated parasites are now considered as lead candidates for vaccination of humans against malaria. Although the skin appears as the preferable administration route, most studies in rodents, which have served as model systems, have been performed after i.v. injection of attenuated sporozoites. In this study, we analyzed the early response to Plasmodium berghei RAS or wild-type sporozoites (WTS) injected intradermally into C57BL/6 mice. We show that RAS have a similar in vivo distribution to WTS and that both induce a similar inflammatory response consisting of a biphasic recruitment of polymorphonuclear neutrophils and inflammatory monocytes in the skin injection site and proximal draining lymph node (dLN). Both WTS and RAS associate with neutrophils and resident myeloid cells in the skin and the dLN, transform inside CD11b(+) cells, and induce a Th1 cytokine profile in the dLN. WTS and RAS are also similarly capable of priming parasite-specific CD8(+) T cells. These studies delineate the early and local response to sporozoite injection into the skin, and suggest that WTS and RAS prime the host immune system in a similar fashion.

Translating the immunogenicity of prime-boost immunization with ChAd63 and MVA ME-TRAP from malaria naive to malaria-endemic populations

To induce a deployable level of efficacy, a successful malaria vaccine would likely benefit from both potent cellular and humoral immunity. These requirements are met by a heterologous prime-boost immunization strategy employing a chimpanzee adenovirus vector followed by modified vaccinia Ankara (MVA), both encoding the pre-erythrocytic malaria antigen ME-thrombospondin-related adhesive protein (TRAP), with high immunogenicity and significant efficacy in UK adults. We undertook two phase 1b open-label studies in adults in Kenya and The Gambia in areas of similar seasonal malaria transmission dynamics and have previously reported safety and basic immunogenicity data. We now report flow cytometry and additional interferon (IFN)-γ enzyme-linked immunospot (ELISPOT) data characterizing pre-existing and induced cellular immunity as well as anti-TRAP IgG responses. T-cell responses induced by vaccination averaged 1,254 spot-forming cells (SFC) per million peripheral blood mononuclear cells (PBMC) across both trials and flow cytometry revealed cytokine production from both CD4(+) and CD8(+) T cells with the frequency of CD8(+) IFN-γ-secreting monofunctional T cells (previously shown to associate with vaccine efficacy) particularly high in Kenyan adults. Immunization with ChAd63 and MVA ME-TRAP induced strong cellular and humoral immune responses in adults living in two malaria-endemic regions of Africa. This prime-boost approach targeting the pre-erythrocytic stage of the malaria life-cycle is now being assessed for efficacy in a target population.

CD8 T-cell-mediated protection against liver-stage malaria: lessons from a mouse model

Malaria is a major global health problem, with severe mortality in children living in sub-Saharan Africa, and there is currently no licensed, effective vaccine. However, vaccine-induced protection from Plasmodium infection, the causative agent of malaria, was established for humans in small clinical trials and for rodents in the 1960s. Soon after, a critical role for memory CD8 T cells in vaccine-induced protection against Plasmodium liver-stage infection was established in rodent models and is assumed to apply to humans. However, these seminal early studies have led to only modest advances over the ensuing years in our understanding the basic features of memory CD8 T cells required for protection against liver-stage Plasmodium infection, an issue which has likely impeded the development of effective vaccines for humans. Given the ethical and practical limitations in gaining mechanistic insight from human vaccine and challenge studies, animal models still have an important role in dissecting the basic parameters underlying memory CD8 T-cell immunity to Plasmodium. Here, we will highlight recent data from our own work in the mouse model of Plasmodium infection that identify quantitative and qualitative features of protective memory CD8 T-cell responses. Finally, these lessons will be discussed in the context of recent findings from clinical trials of vaccine-induced protection in controlled human challenge models.

Current trends of immunization in Nigeria: prospect and challenges

Immunization is aimed at the prevention of infectious diseases. In Nigeria, the National Programme on Immunization (NPI) suffers recurrent setbacks due to many factors including ethnicity and religious beliefs. Nigeria is made up of 36 states with its federal capital in Abuja. The country is divided into six geo-political zones; north central, north west, north east, south east, south west and south south. The population is unevenly distributed across the country. The average population density in 2006 was estimated at 150 people per square kilometres with Lagos, Anambra, Imo, Abia, and Akwa Ibom being the most densely populated states. Most of the densely populated states are found in the south east. Kano with an average density of 442 persons per square kilometre, is the most densely populated state in the northern part of the country. This study presents a review on the current immunization programme and the many challenges affecting its success in the eradication of childhood diseases in Nigeria.

Cytotoxic markers associate with protection against malaria in human volunteers immunized with Plasmodium falciparum sporozoites

BACKGROUND

Immunization of healthy volunteers by bites from Plasmodium falciparum-infected mosquitoes during chloroquine chemoprophylaxis (hereafter, chemoprophylaxis and sporozoites [CPS] immunization) induces sterile protection against malaria. CPS-induced protection is mediated by immunity against pre-erythrocytic stages, presumably at least partially by cytotoxic cellular responses. We therefore aimed to investigate the association of CPS-induced cytotoxic T-cell markers with protection.

METHODS

In a double-blind randomized controlled trial, we performed dose titration of CPS immunization followed by homologous challenge infection in 29 subjects. Immune responses were assessed by in vitro restimulation of peripheral blood mononuclear cells and flow cytometry.

RESULTS

Dose-dependent complete protection was obtained in 4 of 5 volunteers after immunization with bites from 45 P. falciparum-infected mosquitoes, in 8 of 9 volunteers with bites from 30, and in 5 of 10 volunteers with bites from 15 (odds ratio [OR], 5.0; 95% confidence interval [CI], 1.5-17). Completely protected subjects had significantly higher proportions of CD4 T cells expressing the degranulation marker CD107a (OR, 8.4; 95% CI, 1.5-123; P = .011) and CD8 cells producing granzyme B (OR, 11; 95% CI, 1.9-212; P = .004) after P. falciparum restimulation.

CONCLUSIONS

These data underline the efficiency of CPS immunization to induce sterile protection and support a possible role for cytotoxic CD4 and CD8 T-cell responses in pre-erythrocytic immunity.

CLINICAL TRIALS REGISTRATION

NCT01218893.

CD8+ T cells from a novel T cell receptor transgenic mouse induce liver-stage immunity that can be boosted by blood-stage infection in rodent malaria

To follow the fate of CD8⁺ T cells responsive to Plasmodium berghei ANKA (PbA) infection, we generated an MHC I-restricted TCR transgenic mouse line against this pathogen. T cells from this line, termed PbT-I T cells, were able to respond to blood-stage infection by PbA and two other rodent malaria species, P. yoelii XNL and P. chabaudi AS. These PbT-I T cells were also able to respond to sporozoites and to protect mice from liver-stage infection. Examination of the requirements for priming after intravenous administration of irradiated sporozoites, an effective vaccination approach, showed that the spleen rather than the liver was the main site of priming and that responses depended on CD8α⁺ dendritic cells. Importantly, sequential exposure to irradiated sporozoites followed two days later by blood-stage infection led to augmented PbT-I T cell expansion. These findings indicate that PbT-I T cells are a highly versatile tool for studying multiple stages and species of rodent malaria and suggest that cross-stage reactive CD8⁺ T cells may be utilized in liver-stage vaccine design to enable boosting by blood-stage infections.

Malaria is a disease caused by Plasmodium species, which have a highly complex life cycle involving both liver and blood stages of mammalian infection. To prevent disease, one strategy has been to induce CD8⁺ T cells against liver-stage parasites, usually by immunization with stage-specific antigens. Here we describe a T cell receptor specificity that recognizes an antigen expressed in both the liver and blood stages of several rodent Plasmodium species. We generated a T cell receptor transgenic mouse with this specificity and showed that T cells from this line could protect against liver-stage infection. We used this novel tool to identify the site and cell-type involved in priming to a recently developed intravenous attenuated sporozoite vaccine shown to have efficacy in humans. We showed that CD8⁺ T cells with this specificity could protect against liver-stage infection while causing pathology to the blood stage. Finally, we provided evidence that T cells with cross-stage specificity can be primed and boosted on alternative stages, raising the possibility that antigens expressed in multiple stages might be ideal vaccine candidates for generating strong immunity to liver-stage parasites.

External quality assurance of malaria nucleic acid testing for clinical trials and eradication surveillance

Nucleic acid testing (NAT) for malaria parasites is an increasingly recommended diagnostic endpoint in clinical trials of vaccine and drug candidates and is also important in surveillance of malaria control and elimination efforts. A variety of reported NAT assays have been described, yet no formal external quality assurance (EQA) program provides validation for the assays in use. Here, we report results of an EQA exercise for malaria NAT assays. Among five centers conducting controlled human malaria infection trials, all centers achieved 100% specificity and demonstrated limits of detection consistent with each laboratory's pre-stated expectations. Quantitative bias of reported results compared to expected results was generally <0.5 log₁₀ parasites/mL except for one laboratory where the EQA effort identified likely reasons for a general quantitative shift. The within-laboratory variation for all assays was low at <10% coefficient of variation across a range of parasite densities. Based on this study, we propose to create a Molecular Malaria Quality Assessment program that fulfills the need for EQA of malaria NAT assays worldwide.

Malaria vaccine R&D in the Decade of Vaccines: breakthroughs, challenges and opportunities

While recent progress has been made in reducing malaria mortality with other interventions, vaccines are still urgently needed to further reduce the incidence of clinical disease, including during pregnancy, and to provide "herd protection" by blocking parasite transmission. The most clinically advanced candidate, RTS,S, is presently undergoing Phase 3 evaluation in young African children across 13 clinical sites in eight African countries. In the 12-month period following vaccination, RTS,S conferred approximately 50% protection from clinical Plasmodium falciparum disease in children aged 5-17 months, and approximately 30% protection in children aged 6-12 weeks when administered in conjunction with Expanded Program for Immunization (EPI) vaccines. The development of more highly efficacious vaccines to prevent clinical disease caused by both P. falciparum and Plasmodium vivax, as well as vaccines to support elimination efforts by inducing immunity that blocks malaria parasite transmission, are priorities. Some key barriers to malaria vaccine development include: a paucity of well-characterized target immunogens and an absence of clear correlates of protection to enable vaccine development targeting all stages of the P. falciparum and P. vivax lifecycles; a limited number of safe and effective delivery systems, including adjuvants, that induce potent, long-lived protective immunity, be it by antibody, CD4+, and/or CD8+ T cell responses; and, for vaccines designed to provide "herd protection" by targeting sexual stage and/or mosquito antigens, the lack of a clear clinical and regulatory pathway to licensure using non-traditional endpoints. Recommendations to overcome these, and other key challenges, are suggested in this document.

Irradiated wild-type and Spa mutant Staphylococcus aureus induce anti-S. aureus immune responses in mice which do not protect against subsequent intravenous challenge

Staphylococcus aureus remains an important human and animal pathogen. Its pathogenicity is determined in part by expression of the Spa-immune subversion protein, neutralising the activity of which provides partial protection in murine models, as does experimental infection with live S. aureus with Spa gene deletions followed by antibiotic-mediated cure in mice. Together, these data raise the question of whether Spa mutant S. aureus might represent a viable vaccine. Here, we find that gamma-irradiated S. aureus strains, both wild-type and null mutant of spa, are immunogenic in mice when administered intramuscularly, eliciting large amounts of anti-S. aureus antibodies, as judged by whole-cell immunoassay on fixed microorganisms. We used an intravenous challenge system to assess vaccine efficacy, the sensitivity of which was increased by studying renal bacterial concentrations in both kidneys. Despite this, protection from intravenous challenge was not observed (mean difference between vaccinated and unvaccinated mice 0.27 log(10) with 95% confidence interval -0.922 to 1.467). Surprisingly, antibody responses elicited against a panel of protective cell surface proteins were very low, indicating that most antibody induced is not protective. Additionally, these data suggest a limited role for irradiated wild-type or spa mutant S. aureus as vaccines.

Immunobiology of Plasmodium in liver and brain

Malaria remains one of the most serious health problems globally, but our understanding of the biology of the parasite and the pathogenesis of severe disease is still limited. Multiple cellular effector mechanisms that mediate parasite elimination from the liver have been described, but how effector cells use classical granule-mediated cytotoxicity to attack infected hepatocytes and how cytokines and chemokines spread via the unique fluid pathways of the liver to reach the parasites over considerable distances remains unknown. Similarly, a wealth of information on cerebral malaria (CM), one of the most severe manifestations of the disease, was gained from post-mortem analyses of human brain and murine disease models, but the cellular processes that ultimately cause disease are not fully understood. Here, we discuss how imaging of the local dynamics of parasite infection and host response as well as consideration of anatomical and physiological features of liver and brain can provide a better understanding of the initial asymptomatic hepatic phase of the infection and the cascade of events leading to CM. Given the increasing drug resistance of both parasite and vector and the unavailability of a protective vaccine, the urgency to reduce the tremendous morbidity and mortality associated with severe malaria is obvious.

Phenotypic characterization of Plasmodium berghei responsive CD8+ T cells after immunization with live sporozoites under chloroquine cover

BACKGROUND

An effective malaria vaccine remains elusive. The most effective experimental vaccines confer only limited and short-lived protection despite production of protective antibodies. However, immunization with irradiated sporozoites, or with live sporozoites under chloroquine cover, has resulted in long-term protection apparently due to the generation of protective CD8+ T cells. The nature and function of these protective CD8+ T cells has not been elucidated. In the current study, the phenotype of CD8+ T cells generated after immunization of C57BL/6 mice with live Plasmodium berghei sporozoites under chloroquine cover was investigated.

METHODS

Female C57BL/6 mice, C57BL/6 mice B2 macroglobulin -/- [KO], or invariant chain-/- [Ic KO] [6-8 weeks old] were immunized with P. berghei sporozoites and treated daily with 800 μg/mouse of chloroquine for nine days. This procedure of immunization is referred to as "infection/cure". Mice were challenged by inoculating intravenously 1,000 infectious sporozoites. Appearance of parasitaemia was monitored by Giemsa-stained blood smears.

RESULTS

By use of MHC I and MHC II deficient animals, results indicate that CD8+ T cells are necessary for full protection and that production of protective antibodies is either CD4+ T helper cells dependent and/or lymphokines produced by CD4 cells contribute to the protection directly or by helping CD8+ T cells. Further, the phenotype of infection/cure P. berghei responsive CD8+ T cells was determined to be KLRG1high CD27low CD44high and CD62Llow.

CONCLUSION

The KLRG1high CD27low CD44high and CD62Llow phenotype of CD8+ T cells is associated with protection and should be investigated further as a candidate correlate of protection.

The whole parasite, pre-erythrocytic stage approach to malaria vaccine development: a review

PURPOSE OF REVIEW

The whole sporozoite (SPZ) vaccine platform provides the only established approach for inducing high-level sustained protective immunity in humans against malaria. We introduce this platform, highlight literature published since 2011, and discuss the challenges of further development.

RECENT FINDINGS

There are three major approaches to development of a whole parasite vaccine to prevent malaria infection using the SPZ platform: radiation-attenuated sporozoites (irrSPZ), chemoprophylaxis with infectious sporozoites (CPS), and genetically attenuated parasites (GAPs). In all three, SPZ are administered to the vaccinee. All three protect animals against infection when administered by injection with a needle and syringe, and irrSPZ and CPS protect against Plasmodium falciparum malaria in humans when P. falciparum SPZ (PfSPZ) are administered by mosquito bite. Metabolically active, nonreplicating (radiation attenuated) aseptic, purified, cryopreserved PfSPZ (PfSPZ Vaccine), and infectious, aseptic, purified, cryopreserved PfSPZ administered with chemoprophylaxis (PfSPZ-CVac approach) administered by needle and syringe have entered clinical trials. Preliminary data indicate that the PfSPZ Vaccine is safe, well tolerated and highly protective when administered intravenously.

SUMMARY

With proof-of-concept now established for high-grade protection induced by parenteral administration of a whole sporozoite vaccine, pathways for further development are currently being defined. Demonstration of high-level, durable, cross-strain P. falciparum protection would set the stage for licensure of a vaccine that could lead to dramatic reductions in malaria morbidity and mortality, and eventually elimination of this ancient scourge.

Two Plasmodium 6-Cys family-related proteins have distinct and critical roles in liver-stage development

The 10 Plasmodium 6-Cys proteins have critical roles throughout parasite development and are targets for antimalaria vaccination strategies. We analyzed the conserved 6-cysteine domain of this family and show that only the last 4 positionally conserved cysteine residues are diagnostic for this domain and identified 4 additional "6-Cys family-related" proteins. Two of these, sequestrin and B9, are critical to Plasmodium liver-stage development. RT-PCR and immunofluorescence assays show that B9 is translationally repressed in sporozoites and is expressed after hepatocyte invasion where it localizes to the parasite plasma membrane. Mutants lacking B9 expression in the rodent malaria parasites P. berghei and P. yoelii and the human parasite P. falciparum developmentally arrest in hepatocytes. P. berghei mutants arrest in the livers of BALB/c (100%) and C57BL6 mice (>99.9%), and in cultures of Huh7 human-hepatoma cell line. Similarly, P. falciparum mutants while fully infectious to primary human hepatocytes abort development 3 d after infection. This growth arrest is associated with a compromised parasitophorous vacuole membrane a phenotype similar to, but distinct from, mutants lacking the 6-Cys sporozoite proteins P52 and P36. Our results show that 6-Cys proteins have critical but distinct roles in establishment and maintenance of a parasitophorous vacuole and subsequent liver-stage development.

Immunogenicity of a prime-boost vaccine containing the circumsporozoite proteins of Plasmodium vivax in rodents

Plasmodium vivax is the most widespread and the second most prevalent malaria-causing species in the world. Current measures used to control the transmission of this disease would benefit from the development of an efficacious vaccine. In the case of the deadly parasite P. falciparum, the recombinant RTS,S vaccine containing the circumsporozoite antigen (CSP) consistently protects 30 to 50% of human volunteers against infection and is undergoing phase III clinical trials in Africa with similar efficacy. These findings encouraged us to develop a P. vivax vaccine containing the three circulating allelic forms of P. vivax CSP. Toward this goal, we generated three recombinant bacterial proteins representing the CSP alleles, as well as a hybrid polypeptide called PvCSP-All-CSP-epitopes. This hybrid contains the conserved N and C termini of P. vivax CSP and the three variant repeat domains in tandem. We also generated simian and human recombinant replication-defective adenovirus vectors expressing PvCSP-All-CSP-epitopes. Mice immunized with the mixture of recombinant proteins in a formulation containing the adjuvant poly(I·C) developed high and long-lasting serum IgG titers comparable to those elicited by proteins emulsified in complete Freund's adjuvant. Antibody titers were similar in mice immunized with homologous (protein-protein) and heterologous (adenovirus-protein) vaccine regimens. The antibodies recognized the three allelic forms of CSP, reacted to the repeated and nonrepeated regions of CSP, and recognized sporozoites expressing the alleles VK210 and VK247. The vaccine formulations described in this work should be useful for the further development of an anti-P. vivax vaccine.

CD8+ T Cell Responses to Plasmodium and Intracellular Parasites

Parasitic protozoa are major threats to human health affecting millions of people around the world. Control of these infections by the host immune system relies on a myriad of immunological mechanisms that includes both humoral and cellular immunity. CD8⁺ T cells contribute to the control of these parasitic infections in both animals and humans. Here, we will focus on the CD8⁺ T cell response against a subset of these protozoa: Plasmodium, Toxoplasma gondii, Leishmania and Trypanosoma cruzi, with an emphasis on experimental rodent systems. It is evident a complex interaction occurs between CD8⁺ T cells and the invading protozoa. A detailed understanding of how CD8⁺ T cells mediate protection should provide the basis for the development of effective vaccines that prevent and control infections by these parasites.

Idiopathic acute myocarditis during treatment for controlled human malaria infection: a case report

A 23-year-old healthy male volunteer took part in a clinical trial in which the volunteer took chloroquine chemoprophylaxis and received three intradermal doses at four-week intervals of aseptic, purified Plasmodium falciparum sporozoites to induce protective immunity against malaria. Fifty-nine days after the last administration of sporozoites and 32 days after the last dose of chloroquine the volunteer underwent controlled human malaria infection (CHMI) by the bites of five P. falciparum-infected mosquitoes. Eleven days post-CHMI a thick blood smear was positive (6 P. falciparum/μL blood) and treatment was initiated with atovaquone/proguanil (Malarone®). On the second day of treatment, day 12 post-CHMI, troponin T, a marker for cardiac tissue damage, began to rise above normal, and reached a maximum of 1,115 ng/L (upper range of normal = 14 ng/L) on day 16 post-CHMI. The volunteer had one ~20 minute episode of retrosternal chest pain and heavy feeling in his left arm on day 14 post-CHMI. ECG at the time revealed minor repolarization disturbances, and cardiac MRI demonstrated focal areas of subepicardial and midwall delayed enhancement of the left ventricle with some oedema and hypokinesia. A diagnosis of myocarditis was made. Troponin T levels were normal within 16 days and the volunteer recovered without clinical sequelae. Follow-up cardiac MRI at almost five months showed normal function of both ventricles and disappearance of oedema. Delayed enhancement of subepicardial and midwall regions decreased, but was still present. With the exception of a throat swab that was positive for rhinovirus on day 14 post-CHMI, no other tests for potential aetiologies of the myocarditis were positive. A number of possible aetiological factors may explain or have contributed to this case of myocarditis including, i) P. falciparum infection, ii) rhinovirus infection, iii) unidentified pathogens, iv) hyper-immunization (the volunteer received six travel vaccines between the last immunization and the CHMI), v) atovaquone/proguanil treatment, or vi) a combination of these factors. Definitive aetiology and pathophysiological mechanism for the myocarditis have not been established.

IFNγ/IL-10 co-producing cells dominate the CD4 response to malaria in highly exposed children

Although evidence suggests that T cells are critical for immunity to malaria, reliable T cell correlates of exposure to and protection from malaria among children living in endemic areas are lacking. We used multiparameter flow cytometry to perform a detailed functional characterization of malaria-specific T cells in 78 four-year-old children enrolled in a longitudinal cohort study in Tororo, Uganda, a highly malaria-endemic region. More than 1800 episodes of malaria were observed in this cohort, with no cases of severe malaria. We quantified production of IFNγ, TNFα, and IL-10 (alone or in combination) by malaria-specific T cells, and analyzed the relationship of this response to past and future malaria incidence. CD4⁺ T cell responses were measurable in nearly all children, with the majority of children having CD4⁺ T cells producing both IFNγ and IL-10 in response to malaria-infected red blood cells. Frequencies of IFNγ/IL10 co-producing CD4⁺ T cells, which express the Th1 transcription factor T-bet, were significantly higher in children with ≥2 prior episodes/year compared to children with <2 episodes/year (P<0.001) and inversely correlated with duration since malaria (Rho = −0.39, P<0.001). Notably, frequencies of IFNγ/IL10 co-producing cells were not associated with protection from future malaria after controlling for prior malaria incidence. In contrast, children with <2 prior episodes/year were significantly more likely to exhibit antigen-specific production of TNFα without IL-10 (P = 0.003). While TNFα-producing CD4⁺ T cells were not independently associated with future protection, the absence of cells producing this inflammatory cytokine was associated with the phenotype of asymptomatic infection. Together these data indicate that the functional phenotype of the malaria-specific T cell response is heavily influenced by malaria exposure intensity, with IFNγ/IL10 co-producing CD4⁺ T cells dominating this response among highly exposed children. These CD4⁺ T cells may play important modulatory roles in the development of antimalarial immunity.

Despite reports of decreasing malaria morbidity across many parts of Africa, the incidence of malaria among children continues to be very high in Uganda, even in the setting of insecticide-treated bednets and artemisinin-based combination therapy. Additional control measures, including a vaccine, are sorely needed in these settings, but progress has been limited by our lack of understanding of immunologic correlates of exposure and protection. T cell responses to malaria are thought to be important for protection in experimental models, but their role in protecting against naturally acquired infection is not clear. In this study, we performed detailed assessments of the malaria-specific T cell response among 4-year-old children living in Tororo, Uganda, an area of high malaria transmission. We found that recent malaria infection induces a malaria-specific immune response dominated by Th1 T cells co-producing IFNγ and IL-10, and that these cells are not associated with protection from future infection. IFNγ/IL-10 co-producing cells have been described in several parasitic infections and are hypothesized to be important in limiting CD4-mediated pathology, but they may also prevent the development of sterilizing immunity. These observations have important implications for understanding the pathophysiology of malaria in humans and for malaria vaccine development.

Plasmodium berghei sporozoites acquire virulence and immunogenicity during mosquito hemocoel transit

Malaria is a vector-borne disease caused by the single-cell eukaryote Plasmodium. The infectious parasite forms are sporozoites, which originate from midgut-associated oocysts, where they eventually egress and reach the mosquito hemocoel. Sporozoites actively colonize the salivary glands in order to be transmitted to the mammalian host. Whether residence in the salivary glands provides distinct and vital cues for the development of infectivity remains unsolved. In this study, we systematically compared the infectivity of Plasmodium berghei sporozoites isolated from the mosquito hemocoel and salivary glands. Hemocoel sporozoites display a lower proportion of gliding motility but develop into liver stages when added to cultured hepatoma cells or after intravenous injection into mice. Mice infected by hemocoel sporozoites had blood infections similar to those induced by sporozoites liberated from salivary glands. These infected mice display indistinguishable systemic inflammatory cytokine responses and develop experimental cerebral malaria. When used as metabolically active, live attenuated vaccine, hemocoel sporozoites elicit substantial protection against sporozoite challenge infections. Collectively, these findings show that salivary gland colonization does not influence parasite virulence in the mammalian host when sporozoites are administered intravenously. This conclusion has important implications for in vitro sporozoite production and manufacturing of whole-sporozoite vaccines.

Model for in vivo assessment of humoral protection against malaria sporozoite challenge by passive transfer of monoclonal antibodies and immune serum

Evidence from clinical trials of malaria vaccine candidates suggests that both cell-mediated and humoral immunity to pre-erythrocytic parasite stages can provide protection against infection. Novel pre-erythrocytic antibody (Ab) targets could be key to improving vaccine formulations, which are currently based on targeting antigens such as the circumsporozoite protein (CSP). However, methods to assess the effects of sporozoite-specific Abs on pre-erythrocytic infection in vivo remain underdeveloped. Here, we combined passive transfer of monoclonal Abs (MAbs) or immune serum with a luciferase-expressing Plasmodium yoelii sporozoite challenge to assess Ab-mediated inhibition of liver infection in mice. Passive transfer of a P. yoelii CSP MAb showed inhibition of liver infection when mice were challenged with sporozoites either intravenously or by infectious mosquito bite. However, inhibition was most potent for the mosquito bite challenge, leading to a more significant reduction of liver-stage burden and even a lack of progression to blood-stage parasitemia. This suggests that Abs provide effective protection against a natural infection. Inhibition of liver infection was also achieved by passive transfer of immune serum from whole-parasite-immunized mice. Furthermore, we demonstrated that passive transfer of a MAb against P. falciparum CSP inhibited liver-stage infection in a humanized mouse/P. falciparum challenge model. Together, these models constitute unique and sensitive in vivo methods to assess serum-transferable protection against Plasmodium sporozoite challenge.

Malaria vaccines: past, present and future

The currently available malaria control tools have allowed malaria elimination in many regions but there remain many regions where malaria control has made little progress. A safe and protective malaria vaccine would be a huge asset for malaria control. Despite the many challenges, efforts continue to design and evaluate malaria vaccine candidates. These candidates target different stages in the life cycle of Plasmodia. The most advanced vaccine candidates target the pre-erythrocytic stages in the life cycle of the parasite and include RTS,S/AS01, which has progressed through clinical development to the stage that it may be licensed in 2015. Attenuated whole-parasite vaccine candidates are highly protective, but there are challenges to manufacture and to administration. Cellular immunity is targeted by the prime-boost approach. Priming vectors trigger only modest responses but these are focused on the recombinant antigen. Boosting vectors trigger strong but broad non-specific responses. The heterologous sequence produces strong immunological responses to the recombinant antigen. Candidates that target the blood stages of the parasite have to result in an immune response that is more effective than the response to an infection to abort or control the infection of merozoites and hence disease. Finally, the sexual stages of the parasite offer another target for vaccine development, which would prevent the transmission of malaria. Today it seems unlikely that any candidate targeting a single antigen will provide complete protection against an organism of the complexity of Plasmodium. A systematic search for vaccine targets and combinations of antigens may be a more promising approach.

A phase 1b randomized, controlled, double-blinded dosage-escalation trial to evaluate the safety, reactogenicity and immunogenicity of an adenovirus type 35 based circumsporozoite malaria vaccine in Burkinabe healthy adults 18 to 45 years of age

Background

Ad35.CS.01 is a pre-erythrocytic malaria candidate vaccine. It is a codon optimized nucleotide sequence representing the P. falciparum circumsporozoite (CS) surface antigen inserted in a replication deficient Adenovirus 35 backbone. A Phase 1a trial has been conducted in the USA in naïve adults and showed that the vaccine was safe. The aim of this study is to assess the safety and immunogenicity of ascending dosages in sub Saharan Africa.

Methods

A double blind, randomized, controlled, dose escalation, phase Ib trial was conducted in a rural area of Balonghin, the Saponé health district (Burkina Faso). Forty-eight healthy adults aged 18-45 years were randomized into 4 cohorts of 12 to receive three vaccine doses (day 0, 28 and 84) of 10⁹, 10¹⁰, 5X10¹⁰, 10¹¹ vp of Ad35.CS.01 or normal saline by intra muscular injection. Subjects were monitored carefully during the 14 days following each vaccination for non serious adverse events. Severe and serious adverse events were collected throughout the participant study duration (12 months from the first vaccination). Humoral and cellular immune responses were measured on study days 0, 28, 56, 84, 112 and 140.

Results

Of the forty-eight subjects enrolled, forty-four (91.7%) received all three scheduled vaccine doses. Local reactions, all of mild severity, occurred in thirteen (27.1%) subjects. Severe (grade 3) laboratory abnormalities occurred in five (10.4%) subjects. One serious adverse event was reported and attributed to infection judged unrelated to vaccine. The vaccine induced both antibody titers and CD8 T cells producing IFNγ and TNFα with specificity to CS while eliciting modest neutralizing antibody responses against Ad35.

Conclusion

Study vaccine Ad35.CS.01 at four different dose levels was well-tolerated and modestly immunogenic in this population. These results suggest that Ad35.CS.01 should be further investigated for preliminary efficacy in human challenge models and as part of heterologous prime-boost vaccination strategies.

Trial Registration

ClinicalTrials.gov NCT01018459 http://clinicaltrials.gov/ct2/show/NCT01018459

Looking under the skin: the first steps in malarial infection and immunity

Malaria, which is caused by Plasmodium spp., starts with an asymptomatic phase, during which sporozoites, the parasite form that is injected into the skin by a mosquito, develop into merozoites, the form that infects erythrocytes. This pre-erythrocytic phase is still the most enigmatic in the parasite life cycle, but has long been recognized as an attractive vaccination target. In this Review, we present what has been learned in recent years about the natural history of the pre-erythrocytic stages, mainly using intravital imaging in rodents. We also consider how this new knowledge is in turn changing our understanding of the immune response mounted by the host against the pre-erythrocytic forms.

A glycolipid adjuvant, 7DW8-5, enhances CD8+ T cell responses induced by an adenovirus-vectored malaria vaccine in non-human primates

A key strategy to a successful vaccine against malaria is to identify and develop new adjuvants that can enhance T-cell responses and improve protective immunity. Upon co-administration with a rodent malaria vaccine in mice, 7DW8-5, a recently identified novel analog of α-galactosylceramide (α-GalCer), enhances the level of malaria-specific protective immune responses more strongly than the parent compound. In this study, we sought to determine whether 7DW8-5 could provide a similar potent adjuvant effect on a candidate human malaria vaccine in the more relevant non-human primate (NHP) model, prior to committing to clinical development. The candidate human malaria vaccine, AdPfCA (NMRC-M3V-Ad-PfCA), consists of two non-replicating recombinant adenoviral (Ad) vectors, one expressing the circumsporozoite protein (CSP) and another expressing the apical membrane antigen-1 (AMA1) of Plasmodium falciparum. In several phase 1 clinical trials, AdPfCA was well tolerated and demonstrated immunogenicity for both humoral and cell-mediated responses. In the study described herein, 25 rhesus macaques received prime and boost intramuscular (IM) immunizations of AdPfCA alone or with an ascending dose of 7DW8-5. Our results indicate that 7DW8-5 is safe and well-tolerated and provides a significant enhancement (up to 9-fold) in malaria-specific CD8+ T-cell responses after both priming and boosting phases, supporting further clinical development.