The Effects of radiation on the safety and protective efficacy of an attenuated Plasmodium yoelii sporozoite malaria vaccine

Ultra-low volume intradermal administration of radiation-attenuated sporozoites with the glycolipid adjuvant 7DW8-5 completely protects mice against malaria

Radiation-attenuated sporozoite (RAS) vaccines can completely prevent blood stage Plasmodium infection by inducing liver-resident memory CD8+ T cells to target parasites in the liver. Such T cells can be induced by 'Prime-and-trap' vaccination, which here combines DNA priming against the P. yoelii circumsporozoite protein (CSP) with a subsequent intravenous (IV) dose of liver-homing RAS to "trap" the activated and expanding T cells in the liver. Prime-and-trap confers durable protection in mice, and efforts are underway to translate this vaccine strategy to the clinic. However, it is unclear whether the RAS trapping dose must be strictly administered by the IV route. Here we show that intradermal (ID) RAS administration can be as effective as IV administration if RAS are co-administrated with the glycolipid adjuvant 7DW8-5 in an ultra-low inoculation volume. In mice, the co-administration of RAS and 7DW8-5 in ultra-low ID volumes (2.5 µL) was completely protective and dose sparing compared to standard volumes (10-50 µL) and induced protective levels of CSP-specific CD8+ T cells in the liver. Our finding that adjuvants and ultra-low volumes are required for ID RAS efficacy may explain why prior reports about higher volumes of unadjuvanted ID RAS proved less effective than IV RAS. The ID route may offer significant translational advantages over the IV route and could improve sporozoite vaccine development.

Safety and protective efficacy of PfSPZ Vaccine administered to HIV-negative and -positive Tanzanian adults

BACKGROUNDSanaria PfSPZ Vaccine, composed of attenuated Plasmodium falciparum (Pf) sporozoites (SPZ), protects against malaria. We conducted this clinical trial to assess the safety and efficacy of PfSPZ Vaccine in HIV-positive (HIV+) individuals, since the HIV-infection status of participants in mass vaccination programs may be unknown.METHODSThis randomized, double-blind, placebo-controlled trial enrolled 18- to 45-year-old HIV-negative (HIV-) and well-controlled HIV+ Tanzanians (HIV viral load <40 copies/mL, CD4 counts >500 cells/μL). Participants received 5 doses of PfSPZ Vaccine or normal saline (NS) over 28 days, followed by controlled human malaria infection (CHMI) 3 weeks later.RESULTSThere were no solicited adverse events in the 9 HIV- and 12 HIV+ participants. After CHMI, 6 of 6 NS controls, 1 of 5 HIV- vaccinees, and 4 of 4 HIV+ vaccinees were Pf positive by quantitative PCR (qPCR). After immunization, anti-Pf circumsporozoite protein (anti-PfCSP) (isotype and IgG subclass) and anti-PfSPZ antibodies, anti-PfSPZ CD4+ T cell responses, and Vδ2+ γδ CD3+ T cells were nonsignificantly higher in HIV- than in HIV+ vaccinees. Sera from HIV- vaccinees had significantly higher inhibition of PfSPZ invasion of hepatocytes in vitro and antibody-dependent complement deposition (ADCD) and Fcγ3B binding by anti-PfCSP and ADCD by anti-cell-traversal protein for ookinetes and SPZ (anti-PfCelTOS) antibodies.CONCLUSIONSPfSPZ Vaccine was safe and well tolerated in HIV+ vaccinees, but not protective. Vaccine efficacy was 80% in HIV- vaccinees (P = 0.012), whose sera had significantly higher inhibition of PfSPZ invasion of hepatocytes and enrichment of multifunctional PfCSP antibodies. A more potent PfSPZ vaccine or regimen is needed to protect those living with HIV against Pf infection in Africa.TRIAL REGISTRATIONClinicalTrials.gov NCT03420053.FUNDINGEquatorial Guinea Malaria Vaccine Initiative (EGMVI), made up of the Government of Equatorial Guinea Ministries of Mines and Hydrocarbons, and Health and Social Welfare, Marathon Equatorial Guinea Production Limited, Noble Energy, Atlantic Methanol Production Company, and EG LNG; Swiss government, through ESKAS scholarship grant no. 2016.0056; Intramural Research Program of the National Institute of Allergy and Infectious Diseases, NIH; NIH grant 1U01AI155354-01.

Ultra-low volume intradermal administration of radiation-attenuated sporozoites with the glycolipid adjuvant 7DW8-5 completely protects mice against malaria

Malaria is caused by Plasmodium parasites and was responsible for over 247 million infections and 619,000 deaths in 2021. Radiation-attenuated sporozoite (RAS) vaccines can completely prevent blood stage infection by inducing protective liver-resident memory CD8+ T cells. Such T cells can be induced by 'prime-and-trap' vaccination, which here combines DNA priming against the P. yoelii circumsporozoite protein (CSP) with a subsequent intravenous (IV) dose of liver-homing RAS to "trap" the activated and expanding T cells in the liver. Prime-and-trap confers durable protection in mice, and efforts are underway to translate this vaccine strategy to the clinic. However, it is unclear whether the RAS trapping dose must be strictly administered by the IV route. Here we show that intradermal (ID) RAS administration can be as effective as IV administration if RAS are co-administrated with the glycolipid adjuvant 7DW8-5 in an ultra-low inoculation volume. In mice, the co-administration of RAS and 7DW8-5 in ultra-low ID volumes (2.5 μL) was completely protective and dose sparing compared to standard volumes (10-50 μL) and induced protective levels of CSP-specific CD8+ T cells in the liver. Our finding that adjuvants and ultra-low volumes are required for ID RAS efficacy may explain why prior reports about higher volumes of unadjuvanted ID RAS proved less effective. The ID route may offer significant translational advantages over the IV route and could improve sporozoite vaccine development.

Distinct immune responses associated with vaccination status and protection outcomes after malaria challenge

Understanding immune mechanisms that mediate malaria protection is critical for improving vaccine development. Vaccination with radiation-attenuated Plasmodium falciparum sporozoites (PfRAS) induces high level of sterilizing immunity against malaria and serves as a valuable tool for the study of protective mechanisms. To identify vaccine-induced and protection-associated responses during malarial infection, we performed transcriptome profiling of whole blood and in-depth cellular profiling of PBMCs from volunteers who received either PfRAS or noninfectious mosquito bites, followed by controlled human malaria infection (CHMI) challenge. In-depth single-cell profiling of cell subsets that respond to CHMI in mock-vaccinated individuals showed a predominantly inflammatory transcriptome response. Whole blood transcriptome analysis revealed that gene sets associated with type I and II interferon and NK cell responses were increased in prior to CHMI while T and B cell signatures were decreased as early as one day following CHMI in protected vaccinees. In contrast, non-protected vaccinees and mock-vaccinated individuals exhibited shared transcriptome changes after CHMI characterized by decreased innate cell signatures and inflammatory responses. Additionally, immunophenotyping data showed different induction profiles of vδ2+ γδ T cells, CD56+ CD8+ T effector memory (Tem) cells, and non-classical monocytes between protected vaccinees and individuals developing blood-stage parasitemia, following treatment and resolution of infection. Our data provide key insights in understanding immune mechanistic pathways of PfRAS-induced protection and infective CHMI. We demonstrate that vaccine-induced immune response is heterogenous between protected and non-protected vaccinees and that inducted-malaria protection by PfRAS is associated with early and rapid changes in interferon, NK cell and adaptive immune responses. Trial Registration: ClinicalTrials.gov NCT01994525.

A First for Human Vaccinology: GMP Compliant Radiation Attenuation of Plasmodium falciparum Sporozoites for Production of a Vaccine Against Malaria

Ionizing radiation (UV, X-ray and ɣ) administered at an appropriate dose to pathogenic organisms can prevent replication while preserving metabolic activity. We have established the GMP process for attenuation by ionizing radiation of the Plasmodium falciparum (Pf) sporozoites (SPZ) in Sanaria^® PfSPZ Vaccine, a protective vaccine against malaria. Mosquitoes raised and infected aseptically with Pf were transferred into infected mosquito transport containers (IMTC) and ɣ-irradiated using a ⁶⁰Co source. PfSPZ were then extracted, purified, vialed, and cryopreserved. To establish the appropriate radiation conditions, the irradiation field inside the IMTCs was mapped using radiochromic film and alanine transfer dosimeters. Dosimeters were irradiated for times calculated to provide 120-170 Gy at the minimum dose location inside the IMTC and regression analysis was used to determine the time required to achieve a lower 95% confidence interval for 150 Gy. A formula incorporating the half-life of ⁶⁰Co was then used to construct tables of irradiation times for each calendar day. From the mapping studies, formulae were derived to estimate the minimum and maximum doses of irradiation received inside the IMTC from a reference dosimeter mounted on the outside wall. For PfSPZ Vaccine manufacture a dose of 150 Gy was targeted for each irradiation event, a dose known to completely attenuate PfSPZ. The reference dosimeters were processed by the National Institute of Standards and Technology. There have been 587 irradiation events to produce PfSPZ Vaccine during 13 years which generated multiple lots released for pre-clinical studies and clinical trials. The estimated doses at the minimum dose location (mean 154.3 ± 1.77 Gy; range 150.0-159.3 Gy), and maximum dose location (mean 166.3 ± 3.65 Gy, range 155.7 to 175.3 Gy), in IMTCs were normally distributed. Overall dose uniformity was 1.078 ± 0.012. There was no siginifcant change in measured dose over 13 years. As of January 2022, 21 clinical trials of PfSPZ Vaccine have been conducted, with 1,740 volunteers aged 5 months to 61 years receiving 5,648 doses of PfSPZ Vaccine totalling >5.3 billion PfSPZ administered. There have been no breakthrough infections, confirming the consistency and robustness of the radiation attenuation process.

Evaluation of X-ray ionizing radiation on Plasmodium berghei invasion of erythrocytes

Developing new strategies for designing effective vaccines has become a priority for parasitologists worldwide. There is high interest in designing a vaccine against malaria since it is considered one of the most prevalent infectious diseases in the tropics. We evaluated the effects of X-rays irradiation on the erythrocytic stage of Plasmodium berghei ANKA merozoites and schizonts using doses of ionizing radiation ranging between 10 and 300 Gy on parasitized red blood cells (PRBC) to study the attenuating effects of radiation on the merozoites. Parasitic activity diminution was observed starting at 50 Gy, and the dose for complete attenuation was established at 200 Gy, corresponding with a 100% survival rate of mice. In vivo invasion experiments and immunofluorescence assays (IFA) showed inhibition of merozoite invasion of the host red blood cells (RBC). Nonetheless, immunization with irradiated parasitized red blood cells (IPRBC) was ineffective in protective assays. We perform cytoadherence and inhibition of cytoadhesion assays on irradiated merozoites. The results showed that high irradiation doses caused an unspecific cellular adhesion phenomenon independent of the ICAM-1 and CD36 interaction, which was determined by Cytoadhesion assays. Our results show that, even though X-ray irradiation is an effective method to induce complete parasite attenuation, it might affect the parasite's membrane surface structures triggering unspecific adhesion.

Identification of the Glycan Binding Profile of Human and Rodent Plasmodium Sporozoites

The transmission of Plasmodium spp. sporozoites to the mammalian host is the first step in the initiation of the mosquito-borne disease known as malaria. The exact route of transmission from the bloodstream to the liver is still not clearly elucidated, and identification of the host glycan structures bound by the sporozoites may inform as to which host cells are involved. Here, we provide a comprehensive analysis of the glycan structures that sporozoites from the human pathogen, P. falciparum, and the rodent pathogen, P. yoelii, recognize and bind. Glycan array analysis was used to profile the glycans bound by the sporozoites, and the binding affinities of these sporozoite-glycan interactions were then determined by surface plasmon resonance. Data showed that the different Plasmodium spp. bind different classes of glycans. P. falciparum was observed to bind to glycans with terminal N-acetylgalactosamine (GalNAc) or Galactose (Gal) linked to a GalNAc, and the highest-affinity observed was with the GalNAc monosaccharide (12.5 nM). P. yoelii bound glycosaminoglycans, mannosyl glycans, Gal linked to N-acetylglucosamine structures, and the αGal epitope. The highest-affinity interaction for P. yoelii was with the αGal epitope (31.4 nM). This is the first study to identify the key host glycan structures recognized by human and rodent Plasmodium spp. sporozoites. An understanding of how Plasmodium sporozoites interact with the specific glycan structures identified here may provide further insight into this infectious disease that could help direct the design of an effective therapeutic.

A double-blind, placebo-controlled phase 1/2a trial of the genetically attenuated malaria vaccine PfSPZ-GA1

Immunization with attenuated Plasmodium sporozoites can induce protection against malaria infection, as shown by Plasmodium falciparum (Pf) sporozoites attenuated by radiation in multiple clinical trials. As alternative attenuation strategy with a more homogeneous population of Pf sporozoites (PfSPZ), genetically engineered Plasmodium berghei sporozoites (SPZ) lacking the genes b9 and slarp induced sterile protection against malaria in mice. Consequently, PfSPZ-GA1 Vaccine, a Pf identical double knockout (Pf∆b9∆slarp), was generated as a genetically attenuated malaria parasite vaccine and tested for safety, immunogenicity, and preliminary efficacy in malaria-naïve Dutch volunteers. Dose-escalation immunizations up to 9.0 × 10⁵ PfSPZ of PfSPZ-GA1 Vaccine were well tolerated without breakthrough blood-stage infection. Subsequently, groups of volunteers were immunized three times by direct venous inoculation with cryopreserved PfSPZ-GA1 Vaccine (9.0 × 10⁵ or 4.5 × 10⁵ PfSPZ, N = 13 each), PfSPZ Vaccine (radiation-attenuated PfSPZ, 4.5 × 10⁵ PfSPZ, N = 13), or normal saline placebo at 8-week intervals, followed by exposure to mosquito bite controlled human malaria infection (CHMI). After CHMI, 3 of 25 volunteers from both PfSPZ-GA1 groups were sterilely protected, and the remaining 17 of 22 showed a patency ≥9 days (median patency in controls, 7 days; range, 7 to 9). All volunteers in the PfSPZ Vaccine control group developed parasitemia (median patency, 9 days; range, 7 to 12). Immunized groups exhibited a significant, dose-related increase in anti-Pf circumsporozoite protein (CSP) antibodies and Pf-specific interferon-γ (IFN-γ)-producing T cells. Although no definite conclusion can be drawn on the potential strength of protective efficacy of PfSPZ-GA1 Vaccine, the favorable safety profile and induced immune responses by PfSPZ-GA1 Vaccine warrant further clinical evaluation.

Experimental determination of the force of malaria infection reveals a non-linear relationship to mosquito sporozoite loads

Plasmodium sporozoites are the infective stage of the malaria parasite. Though this is a bottleneck for the parasite, the quantitative dynamics of transmission, from mosquito inoculation of sporozoites to patent blood-stage infection in the mammalian host, are poorly understood. Here we utilize a rodent model to determine the probability of malaria infection after infectious mosquito bite, and consider the impact of mosquito parasite load, blood-meal acquisition, probe-time, and probe location, on infection probability. We found that infection likelihood correlates with mosquito sporozoite load and, to a lesser degree, the duration of probing, and is not dependent upon the mosquito’s ability to find blood. The relationship between sporozoite load and infection probability is non-linear and can be described by a set of models that include a threshold, with mosquitoes harboring over 10,000 salivary gland sporozoites being significantly more likely to initiate a malaria infection. Overall, our data suggest that the small subset of highly infected mosquitoes may contribute disproportionally to malaria transmission in the field and that quantifying mosquito sporozoite loads could aid in predicting the force of infection in different transmission settings.

Malaria is a leading cause of death in many parts of the world. Infection is initiated when infected Anopheles mosquitoes inject sporozoites as they look for blood. Though transmission is a bottleneck for the parasite and thus a good point for intervention, many aspects of transmission remain poorly understood. In this study, using a rodent model of malaria, we found that the majority of infective bites do not result in malaria infection. Furthermore, we found that the bites of mosquitoes with heavy parasite burdens are significantly more likely to result in blood stage infection. These data have important implications for designing interventions targeting transmission stages of the malaria parasite as they suggest that reducing parasite loads, even without completely eliminating them, could be effective against disease spread. We also found that mosquitoes that probe but do not succeed in finding blood are equally likely to initiate infection, an important finding for human vaccine trials. Overall this work contributes to our understanding of the epidemiology of malaria and should aid in the development of malaria elimination strategies.

Grammomys surdaster, the Natural Host for Plasmodium berghei Parasites, as a Model to Study Whole-Organism Vaccines Against Malaria

AbstractInbred mice are commonly used to test candidate malaria vaccines, but have been unreliable for predicting efficacy in humans. To establish a more rigorous animal model, we acquired African woodland thicket rats of the genus Grammomys, the natural hosts for Plasmodium berghei. Thicket rats were acquired and identified as Grammomys surdaster by skull and teeth measurements and mitochondrial DNA genotyping. Herein, we demonstrate that thicket rats are highly susceptible to infection by P. berghei, and moderately susceptible to Plasmodium yoelii and Plasmodium chabaudi: 1-2 infected mosquito bites or 25-100 sporozoites administered by intravenous injection consistently resulted in patent parasitemia with P. berghei, and resulted in patent parasitemia with P. yoelii and P. chabaudi strains for at least 50% of animals. We then assessed efficacy of whole-organism vaccines to induce sterile immunity, and compared the thicket rat model to conventional mouse models. Using P. berghei ANKA radiation-attenuated sporozoites, and P. berghei ANKA and P. yoelii chemoprophylaxis vaccination approaches, we found that standard doses of vaccine sufficient to protect laboratory mice for a long duration against malaria challenge, are insufficient to protect thicket rats, which require higher doses of vaccine to achieve even short-term sterile immunity. Thicket rats may offer a more stringent and pertinent model for evaluating whole-organism vaccines.

Time for Genome Editing: Next-Generation Attenuated Malaria Parasites

Immunization with malaria parasites that developmentally arrest in or immediately after the liver stage is the only way currently known to confer sterilizing immunity in both humans and rodent models. There are various ways to attenuate parasite development resulting in different timings of arrest, which has a significant impact on vaccination efficiency. To understand what most impacts vaccination efficiency, newly developed gain-of-function methods can now be used to generate a wide array of differently attenuated parasites. The combination of multiple attenuation approaches offers the potential to engineer efficiently attenuated Plasmodium parasites and learn about their fascinating biology at the same time. Here we discuss recent studies and the potential of targeted parasite manipulation using genome editing to develop live attenuated malaria vaccines.

Trimethoprim-Sulfamethoxazole Prophylaxis During Live Malaria Sporozoite Immunization Induces Long-Lived, Homologous, and Heterologous Protective Immunity Against Sporozoite Challenge

Trimethoprim-sulfamethoxazole (TMP-SMX) is widely used in malaria-endemic areas in human immunodeficiency virus (HIV)-infected children and HIV-uninfected, HIV-exposed children as opportunistic infection prophylaxis. Despite the known effects that TMP-SMX has in reducing clinical malaria, its impact on development of malaria-specific immunity in these children remains poorly understood. Using rodent malaria models, we previously showed that TMP-SMX, at prophylactic doses, can arrest liver stage development of malaria parasites and speculated that TMP-SMX prophylaxis during repeated malaria exposures would induce protective long-lived sterile immunity targeting pre-erythrocytic stage parasites in mice. Using the same models, we now demonstrate that repeated exposures to malaria parasites during TMP-SMX administration induces stage-specific and long-lived pre-erythrocytic protective anti-malarial immunity, mediated primarily by CD8⁺ T-cells. Given the HIV infection and malaria coepidemic in sub-Saharan Africa, clinical studies aimed at determining the optimum duration of TMP-SMX prophylaxis in HIV-infected or HIV-exposed children must account for the potential anti-infection immunity effect of TMP-SMX prophylaxis.

Regulatory issues in immunity to liver and blood-stage malaria

T cells play a major role in control of both blood and liver stage of plasmodium infection. While immunization with certain attenuated whole-parasite vaccines that are attenuated at the liver stage of the infection induces protective T cell responses, even multiple exposures to natural infection in endemic areas do not lead to stable T cell memory or humoral immunity and sterilizing protection. One of the key differences between vaccination and natural exposure is the absence of blood stage during vaccination. Here we will discuss possible immunoregulatory strategies employed by blood stage of malaria leading to generation of severely compromised T cell and humoral immune responses and subsequent lack of sterilizing immunity.

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.

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.

CD8 T cell independent immunity after single dose infection-treatment-vaccination (ITV) against Plasmodium yoelii

Sporozoite vaccination of both humans and rodents elicits potent anti-malarial immunity, but the dose of sporozoites and the number of immunizations required varies with vaccination approach. Here we examine the immunological basis for superior protection afforded from single-dose vaccination with virulent sporozoites administered under prophylatic chloroquine-cover, referred to as infection-treatment-vaccination (ITV), compared to the well-studied approach of administering radiation-attenuated Plasmodium sporozoites (RAS). Earlier rodent studies utilizing ITV and RAS vaccination suggested a major role of CD8 T cells in reducing liver parasite burden after sporozoite challenge in a BALB/c mouse model. Consistent with this, we find that in C57Bl/6 mice ITV elicits substantially higher parasite-specific CD8 T cell responses than RAS vaccination and enhances immunity against P. yoelii infection. However, we show ITV-induced CD8 T cells are not necessary for protection following liver-stage sporozoite or blood-stage parasite challenge. Mechanistically, we found protection afforded from single-dose ITV is associated with low grade, transient parasitemia shortly following cessation of chloroquine treatment and generation of potent antibody responses to blood-stage parasites. Collectively, our data show the mechanistic basis for enhanced protective immunity against P. yoelli elicited by ITV in highly susceptible C57Bl/6 mice is independent of CD8 T cells. These studies may be relevant in understanding the potent immunity observed with ITV in humans.

Liver or blood-stage arrest during malaria sporozoite immunization: the later the better?

So far, the best immunization strategies to achieve high levels of protection against malaria are based on whole parasites. Complete sterile protection can be obtained in rodent models after immunization with sporozoites and chemoprophylaxis, or with sporozoites attenuated either genetically or by radiation. These approaches target specific stages, with arrests occurring at different time-points of the parasite life cycle. Here, we review these different approaches in relation to their capacity to induce protection in both Plasmodium berghei and Plasmodium yoelii models. The combined data suggest that maximal liver-stage exposure without further development into blood stages may induce the most efficient protection in mice.

A nonintegrative lentiviral vector-based vaccine provides long-term sterile protection against malaria

Trials testing the RTS,S candidate malaria vaccine and radiation-attenuated sporozoites (RAS) have shown that protective immunity against malaria can be induced and that an effective vaccine is not out of reach. However, longer-term protection and higher protection rates are required to eradicate malaria from the endemic regions. It implies that there is still a need to explore new vaccine strategies. Lentiviral vectors are very potent at inducing strong immunological memory. However their integrative status challenges their safety profile. Eliminating the integration step obviates the risk of insertional oncogenesis. Providing they confer sterile immunity, nonintegrative lentiviral vectors (NILV) hold promise as mass pediatric vaccine by meeting high safety standards. In this study, we have assessed the protective efficacy of NILV against malaria in a robust pre-clinical model. Mice were immunized with NILV encoding Plasmodium yoelii Circumsporozoite Protein (Py CSP) and challenged with sporozoites one month later. In two independent protective efficacy studies, 50% (37.5-62.5) of the animals were fully protected (p = 0.0072 and p = 0.0008 respectively when compared to naive mice). The remaining mice with detectable parasitized red blood cells exhibited a prolonged patency and reduced parasitemia. Moreover, protection was long-lasting with 42.8% sterile protection six months after the last immunization (p = 0.0042). Post-challenge CD8+ T cells to CSP, in contrast to anti-CSP antibodies, were associated with protection (r = -0.6615 and p = 0.0004 between the frequency of IFN-g secreting specific T cells in spleen and parasitemia). However, while NILV and RAS immunizations elicited comparable immunity to CSP, only RAS conferred 100% of sterile protection. Given that a better protection can be anticipated from a multi-antigen vaccine and an optimized vector design, NILV appear as a promising malaria vaccine.

Radiation-induced cellular and molecular alterations in asexual intraerythrocytic Plasmodium falciparum

BACKGROUND

γ-irradiation is commonly used to create attenuation in Plasmodium parasites. However, there are no systematic studies on the survival, reversion of virulence, and molecular basis for γ-radiation-induced cell death in malaria parasites.

METHODS

The effect of γ-irradiation on the growth of asexual Plasmodium falciparum was studied in erythrocyte cultures. Cellular and ultrastructural changes within the parasite were studied by fluorescence and electron microscopy, and genome-wide transcriptional profiling was performed to identify parasite biomarkers of attenuation and cell death.

RESULTS

γ-radiation induced the death of P. falciparum in a dose-dependent manner. These parasites had defective mitosis, sparse cytoplasm, fewer ribosomes, disorganized and clumped organelles, and large vacuoles-observations consistent with "distressed" or dying parasites. A total of 185 parasite genes were transcriptionally altered in response to γ-irradiation (45.9% upregulated, 54.1% downregulated). Loss of parasite survival was correlated with the downregulation of genes encoding translation factors and with upregulation of genes associated with messenger RNA-sequestering stress granules. Genes pertaining to cell-surface interactions, host-cell remodeling, and secreted proteins were also altered.

CONCLUSIONS

These studies provide a framework to assess the safety of γ-irradiation attenuation and promising targets for genetic deletion to produce whole parasite-based attenuated vaccines.

Killed but metabolically active Leishmania infantum as a novel whole-cell vaccine for visceral leishmaniasis

There are currently no effective vaccines for visceral leishmaniasis, the second most deadly parasitic infection in the world. Here, we describe a novel whole-cell vaccine approach using Leishmania infantum chagasi promastigotes treated with the psoralen compound amotosalen (S-59) and low doses of UV A radiation. This treatment generates permanent, covalent DNA cross-links within parasites and results in Leishmania organisms termed killed but metabolically active (KBMA). In this report, we characterize the in vitro growth characteristics of both KBMA L. major and KBMA L. infantum chagasi. Concentrations of S-59 that generate optimally attenuated parasites were identified. Like live L. infantum chagasi, KBMA L. infantum chagasi parasites were able to initially enter liver cells in vivo after intravenous infection. However, whereas live L. infantum chagasi infection leads to hepatosplenomegaly in mice after 6 months, KBMA L. infantum chagasi parasites were undetectable in the organs of mice at this time point. In vitro, KBMA L. infantum chagasi retained the ability to enter macrophages and induce nitric oxide production. These characteristics of KBMA L. infantum chagasi correlated with the ability to prophylactically protect mice via subcutaneous vaccination at levels similar to vaccination with live, virulent organisms. Splenocytes from mice vaccinated with either live L. infantum chagasi or KBMA L. infantum chagasi displayed similar cytokine patterns in vitro. These results suggest that KBMA technology is a potentially safe and effective novel vaccine strategy against the intracellular protozoan L. infantum chagasi. This approach may represent a new method for whole-cell vaccination against other complex intracellular pathogens.

Superior antimalarial immunity after vaccination with late liver stage-arresting genetically attenuated parasites

While subunit vaccines have shown partial efficacy in clinical trials, radiation-attenuated sporozoites (RAS) remain the "gold standard" for sterilizing protection against Plasmodium infection in human vaccinees. The variability in immunogenicity and replication introduced by the extensive, random DNA damage necessary to generate RAS could be overcome by genetically attenuated parasites (GAP) designed via gene deletion to arrest at defined points during liver-stage development. Here, we demonstrate the principle that late liver stage-arresting GAP induce larger and broader CD8 T cell responses that provide superior protection in inbred and outbred mice compared to RAS or early-arresting GAP immunizations. Late liver stage-arresting GAP also engender high levels of cross-stage and cross-species protection and complete protection when administered by translationally relevant intradermal or subcutaneous routes. Collectively, our results underscore the potential utility of late liver stage-arresting GAP as broadly protective next-generation live-attenuated malaria vaccines and support their potential as a powerful model for identifying antigens to generate cross-stage protection.

Intradermal immunization of mice with radiation-attenuated sporozoites of Plasmodium yoelii induces effective protective immunity

BACKGROUND

Intravenous injection of mice with attenuated Plasmodium berghei sporozoites induces sterile immunity to challenge with viable sporozoites. Non-intravenous routes have been reported to yield poor immunity. Because intravenous immunization has been considered to be unacceptable for large scale vaccination of humans, assessment was made of the results of intradermal immunization of mice with Plasmodium yoelii, a rodent malaria parasite whose infectivity resembles that of human malaria.

METHODS

Mice were immunized with two injections of isolated, radiation-attenuated P. yoelii sporozoites, either by intravenous (IV) or intradermal (ID) inoculation. In an attempt to enhance protective immunogenicity of ID-injections, one group of experimental mice received topical application of an adjuvant, Imiquimod, while another group had their injections accompanied by local "tape-stripping" of the skin, a procedure known to disrupt the stratum corneum and activate local immunocytes. Challenge of immunized and non-immunized control mice was by bite of sporozoite-infected mosquitoes. Degree of protection among the various groups of mice was determined by microscopic examination of stained blood smears. Statistical significance of protection was determined by a one-way ANOVA followed by Tukey's post hoc test.

RESULTS

Two intravenous immunizations produced 94% protection to mosquito bite challenge; intradermal immunization produced 78% protection, while intradermal immunization accompanied by "tape-stripping" produced 94% protection. There were no statistically significant differences in degree of protective immunity between immunizations done by intravenous versus intradermal injection.

CONCLUSIONS

The use of a sub-microlitre syringe for intradermal injections yielded excellent protective immunity. ID-immunization with large numbers of radiation-attenuated P. yoelii sporozoites led to levels of protective immunity comparable to those achieved by IV-immunization. It remains to be determined whether an adjuvant treatment can be found to substantially reduce the numbers of attenuated sporozoites required to achieve a strong protective immunity with as few doses as possible for possible extension to immunization of humans.

Malaria vaccine efficacy: overcoming the helminth hurdle

Several studies have documented that helminth infections can interfere with the development of the immune response of vaccines against different diseases, although some results have been contradictory. The mechanisms involved in the inhibition of the immune response to vaccination by helminth are still unclear, and murine models of helminth-malaria coinfections have proven helpful in investigating some aspects of the interactions involved. The study evaluated here focuses on the effect of helminth infection in mice on the immunogenicity and protective efficacy of two distinct malaria vaccine candidates, a transmission-blocking DNA vaccine based on Pfs25 antigen and a pre-erythrocytic vaccine based on irradiated sporozoites. Interestingly, the authors found that helminth infection dramatically reduced DNA vaccine immunogenicity, while immunization with irradiated sporozoites was able to induce a high level of antibodies and protection, independently of helminth infection. Immune suppression by helminth infection affected all IgG isotypes, suggesting no particular polarization of the immune response, but the generation of memory B cells was not affected. It will be of key interest to understand the mechanisms underlying the efficacy of the sporozoite vaccine, and its ability to overcome helminth immunosuppression, as this may help in the design of more effective vaccines.

Extreme CD8 T cell requirements for anti-malarial liver-stage immunity following immunization with radiation attenuated sporozoites

Radiation-attenuated Plasmodium sporozoites (RAS) are the only vaccine shown to induce sterilizing protection against malaria in both humans and rodents. Importantly, these “whole-parasite” vaccines are currently under evaluation in human clinical trials. Studies with inbred mice reveal that RAS-induced CD8 T cells targeting liver-stage parasites are critical for protection. However, the paucity of defined T cell epitopes for these parasites has precluded precise understanding of the specific characteristics of RAS-induced protective CD8 T cell responses. Thus, it is not known whether quantitative or qualitative differences in RAS-induced CD8 T cell responses underlie the relative resistance or susceptibility of immune inbred mice to sporozoite challenge. Moreover, whether extraordinarily large CD8 T cell responses are generated and required for protection following RAS immunization, as has been described for CD8 T cell responses following single-antigen subunit vaccination, remains unknown. Here, we used surrogate T cell activation markers to identify and track whole-parasite, RAS-vaccine-induced effector and memory CD8 T cell responses. Our data show that the differential susceptibility of RAS-immune inbred mouse strains to Plasmodium berghei or P. yoelii sporozoite challenge does not result from host- or parasite-specific decreases in the CD8 T cell response. Moreover, the surrogate activation marker approach allowed us for the first time to evaluate CD8 T cell responses and protective immunity following RAS-immunization in outbred hosts. Importantly, we show that compared to a protective subunit vaccine that elicits a CD8 T cell response to a single epitope, diversifying the targeted antigens through whole-parasite RAS immunization only minimally, if at all, reduced the numerical requirements for memory CD8 T cell-mediated protection. Thus, our studies reveal that extremely high frequencies of RAS-induced memory CD8 T cells are required, but may not suffice, for sterilizing anti-Plasmodial immunity. These data provide new insights into protective CD8 T cell responses elicited by RAS-immunization in genetically diverse hosts, information with relevance to developing attenuated whole-parasite vaccines.

Plasmodium infections are a global health crisis resulting in ∼300 million cases of malaria each year and ∼1 million deaths. Radiation-attenuated Plasmodium sporozoites (RAS) are the only vaccines that induce sterilizing anti-malarial immunity in humans. Importantly, “whole parasite” anti-malarial RAS vaccines are currently under evaluation in clinical trials. In rodents, RAS-induced protection is largely mediated by CD8 T cells. However, the quantitative and qualitative characteristics of RAS-induced protective CD8 T cell responses are unknown. Here, we used surrogate markers of T cell activation to reveal the magnitude and kinetics of Plasmodium-specific CD8 T cell responses following RAS-immunization in both inbred and outbred mice. Our data show that, independent of host genetic background, extremely large memory CD8 T cell responses were required, but not always sufficient for sterilizing protection. These data have broad implications for evaluating total T cell responses to attenuated pathogen-vaccines and direct relevance for efforts to translate attenuated whole-Plasmodium vaccines to humans.

Why functional pre-erythrocytic and bloodstage malaria vaccines fail: a meta-analysis of fully protective immunizations and novel immunological model

BACKGROUND

Clinically protective malaria vaccines consistently fail to protect adults and children in endemic settings, and at best only partially protect infants.

METHODOLOGY/PRINCIPAL FINDINGS

We identify and evaluate 1916 immunization studies between 1965-February 2010, and exclude partially or nonprotective results to find 177 completely protective immunization experiments. Detailed reexamination reveals an unexpectedly mundane basis for selective vaccine failure: live malaria parasites in the skin inhibit vaccine function. We next show published molecular and cellular data support a testable, novel model where parasite-host interactions in the skin induce malaria-specific regulatory T cells, and subvert early antigen-specific immunity to parasite-specific immunotolerance. This ensures infection and tolerance to reinfection. Exposure to Plasmodium-infected mosquito bites therefore systematically triggers immunosuppression of endemic vaccine-elicited responses. The extensive vaccine trial data solidly substantiate this model experimentally.

CONCLUSIONS/SIGNIFICANCE

We conclude skinstage-initiated immunosuppression, unassociated with bloodstage parasites, systematically blocks vaccine function in the field. Our model exposes novel molecular and procedural strategies to significantly and quickly increase protective efficacy in both pipeline and currently ineffective malaria vaccines, and forces fundamental reassessment of central precepts determining vaccine development. This has major implications for accelerated local eliminations of malaria, and significantly increases potential for eradication.

Genetically engineered, attenuated whole-cell vaccine approaches for malaria

Malaria remains one of the most significant infectious diseases affecting human populations in developing countries. The quest for an efficacious malaria vaccine has been ongoing for nearly a century with limited success. The identification of malaria parasite antigens focused efforts on the development of subunit vaccines but has so far yielded only one partially efficacious vaccine candidate, RTS/S. The lack of high vaccine efficacy observed to date with subunit vaccine candidates raises doubts that the development of a single antigen or even a multi-antigen malaria subunit vaccine is possible. Fortunately, it has been demonstrated in animal studies and experimental clinical studies that immunizations with live-attenuated sporozoite stages of the malaria parasite confer long lasting, sterile protection against infection, providing a benchmark for vaccine development. These early successful vaccinations with live-attenuated malaria parasites did not however, promote a developmental path forward for such a vaccine approach. The discovery of genetically engineered parasite strains that are fully attenuated during the early asymptomatic liver infection and confer complete sterile protection in animal malaria models support the development of a live attenuated sporozoite vaccine for Plasmodium falciparum and its accelerated safety and efficacy testing in malaria challenge models and in malaria endemic areas.

Plasmodium yoelii-infected A. stephensi inefficiently transmit malaria compared to intravenous route

It was recently reported that when mosquitoes infected with P. berghei sporozoites feed on mice, they deposit approximately 100–300 sporozoites in the dermis. When we inoculate P. yoelii (Py) sporozoites intravenously (IV) into BALB/c mice, the 50% infectious dose (ID₅₀) is often less than 3 sporozoites, indicating that essentially all Py sporozoites in salivary glands are infectious. Thus, it should only take the bite of one infected mosquito to infect 100% of mice. In human subjects, it takes the bite of at least 5 P. falciparum-infected mosquitoes to achieve 100% blood stage infection. Exposure to 1–2 infected mosquitoes only leads to blood stage infection in approximately 50% of subjects. If mosquitoes carrying Py sporozoites inoculate 100–300 sporozoites per bite, and 1 to 2 mosquito bites achieve 50% blood stage infection rates, then this would suggest that the majority of sporozoites inoculated by mosquitoes into the dermis are not responsible for a productive infection, or that a significant number of sporozoite-infected mosquitoes do not inoculate any sporozoites. The objective of this study was to determine if this is the case. We therefore studied the infectivity to mice of the bites of 1, 2, 4, or 5–8 Py-infected mosquitoes. The bite of one Py sporozoite-infected mosquito caused blood stage infection in 41.4% (12/29) of mice, two bites infected 66.7% (22/33), four bites infected 75% (18/24), and five to eight bites infected 100% (21/21). These findings demonstrate that inoculation of sporozoites by mosquito bite is much less efficient than IV inoculation of Py sporozoites by needle and syringe. Such data may have implications for determining the best route and dose of administration to humans of our attenuated P. falciparum sporozoite vaccine, the scientific basis of which is immunity by bites from irradiated infected mosquitoes, and suggest that the challenge is to develop a method of administration that approximates IV inoculation, not one that mimics mosquito bite.

Advances and challenges in malaria vaccine development

Malaria remains one of the most devastating infectious diseases that threaten humankind. Human malaria is caused by five different species of Plasmodium parasites, each transmitted by the bite of female Anopheles mosquitoes. Plasmodia are eukaryotic protozoans with more than 5000 genes and a complex life cycle that takes place in the mosquito vector and the human host. The life cycle can be divided into pre-erythrocytic stages, erythrocytic stages and mosquito stages. Malaria vaccine research and development faces formidable obstacles because many vaccine candidates will probably only be effective in a specific species at a specific stage. In addition, Plasmodium actively subverts and escapes immune responses, possibly foiling vaccine-induced immunity. Although early successful vaccinations with irradiated, live-attenuated malaria parasites suggested that a vaccine is possible, until recently, most efforts have focused on subunit vaccine approaches. Blood-stage vaccines remain a primary research focus, but real progress is evident in the development of a partially efficacious recombinant pre-erythrocytic subunit vaccine and a live-attenuated sporozoite vaccine. It is unlikely that partially effective vaccines will eliminate malaria; however, they might prove useful in combination with existing control strategies. Elimination of malaria will probably ultimately depend on the development of highly effective vaccines.

Comparison of the immunological responses and efficacy of gamma-irradiated V3526 vaccine formulations against subcutaneous and aerosol challenge with Venezuelan equine encephalitis virus subtype IAB

We recently developed a gamma-irradiation method to inactivate V3526, a live-attenuated Venezuelan equine encephalitis virus (VEEV) vaccine candidate. Dosage and schedule studies were conducted to evaluate the immunogenicity and efficacy of gamma-irradiated V3526 (gV3526). Subcutaneous (SC) and low dosage intramuscular (IM) administration of gV3526 were highly effective in protecting mice against a SC challenge with VEEV IA/B Trinidad Donkey strain, but not against an equivalent aerosol challenge. More robust immune responses and increased protective efficacy were noted when the IM dosage of gV3526 was increased. IM administration of gV3526 formulated with either CpG or CpG plus Alhydrogel further augmented the immune response in mice and resulted in 100% protection against aerosol challenge.

Neither mosquito saliva nor immunity to saliva has a detectable effect on the infectivity of Plasmodium sporozoites injected into mice

Malaria infection is initiated when a female Anopheles mosquito probing for blood injects saliva, together with sporozoites, into the skin of its mammalian host. Prior studies had suggested that saliva may enhance sporozoite infectivity. Using rodent malaria models (Plasmodium berghei and P. yoelii), we were unable to show that saliva had any detectable effect on sporozoite infectivity. This is encouraging for plans to immunize humans with washed, attenuated P. falciparum sporozoites because many individuals develop cutaneous, hypersensitivity reactions to mosquito saliva after repeated exposure. If washed sporozoites have no appreciable loss of infectivity, they likely do not have decreased immunogenicity; thus, vaccinees are unlikely to develop cutaneous reactions against mosquito saliva during attempted immunization with such sporozoites. Earlier studies also suggested that repeated prior exposure to mosquito saliva reduces infectivity of sporozoites injected by mosquitoes into sensitized hosts. However, our own studies show that prior exposure of mice to saliva had no detectable effect on numbers of sporozoites delivered by infected mosquitoes, the rate of disappearance of these sporozoites from the skin or infectivity of the sporozoites. Under natural conditions, sporozoites are delivered both to individuals who may exhibit cutaneous hypersensitivity to mosquito bite and to others who may have not yet developed such reactivity. It was tempting to hypothesize that differences in responsiveness to mosquito bite by different individuals might modulate the infectivity of sporozoites delivered into a milieu of changes induced by cutaneous hypersensitivity. Our results with rodent malaria models, however, were unable to support such a hypothesis.