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

γδ T Cells Are Required for the Induction of Sterile Immunity during Irradiated Sporozoite Vaccinations

Whole-sporozoite vaccines confer sterilizing immunity to malaria-naive individuals by unknown mechanisms. In the first PfSPZ Vaccine trial ever in a malaria-endemic population, Vδ2 γδ T cells were significantly elevated and Vγ9/Vδ2 transcripts ranked as the most upregulated in vaccinees who were protected from Plasmodium falciparum infection. In a mouse model, absence of γδ T cells during vaccination impaired protective CD8 T cell responses and ablated sterile protection. γδ T cells were not required for circumsporozoite protein-specific Ab responses, and γδ T cell depletion before infectious challenge did not ablate protection. γδ T cells alone were insufficient to induce protection and required the presence of CD8α+ dendritic cells. In the absence of γδ T cells, CD8α+ dendritic cells did not accumulate in the livers of vaccinated mice. Altogether, our results show that γδ T cells were essential for the induction of sterile immunity during whole-organism vaccination.

Advances in malaria vaccine development: report from the 2017 malaria vaccine symposium

The Malaria Vaccine Symposium occurred at Johns Hopkins University in Baltimore, MD, USA on April 25th, 2017, coinciding with World Malaria Day and the WHO announcement that the RTS,S malaria vaccine would begin pilot implementation programs in Ghana, Kenya, and Malawi in 2018. Scientists from several disciplines reported progress on an array of malaria vaccine concepts and product candidates, including pre-erythrocytic vaccines that prevent infection, blood-stage vaccines that limit infection and disease, and transmission-blocking vaccines that interrupt the spread of infection. Other speakers highlighted the immunological and genetic considerations that must be addressed by vaccinologists to yield the most efficacious vaccines. Here, we highlight the advances in malaria vaccinology that were reported at the symposium.

Novel ELISA method as exploratory tool to assess immunity induced by radiated attenuated sporozoites to decipher protective immunity

Background

Whole parasite vaccines provide a unique opportunity for dissecting immune mechanisms and identify antigens that are targeted by immune responses which have the potential to mediate sterile protection against malaria infections. The radiation attenuated sporozoite (PfSPZ) vaccine has been considered the gold standard for malaria vaccines because of its unparalleled efficacy. The immunogenicity of this and other vaccines continues to be evaluated by using recombinant proteins or peptides of known sporozoite antigens. This approach, however, has significant limitations by relying solely on a limited number of known pathogen-associated immune epitopes. Using the full range of antigens expressed by the sporozoite will enable the comprehensive immune-profiling of humoral immune responses induced by whole parasite vaccines. To address this challenge, a novel ELISA based on sporozoites was developed.

Results

The SPZ-ELISA method described in this report can be performed with either freshly dissected sporozoites or with cryopreserved sporozoite lysates. The use of a fixative for reproducible coating is not required. The SPZ-ELISA was first validated using monoclonal antibodies specific for CSP and TRAP and then used for the characterization of immune sera from radiation attenuated sporozoite vaccinees.

Conclusion

Applying this simple and highly reproducible approach to assess immune responses induced by malaria vaccines, both recombinant and whole parasite vaccines, (1) will help in the evaluation of immune responses induced by antigenically complex malaria vaccines such as the irradiated SPZ-vaccine, (2) will facilitate and accelerate the identification of immune correlates of protection, and (3) can also be a valuable assessment tool for antigen discovery as well as down-selection of vaccine formulations and, thereby, guide vaccine design.

Electronic supplementary material

The online version of this article (10.1186/s12936-017-2129-9) contains supplementary material, which is available to authorized users.

Innovative Approaches to Improve Anti-Infective Vaccine Efficacy

Safe and efficacious vaccines are arguably the most successful medical interventions of all time. Yet the ongoing discovery of new pathogens, along with emergence of antibiotic-resistant pathogens and a burgeoning population at risk of such infections, imposes unprecedented public health challenges. To meet these challenges, innovative strategies to discover and develop new or improved anti-infective vaccines are necessary. These approaches must intersect the most meaningful insights into protective immunity and advanced technologies with capabilities to deliver immunogens for optimal immune protection. This goal is considered through several recent advances in host-pathogen relationships, conceptual strides in vaccinology, and emerging technologies. Given a clear and growing risk of pandemic disease should the threat of infection go unmet, developing vaccines that optimize protective immunity against high-priority and antibiotic-resistant pathogens represents an urgent and unifying imperative.

In silico design of knowledge-based Plasmodium falciparum epitope ensemble vaccines

Malaria is a global health burden, and a major cause of mortality and morbidity in Africa. Here we designed a putative malaria epitope ensemble vaccine by selecting an optimal set of pathogen epitopes. From the IEDB database, 584 experimentally-verified CD8+ epitopes and 483 experimentally-verified CD4+ epitopes were collected; 89% of which were found in 8 proteins. Using the PVS server, highly conserved epitopes were identified from variability analysis of multiple alignments of Plasmodium falciparum protein sequences. The allele-dependent binding of epitopes was then assessed using IEDB analysis tools, from which the population protection coverage of single and combined epitopes was estimated. Ten conserved epitopes from four well-studied antigens were found to have a coverage of 97.9% of the world population: 7 CD8+ T cell epitopes (LLMDCSGSI, FLIFFDLFLV, LLACAGLAYK, TPYAGEPAPF, LLACAGLAY, SLKKNSRSL, and NEVVVKEEY) and 3 CD4+ T cell epitopes (MRKLAILSVSSFLFV, KSKYKLATSVLAGLL and GLAYKFVVPGAATPYE). The addition of four heteroclitic peptides - single point mutated epitopes - increased HLA binding affinity and raised the predicted world population coverage above 99%.

Humoral protection against mosquito bite-transmitted Plasmodium falciparum infection in humanized mice

A malaria vaccine that prevents infection will be an important new tool in continued efforts of malaria elimination, and such vaccines are under intense development for the major human malaria parasite Plasmodium falciparum (Pf). Antibodies elicited by vaccines can block the initial phases of parasite infection when sporozoites are deposited into the skin by mosquito bite and then target the liver for further development. However, there are currently no standardized in vivo preclinical models that can measure the inhibitory activity of antibody specificities against Pf sporozoite infection via mosquito bite. Here, we use human liver-chimeric mice as a challenge model to assess prevention of natural Pf sporozoite infection by antibodies. We demonstrate that these mice are consistently infected with Pf by mosquito bite and that this challenge can be combined with passive transfer of either monoclonal antibodies or polyclonal human IgG from immune serum to measure antibody-mediated blocking of parasite infection using bioluminescent imaging. This methodology is useful to down-select functional antibodies and to investigate mechanisms or immune correlates of protection in clinical trials, thereby informing rational vaccine optimization.

Modest heterologous protection after Plasmodium falciparum sporozoite immunization: a double-blind randomized controlled clinical trial

BACKGROUND

A highly efficacious vaccine is needed for malaria control and eradication. Immunization with Plasmodium falciparum NF54 parasites under chemoprophylaxis (chemoprophylaxis and sporozoite (CPS)-immunization) induces the most efficient long-lasting protection against a homologous parasite. However, parasite genetic diversity is a major hurdle for protection against heterologous strains.

METHODS

We conducted a double-blind, randomized controlled trial in 39 healthy participants of NF54-CPS immunization by bites of 45 NF54-infected (n = 24 volunteers) or uninfected mosquitoes (placebo; n = 15 volunteers) against a controlled human malaria infection with the homologous NF54 or the genetically distinct NF135.C10 and NF166.C8 clones. Cellular and humoral immune assays were performed as well as genetic characterization of the parasite clones.

RESULTS

NF54-CPS immunization induced complete protection in 5/5 volunteers against NF54 challenge infection at 14 weeks post-immunization, but sterilely protected only 2/10 and 1/9 volunteers against NF135.C10 and NF166.C8 challenge infection, respectively. Post-immunization plasma showed a significantly lower capacity to block heterologous parasite development in primary human hepatocytes compared to NF54. Whole genome sequencing showed that NF135.C10 and NF166.C8 have amino acid changes in multiple antigens targeted by CPS-induced antibodies. Volunteers protected against heterologous challenge were among the stronger immune responders to in vitro parasite stimulation.

CONCLUSIONS

Although highly protective against homologous parasites, NF54-CPS-induced immunity is less effective against heterologous parasite clones both in vivo and in vitro. Our data indicate that whole sporozoite-based vaccine approaches require more potent immune responses for heterologous protection.

TRIAL REGISTRATION

This trial is registered in clinicaltrials.gov, under identifier NCT02098590 .

Protective immunity differs between routes of administration of attenuated malaria parasites independent of parasite liver load

In humans and murine models of malaria, intradermal immunization (ID-I) with genetically attenuated sporozoites that arrest in liver induces lower protective immunity than intravenous immunization (IV-I). It is unclear whether this difference is caused by fewer sporozoites migrating into the liver or by suboptimal hepatic and injection site-dependent immune responses. We therefore developed a Plasmodium yoelii immunization/boost/challenge model to examine parasite liver loads as well as hepatic and lymph node immune responses in protected and unprotected ID-I and IV-I animals. Despite introducing the same numbers of genetically attenuated parasites in the liver, ID-I resulted in lower sterile protection (53-68%) than IV-I (93-95%). Unprotected mice developed less sporozoite-specific CD8⁺ and CD4⁺ effector T-cell responses than protected mice. After immunization, ID-I mice showed more interleukin-10-producing B and T cells in livers and skin-draining lymph nodes, but fewer hepatic CD8 memory T cells and CD8⁺ dendritic cells compared to IV-I mice. Our results indicate that the lower protection efficacy obtained by intradermal sporozoite administration is not linked to low hepatic parasite numbers as presumed before, but correlates with a shift towards regulatory immune responses. Overcoming these immune suppressive responses is important not only for live-attenuated malaria vaccines but also for other live vaccines administered in the skin.

Vaccines to Accelerate Malaria Elimination and Eventual Eradication

Remarkable progress has been made in coordinated malaria control efforts with substantial reductions in malaria-associated deaths and morbidity achieved through mass administration of drugs and vector control measures including distribution of long-lasting insecticide-impregnated bednets and indoor residual spraying. However, emerging resistance poses a significant threat to the sustainability of these interventions. In this light, the malaria research community has been charged with the development of a highly efficacious vaccine to complement existing malaria elimination measures. As the past 40 years of investment in this goal attests, this is no small feat. The malaria parasite is a highly complex organism, exquisitely adapted for survival under hostile conditions within human and mosquito hosts. Here we review current vaccine strategies to accelerate elimination and the potential for novel and innovative approaches to vaccine design through a better understanding of the host-parasite interaction.

Engineering of a self-adjuvanted iTEP-delivered CTL vaccine

Cytotoxic T lymphocyte (CTL) epitope peptide-based vaccines are widely used in cancer and infectious disease therapy. We previously generated an immune-tolerant elastin-like polypeptides (iTEPs)-based carrier to deliver a peptide CTL vaccine and enhance the efficiency of the vaccine. To further optimize the vaccine carrier, we intended to potentiate its function by designing an iTEP-based carrier that was able to deliver adjuvant and a vaccine epitope as one molecule. Thus, we fused a 9-mer H₁₀₀, a peptide derived from the high-mobility group box 1 protein (HMGB1) that could induce activation of dendritic cells (DCs), with an iTEP polymer to generate a new iTEP polymer named H₁₀₀-iTEP. The H₁₀₀-iTEP still kept the feature of reversible phase transition of iTEPs and should be able to be used as a polymer carrier to deliver peptide vaccines. The expression levels of CD80/CD86 on DCs were assessed using flow cytometry. The iTEP fusion-stimulated IL-6 secretion by DCs was measured with ELISA. Activation of antigen-specific CD8⁺ T cells induced by iTEP fusions was examined through a B3Z hybridoma cell activation assay. In vivo CTL activation promoted by iTEP fusions was detected by an IFN-γ-based ELISPOT assay. The iTEP fused with H₁₀₀ could induce maturation of DCs in vitro as evidenced by increased CD80 and CD86 expression. The iTEP fusion also promoted activation of DCs by increasing secretion of a proinflammatory cytokine IL-6. The N-terminus or C-terminus fusion of H₁₀₀ to iTEP had a similar effect and a reduced form of cysteine in iTEP fusions was required for DC stimulation. iTEP fusions potentiated a co-administrated CTL vaccine by increasing an antigen-specific CTL response in vitro and in vivo. When the H₁₀₀-iTEP was fused to a CTL epitope to generate a one-molecule vaccine, this self-adjuvanted vaccine elicited a stronger antigen-specific CTL response than a vaccine adjuvanted by Incomplete Freund's Adjuvant. Thus, we have successfully generated a functional, one-molecule iTEP-based self-adjuvanted vaccine.

Cellular immune response to DNA and vaccinia prime-boost immunization kills Plasmodium yoelii-infected hepatocytes in vitro

Background

Plasmid DNA encoding Plasmodium yoelii circumsporozoite protein (PyCSP) followed by boosting with recombinant vaccinia virus containing the PyCSP elicited significant protective immunity in mice that was primarily mediated by CD8+ T-cell responses directed to P. yoelii -infected hepatocytes. This study was to further explore protection using in vitro cultures of P. yoelii parasites in mouse hepatocytes. Spleen cells from DNA/vaccinia virus-immunized mice were co-cultured in vitro with mouse hepatocytes containing developing P. yoelii liver stage parasites. A semipermeable membrane separating spleen cells and hepatocytes was used to demonstrate if cell-to-cell contact was required. Inhibitors of mediators likely involved in spleen cell killing were added to these co-cultures. Spleen cells from immunized mice inhibited in vitro P. yoelii parasite development, and inhibition was eliminated by separating effectors and targets with the semipermeable membrane. Additionally, inhibitors of inducible nitric oxide synthase, caspase activation, NF-κB activation as well as antibodies against interferon-gamma (IFN-γ) and ICAM-1 reduced parasite inhibition. These findings suggest that direct contact between spleen cells from immunized mice and P. yoelii-infected hepatocytes is required for eliminating liver stage parasites and provide more insight into CD8+ T-cell-mediated inhibition of malaria liver stage development.

Engineering of Genetically Arrested Parasites (GAPs) For a Precision Malaria Vaccine

Continuous stage conversion and swift changes in the antigenic repertoire in response to acquired immunity are hallmarks of complex eukaryotic pathogens, including Plasmodium species, the causative agents of malaria. Efficient elimination of Plasmodium liver stages prior to blood infection is one of the most promising malaria vaccine strategies. Here, we describe different genetically arrested parasites (GAPs) that have been engineered in Plasmodium berghei, P. yoelii and P. falciparum and compare their vaccine potential. A better understanding of the immunological mechanisms of prime and boost by arrested sporozoites and experimental strategies to enhance vaccine efficacy by further engineering existing GAPs into a more immunogenic form hold promise for continuous improvements of GAP-based vaccines. A critical hurdle for vaccines that elicit long-lasting protection against malaria, such as GAPs, is safety and efficacy in vulnerable populations. Vaccine research should focus on solutions toward turning malaria into a vaccine-preventable disease, which would offer an exciting new path of malaria control.

Combinatorial Delivery of Dual and Triple TLR Agonists via Polymeric Pathogen-like Particles Synergistically Enhances Innate and Adaptive Immune Responses

Despite decades of research very few vaccine-adjuvants have received FDA approval. Two fundamental challenges plague clinical translation of vaccine-adjuvants: reducing acute toxicities that result from systemic diffusion of many soluble adjuvants, and delivering multiple adjuvants at the same time to mimic the synergistic immune-stimulation of pathogens, while being safe. In order to address these barriers, we evaluated combinations of four clinically relevant immune-agonists, specifically Toll-like receptor (TLR) ligands, using biodegradable, polymer microparticles. We tested them alone and in combinations of 2 or 3, for a total of 10 unique conditions. We evaluated primary bone-marrow-derived Dendritic Cell phenotypes and functionality, and identified several synergistic combinations. We picked a dual and a triple adjuvant combination, TLR4/TLR9 and TLR4/TLR7/TLR9, for further evaluation and found that both combinations promoted antigen cross-presentation in vitro. Studies in mice using the model antigen Ovalbumin, showed that both combinations enhanced lymph node germinal center and T follicular helper cell responses. The triple adjuvant combination showed increased antigen-specific antibody titer with an overall balanced Th1/Th2 response, while the dual combination promoted Th1-polarized IgG responses. Our results show how polymeric particulate-carriers can be adopted to safely deliver combinatorial adjuvants and selectively synergize specific types of immune responses for vaccine applications.

Experimental evaluation of cryopreservative solutions to maintain in vitro and in vivo infectivity of P. berghei sporozoites

The rodent malaria parasite Plasmodium berghei is an excellent model organism for laboratory-based experimental evaluation of anti-malarial therapeutics prior to studies with human malaria parasites. The rodent model is especially important for evaluation of pre-erythrocytic (PE) stage therapies, especially as current efforts to develop new PE vaccines and drugs is limited by access to P. falciparum and P. vivax sporozoites. Developing a more effective method for cryopreservation of sporozoites would help improve access to sporozoites for laboratories lacking suitable insectary facilities. In this study, P. berghei GFP-expressing sporozoites were purified from infected mosquitoes by manual dissection of salivary glands and different commercially-available, serum-free cryopreservative solutions were evaluated for efficient cryopreservation of the sporozoites. The cryopreservative solutions evaluated included CryoStor CS2, CryoSolutions DX5, CryoSolutions MC, Hestar 200, Voluven, Hetastarch, and Glycerolyte 57. The viability of fresh and post-thaw cryopreserved sporozoites was determined as a function of the relative sporozoite infectivity by infecting HC-04 cells in vitro, monitoring invasion and growth and development of liver stage parasites. Flow cytometer-based counting provided unbiased and fast quantitative assessment of parasite in vitro infection in infected HC-04 and in vivo infectivity was validated by injecting sporozoites IV into mice. CryoStor CS2 delivered the highest post-thaw recovery and infectivity of cryopreserved sporozoites. Sporozoites cryopreserved in CryoStor CS2 achieved 38% complete development of hepatic stages in HC-04 and 100% infectivity in mice. The cryopreservation method described here demonstrates a viable alternative for fresh Plasmodium sporozoites. The use of cryopreserved sporozoites should facilitate greater access to sporozoites for chemotherapeutic and vaccine research.

A Prime/Boost PfCS14KM/MVA-sPfCSM Vaccination Protocol Generates Robust CD8+ T Cell and Antibody Responses to Plasmodium falciparum Circumsporozoite Protein and Protects Mice against Malaria

Vaccines against the preerythrocytic stages of malaria are appealing because the parasite can be eliminated before disease onset and because they offer the unique possibility of targeting the parasite with both antibodies and T cells. Although the role of CD8⁺ T cells in preerythrocytic malaria stages is well documented, a highly effective T cell-inducing vaccine remains to be advanced. Here we report the development of a prime-boost immunization regimen with the Plasmodium falciparum circumsporozoite protein (PfCS) fused to the oligomer-forming vaccinia virus A27 protein and a modified vaccinia virus Ankara (MVA) vector expressing PfCS. This protocol induced polyfunctional CD8⁺ T cells with an effector memory phenotype and high PfCS antibody levels. These immune responses correlated with inhibition of liver-stage parasitemia in 80% and sterile protection in 40% of mice challenged with a transgenic P. berghei parasite line that expressed PfCS. Our findings underscore the potential of T and B cell immunization strategies for improving protective effectiveness against malaria.

Frequent inoculations with radiation attenuated sporozoite is essential for inducing sterile protection that correlates with a threshold level of Plasmodia liver-stage specific CD8+ T cells

Whole sporozoite vaccine (WSV) is shown to induce sterile protection that targets Plasmodium liver-stage infection. There are many underlying issues associated with induction of effective sterile protracted protection. In this study, we have addressed how the alterations in successive vaccine regimen could possibly affect the induction of sterile protection. We have demonstrated that the pattern of vaccination with RAS (radiation attenuated sporozoites) induces varying degrees of protection among B6 mice. Animals receiving four successive doses generated 100% sterile protection. However, three successive doses, though with the same parasite inoculum as four doses, could induce sterile protection in ∼50% mice. Interestingly, mice immunized with the same 3 doses, but with longer gap, could not survive the challenge. We demonstrate that degree of protection correlates with the frequencies of IFN-γ⁺ and multifunctional (IFN-γ⁺ CD107a⁺) CD8⁺ T_EM cells present in liver. The failure to achieve protective threshold frequency of these cells in liver might make the host more vulnerable to parasite infection during infectious sporozoite challenge.

Rhesus macaque and mouse models for down-selecting circumsporozoite protein based malaria vaccines differ significantly in immunogenicity and functional outcomes

BACKGROUND

Non-human primates, such as the rhesus macaques, are the preferred model for down-selecting human malaria vaccine formulations, but the rhesus model is expensive and does not allow for direct efficacy testing of human malaria vaccines. Transgenic rodent parasites expressing genes of human Plasmodium are now routinely used for efficacy studies of human malaria vaccines. Mice have however rarely predicted success in human malaria trials and there is scepticism whether mouse studies alone are sufficient to move a vaccine candidate into the clinic.

METHODS

A comparison of immunogenicity, fine-specificity and functional activity of two Alum-adjuvanted Plasmodium falciparum circumsporozoite protein (CSP)-based vaccines was conducted in mouse and rhesus models. One vaccine was a soluble recombinant protein (CSP) and the other was the same CSP covalently conjugated to the Qβ phage particle (Qβ-CSP).

RESULTS

Mice showed different kinetics of antibody responses and different sensitivity to the NANP-repeat and N-terminal epitopes as compared to rhesus. While mice failed to discern differences between the protective efficacy of CSP versus Qβ-CSP vaccine following direct challenge with transgenic Plasmodium berghei parasites, rhesus serum from the Qβ-CSP-vaccinated animals induced higher in vivo sporozoite neutralization activity.

CONCLUSIONS

Despite some immunologic parallels between models, these data demonstrate that differences between the immune responses induced in the two models risk conflicting decisions regarding potential vaccine utility in humans. In combination with historical observations, the data presented here suggest that although murine models may be useful for some purposes, non-human primate models may be more likely to predict the human response to investigational vaccines.

Assessment of the Plasmodium falciparum Preerythrocytic Antigen UIS3 as a Potential Candidate for a Malaria Vaccine

Efforts are under way to improve the efficacy of subunit malaria vaccines through assessments of new adjuvants, vaccination platforms, and antigens. In this study, we further assessed the Plasmodium falciparum antigen upregulated in infective sporozoites 3 (PfUIS3) as a vaccine candidate. PfUIS3 was expressed in the viral vectors chimpanzee adenovirus 63 (ChAd63) and modified vaccinia virus Ankara (MVA) and used to immunize mice in a prime-boost regimen. We previously demonstrated that this regimen could provide partial protection against challenge with chimeric P. berghei parasites expressing PfUIS3. We now show that ChAd63-MVA PfUIS3 can also provide partial cross-species protection against challenge with wild-type P. berghei parasites. We also show that PfUIS3-specific cellular memory responses could be recalled in human volunteers exposed to P. falciparum parasites in a controlled human malaria infection study. When ChAd63-MVA PfUIS3 was coadministered with the vaccine candidate P. falciparum thrombospondin-related adhesion protein (PfTRAP) expressed in the ChAd63-MVA system, there was no significant change in immunogenicity to either vaccine. However, when mice were challenged with double chimeric P. berghei-P. falciparum parasites expressing both PfUIS3 and PfTRAP, vaccine efficacy was improved to 100% sterile protection. This synergistic effect was evident only when the two vaccines were mixed and administered at the same site. We have therefore demonstrated that vaccination with PfUIS3 can induce a consistent delay in patent parasitemia across mouse strains and against chimeric parasites expressing PfUIS3 as well as wild-type P. berghei; when this vaccine is combined with another partially protective regimen (ChAd63-MVA PfTRAP), complete protection is induced.

Sterile protection against human malaria by chemoattenuated PfSPZ vaccine

A highly protective malaria vaccine would greatly facilitate the prevention and elimination of malaria and containment of drug-resistant parasites. A high level (more than 90%) of protection against malaria in humans has previously been achieved only by immunization with radiation-attenuated Plasmodium falciparum (Pf) sporozoites (PfSPZ) inoculated by mosquitoes; by intravenous injection of aseptic, purified, radiation-attenuated, cryopreserved PfSPZ ('PfSPZ Vaccine'); or by infectious PfSPZ inoculated by mosquitoes to volunteers taking chloroquine or mefloquine (chemoprophylaxis with sporozoites). We assessed immunization by direct venous inoculation of aseptic, purified, cryopreserved, non-irradiated PfSPZ ('PfSPZ Challenge') to malaria-naive, healthy adult volunteers taking chloroquine for antimalarial chemoprophylaxis (vaccine approach denoted as PfSPZ-CVac). Three doses of 5.12 × 104 PfSPZ of PfSPZ Challenge at 28-day intervals were well tolerated and safe, and prevented infection in 9 out of 9 (100%) volunteers who underwent controlled human malaria infection ten weeks after the last dose (group III). Protective efficacy was dependent on dose and regimen. Immunization with 3.2 × 103 (group I) or 1.28 × 104 (group II) PfSPZ protected 3 out of 9 (33%) or 6 out of 9 (67%) volunteers, respectively. Three doses of 5.12 × 104 PfSPZ at five-day intervals protected 5 out of 8 (63%) volunteers. The frequency of Pf-specific polyfunctional CD4 memory T cells was associated with protection. On a 7,455 peptide Pf proteome array, immune sera from at least 5 out of 9 group III vaccinees recognized each of 22 proteins. PfSPZ-CVac is a highly efficacious vaccine candidate; when we are able to optimize the immunization regimen (dose, interval between doses, and drug partner), this vaccine could be used for combination mass drug administration and a mass vaccination program approach to eliminate malaria from geographically defined areas.

Attenuated PfSPZ Vaccine induces strain-transcending T cells and durable protection against heterologous controlled human malaria infection

A live-attenuated malaria vaccine, Plasmodium falciparum sporozoite vaccine (PfSPZ Vaccine), confers sterile protection against controlled human malaria infection (CHMI) with Plasmodium falciparum (Pf) parasites homologous to the vaccine strain up to 14 mo after final vaccination. No injectable malaria vaccine has demonstrated long-term protection against CHMI using Pf parasites heterologous to the vaccine strain. Here, we conducted an open-label trial with PfSPZ Vaccine at a dose of 9.0 × 10⁵ PfSPZ administered i.v. three times at 8-wk intervals to 15 malaria-naive adults. After CHMI with homologous Pf parasites 19 wk after final immunization, nine (64%) of 14 (95% CI, 35-87%) vaccinated volunteers remained without parasitemia compared with none of six nonvaccinated controls (P = 0.012). Of the nine nonparasitemic subjects, six underwent repeat CHMI with heterologous Pf7G8 parasites 33 wk after final immunization. Five (83%) of six (95% CI, 36-99%) remained without parasitemia compared with none of six nonvaccinated controls. PfSPZ-specific T-cell and antibody responses were detected in all vaccine recipients. Cytokine production by T cells from vaccinated subjects after in vitro stimulation with homologous (NF54) or heterologous (7G8) PfSPZ were highly correlated. Interestingly, PfSPZ-specific T-cell responses in the blood peaked after the first immunization and were not enhanced by subsequent immunizations. Collectively, these data suggest durable protection against homologous and heterologous Pf parasites can be achieved with PfSPZ Vaccine. Ongoing studies will determine whether protective efficacy can be enhanced by additional alterations in the vaccine dose and number of immunizations.

Safety and efficacy of PfSPZ Vaccine against Plasmodium falciparum via direct venous inoculation in healthy malaria-exposed adults in Mali: a randomised, double-blind phase 1 trial

BACKGROUND

Plasmodium falciparum sporozite (PfSPZ) Vaccine is a metabolically active, non-replicating, whole malaria sporozoite vaccine that has been reported to be safe and protective against P falciparum controlled human malaria infection in malaria-naive individuals. We aimed to assess the safety and protective efficacy of PfSPZ Vaccine against naturally acquired P falciparum in malaria-experienced adults in Mali.

METHODS

After an open-label dose-escalation study in a pilot safety cohort, we did a double-blind, randomised, placebo-controlled trial based in Donéguébougou and surrounding villages in Mali. We recruited 18-35-year-old healthy adults who were randomly assigned (1:1) in a double-blind manner, with stratification by village and block randomisation, to receive either five doses of 2·7 × 10⁵ PfSPZ or normal saline at days 0, 28, 56, 84, and 140 during the dry season (January to July inclusive). Participants and investigators were masked to group assignments, which were unmasked at the final study visit, 6 months after receipt of the last vaccination. Participants received combined artemether and lumefantrine (four tablets, each containing 20 mg artemether and 120 mg lumefantrine, given twice per day over 3 days for a total of six doses) to eliminate P falciparum before the first and last vaccinations. We collected blood smears every 2 weeks and during any illness for 24 weeks after the fifth vaccination. The primary outcome was the safety and tolerability of the vaccine, assessed as local and systemic reactogenicity and adverse events. The sample size was calculated for the exploratory efficacy endpoint of time to first P falciparum infection beginning 28 days after the fifth vaccination. The safety analysis included all participants who received at least one dose of investigational product, whereas the efficacy analyses included only participants who received all five vaccinations. This trial is registered at ClinicalTrials.gov, number NCT01988636.

FINDINGS

Between Jan 18 and Feb 24, 2014, we enrolled 93 participants into the main study cohort with 46 participants assigned PfSPZ Vaccine and 47 assigned placebo, all of whom were evaluable for safety. We detected no significant differences in local or systemic adverse events or laboratory abnormalities between the PfSPZ Vaccine and placebo groups, and only grade 1 (mild) local or systemic adverse events occurred in both groups. The most common solicited systemic adverse event in the vaccine and placebo groups was headache (three [7%] people in the vaccine group vs four [9%] in the placebo group) followed by fatigue (one [2%] person in the placebo group), fever (one [2%] person in the placebo group), and myalgia (one [2%] person in each group). The exploratory efficacy analysis included 41 participants from the vaccine group and 40 from the placebo group. Of these participants, 37 (93%) from the placebo group and 27 (66%) from the vaccine group developed P falciparum infection. The hazard ratio for vaccine efficacy was 0·517 (95% CI 0·313-0·856) by time-to-infection analysis (log-rank p=0·01), and 0·712 (0·528-0·918) by proportional analysis (p=0·006).

INTERPRETATION

PfSPZ Vaccine was well tolerated and safe. PfSPZ Vaccine showed significant protection in African adults against P falciparum infection throughout an entire malaria season.

FUNDING

US National Institutes of Health Intramural Research Program, Sanaria.

Systems analysis of protective immune responses to RTS,S malaria vaccination in humans

RTS,S is an advanced malaria vaccine candidate and confers significant protection against Plasmodium falciparum infection in humans. Little is known about the molecular mechanisms driving vaccine immunity. Here, we applied a systems biology approach to study immune responses in subjects receiving three consecutive immunizations with RTS,S (RRR), or in those receiving two immunizations of RTS,S/AS01 following a primary immunization with adenovirus 35 (Ad35) (ARR) vector expressing circumsporozoite protein. Subsequent controlled human malaria challenge (CHMI) of the vaccinees with Plasmodium-infected mosquitoes, 3 wk after the final immunization, resulted in ∼50% protection in both groups of vaccinees. Circumsporozoite protein (CSP)-specific antibody titers, prechallenge, were associated with protection in the RRR group. In contrast, ARR-induced lower antibody responses, and protection was associated with polyfunctional CD4⁺ T-cell responses 2 wk after priming with Ad35. Molecular signatures of B and plasma cells detected in PBMCs were highly correlated with antibody titers prechallenge and protection in the RRR cohort. In contrast, early signatures of innate immunity and dendritic cell activation were highly associated with protection in the ARR cohort. For both vaccine regimens, natural killer (NK) cell signatures negatively correlated with and predicted protection. These results suggest that protective immunity against P. falciparum can be achieved via multiple mechanisms and highlight the utility of systems approaches in defining molecular correlates of protection to vaccination.

Chemoprophylaxis with sporozoite immunization in P. knowlesi rhesus monkeys confers protection and elicits sporozoite-specific memory T cells in the liver

Whole malaria sporozoite vaccine regimens are promising new strategies, and some candidates have demonstrated high rates of durable clinical protection associated with memory T cell responses. Little is known about the anatomical distribution of memory T cells following whole sporozoite vaccines, and immunization of nonhuman primates can be used as a relevant model for humans. We conducted a chemoprophylaxis with sporozoite (CPS) immunization in P. knowlesi rhesus monkeys and challenged via mosquito bites. Half of CPS immunized animals developed complete protection, with a marked delay in parasitemia demonstrated in the other half. Antibody responses to whole sporozoites, CSP, and AMA1, but not CelTOS were detected. Peripheral blood T cell responses to whole sporozoites, but not CSP and AMA1 peptides were observed. Unlike peripheral blood, there was a high frequency of sporozoite-specific memory T cells observed in the liver and bone marrow. Interestingly, sporozoite-specific CD4+ and CD8+ memory T cells in the liver highly expressed chemokine receptors CCR5 and CXCR6, both of which are known for liver sinusoid homing. The majority of liver sporozoite-specific memory T cells expressed CD69, a phenotypic marker of tissue-resident memory (TRM) cells, which are well positioned to rapidly control liver-stage infection. Vaccine strategies that aim to elicit large number of liver TRM cells may efficiently increase the efficacy and durability of response against pre-erythrocytic parasites.

The Threshold of Protection from Liver-Stage Malaria Relies on a Fine Balance between the Number of Infected Hepatocytes and Effector CD8+ T Cells Present in the Liver

Since the demonstration of sterile protection afforded by injection of irradiated sporozoites, CD8⁺ T cells have been shown to play a significant role in protection from liver-stage malaria. This is, however, dependent on the presence of an extremely high number of circulating effector cells, thought to be necessary to scan, locate, and kill infected hepatocytes in the short time that parasites are present in the liver. We used an adoptive transfer model to elucidate the kinetics of the effector CD8⁺ T cell response in the liver following Plasmodium berghei sporozoite challenge. Although effector CD8⁺ T cells require <24 h to find, locate, and kill infected hepatocytes, active migration of Ag-specific CD8⁺ T cells into the liver was not observed during the 2-d liver stage of infection, as divided cells were only detected from day 3 postchallenge. However, the percentage of donor cells recruited into division was shown to indicate the level of Ag presentation from infected hepatocytes. By titrating the number of transferred Ag-specific effector CD8⁺ T cells and sporozoites, we demonstrate that achieving protection toward liver-stage malaria is reliant on CD8⁺ T cells being able to locate infected hepatocytes, resulting in a protection threshold dependent on a fine balance between the number of infected hepatocytes and CD8⁺ T cells present in the liver. With such a fine balance determining protection, achieving a high number of CD8⁺ T cells will be critical to the success of a cell-mediated vaccine against liver-stage malaria.

Protection against Plasmodium falciparum malaria by PfSPZ Vaccine

BACKGROUND: A radiation-attenuated Plasmodium falciparum (Pf) sporozoite (SPZ) malaria vaccine, PfSPZ Vaccine, protected 6 of 6 subjects (100%) against homologous Pf (same strain as in the vaccine) controlled human malaria infection (CHMI) 3 weeks after 5 doses administered intravenously. The next step was to assess protective efficacy against heterologous Pf (different from Pf in the vaccine), after fewer doses, and at 24 weeks. METHODS: The trial assessed tolerability, safety, immunogenicity, and protective efficacy of direct venous inoculation (DVI) of 3 or 5 doses of PfSPZ Vaccine in non-immune subjects. RESULTS: Three weeks after final immunization, 5 doses of 2.7 × 10⁵ PfSPZ protected 12 of 13 recipients (92.3% [95% CI: 48.0, 99.8]) against homologous CHMI and 4 of 5 (80.0% [10.4, 99.5]) against heterologous CHMI; 3 doses of 4.5 × 10⁵ PfSPZ protected 13 of 15 (86.7% [35.9, 98.3]) against homologous CHMI. Twenty-four weeks after final immunization, the 5-dose regimen protected 7 of 10 (70.0% [17.3, 93.3]) against homologous and 1 of 10 (10.0% [-35.8, 45.6]) against heterologous CHMI; the 3-dose regimen protected 8 of 14 (57.1% [21.5, 76.6]) against homologous CHMI. All 22 controls developed Pf parasitemia. PfSPZ Vaccine was well tolerated, safe, and easy to administer. No antibody or T cell responses correlated with protection. CONCLUSIONS: We have demonstrated for the first time to our knowledge that PfSPZ Vaccine can protect against a 3-week heterologous CHMI in a limited group of malaria-naive adult subjects. A 3-dose regimen protected against both 3-week and 24-week homologous CHMI (87% and 57%, respectively) in this population. These results provide a foundation for developing an optimized immunization regimen for preventing malaria. TRIAL REGISTRATION: ClinicalTrials.gov NCT02215707. FUNDING: Support was provided through the US Army Medical Research and Development Command, Military Infectious Diseases Research Program, and the Naval Medical Research Center's Advanced Medical Development Program.

A potent adjuvant effect of a CD1d-binding NKT cell ligand in human immune system mice

OBJECTIVES

A CD1d-binding invariant natural killer T (iNKT)-cell stimulatory glycolipid, namely 7DW8-5, is shown to enhance the efficacy of radiation-attenuated sporozoites (RAS)-based malaria vaccine in mice. In the current study, we aim to determine whether 7DW8-5 can display a potent adjuvant effect in human immune system (HIS) mice.

METHODS

HIS-A2/hCD1d mice, which possess both functional human iNKT cells and CD8+ T cells, were generated by the transduction of NSG mice with adeno-associated virus serotype 9 expressing genes that encode human CD1d molecules and HLA-A*0201, followed by the engraftment of human hematopoietic stem cells. The magnitudes of human iNKT-cell response against 7DW8-5 and HLA-A*0201-restricted human CD8+ T-cell response against a human malaria antigen in HIS-A2/hCD1d mice were determined by using human CD1d tetramer and human HLA-A*0201 tetramer, respectively.

RESULTS

We found that 7DW8-5 stimulates human iNKT cells in HIS-A2/hCD1d mice, as well as those derived from HIS-A2/hCD1d mice in vitro. We also found that 7DW8-5 significantly increases the level of a human malarial antigen-specific HLA-A*0201-restricted human CD8+ T-cell response in HIS-A2/hCD1d mice.

CONCLUSIONS

Our study indicates that 7DW8-5 can display a potent adjuvant effect on RAS vaccine-induced anti-malarial immunity by augmenting malaria-specific human CD8+ T-cell response.

Eliciting Epitope-Specific CD8+ T Cell Response by Immunization with Microbial Protein Antigens Formulated with α-Galactosylceramide: Theory, Practice, and Protocols

CD8+ cytotoxic T lymphocytes confer protection against infectious diseases caused by viruses, bacteria, and parasites. Hence, significant efforts have been invested into devising ways to generate CD8+ T cell-targeted vaccines. Generation of microbe-free protein subunit vaccines requires a thorough knowledge of protective target antigens. Such antigens are proteolytically processed peptides presented by MHC class I molecules. To induce a robust antigen-specific CD8+ T cell response through vaccination, it is essential to formulate the antigen with an effective adjuvant. Here, we describe a versatile method for generating high-frequency antigen-specific CD8+ T cells through immunization of mice using the invariant natural killer T cell agonist α-galactosylceramide as the adjuvant.

Plasmodium knowlesi: a relevant, versatile experimental malaria model

The primate malaria Plasmodium knowlesi has a long-standing history as an experimental malaria model. Studies using this model parasite in combination with its various natural and experimental non-human primate hosts have led to important advances in vaccine development and in our understanding of malaria invasion, immunology and parasite-host interactions. The adaptation to long-term in vitro continuous blood stage culture in rhesus monkey, Macaca fascicularis and human red blood cells, as well as the development of various transfection methodologies has resulted in a highly versatile experimental malaria model, further increasing the potential of what was already a very powerful model. The growing evidence that P. knowlesi is an important human zoonosis in South-East Asia has added relevance to former and future studies of this parasite species.

Molecular Markers of Radiation Induced Attenuation in Intrahepatic Plasmodium falciparum Parasites

Experimental immunization with radiation attenuated sporozoites (RAS) and genetically attenuated sporozoites has proved to be a promising approach for malaria vaccine development. However, parasite biomarkers of growth attenuation and enhanced immune protection in response to radiation remain poorly understood. Here, we report on the effect of an attenuating dose of γ-irradiation (15 krad) on the Plasmodium falciparum sporozoite (PfSPZ) ultrastructure by electron microscopy, growth rate of liver stage P. falciparum in liver cell cultures, and genome-wide transcriptional profile of liver stage parasites by microarray. We find that γ-irradiation treated PfSPZ retained a normal cellular structure except that they were vacuous with a partially disrupted plasma membrane and inner membrane complex. A similar infection rate was observed by γ-irradiation-treated and untreated PfSPZ in human HCO-4 liver cells (0.47% versus 0.49%, respectively) on day 3 post-infection. In the microarray studies, cumulatively, 180 liver stage parasite genes were significantly transcriptionally altered on day 3 and/or 6 post-infection. Among the transcriptionally altered biomarkers, we identified a signature of seven candidate parasite genes that associated with functionally diverse pathways that may regulate radiation induced cell cycle arrest of the parasite within the hepatocyte. A repertoire of 14 genes associated with protein translation is transcriptionally overexpressed within the parasite by radiation. Additionally, 37 genes encode proteins expressed on the cell surface or exported into the host cell, 4 encode membrane associated transporters, and 10 encode proteins related to misfolding and stress-related protein processing. These results have significantly increased the repertoire of novel targets for 1) biomarkers of safety to define proper attenuation, 2) generating genetically attenuated parasite vaccine candidates, and 3) subunit candidate vaccines against liver stage malaria.

An expanding toolkit for preclinical pre-erythrocytic malaria vaccine development: bridging traditional mouse malaria models and human trials

Malaria remains a significant public health burden with 214 million new infections and over 400,000 deaths in 2015. Elucidating relevant Plasmodium parasite biology can lead to the identification of novel ways to control and ultimately eliminate the parasite within geographic areas. Particularly, the development of an effective vaccine that targets the clinically silent pre-erythrocytic stages of infection would significantly augment existing malaria elimination tools by preventing both the onset of blood-stage infection/disease as well as spread of the parasite through mosquito transmission. In this Perspective, we discuss the role of small animal models in pre-erythrocytic stage vaccine development, highlighting how human liver-chimeric and human immune system mice are emerging as valuable components of these efforts.

Protective immunity to liver-stage malaria

Despite decades of research and recent clinical trials, an efficacious long-lasting preventative vaccine for malaria remains elusive. This parasite infects mammals via mosquito bites, progressing through several stages including the relatively short asymptomatic liver stage followed by the more persistent cyclic blood stage, the latter of which is responsible for all disease symptoms. As the liver acts as a bottleneck to blood-stage infection, it represents a potential site for parasite and disease control. In this review, we discuss immunity to liver-stage malaria. It is hoped that the knowledge gained from animal models of malaria immunity will translate into a more powerful and effective vaccine to reduce this global health problem.

A whole-killed, blood-stage lysate vaccine protects against the malaria liver stage

Although the attenuated sporozoite is the most efficient vaccine to prevent infection with the malaria parasite, the limitation of a source of sterile sporozoites greatly hampers its application. In this study, we found that the whole-killed, blood-stage lysate vaccine could confer protection against the blood stage as well as the liver stage. Although the protective immunity induced by the whole-organism vaccine against the blood stage is dependent on parasite-specific CD4⁺ T-cell responses and antibodies, in mice immunized with the whole-killed, blood-stage lysate vaccine, CD8⁺ , but not CD4⁺ effector T-cell responses greatly contributed to protection against the liver stage. Thus, our data suggested that the whole-killed, blood-stage lysate vaccine could be an alternative promising strategy to prevent malaria infection and to reduce the morbidity and mortality of patients with malaria.

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.

Development of replication-deficient adenovirus malaria vaccines

INTRODUCTION

Malaria remains a major threat to endemic populations and travelers, including military personnel to these areas. A malaria vaccine is feasible, as radiation attenuated sporozoites induce nearly 100% efficacy. Areas covered: This review covers current malaria clinical trials using adenoviruses and pre-clinical research. Heterologous prime-boost regimens, including replication-deficient human adenovirus 5 (HuAd5) carrying malaria antigens, are efficacious. However, efficacy appears to be adversely affected by pre-existing anti-HuAd5 antibodies. Current strategies focus on replacing HuAd5 with rarer human adenoviruses or adenoviruses isolated from non-human primates (NHPs). The chimpanzee adenovirus ChAd63 is undergoing evaluation in clinical trials including infants in malaria-endemic areas. Key antigens have been identified and are being used alone, in combination, or with protein subunit vaccines. Gorilla adenoviruses carrying malaria antigens are also currently being evaluated in preclinical models. These replacement adenovirus vectors will be successfully used to develop vaccines against malaria, as well as other infectious diseases. Expert commentary: Simplified prime-boost single shot regimens, dry-coated live vector vaccines or silicon microneedle arrays could be developed for malaria or other vaccines. Replacement vectors with similar or superior immunogenicity have rapidly advanced, and several are now in extensive Phase 2 and beyond in malaria as well as other diseases, notably Ebola.

Polymeric hepatitis C virus non-structural protein 5A nanocapsules induce intrahepatic antigen-specific immune responses

Targeting antigen combined with adjuvants to hepatic antigen-presenting cells (APCs) is essential for the induction of intrahepatic T cellular immunity controlling and resolving viral infections of the liver. Intravenous injection of antigen-loaded nanoparticles is a promising approach for the delivery of antigens to liver APCs. Accordingly, polymeric nanocapsules (NCs) synthesized exclusively of hepatitis C virus non-structural protein 5A (NS5A) and the adjuvant monophosphoryl lipid A (MPLA) adsorbed to the nanocapsule surface were developed. Aim of the present study was the evaluation of the in vitro and in vivo behavior of MPLA-functionalized NS5A-NCs regarding the interaction with liver dendritic cells (DCs) and the potential to induce intrahepatic immune responses in a mouse model. Maturation of DCs was significantly increased by application of NS5A+MPLA-NCs compared to non-functionalized NS5A-NCs promoting a vigorous expression of CD40, CD80, CD86 and a strong secretion of the Th1-related cytokine IL-12. NS5A-NCs were preferentially deposited in DCs and Kupffer cells residing in the liver after intravenous administration. Immunization with NS5A-NCs induced intrahepatic antigen-specific CD4(+) T cellular immune responses determined by the secretion of IFNγ and IL-2. Furthermore, supplementation with MPLA induced significant levels of NS5A-specific antibodies. The application of polymeric nanocapsules synthesized exclusively out of antigen avoids the risk of unintended side effects caused by additional carrier substances. Functionalization with adjuvants like MPLA and the efficient targeting to liver-resident APCs inherits the potential for application of antigen nanocapsules in further vaccination approaches against pathogens affecting the liver.

Human CD8+ T cells mediate protective immunity induced by a human malaria vaccine in human immune system mice

A number of studies have shown that CD8+ T cells mediate protective anti-malaria immunity in a mouse model. However, whether human CD8+ T cells play a role in protection against malaria remains unknown. We recently established human immune system (HIS) mice harboring functional human CD8+ T cells (HIS-CD8 mice) by transduction with HLA-A∗0201 and certain human cytokines using recombinant adeno-associated virus-based gene transfer technologies. These HIS-CD8 mice mount a potent, antigen-specific HLA-A∗0201-restricted human CD8+ T-cell response upon immunization with a recombinant adenovirus expressing a human malaria antigen, the Plasmodium falciparum circumsporozoite protein (PfCSP), termed AdPfCSP. In the present study, we challenged AdPfCSP-immunized HIS-CD8 mice with transgenic Plasmodium berghei sporozoites expressing full-length PfCSP and found that AdPfCSP-immunized (but not naïve) mice were protected against subsequent malaria challenge. The level of the HLA-A∗0201-restricted, PfCSP-specific human CD8+ T-cell response was closely correlated with the level of malaria protection. Furthermore, depletion of human CD8+ T cells from AdPfCSP-immunized HIS-CD8 mice almost completely abolished the anti-malaria immune response. Taken together, our data show that human CD8+ T cells mediate protective anti-malaria immunity in vivo.

Purification of Plasmodium Sporozoites Enhances Parasite-Specific CD8+ T Cell Responses

Malaria infection caused by Plasmodium parasites continues to cause enormous morbidity and mortality in areas where it is endemic, and there is no licensed vaccine capable of inducing sterile protection. Hyperimmunization with attenuated whole sporozoites can induce sterile protective immune responses targeting preerythrocytic antigens. Most animal models of hyperimmunization rely on sporozoites dissected from mosquito salivary glands and injected without further purification. In BALB/c mice, repeated small doses of P. yoelii sporozoites progressively expand the population of sporozoite-specific CD8(+) T cells. In this study, large secondary doses of unpurified sporozoites unexpectedly led to contraction of sporozoite-specific CD8(+) T cell responses in sporozoite-primed mice. While sporozoite-primed CD8(+) T cells alternatively can be expanded by secondary exposure to Listeria monocytogenes expressing recombinant Plasmodium antigens, such expansion was potently inhibited by coinjection of large doses of unpurified sporozoites and by uninfected salivary glands alone. Purification of sporozoites away from mosquito salivary gland debris by density gradient centrifugation eliminated salivary gland-associated inhibition. Thus, the inhibitory effect appears to be due to exposure to uninfected mosquito salivary glands rather than sporozoites. To further assess the effect of salivary gland exposure on later sporozoite vaccinations, mice were immunized with uninfected salivary glands from a single mosquito. Compared to naive mice, salivary gland presensitization reduced subsequent liver burdens by 71%. These data show that a component(s) in mosquito salivary glands reduces liver infection, thereby limiting antigen dose and contributing to lower-magnitude T cell responses. These findings suggest that sporozoite immunogenicity studies be performed using purified sporozoites whenever feasible.

Chemically Attenuated Blood-Stage Plasmodium yoelii Parasites Induce Long-Lived and Strain-Transcending Protection

The development of a vaccine is essential for the elimination of malaria. However, despite many years of effort, a successful vaccine has not been achieved. Most subunit vaccine candidates tested in clinical trials have provided limited efficacy, and thus attenuated whole-parasite vaccines are now receiving close scrutiny. Here, we test chemically attenuated Plasmodium yoelii 17X and demonstrate significant protection following homologous and heterologous blood-stage challenge. Protection against blood-stage infection persisted for at least 9 months. Activation of both CD4⁺ and CD8⁺ T cells was shown after vaccination; however, in vivo studies demonstrated a pivotal role for both CD4⁺ T cells and B cells since the absence of either cell type led to loss of vaccine-induced protection. In spite of significant activation of circulating CD8⁺ T cells, liver-stage immunity was not evident. Neither did vaccine-induced CD8⁺ T cells contribute to blood-stage protection; rather, these cells contributed to pathogenesis, since all vaccinated mice depleted of both CD4⁺ and CD8⁺ T cells survived a challenge infection. This study provides critical insight into whole-parasite vaccine-induced immunity and strong support for testing whole-parasite vaccines in humans.

Development of whole sporozoite malaria vaccines

INTRODUCTION

Despite recent advances, malaria remains a major health threat both to populations in endemic areas as well travelers, including military personnel, to these areas. Subunit vaccines have not yet achieved sufficient efficacy needed for use in any of these at risk populations. Areas covered: This review discusses the current status of various whole sporozoite vaccine approaches and is mainly focused on current clinical trials. Expert commentary: Nearly 100% efficacy was achieved by administering multiple bites of radiation-attenuated sporozoite (RAS) Plasmodium falciparum-infected mosquitoes; this is impractical for widespread use. Now, this high level efficacy has been reproduced using purified, metabolically active RAS (PfSPZ Sanaria® Vaccine), which is undergoing extensive clinical testing. Alternative whole sporozoite vaccines include immunization with fully infectious sporozoites under chloroquine prophylaxis (CPS) or as genetically-attenuated parasites (GAP). By also manufacturing purified infectious sporozoites, it is now possible to combine these with CPS and GAP, as well as perform challenge studies using controlled doses of sporozoites.

The Ex Vivo IFN-γ Enzyme-Linked Immunospot (ELISpot) Assay

The quantification of single cell interferon-gamma (IFN-γ) release for assessing cellular immune responses using the Enzyme-linked immunospot (ELISPOT) assay is an invaluable technique in immunology. Peripheral blood mononuclear cells (PBMC) are stimulated in vitro with recombinant proteins, peptides and recently whole malaria organisms. Stimulation may be short term (20-36 h) or long term (cultured ELISpot, up to 7 days). ELISpot is also able to quantify other cytokines secreted by antigen-specific T-cells, such as interleukin-2, interleukin-5, and other interleukins. ELISpot is playing an important role especially in vaccine research studies.

Laser-induced capillary leakage for blood biomarker detection and vaccine delivery via the skin

Circulation system is the center for coordination and communication of all organs in our body. Examination of any change in its analytes or delivery of therapeutic drugs into the system consists of important medical practice in today's medicine. Two recent studies prove that brief illumination of skin with a low powered laser, at wavelengths preferentially absorbed by hemoglobin, increases the amount of circulating biomarkers in the epidermis and upper dermis by more than 1,000-fold. When probe-coated microneedle arrays are applied into laser-treated skin, plasma blood biomarkers can be reliably, accurately, and sufficiently quantified in 15∼30 min assays, with a maximal detection in one hr in a manner independent of penetration depth or a molecular mass of the biomarker. Moreover, the laser treatment permits a high efficient delivery of radiation-attenuated malarial sporozoites (RAS) into the circulation, leading to robust immunity against malaria infections, whereas similar immunization at sham-treated skin elicits poor immune responses. Thus this technology can potentially instruct designs of small, portable devices for onsite, in mobile clinics, or at home for point-of-care diagnosis and drug/vaccine delivery via the skin. Laser-induced capillary leakage (a) to induce extravasation of circualing molecules only (b) or facilitate entry of attenuated malaria sporozoites into the capillary (c). Skin illumination with a laser preferably absorbed by hemoglobin causes dilation of the capillary beneath the skin. The extravasated molecules can be sufficiently measured in the skin or guide sporozoites to enter the vessel.

Route of administration of attenuated sporozoites is instrumental in rendering immunity against Plasmodia infection

Whole sporozoite vaccine (WSV) approach has been shown to induce efficient CD8(+) T cell response, critical for developing of long-lasting sterile protection against Plasmodium. Although WSV was initiated over four decades ago, we still do not fully understand about the absolute requirements for the generation of liver-stage specific CD8(+) T memory cells. For more than a decade intravenous (IV) route of immunization has been shown to be protective in pre-clinical studies. However, the intradermal (ID) route is preferred over IV route by many researchers as it is perceived to mimic the natural route of parasite delivery through mosquito bite. Various clinical studies have shown that ID route provokes poor protective responses compared to those seen with IV route of administration. The present study highlights the importance of circumsporozoite (CS) protein in preventing sporozoite entry to the hepatocytes, which however, it is not necessarily sufficient to ensure sterile protection. Instead, this article favors the idea that liver-stage development is a prime requirement for generation of antigen specific CD8(+) T cells and suggests the conditions favored by IV inoculation of sporozoite.

Protection against malaria at 1 year and immune correlates following PfSPZ vaccination

An attenuated Plasmodium falciparum (Pf) sporozoite (SPZ) vaccine, PfSPZ Vaccine, is highly protective against controlled human malaria infection (CHMI) 3 weeks after immunization, but the durability of protection is unknown. We assessed how vaccine dosage, regimen, and route of administration affected durable protection in malaria-naive adults. After four intravenous immunizations with 2.7 × 10(5) PfSPZ, 6/11 (55%) vaccinated subjects remained without parasitemia following CHMI 21 weeks after immunization. Five non-parasitemic subjects from this dosage group underwent repeat CHMI at 59 weeks, and none developed parasitemia. Although Pf-specific serum antibody levels correlated with protection up to 21-25 weeks after immunization, antibody levels waned substantially by 59 weeks. Pf-specific T cell responses also declined in blood by 59 weeks. To determine whether T cell responses in blood reflected responses in liver, we vaccinated nonhuman primates with PfSPZ Vaccine. Pf-specific interferon-γ-producing CD8 T cells were present at ∼100-fold higher frequencies in liver than in blood. Our findings suggest that PfSPZ Vaccine conferred durable protection to malaria through long-lived tissue-resident T cells and that administration of higher doses may further enhance protection.

Linking Murine and Human Plasmodium falciparum Challenge Models in a Translational Path for Antimalarial Drug Development

Effective progression of candidate antimalarials is dependent on optimal dosing in clinical studies, which is determined by a sound understanding of pharmacokinetics and pharmacodynamics (PK/PD). Recently, two important translational models for antimalarials have been developed: the NOD/SCID/IL2Rγ^−/− (NSG) model, whereby mice are engrafted with noninfected and Plasmodium falciparum-infected human erythrocytes, and the induced blood-stage malaria (IBSM) model in human volunteers. The antimalarial mefloquine was used to directly measure the PK/PD in both models, which were compared to previously published trial data for malaria patients. The clinical part was a single-center, controlled study using a blood-stage Plasmodium falciparum challenge inoculum in volunteers to characterize the effectiveness of mefloquine against early malaria. The study was conducted in three cohorts (n = 8 each) using different doses of mefloquine. The characteristic delay in onset of action of about 24 h was seen in both NSG and IBSM systems. In vivo 50% inhibitory concentrations (IC₅₀s) were estimated at 2.0 μg/ml and 1.8 μg/ml in the NSG and IBSM models, respectively, aligning with 1.8 μg/ml reported previously for patients. In the IBSM model, the parasite reduction ratios were 157 and 195 for the 10- and 15-mg/kg doses, within the range of previously reported clinical data for patients but significantly lower than observed in the mouse model. Linking mouse and human challenge models to clinical trial data can accelerate the accrual of critical data on antimalarial drug activity. Such data can guide large clinical trials required for development of urgently needed novel antimalarial combinations. (This trial was registered at the Australian New Zealand Clinical Trials Registry [http://anzctr.org.au] under registration number ACTRN12612000323820.)

Myeloid Cell Isolation from Mouse Skin and Draining Lymph Node Following Intradermal Immunization with Live Attenuated Plasmodium Sporozoites

Malaria infection begins when the sporozoite stage of Plasmodium is inoculated into the skin of a mammalian host through a mosquito bite. The highly motile parasite not only reaches the liver to invade hepatocytes and transform into erythrocyte-infective form. It also migrates into the skin and to the proximal lymph node draining the injection site, where it can be recognized and degraded by resident and/or recruited myeloid cells. Intravital imaging reported the early recruitment of brightly fluorescent Lys-GFP positive leukocytes in the skin and the interactions between sporozoites and CD11c(+) cells in the draining lymph node. We present here an efficient procedure to recover, identify and enumerate the myeloid cell subsets that are recruited to the mouse skin and draining lymph node following intradermal injection of immunizing doses of sporozoites in a murine model. Phenotypic characterization using multi-parametric flow cytometry provides a reliable assay to assess early dynamic cellular changes during inflammatory response to Plasmodium infection.