Protection against malaria in mice is induced by blood stage-arresting histamine-releasing factor (HRF)-deficient parasites

Experimental vaccination by single dose sporozoite injection of blood-stage attenuated malaria parasites

Malaria vaccination approaches using live Plasmodium parasites are currently explored, with either attenuated mosquito-derived sporozoites or attenuated blood-stage parasites. Both approaches would profit from the availability of attenuated and avirulent parasites with a reduced blood-stage multiplication rate. Here we screened gene-deletion mutants of the rodent parasite P. berghei and the human parasite P. falciparum for slow growth. Furthermore, we tested the P. berghei mutants for avirulence and resolving blood-stage infections, while preserving sporozoite formation and liver infection. Targeting 51 genes yielded 18 P. berghei gene-deletion mutants with several mutants causing mild infections. Infections with the two most attenuated mutants either by blood stages or by sporozoites were cleared by the immune response. Immunization of mice led to protection from disease after challenge with wild-type sporozoites. Two of six generated P. falciparum gene-deletion mutants showed a slow growth rate. Slow-growing, avirulent P. falciparum mutants will constitute valuable tools to inform on the induction of immune responses and will aid in developing new as well as safeguarding existing attenuated parasite vaccines.

Size-dependent enhancement of gene expression by Plasmodium 5'UTR introns

BACKGROUND

Eukaryotic genes contain introns that are removed by the spliceosomal machinery during mRNA maturation. Introns impose a huge energetic burden on a cell; therefore, they must play an essential role in maintaining genome stability and/or regulating gene expression. Many genes (> 50%) in Plasmodium parasites contain predicted introns, including introns in 5' and 3' untranslated regions (UTR). However, the roles of UTR introns in the gene expression of malaria parasites remain unknown.

METHODS

In this study, an episomal dual-luciferase assay was developed to evaluate gene expression driven by promoters with or without a 5'UTR intron from four Plasmodium yoelii genes. To investigate the effect of the 5'UTR intron on endogenous gene expression, the pytctp gene was tagged with 3xHA at the N-terminal of the coding region, and parasites with or without the 5'UTR intron were generated using the CRISPR/Cas9 system.

RESULTS

We showed that promoters with 5'UTR introns had higher activities in driving gene expression than those without 5'UTR introns. The results were confirmed in recombinant parasites expressing an HA-tagged gene (pytctp) driven by promoter with or without 5'UTR intron. The enhancement of gene expression was intron size dependent, but not the DNA sequence, e.g. the longer the intron, the higher levels of expression. Similar results were observed when a promoter from one strain of P. yoelii was introduced into different parasite strains. Finally, the 5'UTR introns were alternatively spliced in different parasite development stages, suggesting an active mechanism employed by the parasites to regulate gene expression in various developmental stages.

CONCLUSIONS

Plasmodium 5'UTR introns enhance gene expression in a size-dependent manner; the presence of alternatively spliced mRNAs in different parasite developmental stages suggests that alternative slicing of 5'UTR introns is one of the key mechanisms in regulating parasite gene expression and differentiation.

A CAF01-adjuvanted whole asexual blood-stage liposomal malaria vaccine induces a CD4+ T-cell-dependent strain-transcending protective immunity in rodent models

IMPORTANCE

Malaria is a devastating disease that has claimed many lives, especially children <5 years of age in Sub-Saharan Africa, as documented in World Malaria Reports by WHO. Even though vector control and chemoprevention tools have helped with elimination efforts in some, if not all, endemic areas, these efforts have been hampered by serious issues (including drug and insecticide resistance and disruption to social cohesion caused by the COVID-19 pandemic). Development of an effective malaria vaccine is the alternative preventative tool in the fight against malaria. Vaccines save millions of lives each year and have helped in elimination and/or eradication of global diseases. Development of a highly efficacious malaria vaccine that will ensure long-lasting protective immunity will be a "game-changing" prevention strategy to finally eradicate the disease. Such a vaccine will need to counteract the significant obstacles that have been hampering subunit vaccine development to date, including antigenic polymorphism, sub-optimal immunogenicity, and waning vaccine efficacy.

Plasmodium-encoded murine IL-6 impairs liver stage infection and elicits long-lasting sterilizing immunity

Introduction

Plasmodium sporozoites (SPZ) inoculated by Anopheles mosquitoes into the skin of the mammalian host migrate to the liver before infecting hepatocytes. Previous work demonstrated that early production of IL-6 in the liver is detrimental for the parasite growth, contributing to the acquisition of a long-lasting immune protection after immunization with live attenuated parasites.

Methods

Considering that IL-6 as a critical pro-inflammatory signal, we explored a novel approach whereby the parasite itself encodes for the murine IL-6 gene. We generated transgenic P. berghei parasites that express murine IL-6 during liver stage development.

Results and Discussion

Though IL-6 transgenic SPZ developed into exo-erythrocytic forms in hepatocytes in vitro and in vivo, these parasites were not capable of inducing a blood stage infection in mice. Furthermore, immunization of mice with transgenic IL-6-expressing P. berghei SPZ elicited a long-lasting CD8⁺ T cell-mediated protective immunity against a subsequent infectious SPZ challenge. Collectively, this study demonstrates that parasite-encoded IL-6 attenuates parasite virulence with abortive liver stage of Plasmodium infection, forming the basis of a novel suicide vaccine strategy to elicit protective antimalarial immunity.

Antibody-dependent immune responses elicited by blood stage-malaria infection contribute to protective immunity to the pre-erythrocytic stages

Advances in transcriptomics and proteomics have revealed that different life-cycle stages of the malaria parasite, Plasmodium, share antigens, thus allowing for the possibility of eliciting immunity to a parasite life-cycle stage that has not been experienced before. Using the Plasmodium chabaudi (AS strain) model of malaria in mice, we investigated how isolated exposure to blood-stage infection, bypassing a liver-stage infection, yields significant protection to sporozoite challenge resulting in lower liver parasite burdens. Antibodies are the main immune driver of this protection. Antibodies induced by blood-stage infection recognise proteins on the surface of sporozoites and can impair sporozoite gliding motility in vitro, suggesting a possible function in vivo. Furthermore, mice lacking B cells and/or secreted antibodies are not protected against a sporozoite challenge in mice that had a previous blood-stage infection. Conversely, effector CD4⁺ and CD8⁺ T cells do not seem to play a role in protection from sporozoite challenge of mice previously exposed only to the blood stages of P. chabaudi. The protective response against pre-erythrocytic stages can be induced by infections initiated by serially passaged blood-stage parasites as well as recently mosquito transmitted parasites and is effective against a different strain of P. chabaudi (CB strain), but not against another rodent malaria species, P. yoelii. The possibility to induce protective cross-stage antibodies advocates the need to consider both stage-specific and cross-stage immune responses to malaria, as natural infection elicits exposure to all life-cycle stages. Future investigation into these cross-stage antibodies allows the opportunity for candidate antigens to contribute to malaria vaccine development.

AFF3, a susceptibility factor for autoimmune diseases, is a molecular facilitator of immunoglobulin class switch recombination

Immunoglobulin class switch recombination (CSR) plays critical roles in controlling infections and inflammatory tissue injuries. Here, we show that AFF3, a candidate gene for both rheumatoid arthritis and type 1 diabetes, is a molecular facilitator of CSR with an isotype preference. Aff3-deficient mice exhibit low serum levels of immunoglobulins, predominantly immunoglobulin G2c (IgG2c) followed by IgG1 and IgG3 but not IgM. Furthermore, Aff3-deficient mice show weak resistance to Plasmodium yoelii infection, confirming that Aff3 modulates immunity to this pathogen. Mechanistically, the AFF3 protein binds to the IgM and IgG1 switch regions via a C-terminal domain, and Aff3 deficiency reduces the binding of AID to the switch regions less efficiently. One AFF3 risk allele for rheumatoid arthritis is associated with high mRNA expression of AFF3, IGHG2, and IGHA2 in human B cells. These findings demonstrate that AFF3 directly regulates CSR by facilitating the recruitment of AID to the switch regions.

Live attenuated vaccines, a favorable strategy to provide long-term immunity against protozoan diseases

The control of diseases caused by protozoan parasites is one of the United Nations' Sustainable Development Goals. In recent years much research effort has gone into developing a new generation of live attenuated vaccines (LAVs) against malaria, Chagas disease and leishmaniasis. However, there is a bottleneck related to their biosafety, production, and distribution that slows downs further development. The success of irradiated or genetically attenuated sporozoites against malaria, added to the first LAV against leishmaniasis to be evaluated in clinical trials, is indicative that the drawbacks of LAVs are gradually being overcome. However, whether persistence of LAVs is a prerequisite for sustained long-term immunity remains to be clarified, and the procedures necessary for clinical evaluation of vaccine candidates need to be standardized.

Development and Evaluation of a Cryopreserved Whole-Parasite Vaccine in a Rodent Model of Blood-Stage Malaria

Infection with malaria parasites continues to be a major global public health issue. While current control measures have enabled a significant decrease in morbidity and mortality over the last 20 years, additional tools will be required if we are to progress toward malaria parasite eradication. Malaria vaccine research has focused on the development of subunit vaccines; however, more recently, interest in whole-parasite vaccines has reignited. Whole-parasite vaccines enable the presentation of a broad repertoire of antigens to the immune system, which limits the impact of antigenic polymorphism and genetic restriction of the immune response. We previously reported that whole-parasite vaccines can be prepared using chemically attenuated parasites within intact red blood cells or using killed parasites in liposomes, although liposomes were less immunogenic than attenuated parasites. If they could be frozen or freeze-dried and be made more immunogenic, liposomal vaccines would be ideal for vaccine deployment in areas where malaria is endemic. Here, we develop and evaluate a Plasmodium yoelii liposomal vaccine with enhanced immunogenicity and efficacy due to incorporation of TLR4 agonist, 3D(6-acyl) PHAD, and mannose to target the liposome to antigen-presenting cells. Following vaccination, mice were protected, and strong cellular immune responses were induced, characterized by parasite-specific splenocyte proliferation and a mixed Th1/Th2/Th17 cytokine response. Parasite-specific antibodies were induced, predominantly of the IgG1 subclass. CD4⁺ T cells and gamma interferon were critical components of the protective immune response. This study represents an important development toward evaluation of this whole-parasite blood-stage vaccine in a phase I clinical trial. IMPORTANCE Malaria is a mosquito-borne infectious disease that is caused by parasites of the genus, Plasmodium. There are seven different Plasmodium spp. that can cause malaria in humans, with P. falciparum causing the majority of the morbidity and mortality. Malaria parasites are endemic in 87 countries and continue to result in >200 million cases of malaria and >400,000 deaths/year, mostly children <5 years of age. Malaria infection initially presents as a flu-like illness but can rapidly progress to severe disease in nonimmune individuals if treatment is not initiated promptly. Existing control strategies for the mosquito vector (insecticides) and parasite (antimalarial drugs) are becoming increasingly less effective due to the development of resistance. While artemisinin combination therapies are frontline treatment for P. falciparum malaria, resistance has been documented in numerous countries. A highly effective malaria vaccine is urgently required to reduce malaria-attributable clinical disease and death and enable progression toward the ultimate goal of eradication.

Whole-Killed Blood-Stage Vaccine: Is It Worthwhile to Further Develop It to Control Malaria?

Major challenges have been encountered regarding the development of highly efficient subunit malaria vaccines, and so whole-parasite vaccines have regained attention in recent years. The whole-killed blood-stage vaccine (WKV) is advantageous as it can be easily manufactured and efficiently induced protective immunity against a blood-stage challenge, as well as inducing cross-stage protection against both the liver and sexual-stages. However, it necessitates a high dose of parasitized red blood cell (pRBC) lysate for immunization, and this raises concerns regarding its safety and low immunogenicity. Knowledge of the major components of WKV that can induce or evade the host immune response, and the development of appropriate human-compatible adjuvants will greatly help to optimize the WKV. Therefore, we argue that the further development of the WKV is worthwhile to control and potentially eradicate malaria worldwide.

Hepatic Inflammation Confers Protective Immunity Against Liver Stages of Malaria Parasite

Deciphering the mechanisms by which Plasmodium parasites develop inside hepatocytes is an important step toward the understanding of malaria pathogenesis. We propose that the nature and the magnitude of the inflammatory response in the liver are key for the establishment of the infection. Here, we used mice deficient in the multidrug resistance-2 gene (Mdr2-/-)-encoded phospholipid flippase leading to the development of liver inflammation. Infection of Mdr2-/- mice with Plasmodium berghei ANKA (PbANKA) sporozoites (SPZ) resulted in the blockade of hepatic exo-erythrocytic forms (EEFs) with no further development into blood stage parasites. Interestingly, cultured primary hepatocytes from mutant and wild-type mice are equally effective in supporting EEF development. The abortive infection resulted in a long-lasting immunity in Mdr2-/- mice against infectious SPZ where neutrophils and IL-6 appear as key effector components along with CD8+ and CD4+ effector and central memory T cells. Inflammation-induced breakdown of liver tolerance promotes anti-parasite immunity and provides new approaches for the design of effective vaccines against malaria disease.

Recombinant measles vaccine expressing malaria antigens induces long-term memory and protection in mice

Following the RTS,S malaria vaccine, which showed only partial protection with short-term memory, there is strong support to develop second-generation malaria vaccines that yield higher efficacy with longer duration. The use of replicating viral vectors to deliver subunit vaccines is of great interest due to their capacity to induce efficient cellular immune responses and long-term memory. The measles vaccine virus offers an efficient and safe live viral vector that could easily be implemented in the field. Here, we produced recombinant measles viruses (rMV) expressing malaria “gold standard” circumsporozoïte antigen (CS) of Plasmodium berghei (Pb) and Plasmodium falciparum (Pf) to test proof of concept of this delivery strategy. Immunization with rMV expressing PbCS or PfCS induced high antibody responses in mice that did not decrease for at least 22 weeks post-prime, as well as rapid development of cellular immune responses. The observed long-term memory response is key for development of second-generation malaria vaccines. Sterile protection was achieved in 33% of immunized mice, as usually observed with the CS antigen, and all other immunized animals were clinically protected from severe and lethal Pb ANKA-induced cerebral malaria. Further rMV-vectored malaria vaccine candidates expressing additional pre-erythrocytic and blood-stage antigens in combination with rMV expressing PfCS may provide a path to development of next generation malaria vaccines with higher efficacy.

Following the limited success of the RTS,S recombinant malaria vaccine there is a pressing need for second generation malaria vaccines. Frédéric Tangy and colleagues at the Pasteur Institute, Paris, generate novel vaccines based on recombinant measles virus (rMV) expressing the major circumsporozoite antigen CS from either Plasmodium berghei (rMV-CSPb) or P. falciparum (rMV-CSPf). rMV is a strong vector candidate because of its widespread use, safety profile and efficacy. Mice permissive to rMV infection show rapid and durable (at least 22 weeks) CS antibody responses as well as activation of cell-mediated immunity and type 1 helper responses following vaccination with rMV-CSPb or rMV-CSPf. rMV-CSPb vaccination protects mice from lethal challenge with Pb sporozoites, and in a subset of mice leads to sterile immunity. The rMV vector offers the potential of incorporating further antigens from other Plasmodium infection stages and thereby enhancement of vaccine efficacy.

Vaccination with chemically attenuated Plasmodium falciparum asexual blood-stage parasites induces parasite-specific cellular immune responses in malaria-naïve volunteers: a pilot study

BACKGROUND

The continuing morbidity and mortality associated with infection with malaria parasites highlights the urgent need for a vaccine. The efficacy of sub-unit vaccines tested in clinical trials in malaria-endemic areas has thus far been disappointing, sparking renewed interest in the whole parasite vaccine approach. We previously showed that a chemically attenuated whole parasite asexual blood-stage vaccine induced CD4⁺ T cell-dependent protection against challenge with homologous and heterologous parasites in rodent models of malaria.

METHODS

In this current study, we evaluated the immunogenicity and safety of chemically attenuated asexual blood-stage Plasmodium falciparum (Pf) parasites in eight malaria-naïve human volunteers. Study participants received a single dose of 3 × 10⁷ Pf pRBC that had been treated in vitro with the cyclopropylpyrolloindole analogue, tafuramycin-A.

RESULTS

We demonstrate that Pf asexual blood-stage parasites that are completely attenuated are immunogenic, safe and well tolerated in malaria-naïve volunteers. Following vaccination with a single dose, species and strain transcending Plasmodium-specific T cell responses were induced in recipients. This included induction of Plasmodium-specific lymphoproliferative responses, T cells secreting the parasiticidal cytokines, IFN-γ and TNF, and CD3⁺CD45RO⁺ memory T cells. Pf-specific IgG was not detected.

CONCLUSIONS

This is the first clinical study evaluating a whole parasite blood-stage malaria vaccine. Following administration of a single dose of completely attenuated Pf asexual blood-stage parasites, Plasmodium-specific T cell responses were induced while Pf-specific antibodies were not detected. These results support further evaluation of this chemically attenuated vaccine in humans.

TRIAL REGISTRATION

Trial registration: ACTRN12614000228684 . Registered 4 March 2014.

Profiling MHC II immunopeptidome of blood-stage malaria reveals that cDC1 control the functionality of parasite-specific CD4 T cells

In malaria, CD4 Th1 and T follicular helper (T_FH) cells are important for controlling parasite growth, but Th1 cells also contribute to immunopathology. Moreover, various regulatory CD4 T‐cell subsets are critical to hamper pathology. Yet the antigen‐presenting cells controlling Th functionality, as well as the antigens recognized by CD4 T cells, are largely unknown. Here, we characterize the MHC II immunopeptidome presented by DC during blood‐stage malaria in mice. We establish the immunodominance hierarchy of 14 MHC II ligands derived from conserved parasite proteins. Immunodominance is shaped differently whether blood stage is preceded or not by liver stage, but the same ETRAMP‐specific dominant response develops in both contexts. In naïve mice and at the onset of cerebral malaria, CD8α⁺ dendritic cells (cDC1) are superior to other DC subsets for MHC II presentation of the ETRAMP epitope. Using in vivo depletion of cDC1, we show that cDC1 promote parasite‐specific Th1 cells and inhibit the development of IL‐10⁺CD4 T cells. This work profiles the P. berghei blood‐stage MHC II immunopeptidome, highlights the potency of cDC1 to present malaria antigens on MHC II, and reveals a major role for cDC1 in regulating malaria‐specific CD4 T‐cell responses.

Immunological memory to blood-stage malaria infection is controlled by the histamine releasing factor (HRF) of the parasite

While most subunit malaria vaccines provide only limited efficacy, pre-erythrocytic and erythrocytic genetically attenuated parasites (GAP) have been shown to confer complete sterilizing immunity. We recently generated a Plasmodium berghei (PbNK65) parasite that lacks a secreted factor, the histamine releasing factor (HRF) (PbNK65 hrfΔ), and induces in infected mice a self-resolving blood stage infection accompanied by a long lasting immunity. Here, we explore the immunological mechanisms underlying the anti-parasite protective properties of the mutant PbNK65 hrfΔ and demonstrate that in addition to an up-regulation of IL-6 production, CD4⁺ but not CD8⁺ T effector lymphocytes are indispensable for the clearance of malaria infection. Maintenance of T cell-associated protection is associated with the reduction in CD4⁺PD-1⁺ and CD8⁺PD-1⁺ T cell numbers. A higher number of central and effector memory B cells in mutant-infected mice also plays a pivotal role in protection. Importantly, we also demonstrate that prior infection with WT parasites followed by a drug cure does not prevent the induction of PbNK65 hrfΔ-induced protection, suggesting that such protection in humans may be efficient even in individuals that have been infected and who repeatedly received antimalarial drugs.

Structure-Function Relationship of TCTP

The translationally controlled tumor protein (TCTP) is a small, multifunctional protein found in most, if not all, eukaryotic lineages, involved in a myriad of key regulatory processes. Among these, the control of proliferation and inhibition of cell death, as well as differentiation, are the most important, and it is probable that other responses are derived from the ability of TCTP to influence them in both unicellular and multicellular organisms. In the latter, an additional function for TCTP stems from its capacity to be secreted via a nonclassical pathway and function in a non-cell autonomous (paracrine) manner, thus affecting the responses of neighboring or distant cells to developmental or environmental stimuli (as in the case of serum TCTP/histamine-releasing factor in mammals and phloem TCTP in Arabidopsis). The additional ability to traverse membranes without a requirement for transmembrane receptors adds to its functional flexibility. The long-distance transport of TCTP mRNA and protein in plants via the vascular system supports the notion that an important aspect of TCTP function is its ability to influence the response of neighboring and distant cells to endogenous and exogenous signals in a supracellular manner. The predicted tridimensional structure of TCTPs indicates a high degree of conservation, more than its amino acid sequence similarity could suggest. However, subtle differences in structure could lead to different activities, as evidenced by TCTPs secreted by Plasmodium spp. Similar structural variations in animal and plant TCTPs, likely the result of convergent evolution, could lead to deviations from the canonical function of this group of proteins, which could have an impact from a biomedical and agricultural perspectives.

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.