Malaria blood stage infection suppresses liver stage infection via host-induced interferons but not hepcidin

Malaria-causing Plasmodium parasites first replicate as liver stages (LS), which then seed symptomatic blood stage (BS) infection. Emerging evidence suggests that these stages impact each other via perturbation of host responses, and this influences the outcome of natural infection. We sought to understand whether the parasite stage interplay would affect live-attenuated whole parasite vaccination, since the efficacy of whole parasite vaccines strongly correlates with their extend of development in the liver. We thus investigated the impact of BS infection on LS development of genetically attenuated and wildtype parasites in female rodent malaria models and observed that for both, LS infection suffered severe suppression during concurrent BS infection. Strikingly and in contrast to previously published studies, we find that the BS-induced iron-regulating hormone hepcidin is not mediating suppression of LS development. Instead, we demonstrate that BS-induced host interferons are the main mediators of LS developmental suppression. The type of interferon involved depended on the BS-causing parasite species. Our study provides important mechanistic insights into the BS-mediated suppression of LS development. This has direct implications for understanding the outcomes of live-attenuated Plasmodium parasite vaccination in malaria-endemic areas and might impact the epidemiology of natural malaria infection.

Plasmodium GPI-anchored micronemal antigen is essential for parasite transmission through the mosquito host

Plasmodium parasites, the eukaryotic pathogens that cause malaria, feature three distinct invasive forms tailored to the host environment they must navigate and invade for life cycle progression. One conserved feature of these invasive forms is the micronemes, apically oriented secretory organelles involved in egress, motility, adhesion, and invasion. Here we investigate the role of GPI-anchored micronemal antigen (GAMA), which shows a micronemal localization in all zoite forms of the rodent-infecting species Plasmodium berghei. ∆GAMA parasites are severely defective for invasion of the mosquito midgut. Once formed, oocysts develop normally, however, sporozoites are unable to egress and exhibit defective motility. Epitope-tagging of GAMA revealed tight temporal expression late during sporogony and showed that GAMA is shed during sporozoite gliding motility in a similar manner to circumsporozoite protein. Complementation of P. berghei knockout parasites with full-length P. falciparum GAMA partially restored infectivity to mosquitoes, indicating conservation of function across Plasmodium species. A suite of parasites with GAMA expressed under the promoters of CTRP, CAP380, and TRAP, further confirmed the involvement of GAMA in midgut infection, motility, and vertebrate infection. These data show GAMA's involvement in sporozoite motility, egress, and invasion, implicating GAMA as a regulator of microneme function.

Type I Interferon remodels intrahepatocytic signaling to promote T cell dysfunction during liver stage Plasmodium infection

Malaria is caused by Plasmodium parasites and accounted for 627,000 deaths in 2021. Upon deposition into the dermis, Plasmodium parasites traffic to, infect, and develop within hepatocytes before egressing to cause symptomatic erythrocytic infection. We recently reported that liver stage Plasmodium replication induces type I Interferon (IFN-I) signaling that compromises anti-malaria T cells. We now show that this IFN-I-mediated, inferior T cell response is mostly limited to hepatic CD8 T cells and is characterized by diminished cytokine production, impaired homeostatic CD8 T cell survival, and the expression of receptors and transcription factors associated with T cell exhaustion. We set out to identify the cells and molecular events involved in the induction of T cell exhaustion during the transient and non-persistent liver stage Plasmodium infection and observed that infected mice lacking IFNAR expression solely on hepatocytes do not generate significant frequencies of exhausted T cells. Furthermore, transcriptomic analyses of hepatocytes isolated during infection indicate that IFN-I signaling coincides with a remodeling of the hepatocyte transcriptome such that antigen processing and presentation, chemokine transcripts and immunoregulatory receptors and soluble factors are upregulated. We hypothesize that IFN-I signaling reshapes the hepatocyte transcriptome to promote an immunosuppressive liver microenvironment that impairs anti-malaria T cells. Together, our studies represent the first description of a negative role for liver stage IFN-I signaling on anti-malaria adaptive immunity and may necessitate a paradigm shift in the rational design of efficacious anti-malaria whole parasite vaccines.

Supported by 1U01AI42001

The immunological perturbations induced by ongoing blood-stage infection during Plasmodium GAP (Genetically Attenuated Parasites) vaccination

Plasmodium sporozoites (SPZ) are transmitted by mosquito bite, infect the liver, grow and differentiate as intrahepatocytic liver stages (LS) and ultimately form the first generation of red blood cell-infectious merozoites, which initiate the symptomatic blood stage infection. Immunization with replication-deficient radiation attenuated SPZ vaccine has shown to confer sterilizing immunity in animal models by preventing parasite growth into the liver before onset of symptomatic blood stage infection. This attenuated SPZ vaccine has undergone clinical testing and it was observed that sterilizing immunity was achieved high in malaria naïve subjects but lower in malaria pre-exposed subjects residing in malaria-endemic regions. Previous studies in rodent models indicate that the ongoing blood stage infection suppresses protection after immunization with replication deficient irradiated SPZ. We have shown that replication competent late liver stage arresting genetically attenuated parasites (RC-GAP) immunization confer superior protection when compared to irradiated SPZ. However, whether the blood stage infection would also affect the RC-GAP parasite vaccine mediated protective immunity by altering specific immune responses remains unknown. Here, we show that ongoing homologous blood stage infection during Plasmodium yoelii RC-GAP immunization suppress the vaccine mediated immunity in Balb/c mice. Our data suggest that the reduction in protection was associated with the perturbations of long-lived Ag specific antibody as well as tissue resident memory CD8 T cell (TRMs) responses generated against pre-erythrocytic SPZ and LS stages.

Superior CD8 T cell-mediated protection against malaria parasite liver stage infection after vaccination with Replication Competent Genetically Attenuated Parasites (RC-GAP) and impact of blood stage infection on immunity

Plasmodium sporozoites (SPZ) are transmitted by mosquito bite, infect the liver, grow and differentiate as intrahepatocytic liver stages (LS) and ultimately form the first generation of red blood cell-infectious merozoites, which initiate the symptomatic blood stage infection. A vaccine against the clinically silent pre-erythrocytic SPZ and LS stages is attractive as it confers protection against infection before onset of symptomatic blood stage infection. Immunization with replication-deficient attenuated SPZ constitutes a promising vaccination strategy to engender protective humoral and CD8 T cell responses, with tissue resident memory CD8 T cell (TRMs) mediating sterilizing immunity in animal models. Attenuated parasite vaccines have undergone clinical testing and it was observed that sterilizing immunity is high in malaria naïve subjects but lower in malaria pre-exposed subjects residing in malaria-endemic regions. Previous studies in rodent models indicate that the ongoing blood stage infection suppresses protection after immunization with replication-deficient irradiated sporozoites. We have shown that replication competent late liver stage arresting genetically attenuated parasites (RC-GAP) confer superior protection when compared to irradiated sporozoites. We show that this protection is dependent on CD8 TRMs in the liver. However, whether the blood stage infection also suppresses the RC-GAP parasite vaccine mediated protective immunity remains unknown. Here, we examined whether blood stage infection either before or during RC-GAP immunization affects vaccine induced protection in the Plasmodium yoelii/Balb/c mouse model and how this correlated with perturbation of memory T and B cell responses.

Designer Parasites: Genetically Engineered Plasmodium as Vaccines To Prevent Malaria Infection

A highly efficacious malaria vaccine that prevents disease and breaks the cycle of infection remains an aspirational goal of medicine. Whole parasite vaccines based on the sporozoite forms of the parasite that target the clinically silent pre-erythrocytic stages of infection have emerged as one of the leading candidates. In animal models of malaria, these vaccines elicit potent neutralizing Ab responses against the sporozoite stage and cytotoxic T cells that eliminate parasite-infected hepatocytes. Among whole-sporozoite vaccines, immunization with live, replication-competent whole parasites engenders superior immunity and protection when compared with live replication-deficient sporozoites. As such, the genetic design of replication-competent vaccine strains holds the promise for a potent, broadly protective malaria vaccine. In this report, we will review the advances in whole-sporozoite vaccine development with a particular focus on genetically attenuated parasites both as malaria vaccine candidates and also as valuable tools to interrogate protective immunity against Plasmodium infection.

Human Antibodies that Slow Erythrocyte Invasion Potentiate Malaria-Neutralizing Antibodies

The Plasmodium falciparum reticulocyte-binding protein homolog 5 (PfRH5) is the leading target for next-generation vaccines against the disease-causing blood-stage of malaria. However, little is known about how human antibodies confer functional immunity against this antigen. We isolated a panel of human monoclonal antibodies (mAbs) against PfRH5 from peripheral blood B cells from vaccinees in the first clinical trial of a PfRH5-based vaccine. We identified a subset of mAbs with neutralizing activity that bind to three distinct sites and another subset of mAbs that are non-functional, or even antagonistic to neutralizing antibodies. We also identify the epitope of a novel group of non-neutralizing antibodies that significantly reduce the speed of red blood cell invasion by the merozoite, thereby potentiating the effect of all neutralizing PfRH5 antibodies as well as synergizing with antibodies targeting other malaria invasion proteins. Our results provide a roadmap for structure-guided vaccine development to maximize antibody efficacy against blood-stage malaria.

Innate immunity limits protective adaptive immune responses against pre-erythrocytic malaria infection

Immunization with whole pre-erythrocytic stage malaria parasites that infect hepatocytes but are unable to cause blood stage infection constitutes one of the most promising strategies to protect against malaria infection. In rodent malaria models, whole parasite vaccines that exhibit substantial replication within hepatocytes engender superior, long-lasting strain- and stage-transcending immunity as compared to replication-deficient whole parasite vaccines. Liver stage infection with replication-competent Plasmodium parasites induces an innate immune response that is dependent on type I Interferon (IFN-1) signaling. However, it remains unknown how this IFN-1 response contributes to vaccine-engendered adaptive immunity. Using a replication-competent genetically attenuated parasite vaccine in an immunization regimen that confers partial protection in wildtype (WT) mice, we examined vaccine efficacy in mice lacking IRF3 or IFNAR. Surprisingly, IRF3−/− and IFNAR−/− mice showed superior protection against an infectious sporozoite challenge when compared to WT mice. Better protection correlated with a greater frequency of liver-resident memory CD8 T cells in immunized IFNAR−/− mice. Furthermore, IFNAR−/− mice exhibited lower levels of the T cell co-inhibitory receptors, PD-1 and LAG-3, a greater percentage of IFNγ-producing CD8 T cells, and eliminated liver stages more effectively. Taken together, we show that the Plasmodium engendered IFN-1 response impairs the generation of an optimal memory T cell response. To our knowledge, this study is the first description of a detrimental role for IFN-1 signaling on malaria vaccine efficacy and as such may necessitate a paradigm shift in rational malaria vaccine design.

Humanized Mouse Models for the Study of Human Malaria Parasite Biology, Pathogenesis, and Immunity

Malaria parasite infection continues to inflict extensive morbidity and mortality in resource-poor countries. The insufficiently understood parasite biology, continuously evolving drug resistance and the lack of an effective vaccine necessitate intensive research on human malaria parasites that can inform the development of new intervention tools. Humanized mouse models have been greatly improved over the last decade and enable the direct study of human malaria parasites in vivo in the laboratory. Nevertheless, no small animal model developed so far is capable of maintaining the complete life cycle of Plasmodium parasites that infect humans. The ultimate goal is to develop humanized mouse systems in which a Plasmodium infection closely reproduces all stages of a parasite infection in humans, including pre-erythrocytic infection, blood stage infection and its associated pathology, transmission as well as the human immune response to infection. Here, we discuss current humanized mouse models and the future directions that should be taken to develop next-generation models for human malaria parasite research.

Plasmodium falciparum Liver Stage Infection and Transition to Stable Blood Stage Infection in Liver-Humanized and Blood-Humanized FRGN KO Mice Enables Testing of Blood Stage Inhibitory Antibodies (Reticulocyte-Binding Protein Homolog 5) In Vivo

The invention of liver-humanized mouse models has made it possible to directly study the preerythrocytic stages of Plasmodium falciparum. In contrast, the current models to directly study blood stage infection in vivo are extremely limited. Humanization of the mouse blood stream is achievable by frequent injections of human red blood cells (hRBCs) and is currently the only system with which to study human malaria blood stage infections in a small animal model. Infections have been primarily achieved by direct injection of P. falciparum-infected RBCs but as such, this modality of infection does not model the natural route of infection by mosquito bite and lacks the transition of parasites from liver stage infection to blood stage infection. Including these life cycle transition points in a small animal model is of relevance for testing therapeutic interventions. To this end, we used FRGN KO mice that were engrafted with human hepatocytes and performed a blood exchange under immune modulation to engraft the animals with more than 50% hRBCs. These mice were infected by mosquito bite with sporozoite stages of a luciferase-expressing P. falciparum parasite, resulting in noninvasively measurable liver stage burden by in vivo bioluminescent imaging (IVIS) at days 5–7 postinfection. Transition to blood stage infection was observed by IVIS from day 8 onward and then blood stage parasitemia increased with a kinetic similar to that observed in controlled human malaria infection. To assess the utility of this model, we tested whether a monoclonal antibody targeting the erythrocyte invasion ligand reticulocyte-binding protein homolog 5 (with known growth inhibitory activity in vitro) was capable of blocking blood stage infection in vivo when parasites emerge from the liver and found it highly effective. Together, these results show that a combined liver-humanized and blood-humanized FRGN mouse model infected with luciferase-expressing P. falciparum will be a useful tool to study P. falciparum preerythrocytic and erythrocytic stages and enables the testing of interventions that target either one or both stages of parasite infection.

Viral FGARAT ORF75A promotes early events in lytic infection and gammaherpesvirus pathogenesis in mice

Gammaherpesviruses encode proteins with homology to the cellular purine metabolic enzyme formyl-glycinamide-phosphoribosyl-amidotransferase (FGARAT), but the role of these viral FGARATs (vFGARATs) in the pathogenesis of a natural host has not been investigated. We report a novel role for the ORF75A vFGARAT of murine gammaherpesvirus 68 (MHV68) in infectious virion production and colonization of mice. MHV68 mutants with premature stop codons in orf75A exhibited a log reduction in acute replication in the lungs after intranasal infection, which preceded a defect in colonization of multiple host reservoirs including the mediastinal lymph nodes, peripheral blood mononuclear cells, and the spleen. Intraperitoneal infection rescued splenic latency, but not reactivation. The 75A.stop virus also exhibited defective replication in primary fibroblast and macrophage cells. Viruses produced in the absence of ORF75A were characterized by an increase in the ratio of particles to PFU. In the next round of infection this led to the alteration of early events in lytic replication including the deposition of the ORF75C tegument protein, the accelerated kinetics of viral gene expression, and induction of TNFα release and cell death. Infecting cells to deliver equivalent genomes revealed that ORF75A was required for initiating early events in infection. In contrast with the numerous phenotypes observed in the absence of ORF75A, ORF75B was dispensable for replication and pathogenesis. These studies reveal that murine rhadinovirus vFGARAT family members ORF75A and ORF75C have evolved to perform divergent functions that promote replication and colonization of the host.

Gammaherpesviruses are infectious agents that cause cancer. The study of viral genes unique to this subfamily may offer insight into the strategies that these viruses use to persist in the host and drive disease. The vFGARATs are a family of viral proteins found only in gammaherpesviruses, and are critical for replication in cell culture. Here we report that a rhadinovirus of rodents requires a previously uncharacterized vFGARAT family member, ORF75A, to support viral growth and persistence in mice. In addition, viruses lacking ORF75A are defective in the production of infectious viral particles. Thus, duplications and functional divergence of the various vFGARATs in the rhadinovirus lineage have likely been driven by selective pressures to disseminate within and colonize the host. Identification of the shared host processes that are targeted by the diverse family of vFGARATs may reveal novel targets for therapeutic agents to prevent life-long infections by these oncogenic viruses.

Characterization of the type I IFN response to liver stage infection with genetically attenuated malaria parasite vaccines

Malaria, a disease caused by Plasmodium parasites, kills nearly 600,000 people annually. Inoculated by an infectious mosquito bite, Plasmodium sporozoites travel to the liver and infect hepatocytes. Here, they develop into merozoites that are released into the blood to cause malaria associated mortality and morbidity. No fully protective malaria vaccine exists, but immunizations with genetically attenuated parasites (GAPs) that arrest in the liver confer sterile protection from challenge in mice. Moreover, using a novel super-infection assay, we observed that GAP immunization induces a potent type I IFN response to control liver stage infection. However, the factors required for the induction and propagation of the IFN response, and the influence of this response on adaptive immunity had not been elucidated. To identify the upstream inducers of IFN signaling, we immunized mice with either mosquito debris or GAPs and measured gene expression in hepatocytes by RNA-Seq. 800 genes were significantly upregulated upon GAP immunization, including transcripts for cytosolic RNA and DNA sensors. To examine the influence of the IFN response on adaptive immunity, WT, IRF3−/− and IFNAR−/− mice were immunized with a suboptimal dose of GAP, and challenged 4 weeks later with WT parasite. The immunization regimen elicited sterile protection in 20% of the WT mice, while 100% protection was achieved in IRF3−/− or IFNAR−/− mice. Taken together, the data suggest that hepatocytes utilize RNA and DNA sensors to induce a type I IFN response that is necessary for early control of liver stage infection, yet detrimental to adaptive immunity. Current studies are examining the mechanisms behind the IFN-mediated suppression of adaptive immunity.