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.

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.

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.

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

[This corrects the article DOI: 10.1371/journal.ppat.1006843.].

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.

A Plasmodium Parasite with Complete Late Liver Stage Arrest Protects against Preerythrocytic and Erythrocytic Stage Infection in Mice.

Genetically attenuated malaria parasites (GAP) that arrest during liver stage development are powerful immunogens and afford complete and durable protection against sporozoite infection. Late liver stage-arresting GAP provide superior protection against sporozoite challenge in mice compared to early live stage-arresting attenuated parasites. However, very few late liver stage-arresting GAP have been generated to date. Therefore, identification of additional loci that are critical for late liver stage development and can be used to generate novel late liver stage-arresting GAPs is of importance. We further explored genetic attenuation in Plasmodium yoelii by combining two gene deletions, PlasMei2 and liver-specific protein 2 (LISP2), that each cause late liver stage arrest with various degrees of infrequent breakthrough to blood stage infection. The dual gene deletion resulted in a synthetic lethal phenotype that caused complete attenuation in a highly susceptible mouse strain. P. yoeliiplasmei2-lisp2- arrested late in liver stage development and did not persist in livers beyond 3 days after infection. Immunization with this GAP elicited robust protective antibody responses in outbred and inbred mice against sporozoites, liver stages, and blood stages as well as eliciting protective liver-resident T cells. The immunization afforded protection against both sporozoite challenge and blood stage challenge. These findings provide evidence that completely attenuated late liver stage-arresting GAP are achievable via the synthetic lethal approach and might enable a path forward for the creation of a completely attenuated late liver stage-arresting P. falciparum GAP.

Absence of the uracil DNA glycosylase of murine gammaherpesvirus 68 impairs replication and delays the establishment of latency in vivo.

UNLABELLED: Uracil DNA glycosylases (UNG) are highly conserved proteins that preserve DNA fidelity by catalyzing the removal of mutagenic uracils. All herpesviruses encode a viral UNG (vUNG), and yet the role of the vUNG in a pathogenic course of gammaherpesvirus infection is not known. First, we demonstrated that the vUNG of murine gammaherpesvirus 68 (MHV68) retains the enzymatic function of host UNG in an in vitro class switch recombination assay. Next, we generated a recombinant MHV68 with a stop codon in ORF46/UNG (ΔUNG) that led to loss of UNG activity in infected cells and a replication defect in primary fibroblasts. Acute replication of MHV68ΔUNG in the lungs of infected mice was reduced 100-fold and was accompanied by a substantial delay in the establishment of splenic latency. Latency was largely, yet not fully, restored by an increase in virus inoculum or by altering the route of infection. MHV68 reactivation from latent splenocytes was not altered in the absence of the vUNG. A survey of host UNG activity in cells and tissues targeted by MHV68 indicated that the lung tissue has a lower level of enzymatic UNG activity than the spleen. Taken together, these results indicate that the vUNG plays a critical role in the replication of MHV68 in tissues with limited host UNG activity and this vUNG-dependent expansion, in turn, influences the kinetics of latency establishment in distal reservoirs. IMPORTANCE: Herpesviruses establish chronic lifelong infections using a strategy of replicative expansion, dissemination to latent reservoirs, and subsequent reactivation for transmission and spread. We examined the role of the viral uracil DNA glycosylase, a protein conserved among all herpesviruses, in replication and latency of murine gammaherpesvirus 68. We report that the viral UNG of this murine pathogen retains catalytic activity and influences replication in culture. The viral UNG was impaired for productive replication in the lung. This defect in expansion at the initial site of acute replication was associated with a substantial delay of latency establishment in the spleen. The levels of host UNG were substantially lower in the lung compared to the spleen, suggesting that herpesviruses encode a viral UNG to compensate for reduced host enzyme levels in some cell types and tissues. These data suggest that intervention at the site of initial replicative expansion can delay the establishment of latency, a hallmark of chronic herpesvirus infection.

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.

Liver-stage Plasmodium infection tunes clinical outcomes.

Chora and colleagues show that infection of the liver by Plasmodium modulates severity of disease in the experimental cerebral malaria (ECM) model by generating gamma delta (ɣδ) T cells that produce IL-17. This work calls into question the long-standing assumption that liver infection does not modulate severity of malaria.

Spike-specific T cells are enriched in breastmilk following SARS-CoV-2 mRNA vaccination.

Human breastmilk is rich in T cells; however, their specificity and function are largely unknown. We compared the phenotype, diversity, and antigen specificity of T cells in breastmilk and peripheral blood of lactating individuals who received SARS-CoV-2 messenger RNA (mRNA) vaccination. Relative to blood, breastmilk contained higher frequencies of T effector and central memory populations that expressed mucosal-homing markers. T cell receptor sequence overlap was limited between blood and breastmilk. Overabundant breastmilk clones were observed in all individuals, were diverse, and contained complementarity-determining regions in three sequences with known epitope specificity, including to SARS-CoV-2 spike. SARS-CoV-2 spike-specific T cell receptors were more frequent in breastmilk compared to blood and expanded in breastmilk following a 3rd mRNA vaccine dose. Our observations indicate that the lactating breast contains a distinct T cell population that can be modulated by maternal vaccination with potential implications for passive infant protection.

Longitudinal immune profiling after radiation-attenuated sporozoite vaccination reveals coordinated immune processes correlated with malaria protection.

Background: Identifying immune processes required for liver-stage sterilizing immunity to malaria remains an open problem. The IMRAS trial comprised 5x immunizations with radiation-attenuated sporozoites resulting in 55% protection from subsequent challenge. Methods: To identify correlates of vaccination and protection, we performed detailed systems immunology longitudinal profiling of the entire trial time course including whole blood transcriptomics, detailed PBMC cell phenotyping and serum antigen array profiling of 11 IMRAS radiation-attenuated sporozoite (RAS) vaccinees at up to 21 timepoints each. Results: RAS vaccination induced serum antibody responses to CSP, TRAP, and AMA1 in all vaccinees. We observed large numbers of differentially expressed genes associated with vaccination response and protection, with distinctly differing transcriptome responses elicited after each immunization. These included inflammatory and proliferative responses, as well as increased abundance of monocyte and DC subsets after each immunization. Increases in Vδ2 γδ; T cells and MAIT cells were observed in response to immunization over the course of study, and CD1c+ CD40+ DC abundance was significantly associated with protection. Interferon responses strongly differed between protected and non-protected individuals with high interferon responses after the 1st immunization, but not the 2nd-5th. Blood transcriptional interferon responses were correlated with abundances of different circulating classical and non-classical monocyte populations. Conclusions: This study has revealed multiple coordinated immunological processes induced by vaccination and associated with protection. Our work represents the most detailed immunological profiling of a RAS vaccine trial performed to date and will guide the design and interpretation of future malaria vaccine trials.

Anti-TRAP/SSP2 monoclonal antibodies can inhibit sporozoite infection and may enhance protection of anti-CSP monoclonal antibodies.

Vaccine-induced sterilizing protection from infection by Plasmodium parasites, the pathogens that cause malaria, will be essential in the fight against malaria as it would prevent both malaria-related disease and transmission. Stopping the relatively small number of parasites injected by the mosquito before they can migrate from the skin to the liver is an attractive means to this goal. Antibody-eliciting vaccines have been used to pursue this objective by targeting the major parasite surface protein present during this stage, the circumsporozoite protein (CSP). While CSP-based vaccines have recently had encouraging success in disease reduction, this was only achieved with extremely high antibody titers and appeared less effective for a complete block of infection (i.e., sterile protection). While such disease reduction is important, these and other results indicate that strategies focusing on CSP alone may not achieve the high levels of sterile protection needed for malaria eradication. Here, we show that monoclonal antibodies (mAbs) recognizing another sporozoite protein, TRAP/SSP2, exhibit a range of inhibitory activity and that these mAbs may augment CSP-based protection despite conferring no sterile protection on their own. Therefore, pursuing a multivalent subunit vaccine immunization is a promising strategy for improving infection-blocking malaria vaccines.

Spike-specific T cells are enriched in breastmilk following SARS-CoV-2 mRNA vaccination.

Human breastmilk is rich in T cells; however, their specificity and function are largely unknown. We compared the phenotype, diversity, and antigen specificity of T cells in the breastmilk and peripheral blood of lactating individuals who received SARS-CoV-2 mRNA vaccination. Relative to blood, breastmilk contained higher frequencies of T effector and central memory populations that expressed mucosal-homing markers. T cell receptor (TCR) sequence overlap was limited between blood and breastmilk. Overabundan t breastmilk clones were observed in all individuals, were diverse, and contained CDR3 sequences with known epitope specificity including to SARS-CoV-2 Spike. Spike-specific TCRs were more frequent in breastmilk compared to blood and expanded in breastmilk following a third mRNA vaccine dose. Our observations indicate that the lactating breast contains a distinct T cell population that can be modulated by maternal vaccination with potential implications for infant passive protection. One-Sentence Summary: The breastmilk T cell repertoire is distinct and enriched for SARS-CoV-2 Spike-specificity after maternal mRNA vaccination.

The macaque gut microbiome in health, lentiviral infection, and chronic enterocolitis.

The vertebrate gut harbors a vast community of bacterial mutualists, the composition of which is modulated by the host immune system. Many gastrointestinal (GI) diseases are expected to be associated with disruptions of host-bacterial interactions, but relatively few comprehensive studies have been reported. We have used the rhesus macaque model to investigate forces shaping GI bacterial communities. We used DNA bar coding and pyrosequencing to characterize 141,000 sequences of 16S rRNA genes obtained from 100 uncultured GI bacterial samples, allowing quantitative analysis of community composition in health and disease. Microbial communities of macaques were distinct from those of mice and humans in both abundance and types of taxa present. The macaque communities differed among samples from intestinal mucosa, colonic contents, and stool, paralleling studies of humans. Communities also differed among animals, over time within individual animals, and between males and females. To investigate changes associated with disease, samples of colonic contents taken at necropsy were compared between healthy animals and animals with colitis and undergoing antibiotic therapy. Communities from diseased and healthy animals also differed significantly in composition. This work provides comprehensive data and improved methods for studying the role of commensal microbiota in macaque models of GI diseases and provides a model for the large-scale screening of the human gut microbiome.

Community-wide response of the gut microbiota to enteropathogenic Citrobacter rodentium infection revealed by deep sequencing.

We investigated the spatial and temporal response of the murine gut microbiome to infection with Citrobacter rodentium, an attaching-and-effacing bacterium that provokes innate and adaptive immune responses, resulting in transient bacterial colitis. Previous studies have suggested that C. rodentium-induced inflammation is associated with an increased abundance of Enterobacteriaceae. We report here a deeper analysis of this model using DNA bar coding and 454 pyrosequencing to characterize 101,894 partial 16S rRNA gene sequences from 85 microbial samples from tissue-adhered and luminal bacteria of the cecum, proximal colon, and distal colon, which allowed us to identify previously unappreciated spatial and kinetic changes in multiple bacterial lineages. The deep sequencing data revealed that C. rodentium was most abundantly associated with the cecal mucosa at day 9 postinfection and then diminished in abundance, providing the first reported use of deep sequencing to track a pathogen in vivo through the course of infection. Notable changes were associated with both the mucosally adhered and luminal microbiota at both day 9 and day 14 postinfection. Alterations in abundance were seen for Proteobacteria, Deferribacteres, Clostridia, and others; however, changes in Enterobacteriaceae could be accounted for by the presence of C. rodentium itself, which is a member of this family. The Lactobacillus group decreased in abundance during infection, which may be important for pathogenesis because members of this lineage modulate the composition of the gut microbiota and are used as probiotics. Thus, deep sequencing provides previously inaccessible information on how Citrobacter infection and clearance reshapes the gut microbial community in space and time.

DNA bar coding and pyrosequencing to identify rare HIV drug resistance mutations.

Treatment of HIV-infected individuals with antiretroviral agents selects for drug-resistant mutants, resulting in frequent treatment failures. Although the major antiretroviral resistance mutations are routinely characterized by DNA sequencing, treatment failures are still common, probably in part because undetected rare resistance mutations facilitate viral escape. Here we combined DNA bar coding and massively parallel pyrosequencing to quantify rare drug resistance mutations. Using DNA bar coding, we were able to analyze seven viral populations in parallel, overall characterizing 118 093 sequence reads of average length 103 bp. Analysis of a control HIV mixture showed that resistance mutations present as 5% of the population could be readily detected without false positive calls. In three samples of multidrug-resistant HIV populations from patients, all the drug-resistant mutations called by conventional analysis were identified, as well as four additional low abundance drug resistance mutations, some of which would be expected to influence the response to antiretroviral therapy. Methods for sensitive characterization of HIV resistance alleles have been reported, but only the pyrosequencing method allows all the positions at risk for drug resistance mutations to be interrogated deeply for many HIV populations in a single experiment.

Malaria Immunity: The Education of an Unnatural Response.

In this issue of Cell Host & Microbe, Kurup et al. report that infection of the liver by Plasmodium parasites promotes the recruitment of dendritic cells that acquire and present parasite antigen from infected hepatocytes. These cells then prime parasite-specific CD8 T cells in liver-draining lymph nodes.

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

Immunization with attenuated whole Plasmodium sporozoites constitutes a promising vaccination strategy. Compared to replication-deficient parasites, immunization with replication-competent parasites confers better protection and also induces a type I IFN (IFN-1) response, but whether this IFN-1 response has beneficial or adverse effects on vaccine-induced adaptive immunity is not known. Here, we show that IFN-1 signaling-deficient mice immunized with replication-competent sporozoites exhibit superior protection against infection. This correlates with superior CD8 T cell memory including reduced expression of the exhaustion markers PD-1 and LAG-3 on these cells and increased numbers of memory CD8 T cells in the liver. Moreover, the adoptive transfer of memory CD8 T cells from the livers of previously immunized IFN-1 signaling-deficient mice confers greater protection against liver stage parasites. However, the detrimental role of IFN-1 signaling is not CD8 T cell intrinsic. Together, our data demonstrate that liver stage-engendered IFN-1 signaling impairs hepatic CD8 T cell memory via a CD8 T cell-extrinsic mechanism.