Plasmodium-host interactions directly influence the threshold of memory CD8 T cells required for protective immunity

Heterogeneity in killing efficacy of individual effector CD8+ T cells against Plasmodium liver stages

Vaccination strategies in mice inducing high numbers of memory CD8+ T cells specific to a single epitope are able to provide sterilizing protection against infection with Plasmodium sporozoites. We have recently found that Plasmodium-specific CD8+ T cells cluster around sporozoite-infected hepatocytes but whether such clusters are important in elimination of the parasite remains incompletely understood. Here, we used our previously generated data in which we employed intravital microscopy to longitudinally image 32 green fluorescent protein (GFP)-expressing Plasmodium yoelii parasites in livers of mice that had received activated Plasmodium-specific CD8+ T cells after sporozoite infection. We found significant heterogeneity in the dynamics of the normalized GFP signal from the parasites (termed 'vitality index' or VI) that was weakly correlated with the number of T cells near the parasite. We also found that a simple model assuming mass-action, additive killing by T cells well describes the VI dynamics for most parasites and predicts a highly variable killing efficacy by individual T cells. Given our estimated median per capita kill rate of k = 0.031/h we predict that a single T cell is typically incapable of killing a parasite within the 48 h lifespan of the liver stage in mice. Stochastic simulations of T cell clustering and killing of the liver stage also suggested that: (i) three or more T cells per infected hepatocyte are required to ensure sterilizing protection; (ii) both variability in killing efficacy of individual T cells and resistance to killing by individual parasites may contribute to the observed variability in VI decline, and (iii) the stable VI of some clustered parasites cannot be explained by measurement noise. Taken together, our analysis for the first time provides estimates of efficiency at which individual CD8+ T cells eliminate intracellular parasitic infection in vivo.

Memory CD8+ T cell-mediated protection against liver-stage malaria

Nearly half of the world's population is at risk of malaria, a disease caused by the protozoan parasite Plasmodium, which is estimated to cause more than 240,000,000 infections and kill more than 600,000 people annually. The emergence of Plasmodia resistant to chemoprophylactic treatment highlights the urgency to develop more effective vaccines. In this regard, whole sporozoite vaccination approaches in murine models and human challenge studies have provided substantial insight into the immune correlates of protection from malaria. From these studies, CD8+ T cells have come to the forefront, being identified as critical for vaccine-mediated liver-stage immunity that can prevent the establishment of the symptomatic blood stages and subsequent transmission of infection. However, the unique biological characteristics required for CD8+ T cell protection from liver-stage malaria dictate that more work must be done to design effective vaccines. In this review, we will highlight a subset of studies that reveal basic aspects of memory CD8+ T cell-mediated protection from liver-stage malaria infection.

Mathematical Modeling to Guide Experimental Design: T Cell Clustering as a Case Study

Mathematical modeling provides a rigorous way to quantify immunological processes and discriminate between alternative mechanisms driving specific biological phenomena. It is typical that mathematical models of immunological phenomena are developed by modelers to explain specific sets of experimental data after the data have been collected by experimental collaborators. Whether the available data are sufficient to accurately estimate model parameters or to discriminate between alternative models is not typically investigated. While previously collected data may be sufficient to guide development of alternative models and help estimating model parameters, such data often do not allow to discriminate between alternative models. As a case study, we develop a series of power analyses to determine optimal sample sizes that allow for accurate estimation of model parameters and for discrimination between alternative models describing clustering of CD8 T cells around Plasmodium liver stages. In our typical experiments, mice are infected intravenously with Plasmodium sporozoites that invade hepatocytes (liver cells), and then activated CD8 T cells are transferred into the infected mice. The number of T cells found in the vicinity of individual infected hepatocytes at different times after T cell transfer is counted using intravital microscopy. We previously developed a series of mathematical models aimed to explain highly variable number of T cells per parasite; one of such models, the density-dependent recruitment (DDR) model, fitted the data from preliminary experiments better than the alternative models, such as the density-independent exit (DIE) model. Here, we show that the ability to discriminate between these alternative models depends on the number of parasites imaged in the analysis; analysis of about [Formula: see text] parasites at 2, 4, and 8 h after T cell transfer will allow for over 95% probability to select the correct model. The type of data collected also has an impact; following T cell clustering around individual parasites over time (called as longitudinal (LT) data) allows for a more precise and less biased estimates of the parameters of the DDR model than that generated from a more traditional way of imaging individual parasites in different liver areas/mice (cross-sectional (CS) data). However, LT imaging comes at a cost of a need to keep the mice alive under the microscope for hours which may be ethically unacceptable. We finally show that the number of time points at which the measurements are taken also impacts the precision of estimation of DDR model parameters; in particular, measuring T cell clustering at one time point does not allow accurately estimating all parameters of the DDR model. Using our case study, we propose a general framework on how mathematical modeling can be used to guide experimental designs and power analyses of complex biological processes.

Infectious sporozoite challenge modulates radiation attenuated sporozoite vaccine-induced memory CD8+ T cells for better survival characteristics

Radiation attenuated sporozoite (RAS), a whole parasite vaccine approach provides sterile protection against malaria. However, RAS immunization does not confer protection for long, and that has been correlated with the waning parasite-induced memory CD8⁺ T cell responses. Interestingly, an intermittent infectious (wild-type) sporozoite challenge to the RAS vaccinated mice lengthened the protection period from 6 to 18 months. Herein, we have studied the changes that infectious sporozoite brought in RAS-induced memory CD8⁺ T cells for conferring lengthened protection. We observed that the infectious sporozoite challenge has boosted the frequency of foreign antigen-experienced memory CD8⁺ T cells. In those CD8⁺ T cells, it has reduced the Annexin-V reactivity, raised Bcl-2 expression, and also more cells undergone homeostatic proliferation (Ki-67⁺ ). It has also scaled down the frequency of Nur77 and CX3CR1 high expressing cells in those memory CD8⁺ T cell populations which we further correlated with better survival signals. This article is protected by copyright. All rights reserved.

T cell-mediated immunity to malaria

Immunity to malaria has been linked to the availability and function of helper CD4⁺ T cells, cytotoxic CD8⁺ T cells and γδ T cells that can respond to both the asymptomatic liver stage and the symptomatic blood stage of Plasmodium sp. infection. These T cell responses are also thought to be modulated by regulatory T cells. However, the precise mechanisms governing the development and function of Plasmodium-specific T cells and their capacity to form tissue-resident and long-lived memory populations are less well understood. The field has arrived at a point where the push for vaccines that exploit T cell-mediated immunity to malaria has made it imperative to define and reconcile the mechanisms that regulate the development and functions of Plasmodium-specific T cells. Here, we review our current understanding of the mechanisms by which T cell subsets orchestrate host resistance to Plasmodium infection on the basis of observational and mechanistic studies in humans, non-human primates and rodent models. We also examine the potential of new experimental strategies and human infection systems to inform a new generation of approaches to harness T cell responses against malaria.

Using two phases of the CD4 T cell response to blood-stage murine malaria to understand regulation of systemic immunity and placental pathology in Plasmodium falciparum infection

Plasmodium falciparum infection and malaria remain a risk for millions of children and pregnant women. Here, we seek to integrate knowledge of mouse and human T helper cell (Th) responses to blood-stage Plasmodium infection to understand their contribution to protection and pathology. Although there is no complete Th subset differentiation, the adaptive response occurs in two phases in non-lethal rodent Plasmodium infection, coordinated by Th cells. In short, cellular immune responses limit the peak of parasitemia during the first phase; in the second phase, humoral immunity from T cell-dependent germinal centers is critical for complete clearance of rapidly changing parasite. A strong IFN-γ response kills parasite, but an excess of TNF compared with regulatory cytokines (IL-10, TGF-β) can cause immunopathology. This common pathway for pathology is associated with anemia, cerebral malaria, and placental malaria. These two phases can be used to both understand how the host responds to rapidly growing parasite and how it attempts to control immunopathology and variation. This dual nature of T cell immunity to Plasmodium is discussed, with particular reference to the protective nature of the continuous generation of effector T cells, and the unique contribution of effector memory T cells.

Cytotoxic T Cell-Derived Granzyme B Is Increased in Severe Plasmodium Falciparum Malaria

In Plasmodium falciparum malaria, CD8⁺ T cells play a double-edged role. Liver-stage specific CD8⁺ T cells can confer protection, as has been shown in several vaccine studies. Blood-stage specific CD8⁺ T cells, on the other hand, contribute to the development of cerebral malaria in murine models of malaria. The role of CD8⁺ T cells in humans during the blood-stage of P. falciparum remains unclear. As part of a cross-sectional malaria study in Ghana, granzyme B levels and CD8⁺ T cells phenotypes were compared in the peripheral blood of children with complicated malaria, uncomplicated malaria, afebrile but asymptomatically infected children and non-infected children. Granzyme B levels in the plasma were significantly higher in children with febrile malaria than in afebrile children. CD8⁺ T cells were the main T cell subset expressing granzyme B. The proportion of granzyme B⁺ CD8⁺ T cells was significantly higher in children with complicated malaria than in uncomplicated malaria, whereas the activation marker CD38 on CD8⁺ T cells showed similar expression levels. This suggests a pathogenic role of cytotoxic CD8⁺ T cells in the development of malaria complications in humans.

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.

Malaria vaccines in the eradication era: current status and future perspectives

INTRODUCTION

The challenge to eradicate malaria is an enormous task that will not be achieved by current control measures, thus an efficacious and long-lasting malaria vaccine is required. The licensing of RTS, S/AS01 is a step forward in providing some protection, but a malaria vaccine that protects across multiple transmission seasons is still needed. To achieve this, inducing beneficial immune responses while minimising deleterious non-targeted effects will be essential.

AREAS COVERED

This article discusses the current challenges and advances in malaria vaccine development and reviews recent human clinical trials for each stage of infection. Pubmed and ScienceDirect were searched, focusing on cell mediated immunity and how T cell subsets might be targeted in future vaccines using novel adjuvants and emerging vaccine technologies.

EXPERT COMMENTARY

Despite decades of research there is no highly effective licensed malaria vaccine. However, there is cause for optimism as new adjuvants and vaccine systems emerge, and our understanding of correlates of protection increases, especially regarding cellular immunity. The new field of heterologous (non-specific) effects of vaccines also highlights the broader consequences of immunization. Importantly, the WHO led Malaria Vaccine Technology Roadmap illustrates that there is a political will among the global health community to make it happen.

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.

Sendai Virus Mucosal Vaccination Establishes Lung-Resident Memory CD8 T Cell Immunity and Boosts BCG-Primed Protection against TB in Mice

Accumulating evidence has shown the protective role of CD8⁺ T cells in vaccine-induced immunity against Mycobacterium tuberculosis (Mtb) despite controversy over their role in natural immunity. However, the current vaccine BCG is unable to induce sufficient CD8⁺ T cell responses, especially in the lung. Sendai virus, a respiratory RNA virus, is here engineered firstly as a novel recombinant anti-TB vaccine (SeV85AB) that encodes Mtb immuno-dominant antigens, Ag85A and Ag85B. A single mucosal vaccination elicited potent antigen-specific T cell responses and a degree of protection against Mtb challenge similar to the effect of BCG in mice. Depletion of CD8⁺ T cells abrogated the protective immunity afforded by SeV85AB vaccination. Interestingly, only SeV85AB vaccination induced high levels of lung-resident memory CD8⁺ T (T_RM) cells, and this led to a rapid and strong recall of antigen-specific CD8⁺ T cell responses against Mtb challenge infection. Furthermore, when used in a BCG prime-SeV85AB boost strategy, SeV85AB vaccine significantly enhanced protection above that seen after BCG vaccination alone. Our findings suggest that CD8⁺ T_RM cells that arise in lungs responding to this mucosal vaccination might help to protect against TB, and SeV85AB holds notable promise to improve BCG’s protective efficacy in a prime-boost immunization regimen.

Immune Signature of Enhanced Functional Avidity CD8+ T Cells in vivo Induced by Vaccinia Vectored Vaccine

Functional avidity of T cells is a critical determinant for clearing viral infection and eliminating tumor. Understanding how functional avidity is maintained in T cells is imperative for immunotherapy. However, studies systematically characterize T cell with high functional avidity induced in vivo are still lacking. Previously, we and others found vaccinia vectored vaccine (VACV) induced antigen-specific CD8+ T cells with relatively high functional avidity to those from DNA vaccine. Herein, we used functional, immune phenotyping and transcriptomic studies to define the immune signature of these CD8+ T cells with high functional avidity. Antigen-specific CD8+ T cells induced by VACV executed superior in vivo killing activity and displayed a distinct transcriptional profile, whereas no significantly differences were found in composition of memory sub-populations and cytokine poly-functionality. Transcriptional analyses revealed unique features of VACV induced CD8+ T cells in several biological processes, including transport, cell cycle, cell communication and metabolic processes. In summary, we characterize CD8+ T cells of high functional avidity induced in vivo by VACV, which not only improves our understanding of adaptive T cell immunity in VACV vaccination, but also provides clues to modulate functional avidity of CD8+ T cells for T cell based immunotherapy.

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.

Discriminating Protective from Nonprotective Plasmodium-Specific CD8+ T Cell Responses

Despite decades of research, malaria remains a global health crisis. Current subunit vaccine approaches do not provide efficient long-term, sterilizing immunity against Plasmodium infections in humans. Conversely, whole parasite vaccinations with their larger array of target Ags have conferred long-lasting sterilizing protection to humans. Similar studies in rodent models of malaria reveal that CD8(+) T cells play a critical role in liver-stage immunity after whole parasite vaccination. However, it is unknown whether all CD8(+) T cell specificities elicited by whole parasite vaccination contribute to protection, an issue of great relevance for enhanced subunit vaccination. In this article, we show that robust CD8(+) T cell responses of similar phenotype are mounted after prime-boost immunization against Plasmodium berghei glideosome-associated protein 5041-48-, sporozoite-specific protein 20318-325-, thrombospondin-related adhesion protein (TRAP) 130-138-, or circumsporozoite protein (CSP) 252-260-derived epitopes in mice, but only CSP252-260- and TRAP130-138-specific CD8(+) T cells provide sterilizing immunity and reduce liver parasite burden after sporozoite challenge. Further, CD8(+) T cells specific to sporozoite surface-expressed CSP and TRAP proteins, but not intracellular glideosome-associated protein 50 and sporozoite-specific protein 20, efficiently recognize sporozoite-infected hepatocytes in vitro. These results suggest that: 1) protection-relevant antigenic targets, regardless of their immunogenic potential, must be efficiently presented by infected hepatocytes for CD8(+) T cells to eliminate liver-stage Plasmodium infection; and 2) proteins expressed on the surface of sporozoites may be good target Ags for protective CD8(+) T cells.

Plasmodium cellular effector mechanisms and the hepatic microenvironment

Plasmodium falciparum malaria remains one of the most serious health problems globally. Immunization with attenuated parasites elicits multiple cellular effector mechanisms capable of eliminating Plasmodium liver stages. However, malaria liver stage (LS) immunity is complex and the mechanisms effector T cells use to locate the few infected hepatocytes in the large liver in order to kill the intracellular LS parasites remain a mystery to date. Here, we review our current knowledge on the behavior of CD8 effector T cells in the hepatic microvasculature, in malaria and other hepatic infections. Taking into account the unique immunological and lymphogenic properties of the liver, we discuss whether classical granule-mediated cytotoxicity might eliminate infected hepatocytes via direct cell contact or whether cytokines might operate without cell–cell contact and kill Plasmodium LSs at a distance. A thorough understanding of the cellular effector mechanisms that lead to parasite death hence sterile protection is a prerequisite for the development of a successful malaria vaccine to protect the 40% of the world’s population currently at risk of Plasmodium infection.

Correlates of protective immunity following whole sporozoite vaccination against malaria

Human infection with Plasmodium parasites remains a serious global health crisis, leading to more than 600,000 deaths annually. Currently, no licensed vaccine is available to alleviate this malaria disease burden and vaccination with the most advanced antimalarial vaccine candidate, RTS,S, provides limited protection that wanes over time. To date, the only vaccination strategy capable of inducing complete, long-lasting protection in human subjects is administration of attenuated whole sporozoites. Several approaches for vaccination with attenuated whole sporozoites have been clinically tested in humans and include vaccination with radiation or genetically attenuated sporozoites or with virulent sporozoites concurrent with administration of antimalarial drug cover. Rodent studies with these three attenuated whole sporozoite vaccination (WSV) approaches provide insights into the immune correlates of vaccine-induced protection. The majority of these studies have identified a critical role for liver-stage parasite-directed CD8 T cells in providing protection with possible contributions from Plasmodium-specific CD4 T cells or antibodies. Together, rodent and human vaccination studies with attenuated WSV may lead to an understanding of the correlates of protective immunity against malarial disease, and the development of new, highly efficacious vaccines.

Measurement of the T Cell Response to Preerythrocytic Vaccination in Mice

Whole attenuated parasite vaccines designed to elicit immunity against the clinically silent preerythrocytic stage of Plasmodium infection represent the most efficacious experimental platforms currently in clinical trial. Studies in rodents and humans show that T cells mediate vaccine-induced protection. Thus, determining the quantitative and qualitative properties of these T cells remains a major research focus. Most rodent models of preerythrocytic anti-Plasmodium vaccination focus on circumsporozoite-specific CD8 T cell responses in BALB/c mice. However, CD4 T cells and non-circumsporozoite-specific CD8 T cells also significantly contribute to protection. Here we describe alternative approaches that enable detection and functional characterization of total CD8 and CD4 T cell responses induced by preerythrocytic vaccination in mice. These flow cytometry-based approaches rely on monitoring the modulation of expressed integrins and co-receptors on the surface of T cells in vaccinated mice. The approaches enable direct determination of the magnitude, kinetics, distribution, phenotype, and functional features of T cell responses induced by infection or whole-parasite vaccination using any mouse-parasite species combination.

Diet-induced obesity does not impact the generation and maintenance of primary memory CD8 T cells

The extent to which obesity compromises the differentiation and maintenance of protective memory CD8 T cell responses and renders obese individuals susceptible to infection remains unknown. In this study, we show that diet-induced obesity did not impact the maintenance of pre-existing memory CD8 T cells, including acquisition of a long-term memory phenotype (i.e., CD27(hi), CD62L(hi), KLRG1(lo)) and function (i.e., cytokine production, secondary expansion, and memory CD8 T cell-mediated protection). Additionally, obesity did not influence the differentiation and maintenance of newly evoked memory CD8 T cell responses in inbred and outbred hosts generated in response to different types of systemic (LCMV, L. monocytogenes) and/or localized (influenza virus) infections. Interestingly, the rate of naive-to-memory CD8 T cell differentiation after a peptide-coated dendritic cell immunization was similar in lean and obese hosts, suggesting that obesity-associated inflammation, unlike pathogen- or adjuvant-induced inflammation, did not influence the development of endogenous memory CD8 T cell responses. Therefore, our studies reveal that the obese environment does not influence the development or maintenance of memory CD8 T cell responses that are either primed before or after obesity is established, a surprising notion with important implications for future studies aiming to elucidate the role obesity plays in host susceptibility to infections.

CD8 T-cell-mediated protection against liver-stage malaria: lessons from a mouse model

Malaria is a major global health problem, with severe mortality in children living in sub-Saharan Africa, and there is currently no licensed, effective vaccine. However, vaccine-induced protection from Plasmodium infection, the causative agent of malaria, was established for humans in small clinical trials and for rodents in the 1960s. Soon after, a critical role for memory CD8 T cells in vaccine-induced protection against Plasmodium liver-stage infection was established in rodent models and is assumed to apply to humans. However, these seminal early studies have led to only modest advances over the ensuing years in our understanding the basic features of memory CD8 T cells required for protection against liver-stage Plasmodium infection, an issue which has likely impeded the development of effective vaccines for humans. Given the ethical and practical limitations in gaining mechanistic insight from human vaccine and challenge studies, animal models still have an important role in dissecting the basic parameters underlying memory CD8 T-cell immunity to Plasmodium. Here, we will highlight recent data from our own work in the mouse model of Plasmodium infection that identify quantitative and qualitative features of protective memory CD8 T-cell responses. Finally, these lessons will be discussed in the context of recent findings from clinical trials of vaccine-induced protection in controlled human challenge models.

TLR4 ligands lipopolysaccharide and monophosphoryl lipid a differentially regulate effector and memory CD8+ T Cell differentiation

Vaccines formulated with nonreplicating pathogens require adjuvants to help bolster immunogenicity. The role of adjuvants in Ab production has been well studied, but how they influence memory CD8(+) T cell differentiation remains poorly defined. In this study we implemented dendritic cell-mediated immunization to study the effects of commonly used adjuvants, TLR ligands, on effector and memory CD8(+) T cell differentiation in mice. Intriguingly, we found that the TLR4 ligand LPS was far more superior to other TLR ligands in generating memory CD8(+) T cells upon immunization. LPS boosted clonal expansion similar to the other adjuvants, but fewer of the activated CD8(+) T cells died during contraction, generating a larger pool of memory cells. Surprisingly, monophosphoryl lipid A (MPLA), another TLR4 ligand, enhanced clonal expansion of effector CD8(+) T cells, but it also promoted their terminal differentiation and contraction; thus, fewer memory CD8(+) T cells formed, and MPLA-primed animals were less protected against secondary infection compared with those primed with LPS. Furthermore, gene expression profiling revealed that LPS-primed effector cells displayed a stronger pro-memory gene expression signature, whereas the gene expression profile of MPLA-primed effector cells aligned closer with terminal effector CD8(+) T cells. Lastly, we demonstrated that the LPS-TLR4-derived "pro-memory" signals were MyD88, but not Toll/IL-1R domain-containing adapter inducing IFN-β, dependent. This study reveals the influential power of adjuvants on the quantity and quality of CD8(+) T cell memory, and that attention to adjuvant selection is crucial because boosting effector cell expansion may not always equate with more memory T cells or greater protection.

Memory T cells maintain protracted protection against malaria

Immunologic memory is one of the cardinal features of antigen-specific immune responses, and the persistence of memory cells contributes to prophylactic immunizations against infectious agents. Adequately maintained memory T and B cell pools assure a fast, effective and specific response against re-infections. However, many aspects of immunologic memory are still poorly understood, particularly immunologic memory inducible by parasites, for example, Plasmodium spp., the causative agents of malaria. For example, memory responses to Plasmodium antigens amongst residents of malaria endemic areas appear to be either inadequately developed or maintained, because persons who survive episodes of childhood malaria remain vulnerable to intermittent malaria infections. By contrast, multiple exposures of humans and laboratory rodents to radiation-attenuated Plasmodium sporozoites (γ-spz) induce sterile and long-lasting protection against experimental sporozoite challenge. Multifactorial immune mechanisms maintain this protracted and sterile protection. While the presence of memory CD4 T cell subsets has been associated with lasting protection in humans exposed to multiple bites from Anopheles mosquitoes infected with attenuated Plasmodium falciparum, memory CD8 T cells maintain protection induced with Plasmodium yoelii and Plasmodium berghei γ-spz in murine models. In this review, we discuss our observations that show memory CD8 T cells specific for antigens expressed by P. berghei liver stage parasites as an indispensable component for the maintenance of protracted protective immunity against experimental malaria infection; moreover, the provision of an Ag-depot assures a quick recall of memory T cells as IFN-γ-producing effector CD8 T cells and IL-4- producing CD4 T cells that collaborate with B cells for an effective antibody response.

CD8+ T Cell Responses to Plasmodium and Intracellular Parasites

Parasitic protozoa are major threats to human health affecting millions of people around the world. Control of these infections by the host immune system relies on a myriad of immunological mechanisms that includes both humoral and cellular immunity. CD8⁺ T cells contribute to the control of these parasitic infections in both animals and humans. Here, we will focus on the CD8⁺ T cell response against a subset of these protozoa: Plasmodium, Toxoplasma gondii, Leishmania and Trypanosoma cruzi, with an emphasis on experimental rodent systems. It is evident a complex interaction occurs between CD8⁺ T cells and the invading protozoa. A detailed understanding of how CD8⁺ T cells mediate protection should provide the basis for the development of effective vaccines that prevent and control infections by these parasites.

Analysis of lymphocytes in patients with Plasmodium vivax malaria and its relation to the annexin-A1 and IL-10

BACKGROUND

Malaria is the most prevalent parasitic disease in the world. In Brazil, the largest number of malaria cases (98%) is within the Legal Amazon region, where Plasmodium vivax is responsible for over 80% of diagnosed cases. The aim of this study was to investigate the annexin-A1 expression in CD4+, CD8+ T cells, regulatory T cells (Treg) and cytokine IL-10 quantification in plasma from patients with malaria caused by P. vivax.

METHODS

The quantification of the cytokine IL-10 of patients infected with P. vivax and healthy controls were evaluated by enzyme-linked immunosorbent assay (ELISA). The determination of the expression of annexin-A1 in lymphocytes from patients and healthy controls was determined by immunofluorescence staining. All results were correlated with the parasitaemia and the number of previous episodes of malaria.

RESULTS

The cytokine IL-10 plasma levels showed a significant increase in both patients with low (650.4 ± 59.3 pg/mL) and high (2870 ± 185.3 pg/mL) parasitaemia compared to the control (326.1 ± 40.1 pg/mL). In addition, there was an increase of this cytokine in an episode dependent manner (individuals with no previous episodes of malaria--primoinfected: 363.9 ± 31.1 pg/mL; individuals with prior exposure: 659.9 ± 49.4 pg/mL). The quantification of annexin-A1 expression indicated a decrease in CD4+ and CD8+ T cells and an increase in Treg in comparison with the control group. When annexin-A1 expression was compared according to the number of previous episodes of malaria, patients who have been exposed more than once to the parasite was found to have higher levels of CD4+ T cells (96.0 ± 2.5 A.U) compared to primoinfected (50.3 ± 1.7). However, this endogenous protein had higher levels in CD8+ (108.5 ± 3.1) and Treg (87.5 ± 2.5) from patients primoinfected.

CONCLUSION

This study demonstrates that in the patients infected with P. vivax the release of immunoregulatory molecules can be influenced by the parasitaemia level and the number of previous episodes of malaria. annexin-A1 is expressed differently in lymphocyte sub-populations and may have a role in cell proliferation. Furthermore, annexin-A1 may be contributing to IL-10 release in plasma of patients with vivax malaria.

Looking under the skin: the first steps in malarial infection and immunity

Malaria, which is caused by Plasmodium spp., starts with an asymptomatic phase, during which sporozoites, the parasite form that is injected into the skin by a mosquito, develop into merozoites, the form that infects erythrocytes. This pre-erythrocytic phase is still the most enigmatic in the parasite life cycle, but has long been recognized as an attractive vaccination target. In this Review, we present what has been learned in recent years about the natural history of the pre-erythrocytic stages, mainly using intravital imaging in rodents. We also consider how this new knowledge is in turn changing our understanding of the immune response mounted by the host against the pre-erythrocytic forms.

In vivo CD8+ T cell dynamics in the liver of Plasmodium yoelii immunized and infected mice

Plasmodium falciparum malaria remains one of the most serious health problems globally and a protective malaria vaccine is desperately needed. Vaccination with attenuated parasites elicits multiple cellular effector mechanisms that lead to Plasmodium liver stage elimination. While granule-mediated cytotoxicity requires contact between CD8+ effector T cells and infected hepatocytes, cytokine secretion should allow parasite killing over longer distances. To better understand the mechanism of parasite elimination in vivo, we monitored the dynamics of CD8+ T cells in the livers of naïve, immunized and sporozoite-infected mice by intravital microscopy. We found that immunization of BALB/c mice with attenuated P. yoelii 17XNL sporozoites significantly increases the velocity of CD8+ T cells patrolling the hepatic microvasculature from 2.69±0.34 μm/min in naïve mice to 5.74±0.66 μm/min, 9.26±0.92 μm/min, and 7.11±0.73 μm/min in mice immunized with irradiated, early genetically attenuated (Pyuis4-deficient), and late genetically attenuated (Pyfabb/f-deficient) parasites, respectively. Sporozoite infection of immunized mice revealed a 97% and 63% reduction in liver stage density and volume, respectively, compared to naïve controls. To examine cellular mechanisms of immunity in situ, naïve mice were passively immunized with hepatic or splenic CD8+ T cells. Unexpectedly, adoptive transfer rendered the motile CD8+ T cells from immunized mice immotile in the liver of P. yoelii infected mice. Similarly, when mice were simultaneously inoculated with viable sporozoites and CD8+ T cells, velocities 18 h later were also significantly reduced to 0.68±0.10 μm/min, 1.53±0.22 μm/min, and 1.06±0.26 μm/min for CD8+ T cells from mice immunized with irradiated wild type sporozoites, Pyfabb/f-deficient parasites, and P. yoelii CS_280–288 peptide, respectively. Because immobilized CD8+ T cells are unable to make contact with infected hepatocytes, soluble mediators could potentially play a key role in parasite elimination under these experimental conditions.

Pushing the frontiers of T-cell vaccines: accurate measurement of human T-cell responses

There is a need for novel approaches to tackle major vaccine challenges such as malaria, tuberculosis and HIV, among others. Success will require vaccines able to induce a cytotoxic T-cell response--a deficiency of most current vaccine approaches. The successful development of T-cell vaccines faces many hurdles, not least being the lack of consensus on a standardized T-cell assay format able to be used as a correlate of vaccine efficacy. Hence, there remains a need for reproducible measures of T-cell immunity proven in human clinical trials to correlate with vaccine protection. The T-cell equivalent of a neutralizing antibody assay would greatly accelerate the development and commercialization of T-cell vaccines. Recent advances have seen a plethora of new T-cell assays become available, including some like cytometry by time-of-flight with extreme multiparameter T-cell phenotyping capability. However, whether it is historic thymidine-based proliferation assays or sophisticated new cytometry assays, each assay has its relative advantages and disadvantages, and relatively few of these assays have yet to be validated in large-scale human vaccine trials. This review examines the current range of T-cell assays and assesses their suitability for use in human vaccine trials. Should one or more of these assays be accepted as an agreed surrogate of T-cell protection by a regulatory agency, this would significantly accelerate the development of T-cell vaccines.

Identification of targets of CD8⁺ T cell responses to malaria liver stages by genome-wide epitope profiling

CD8⁺ T cells mediate immunity against Plasmodium liver stages. However, the paucity of parasite-specific epitopes of CD8⁺ T cells has limited our current understanding of the mechanisms influencing the generation, maintenance and efficiency of these responses. To identify antigenic epitopes in a stringent murine malaria immunisation model, we performed a systematic profiling of H(2b)-restricted peptides predicted from genome-wide analysis. We describe the identification of Plasmodium berghei (Pb) sporozoite-specific gene 20 (S20)- and thrombospondin-related adhesive protein (TRAP)-derived peptides, termed PbS20₃₁₈ and PbTRAP₁₃₀ respectively, as targets of CD8⁺ T cells from C57BL/6 mice vaccinated by whole parasite strategies known to protect against sporozoite challenge. While both PbS20₃₁₈ and PbTRAP₁₃₀ elicit effector and effector memory phenotypes in both the spleens and livers of immunised mice, only PbTRAP₁₃₀-specific CD8⁺ T cells exhibit in vivo cytotoxicity. Moreover, PbTRAP₁₃₀-specific, but not PbS20₃₁₈-specific, CD8⁺ T cells significantly contribute to inhibition of parasite development. Prime/boost vaccination with PbTRAP demonstrates CD8⁺ T cell-dependent efficacy against sporozoite challenge. We conclude that PbTRAP is an immunodominant antigen during liver-stage infection. Together, our results underscore the presence of CD8⁺ T cells with divergent potencies against distinct Plasmodium liver-stage epitopes. Our identification of antigen-specific CD8⁺ T cells will allow interrogation of the development of immune responses against malaria liver stages.

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

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

The survival of memory CD8 T cells that is mediated by IL-15 correlates with sustained protection against malaria

Ag-specific memory T cell responses elicited by infections or vaccinations are inextricably linked to long-lasting protective immunity. Studies of protective immunity among residents of malaria endemic areas indicate that memory responses to Plasmodium Ags are not adequately developed or maintained, as people who survive episodes of childhood malaria are still vulnerable to either persistent or intermittent malaria infections. In contrast, multiple exposures to radiation-attenuated Plasmodium berghei sporozoites (Pb γ-spz) induce long-lasting protective immunity to experimental sporozoite challenge. We previously demonstrated that sterile protection induced by Pb γ-spz is MHC class I-dependent and CD8 T cells are the key effectors. IFN-γ(+) CD8 T cells that arise in Pb γ-spz-immunized B6 mice are found predominantly in the liver and are sensitive to levels of liver-stage Ag depot and they express CD44(hi)CD62L(lo) markers indicative of effector/effector memory phenotype. The developmentally related central memory CD8 T (TCM) cells express elevated levels of CD122 (IL-15Rβ), which suggests that CD8 TCM cells depend on IL-15 for maintenance. Using IL-15-deficient mice, we demonstrate in this study that although protective immunity is inducible in these mice, protection is short-lived, mainly owing to the inability of CD8 TCM cells to survive in the IL-15-deficient milieu. We present a hypothesis consistent with a model whereby intrahepatic CD8 TCM cells, being maintained by IL-15-mediated survival and basal proliferation, are conscripted into the CD8 effector/effector memory T cell pool during subsequent infections.

Pathogen-specific inflammatory milieux tune the antigen sensitivity of CD8(+) T cells by enhancing T cell receptor signaling

CD8(+) T cells confer host protection through T-cell-receptor (TCR)-mediated recognition of foreign antigens presented by infected cells. Thus, generation of CD8(+) T cell populations with high antigen sensitivity is critical for efficient pathogen clearance. Besides selection of high-affinity TCRs, the molecular mechanisms regulating the antigen sensitivity of CD8(+) T cells remain poorly defined. Herein, we have demonstrated that the antigen sensitivity of effector and memory CD8(+) T cells is dynamically regulated and can be tuned by pathogen-induced inflammatory milieux independently of the selection of cells with higher TCR affinity. Mechanistically, we have demonstrated that the signal-transduction capacity of key TCR proximal molecules is enhanced by inflammatory cytokines, which reduced the antigen density required to trigger antimicrobial functions. Dynamic tuning of CD8(+) T cell antigen sensitivity by inflammatory cytokines most likely optimizes immunity to specific pathogens while minimizing the risk of immunopathology at steady state.

Memory CD8 T cells specific for plasmodia liver-stage antigens maintain protracted protection against malaria

Immunologic memory induced by pathogenic agents or vaccinations is inextricably linked to long-lasting protection. Adequately maintained memory T and B cell pools assure a fast, effective, and specific response against re-infections. Studies of immune responses amongst residents of malaria endemic areas suggest that memory responses to Plasmodia antigens appear to be neither adequately developed nor maintained, because persons who survive episodes of childhood malaria remain vulnerable to persistent or intermittent malaria infections. By contrast, multiple exposures of humans and laboratory rodents to radiation-attenuated Plasmodia sporozoites (γ-spz) induces sterile and long-lasting protection against experimental sporozoite challenge. Protection is associated with MHC-class I-dependent CD8 T cells, the key effectors against pre-erythrocytic stage infection. We have adopted the P. berghei γ-spz mouse model to study memory CD8 T cells that are specific for antigens expressed by Pb liver-stage (LS) parasites and are found predominantly in the liver. On the basis of phenotypic and functional characteristics, we have demonstrated that liver CD8 T cells form two subsets: CD44^hiCD62L^loKLRG-1⁺CD107⁺CD127⁻CD122^loCD8 T effector/effector memory (T_E/EM) cells that are the dominant IFN-γ producers and CD44^hiCD62L^hiKLRG-1⁻CD107⁻CD127⁺CD122^hiCD8 T central memory (T_CM) cells. In this review, we discuss our observations concerning the role of CD8 T_E/EM and CD8 T_CM cells in the maintenance of protracted protective immunity against experimental malaria infection. Finally, we present a hypothesis consistent with a model whereby intrahepatic CD8 T_CM cells, that are maintained in part by LS-Ag depot and by IL-15-mediated survival and homeostatic proliferation, form a reservoir of cells ready for conscription to CD8 T_E/EM cells needed to prevent re-infections.

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

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

T cell control of malaria pathogenesis

Transmission of Plasmodium from mosquito to the mammalian host leads to a clinically silent pre-erythrocytic stage of malaria infection, and subsequent cyclical erythrocytic invasion associated with disease. Recent evidence demonstrates that it is the interplay between CD4+ and CD8+ T cells, and the regulation of their response, throughout infection that dictates immunity and the pathogenesis of malaria. The elicited T cell response is context dependent, influenced by diverse host and parasite factors, necessitating the development of a unifying model of T cell potential during Plasmodium infection. Only then can we predict their capacity to dictate the outcome of human disease.