CSP--a model for in vivo presentation of Plasmodium berghei sporozoite antigens by hepatocytes

Killing the competition: a theoretical framework for liver-stage malaria

The first stage of malaria infections takes place inside the host's hepatocytes. Remarkably, Plasmodium parasites do not infect hepatocytes immediately after reaching the liver. Instead, they migrate through several hepatocytes before infecting their definitive host cells, thus increasing their chances of immune destruction. Considering that malaria can proceed normally without cell traversal, this is indeed a puzzling behaviour. In fact, the role of hepatocyte traversal remains unknown to date, implying that the current understanding of malaria is incomplete. In this work, we hypothesize that the parasites traverse hepatocytes to actively trigger an immune response in the host. This behaviour would be part of a strategy of superinfection exclusion aimed to reduce intraspecific competition during the blood stage of the infection. Based on this hypothesis, we formulate a comprehensive theory of liver-stage malaria that integrates all the available knowledge about the infection. The interest of this new paradigm is not merely theoretical. It highlights major issues in the current empirical approach to the study of Plasmodium and suggests new strategies to fight malaria.

Low immunogenicity of malaria pre-erythrocytic stages can be overcome by vaccination

Immunogenicity is considered one important criterion for progression of candidate vaccines to further clinical evaluation. We tested this assumption in an infection and vaccination model for malaria pre-erythrocytic stages. We engineered Plasmodium berghei parasites that harbour a well-characterised epitope for stimulation of CD8+ T cells, either as an antigen in the sporozoite surface-expressed circumsporozoite protein or the parasitophorous vacuole membrane associated protein upregulated in sporozoites 4 (UIS4) expressed in exo-erythrocytic forms (EEFs). We show that the antigen origin results in profound differences in immunogenicity with a sporozoite antigen eliciting robust, superior antigen-specific CD8+ T-cell responses, whilst an EEF antigen evokes poor responses. Despite their contrasting immunogenic properties, both sporozoite and EEF antigens gain access to antigen presentation pathways in hepatocytes, as recognition and targeting by vaccine-induced effector CD8+ T cells results in high levels of protection when targeting either antigen. Our study is the first demonstration that poorly immunogenic EEF antigens do not preclude their susceptibility to antigen-specific CD8+ T-cell killing, which has wide-ranging implications on antigen prioritisation for next-generation pre-erythrocytic malaria vaccines.

The unique immune microenvironment of liver metastases: Challenges and opportunities

Liver metastases from gastrointestinal and non-gastrointestinal malignancies remain a major cause of cancer-related mortality and a major clinical challenge. The liver has unique properties that facilitate metastatic expansion, including a complex immune system that evolved to dampen immunity to neoantigens entering the liver from the gut, through the portal circulation. In this review, we describe the unique microenvironment encountered by cancer cells in the liver, focusing on elements of the innate and adaptive immune response that can act as a double-edge sword, contributing to the elimination of cancer cells on the one hand and promoting their survival and growth, on the other. We discuss this microenvironment in a clinical context, particularly for colorectal carcinoma, and highlight how a better understanding of the role of the microenvironment has spurred an intense effort to develop novel and innovative strategies for targeting liver metastatic disease, some of which are currently being tested in the clinic.

Identification of Plasmodium falciparum circumsporozoite protein-specific CD8+ T cell epitopes in a malaria exposed population

BACKGROUND

Sterile protection against malaria, most likely mediated by parasite-specific CD8+ T cells, has been achieved by attenuated sporozoite vaccination of animals as well as malaria-naïve and malaria-exposed subjects. The circumsporozoite protein (CSP)-based vaccine, RTS,S, shows low efficacy partly due to limited CD8+ T cell induction, and inclusion of such epitopes could improve RTS,S. This study assessed 8-10mer CSP peptide epitopes, present in predicted or previously positive P. falciparum 3D7 CSP 15mer overlapping peptide pools, for their ability to induce CD8+ T cell IFN-γ responses in natural malaria-exposed subjects.

METHODS

Cryopreserved PBMCs from nine HLA-typed subjects were stimulated with 23 8-10mer CSP peptides from the 3D7 parasite in IFN-ɣ ELISpot assays. The CD8+ T cell specificity of IFN-γ responses was confirmed in ELISpot assays using CD8+ T cell-enriched PBMC fractions after CD4+ cell depletion.

RESULTS

Ten of 23 peptide epitopes elicited responses in whole PBMCs from five of the nine subjects. Four peptides tested positive in CD8+ T cell-enriched PBMCs from two previously positive responders and one new subject. All four immunodominant peptides are restricted by globally common HLA supertypes (A02, A03, B07) and mapped to regions of the CSP antigen with limited or no reported polymorphism. Association of these peptide-specific responses with anti-malarial protection remains to be confirmed.

CONCLUSIONS

The relatively conserved nature of the four identified epitopes and their binding to globally common HLA supertypes makes them good candidates for inclusion in potential multi-epitope malaria vaccines.

Prime-and-Trap Malaria Vaccination To Generate Protective CD8+ Liver-Resident Memory T Cells

Tissue-resident memory CD8⁺ T (Trm) cells in the liver are critical for long-term protection against pre-erythrocytic Plasmodium infection. Such protection can usually be induced with three to five doses of i.v. administered radiation-attenuated sporozoites (RAS). To simplify and accelerate vaccination, we tested a DNA vaccine designed to induce potent T cell responses against the SYVPSAEQI epitope of Plasmodium yoelii circumsporozoite protein. In a heterologous "prime-and-trap" regimen, priming using gene gun-administered DNA and boosting with one dose of RAS attracted expanding Ag-specific CD8⁺ T cell populations to the liver, where they became Trm cells. Vaccinated in this manner, BALB/c mice were completely protected against challenge, an outcome not reliably achieved following one dose of RAS or following DNA-only vaccination. This study demonstrates that the combination of CD8⁺ T cell priming by DNA and boosting with liver-homing RAS enhances formation of a completely protective liver Trm cell response and suggests novel approaches for enhancing T cell-based pre-erythrocytic malaria vaccines.

Immune Responses in the Liver

The liver is a key, frontline immune tissue. Ideally positioned to detect pathogens entering the body via the gut, the liver appears designed to detect, capture, and clear bacteria, viruses, and macromolecules. Containing the largest collection of phagocytic cells in the body, this organ is an important barrier between us and the outside world. Importantly, as portal blood also transports a large number of foreign but harmless molecules (e.g., food antigens), the liver's default immune status is anti-inflammatory or immunotolerant; however, under appropriate conditions, the liver is able to mount a rapid and robust immune response. This balance between immunity and tolerance is essential to liver function. Excessive inflammation in the absence of infection leads to sterile liver injury, tissue damage, and remodeling; insufficient immunity allows for chronic infection and cancer. Dynamic interactions between the numerous populations of immune cells in the liver are key to maintaining this balance and overall tissue health.

Tissue-specific cellular immune responses to malaria pre-erythrocytic stages

Complete and long-lasting protective immunity against malaria can be achieved through vaccination with invasive live attenuated Plasmodium sporozoites, the motile stage inoculated in the host skin during a mosquito bite. Protective immunity relies primarily on effector CD8⁺ T cells targeting the parasite in the liver. Understanding the tissue-specific features of the immune response is emerging as a vital requirement for understanding protective immunity. The small parasite inoculum, the scarcity of infected cells and the tolerogenic properties of the liver represent hurdles for the establishment of protective immunity in endemic areas. In this review, we discuss recent advances on liver-specific features of immunity including innate recognition of malaria pre-erythrocytic stages, CD8⁺ T cell interactions with infected hepatocytes, antigen presentation for effective CD8⁺ T cell responses and generation of liver-resident memory CD8⁺ T cells. A better understanding of the factors involved in the induction and maintenance of effector CD8⁺ T cell immunity against malaria pre-erythrocytic stages is crucial for the development of an effective vaccine targeting the initial phase of malaria infection.

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

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

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.

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

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

Defining rules of CD8(+) T cell expansion against pre-erythrocytic Plasmodium antigens in sporozoite-immunized mice

BACKGROUND

Whole Plasmodium sporozoites serve as both experimental tools and potentially as deployable vaccines in the fight against malaria infection. Live sporozoites infect hepatocytes and induce a diverse repertoire of CD8(+) T cell responses, some of which are capable of killing Plasmodium-infected hepatocytes. Previous studies in Plasmodium yoelii-immunized BALB/c mice showed that some CD8(+) T cell responses expanded with repeated parasite exposure, whereas other responses did not.

RESULTS

Here, similar outcomes were observed using known Plasmodium berghei epitopes in C57BL/6 mice. With the exception of the response to PbTRAP, IFNγ-producing T cell responses to most studied antigens, such as PbGAP50, failed to re-expand in mice immunized with two doses of irradiated P. berghei sporozoites. In an effort to boost secondary CD8(+) T cell responses, heterologous cross-species immunizations were performed. Alignment of P. yoelii 17XNL and P. berghei ANKA proteins revealed that >60 % of the amino acids in syntenic orthologous proteins are continuously homologous in fragments ≥8-amino acids long, suggesting that cross-species immunization could potentially trigger responses to a large number of common Class I epitopes. Heterologous immunization resulted in a larger liver burden than homologous immunization. Amongst seven tested antigen-specific responses, only CSP- and TRAP-specific CD8(+) T cell responses were expanded by secondary homologous sporozoite immunization and only those to the L3 ribosomal protein and S20 could be re-expanded by heterologous immunization. In general, heterologous late-arresting, genetically attenuated sporozoites were better at secondarily expanding L3-specific responses than were irradiated sporozoites. GAP50 and several other antigens shared between P. berghei and P. yoelii induced a large number of IFNγ-positive T cells during primary immunization, yet these responses could not be re-expanded by either homologous or heterologous secondary immunization.

CONCLUSIONS

These studies highlight how responses to different sporozoite antigens can markedly differ in recall following repeated sporozoite vaccinations. Cross-species immunization broadens the secondary response to sporozoites and may represent a novel strategy for candidate antigen discovery.

TAP-mediated processing of exoerythrocytic antigens is essential for protection induced with radiation-attenuated Plasmodium sporozoites

MHC class I dependent CD8(+) T cells are essential for protection induced by radiation-attenuated Plasmodium sporozoites (RAS) in murine malaria models. Apart from the mechanism of activation of CD8(+) T cells specific for the circumsporozoite protein, the major sporozoite antigen (Ag), CD8(+) T cells specific for other exoerythrocytic Ags that have been shown to mediate protection have not been thoroughly investigated. Specifically, mechanisms of processing and presentation of exoerythrocytic Ags, which includes liver stage (LS) Ags, remain poorly understood. We hypothesize that as exogenous proteins, LS Ags are processed by mechanisms involving either the TAP-dependent phagosomal-to-cytosol or TAP-independent vacuolar pathway of cross-presentation. We used TAP-deficient mice to investigate whether LS Ag mediated induction of naïve CD8(+) T cells and their recall during sporozoite challenge occur by the TAP-dependent or TAP-independent pathways. On the basis of functional attributes, CD8(+) T cells were activated via the TAP-independent pathway during immunizations with Plasmodium berghei RAS; however, IFN-γ(+) CD8(+) T cells previously induced by P. berghei RAS in TAP-deficient mice failed to be recalled against sporozoite challenge and the mice became parasitemic. On the basis of these observations, we propose that TAP-associated Ag processing is indispensable for sterile protection induced with P. berghei RAS.

Tailoring a Combination Preerythrocytic Malaria Vaccine

The leading malaria vaccine candidate, RTS,S, based on the Plasmodium falciparum circumsporozoite protein (CSP), will likely be the first publicly adopted malaria vaccine. However, this and other subunit vaccines, such as virus-vectored thrombospondin-related adhesive protein (TRAP), provide only intermediate to low levels of protection. In this study, the Plasmodium berghei homologues of antigens CSP and TRAP are combined. TRAP is delivered using adenovirus- and vaccinia virus-based vectors in a prime-boost regime. Initially, CSP is also delivered using these viral vectors; however, a reduction of anti-CSP antibodies is seen when combined with virus-vectored TRAP, and the combination is no more protective than either subunit vaccine alone. Using an adenovirus-CSP prime, protein-CSP boost regime, however, increases anti-CSP antibody titers by an order of magnitude, which is maintained when combined with virus-vectored TRAP. This combination regime using protein CSP provided 100% protection in C57BL/6 mice compared to no protection using virus-vectored TRAP alone and 40% protection using adenovirus-CSP prime and protein-CSP boost alone. This suggests that a combination of CSP and TRAP subunit vaccines could enhance protection against malaria.

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.

Mechanisms of stage-transcending protection following immunization of mice with late liver stage-arresting genetically attenuated malaria parasites

Malaria, caused by Plasmodium parasite infection, continues to be one of the leading causes of worldwide morbidity and mortality. Development of an effective vaccine has been encumbered by the complex life cycle of the parasite that has distinct pre-erythrocytic and erythrocytic stages of infection in the mammalian host. Historically, malaria vaccine development efforts have targeted each stage in isolation. An ideal vaccine, however, would target multiple life cycle stages with multiple arms of the immune system and be capable of eliminating initial infection in the liver, the subsequent blood stage infection, and would prevent further parasite transmission. We have previously shown that immunization of mice with Plasmodium yoelii genetically attenuated parasites (GAP) that arrest late in liver stage development elicits stage-transcending protection against both a sporozoite challenge and a direct blood stage challenge. Here, we show that this immunization strategy engenders both T- and B-cell responses that are essential for stage-transcending protection, but the relative importance of each is determined by the host genetic background. Furthermore, potent anti-blood stage antibodies elicited after GAP immunization rely heavily on FC-mediated functions including complement fixation and FC receptor binding. These protective antibodies recognize the merozoite surface but do not appear to recognize the immunodominant merozoite surface protein-1. The antigen(s) targeted by stage-transcending immunity are present in both the late liver stages and blood stage parasites. The data clearly show that GAP-engendered protective immune responses can target shared antigens of pre-erythrocytic and erythrocytic parasite life cycle stages. As such, this model constitutes a powerful tool to identify novel, protective and stage-transcending T and B cell targets for incorporation into a multi-stage subunit vaccine.

Priming of CD8(+) T Cell Responses to Liver Stage Malaria Parasite Antigens

While the role of malaria parasite-specific memory CD8⁺ T cells in the control of exo-erythrocytic stages of malaria infection is well documented and generally accepted, a debate is still ongoing regarding both the identity of the anatomic site where the activation of naive pathogen-specific T cells is taking place and contribution of different antigen-presenting cells (APCs) into this process. Whereas some studies infer a role of professional APCs present in the lymph nodes draining the site of parasite injection by the mosquito, others argue in favor of the liver as a primary organ and hepatocytes as stimulators of naïve parasite-specific T cell responses. This review aims to critically analyze the current knowledge and outline new lines of research necessary to understand the induction of protective cellular immunity against the malaria parasite.

Tissue signatures influence the activation of intrahepatic CD8(+) T cells against malaria sporozoites

Plasmodium sporozoites and liver stages express antigens that are targeted to the MHC-Class I antigen-processing pathway. After the introduction of Plasmodium sporozoites by Anopheles mosquitoes, bone marrow-derived dendritic cells in skin-draining lymph nodes are the first cells to cross-present parasite antigens and elicit specific CD8⁺ T cells. One of these antigens is the immunodominant circumsporozoite protein (CSP). The CD8⁺ T cell-mediated protective immune response against CSP is dependent on the interleukin loop involving IL-4 receptor expression on CD8⁺ cells and IL-4 secretion by CD4⁺ T cell helpers. In a few days, these CD8⁺ T cells re-circulate to secondary lymphoid organs and the liver. In the liver, the hepatic sinusoids are enriched with cells, such as dendritic, sinusoidal endothelial and Kupffer cells, that are able to cross-present MHC class I antigens to intrahepatic CD8⁺ T cells. Specific CD8⁺ T cells actively find infected hepatocytes and target intra-cellular parasites through mechanisms that are both interferon-γ-dependent and -independent. Immunity is mediated by CD8⁺ T effector or effector-memory cells and, when present in high numbers, these cells can provide sterilizing immunity. Human vaccination trials with recombinant formulations or attenuated sporozoites have yet to achieve the high numbers of specific effector T cells that are required for sterilizing immunity. In spite of the limited number of specific CD8⁺ T cells, attenuated sporozoites provided multiple times by the endovenous route provided a high degree of protective immunity. These observations highlight that CD8⁺ T cells may be useful for improving antibody-mediated protective immunity to pre-erythrocytic stages of malaria parasites.

Depletion of regulatory T cells augments a vaccine-induced T effector cell response against the liver-stage of malaria but fails to increase memory

Regulatory T cells (T_reg) have been shown to restrict vaccine-induced T cell responses in different experimental models. In these studies CD4⁺CD25⁺ T_reg were depleted using monoclonal antibodies against CD25, which might also interfere with CD25 on non-regulatory T cell populations and would have no effect on Foxp3⁺CD25⁻ T_reg. To obtain more insights in the specific function of T_reg during vaccination we used mice that are transgenic for a bacterial artificial chromosome expressing a diphtheria toxin (DT) receptor-eGFP fusion protein under the control of the foxp3 gene locus (depletion of regulatory T cell mice; DEREG). As an experimental vaccine-carrier recombinant Bordetella adenylate cyclase toxoid fused with a MHC-class I-restricted epitope of the circumsporozoite protein (ACT-CSP) of Plasmodium berghei (Pb) was used. ACT-CSP was shown by us previously to introduce the CD8⁺ epitope of Pb-CSP into the MHC class I presentation pathway of professional antigen-presenting cells (APC). Using this system we demonstrate here that the number of CSP-specific T cells increases when T_reg are depleted during prime but also during boost immunization. Importantly, despite this increase of T effector cells no difference in the number of antigen-specific memory cells was observed.

Antigen export during liver infection of the malaria parasite augments protective immunity

Protective immunity against preerythrocytic malaria parasite infection is difficult to achieve. Intracellular Plasmodium parasites likely minimize antigen presentation by surface-expressed major histocompatibility complex class I (MHC-I) molecules on infected cells, yet they actively remodel their host cells by export of parasite factors. Whether exported liver-stage proteins constitute better candidates for MHC-I antigen presentation to CD8⁺ T lymphocytes remains unknown. Here, we systematically characterized the contribution of protein export to the magnitude of antigen-specific T-cell responses against Plasmodium berghei liver-stage parasites in C57BL/6 mice. We generated transgenic sporozoites that secrete a truncated ovalbumin (OVA) surrogate antigen only in the presence of an amino-terminal protein export element. Immunization with live attenuated transgenic sporozoites revealed that antigen export was not critical for CD8⁺ T-cell priming but enhanced CD8⁺ T-cell proliferation in the liver. Upon transfer of antigen-specific CD8⁺ T cells, liver-stage parasites secreting the target protein were eliminated more efficiently. We conclude that Plasmodium parasites strictly control protein export during liver infection to minimize immune recognition. Strategies that enhance the discharge of parasite proteins into infected hepatocytes could improve the efficacy of candidate preerythrocytic malaria vaccines.

Vaccine development against Plasmodium parasites remains a priority in malaria research. The most advanced malaria subunit vaccine candidates contain Plasmodium surface proteins with important roles for parasite vital functions. A fundamental question is whether recognition by effector CD8⁺ T cells is restricted to sporozoite surface antigens or extends to parasite proteins that are synthesized during the extensive parasite expansion phase in the liver. Using a surrogate model antigen, we found that a cytoplasmic antigen is able to induce robust protective CD8⁺ T-cell responses, but protein export further enhances immunogenicity and protection. Our results show that a cytoplasmic localization does not exclude a protein’s candidacy for malaria subunit vaccines and that protein secretion can enhance protective immunity.

Immunobiology of Plasmodium in liver and brain

Malaria remains one of the most serious health problems globally, but our understanding of the biology of the parasite and the pathogenesis of severe disease is still limited. Multiple cellular effector mechanisms that mediate parasite elimination from the liver have been described, but how effector cells use classical granule-mediated cytotoxicity to attack infected hepatocytes and how cytokines and chemokines spread via the unique fluid pathways of the liver to reach the parasites over considerable distances remains unknown. Similarly, a wealth of information on cerebral malaria (CM), one of the most severe manifestations of the disease, was gained from post-mortem analyses of human brain and murine disease models, but the cellular processes that ultimately cause disease are not fully understood. Here, we discuss how imaging of the local dynamics of parasite infection and host response as well as consideration of anatomical and physiological features of liver and brain can provide a better understanding of the initial asymptomatic hepatic phase of the infection and the cascade of events leading to CM. Given the increasing drug resistance of both parasite and vector and the unavailability of a protective vaccine, the urgency to reduce the tremendous morbidity and mortality associated with severe malaria is obvious.

Dynamics of the major histocompatibility complex class I processing and presentation pathway in the course of malaria parasite development in human hepatocytes: implications for vaccine development

Control of parasite replication exerted by MHC class I restricted CD8+ T-cells in the liver is critical for vaccination-induced protection against malaria. While many intracellular pathogens subvert the MHC class I presentation machinery, its functionality in the course of malaria replication in hepatocytes has not been characterized. Using experimental systems based on specific identification, isolation and analysis of human hepatocytes infected with P. berghei ANKA GFP or P. falciparum 3D7 GFP sporozoites we demonstrated that molecular components of the MHC class I pathway exhibit largely unaltered expression in malaria-infected hepatocytes until very late stages of parasite development. Furthermore, infected cells showed no obvious defects in their capacity to upregulate expression of different molecular components of the MHC class I machinery in response to pro-inflammatory lymphokines or trigger direct activation of allo-specific or peptide-specific human CD8+ T-cells. We further demonstrate that ectopic expression of circumsporozoite protein does not alter expression of critical genes of the MHC class I pathway and its response to pro-inflammatory cytokines. In addition, we identified supra-cellular structures, which arose at late stages of parasite replication, possessed the characteristic morphology of merosomes and exhibited nearly complete loss of surface MHC class I expression. These data have multiple implications for our understanding of natural T-cell immunity against malaria and may promote development of novel, efficient anti-malaria vaccines overcoming immune escape of the parasite in the liver.

Immune surveillance by the liver

Receiving both portal vein blood and arterial blood, the liver is an important and critical component in the defense against blood-borne infection. To accomplish this role, the liver contains numerous innate and adaptive immune cells that specialize in detection and capture of pathogens from the blood. Further, these immune cells participate in coordinated immune responses leading to pathogen clearance, leukocyte recruitment and antigen presentation to lymphocytes within the vasculature. Finally, this role in host defense must be tightly regulated to ensure that inappropriate immune responses are not raised against nonpathogenic exogenous blood-borne molecules, such as those derived from food. It is this balance between activation and tolerance that characterizes the liver as a frontline immunological organ.

A T-cell response to a liver-stage Plasmodium antigen is not boosted by repeated sporozoite immunizations

Development of an antimalarial subunit vaccine inducing protective cytotoxic T lymphocyte (CTL)-mediated immunity could pave the way for malaria eradication. Experimental immunization with sporozoites induces this type of protective response, but the extremely large number of proteins expressed by Plasmodium parasites has so far prohibited the identification of sufficient discrete T-cell antigens to develop subunit vaccines that produce sterile immunity. Here, using mice singly immunized with Plasmodium yoelii sporozoites and high-throughput screening, we identified a unique CTL response against the parasite ribosomal L3 protein. Unlike CTL responses to the circumsporozoite protein (CSP), the population of L3-specific CTLs was not expanded by multiple sporozoite immunizations. CSP is abundant in the sporozoite itself, whereas L3 expression does not increase until the liver stage. The response induced by a single immunization with sporozoites reduces the parasite load in the liver so greatly during subsequent immunizations that L3-specific responses are only generated during the primary exposure. Functional L3-specific CTLs can, however, be expanded by heterologous prime-boost regimens. Thus, although repeat sporozoite immunization expands responses to preformed antigens like CSP that are present in the sporozoite itself, this immunization strategy may not expand CTLs targeting parasite proteins that are synthesized later. Heterologous strategies may be needed to increase CTL responses across the entire spectrum of Plasmodium liver-stage proteins.