Unraveling the role of natural killer cells in leishmaniasis

NK cells are known as frontline responders that are efficient in combating several maladies as well as leishmaniasis caused by Leishmania spp. As such they are being investigated to be used for adoptive transfer therapy and vaccine. In spite of the lack of antigen-specific receptors at their surface, NK cells can selectively recognize pathogens, accomplished by the activation of the receptors on the NK cell surface and also as the result of their effector functions. Activation of NK cells can occur through interaction between TLR-2 expressed on NK cells and. LPG of Leishmania parasites. In addition, NK cell activation can occur by cytokines (e.g., IFN-γ and IL-12) that also lead to producing cytokines and chemokines and lysis of target cells. This review summarizes several evidences that support NK cells activation for controlling leishmaniasis and the potentially lucrative roles of NK cells during leishmaniasis. Furthermore, we discuss strategies of Leishmania parasites in inhibiting NK cell functions. Leishmania LPG can utilizes TLR2 to evade host-immune responses. Also, Leishmania GP63 can directly binds to NK cells and modulates NK cell phenotype. Finally, this review analyzes the potentialities to harness NK cells effectiveness in therapy regimens and vaccinations.

Involvement of TIGIT in Natural Killer Cell Exhaustion and Immune Escape in Patients and Mouse Model With Liver Echinococcus multilocularis Infection

BACKGROUND AND AIMS

Alveolar echinococcosis (AE) is a lethal helminthic liver disease caused by persistent infection with Echinococcus multilocularis. Although more attention has been paid to the immunotolerance of T cells caused by E. multilocularis infection, the role of natural killer (NK) cell, a critical player in liver immunity, is seldom studied.

APPROACH AND RESULTS

Here, we observed that NK cells from the blood and closed liver tissue (CLT) of AE patients expressed a higher level of inhibitory receptor TIGIT and were functionally exhausted with a lower expression of granzyme B, perforin, interferon-gamma (IFN-γ), and TNF-α. Addition of anti-TIGIT (T-cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domain) monoclonal antibody into AE patients' peripheral blood mononuclear cell culture significantly enhanced the synthesis of IFN-γ and TNF-α by NK cells, indicating the reversion of exhausted NK cells by TIGIT blockade. In the mouse model of E. multilocularis infection, liver and splenic TIGIT⁺ NK cells progressively increased dependent of infection dosage and timing and were less activated and less degranulated with lower cytokine secretion. Furthermore, TIGIT deficiency or blockade in vivo inhibited liver metacestode growth, reduced liver injury, and increased the level of IFN-γ produced by liver NK cells. Interestingly, NK cells from mice with persistent chronic infection expressed a higher level of TIGIT compared to self-healing mice. To look further into the mechanisms, more regulatory CD56^bright and murine CD49a⁺ NK cells with higher TIGIT expression existed in livers of AE patients and mice infected with E. multilocularis, respectively. They coexpressed higher surface programmed death ligand 1 and secreted more IL-10, two strong inducers to mediate the functional exhaustion of NK cells.

CONCLUSIONS

Our results indicate that inhibitory receptor TIGIT is involved in NK cell exhaustion and immune escape from E. multilocularis infection.

Host-targeted Interventions as an Exciting Opportunity to Combat Malaria

Terminal and benign diseases alike in adults, children, pregnant women, and others are successfully treated by pharmacological inhibitors that target human enzymes. Despite extensive global efforts to fight malaria, the disease continues to be a massive worldwide health burden, and new interventional strategies are needed. Current drugs and vector control strategies have contributed to the reduction in malaria deaths over the past 10 years, but progress toward eradication has waned in recent years. Resistance to antimalarial drugs is a substantial and growing problem. Moreover, targeting dormant forms of the malaria parasite Plasmodium vivax is only possible with two approved drugs, which are both contraindicated for individuals with glucose-6-phosphate dehydrogenase deficiency and in pregnant women. Plasmodium parasites are obligate intracellular parasites and thus have specific and absolute requirements of their hosts. Growing evidence has described these host necessities, paving the way for opportunities to pharmacologically target host factors to eliminate Plasmodium infection. Here, we describe progress in malaria research and adjacent fields and discuss key challenges that remain in implementing host-directed therapy against malaria.

Characterization of the Gene Expression Patterns in the Murine Liver Following Intramuscular Administration of Baculovirus

Intramuscular administration of wild-type baculovirus is able to both protect against Plasmodium sporozoite challenge and eliminate liver-stage parasites via a Toll-like receptor 9-independent pathway. To investigate its effector mechanism(s), the gene expression profile in the liver of baculovirus-administered mice was characterized by cDNA microarray analysis. The ingenuity pathway analysis gene ontology module revealed that the major gene subsets induced by baculovirus were immune-related signaling, such as interferon signaling. A total of 40 genes commonly upregulated in a Toll-like receptor 9-independent manner were included as possible candidates for parasite elimination. This gene subset consisted of NT5C3, LOC105246895, BTC, APOL9a/b, G3BP3, SLC6A6, USP25, TRIM14, and PSMB8 as the top 10 candidates according to the special unit. These findings provide new insight into effector molecules responsible for liver-stage parasite killing and, possibly, the development of a new baculovirus-mediated prophylactic and therapeutic biopharmaceutical for malaria.

Intestinal transcriptomes in Kazakh sheep with different haplotypes after experimental Echinococcus granulosus infection

Cystic echinococcosis (CE) is a chronic zoonosis caused by infection with the larval stage of the cestode Echinococcus granulosus. As the intermediate host, sheep are highly susceptible to this disease. Our previous studies have shown that sheep with haplotype MHC Mva Ibc-Sac IIab-Hin1I ab were resistant to CE infection, while their counterparts without this haplotype were not. In order to reveal the molecular mechanism of resistance in Kazakh sheep, after selecting the differential miRNA in our previous study, herein, transcriptome analyses were conducted to detect the differential expression genes in the intestinal tissue of Kazakh sheep with resistant and non-resistant MHC haplotypes, after peroral infection with E. granulosus eggs. A total of 3835 differentially expressed genes were identified between the two groups, with 2229 upregulated and 1606 downregulated. Further function analysis showed that the most significant genes were related to both innate immune response and adaptive response participating in the defense against E. granulosus infection and the metabolic changes associated with it. The results suggest that genes related to lectin receptors, NK cells activation, chemokines, and tumor necrosis factor, may play important roles in the response of intestinal tissue to E. granulosus.

Plasmodium yoelii Uses a TLR3-Dependent Pathway to Achieve Mammalian Host Parasitism

Malaria is associated with complicated immunopathogenesis. In this study, we provide evidence for an unexpected role of TLR3 in promoting the establishment of Plasmodium yoelii infection through delayed clearance of parasitemia in wild type C57BL/6jRj (B6) compared with TLR3 knockout mice. In this study, we confirmed an increased expression of Tlr3, Trif, Tbk1, and Irf7/Irf3 in the liver 42 h postinfection and the initiation of an early burst of proinflammatory response such as Ifng, NF-kB, and Tnfa in B6 mice that may promote parasite fitness. Interestingly, in the absence of TLR3, we showed the involvement of high IFN-γ and lower type I IFN response in the early clearance of parasitemia. In parallel, we observed an increase in splenic NK and NKT cells expressing TLR3 in infected B6 mice, suggesting a role for TLR sensing in the innate immune response. Finally, we find evidence that the increase in the frequency of CD19⁺TLR3⁺ B cells along with reduced levels of total IgG in B6 mice possibly suggests the initiation of TLR3-dependent pathway early during P. yoelii infection. Our results thus reveal a new mechanism in which a parasite-activated TLR3 pathway promotes blood stage infection along with quantitative and qualitative differences in Ab responses.

Immune responses to malaria pre-erythrocytic stages: Implications for vaccine development

Radiation-attenuated sporozoites induce sterilizing immunity and remain the 'gold standard' for malaria vaccine development. Despite practical challenges in translating these whole sporozoite vaccines to large-scale intervention programmes, they have provided an excellent platform to dissect the immune responses to malaria pre-erythrocytic (PE) stages, comprising both sporozoites and exoerythrocytic forms. Investigations in rodent models have provided insights that led to the clinical translation of various vaccine candidates-including RTS,S/AS01, the most advanced candidate currently in a trial implementation programme in three African countries. With advances in immunology, transcriptomics and proteomics, and application of lessons from past failures, an effective, long-lasting and wide-scale malaria PE vaccine remains feasible. This review underscores the progress in PE vaccine development, focusing on our understanding of host-parasite immunological crosstalk in the tissue environments of the skin and the liver. We highlight possible gaps in the current knowledge of PE immunity that can impact future malaria vaccine development efforts.

PD-1 Expression on NK Cells in Malaria-Exposed Individuals Is Associated with Diminished Natural Cytotoxicity and Enhanced Antibody-Dependent Cellular Cytotoxicity

Natural killer (NK) cells are key effector cells of innate resistance capable of destroying tumors and virus-infected cells through cytotoxicity and rapid cytokine production. The control of NK cell responses is complex and only partially understood. PD-1 is an inhibitory receptor that regulates T cell function, but a role for PD-1 in regulating NK cell function is only beginning to emerge. Here, we investigated PD-1 expression on NK cells in children and adults in Mali in a longitudinal analysis before, during, and after infection with Plasmodium falciparum malaria. We found that NK cells transiently upregulate PD-1 expression and interleukin-6 (IL-6) production in some individuals during acute febrile malaria. Furthermore, the percentage of PD-1 expressing NK cells increases with age and cumulative malaria exposure. Consistent with this, NK cells of malaria-naive adults upregulated PD-1 following P. falciparum stimulation in vitro Additionally, functional in vitro studies revealed that PD-1 expression on NK cells is associated with diminished natural cytotoxicity but enhanced antibody-dependent cellular cytotoxicity (ADCC). These data indicate that PD-1⁺ NK cells expand in the context of chronic immune activation and suggest that PD-1 may contribute to skewing NK cells toward enhanced ADCC during infections such as malaria.

The immunology of Plasmodium vivax malaria

Plasmodium vivax infection, the predominant cause of malaria in Asia and Latin America, affects ~14 million individuals annually, with considerable adverse effects on wellbeing and socioeconomic development. A clinical hallmark of Plasmodium infection, the paroxysm, is driven by pyrogenic cytokines produced during the immune response. Here, we review studies on the role of specific immune cell types, cognate innate immune receptors, and inflammatory cytokines on parasite control and disease symptoms. This review also summarizes studies on recurrent infections in individuals living in endemic regions as well as asymptomatic infections, a serious barrier to eliminating this disease. We propose potential mechanisms behind these repeated and subclinical infections, such as poor induction of immunological memory cells and inefficient T effector cells. We address the role of antibody-mediated resistance to P. vivax infection and discuss current progress in vaccine development. Finally, we review immunoregulatory mechanisms, such as inhibitory receptors, T regulatory cells, and the anti-inflammatory cytokine, IL-10, that antagonizes both innate and acquired immune responses, interfering with the development of protective immunity and parasite clearance. These studies provide new insights for the clinical management of symptomatic as well as asymptomatic individuals and the development of an efficacious vaccine for vivax malaria.

Antibody-Dependent, Gamma Interferon-Independent Sterilizing Immunity Induced by a Subunit Malaria Vaccine

The development of effective malaria vaccines is hampered by incomplete understanding of the immunological correlates of protective immunity. Recently, the moderate clinical efficacy of the Plasmodium falciparum circumsporozoite protein (CSP)-based RTS,S/AS01_E vaccine in phase 3 studies highlighted the urgency to design and test more efficacious next-generation malaria vaccines. In this study, we report that immunization with recombinant CSP from Plasmodium yoelii (rPyCSP), when delivered in Montanide ISA 51, induced sterilizing immunity against sporozoite challenge in C57BL/6 and BALB/c strains of mice. This immunity was antibody dependent, as evidenced by the complete loss of immunity in B-cell-knockout (KO) mice and by the ability of immune sera to neutralize sporozoite infectivity in mice. Th2-type isotype IgG1 antibody levels were associated with protective immunity. The fact that immunized gamma interferon (IFN-γ)-KO mice and wild-type (WT) mice have similar levels of protective immunity and the absence of IFN-γ-producing CD4⁺ and CD8⁺ T cells in protected mice, as shown by flow cytometry, indicate that the immunity is IFN-γ independent. Protection against sporozoite challenge correlated with higher frequencies of CD4⁺ T cells that express interleukin-2 (IL-2), IL-4, and tumor necrosis factor alpha (TNF-α). In the RTS,S study, clinical immunity was associated with higher IgG levels and frequencies of IL-2- and TNF-α-producing CD4⁺ T cells. The other hallmarks of immunity in our study included an increased number of follicular B cells but a loss in follicular T helper cells. These results provide an excellent model system to evaluate the efficacy of novel adjuvants and vaccine dosage and determine the correlates of immunity in the search for superior malaria vaccine candidates.

Contributions of natural killer cells to the immune response against Plasmodium

Natural killer (NK) cells are important innate effector cells that are well described in their ability to kill virally-infected cells and tumors. However, there is increasing appreciation for the role of NK cells in the control of other pathogens, including intracellular parasites such as Plasmodium, the cause of malaria. NK cells may be beneficial during the early phase of Plasmodium infection—prior to the activation and expansion of antigen-specific T cells—through cooperation with myeloid cells to produce inflammatory cytokines like IFNγ. Recent work has defined how Plasmodium can activate NK cells to respond with natural cytotoxicity, and inhibit the growth of parasites via antibody-dependent cellular cytotoxicity mechanisms (ADCC). A specialized subset of adaptive NK cells that are negative for the Fc receptor γ chain have enhanced ADCC function and correlate with protection from malaria. Additionally, production of the regulatory cytokine IL-10 by NK cells prevents overt pathology and death during experimental cerebral malaria. Now that conditional NK cell mouse models have been developed, previous studies need to be reevaluated in the context of what is now known about other immune populations with similarity to NK cells (i.e., NKT cells and type I innate lymphoid cells). This brief review summarizes recent findings which support the potentially beneficial roles of NK cells during Plasmodium infection in mice and humans. Also highlighted are how the actions of NK cells can be explored using new experimental strategies, and the potential to harness NK cell function in vaccination regimens.

Expression of CD300lf by microglia contributes to resistance to cerebral malaria by impeding the neuroinflammation

Genetic mapping and genome-wide studies provide evidence for the association of several genetic polymorphisms with malaria, a complex pathological disease with multiple severity degrees. We have previously described Berr1and Berr2 as candidate genes identified in the WLA/Pas inbreed mouse strain predisposing to resistance to cerebral malaria (CM) induced by P. berghei ANKA. We report in this study the phenotypic and functional characteristics of a congenic strain we have derived for Berr2^WLA allele on the C57BL/6JR (B6) background. B6.WLA-Berr2 was found highly resistant to CM compared to C57BL/6JR susceptible mice. The mechanisms associated with CM resistance were analyzed by combining genotype, transcriptomic and immune response studies. We found that B6.WLA-Berr2 mice showed a reduced parasite sequestration and blood-brain barrier disruption with low CXCR3⁺ T cell infiltration in the brain along with altered glial cell response upon P. berghei ANKA infection compared to B6. In addition, we have identified the CD300f, belonging to a family of Ig-like encoding genes, as a potential candidate associated with CM resistance. Microglia cells isolated from the brain of infected B6.WLA-Berr2 mice significantly expressed higher level of CD300f compared to CM^S mice and were associated with inhibition of inflammatory response.

Vaccination With Sporozoites: Models and Correlates of Protection

Despite continuous efforts, the century-old goal of eradicating malaria still remains. Multiple control interventions need to be in place simultaneously to achieve this goal. In addition to effective control measures, drug therapies and insecticides, vaccines are critical to reduce mortality and morbidity. Hence, there are numerous studies investigating various malaria vaccine candidates. Most of the malaria vaccine candidates are subunit vaccines. However, they have shown limited efficacy in Phase II and III studies. To date, only whole parasite formulations have been shown to induce sterile immunity in human. In this article, we review and discuss the recent developments in vaccination with sporozoites and the mechanisms of protection involved.

γδ-T cells promote IFN-γ-dependent Plasmodium pathogenesis upon liver-stage infection

Cerebral malaria (CM) is a major cause of death due to Plasmodium infection. Both parasite and host factors contribute to the onset of CM, but the precise cellular and molecular mechanisms that contribute to its pathogenesis remain poorly characterized. Unlike conventional αβ-T cells, previous studies on murine γδ-T cells failed to identify a nonredundant role for this T cell subset in experimental cerebral malaria (ECM). Here we show that mice lacking γδ-T cells are resistant to ECM when infected with Plasmodium berghei ANKA sporozoites, the liver-infective form of the parasite and the natural route of infection, in contrast with their susceptible phenotype if challenged with P. berghei ANKA-infected red blood cells that bypass the liver stage of infection. Strikingly, the presence of γδ-T cells enhanced the expression of Plasmodium immunogenic factors and exacerbated subsequent systemic and brain-infiltrating inflammatory αβ-T cell responses. These phenomena were dependent on the proinflammatory cytokine IFN-γ, which was required during liver stage for modulation of the parasite transcriptome, as well as for downstream immune-mediated pathology. Our work reveals an unanticipated critical role of γδ-T cells in the development of ECM upon Plasmodium liver-stage infection.

Innate Lymphoid Cells in Protection, Pathology, and Adaptive Immunity During Apicomplexan Infection

Apicomplexans are a diverse and complex group of protozoan pathogens including Toxoplasma gondii, Plasmodium spp., Cryptosporidium spp., Eimeria spp., and Babesia spp. They infect a wide variety of hosts and are a major health threat to humans and other animals. Innate immunity provides early control and also regulates the development of adaptive immune responses important for controlling these pathogens. Innate immune responses also contribute to immunopathology associated with these infections. Natural killer (NK) cells have been for a long time known to be potent first line effector cells in helping control protozoan infection. They provide control by producing IL-12 dependent IFNγ and killing infected cells and parasites via their cytotoxic response. Results from more recent studies indicate that NK cells could provide additional effector functions such as IL-10 and IL-17 and might have diverse roles in immunity to these pathogens. These early studies based their conclusions on the identification of NK cells to be CD3–, CD49b+, NK1.1+, and/or NKp46+ and the common accepted paradigm at that time that NK cells were one of the only lymphoid derived innate immune cells present. New discoveries have lead to major advances in understanding that NK cells are only one of several populations of innate immune cells of lymphoid origin. Common lymphoid progenitor derived innate immune cells are now known as innate lymphoid cells (ILC) and comprise three different groups, group 1, group 2, and group 3 ILC. They are a functionally heterogeneous and plastic cell population and are important effector cells in disease and tissue homeostasis. Very little is known about each of these different types of ILCs in parasitic infection. Therefore, we will review what is known about NK cells in innate immune responses during different protozoan infections. We will discuss what immune responses attributed to NK cells might be reconsidered as ILC1, 2, or 3 population responses. We will then discuss how different ILCs may impact immunopathology and adaptive immune responses to these parasites.

Vaccinating for natural killer cell effector functions

Vaccination has proved to be highly effective in reducing global mortality and eliminating infectious diseases. Building on this success will depend on the development of new and improved vaccines, new methods to determine efficacy and optimum dosing and new or refined adjuvant systems. NK cells are innate lymphoid cells that respond rapidly during primary infection but also have adaptive characteristics enabling them to integrate innate and acquired immune responses. NK cells are activated after vaccination against pathogens including influenza, yellow fever and tuberculosis, and their subsequent maturation, proliferation and effector function is dependent on myeloid accessory cell-derived cytokines such as IL-12, IL-18 and type I interferons. Activation of antigen-presenting cells by live attenuated or whole inactivated vaccines, or by the use of adjuvants, leads to enhanced and sustained NK cell activity, which in turn contributes to T cell recruitment and memory cell formation. This review explores the role of cytokine-activated NK cells as vaccine-induced effector cells and in recall responses and their potential contribution to vaccine and adjuvant development.

The Effect of Antiretroviral Naïve HIV-1 Infection on the Ability of Natural Killer Cells to Produce IFN-γ upon Exposure to Plasmodium falciparum-Infected Erythrocytes

Background

In sub-Saharan Africa, intense perennial Plasmodium species transmission coincides with areas of high prevalence of the human immunodeficiency virus type 1 (HIV) infection. This implies that antiretroviral naïve HIV-infected people living within these regions are repeatedly exposed to Plasmodium species infection and consequently malaria. Natural killer (NK) cells are known to contribute to malaria immunity through the production of IFN-γ after exposure to Plasmodium falciparum-infected erythrocytes (infected red blood cells [iRBC]). However, in antiretroviral naïve HIV-1 infection, these functions could be impaired. In this study we assess the ability of NK cells from antiretroviral naïve HIV-1-infected people to respond to iRBC.

Method

Magnetically sorted NK cells from antiretroviral naïve HIV-1-infected people were tested for their ability to respond to iRBC following in vitro coculture. NK cell IFN-γ production after coculture was measured through multiparametric flow cytometry analysis.

Results

Our data show a significant reduction (p = 0.03) in IFN-γ production by NK cells from antiretroviral naïve HIV-1-infected people after coculture with iRBCs. This was in contrast to the NK cell response from healthy controls, which demonstrated elevated IFN-γ production. NK cell IFN-γ production from untreated HIV-1-infected participants correlated inversely with the viral load (r = −0.5, p = 0.02) and positively with total helper CD4⁺ T-cell count (r = 0.4, p = 0.04). Thus, antiretroviral naïve HIV-1 infection can dampen NK cell-mediated immunity to P. falciparum infection in malaria-intense regions. This could in effect escalate morbidity and mortality in people chronically infected with HIV-1.

NK Cells: Uncertain Allies against Malaria

Until recently, studies of natural killer (NK) cells in infection have focused almost entirely on their role in viral infections. However, there is an increasing awareness of the potential for NK cells to contribute to the control of a wider range of pathogens, including intracellular parasites such as Plasmodium spp. Given the high prevalence of parasitic diseases in the developing world and the devastating effects these pathogens have on large numbers of vulnerable people, investigating interactions between NK cells and parasitized host cells presents the opportunity to reveal novel immunological mechanisms with the potential to aid efforts to eradicate these diseases. The capacity of NK cells to produce inflammatory cytokines early after malaria infection, as well as a possible role in direct cytotoxic killing of malaria-infected cells, suggests a beneficial impact of NK cells in this disease. However, NK cells may also contribute to overproduction of pro-inflammatory cytokines and the consequent immunopathology. As comparatively little is known about the role of NK cells later in the course of infection, and growing evidence suggests that heterogeneity in NK cell responses to malaria may be influenced by KIR/HLA interactions, a better understanding of the mechanisms by which NK cells might directly interact with parasitized cells may reveal a new role for these cells in the course of malaria infection.

Systems analysis of protective immune responses to RTS,S malaria vaccination in humans

RTS,S is an advanced malaria vaccine candidate and confers significant protection against Plasmodium falciparum infection in humans. Little is known about the molecular mechanisms driving vaccine immunity. Here, we applied a systems biology approach to study immune responses in subjects receiving three consecutive immunizations with RTS,S (RRR), or in those receiving two immunizations of RTS,S/AS01 following a primary immunization with adenovirus 35 (Ad35) (ARR) vector expressing circumsporozoite protein. Subsequent controlled human malaria challenge (CHMI) of the vaccinees with Plasmodium-infected mosquitoes, 3 wk after the final immunization, resulted in ∼50% protection in both groups of vaccinees. Circumsporozoite protein (CSP)-specific antibody titers, prechallenge, were associated with protection in the RRR group. In contrast, ARR-induced lower antibody responses, and protection was associated with polyfunctional CD4⁺ T-cell responses 2 wk after priming with Ad35. Molecular signatures of B and plasma cells detected in PBMCs were highly correlated with antibody titers prechallenge and protection in the RRR cohort. In contrast, early signatures of innate immunity and dendritic cell activation were highly associated with protection in the ARR cohort. For both vaccine regimens, natural killer (NK) cell signatures negatively correlated with and predicted protection. These results suggest that protective immunity against P. falciparum can be achieved via multiple mechanisms and highlight the utility of systems approaches in defining molecular correlates of protection to vaccination.

The rodent malaria liver stage survives in the rapamycin-induced autophagosome of infected Hepa1-6 cells

It has been reported that non-selective autophagy of infected hepatocytes could facilitate the development of malaria in the liver stage, but the fate of parasites following selective autophagy of infected hepatocytes is still not very clear. Here, we confirmed that sporozoite infection can induce a selective autophagy-like process targeting EEFs (exo-erythrocytic forms) in Hepa1–6. Rapamycin treatment greatly enhanced this process in EEFs and non-selective autophagy of infected Hepa1-6 cells and enhanced the development of the malaria liver stage in vivo. Although rapamycin promoted the fusion of autophagosomes containing the malaria parasite with lysosomes, some parasites inside the autophagosome survived and replicated normally. Further study showed that the maturation of affected autolysosomes was greatly inhibited. Therefore, in addition to the previously described positive role of rapamycin-induced nonselective autophagy of hepatocytes, we provide evidence that the survival of EEFs in the autophagosome of the infected hepatocytes also contributes to rapamycin-enhanced development of the malaria liver stage, possibly due to the suppression of autolysosome maturation by EEFs. These data suggest that the inhibition of autolysosome maturation might be a novel escape strategy used by the malaria liver stage.

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.

Comparative Analysis of Conventional Natural Killer Cell Responses to Acute Infection with Toxoplasma gondii Strains of Different Virulence

Conventional natural killer (cNK) cells, members of group 1 innate lymphoid cells, are a diverse cell subpopulation based on surface receptor expression, maturation, and functional potential. cNK cells are critical for early immunity to Toxoplasma gondii via IFNγ production. Acute cNK cell responses to infection with different strains of T. gondii have not yet been characterized in detail. Here, we comprehensively performed this analysis with Type I virulent RH, Type II avirulent ME49, and fully attenuated Type I cps1-1 strains. In response to these three parasite strains, murine cNK cells produce IFNγ and become cytotoxic and polyfunctional (IFNγ+CD107a+) at the site of infection. In contrast to virulent RH and avirulent ME49 T. gondii strains, attenuated cps1-1 induced only local cNK cell responses. Infections with RH and ME49 parasites significantly decreased cNK cell frequency and numbers in spleen 5 days post infection compared with cps1-1 parasites. cNK cell subsets expressing activating receptors Ly49H, Ly49D, and NKG2D and inhibitory receptors Ly49I and CD94/NKG2A were similar when compared between the strains and at 5 days post infection. cNK cells were not proliferating (Ki67−) 5 days post infection with any of the strains. cNK cell maturation as measured by CD27, CD11b, and KLRG1 was affected after infection with different parasite strains. RH and ME49 infection significantly reduced mature cNK cell frequency and increased immature cNK cell populations compared with cps1-1 infection. Interestingly, KLRG1 was highly expressed on immature cNK cells after RH infection. After RH and ME49 infections, CD69+ cNK cells in spleen were present at higher frequency than after cps1-1 infection, which may correlate with loss of the mature cNK cell population. Cytokine multiplex analysis indicated cNK cell responses correlated with peritoneal exudate cell, spleen, and serum proinflammatory cytokine levels, including IL-12. qPCR analysis of parasite-specific B1 gene revealed that parasite burdens may affect cNK cell responses. This study demonstrates infection with RH and ME49 parasites impacts cNK cell maturation during acute T. gondii infection. Different cNK cell responses could impact early immunity and susceptibility to these strains.

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.

The TLR2 is activated by sporozoites and suppresses intrahepatic rodent malaria parasite development

TLRs (Toll-like receptors) play an important role in the initiation of innate immune responses against invading microorganisms. Although several TLRs have been reported to be involved in the innate immune response against the blood-stage of malaria parasites, the role of TLRs in the development of the pre-erythrocytic stage is still largely unknown. Here, we found that sporozoite and its lysate could significantly activate the TLR2, and induce macrophages to release proinflammatory cytokines, including IL-6, MCP-1 and TNF-α, in a TLR2-dependent manner. Further studies showed that sporozoite and its lysate could be recognized by either TLR2 homodimers or TLR2/1 and TLR2/6 heterodimers, implicating the complexity of TLR2 agonist in sporozoite. Interestingly, the TLR2 signaling can significantly suppress the development of the pre-erythrocytic stage of Plasmodium yoelii, as both liver parasite load and subsequent parasitemia were significantly elevated in both TLR2- and MyD88-deficient mice. Additionally, the observed higher level of parasite burden in TLR2^−/− mice was found to be closely associated with a reduction in proinflammatory cytokines in the liver. Therefore, we provide the first evidence that sporozoites can activate the TLR2 signaling, which in turn significantly inhibits the intrahepatic parasites. This may provide us with novel clues to design preventive anti-malaria therapies.

IL-18-induced expression of high-affinity IL-2R on murine NK cells is essential for NK-cell IFN-γ production during murine Plasmodium yoelii infection

Early production of pro‐inflammatory cytokines, including IFN‐γ, is essential for control of blood‐stage malaria infections. We have shown that IFN‐γ production can be induced among human natural killer (NK) cells by coculture with Plasmodium falciparum infected erythrocytes, but the importance of this response is unclear. To further explore the role of NK cells during malaria infection, we have characterized the NK‐cell response of C57BL/6 mice during lethal (PyYM) or nonlethal (Py17XNL) P. yoelii infection. Ex vivo flow cytometry revealed that NK cells are activated within 24 h of Py17XNL blood‐stage infection, expressing CD25 and producing IFN‐γ; this response was blunted and delayed during PyYM infection. CD25 expression and IFN‐γ production were highly correlated, suggesting a causal relationship between the two responses. Subsequent in vitro experiments revealed that IL‐18 signaling is essential for induction of CD25 and synergizes with IL‐12 to enhance CD25 expression on splenic NK cells. In accordance with this, Py17XNL‐infected erythrocytes induced NK‐cell CD25 expression and IFN‐γ production in a manner that is completely IL‐18‐ and partially IL‐12‐dependent, and IFN‐γ production is enhanced by IL‐2. These data suggest that IL‐2 signaling via CD25 amplifies IL‐18‐ and IL‐12‐mediated NK‐cell activation during malaria infection.

Immune evasion strategies of pre-erythrocytic malaria parasites

Malaria is a mosquito-borne infectious disease of humans. It begins with a bite from an infected female Anopheles mosquito and leads to the development of the pre-erythrocytic and blood stages. Blood-stage infection is the exclusive cause of clinical symptoms of malaria. In contrast, the pre-erythrocytic stage is clinically asymptomatic and could be an excellent target for preventive therapies. Although the robust host immune responses limit the development of the liver stage, malaria parasites have also evolved strategies to suppress host defenses at the pre-erythrocytic stage. This paper reviews the immune evasion strategies of malaria parasites at the pre-erythrocytic stage, which could provide us with potential targets to design prophylactic strategies against malaria.

Contribution of the Ly49E natural killer receptor in the immune response to Plasmodium berghei infection and control of hepatic parasite development

Natural killer (NK) cells have different roles in the host response against Plasmodium-induced malaria depending on the stage of infection. Liver NK cells have a protective role during the initial hepatic stage of infection by production of the T_H1-type cytokines IFN-γ and TNF-α. In the subsequent erythrocytic stage of infection, NK cells also induce protection through Th1-type cytokines but, in addition, may also promote development of cerebral malaria via CXCR3-induction on CD8⁺ T cells resulting in migration of these cells to the brain. We have recently shown that the regulatory Ly49E NK receptor is expressed on liver NK cells in particular. The main objective of this study was therefore to examine the role of Ly49E expression in the immune response upon Plasmodium berghei ANKA infection, for which we compared wild type (WT) to Ly49E knockout (KO) mice. We show that the parasitemia was higher at the early stage, i.e. at days 6–7 of Plasmodium berghei ANKA infection in Ly49E KO mice, which correlated with lower induction of CD69, IFN-γ and TNF-α in DX5⁻ liver NK cells at day 5 post-infection. At later stages, these differences faded. There was also no difference in the kinetics and the percentage of cerebral malaria development and in lymphocyte CXCR3 expression in WT versus Ly49E KO mice. Collectively, we show that the immune response against Plasmodium berghei ANKA infection is not drastically affected in Ly49E KO mice. Although NK cells play a crucial role in Plasmodium infection and Ly49E is highly expressed on liver NK cells, the Ly49E NK receptor only has a temporarily role in the immune control of this parasite.

Requirement and redundancy of the Src family kinases Fyn and Lyn in perforin-dependent killing of Cryptococcus neoformans by NK cells

Natural killer (NK) cells directly recognize and kill fungi, such as the pathogenic fungus Cryptococcus neoformans, via cytolytic mechanisms. However, the precise signaling pathways governing this NK cell microbicidal activity and the implications for fungal recognition are still unknown. Previously, it was reported that NK cell anticryptococcal activity is mediated through a conserved phosphatidylinositol 3-kinase-extracellular signal-regulated kinase 1/2 (PI3K-ERK1/2) pathway. Using YT (a human NK-like cell line) and primary human NK cells, we sought to identify the upstream, receptor-proximal signaling elements that led to fungal cytolysis. We demonstrate that Src family kinases were activated in response to C. neoformans. Furthermore, pharmacologic inhibition with an Src kinase inhibitor blocked C. neoformans-induced downstream activation of PI3K and ERK1/2 and abrogated cryptococcal killing. At the same time, the inhibitor disrupted the polarization of perforin-containing granules toward the NK cell-cryptococcal synapse but had no effect on conjugate formation between the organism and the NK cell. Finally, small interfering RNA (siRNA) double (but not single) knockdown of two Src family kinases, Fyn and Lyn, blocked cryptococcal killing. Together these data demonstrate a mechanism whereby the Src family kinases, Fyn and Lyn, redundantly mediate anticryptococcal activity through the activation of PI3K and ERK1/2, which in turn facilitates killing by inducing the polarization of perforin-containing granules to the NK cell-cryptococcal synapse.

The ly49 gene family. A brief guide to the nomenclature, genetics, and role in intracellular infection

Understanding the Ly49 gene family can be challenging in terms of nomenclature and genetic organization. The Ly49 gene family has two major gene nomenclature systems, Ly49 and Killer Cell Lectin-like Receptor subfamily A (klra). Mice from different strains have varying numbers of these genes with strain specific allelic variants, duplications, deletions, and pseudogene sequences. Some members activate NK lymphocytes, invariant NKT (iNKT) lymphocytes and γδ T lymphocytes while others inhibit killing activity. One family member, Ly49Q, is expressed only on myeloid cells and is not found on NK, iNKT, or γδ T cells. There is growing evidence that these receptors may regulate not just the immune response to viruses, but other intracellular pathogens as well. Thus, this review’s primary goal is to provide a guide for researchers first encountering the Ly49 gene family and a foundation for future studies on the role that these gene products play in the immune response, particularly the response to intracellular viral and bacterial pathogens.

Activation of human NK cells by Plasmodium-infected red blood cells

This chapter describes a protocol to assess activation of human NK cells following in vitro stimulation with malaria-infected red blood cells. Activation is assessed by flow cytometry, staining for cell surface expression of CD69 and accumulation of intracellular IFN-γ. Procedures are described for in vitro propagation and purification of Plasmodium falciparum parasites, separation of peripheral blood mononuclear cells from heparinized blood by density centrifugation, in vitro culture of PBMC and for staining and analysis of PBMC by flow cytometry. Some examples of typical FACS plots are shown.

Plasmodium chabaudi AS: distinct CD4(+)CD25(+)Foxp3(+) regulatory T cell responses during infection in DBA/2 and BALB/c mice

Malaria infections display variation patterns of clinical course and outcome. Although CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells play an essential role in immune homeostasis, the immune regulatory roles involved in malaria infection remains to be elucidated. Herein, we compared the disparity in Treg cells response during the course of blood stage Plasmodium chabaudi chabaudi AS (P. c chabaudi AS) infection in DBA/2 and BALB/c mice. BALB/c mice initiated a Th1/Th2 profile respond to P. c chabaudi AS infection, but DBA/2 mice failed to control P. c chabaudi AS infection and almost of them died post-peak parasitemia. At the peak parasitemia, we found that higher proportion of Treg cells with elevated Foxp3 expression in DBA/2 than in BALB/c mice. We used anti-CD25 mAb to deplete Treg cells and found that the survival time and rate were prolonged in DBA/2 mice treated with anti-CD25 mAb. Treatment with anti-CD25 mAb in vivo led to enhanced pro-inflammation responses and Foxp3 expression decline on Treg cells. In contrast, after DBA/2 was treatment with anti-IL-10R mAb, IL-10R blockade in vivo caused excessive pro-inflammation responses and Foxp3 expression loss on CD4(+)CD25(+) T cells. Earlier death was found in all of DBA/2 mice with anti-IL-10R mAb. It suggested that IL-2 and IL-10 signal involved in maintaining Foxp3 expression on Treg cells. In all, the moderate suppressive activity of Treg cells may facilitate resistance to P. c chabaudi AS infection.

Interferon-γ, a valuable surrogate marker of Plasmodium falciparum pre-erythrocytic stages protective immunity

Immunity against the pre-erythrocytic stages of malaria is the most promising, as it is strong and fully sterilizing. Yet, the underlying immune effectors against the human Plasmodium falciparum pre-erythrocytic stages remain surprisingly poorly known and have been little explored, which in turn prevents any rational vaccine progress. Evidence that has been gathered in vitro and in vivo, in higher primates and in humans, is reviewed here, emphasizing the significant role of IFN-γ, either as a critical immune mediator or at least as a valuable surrogate marker of protection. One may hope that these results will trigger investigations in volunteers immunized either by optimally irradiated or over-irradiated sporozoites, to quickly delineate better surrogates of protection, which are essential for the development of a successful malaria vaccine.

Interferon-γ--central mediator of protective immune responses against the pre-erythrocytic and blood stage of malaria

Immune responses against Plasmodium parasites, the causative organisms of malaria, are traditionally dichotomized into pre-erythrocytic and blood-stage components. Whereas the central role of cellular responses in pre-erythrocytic immunity is well established, protection against blood-stage parasites has generally been ascribed to humoral responses. A number of recent studies, however, have highlighted the existence of cellular immunity against blood-stage parasites, in particular, the prominence of IFN-γ production. Here, we have undertaken to chart the contribution of this prototypical cellular cytokine to immunity against pre-erythrocytic and blood-stage parasites. We summarize the various antiparasitic effector functions that IFN-γ serves to induce, review an array of data about its protective effects, and scrutinize evidence for any deleterious, immunopathological outcome in malaria patients. We discuss the activation and contribution of different cellular sources of IFN-γ production during malaria infection and its regulation in relation to exposure. We conclude that IFN-γ forms a central mediator of protective immune responses against pre-erythrocytic and blood-stage malaria parasites and identify a number of implications for rational malaria vaccine development.

Plasmodium yoelii: influence of immune modulators on the development of the liver stage

Plasmodium sporozoites suppress the respiratory burst and antigen presentation of Kupffer cells, which are regarded as the portal of invasion into hepatocytes. It is not known whether immune modulation of Kupffer cells can affect the liver stage. In the present study, we found that sporozoites inoculated into Wistar rats could be detected in the liver, spleen, and lung; however, most sporozoites were arrested in the liver. Sporozoites were captured by Kupffer cells lined with endothelial cells in the liver sinusoid before hepatocyte invasion. Pretreatment with TLR3 agonist poly(I:C) and TLR2 agonist BCG primarily activated Kupffer cells, inhibiting the sporozoite development into the exoerythrocytic form, whereas Kupffer cell antagonists dexamethasone and cyclophosphamide promoted development of the liver stage. Our data suggests that sporozoite development into its exoerythrocytic form may be associated with Kupffer cell functional status. Immune modulation of Kupffer cells could be a promising strategy to prevent malaria parasite infection.

KIR3DL1/S1 genotypes and KIR2DS4 allelic variants in the AB KIR genotypes are associated with Plasmodium-positive individuals in malaria infection

The importance of innate immunity in malaria has been suggested for early protection from maturation and multiplication of Plasmodium parasites injected via infected mosquitoes. In this study, the killer cell immunoglobulin-like receptor (KIR) genes in innate immunity were investigated for an association with malaria in the comparison between Plasmodium-positive and Plasmodium-negative Melanesian individuals in the Solomon Islands, one of the most hyperendemic malaria regions in the world. The higher frequency of a pair of KIR3DL1 and KIR2DS4 was observed in the Plasmodium-positive individuals, which led to the investigation of KIR3DL1/S1 genotypes in concert with KIR2DS4 allelic variants. The positive individuals showed the highest frequency of KIR3DL1/KIR3DS1 heterozygosity, which might suggest the masking of activating KIR3DS1 by inhibitory KIR3DL1 at allelic levels to maintain the KIR3DS1-driven activation of natural killer cells diminished in controlling Plasmodium proliferation. The extended analysis with A/B genotypes further revealed the trend of parasitic positive individuals to be KIR3DL1/KIR3DS1 heterozygous in pair with KIR2DS4 nondeleted variants in a set of KIR genes inheritable as the AB genotypes. To the best of our knowledge, this study is the first KIR investigation of the malaria-infected population, which strengthened the potential associations of KIR with malaria pathogenesis. The balance of inhibitory and activating KIR3D genes (KIR3DL1/S1) and membrane-bound or secreted status of KIR2DS4 alleles in the interaction with the other KIR genes in the AB genotypes might constitute a part of KIR characteristics to determine resistance or susceptibility to Plasmodium parasitic infection.

The localization and migration of natural killer cells in health and disease

Natural killer (NK) cells comprise a finite lymphocyte lineage with distinctive gene expression patterns. Natural killer (NK) cells develop in the bone marrow (BM) and are not static but populate secondary and primary lymphoid organs. A unique feature of NK cells is their expression of activating and inhibitory receptors, which allow them to respond either when ligands for activating receptors are upregulated or when ligands for inhibitory receptors are downregulated. The unique transcriptome of NK cells renders them capable of protecting the host from a vast array of disease states. Their undisputed importance in host protection is conferred by their ability to eliminate unhealthy cells. However, in order for NK cells to exert their effects, they need to be strategically located at the right places. This chapter provides an overview of the current understanding of the localization of NK cell populations and their ability to migrate in response to homeostatic and pathological conditions. NK cells develop in the BM, which they exit using specific molecular interactions. Exit from the BM is followed by localization to a number of tissues, including secondary lymphoid organs. Within each tissue, NK cells often acquire unique function and phenotype that is regulated by the local microenvironment. Their localization is primarily directed by the action of chemokines and therefore is in tight association with the activation status of the organism. Changes in chemokine expression during disease results in further NK cell mobilization and allows them to protect the host from infection and malignancy. Thus, from their time of production until their end, NK cells travel exhaustively over long distances and visit places that influence their already dynamic life.

Inhibitory role of toll-like receptors agonists in Plasmodium yoelii liver stage development

It is well known that innate immune plays an important role in controlling the development of Plasmodium liver stage. However, little is known about the role of toll-like receptors (TLR) signalling in the pre-erythrocytic immunity against Plasmodium. Here, we found that pre-treatment with individual TLR agonist pam3CSK4 (TLR2), poly(I:C) (TLR3), LPS (TLR4) and CpG (TLR9) could decrease significantly the liver malaria parasite load in mice for 58%, 63%, 75% and 88% respectively. Moreover, no parasitaemia was observed within 14 days in CpG group mice challenged with 100 sporozoites. At 24 h prior to CpG injection, administration of gadolinium chloride (GdCl(3)) led to the rebound of liver Plasmodium load through inhibiting selectively Kupffer cells (KC) phagocytosis capacity but failed to neutralize completely CpG-induced immunity against malaria liver stage. Compared with the control, pre-treatment of CpG up-regulated hepatic pro-inflammatory cytokines IL-12, IFN-gamma and TNF-alpha, but down-regulated anti-inflammatory cytokines IL-10 and TGF-beta. Hence, our data demonstrated the inhibitory role of diverse TLR agonists in the Plasmodium development during pre-erythrocytic stage. As the most robust agonist, CpG might inhibit the development of Plasmodium liver stage through regulation of intrahepatic inflammatory cytokines and enhancement of KC cells phagocytosis capacity.

Effects of CD4+CD25+Foxp3+regulatory T cells on earlyPlasmodium yoelii17XL infection in BALB/c mice

The outcome ofPlasmodium yoelii17XL-infected BALB/c and DBA/2 mice, ranging from death to spontaneous cure, respectively, depends largely on the establishment of effective pro-inflammatory type 1 responses during the early stages of infection and associates with CD4+CD25+Foxp3+regulatory T cells (Tregs). Here, effects of Tregs were analysed on earlyP. yoelii17XL infection in BALB/c and DBA/2 mice.In vivodepletion of Tregs significantly reversed the inhibited establishment of effective pro-inflammatory type 1 responses in BALB/c mice, indicating that this cell population contributed to the suppression of T-cell function in malaria. Moreover, the proportion and absolute numbers of IL-10-secreting Tregs in BALB/c mice were significantly higher than that found in DBA/2 mice by intracytoplasmic staining, and IL-10 production was correlated with the Tregs population. In addition,in vivoTregs depletion decreased the production of IL-10 and the apoptosis of CD4+T cells. Consistently, IL-10R blockade also had the same effect as that of Tregs depletion inP. yoelii17XL-infected BALB/c mice. Our data demonstrate that Tregs perhaps have an important role in regulating pro-inflammatory type 1 responses in an IL-10-dependent manner and induce CD4+T cell apoptosis during the early stage ofP. yoelii17XL infection.

An early burst of IFN-gamma induced by the pre-erythrocytic stage favours Plasmodium yoelii parasitaemia in B6 mice

BACKGROUND

In murine models of malaria, an early proinflammatory response has been associated with the resolution of blood-stage infection. To dissect the protective immune mechanism that allow the control of parasitaemia, the early immune response of C57BL/6 mice induced during a non-lethal plasmodial infection was analysed.

METHODS

Mice were infected with Plasmodium yoelii 265BY sporozoites, the natural invasive form of the parasite, in order to complete its full life cycle. The concentrations of three proinflammatory cytokines in the sera of mice were determined by ELISA at different time points of infection. The contribution of the liver and the spleen to this cytokinic response was evaluated and the cytokine-producing lymphocytes were identified by flow cytometry. The physiological relevance of these results was tested by monitoring parasitaemia in genetically deficient C57BL/6 mice or wild-type mice treated with anti-cytokine neutralizing antibody. Finally, the cytokinic response in sera of mice infected with parasitized-RBCs was analysed.

RESULTS

The early immune response of C57BL/6 mice to sporozoite-induced malaria is characterized by a peak of IFN-gamma in the serum at day 5 of infection and splenic CD4 T lymphocytes are the major producer of this cytokine at this time point. Somewhat unexpected, the parasitaemia is significantly lower in P. yoelii-infected mice in the absence of IFN-gamma. More precisely, at early time points of infection, IFN-gamma favours parasitaemia, whereas helping to clear efficiently the blood-stage parasites at later time points. Interestingly, the early IFN-gamma burst is induced by the pre-erythrocytic stage.

CONCLUSION

These results challenge the current view regarding the role of IFN-gamma on the control of parasite growth since they show that IFN-gamma is not an essential mediator of protection in P. yoelii-infected C57BL/6 mice. Moreover, the mice parasitaemia is more efficiently controlled in the absence of an early IFN-gamma production, suggesting that this cytokine promotes parasite's growth. Finally, this early burst of IFN-gamma is induced by the pre-erythrocytic stage, showing the impact of this stage on the immune response taking place during the subsequent erythrocytic stage.

Cellular effector mechanisms against Plasmodium liver stages

Advances in our understanding of the molecular and cell biology of the malaria parasite have led to new vaccine development efforts resulting in a pipeline of over 40 candidates undergoing clinical phase I-III trials. Vaccine-induced CD4+ and CD8+ T cells specific for pre-erythrocytic stage antigens have been found to express cytolytic and multi-cytokine effector functions that support a key role for these T cells within the hepatic environment. However, little is known of the cellular interactions that occur during the effector phase in which the intracellular hepatic stage of the parasite is targeted and destroyed. This review focuses on cell biological aspects of the interaction between malaria-specific effector cells and the various antigen-presenting cells that are known to exist within the liver, including hepatocytes, dendritic cells, Kupffer cells, stellate cells and sinusoidal endothelia. Considering the unique immune properties of the liver, it is conceivable that these different hepatic antigen-presenting cells fulfil distinct but complementary roles during the effector phase against Plasmodium liver stages.

Evolutionary struggles between NK cells and viruses

Natural killer (NK) cells are well recognized for their ability to provide a first line of defence against viral pathogens and they are increasingly being implicated in immune responses against certain bacterial and parasitic infections. Reciprocally, viruses have devised numerous strategies to evade the activation of NK cells and have influenced the evolution of NK-cell receptors and their ligands. NK cells contribute to host defence by their ability to rapidly secrete cytokines and chemokines, as well as to directly kill infected host cells. In addition to their participation in the immediate innate immune response against infection, interactions between NK cells and dendritic cells shape the nature of the subsequent adaptive immune response to pathogens.

Intrahepatic endothelial and Kupffer cells involved in immunosuppressive cytokines and natural killer (NK)/NK T cell disorders in viral acute hepatitis

During acute viral hepatitis, the intrahepatic tolerance sustained by immunosuppressive cytokines such as interleukin (IL)-4, IL-10, transforming growth factor (TGF)-beta and prostaglandin E2 (PGE2), produced by Kupffer cells (KC), liver sinusoidal endothelial cells (LSEC), natural killer (NK) T cells and natural regulatory T cells may be disturbed. NK cells are recruited normally in the liver and produce interferon (IFN)-gamma to control viral replication. The use of mouse hepatitis virus type 3 (MHV3) attenuated variants showing selected tropisms for KC or LSEC have allowed determining their roles in the disturbances of immune tolerance during viral hepatitis. Groups of C57BL/6 mice were infected with the pathogenic L2-MHV3 (KC+, LSEC+), low attenuated 51.6-MHV3 (KC+, LSEC-) or high attenuated CL12-MHV3 (KC-, LSEC-) variants for the first 3 days. Results showed that IL-10, TGF-beta and PGE2 production in the liver decreased in L2-MHV3-infected mice and increased in 51.6-MHV3- and CL12-MHV3-infected mice. The ratio of IFN-gamma/IL-4 in liver decreased in L2-MHV3-infected mice, while it was not (or low) altered in mice infected with the attenuated MHV3 variant mice. Phenotypic analysis of intrahepatic mononuclear cells revealed that apoptotic NK and NK T cells increased in mice infected with the L2-MHV3, but were minor in 51.6-MHV3- and CL12-MHV3-infected mice. The numbers of CD4+ forkhead box P3+ cells increased in the livers from low pathogenic CL12-MHV3 and YAC-MHV3-infected mice. These results indicate that viral permissivity of KC and LSEC is involved in the decrease of IL-10 and PGE2, while KC may play an additional role in the apoptosis of NK and NK T cells during acute viral hepatitis.

Class II-restricted protective immunity induced by malaria sporozoites

The irradiated-sporozoite vaccine elicits sterile immunity against Plasmodium parasites in experimental rodent hosts and human volunteers. Based on rodent malaria models, it has been proposed that CD8+ T cells are the key protective effector mechanism required in sporozoite-induced immunity. To investigate the role of class II-restricted immunity in protective immunity, we immunized beta2-microglobulin knockout (beta2M-/-) mice with irradiated Plasmodium yoelii or P. berghei sporozoites. Sterile immunity was obtained in the CD8+-T-cell-deficient mice immunized with either P. berghei or P. yoelii sporozoites. beta2M-/- mice with the BALB/c (H-2d) genetic background as well as those with the C57BL (H-2b) genetic background were protected. Effector mechanisms included CD4+ T cells, mediated in part through the production of gamma interferon, and neutralizing antibodies that targeted the extracellular sporozoites. We conclude that in the absence of class I-restricted CD8+ T cells, sporozoite-induced protective immunity can be effectively mediated by class II-restricted immune effector mechanisms. These results support efforts to develop subunit vaccines that effectively elicit high levels of antibody and CD4+ T cells to target Plasmodium pre-erythrocytic stages.

Macrophage-mediated but gamma interferon-independent innate immune responses control the primary wave of Plasmodium yoelii parasitemia

In most models of blood-stage malaria infection, proinflammatory immune responses are required for control of infection and elimination of parasites. We hypothesized therefore that the fulminant infections caused in mice by the lethal strain of Plasmodium yoelii (17XL) might be due to failure to activate a sufficient inflammatory response. Here we have compared the adaptive CD4+ T-cell and innate immune response to P. yoelii 17XL with that induced by the self-resolving, nonlethal strain of P. yoelii, 17X(NL). During the first 7 to 9 days of infection, splenic effector CD4+ T-cell responses were similar in mice with lethal and nonlethal infections with similar levels of activation in vivo and equivalent proliferation in vitro following mitogenic stimulation. Nonspecific T-cell hyporesponsiveness was observed at similar levels during both infections and was due, in part, to suppression mediated by CD11b+ cells. Importantly, however, RAG-/- mice were able to control the initial growth phase of nonlethal P. yoelii infection as effectively as wild-type mice, indicating that T cells and/or B cells play little, if any, role in control of the primary peak of parasitemia. Somewhat unexpectedly, we could find no clear role for either NK cells or gamma interferon (IFN-gamma) in controlling primary P. yoelii infection. In contrast, depletion of monocytes/macrophages exacerbated parasite growth and anemia during both lethal and nonlethal acute P. yoelii infections, indicating that there is an IFN-gamma-, NK cell-, and T-cell-independent pathway for induction of effector macrophages during acute malaria infection.

NK cells stimulate recruitment of CXCR3+ T cells to the brain during Plasmodium berghei-mediated cerebral malaria

NK cells are cytotoxic lymphocytes that also secrete regulatory cytokines and can therefore influence adaptive immune responses. NK cell function is largely controlled by genes present in a genomic region named the NK complex. It has been shown that the NK complex is a genetic determinant of murine cerebral malaria pathogenesis mediated by Plasmodium berghei ANKA. In this study, we show that NK cells are required for cerebral malaria disease induction and the control of parasitemia. NK cells were found infiltrating brains of cerebral malaria-affected mice. NK cell depletion resulted in inhibition of T cell recruitment to the brain of P. berghei-infected animals. NK cell-depleted mice displayed down-regulation of CXCR3 expression and a significant reduction of T cells migrating in response to IFN-gamma-inducible protein 10, indicating that this chemokine pathway plays an essential role in leukocyte trafficking leading to cerebral disease and fatalities.

Genetically attenuated P36p-deficient Plasmodium berghei sporozoites confer long-lasting and partial cross-species protection

Immunisation with live, radiation-attenuated sporozoites (RAS) or genetically attenuated sporozoites (GAS) of rodent plasmodial parasites protects against subsequent challenge infections. We recently showed that immunisation with Plasmodium berghei GAS that lack the microneme protein P36p protects mice for a period of up to 4 months. Here, we show that the period of full protection induced by p36p(-)-sporozoites lasts 12 and 18 months in C57Bl6 and BALB/c mice, respectively. Full protection is also achieved with three doses of only 1000 p36p(-) (but not RAS) sporozoites. Subcutaneous, intradermal or intramuscular routes of administration also lead to partial protection. In addition, immunisation with either P. berghei RAS- or, to a lesser extent, p36p(-)-sporozoites inhibits parasite intrahepatic development in mice challenged with Plasmodium yoelii sporozoites. Since naturally acquired malaria infections or subunit-based vaccines only induce short-term immune responses, the protection conferred by immunisation with p36p(-)-sporozoites described here further emphasises the potential of GAS as a vaccination strategy for malaria.

Primary infection of C57BL/6 mice with Plasmodium yoelii induces a heterogeneous response of NKT cells

NKT cells are a population of innate-like lymphocytes that display effector functions and immunoregulatory properties. We characterized the NKT cell response induced in C57BL/6 mice during a primary infection with Plasmodium yoelii sporozoites. We observed a heterogeneous NKT cell response that differed between liver and spleen. Hepatic NKT cells found in infected livers consisted mainly of CD1d-dependent CD4+ and double-negative (DN) NKT cells, whereas CD1d-independent NKT cells exhibiting a TCR(high) CD4(high) phenotype were prominent among splenic NKT cells during the infection. Hepatic and splenic NKT cells isolated from infected mice were activated and secreted mainly gamma interferon and tumor necrosis factor alpha in response to stimulation. Finally, P. yoelii-activated hepatic DN NKT cells inhibited the parasite's liver stage in a CD1d-dependent manner in vitro. However, experiments using B6.CD1d-deficient mice showed that CD1d and CD1d-restricted NKT cells are not necessary to control the parasite's development in vivo during neither the preerythrocytic stage nor the erythrocytic stage. Thus, our results show that a primary P. yoelii infection induces a heterogeneous and organ-specific response of NKT cells and that CD1d-dependent NKT cells play a minor role in the control of the development of Plasmodium in vivo in our model.

Natural killer cells and malaria

Malaria, caused by the infection with parasites of the germs Plasmodium, is one of the three most important infectious diseases worldwide, along with tuberculosis and infection with human immunodeficiency virus. Natural killer (NK) cells are lymphocytes classically involved in the early defense against viral infections and intracytoplasmic bacterial infections and are also implicated during the course of tumor development and allogeneic transplantation. These cells display important cytotoxic activity and produce high levels of proinflammatory cytokines. In both mouse and human models of malaria, NK cells appear to be a major source of interferon-gamma during the early phase of infection. In humans, indirect signaling through monocytes/macrophages required to optimally stimulate NK cell activity. However, the in vivo functions of NK cells during malaria are still enigmatic, and many issues remain to be dissected, such as the molecular basis of the direct recognition of iRBCs by NK cells.