Dendritic cell biology during malaria

The spleen: "epicenter" in malaria infection and immunity

The spleen is a complex secondary lymphoid organ that plays a crucial role in controlling blood‐stage infection with Plasmodium parasites. It is tasked with sensing and removing parasitized RBCs, erythropoiesis, the activation and differentiation of adaptive immune cells, and the development of protective immunity, all in the face of an intense inflammatory environment. This paper describes how these processes are regulated following infection and recognizes the gaps in our current knowledge, highlighting recent insights from human infections and mouse models.

Tissue-Resident CD169(+) Macrophages Form a Crucial Front Line against Plasmodium Infection

Tissue macrophages exhibit diverse functions, ranging from the maintenance of tissue homeostasis, including clearance of senescent erythrocytes and cell debris, to modulation of inflammation and immunity. Their contribution to the control of blood-stage malaria remains unclear. Here, we show that in the absence of tissue-resident CD169(+) macrophages, Plasmodium berghei ANKA (PbA) infection results in significantly increased parasite sequestration, leading to vascular occlusion and leakage and augmented tissue deposition of the malarial pigment hemozoin. This leads to widespread tissue damage culminating in multiple organ inflammation. Thus, the capacity of CD169(+) macrophages to contain the parasite burden and its sequestration into different tissues and to limit infection-induced inflammation is crucial to mitigating Plasmodium infection and pathogenesis.

Splenic CD11c+ cells derived from semi-immune mice protect naïve mice against experimental cerebral malaria

Background

Immunity to malaria requires innate, adaptive immune responses and Plasmodium-specific memory cells. Previously, mice semi-immune to malaria was developed. Three cycles of infection and cure (‘three-cure’) were required to protect mice against Plasmodium berghei (ANKA strain) infection.

Methods

C57BL/6 J mice underwent three cycles of P. berghei infection and drug-cure to become semi-immune. The spleens of infected semi-immune mice were collected for flow cytometry analysis. CD11c(+) cells of semi-immune mice were isolated and transferred into naïve mice which were subsequently challenged and followed up by survival and parasitaemia.

Results

The percentages of splenic CD4(+) and CD11c(+) cells were increased in semi-immune mice on day 7 post-infection. The proportion and number of B220(+)CD11c(+)low cells (plasmacytoid dendritic cells, DCs) was higher in semi-immune, three-cure mice than in their naïve littermates on day 7 post-infection (2.6 vs 1.1% and 491,031 vs 149,699, respectively). In adoptive transfer experiment, three months after the third cured P. berghei infection, splenic CD11c(+) DCs of non-infected, semi-immune, three-cure mice slowed Plasmodium proliferation and decreased the death rate due to neurological pathology in recipient mice. In addition, anti-P. berghei IgG1 level was higher in mice transferred with CD11c(+) cells of semi-immune, three-cure mice than mice transferred with CD11c(+) cells of naïve counterparts.

Conclusion

CD11c(+) cells of semi-immune mice protect against experimental cerebral malaria three months after the third cured malaria, potentially through protective plasmacytoid DCs and enhanced production of malaria-specific antibody.

Role of IL6, IL12B and VDR gene polymorphisms in Plasmodium vivax malaria severity, parasitemia and gametocytemia levels in an Amazonian Brazilian population

OBJECTIVE

To investigate the influence of IL6, IL12B and VDR single nucleotide polymorphisms (SNPs) in uncomplicated Plasmodium vivax infection symptoms intensity, parasitemia and gametocytemia levels in a Brazilian Amazonian population.

METHODS

A total of 167 malaria patients infected by P. vivax have parasitemia and gametocytemia levels estimated before treatment. Fourteen clinical symptoms were evaluated and included in a principal component analysis to derive a clinical symptom index. Patients were genotyped for IL6-174C>G, IL12B 735T>C, 458A>G, 159A>C, and VDR FokI, TaqI, BsmI SNPs by Taqman 5' nuclease assays. A General Linear Model analysis of covariance with age, gender, exposure period and infection history and genetic ancestry was performed to investigate the association of genotypes with parasitemia and gametocytemia levels and with a clinical symptom index.

RESULTS

Higher parasitemia levels were observed in IL6-174C carriers (p=0.02) whereas IL12B CGT haplotype carriers presented lower parasitemia levels (p=0.008). VDR TaqIC/BsmIA haplotype carriers showed higher gametocyte levels than non-carriers (p=0.013). Based on the clinical index values the IL6-174C>G polymorphism was associated with malaria severity. The IL6-174C carriers presented a more severe clinical index when compared to GG homozygotes (p=0.001).

CONCLUSION

The present study suggests that IL6, IL12 and VDR influence severity, parasitemia and gametocytemia clearance in P. vivax infections, and highlights their potential role in malaria immune response in an Amazonian population.

Immune activation and regulation in simian immunodeficiency virus-Plasmodium fragile-coinfected rhesus macaques

Human immunodeficiency virus (HIV) is characterized by immune activation, while chronic malaria is associated with elevated interleukin-10 (IL-10) levels. How these apparently antagonizing forces interact in the coinfected host is poorly understood. Using a rhesus macaque model of simian immunodeficiency virus (SIV)-Plasmodium fragile coinfection, we evaluated how innate immune effector cells affect the balance between immune activation and regulation. In vitro Toll-like receptor (TLR) responses of peripheral blood myeloid dendritic cells (mDC) and monocytes were temporarily associated with acute parasitemic episodes and elevated plasma IL-10 levels. Prolonged infection resulted in a decline of mDC function. Monocytes maintained TLR responsiveness but, in addition to IL-12 and tumor necrosis factor alpha, also produced IL-10. Consistent with the role of spleen in the clearance of parasite-infected red blood cells, coinfected animals also had increased splenic IL-10 mRNA levels. The main cellular source of IL-10 in the spleens of coinfected animals, however, was not splenic macrophages but T cells, suggesting an impairment of adaptive immunity. In contrast to those in spleen, IL-10-positive cells in axillary lymph nodes of coinfected animals were predominantly mDC, reminiscent of the immunosuppressive phenotype of peripheral blood mDC. Concurrent with IL-10 induction, however, SIV infection promoted elevated systemic IL-12 levels. The continuously increasing ratio of plasma IL-12 to IL-10 suggested that the overall host response in SIV-P. fragile-coinfected animals was shifted toward immune activation versus immune regulation. Therefore, SIV-P. fragile coinfection might be characterized by earlier manifestation of immune dysfunction and exhaustion than that of single-pathogen infections. This could translate into increased morbidity in HIV-malaria-coinfected individuals.

Babesia bovis:lipids from virulent S2P and attenuated R1A strains trigger differential signalling and inflammatory responses in bovine macrophages

The intra-erythrocytic protozoanBabesia bovisis an economically important pathogen that causes an acute and often fatal infection in adult cattle. Babesiosis limitation depends on the early activation of macrophages, essential cells of the host innate immunity, which can generate an inflammatory response mediated by cytokines and nitric oxide (NO). Herein, we demonstrate in bovine macrophages that lipids fromB. bovisattenuated R1A strain (LA) produced a stronger NO release, an early TNFαmRNA induction and 2-fold higher IL-12p35 mRNA levels compared to the lipids of virulent S2P strain (LV). Neither LAnor LVinduced anti-inflammatory IL-10. Regarding signalling pathways, we here report that LAinduced a significant phosphorylation of p38 and extracellular signal-regulated kinases 1 and 2 (ERK1/2) whereas LVonly induced a reduced activation of ERK1/2. Besides, NF-κB was activated by LAand LV, but LAproduced an early degradation of the inhibitor IκB. Interestingly, LVand the majority of its lipid fractions, exerted a significant inhibition of concanavalin A-induced peripheral blood mononuclear cell proliferation with respect to LAand its corresponding lipid fractions. In addition, we determined that animals infected with R1A developed a higher increase in IgM anti-phosphatidylcholine than those inoculated with S2P. Collectively, S2P lipids generated a decreased inflammatory response contributing to the evasion of innate immunity. Moreover, since R1A lipids induced a pro-inflammatory profile, we propose these molecules as good candidates for immunoprophylactic strategies against babesiosis.

Are plasmacytoid dendritic cells the misguided sentinels of malarial immunity?

Dendritic cells (DCs), the sentinels of immunity, reside in almost every organ of the body. These cells are responsible for initiating immune responses against infectious agents. DCs are divided into different subsets based on their biological functions, with plasmacytoid DCs (pDCs) and conventional DCs (cDCs) being two major populations. The ability of DCs to protect against malaria infection was recently questioned when pDCs were reported to be a reservoir for rodent Plasmodium spp. in the spleen. This opinion article explores how the occupation of pDCs by the parasite may corrupt immunity against malaria.

Multiple functions of human T cells generated by experimental malaria challenge

Protective immunity generated following malaria infection may be comprised of Ab or T cells against malaria Ag of different stages; however, the short-lived immunity that is observed suggests deficiency in immune memory or regulatory activity. In this study, cellular immune responses were investigated in individuals receiving Plasmodium falciparum sporozoite challenge by the natural (mosquito bite) route as part of a malaria vaccine efficacy trial. Parasitemia, monitored by blood film microscopy and PCR, was subsequently cleared with drugs. All individuals demonstrated stable IFN-gamma, IL-2 and IL-4 ex vivo ELISPOT effector responses against P. falciparum-infected RBC (iRBC) Ag, 28 and 90 days after challenge. However, infected RBC-specific central memory responses, as measured by IFN-gamma cultured ELISPOT, were low and unstable over time, despite CD4(+) T cells being highly proliferative by CFSE dilution, and showed an inverse relationship to parasite density. In support of the observation of poor memory, co-culture experiments showed reduced responses to common recall Ag, indicating malaria-specific regulatory activity. This activity could not be accounted for by the expression of IL-10, TGF-beta, FOXP3 or CTLA-4, but proliferating T cells expressed high levels of CD95, indicating a pro-apoptotic phenotype. Lastly, there was an inverse relationship between FOXP3 expression, when measured 10 days after challenge, and ex vivo IFN-gamma measured more than 100 days later. This study shows that malaria infection elicits specific Th1 and Th2 effector cells, but concomitant weak central memory and regulatory activity, which may help to explain the short-lived immunity observed.

Genetically attenuated parasite vaccines induce contact-dependent CD8+ T cell killing of Plasmodium yoelii liver stage-infected hepatocytes

The production of IFN-gamma by CD8(+) T cells is an important hallmark of protective immunity induced by irradiation-attenuated sporozoites against malaria. Here, we demonstrate that protracted sterile protection conferred by a Plasmodium yoelii genetically attenuated parasite (PyGAP) vaccine was completely dependent on CD8(+) T lymphocytes but only partially dependent on IFN-gamma. We used live cell imaging to document that CD8(+) CTL from PyGAP-immunized mice directly killed hepatocyte infected with a liver stage parasite. Immunization studies with perforin and IFN-gamma knockout mice also indicated that the protection was largely dependent on perforin-mediated effector mechanisms rather than on IFN-gamma. This was further supported by our observation that both liver and spleen CD8(+) T cells from PyGAP-immunized mice induced massive apoptosis of liver stage-infected hepatocytes in vitro without the release of detectable IFN-gamma and TNF-alpha. Conversely, CD8(+) T cells isolated from naive mice that had survived wild-type P. yoelii sporozoite infection targeted mainly sporozoite-traversed and uninfected hepatocytes, revealing an immune evasion strategy that might be used by wild-type parasites to subvert host immune responses during natural infection. However, CTLs from wild-type sporozoite-challenged mice could recognize and kill infected hepatocytes that were pulsed with circumsporozoite protein. Additionally, protection in PyGAP-immunized mice directly correlated with the magnitude of effector memory CD8(+) T cells. Our findings implicate CTLs as key immune effectors in a highly protective PyGAP vaccine for malaria and emphasize the critical need to define surrogate markers for correlates of protection, apart from IFN-gamma.

What really happens to dendritic cells during malaria?

As dendritic cells (DCs) initiate all adaptive and some innate immune responses, it is not surprising that DC function during malaria is the subject of intensive investigations. However, the results of these investigations have so far been controversial. Here, we discuss various aspects of these studies, including the influence of the species and strain of Plasmodium on DC function, the effects of Plasmodium infection on the activation of CD8(+) T cells by DCs, the effects of haemozoin and the effects of Plasmodium infections on DC Toll-like-receptor signalling.

New insight into the role of dendritic cells in malaria immune pathogenesis

The mechanism by which the host develops protective immunity to malaria remains poorly understood. Dendritic cells (DCs) are central to the initiation and regulation of the adaptive immune response. Modulation of DC function might enable Plasmodium to evade the immune system. Millington et al. propose one mechanism by which malaria inhibits DC-T-cell interactions without interfering directly with T-cell receptor engagement. The consequence is a decrease in the co-stimulation required to develop an effective immune response.

Plasmodium infection and endotoxic shock induce the expansion of regulatory dendritic cells

During an acute Plasmodium infection, uncontrolled proinflammatory responses can cause morbidity and mortality. Regulation of this response is required to prevent immunopathology. We therefore decided to investigate a recently characterized subset of regulatory dendritic cells (DCs) that expresses low levels of CD11c and high levels of CD45RB. During a Plasmodium yoelii infection, these regulatory CD11clowCD45RBhigh DCs become the prevalent CD11c-expressing cells in the spleen, overtaking the conventional CD11chigh DCs. Furthermore, the regulatory CD11clowCD45RBhigh DCs induce IL-10-expressing CD4 T cells. A similar change in splenic DC subsets is seen when mice are injected with sublethal doses of LPS, suggesting that shifting the splenic DC subsets in favor of regulatory CD11clowCD45RBhigh DCs can be triggered solely by a high inflammatory stimulus. This is the first time regulatory DCs have been observed in a natural immune response to an infectious disease or endotoxic shock.

Dendritic cell preactivation impairs MHC class II presentation of vaccines and endogenous viral antigens

When dendritic cells (DCs) encounter signals associated with infection or inflammation, they become activated and undergo maturation. Mature DCs are very efficient at presenting antigens captured in association with their activating signal but fail to present subsequently encountered antigens, at least in vitro. Such impairment of MHC class II (MHC II) antigen presentation has generally been thought to be a consequence of down-regulation of endocytosis, so it might be expected that antigens synthesized by the DCs themselves (for instance, viral antigens) would still be presented by mature DCs. Here, we show that DCs matured in vivo could still capture and process soluble antigens, but were unable to present peptides derived from these antigens. Furthermore, presentation of viral antigens synthesized by the DCs themselves was also severely impaired. Indeed, i.v. injection of pathogen mimics, which caused systemic DC activation in vivo, impaired the induction of CD4 T cell responses against subsequently encountered protein antigens. This immunosuppressed state could be reversed by adoptive transfer of DCs loaded exogenously with antigens, demonstrating that impairment of CD4 T cell responses was due to lack of antigen presentation rather than to overt suppression of T cell activation. The biochemical mechanism underlying this phenomenon was the down-regulation of MHC II-peptide complex formation that accompanied DC maturation. These observations have important implications for the design of prophylactic and therapeutic DC vaccines and contribute to the understanding of the mechanisms causing immunosuppression during systemic blood infections.

Coadaptation and malaria control

Malaria emerges from a disequilibrium of the system 'human-plasmodium-mosquito' (HPM). If the equilibrium is maintained, malaria does not ensue and the result is asymptomatic plasmodium infection. The relationships among the components of the system involve coadaptive linkages that lead to equilibrium. A vast body of evidence supports this assumption, including the strategies involved in the relationships between plasmodium and human and mosquito immune systems, and the emergence of resistance of plasmodia to antimalarial drugs and of mosquitoes to insecticides. Coadaptive strategies for malaria control are based on the following principles: (1) the system HPM is composed of three highly complex and dynamic components, whose interplay involves coadaptive linkages that tend to maintain the equilibrium of the system; (2) human and mosquito immune systems play a central role in the coadaptive interplay with plasmodium, and hence, in the maintenance of the system's equilibrium; the under- or overfunction of human immune system may result in malaria and influence its severity; (3) coadaptation depends on genetic and epigenetic phenomena occurring at the interfaces of the components of the system, and may involve exchange of infectrons (genes or gene fragments) between the partners; (4) plasmodia and mosquitoes have been submitted to selective pressures, leading to adaptation, for an extremely long while and are, therefore, endowed with the capacity to circumvent both natural (immunity) and artificial (drugs, insecticides, vaccines) measures aiming at destroying them; (5) since malaria represents disequilibrium of the system HPM, its control should aim at maintaining or restoring this equilibrium; (6) the disequilibrium of integrated systems involves the disequilibrium of their components, therefore the maintenance or restoration of the system's equilibrium depend on the adoption of integrated and coordinated measures acting on all components, that means, panadaptive strategies. Coadaptive strategies for malaria control should consider that: (1) host immune response has to be induced, since without it, no coadaptation is attained; (2) the immune response has to be sustained and efficient enough to avoid plasmodium overgrowth; (3) the immune response should not destroy all parasites; (4) the immune response has to be well controlled in order to not harm the host. These conditions are mostly influenced by antimalarial drugs, and should also be taken into account for the development of coadaptive malaria vaccines.

Unique functions of splenic CD8alpha+ dendritic cells during infection with intracellular pathogens

Deciphering the prerequisites for the induction of protective cytotoxic T cell responses is essential for future development of more effective CD8(+) T cell-based vaccines against infectious diseases and cancer. Since crucial events for CD8(+) T cell priming and differentiation occur during the first contacts of naïve T cells with distinct antigen-presenting cells (APCs), the identification and therapeutic targeting of these 'master' APCs has become a major quest in the field. A decade ago, dendritic cells (DCs) were discovered as potent APCs, as they combine all major features for the initiation of T cell responses: (1) naïve DCs demonstrate high endocytic activity and scan continuously their environment in strategic positions throughout the whole body; (2) after activation (e.g. during pathogen invasion), DCs migrate into T cell zones of their draining lymphatic compartments, meanwhile processing captured antigen and maturing in order to stimulate encountered antigen-specific T cells. During the last years, different subsets of DCs that can be distinguished by specific surface marker expression and effector functions have been identified in mice. Their distinct functional capabilities have led to the concept of work-sharing; "migrating" DCs primarily transport antigens to the lymph node, where a specialized subset of "resident" DCs, defined by the expression of the CD8alphaalpha homodimer (CD8alpha(+) DCs), primes CD8(+) T cells upon antigen cross-presentation. Accordingly, CD8alpha(+) DCs have been found to prime CD8(+) T cells against different viruses as well as intracellular bacteria such as Listeria monocytogenes (L.m.). Recently, L.m. was found to survive specifically in splenic CD8alpha(+) DCs shortly after intravenous infection. Further experiments revealed a more generalized sampling activity of CD8alpha(+) DCs for blood-borne particles. These findings indicate that splenic CD8alpha(+) DCs might unite efficacious antigen-trapping with the licence to prime CD8(+) T cells. This new aspect of DC function could have evolved to guarantee a more rapid antigen-specific response against generalized infections.

Intrinsic and cooperative antigen-presenting functions of dendritic-cell subsets in vivo

Dendritic cells (DCs) comprise several subsets, and their roles in the presentation of antigens derived from pathogens, vaccines and self tissues are now beginning to be elucidated. Differences in location, life cycle and intrinsic abilities to capture, process and present antigens on their MHC class I and class II molecules enable each DC subset to have distinct roles in immunity to infection and in the maintenance of self tolerance. Unexpected interactions among DC subsets have also been revealed. These interactions, which allow the integration of the intrinsic abilities of different DC types, enhance the ability of the DC network to respond to multiple scenarios of infection.

T cell memory in malaria

The observation that individuals living in malaria endemic areas fail to develop sterilizing immunity to malaria infection has led to the assumption that malaria-specific immune responses are sub-optimal. Recently, T cell receptor (TCR) transgenic mice specific for the sporozoite and blood stages of the malaria parasite have been developed. Studies using these models have found that, unexpectedly, T cell memory in malaria is not noticeably defective. However, if T cell memory is 'normal' why are people not better protected? We suggest this is because protective immunity and T cell memory do not always correlate; moreover, T cells alone may simply not be able to provide the type of antibody-mediated sterilizing immunity induced by traditional vaccines.