Increased levels of soluble Fas ligand in serum in Plasmodium falciparum malaria

SARS-CoV-2-associated lymphopenia: possible mechanisms and the role of CD147

T lymphocytes play a primary role in the adaptive antiviral immunity. Both lymphocytosis and lymphopenia were found to be associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). While lymphocytosis indicates an active anti-viral response, lymphopenia is a sign of poor prognosis. T-cells, in essence, rarely express ACE2 receptors, making the cause of cell depletion enigmatic. Moreover, emerging strains posed an immunological challenge, potentially alarming for the next pandemic. Herein, we review how possible indirect and direct key mechanisms could contribute to SARS-CoV-2-associated-lymphopenia. The fundamental mechanism is the inflammatory cytokine storm elicited by viral infection, which alters the host cell metabolism into a more acidic state. This "hyperlactic acidemia" together with the cytokine storm suppresses T-cell proliferation and triggers intrinsic/extrinsic apoptosis. SARS-CoV-2 infection also results in a shift from steady-state hematopoiesis to stress hematopoiesis. Even with low ACE2 expression, the presence of cholesterol-rich lipid rafts on activated T-cells may enhance viral entry and syncytia formation. Finally, direct viral infection of lymphocytes may indicate the participation of other receptors or auxiliary proteins on T-cells, that can work alone or in concert with other mechanisms. Therefore, we address the role of CD147-a novel route-for SARS-CoV-2 and its new variants. CD147 is not only expressed on T-cells, but it also interacts with other co-partners to orchestrate various biological processes. Given these features, CD147 is an appealing candidate for viral pathogenicity. Understanding the molecular and cellular mechanisms behind SARS-CoV-2-associated-lymphopenia will aid in the discovery of potential therapeutic targets to improve the resilience of our immune system against this rapidly evolving virus.

Association of apoptosis-related variants to malaria infection and parasite density in individuals from the Brazilian Amazon

BACKGROUND

In malaria infection, apoptosis acts as an important immunomodulatory mechanism that leads to the elimination of parasitized cells, thus reducing the parasite density and controlling immune cell populations. Here, it was investigated the association of INDEL variants in apoptotic genes-rs10562972 (FAS), rs4197 (FADD), rs3834129 and rs59308963 (CASP8), rs61079693 (CASP9), rs4647655 (CASP3), rs11269260 (BCL-2), and rs17880560 (TP53)-and the influence of genetic ancestry with susceptibility to malaria and parasite density in an admixed population from the Brazilian Amazon.

METHODS

Total DNA was extracted from 126 malaria patients and 101 uninfected individuals for investigation of genetic ancestries and genotypic distribution of apoptosis-related variants by Multiplex PCR. Association analyses consisted of multivariate logistic regressions, considering the following comparisons: (i) DEL/DEL genotype vs. INS/DEL + INS/INS; and (ii) INS/INS vs. INS/DEL + DEL/DEL.

RESULTS

Individuals infected by Plasmodium falciparum had significantly higher African ancestry proportions in comparison to uninfected controls, Plasmodium vivax, and mixed infections. The INS/INS genotype of rs3834129 (CASP8) seemed to increase the risk for P. falciparum infection (P = 0.038; OR = 1.867; 95% CI 0.736-3.725), while the DEL/DEL genotype presented a significant protective effect against infection by P. falciparum (P = 0.049; OR = 0.446; 95% CI 0.185-0.944) and mixed infection (P = 0.026; OR = 0.545; 95% CI 0.281-0.996), and was associated with lower parasite density in P. falciparum malaria (P = 0.009; OR = 0.383; 95% CI 0.113-1.295). Additionally, the INS/INS genotype of rs10562972 (FAS) was more frequent among individuals infected with P. vivax compared to P. falciparum (P = 0.036; OR = 2.493; 95% CI 1.104-4.551), and the DEL/DEL genotype of rs17880560 (TP53) was significantly more present in patients with mono-infection by P. vivax than in individuals with mixed infection (P = 0.029; OR = 0.667; 95% CI 0.211-1.669).

CONCLUSIONS

In conclusion, variants in apoptosis genes are associated with malaria susceptibility and parasite density, indicating the role of apoptosis-related genetic profiles in immune responses against malaria infection.

Immunogenomic profile at baseline predicts host susceptibility to clinical malaria

Introduction

Host gene and protein expression impact susceptibility to clinical malaria, but the balance of immune cell populations, cytokines and genes that contributes to protection, remains incompletely understood. Little is known about the determinants of host susceptibility to clinical malaria at a time when acquired immunity is developing.

Methods

We analyzed peripheral blood mononuclear cells (PBMCs) collected from children who differed in susceptibility to clinical malaria, all from a small town in Mali. PBMCs were collected from children aged 4-6 years at the start, peak and end of the malaria season. We characterized the immune cell composition and cytokine secretion for a subset of 20 children per timepoint (10 children with no symptomatic malaria age-matched to 10 children with >2 symptomatic malarial illnesses), and gene expression patterns for six children (three per cohort) per timepoint.

Results

We observed differences between the two groups of children in the expression of genes related to cell death and inflammation; in particular, inflammatory genes such as CXCL10 and STAT1 and apoptotic genes such as XAF1 were upregulated in susceptible children before the transmission season began. We also noted higher frequency of HLA-DR+ CD4 T cells in protected children during the peak of the malaria season and comparable levels cytokine secretion after stimulation with malaria schizonts across all three time points.

Conclusion

This study highlights the importance of baseline immune signatures in determining disease outcome. Our data suggests that differences in apoptotic and inflammatory gene expression patterns can serve as predictive markers of susceptibility to clinical malaria.

Memory B cell diversity: insights for optimized vaccine design

The overarching logos of mammalian memory B cells (MBCs) is to cache the potential for enhanced antibody production upon secondary exposure to cognate antigenic determinants. However, substantial phenotypic diversity has been identified across MBCs, hinting at the existence of unique origins or subfunctions within this compartment. Herein, we discuss recent advancements in human circulatory MBC subphenotyping as driven by high-throughput cell surface marker analysis and other approaches, as well as speculated and substantiated subfunctions. With this in mind, we hypothesize that the relative induction of specific circulatory MBC subsets might be used as a biomarker for optimally durable vaccines and inform vaccination strategies to subvert antigenic imprinting in the context of highly mutable pathogens such as influenza virus or SARS-CoV-2.

Soluble CD95L in cancers and chronic inflammatory disorders, a new therapeutic target?

Although CD95L (also known as FasL) is still predominantly considered as a death ligand that induces apoptosis in infected and transformed cells, substantial evidence indicate that it can also trigger non-apoptotic signaling pathways whose pathophysiological roles remain to be fully elucidated. The transmembrane ligand CD95L belongs to the tumor necrosis factor (TNF) superfamily. After cleavage by metalloprotease, its soluble form (s-CD95L) fails to trigger the apoptotic program but instead induces signaling pathways promoting the aggressiveness of certain inflammatory disorders such as autoimmune diseases and cancers. We propose to evaluate the various pathologies in which the metalloprotease-cleaved CD95L is accumulated and analyze whether this soluble ligand may play a significant role in the pathology progression. Based on the TNFα-targeting therapeutics, we envision that targeting the soluble form of CD95L may represent a very attractive therapeutic option in the pathologies depicted herein.

Unraveling Cell Death Pathways during Malaria Infection: What Do We Know So Far?

Malaria is a parasitic disease (caused by different Plasmodium species) that affects millions of people worldwide. The lack of effective malaria drugs and a vaccine contributes to this disease, continuing to cause major public health and socioeconomic problems, especially in low-income countries. Cell death is implicated in malaria immune responses by eliminating infected cells, but it can also provoke an intense inflammatory response and lead to severe malaria outcomes. The study of the pathophysiological role of cell death in malaria in mammalians is key to understanding the parasite-host interactions and design prophylactic and therapeutic strategies for malaria. In this work, we review malaria-triggered cell death pathways (apoptosis, autophagy, necrosis, pyroptosis, NETosis, and ferroptosis) and we discuss their potential role in the development of new approaches for human malaria therapies.

Cytokine Profile Distinguishes Children With Plasmodium falciparum Malaria From Those With Bacterial Blood Stream Infections

Background

Malaria presents with unspecific clinical symptoms that frequently overlap with other infectious diseases and is also a risk factor for coinfections, such as non-Typhi Salmonella. Malaria rapid diagnostic tests are sensitive but unable to distinguish between an acute infection requiring treatment and asymptomatic malaria with a concomitant infection. We set out to test whether cytokine profiles could predict disease status and allow the differentiation between malaria and a bacterial bloodstream infection.

Methods

We created a classification model based on cytokine concentration levels of pediatric inpatients with either Plasmodium falciparum malaria or a bacterial bloodstream infection using the Luminex platform. Candidate markers were preselected using classification and regression trees, and the predictive strength was calculated through random forest modeling.

Results

Analyses revealed that a combination of 7–15 cytokines exhibited a median disease prediction accuracy of 88% (95th percentile interval, 73%–100%). Haptoglobin, soluble Fas-Ligand, and complement component C2 were the strongest single markers with median prediction accuracies of 82% (with 95th percentile intervals of 71%–94%, 62%–94%, and 62%–94%, respectively).

Conclusions

Cytokine profiles possess good median disease prediction accuracy and offer new possibilities for the development of innovative point-of-care tests to guide treatment decisions in malaria-endemic regions.

Changes in total and differential leukocyte counts during the clinically silent liver phase in a controlled human malaria infection in malaria-naïve Dutch volunteers

Background

Both in endemic countries and in imported malaria, changes in total and differential leukocyte count during Plasmodium falciparum infection have been described. To study the exact dynamics of differential leukocyte counts and their ratios, they were monitored in a group of healthy non-immune volunteers in two separate Controlled Human Malaria Infection (CHMI) studies.

Methods

In two CHMI trials, CHMI-a and CHMI-b, 15 and 24 healthy malaria-naïve volunteers, respectively, were exposed to bites of infected mosquitoes, using the P. falciparum research strain NF54 and the novel clones NF135.C10 and NF166.C8. After mosquito bite exposure, twice-daily blood draws were taken to detect parasitaemia and to monitor the total and differential leukocyte counts. All subjects received a course of atovaquone–proguanil when meeting the treatment criteria.

Results

A total of 39 volunteers participated in the two trials. Thirty-five participants, all 15 participants in CHMI-a and 20 of the 24 volunteers in CHMI-b, developed parasitaemia. During liver stage development of the parasite, the median total leukocyte count increased from 5.5 to 6.1 × 10⁹ leukocytes/L (p = 0.005), the median lymphocyte count from 1.9 to 2.2 (p = 0.001) and the monocyte count from 0.50 to 0.54 (p = 0.038). During the subsequent blood stage infection, significant changes in total and differential leukocyte counts lead to a leukocytopenia (nadir median 3.3 × 10⁹ leukocytes/L, p = 0.0001), lymphocytopenia (nadir median 0.7 × 10⁹ lymphocytes/L, p = 0.0001) and a borderline neutropenia (nadir median 1.5 × 10⁹ neutrophils/L, p = 0.0001). The neutrophil to lymphocyte count ratio (NLCR) reached a maximum of 4.0. Significant correlations were found between parasite load and absolute lymphocyte count (p < 0.001, correlation coefficient − 0.46) and between parasite load and NLCR (p < 0.001, correlation coefficient 0.50). All parameters normalized after parasite clearance.

Conclusions

During the clinically silent liver phase of malaria, an increase of peripheral total leukocyte count and differential lymphocytes and monocytes occurs. This finding has not been described previously. This increase is followed by the appearance of parasites in the peripheral blood after 2–3 days, accompanied by a marked decrease in total leukocyte count, lymphocyte count and the neutrophil count and a rise of the NLCR.

Electronic supplementary material

The online version of this article (10.1186/s12936-017-2108-1) contains supplementary material, which is available to authorized users.

A novel in vitro model reveals distinctive modulatory roles of Plasmodium falciparum and Plasmodium vivax on naïve cell-mediated immunity

BACKGROUND

To date, human peripheral blood mononuclear cells (PBMCs) have been used mainly in immune stimulation assays and the interpretation of data can be influenced by the previous immunological history of donors and cross reactivity with other infectious agents. Resolving these limitations requires an alternative in vitro model to uncover the primary response profiles.

METHODS

A novel in vitro model of mononuclear cells (MNCs) generated from haematopoietic stem cells (HSCs) was developed and these cells were then co-cultured with various antigens from Plasmodium falciparum and Plasmodium vivax to investigate the response of naïve immune cells to malaria antigens by flow cytometry.

RESULTS

In vitro stimulation of naïve lymphocytes showed that CD4+ and CD8+ T lymphocytes were significantly reduced (P < 0.01) by exposure to lysates of infected erythrocytes or intact erythrocytes infected with P. falciparum. The depletion was associated with the expression of CD95 (Fas receptor) on the surface of T lymphocytes. Maturation of T lymphocytes was affected differently, showing elevated CD3+CD4+CD8+ and CD3+CD4-CD8- T lymphocytes after stimulation with cell lysates of P. falciparum and P. vivax, respectively. In addition, antigen presenting monocytes and dendritic cells derived from haematopoietic stem cells showed impaired HLA-DR expression as a consequence of exposure to different species of malaria parasites.

CONCLUSION

These results suggest that naïve mononuclear cells differentiated in vitro from HSCs could provide a valid model for the assessment of immunity. P. falciparum and P. vivax malaria parasites could modulate various populations of immune cells starting from newly differentiated mononuclear cells.

Parasite and the Circulating Pool- Characterisation of Leukocyte Number and Morphology in Malaria

INTRODUCTION

Haematological changes are the most common complications encountered in malaria. There is significant correlation between several of the haematological parameters and the clinical profile, prognosis and mortality in malaria. White cell counts and differentials are among the most basic and primary investigations done in a patient presenting with fever of short duration.

AIM

This study analyzes the numerical and morphological changes in White Blood Cells (WBCs) in peripheral blood in patients with acute malaria in endemic region in an effort to get a picture of specific changes that could be identified by basic investigations.

MATERIALS AND METHODS

This study was conducted in tertiary care hospital in a region endemic for malaria. EDTA anticoagulated venous blood samples from 600 patients diagnosed with vivax and falciparum malaria was analysed in Coulter counter LH 500 for the white cell count and differentials. Morphological changes were looked for in Leishman stained peripheral blood smear. Comparison with age matched healthy controls was done by ANOVA with Bonferroni test wherever applicable.

RESULTS

Patients with malaria showed significant leucopenia, neutrophilia, lymphocytopenia, monocytosis and eosinopenia. Lymphocytopenia was more severe in the falciparum group as compared to the vivax group. A higher White Cell Count (WCC) was seen in patients with higher haemoglobin levels in vivax group. The total leukocyte count showed a negative correlation with neutrophil count in falciparum malaria and a strong positive correlation with neutrophil count in vivax malaria. Band neutrophils were seen in 10% of the patients with falciparum and 1.1% of patients with vivax malaria. Atypical plasmacytoid lymphocytes were the only notable morphological finding.

CONCLUSION

Changes in leukocyte number and morphology in the peripheral blood are common. A combination of monocytosis and eosinopenia in a patient presenting with fever should alert the observer to the presence of malaria and should prompt a repeat blood smear examination in case of initial negative results for the parasite.

The role of haematological parameters in predicting malaria with special emphasis on neutrophil lymphocyte count ratio and monocyte lymphocyte ratio: A single Institutional experience

Introduction:

Malaria is a mosquito borne disease which is a major public health problem and a leading cause of morbidity and mortality worldwide. Various haematological parameters have been studied to help predict malaria, such as alteration in the leucocyte count, platelet counts and erythrocyte counts. The neutrophil lymphocyte count ratio (NLCR) was found to have a good predictive value in systemic inflammation, particularly in critical care setting.

Aims and Objectives:

The present study aims to study the various haematological parameters and acertain the predictive value of NLCR and MLR in the detection of malaria.

Materials and Methods:

A prospective cross sectional study was conducted at a tertiary care hospital between the period of August to December, 2014. A total of 200 smear positive malaria patients and a control group of 100 patients who were smear negative for malaria were included. Hemoglobin, Total leucocyte count, Differential leucocyte count, platelet counts and absolute counts were obtained. The NLCR and MLR were obtained from the above data. The data was analysed by statistical tools.

Results:

A total of 200 smear positive malaria cases were analysed of which, 180 cases were caused by the Plasmodium vivax parasite and 2 cases by Plasmodium Falciparum. Thrombocytopenia and leucopenia were found to have significant association with malaria. In the present study, the NLCR and MLR was not found to have significant association with malaria.

Discussion:

Although NLCR has been proven to be a useful marker for inflammation in many acute conditions5, it is albeit not of much significance in the prediction of malaria. Similarly we have found no significance of MLR in prediction of malaria.

Lymphocyte Perturbations in Malawian Children with Severe and Uncomplicated Malaria

Lymphocytes are implicated in immunity and pathogenesis of severe malaria. Since lymphocyte subsets vary with age, assessment of their contribution to different etiologies can be difficult. We immunophenotyped peripheral blood from Malawian children presenting with cerebral malaria, severe malarial anemia, and uncomplicated malaria (n = 113) and healthy aparasitemic children (n = 42) in Blantyre, Malawi, and investigated lymphocyte subset counts, activation, and memory status. Children with cerebral malaria were older than those with severe malarial anemia. We found panlymphopenia in children presenting with cerebral malaria (median lymphocyte count, 2,100/μl) and uncomplicated malaria (3,700/μl), which was corrected in convalescence and was absent in severe malarial anemia (5,950/μl). Median percentages of activated CD69⁺ NK (73%) and γδ T (60%) cells were higher in cerebral malaria than in other malaria types. Median ratios of memory to naive CD4⁺ lymphocytes were higher in cerebral malaria than in uncomplicated malaria and low in severe malarial anemia. The polarized lymphocyte subset profiles of different forms of severe malaria are independent of age. In conclusion, among Malawian children cerebral malaria is characterized by lymphocyte activation and increased memory cells, consistent with immune priming. In contrast, there are reduced memory cells and less activation in severe malaria anemia. Further studies are required to understand whether these immunological profiles indicate predisposition of some children to one or another form of severe malaria.

Leukogram Profile and Clinical Status in vivax and falciparum Malaria Patients from Colombia

Introduction. Hematological alterations are frequent in malaria patients; the relationship between alterations in white blood cell counts and clinical status in malaria is not well understood. In Colombia, with low endemicity and unstable transmission for malaria, with malaria vivax predominance, the hematologic profile in malaria patients is not well characterized. The aim of this study was to characterize the leukogram in malaria patients and to analyze its alterations in relation to the clinical status. Methods. 888 leukogram profiles of malaria patients from different Colombian regions were studied: 556 with P. falciparum infection (62.6%), 313 with P. vivax infection (35.2%), and 19 with mixed infection by these species (2.1%). Results. Leukocyte counts at diagnosis were within normal range in 79% of patients and 18% had leucopenia; the most frequent alteration was lymphopenia (54%) followed by monocytosis (11%); the differential granulocyte count in 298 patients revealed eosinophilia (15%) and high basophil counts (8%). Leukocytosis, eosinopenia, and neutrophilia were associated with clinical complications. The utility of changes in leukocyte counts as markers of severity should be explored in depth. A better understanding of these hematological parameters will allow their use in prompt diagnosis of malaria complications and monitoring treatment response.

Human Gene Expression in Uncomplicated Plasmodium falciparum Malaria

To examine human gene expression during uncomplicated P. falciparum malaria, we obtained three samples (acute illness, treatment, and recovery) from 10 subjects and utilized each subject's recovery sample as their baseline. At the time of acute illness (day 1), subjects had upregulation of innate immune response, cytokine, and inflammation-related genes (IL-1β, IL-6, TNF, and IFN-γ), which was more frequent with parasitemias >100,000 per μL and body temperatures ≥39°C. Apoptosis-related genes (Fas, BAX, and TP53) were upregulated acutely and for several days thereafter (days 1–3). In contrast, the expression of immune-modulatory (transcription factor 7, HLV-DOA, and CD6) and apoptosis inhibitory (c-myc, caspase 8, and Fas Ligand G) genes was downregulated initially and returned to normal with clinical recovery (days 7–10). These results indicate that the innate immune response, cytokine, and apoptosis pathways are upregulated acutely in uncomplicated malaria with concomitant downregulation of immune-modulatory and apoptosis inhibitory genes.

Induction of Inhibitory Receptors on T Cells During Plasmodium vivax Malaria Impairs Cytokine Production

The function and regulation of the immune response triggered during malaria is complex and poorly understood, and there is a particular paucity of studies conducted in humans infected with Plasmodium vivax. While it has been proposed that T-cell-effector responses are crucial for protection against blood-stage malaria in mice, the mechanisms behind this in humans remain poorly understood. Experimental models of malaria have shown that the regulatory molecules, cytotoxic T-lymphocyte attenuator-4 (CTLA-4), lymphocyte activation gene-3 (LAG-3), and programmed death-1 (PD-1) are involved in the functional impairment of T cells during infection. Our goal was to define the role of these molecules during P. vivax malaria. We demonstrate that infection triggers the expression of regulatory molecules on T cells. The pattern of expression differs in CD4(+) and CD8(+) T cells. Higher frequencies of CD4(+) express more than 1 regulatory molecule compared to CD8(+) T cells. Moreover, lower proportions of CD4(+) T cells coexpress regulatory molecules, but are still able to proliferate. Importantly, simultaneously blockade of the CLTA-4, PD-1, and T-cell immunoglobulin and mucin-3 signaling restores the cytokine production by antigen-specific cells. These data support the hypothesis that upregulation of inhibitory receptors on T cells during P. vivax malaria impairs parasite-specific T-cell effector function.

CD4+ T cells apoptosis in Plasmodium vivax infection is mediated by activation of both intrinsic and extrinsic pathways

BACKGROUND

Reduction in the number of circulating blood lymphocytes (lymphocytopaenia) has been reported during clinical episodes of malaria and is normalized after treatment with anti-malaria drugs. While this phenomenon is well established in malaria infection, the underlying mechanisms are still not fully elucidated. In the present study, the occurrence of apoptosis and its pathways in CD4+ T cells was investigated in naturally Plasmodium vivax-infected individuals from a Brazilian endemic area (Porto Velho - RO).

METHODS

Blood samples were collected from P. vivax-infected individuals and healthy donors. The apoptosis was characterized by cell staining with Annexin V/FITC and propidium iodide and the apoptosis-associated gene expression profile was carried out using RT2 Profiler PCR Array-Human Apoptosis. The plasma TNF level was determined by ELISA. The unpaired t-test or Mann-Whitney test was applied according to the data distribution.

RESULTS

Plasmodium vivax-infected individuals present low number of leukocytes and lymphocytes with a higher percentage of CD4+ T cells in early and/or late apoptosis. Increased gene expression was observed for TNFRSF1B and Bid, associated with a reduction of Bcl-2, in individuals with P. vivax malaria. Furthermore, these individuals showed increased plasma levels of TNF compared to malaria-naive donors.

CONCLUSIONS

The results of the present study suggest that P. vivax infection induces apoptosis of CD4+ T cells mediated by two types of signaling: by activation of the TNFR1 death receptor (extrinsic pathway), which is amplified by Bid, and by decreased expression of the anti-apoptotic protein Bcl-2 (intrinsic pathway). The T lymphocytes apoptosis could reflect a strategy of immune evasion triggered by the parasite, enabling their persistence but also limiting the occurrence of immunopathology.

Increased expression of Fas receptor and Fas ligand in the culture of the peripheral blood mononuclear cells stimulated with Borrelia burgdorferi sensu lato

Apoptosis of the lymphocytes plays an essential role in the regulation of inflammatory/immune responses and its abnormalities may contribute to a chronic infection, persistent inflammation and autoimmunity. Its role in the pathogenesis of the late Lyme borreliosis manifestations has not been studied so far. We have measured Th lymphocyte apoptosis rate, membrane expression of pro-apoptotic Fas receptor, and supernatant concentrations of selected soluble pro- and anti-apoptotic mediators in cultures of peripheral blood mononuclear cells from 16 patients with disseminated Lyme borreliosis (6 with osteoarticular symptoms, 7 with neuroborreliosis and 3 with acrodermatitis chronica atrophicans) and 8 healthy controls. The cultures stimulated for 48h with live Borrelia burgdorferi sensu stricto, B. garinii or B. afzelii spirochetes. Fraction of the apoptotic Th (CD3+CD4+) lymphocytes and expression of Fas in this cell population was measured cytometrically and concentrations of soluble Fas, soluble Fas ligand, IL-10, IL-12 and TGF-β in culture supernatant with ELISA assays. The expression of IL-10, soluble and membrane Fas and soluble Fas ligand was increased under stimulation and higher in the presence of B. burgdorferi sensu stricto than the other species. Apoptosis rate was not affected. There was no difference between Lyme borreliosis patients and controls. IL-10 concentration correlated negatively with the membrane Fas expression and apoptosis under stimulation with B. afzelii and B. garinii. Expression of Fas/FasL system is up-regulated under stimulation with B. burgdorferi, but without corresponding increase in lymphocyte apoptosis. Variable responses observed with different B. burgdorferi species may reflect differences in the pathogenesis of the infection in vivo.

Impairment of T cell function in parasitic infections

In mammals subverted as hosts by protozoan parasites, the latter and/or the agonists they release are detected and processed by sensors displayed by many distinct immune cell lineages, in a tissue(s)-dependent context. Focusing on the T lymphocyte lineage, we review our present understanding on its transient or durable functional impairment over the course of the developmental program of the intracellular parasites Leishmania spp., Plasmodium spp., Toxoplasma gondii, and Trypanosoma cruzi in their mammalian hosts. Strategies employed by protozoa to down-regulate T lymphocyte function may act at the initial moment of naïve T cell priming, rendering T cells anergic or unresponsive throughout infection, or later, exhausting T cells due to antigen persistence. Furthermore, by exploiting host feedback mechanisms aimed at maintaining immune homeostasis, parasites can enhance T cell apoptosis. We will discuss how infections with prominent intracellular protozoan parasites lead to a general down-regulation of T cell function through T cell anergy and exhaustion, accompanied by apoptosis, and ultimately allowing pathogen persistence.

How Reliable Are Hematological Parameters in Predicting Uncomplicated Plasmodium falciparum Malaria in an Endemic Region?

BACKGROUND

Malaria remains endemic in Sub-Saharan Africa. Hematological changes that occur have been suggested as potential predictors of malaria. This study was aimed at evaluating the diagnostic relevance of hematological parameters in predicting malaria.

METHODS

A cross-sectional study involving 370 patients with signs and symptoms of malaria was conducted at Mulago Hospital, Kampala, from May, 2012 to February, 2013. Thin and thick blood films were prepared for each patient and stained with Giemsa to aid the detection of malaria parasites. Patients' hematological parameters were determined.

RESULTS

Out of the 370 patients, 61 (16.5%) had malaria. Significant differences in the hematological parameters between P. falciparum malaria parasitemic patients and nonparasitemic patients were only observed in mean (±SD) of the differential monocyte count (10.89 ± 6.23% versus 8.98 ± 5.02%, p = 0.01) and the platelet count (172.43 (± 80.41) ×10³ cells/μl versus 217.82 ± (95.96) ×10³ cells/μl p = 0.00). The mean (±SD) values of the red blood cell indices (hemoglobin count, MCV, MCH, and MCHC), the differential neutrophil and lymphocyte counts, and the mean platelet volume (MPV) did not significantly differ between the two groups.

CONCLUSION

Hematological changes are unreliable laboratory indicators of malaria in acute uncomplicated Plasmodium falciparum malaria.

Predictive value of lymphocytopenia and the neutrophil-lymphocyte count ratio for severe imported malaria

BACKGROUND

Lymphocytopenia has frequently been described in patients with malaria, but studies on its association with disease severity have yielded conflicting results. The neutrophil/lymphocyte count ratio (NLCR) has been introduced as a parameter for systemic inflammation in critically ill patients and was found, together with lymphocytopenia, to be a better predictor of bacteraemia than routine parameters like C-reactive protein and total leukocyte count. In the present study, the predictive value of the NLCR and lymphocytopenia for severe disease was evaluated in patients with imported malaria.

METHODS

All patients diagnosed with malaria at the Harbour Hospital between January 1st 1999 and January 1st 2012 with differential white cell counts determined within the first 24 hours after admission were included in this retrospective study. Severe malaria was defined according to the WHO criteria. The performance of the NLCR and lymphocytopenia as a marker of severe malarial disease was compared back-to-back with that of C-reactive protein as a reference biomarker.

RESULTS

A total of 440 patients (severe falciparum malaria n = 61, non-severe falciparum malaria n = 259, non-falciparum malaria n=120) were included in the study. Lymphocytopenia was present in 52% of all patients and the median NLCR of all patients was 3.2. Total lymphocyte counts and NLCR did not differ significantly between groups. A significant correlation of total leukocyte count and NLCR, but not lymphocyte count, with parasitaemia was found. ROC analysis revealed a good negative predictive value but a poor positive predictive value of both lymphocytopenia and NLCR and performance was inferior to that of C-reactive protein. After complete parasite clearance a significant rise in total leukocyte count and lymphocyte count and a significant decrease in NLCR was observed.

CONCLUSION

The NLCR was found to correlate with parasitaemia, but both lymphocytopenia and the NLCR were inferior to C-reactive protein as markers for severe disease in patients with imported malaria. The NLCR and lymphocytopenia are not useful as predictive markers for severe disease in imported malaria in the acute care setting.

Malaria: a haematological disease

Plasmodium falciparum malaria remains a major cause of mortality throughout the tropical world. Haematological abnormalities are considered a hallmark of malaria, bearing an impact on final outcome and representing indices of prognostic and follow-up value. These include severe anaemia, coagulation disturbances, leukocyte numerical or functional changes and spleen involvement. Anaemia involves red blood cell lysis due to parasite invasion, as well as mechanisms of intravascular haemolysis and decreased erythropoiesis. Exchange or blood transfusion is mainly recommended in the management of these patients. Haemorrhagic complications in severe malaria are relatively rare despite prominent thrombocytopenia and dysfunction in the coagulation pathway. Numerical, as well as functional changes in the white blood cell are less dramatic than other blood cell series, but still, remain a significant index of disease progression and ultimate prognosis. Finally, the role of the spleen in severe malaria is multifactorial. Care and vigilance should be taken against splenic rupture which is fatal and can occur despite appropriate antimalarial prophylaxis and treatment.

A -436C>A polymorphism in the human FAS gene promoter associated with severe childhood malaria

Human genetics and immune responses are considered to critically influence the outcome of malaria infections including life-threatening syndromes caused by Plasmodium falciparum. An important role in immune regulation is assigned to the apoptosis-signaling cell surface receptor CD95 (Fas, APO-1), encoded by the gene FAS. Here, a candidate-gene association study including variant discovery at the FAS gene locus was carried out in a case-control group comprising 1,195 pediatric cases of severe falciparum malaria and 769 unaffected controls from a region highly endemic for malaria in Ghana, West Africa. We found the A allele of c.−436C>A (rs9658676) located in the promoter region of FAS to be significantly associated with protection from severe childhood malaria (odds ratio 0.71, 95% confidence interval 0.58–0.88, p_empirical = 0.02) and confirmed this finding in a replication group of 1,412 additional severe malaria cases and 2,659 community controls from the same geographic area. The combined analysis resulted in an odds ratio of 0.71 (95% confidence interval 0.62–0.80, p = 1.8×10⁻⁷, n = 6035). The association applied to c.−436AA homozygotes (odds ratio 0.47, 95% confidence interval 0.36–0.60) and to a lesser extent to c.−436AC heterozygotes (odds ratio 0.73, 95% confidence interval 0.63–0.84), and also to all phenotypic subgroups studied, including severe malaria anemia, cerebral malaria, and other malaria complications. Quantitative FACS analyses assessing CD95 surface expression of peripheral blood mononuclear cells of naïve donors showed a significantly higher proportion of CD69⁺CD95⁺ cells among persons homozygous for the protective A allele compared to AC heterozygotes and CC homozygotes, indicating a functional role of the associated CD95 variant, possibly in supporting lymphocyte apoptosis.

Severe malaria caused by infection with the protozoan parasite Plasmodium falciparum is a major health burden, causing approximately one million fatalities annually, predominantly among young children in Sub-Saharan Africa. The occurrence of severe malaria may depend on a complex interplay of transmission dynamics and the development of a protective immune response but also on heritable differences in the susceptibility to the disease. In two large studies including a total of 2,607 affected children and 3,428 apparently healthy individuals from Ghana, West Africa, we investigated genetic variants of the FAS gene, which encodes CD95, a molecule critically involved in the programmed cell death of lymphocytes. We found that a single nucleotide variant in the FAS promoter was associated with a 29%–reduced risk of developing severe malaria. In individuals carrying two copies of the protective allele, a higher proportion of activated lymphocytes was found to express CD95. These findings indicate that a predisposition to an increased expression of CD95 may help to protect from severe malaria, possibly by rendering activated T-lymphocytes more susceptible to programmed cell death.

Blood coagulation, inflammation, and malaria

Malaria remains a highly prevalent disease in more than 90 countries and accounts for at least 1 million deaths every year. Plasmodium falciparum infection is often associated with a procoagulant tonus characterized by thrombocytopenia and activation of the coagulation cascade and fibrinolytic system; however, bleeding and hemorrhage are uncommon events, suggesting that a compensated state of blood coagulation activation occurs in malaria. This article (i) reviews the literature related to blood coagulation and malaria in a historic perspective, (ii) describes basic mechanisms of coagulation, anticoagulation, and fibrinolysis, (iii) explains the laboratory changes in acute and compensated disseminated intravascular coagulation (DIC), (iv) discusses the implications of tissue factor (TF) expression in the endothelium of P. falciparum infected patients, and (v) emphasizes the procoagulant role of parasitized red blood cells (RBCs) and activated platelets in the pathogenesis of malaria. This article also presents the Tissue Factor Model (TFM) for malaria pathogenesis, which places TF as the interface between sequestration, endothelial cell (EC) activation, blood coagulation disorder, and inflammation often associated with the disease. The relevance of the coagulation-inflammation cycle for the multiorgan dysfunction and coma is discussed in the context of malaria pathogenesis.

A decrease of plasma macrophage migration inhibitory factor concentration is associated with lower numbers of circulating lymphocytes in experimental Plasmodium falciparum malaria

Macrophage migration inhibitory factor (MIF) has recently been implicated in the pathogenesis of malarial anaemia. However, field studies have reported contradictory results on circulating MIF concentrations in patients with clinically overt Plasmodium falciparum malaria. We determined plasma MIF levels over time in 10 healthy volunteers during experimental P. falciparum infection. Under fully controlled conditions, MIF levels decreased significantly during early blood-stage infection and reached a nadir at day 8 post-infection. A decrease in the number of circulating lymphocytes, which are an important source of MIF production, paralleled the decrease in MIF levels. Monocyte/macrophage counts remained unchanged. At MIF nadir, the anti-inflammatory cytokine interleukin (IL)-10, which is an inhibitor of T-cell MIF production, was detectable in only 2 of 10 volunteers. Plasma concentrations of the pro-inflammatory cytokines IL-8 and IL-1beta were only marginally elevated. We conclude that circulating MIF levels decrease early in blood-stage malaria as a result of the decline in circulating lymphocytes.

Cerebrospinal fluid and serum biomarkers of cerebral malaria mortality in Ghanaian children

Background

Plasmodium falciparum can cause a diffuse encephalopathy known as cerebral malaria (CM), a major contributor to malaria associated mortality. Despite treatment, mortality due to CM can be as high as 30% while 10% of survivors of the disease may experience short- and long-term neurological complications. The pathogenesis of CM and other forms of severe malaria is multi-factorial and appear to involve cytokine and chemokine homeostasis, inflammation and vascular injury/repair. Identification of prognostic markers that can predict CM severity will enable development of better intervention.

Methods

Postmortem serum and cerebrospinal fluid (CSF) samples were obtained within 2–4 hours of death in Ghanaian children dying of CM, severe malarial anemia (SMA), and non-malarial (NM) causes. Serum and CSF levels of 36 different biomarkers (IL-1β, IL-1ra, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-12 (p70), IL-13, IL-15, IL-17, Eotaxin, FGF basic protein, CRP, G-CSF, GM-CSF, IFN-γ, TNF-α, IP-10, MCP-1 (MCAF), MIP-1α, MIP-1β, RANTES, SDF-1α, CXCL11 (I-TAC), Fas-ligand [Fas-L], soluble Fas [sFas], sTNF-R1 (p55), sTNF-R2 (p75), MMP-9, TGF-β1, PDGF bb and VEGF) were measured and the results compared between the 3 groups.

Results

After Bonferroni adjustment for other biomarkers, IP-10 was the only serum biomarker independently associated with CM mortality when compared to SMA and NM deaths. Eight CSF biomarkers (IL-1ra, IL-8, IP-10, PDGFbb, MIP-1β, Fas-L, sTNF-R1, and sTNF-R2) were significantly elevated in CM mortality group when compared to SMA and NM deaths. Additionally, CSF IP-10/PDGFbb median ratio was statistically significantly higher in the CM group compared to SMA and NM groups.

Conclusion

The parasite-induced local cerebral dysregulation in the production of IP-10, 1L-8, MIP-1β, PDGFbb, IL-1ra, Fas-L, sTNF-R1, and sTNF-R2 may be involved in CM neuropathology, and their immunoassay may have potential utility in predicting mortality in CM.

Pro- and anti-apoptotic activities of protozoan parasites

During infection, programmed cell death, i.e. apoptosis, is an important effector mechanism of innate and adaptive host responses to parasites. In addition, it fulfils essential functions in regulating host immunity and tissue homeostasis. Not surprisingly, however, adaptation of parasitic protozoa to their hosts also involves modulation or even exploitation of cell death in order to facilitate parasite survival in a hostile environment. During recent years, considerable progress has been made in our understanding of apoptosis during parasitic infections and there is now convincing evidence that apoptosis and its modulation by protozoan parasites has a major impact on the parasite-host interaction and on the pathogenesis of disease. This review updates our current knowledge on the diverse functions apoptosis may fulfil during infections with diverse protozoan parasites including apicomplexans, kinetoplastids and amoebae. Furthermore, we also summarize common mechanistic themes of the pro- and anti-apoptotic activities of protozoan parasites. The diverse and complex effects which parasitic protozoa exert on apoptotic cell death within the host highlight fascinating interactions of parasites and their hosts. Importantly, they also stress the importance of further investigations before the modulation of host cell apoptosis can be exploited to combat parasitic infections.

Fas has a role in cerebral malaria, but not in proliferation or exclusion of the murine parasite in mice

We examined the susceptibility of murine Fas-deficient mutants to malaria infection in order to investigate the role of Fas in an experimental murine model of cerebral malaria (CM). We infected mice of B6 and CBA wild-type and mutant backgrounds with Plasmodium berghei ANKA. The incidence of CM in the mutant mice (B6-lpr, CBA-lprcg) was decreased by about 50% compared with wild-type control strains at 2 weeks after infection. We did not observe significant differences of parasitemia during a murine malaria infection with nonlethal Plasmodium yoelii 17XNL between wild-type and lymphoproliferative (lpr) mutant mice of C3H and MRL genetic backgrounds, although B6-lpr mice exhibited significantly higher parasitemia than did B6 mice 12 to 18 days after infection. These results suggest Fas has a possible role in CM but may not play a major role in the proliferation or exclusion of a murine malaria parasite in a nonlethal infection.

Plasmodium falciparum Malaria: reduction of endothelial cell apoptosis in vitro

Organ failure in Plasmodium falciparum malaria is associated with neutrophil activation and endothelial damage. This study investigates whether neutrophil-induced endothelial damage involves apoptosis and whether it can be prevented by neutralization of neutrophil secretory products. Endothelial cells from human umbilical veins were coincubated with neutrophils from healthy donors and with sera from eight patients with P. falciparum malaria, three patients with P. vivax malaria, and three healthy controls. Endothelial apoptosis was demonstrated by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and annexin V staining. The rate of apoptosis of cells was markedly increased after incubation with patient serum compared to that with control serum. Apoptosis was most pronounced after incubation with sera from two patients with fatal cases of P. falciparum malaria, followed by sera of survivors with severe P. falciparum malaria and, finally, by sera of patients with mild P. falciparum and P. vivax malaria. Ascorbic acid, tocopherol, and ulinastatin reduced the apoptosis rate, but gabexate mesilate and pentoxifylline did not. Furthermore, in fatal P. falciparum malaria, apoptotic endothelial cells were identified in renal and pulmonary tissue by TUNEL staining. These findings show that apoptosis caused by neutrophil secretory products plays a major role in endothelial cell damage in malaria. The antioxidants ascorbic acid and tocopherol and the protease inhibitor ulinastatin can reduce malaria-associated endothelial apoptosis in vitro.

Does malaria suffer from lack of memory?

It is widely perceived that immunity to malaria is, to an extent, defective and that one component of this defective immune response is the inability to induce or maintain long-term memory responses. If true, this is likely to pose problems for development of an effective vaccine against malaria. In this article, we critically review and challenge this interpretation of the epidemiological and experimental evidence. While evasion and modulation of host immune responses clearly occurs and naturally acquired immunity is far from optimal, mechanisms to control blood-stage parasites are acquired and maintained by individuals living in endemic areas, allowing parasite density to be kept below the threshold for induction of acute disease. Furthermore, protective immunity to severe pathology is achieved relatively rapidly and is maintained in the absence of boosting by re-infection. Nevertheless, there are significant challenges to overcome. The need for multiple infections to acquire immunity means that young children remain at risk of infection for far too long. Persistent or frequent exposure to antigen seems to be required to maintain anti-parasite immunity (premunition). Lastly, pre-erythrocytic and sexual stages of the life cycle are poorly immunogenic, and there is little evidence of effective pre-erythrocytic or transmission-blocking immunity at the population level. While these problems might theoretically be due to defective immunological memory, we suggest alternative explanations. Moreover, we question the extent to which these problems are malaria-specific rather than generic (i.e. result from inherent limitations of the vertebrate immune system).

The role of programmed cell death in Plasmodium-mosquito interactions

Many host-parasite interactions are regulated in part by the programmed cell death of host cells or the parasite. Here we review evidence suggesting that programmed cell death occurs during the early stages of the development of the malaria parasite in its vector. Zygotes and ookinetes of Plasmodium berghei have been shown to die by programmed cell death (apoptosis) in the midgut lumen of the vector Anopheles stephensi, or whilst developing in vitro. Several morphological markers, indicative of apoptosis, are described and evidence for the involvement of a biochemical pathway involving cysteine proteases discussed in relationship to other protozoan parasites. Malaria infection induces apoptosis in the cells of two mosquito tissues, the midgut and the follicular epithelium. Observations on cell death in both these tissues are reviewed including the role of caspases as effector molecules and the rescue of resorbing follicles resulting from inhibition of caspases. Putative signal molecules that might induce parasite and vector apoptosis are suggested including nitric oxide, reactive nitrogen intermediates, oxygen radicals and endocrine balances. Finally, we suggest that programmed cell death may play a critical role in regulation of infection by the parasite and the host, and contribute to the success or not of parasite establishment and host survival.

Malaria associated apoptosis is not significantly correlated with either parasitemia or the number of previous malaria attacks

The occurrence and intensity of lymphocyte apoptosis in blood samples from 79 outclinic patients with uncomplicated Plasmodium falciparum or Plasmodium vivax malaria and 30 healthy individuals were investigated. No difference in apoptosis percentages was detected between healthy individuals and malaria patients when ex vivo lymphocytes were analyzed. However, significantly increased apoptosis levels were observed in lymphocytes from both P. falciparum- and P. vivax-infected patients when the cells were cultured for 24 h. CD4(+)and CD8(+) T cells were affected to a comparable extent in P.falciparum- and P.vivax-infected patients. However, when we compared apoptosis values in infected and non-infected individuals it appeared that CD4(+) T cells were more susceptible than CD8(+) T cells. A significant increase in the sIL-2R plasma levels was observed in malaria patients when compared with healthy individuals and a positive correlation was observed between sIL-2R levels and apoptosis rates in infected patients presenting increased rates of apoptosis. An increased expression of Fas antigen was recorded after stimulation with P. falciparum antigen or anti-CD3 monoclonal antibody. These data show that a consistent proportion of the lymphocyte population dies by apoptosis during a malaria infection and that a period of time is necessary before in vivo activated cells can express the apoptotic process in vitro.

Alteration of Fas and Fas ligand expression during human visceral leishmaniasis

Several studies in murine systems have suggested a role of apoptosis in the pathogenesis of leishmaniasis. However, the role of apoptosis in visceral leishmaniasis in man has not been explored. In this study, we show that patients with visceral leishmaniasis demonstrate significant dysregulation of Fas and Fas ligand. Levels of soluble Fas (sFas) and soluble Fas ligand (sFasL) were elevated in plasma of patients with active visceral leishmaniasis (VL) and individuals co-infected with VL-HIV-1 compared to healthy controls. The levels of sFas and sFasL were normalized 6 months after successful treatment. In VL patients, the expression of membrane bound Fas, and to a lower extent FasL, were up-regulated on Leishmania donovani-infected spleen cells, the site of parasite multiplication. Expression of Fas and FasL on peripheral blood mononuclear cells was within normal range, probably reflecting that the blood is not a normal site of L. donovani infection. Furthermore, this is suggested by the finding that in vitro infection of macrophages with L. donovani up-regulated Fas expression on the surface of infected cells and enhanced the levels of sFasL in supernatants from infected cultures. How this dysregulation may affect the pathogenesis of human visceral leishmaniasis is discussed.

Cytokine production and apoptosis among T cells from patients under treatment for Plasmodium falciparum malaria

Available evidence suggests that Plasmodium falciparum malaria causes activation and reallocation of T cells, and that these in vivo primed cells re-emerge into the periphery following drug therapy. Here we have examined the cytokine production capacity and susceptibility to programmed cell death of peripheral T cells during and after the period of antimalarial treatment. A high proportion of peripheral CD3+ cells had an activated phenotype at and shortly after time of admission (day 0) and initiation of therapy. This activation peaked around day 2, and at this time-point peripheral T cells from the patients could be induced to produce cytokines at conditions of limited cytokine response in cells from healthy control donors. Activated CD8hi and TCR-gammadelta+ cells were the primary IFN-gamma producers, whereas CD4+ cells constituted an important source of TNF-alpha. The proportion of apoptotic T cells was elevated at admission and peaked 2 days later, while susceptibility to activation-induced cell death in vitro remained increased for at least 1 week after admission. Taken together, the data are consistent with the concept of malaria-induced reallocation of activated T cells to sites of inflammation, followed by their release back into the peripheral blood where they undergo apoptotic death to re-establish immunological homeostasis as inflammation subsides. However, the high proportion of pre-apoptotic cells from the time of admission suggests that apoptosis also contributes to the low frequency and number of T cells in the peripheral circulation during active disease.

Apoptotic deletion of Th cells specific for the 19-kDa carboxyl-terminal fragment of merozoite surface protein 1 during malaria infection

Immunity induced by the 19-kDa fragment of merozoite surface protein 1 is dependent on CD4+ Th cells. However, we found that adoptively transferred CFSE-labeled Th cells specific for an epitope on Plasmodium yoelii 19-kDa fragment of merozoite surface protein 1 (peptide (p)24), but not OVA-specific T cells, were deleted as a result of P. yoelii infection. As a result of infection, spleen cells recovered from infected p24-specific T cell-transfused mice demonstrated reduced response to specific Ag. A higher percentage of CFSE-labeled p24-specific T cells stained positive with annexin and anti-active caspase-3 in infected compared with uninfected mice, suggesting that apoptosis contributed to deletion of p24-specific T cells during infection. Apoptosis correlated with increased percentages of p24-specific T cells that stained positive for Fas from infected mice, suggesting that P. yoelii-induced apoptosis is, at least in part, mediated by Fas. However, bystander cells of other specificities also showed increased Fas expression during infection, suggesting that Fas expression alone is not sufficient for apoptosis. These data have implications for the development of immunity in the face of endemic parasite exposure.

Intracellular protozoan parasites and apoptosis: diverse strategies to modulate parasite-host interactions

Programmed cell death (apoptosis) is an important regulator of the host's response during infection with a variety of intracellular protozoan parasites. Parasitic pathogens have evolved diverse strategies to induce or inhibit host-cell apoptosis, thereby modulating the host's immune response, aiding dissemination within the host or facilitating intracellular survival. Here, we review the molecular and cell-biological mechanisms of the pathogen-induced modulation of host-cell apoptosis and its effects on the parasite-host interaction and the pathogenesis of parasitic diseases. We also discuss the previously unrecognized phenomenon of apoptotic cell death in (unicellular) protozoan parasites and its potential implications.