Plasmodium falciparum Activates CD16+ Dendritic Cells to Produce Tumor Necrosis Factor and Interleukin-10 in Subpatent Malaria

Surface proteome of plasma extracellular vesicles as mechanistic and clinical biomarkers for malaria

PURPOSE

Malaria is a life-threatening mosquito-borne disease caused by Plasmodium parasites, mainly in tropical and subtropical countries. Plasmodium falciparum (P. falciparum) is the most prevalent cause on the African continent and responsible for most malaria-related deaths globally. Important medical needs are biomarkers for disease severity or disease outcome. A potential source of easily accessible biomarkers are blood-borne small extracellular vesicles (sEVs).

METHODS

We performed an EV Array to find proteins on plasma sEVs that are differentially expressed in malaria patients. Plasma samples from 21 healthy subjects and 15 malaria patients were analyzed. The EV array contained 40 antibodies to capture sEVs, which were then visualized with a cocktail of biotin-conjugated CD9, CD63, and CD81 antibodies.

RESULTS

We detected significant differences in the protein decoration of sEVs between healthy subjects and malaria patients. We found CD106 to be the best discrimination marker based on receiver operating characteristic (ROC) analysis with an area under the curve of > 0.974. Additional ensemble feature selection revealed CD106, Osteopontin, CD81, major histocompatibility complex class II DR (HLA-DR), and heparin binding EGF like growth factor (HBEGF) together with thrombocytes to be a feature panel for discrimination between healthy and malaria. TNF-R-II correlated with HLA-A/B/C as well as CD9 with CD81, whereas Osteopontin negatively correlated with CD81 and CD9. Pathway analysis linked the herein identified proteins to IFN-γ signaling.

CONCLUSION

sEV-associated proteins can discriminate between healthy individuals and malaria patients and are candidates for future predictive biomarkers.

TRIAL REGISTRATION

The trial was registered in the Deutsches Register Klinischer Studien (DRKS-ID: DRKS00012518).

Whole Blood Dendritic Cell Cytokine Production Assay

This protocol outlines a method for the timely detection of intracellular cytokines produced by activated dendritic cells (DC) in human whole blood. The quantification of cytokines is used to measure DC immune responsiveness, providing information on the breadth, strength, and DC subtypes responding spontaneously and to specific stimulation with toll-like receptor (TLR) ligands or parasite-infected erythrocytes. DC subsets, plasmacytoid DC, CD1c⁺ DC, CD141⁺ DC, and CD16⁺ DC, are examined in their natural environment of plasma and blood cells (erythrocytes, neutrophils, platelets, and leukocytes) enabling disease, medication, nutritional, and hematological effects on DC function to be examined in vaccine studies, ageing, health, and disease.

Recent Advances in Understanding the Inflammatory Response in Malaria: A Review of the Dual Role of Cytokines

Malaria is a serious and, in some unfortunate cases, fatal disease caused by a parasite of the Plasmodium genus. It predominantly occurs in tropical areas where it is transmitted through the bite of an infected Anopheles mosquito. The pathogenesis of malaria is complex and incompletely elucidated. During blood-stage infection, in response to the presence of the parasite, the host's immune system produces proinflammatory cytokines including IL-6, IL-8, IFN-γ, and TNF, cytokines which play a pivotal role in controlling the growth of the parasite and its elimination. Regulatory cytokines such as transforming growth factor- (TGF-) β and IL-10 maintain the balance between the proinflammatory and anti-inflammatory responses. However, in many cases, cytokines have a double role. On the one hand, they contribute to parasitic clearance, and on the other, they are responsible for pathological changes encountered in malaria. Cytokine-modulating strategies may represent a promising modern approach in disease management. In this review, we discuss the host immune response in malaria, analyzing the latest studies on the roles of pro- and anti-inflammatory cytokines.

Reduced circulating dendritic cells in acute Plasmodium knowlesi and Plasmodium falciparum malaria despite elevated plasma Flt3 ligand levels

Background

Plasmodium falciparum malaria increases plasma levels of the cytokine Fms-like tyrosine kinase 3 ligand (Flt3L), a haematopoietic factor associated with dendritic cell (DC) expansion. It is unknown if the zoonotic parasite Plasmodium knowlesi impacts Flt3L or DC in human malaria. This study investigated circulating DC and Flt3L associations in adult malaria and in submicroscopic experimental infection.

Methods

Plasma Flt3L concentration and blood CD141⁺ DC, CD1c⁺ DC and plasmacytoid DC (pDC) numbers were assessed in (i) volunteers experimentally infected with P. falciparum and in Malaysian patients with uncomplicated (ii) P. falciparum or (iii) P. knowlesi malaria.

Results

Plasmodium knowlesi caused a decline in all circulating DC subsets in adults with malaria. Plasma Flt3L was elevated in acute P. falciparum and P. knowlesi malaria with no increase in a subclinical experimental infection. Circulating CD141⁺ DCs, CD1c⁺ DCs and pDCs declined in all adults tested, for the first time extending the finding of DC subset decline in acute malaria to the zoonotic parasite P. knowlesi.

Conclusions

In adults, submicroscopic Plasmodium infection causes no change in plasma Flt3L but does reduce circulating DCs. Plasma Flt3L concentrations increase in acute malaria, yet this increase is insufficient to restore or expand circulating CD141⁺ DCs, CD1c⁺ DCs or pDCs. These data imply that haematopoietic factors, yet to be identified and not Flt3L, involved in the sensing/maintenance of circulating DC are impacted by malaria and a submicroscopic infection. The zoonotic P. knowlesi is similar to other Plasmodium spp in compromising DC in adult malaria.

Role of Heat Shock Proteins in Immune Modulation in Malaria

Malaria is one of the major parasitic killer diseases worldwide. Severe cases of malaria are mostly in children under the age of 5 years due to their naïve immune system and in pregnant women with weakened immune responses. Inflammatory immune responses against the parasite involve complement activation as well as the antibody and effector cell-mediated immune system. However, after an infection with Plasmodium falciparum (P. falciparum), the most dangerous malaria species, the host-derived immunity is often insufficient to completely inhibit the infection cycles of the parasite in red blood cells for yet unknown reasons. In the present chapter we aim to elucidate the role of the host's and the parasite's heat shock proteins (HSPs) in the development of a novel anti-malaria therapeutic approach.

Setting the stage: The initial immune response to blood-stage parasites

Individuals growing up in malaria endemic areas gradually develop protection against clinical malaria and passive transfer experiments in humans have demonstrated that this protection is mediated in part by protective antibodies. However, neither the target antigens, specific effector mechanisms, nor the role of continual parasite exposure have been elucidated, which complicates vaccine development. Progress has been made in defining the innate signaling pathways activated by parasite components, including DNA, RNA, hemozoin, and phospholipids, which initiate the immune response and will be the focus of this review. The challenge that remains within the field is to understand the role of these early responses in the development of protective adaptive responses that clear iRBC and block merozoite invasion so that optimal vaccines and therapeutics may be produced.

Risk Factors for Kaposi's Sarcoma-Associated Herpesvirus DNA in Blood and in Saliva in Rural Uganda

BACKGROUND

Detectable Kaposi's sarcoma-associated herpesvirus (KSHV) DNA in blood and increased antibody titres may indicate KSHV reactivation, while the transmission of KSHV occurs via viral shedding in saliva.

METHODS

We investigated the risk factors for KSHV DNA detection by real-time polymerase chain reaction in blood and by viral shedding in saliva, in 878 people aged 3 to 89 years of both sexes in a rural Ugandan population cohort. Helminths were detected using microscopy and the presence of malaria parasitaemia was identified using rapid diagnostic tests. Regression modelling was used for a statistical analysis.

RESULTS

The KSHV viral load in blood did not correlate with the viral load in saliva, suggesting separate immunological controls within each compartment. The proportions of individuals with a detectable virus in blood were 23% among children aged 3-5 years and 22% among those 6-12 years, thereafter reducing with increasing age. The proportions of individuals with a detectable virus in saliva increased from 30% in children aged 3-5 years to 45% in those aged 6-12 years, and decreased subsequently with increasing age. Overall, 29% of males shed in saliva, compared to 19% of females (P = .008).

CONCLUSIONS

Together, these data suggest that young males may be responsible for much of the onward transmission of KSHV. Individuals with a current malaria infection had higher levels of viral DNA in their blood (P = .031), compared to uninfected individuals. This suggests that malaria may lead to KSHV reactivation, thereby increasing the transmission and pathogenicity of the virus.

Transcriptional profiling and immunophenotyping show sustained activation of blood monocytes in subpatent Plasmodium falciparum infection

OBJECTIVES

Malaria, caused by Plasmodium infection, remains a major global health problem. Monocytes are integral to the immune response, yet their transcriptional and functional responses in primary Plasmodium falciparum infection and in clinical malaria are poorly understood.

METHODS

The transcriptional and functional profiles of monocytes were examined in controlled human malaria infection with P. falciparum blood stages and in children and adults with acute malaria. Monocyte gene expression and functional phenotypes were examined by RNA sequencing and flow cytometry at peak infection and compared to pre-infection or at convalescence in acute malaria.

RESULTS

In subpatent primary infection, the monocyte transcriptional profile was dominated by an interferon (IFN) molecular signature. Pathways enriched included type I IFN signalling, innate immune response and cytokine-mediated signalling. Monocytes increased TNF and IL-12 production upon in vitro toll-like receptor stimulation and increased IL-10 production upon in vitro parasite restimulation. Longitudinal phenotypic analyses revealed sustained significant changes in the composition of monocytes following infection, with increased CD14⁺CD16^- and decreased CD14^-CD16⁺ subsets. In acute malaria, monocyte CD64/FcγRI expression was significantly increased in children and adults, while HLA-DR remained stable. Although children and adults showed a similar pattern of differentially expressed genes, the number and magnitude of gene expression change were greater in children.

CONCLUSIONS

Monocyte activation during subpatent malaria is driven by an IFN molecular signature with robust activation of genes enriched in pathogen detection, phagocytosis, antimicrobial activity and antigen presentation. The greater magnitude of transcriptional changes in children with acute malaria suggests monocyte phenotypes may change with age or exposure.

Fast and fierce versus slow and smooth: Heterogeneity in immune responses to Plasmodium in the controlled human malaria infection model

Controlled human malaria infection (CHMI) is an established model in clinical malaria research. Upon exposure to Plasmodium falciparum parasites, malaria-naive volunteers differ in dynamics and composition of their immune profiles and subsequent capacity to generate protective immunity. CHMI volunteers are either inflammatory responders who have prominent cellular IFN-γ production primarily driven by adaptive T cells, or tempered responders who skew toward antibody-mediated humoral immunity. When exposed to consecutive CHMIs under antimalarial chemoprophylaxis, individuals who can control parasitemia after a single immunization (fast responders) are more likely to be protected against a subsequent challenge infection. Fast responders tend to be inflammatory responders who can rapidly induce long-lived IFN-γ+ T cell responses. Slow responders or even non-responders can also be protected, but via a more diverse range of responses that take a longer time to reach full protective efficacy, in part due to their tempered phenotype. The latter group can be identified at baseline before CHMI by higher expression of inhibitory ligands CTLA-4 and TIM-3 on CD4+ T cells. Delineating heterogeneity in human immune responses to P. falciparum will facilitate rational design and strategy towards effective malaria vaccines.

Transcriptional Modulation of the Host Immunity Mediated by Cytokines and Transcriptional Factors in Plasmodium falciparum-Infected Patients of North-East India

Complications due to malaria are caused mostly by host immunological responses. Plasmodium falciparum subverts host immunity by various strategies, including modulation in the host immune responses by regulating cytokines. The transcriptional alterations of major cytokines and immunoregulators were analyzed in this study through gene expression profiling in clinically defined subgroups of P. falciparum patients. Malaria patients were included from Dhalai district hospital of Tripura with uncomplicated malaria (UC) and severe malaria (SM) and healthy controls from endemic and non-endemic areas of India. qPCR gene expression analysis was performed for all factors and they were grouped into three clusters based on their altered expressions. The first cluster was downregulated with an increased parasitic burden which included T-BET, GATA3, EOMES, TGF-β, STAT4, STAT6 and cytokines IFN-γ, IL-12, IL-4, IL-5, and IL-13. RANTES, IL-8, CCR8, and CXCR3 were decreased in the SM group. The second cluster was upregulated with severity and included TNF-α, IL-10, IL-1β and IL-7. PD-1 and BCL6 were increased in the SM group. The third cluster comprised of NF-κB and was not altered. The level of perforin was suppressed while GrB expression was elevated in SM. P. falciparum malaria burden is characterized by the modulation of host immunity via compromization of T cell-mediated responses and suppression of innate immune-regulators.

Monocyte-derived dendritic cells in malaria

The pathogenesis of malaria is a multifactorial syndrome associated with a deleterious inflammatory response that is responsible for many of the clinical manifestations. While dendritic cells (DCs) play a critical role in initiating acquired immunity and host resistance to infection, they also play a pathogenic role in inflammatory diseases. In our recent studies, we found in different rodent malaria models that the monocyte-derived DCs (MO-DCs) become, transiently, a main DC population in spleens and inflamed non-lymphoid organs. These studies suggest that acute infection with Plasmodium berghei promotes the differentiation of splenic monocytes into inflammatory monocytes (iMOs) and thereafter into MO-DCs that play a pathogenic role by promoting inflammation and tissue damage. The recruitment of MO-DCs to the lungs and brain are dependent on expression of CCR4 and CCR5, respectively, and expression of respective chemokine ligands in each organ. Once they reach the target organ the MO-DCs produce the CXCR3 ligands (CXCL9 and CXCL10), recruit CD8⁺ T cells, and produce toxic metabolites that play an important role in the development of experimental cerebral malaria (ECM) and acute respiratory distress syndrome (ARDS).

Trial watch: dendritic cell vaccination for cancer immunotherapy

Dendritic- cells (DCs) have received considerable attention as potential targets for the development of anticancer vaccines. DC-based anticancer vaccination relies on patient-derived DCs pulsed with a source of tumor-associated antigens (TAAs) in the context of standardized maturation-cocktails, followed by their reinfusion. Extensive evidence has confirmed that DC-based vaccines can generate TAA-specific, cytotoxic T cells. Nonetheless, clinical efficacy of DC-based vaccines remains suboptimal, reflecting the widespread immunosuppression within tumors. Thus, clinical interest is being refocused on DC-based vaccines as combinatorial partners for T cell-targeting immunotherapies. Here, we summarize the most recent preclinical/clinical development of anticancer DC vaccination and discuss future perspectives for DC-based vaccines in immuno-oncology.

Human Dendritic Cell Subsets, Ontogeny, and Impact on HIV Infection

Dendritic cells (DCs) play important roles in orchestrating host immunity against invading pathogens, representing one of the first responders to infection by mucosal invaders. From their discovery by Ralph Steinman in the 1970s followed shortly after with descriptions of their in vivo diversity and distribution by Derek Hart, we are still continuing to progressively elucidate the spectrum of DCs present in various anatomical compartments. With the power of high-dimensional approaches such as single-cell sequencing and multiparameter cytometry, recent studies have shed new light on the identities and functions of DC subtypes. Notable examples include the reclassification of plasmacytoid DCs as purely interferon-producing cells and re-evaluation of intestinal conventional DCs and macrophages as derived from monocyte precursors. Collectively, these observations have changed how we view these cells not only in steady-state immunity but also during disease and infection. In this review, we will discuss the current landscape of DCs and their ontogeny, and how this influences our understanding of their roles during HIV infection.

Dendritic Cell Responses and Function in Malaria

Malaria remains a serious threat to global health. Sustained malaria control and, eventually, eradication will only be achieved with a broadly effective malaria vaccine. Yet a fundamental lack of knowledge about how antimalarial immunity is acquired has hindered vaccine development efforts to date. Understanding how malaria-causing parasites modulate the host immune system, specifically dendritic cells (DCs), key initiators of adaptive and vaccine antigen-based immune responses, is vital for effective vaccine design. This review comprehensively summarizes how exposure to Plasmodium spp. impacts human DC function in vivo and in vitro. We have highlighted the heterogeneity of the data observed in these studies, compared and critiqued the models used to generate our current understanding of DC function in malaria, and examined the mechanisms by which Plasmodium spp. mediate these effects. This review highlights potential research directions which could lead to improved efficacy of existing vaccines, and outlines novel targets for next-generation vaccine strategies to target malaria.

Novel Strategies for Malaria Vaccine Design

The quest for a licensed effective vaccine against malaria remains a global priority. Even though classical vaccine design strategies have been successful for some viral and bacterial pathogens, little success has been achieved for Plasmodium falciparum, which causes the deadliest form of malaria due to its diversity and ability to evade host immune responses. Nevertheless, recent advances in vaccinology through high throughput discovery of immune correlates of protection, lymphocyte repertoire sequencing and structural design of immunogens, provide a comprehensive approach to identifying and designing a highly efficacious vaccine for malaria. In this review, we discuss novel vaccine approaches that can be employed in malaria vaccine design.