Plasmodium and mononuclear phagocytes

Monocyte phagocytosis of malaria β-haematin in the presence of artemisinin, amodiaquine, chloroquine, doxycycline, primaquine, pyrimethamine and quinine

The intraerythrocytic malaria parasite digests haemoglobin to provide amino acids for metabolism and releases toxic haem that is sequestered into haemozoin, a non-toxic, insoluble, crystalline pigment. Following erythrocyte rupture, haemozoin is released into circulation and phagocytosed by monocytes. Phagocytosed haemozoin and antimalarial drugs have both been reported to modulate monocyte functions. This study determined the effects of therapeutic concentrations of seven antimalarial drugs; amodiaquine, artemisinin, chloroquine, doxycycline, primaquine, pyrimethamine and quinine, on the phagocytosis of β-haematin (synthetic haemozoin) by two monocytic cell lines, J774A.1 and U937, and human peripheral blood mononuclear cells. A novel spectrophotometric method based on the absorbance (O.D 400 nm) of alkali/SDS treated monocytes containing β-haematin was developed to complement counting phagocytosis with microscopy. The method has potential use for the large scale screening of monocyte phagocytic activity. Artemisinin, quinine, primaquine and pyrimethamine activated β-haematin phagocytosis by 12% or more, whereas amodiaquine, chloroquine and doxycyline inhibited β-haematin phagocytosis. In contrast, antimalarial drugs had minimal inhibitory effects on the phagocytosis of latex beads with only quinine resulting in more than 20% inhibition. Antimalarial drugs appear to alter monocyte phagocytic activity which has implications for the treatment, pathogenicity and adjunct therapies for malaria.

Elevated plasma abscisic acid is associated with asymptomatic falciparum malaria and with IgG-/caspase-1-dependent immunity in Plasmodium yoelii-infected mice

Abscisic acid (ABA) is an ancient stress hormone and is detectable in a wide variety of organisms where it regulates innate immunity and inflammation. Previously, we showed that oral supplementation with ABA decreased parasitemia in a mouse model of malaria, decreased liver and spleen pathology and reduced parasite transmission to mosquitoes. Here, we report that higher circulating ABA levels were associated with a reduced risk of symptomatic malaria in a cohort of Plasmodium falciparum-infected Ugandan children. To understand possible mechanisms of ABA protection in malaria, we returned to our mouse model to show that ABA effects on Plasmodium yoelii 17XNL infection were accompanied by minimal effects on complete blood count and blood chemistry analytes, suggesting a benefit to host health. In addition, orally delivered ABA induced patterns of gene expression in mouse liver and spleen that suggested enhancement of host anti-parasite defenses. To test these inferences, we utilized passive immunization and knockout mice to demonstrate that ABA supplementation increases circulating levels of protective, parasite-specific IgG and requires caspase-1 to reduce parasitemia. Collectively, ABA induces host responses that ameliorate infection and disease in an animal model and suggest that further studies of ABA in the context of human malaria are warranted.

An Important Milestone in Parasitology: Celebrating a Hundred Volumes of Advances in Parasitology

Beginning in 1963, the founding rationale of Advances in Parasitology was to provide authentic, well-documented reviews by leading experts, about the progress being made in their area of specialism to inform the wider cadre of parasitologists, disseminating this information across allied disciplines and all users. Some 55 years later, the Series has accumulated over 667 published articles, with just over 650 authors contributing either alone or in collaboration, and has successfully served the parasitological needs of medical, veterinary and wildlife scientific communities with equity, notwithstanding treatises on vectors or intermediate hosts, as well as 'honorary parasites' such as viruses, bacteria and fungi. The first production of Advances in Parasitology united the publishing offices of Academic Press in the USA (New York) and the UK (London), maintaining Webster or Oxford writing styles, but unlike its production, all seven editors, beginning with Professor Ben Dawes, have been UK-based. While Advances in Parasitology is now published by Elsevier from their London Office, it still follows the tradition of hard backed book production, in either eclectic or thematic volume formats. But now, following academic imperatives, the Series supports online posting, allowing chapter(s) to be downloaded ahead of final production of the hard back volume. With the 100th volume of Advances in Parasitology, in eclectic format like the very first, there is good reason to celebrate and reflect on the academic impact and enduring legacy of this Series. Seen not only as a yardstick of publishing success but also as a testament, in part, to our fascination with parasites, these cursorily simple yet wonderfully complex organisms that often cause undue harm and much suffering, is still as vibrant, expanding and relevant as ever before.

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.

Indocyanine Green Liposomes for Diagnosis and Therapeutic Monitoring of Cerebral Malaria

Cerebral malaria (CM) is a major cause of death of Plasmodium falciparum infection. Misdiagnosis of CM often leads to treatment delay and mortality. Conventional brain imaging technologies are rarely applicable in endemic areas. Here we address the unmet need for a simple, non-invasive imaging methodology for early diagnosis of CM. This study presents the diagnostic and therapeutic monitoring using liposomes containing the FDA-approved fluorescent dye indocyanine green (ICG) in a CM murine model. Increased emission intensity of liposomal ICG was demonstrated in comparison with free ICG. The Liposomal ICG's emission was greater in the brains of the infected mice compared to naïve mice and drug treated mice (where CM was prevented). Histological analyses suggest that the accumulation of liposomal ICG in the cerebral vasculature is due to extensive uptake mediated by activated phagocytes. Overall, liposomal ICG offers a valuable diagnostic tool and a biomarker for effectiveness of CM treatment, as well as other diseases that involve inflammation and blood vessel occlusion.

Nrf2-driven CD36 and HO-1 gene expression in circulating monocytes correlates with favourable clinical outcome in pregnancy-associated malaria

Background

Pregnancy-associated malaria (PAM) constitutes one of the most severe forms of malaria infection leading to fetal growth restriction and high risk of infant death. The severity of the pathology is largely attributed to the recruitment of monocytes and macrophages in the placenta which is evidenced by dysregulated inflammation found in placental blood. Importantly, CD36⁺ monocytes/macrophages are also thought to participate in the tight control of the pro- and anti-inflammatory responses following Plasmodium detection through elimination of apoptotic cells and malaria-infected erythrocytes, internalization and recycling of oxidized forms of low-density lipoprotein and collaboration with TLR2 in pro-inflammatory response. Interestingly, previous work demonstrated that CD36 expression was upregulated on inflammatory macrophages following stimulation of the Nrf2 transcription factor, whilst the PPARγ pathway was inhibited and non-functional in the same inflammatory conditions. This current study examined the possible role of Nrf2-driven gene expression, CD36 and Haem-Oxygenase-1 (HO-1), in PAM clinical outcomes.

Methods

Clinical data and biological samples including peripheral blood mononuclear cells were collected from 27 women presenting PAM. Polychromatic flow cytometry was used to characterize innate immune cell subpopulations and quantify CD36 protein expression level on monocytes. mRNA levels of CD36, PPARγ, Nrf2 and HO-1 were determined by qPCR and related to clinical outcomes. Finally, the capacity of monocytes to modulate CD36 expression upon rosiglitazone or sulforaphane treatment, two respective PPARγ or Nrf2 activators, was also investigated.

Results

The CD36 receptor, mostly expressed by CD14⁺ circulating monocytes, statistically correlated with increased infant birth weights. Interestingly, mRNA levels of the transcription factor Nrf2 and the enzyme HO-1 also correlated with lower parasitaemia and increased infant birth weight, while PPARγ mRNA levels did not. Finally, monocytes isolated from low infant birth weight pregnant women were capable of up-regulating CD36 via the Nrf2 pathway ex vivo.

Conclusions

Altogether these results suggest that Nrf2-driven CD36 and HO-1 expression on innate immune cells could contribute to a protective and detoxifying mechanism during PAM. More powered and mechanistical studies are however needed to strengthen the conclusions of this study.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0888-8) contains supplementary material, which is available to authorized users.