Understanding Host-Pathogen-Vector Interactions with Chronic Asymptomatic Malaria Infections

Comparative analysis of peripheral whole blood transcriptome from asymptomatic carriers reveals upregulation of subsets of surface proteins implicated in Plasmodium falciparum phenotypic plasticity

The molecular mechanism underlying Plasmodium falciparum's persistence in the asymptomatic phase of infection remains largely unknown. However, large-scale shifts in the parasites' gene expression during asymptomatic infections may enhance phenotypic plasticity, maximizing their fitness and leading to the persistence of the asymptomatic infections. To uncover these mechanisms, we aimed to identify parasite genetic factors implicated in asymptomatic infections through whole transcriptome analysis. We analyzed publicly available transcriptome datasets containing asymptomatic malaria (ASM), uncomplicated malaria (SM), and malaria-naïve (NSM) samples from 35 subjects for differentially expressed genes (DEGs) and long noncoding RNAs. Our analysis identified 755 and 1773 DEGs in ASM vs SM and NSM, respectively. These DEGs revealed sets of genes coding for proteins of unknown functions (PUFs) upregulated in ASM vs SM and ASM, suggesting their role in underlying fundamental molecular mechanisms during asymptomatic infections. Upregulated genes in ASM vs SM revealed a subset of 24 clonal variant genes (CVGs) involved in host-parasite and symbiotic interactions and modulation of the symbiont of host erythrocyte aggregation pathways. Moreover, we identified 237 differentially expressed noncoding RNAs in ASM vs SM, of which 11 were found to interact with CVGs, suggesting their possible role in regulating the expression of CVGs. Our results suggest that P. falciparum utilizes phenotypic plasticity as an adaptive mechanism during asymptomatic infections by upregulating clonal variant genes, with long noncoding RNAs possibly playing a crucial role in their regulation. Thus, our study provides insights into the parasites' genetic factors that confer a fitness advantage during asymptomatic infections.

Biology and epidemiology of Plasmodium falciparum and Plasmodium vivax gametocyte carriage: Implication for malaria control and elimination

Malaria is among the leading public health problems worldwide. Female anopheles mosquito orchestrates the transmission of malaria by taking gametocytes and introducing sporozoite while taking blood meals. Interrupting transmission is the major strategy for malaria elimination. The gametocyte stage is essential for the onward transmission of malaria. Thus, understanding its basic biology and epidemiology is key to malaria control and elimination. Therefore, the current review focuses on revealing the biology, prevalence, and determinants of gametocyte carriage as well as its implication on mitigation of malaria. It also illustrates the role of asymptomatic and sub-microscopic Plasmodium infections and G-6-PD deficiency in gametocyte carriage and hence malaria transmission. Gametocytogenesis is initiated at committed merozoites and gives rise to the development of gametocytes. The trigger for gametocytogenesis depends on the host, parasite, and intervention factors. Gametocytes pass through five developmental stages identifiable by molecular markers. A considerable number of malaria patients carry gametocytes at a sub-microscopic level, thereby serving as a potential infectious reservoir of transmission. Factors involving the human host, Plasmodium parasite, and intervention parameters play a critical role in gametocyte biology and prevalence. The contribution of asymptomatic and sub-microscopic infections to malaria transmission is unknown. The clear impact of G-6-PD deficiency on malaria control and elimination remains unclear. Lack of clarity on such issues might impede the success of interventions. Basic science and epidemiological studies should continue to overcome the challenges and cope with the ever-evolving parasite and guide interventions.

Imported malaria from land bordering countries in China: A challenge in preventing the reestablishment of malaria transmission

OBJECTIVE

No indigenous malaria cases have been reported since 2017 in China, but a large number of imported cases are still reported every year, including those from the land bordering countries. To characterize their epidemiological profiles will provide evidence for the development of appropriate strategies to effectively address the challenges of border malaria in the post-elimination phase.

METHODS

Individual-level data of imported malaria cases from the land bordering countries were collected from 2017 to 2021 in China via the web-based surveillance systems, and analyzed by SPSS, ArcGIS and WPS software, to explore their epidemiological profiles.

RESULTS

A total of 1170 malaria cases imported into China from six of the fourteen land bordering countries were reported between 2017 and 2021 with a decline trend. Overall, cases were widely distributed in 31-97 counties from 11 to 21 provinces but mainly in Yunnan. Moreover, these imported cases were mainly infected with P. vivax (94.8%), and a total of 68 recurrent cases were reported in 6-14 counties from 4 to 8 provinces. In addition, nearly 57.1% of the total reported cases could seek healthcare within 2 days of getting sick, and 71.3% of the reported cases could be confirmed as malaria on the day they sought medical care.

CONCLUSIONS

China still needs to attach great importance to the risk and challenge of the imported malaria from bordering countries particularly from Myanmar in preventing reestablishment of malaria transmission in the post-elimination phase. It is necessary not only to strengthen collaboration and cooperation with the bordering countries, but also coordinate multiple departments at home to improve malaria surveillance and response system and prevent the reestablishment of malaria transmission in China.

Malaria-derived exosomes exacerbate liver injury during blood stage of Plasmodium berghei infection

Liver injury is a common clinical feature of Plasmodium spp. infection and contributes to multi-organ failure of severe malaria. Malaria-derived exosomes (MD-Exos) have recently engaged as key mediators in parasite-host interactions, modulating the subsequent pathogenic process. However, the role of MD-Exos in malaria-related liver injury and the underlying mechanisms remain unclear. Herein, exosomes from C57BL/6 mice infected with or without P. berghei ANKA serum (namely inf-Exos or un-Exos) were isolated and characterized by transmission electron microscopy, western blotting, and nanoparticle tracking analysis. The miRNAs profiling between inf-Exos and un-Exos were generated using RNA-seq and qPCR. The functions of inf-Exos on liver injury were investigated after two types of exosomes injected into mice intravenously (i.v.), by examining histopathological and apoptotic changes, macrophage polarization, and pro-inflammatory response. The infected red blood cells-stimulated mouse Raw264.7 macrophage cells targeted by inf-Exos or un-Exos were cultured for further study and verification the potential mechanisms. We found that both inf-Exos and un-Exos displayed a typical cup-shaped structure with a diameter of 60-200 nm, and had a positive expression of exosomal markers (e.g., CD9, CD63, and CD81). Compared with infected control mice, the treatment of inf-Exos but not un-Exos dramatically enhanced peripheral blood parasitemia and ECM incidence, exacerbated liver histopathological damage, elevated numbers of liver apoptotic cells, CD68⁺and CD86⁺ macrophages. The CD68⁺-TREM-1⁺ macrophages in liver tissues and the mRNA levels of pro-inflammatory cytokines (e.g., iNOS, TNF-α, IL-1β, and IL-6) were increased by inf-Exos treatment in vivo. Meanwhile, the treatment of inf-Exos resulted in a substantial increase of the mRNA levels of CD86, iNOS, TNF-α, IL-1β, and IL-6, but led to a remarkable decrease of Bcl-6 and SOCS-1 in Raw264.7 cells stimulated with iRBC in vitro. Notably, compared to un-Exos, five types of miRNAs (including miR-10a-5p, miR-10b-5p, miR-155-5p, miR-205-5p, and miR-21a-5p), that were previously reported to target Bcl-6 or SOCS-1, present higher abundance on inf-Exos, as demonstrated by RNA-seq and qPCR. Collectively, our data suggest that inf-Exos exacerbate malaria-induced liver pathology via triggering excessive pro-inflammatory response and promoting macrophage M1 polarization. Our findings will provide new insights into the roles of inf-Exos in malaria parasite-host interaction and pathogenesis of liver injury.

The mystery of persistent, asymptomatic Plasmodium falciparum infections

Plasmodium falciparum causes millions of malaria infections and hundreds of thousands of deaths annually. These parasites avoid the adaptive immune response by systematically cycling through a limited repertoire of variant surface antigens after which the number of circulating parasites drops to extremely low levels, coinciding with a loss of symptoms and eventual clearance of the infection. However, in regions with extended dry seasons or in individuals who no longer reside in endemic areas, asymptomatic infections have been observed to persist for many months or years, potentially serving as reservoirs for transmission. Recent work suggests the possibility that parasites can assume a state in which no variant surface antigens are expressed, thus rendering them virtually invisible to the immune system and enabling them to persist at low levels indefinitely.

CD36-A Host Receptor Necessary for Malaria Parasites to Establish and Maintain Infection

Plasmodium falciparum-infected erythrocytes (PfIEs) present P. falciparum erythrocyte membrane protein 1 proteins (PfEMP1s) on the cell surface, via which they cytoadhere to various endothelial cell receptors (ECRs) on the walls of human blood vessels. This prevents the parasite from passing through the spleen, which would lead to its elimination. Each P. falciparum isolate has about 60 different PfEMP1s acting as ligands, and at least 24 ECRs have been identified as interaction partners. Interestingly, in every parasite genome sequenced to date, at least 75% of the encoded PfEMP1s have a binding domain for the scavenger receptor CD36 widely distributed on host endothelial cells and many other cell types. Here, we discuss why the interaction between PfIEs and CD36 is optimal to maintain a finely regulated equilibrium that allows the parasite to multiply and spread while causing minimal harm to the host in most infections.

Epidemiological characteristics of P. vivax asymptomatic infections in the Peruvian Amazon

Introduction

Herein, we tested the hypothesis that Asymptomatic P. vivax (Pv) infected individuals (Asym) feature different epidemiological, clinical and biochemical characteristics, as well as hematological parameters, potentially predictive of clinical immunity in comparison to symptomatic Pv infected individuals (Sym).

Methodology

Between 2018 - 2021, we conducted 11 population screenings (PS, Day 0 (D0)) in 13 different riverine communities around Iquitos city, in the Peruvian Amazon, to identify Pv Sym and Asym individuals. A group of these individuals agreed to participate in a nested case - control study to evaluate biochemical and hematological parameters. Pv Asym individuals did not present common malaria symptoms (fever, headache, and chills), had a positive/negative microscopy result, a positive qPCR result, reported no history of antimalarial treatment during the last month, and were followed-up weekly until Day 21 (D21). Control individuals, had a negative malaria microscopy and qPCR result, no history of antimalarial treatment or malaria infections during the last three years, and no history of comorbidities or chronic infections.

Results

From the 2159 individuals screened during PS, data revealed a low but heterogeneous Pv prevalence across the communities (11.4%), where most infections were Asym (66.7%) and submicroscopic (82.9%). A total of 29 Asym, 49 Sym, and 30 control individuals participated in the nested case - control study (n=78). Ten of the individuals that were initially Asym at D0, experienced malaria symptoms during follow up and therefore, were included in the Sym group. 29 individuals remained Asym throughout all follow-ups. High levels of eosinophils were found in Asym individuals in comparison to Sym and controls.

Conclusion

For the first-time, key epidemiological, hematological, and biochemical features are reported from Pv Asym infections from the Peruvian Amazon. These results should be considered for the design and reshaping of malaria control measures as the country moves toward malaria elimination.

Systems biology of malaria explored with nonhuman primates

"The Primate Malarias" book has been a uniquely important resource for multiple generations of scientists, since its debut in 1971, and remains pertinent to the present day. Indeed, nonhuman primates (NHPs) have been instrumental for major breakthroughs in basic and pre-clinical research on malaria for over 50 years. Research involving NHPs have provided critical insights and data that have been essential for malaria research on many parasite species, drugs, vaccines, pathogenesis, and transmission, leading to improved clinical care and advancing research goals for malaria control, elimination, and eradication. Whilst most malaria scientists over the decades have been studying Plasmodium falciparum, with NHP infections, in clinical studies with humans, or using in vitro culture or rodent model systems, others have been dedicated to advancing research on Plasmodium vivax, as well as on phylogenetically related simian species, including Plasmodium cynomolgi, Plasmodium coatneyi, and Plasmodium knowlesi. In-depth study of these four phylogenetically related species over the years has spawned the design of NHP longitudinal infection strategies for gathering information about ongoing infections, which can be related to human infections. These Plasmodium-NHP infection model systems are reviewed here, with emphasis on modern systems biological approaches to studying longitudinal infections, pathogenesis, immunity, and vaccines. Recent discoveries capitalizing on NHP longitudinal infections include an advanced understanding of chronic infections, relapses, anaemia, and immune memory. With quickly emerging new technological advances, more in-depth research and mechanistic discoveries can be anticipated on these and additional critical topics, including hypnozoite biology, antigenic variation, gametocyte transmission, bone marrow dysfunction, and loss of uninfected RBCs. New strategies and insights published by the Malaria Host-Pathogen Interaction Center (MaHPIC) are recapped here along with a vision that stresses the importance of educating future experts well trained in utilizing NHP infection model systems for the pursuit of innovative, effective interventions against malaria.

HLA Alleles B*53:01 and C*06:02 Are Associated With Higher Risk of P. falciparum Parasitemia in a Cohort in Uganda

Variation within the HLA locus been shown to play an important role in the susceptibility to and outcomes of numerous infections, but its influence on immunity to P. falciparum malaria is unclear. Increasing evidence indicates that acquired immunity to P. falciparum is mediated in part by the cellular immune response, including NK cells, CD4 and CD8 T cells, and semi-invariant γδ T cells. HLA molecules expressed by these lymphocytes influence the epitopes recognized by P. falciparum-specific T cells, and class I HLA molecules also serve as ligands for inhibitory receptors including KIR. Here we assessed the relationship of HLA class I and II alleles to the risk of P. falciparum infection and symptomatic malaria in a cohort of 892 Ugandan children and adults followed prospectively via both active and passive surveillance. We identified two HLA class I alleles, HLA-B^*53:01 and HLA-C^*06:02, that were associated with a higher prevalence of P. falciparum infection. Notably, no class I or II HLA alleles were found to be associated with protection from P. falciparum parasitemia or symptomatic malaria. These findings suggest that class I HLA plays a role in the ability to restrict parasitemia, supporting an essential role for the cellular immune response in P. falciparum immunity. Our findings underscore the need for better tools to enable mechanistic studies of the T cell response to P. falciparum at the epitope level and suggest that further study of the role of HLA in regulating pre-erythrocytic stages of the P. falciparum life cycle is warranted.