Malaria-associated L-arginine deficiency induces mast cell-associated disruption to intestinal barrier defenses against nontyphoidal Salmonella bacteremia

Basophil-Derived IL-4 and IL-13 Protect Intestinal Barrier Integrity and Control Bacterial Translocation during Malaria

Our previous work demonstrated that basophils regulate a suite of malaria phenotypes, including intestinal mastocytosis and permeability, the immune response to infection, gametocytemia, and parasite transmission to the malaria mosquito Anopheles stephensi. Given that activated basophils are primary sources of the regulatory cytokines IL-4 and IL-13, we sought to examine the contributions of these mediators to basophil-dependent phenotypes in malaria. We generated mice with basophils depleted for IL-4 and IL-13 (baso IL-4/IL-13 (-)) and genotype controls (baso IL-4/IL-13 (+)) by crossing mcpt8-Cre and Il4/Il13fl/fl mice and infected them with Plasmodium yoelii yoelii 17XNL. Conditional deletion was associated with ileal mastocytosis and mast cell (MC) activation, increased intestinal permeability, and increased bacterial 16S levels in blood, but it had no effect on neutrophil activation, parasitemia, or transmission to A. stephensi. Increased intestinal permeability in baso IL-4/IL-13 (-) mice was correlated with elevated plasma eotaxin (CCL11), a potent eosinophil chemoattractant, and increased ileal MCs, proinflammatory IL-17A, and the chemokines MIP-1α (CCL3) and MIP-1β (CCL4). Blood bacterial 16S copies were positively but weakly correlated with plasma proinflammatory cytokines IFN-γ and IL-12p40, suggesting that baso IL-4/IL-13 (-) mice failed to control bacterial translocation into the blood during malaria infection. These observations suggest that basophil-derived IL-4 and IL-13 do not contribute to basophil-dependent regulation of parasite transmission, but these cytokines do orchestrate protection of intestinal barrier integrity after P. yoelii infection. Specifically, basophil-dependent IL-4/IL-13 control MC activation and prevent infection-induced intestinal barrier damage and bacteremia, perhaps via regulation of eosinophils, macrophages, and Th17-mediated inflammation.

Mast cell-derived IL-10 protects intestinal barrier integrity during malaria in mice and regulates parasite transmission to Anopheles stephensi with a female-biased immune response

Malaria is strongly predisposed to bacteremia, which is associated with increased gastrointestinal permeability and a poor clinical prognosis. We previously identified mast cells (MCs) as mediators of intestinal permeability in malaria and described multiple cytokines that rise with parasitemia, including interleukin (IL)-10, which could protect the host from an inflammatory response and alter parasite transmission to Anopheles mosquitoes. Here, we used the Cre-loxP system and non-lethal Plasmodium yoelii yoelii 17XNL to study the roles of MC-derived IL-10 in malaria immunity and transmission. Our data suggest a sex-biased and local inflammatory response mediated by MC-derived IL-10, supported by early increased number and activation of MCs in females relative to males. Increased parasitemia in female MC IL-10 (-) mice was associated with increased ileal levels of chemokines and plasma myeloperoxidase (MPO). We also observed increased intestinal permeability in female and male MC IL-10 (-) mice relative to MC IL-10 (+) mice but no differences in blood bacterial 16S DNA levels. Transmission success of P. yoelii to A. stephensi was higher in female relative to male mice and from female and male MC IL-10 (-) mice relative to MC IL-10 (+) mice. These patterns were associated with increased plasma levels of pro-inflammatory cytokines in female MC IL-10 (-) mice and increased plasma levels of chemokines and markers of neutrophil activation in male MC IL-10 (-) mice. Overall, these data suggest that MC-derived IL-10 protects intestinal barrier integrity, regulates parasite transmission, and controls local and systemic host immune responses during malaria, with a female bias.

Intestinal barrier disruption with Plasmodium falciparum infection in pregnancy and risk of preterm birth: a cohort study

BACKGROUND

Malaria in early pregnancy is a risk factor for preterm birth and is associated with sustained inflammation and dysregulated angiogenesis across gestation. This study investigated whether malaria is associated with increased gut leak and whether this contributes to systemic inflammation, altered angiogenesis, and preterm birth.

METHODS

We quantified plasma concentrations of gut leak markers, soluble CD14 (sCD14) and lipopolysaccharide binding protein (LBP) from 1339 HIV-negative pregnant Malawians at <24 weeks gestational age. We assessed the relationship of sCD14 and LBP concentrations with markers of inflammation, angiogenesis, and L-arginine bioavailability and compared them between participants with and without malaria, and with and without preterm birth.

FINDINGS

Plasma concentrations of sCD14 and LBP were significantly higher in participants with malaria and were associated with parasite burden (p <0.0001, both analyses and analytes). The odds ratio for preterm birth associated with one log sCD14 was 2.67 (1.33 to 5.35, p = 0.006) and 1.63 (1.07-2.47, p = 0.023) for LBP. Both gut leak analytes were positively associated with increases in proinflammatory cytokines CRP, sTNFR2, IL18-BP, CHI3L1 and Angptl3 (p <0.05, all analytes) and sCD14 was significantly associated with angiogenic proteins Angpt-2, sENG and the sFLT:PlGF ratio (p <0.05, all analytes). sCD14 was negatively associated with L-arginine bioavailability (p <0.001).

INTERPRETATION

Malaria in early pregnancy is associated with intestinal barrier dysfunction, which is linked to an increased risk of preterm birth.

FUNDING

Open Philanthropy, Canadian Institutes of Health Research, Canada Research Chair program, European and Developing Countries Clinical Trials Partnership, Bill & Melinda Gates Foundation.

Intestinal injury and the gut microbiota in patients with Plasmodium falciparum malaria

The pathophysiology of severe falciparum malaria involves a complex interaction between the host, parasite, and gut microbes. In this review, we focus on understanding parasite-induced intestinal injury and changes in the human intestinal microbiota composition in patients with Plasmodium falciparum malaria. During the blood stage of P. falciparum infection, infected red blood cells adhere to the vascular endothelium, leading to widespread microcirculatory obstruction in critical tissues, including the splanchnic vasculature. This process may cause intestinal injury and gut leakage. Epidemiological studies indicate higher rates of concurrent bacteraemia in severe malaria cases. Furthermore, severe malaria patients exhibit alterations in the composition and diversity of the intestinal microbiota, although the exact contribution to pathophysiology remains unclear. Mouse studies have demonstrated that the gut microbiota composition can impact susceptibility to Plasmodium infections. In patients with severe malaria, the microbiota shows an enrichment of pathobionts, including pathogens that are known to cause concomitant bloodstream infections. Microbial metabolites have also been detected in the plasma of severe malaria patients, potentially contributing to metabolic acidosis and other clinical complications. However, establishing causal relationships requires intervention studies targeting the gut microbiota.

Mast cell-mediated immune regulation in health and disease

Mast cells are important components of the immune system, and they perform pro-inflammatory as well as anti-inflammatory roles in the complex process of immune regulation in health and disease. Because of their strategic perivascular localization, sensitivity and adaptability to the microenvironment, and ability to release a variety of preformed and newly synthesized effector molecules, mast cells perform unique functions in almost all organs. Additionally, Mast cells express a wide range of surface and cytoplasmic receptors which enable them to respond to a variety of cytokines, chemicals, and pathogens. The mast cell's role as a cellular interface between external and internal environments as well as between vasculature and tissues is critical for protection and repair. Mast cell interactions with different immune and nonimmune cells through secreted inflammatory mediators may also turn in favor of disease promoting agents. First and forefront, mast cells are well recognized for their multifaceted functions in allergic diseases. Reciprocal communication between mast cells and endothelial cells in the presence of bacterial toxins in chronic/sub-clinical infections induce persistent vascular inflammation. We have shown that mast cell proteases and histamine induce endothelial inflammatory responses that are synergistically amplified by bacterial toxins. Mast cells have been shown to exacerbate vascular changes in normal states as well as in chronic or subclinical infections, particularly among cigarette smokers. Furthermore, a potential role of mast cells in SARS-CoV-2-induced dysfunction of the capillary-alveolar interface adds to the growing understanding of mast cells in viral infections. The interaction between mast cells and microglial cells in the brain further highlights their significance in neuroinflammation. This review highlights the significant role of mast cells as the interface that acts as sensor and early responder through interactions with cells in systemic organs and the nervous system.

Determining the association between gut microbiota and its metabolites with higher intestinal Immunoglobulin A response

Immunoglobulin A (IgA) is one of the important and most abundant immunoglobulins which neutralize invading pathogens at mucosal sites. Gut microbial community and their metabolites which are responsible for higher IgA are poorly known. The current study was carried out to determine those microbial community and their metabolites. Twenty-two healthy, 26 days wean piglets were used in the study. After 10 days of weaning, piglets were divided into two groups. Group 1 with significantly higher fecal IgA while group 2 with significantly lower IgA concentrations from each other. Both groups were analyzed for the fecal inflammatory cytokine, fecal microbial community using 16S ribosomal sequencing, and microbial metabolites using GC-MS. Results showed that Firmicutes and Bacteroidetes constituted 90.56% of the microbiome population in the fecal matter of pigs with higher IgA concentration while pigs with lower fecal IgA had Firmicutes and Bacteroidetes abundance as of 95.56%. Pigs with higher IgA had significantly higher Bacteroidota and Desulfobacterota populations, while significantly lower Firmicutes and Firmicutes/ Bacteroidota ratio (p <0.05). Roughly at the species level, animals with higher fecal IgA concentration had significantly higher bacteria which are associated with gut inflammation and infectious such Prevotella spp and Lachnospiraceae AC2044. Pigs with higher IgA had comparatively lower short-chain fatty acid (SCFA) such as acetic acid, butyric, formic acid, isovaleric acid, and propionic acid which has been associated with gut immune tolerance and immune homeostasis.

The Prevalence of Malaria and Bacteremia Co-Infections among Febrile Patients: A Systematic Review and Meta-Analysis

Comprehensive data on the relative contribution of bacteremia to malaria outcomes in a large number of participants are lacking. Therefore, we collated data on the co-existence of malaria and bacteremia in the literature to provide evidence-based information for future studies investigating the clinical significance of this co-infection. The study protocol was registered at PROSPERO (ID: CRD42021287971). Relevant studies were identified from PubMed, Web of Science, and Scopus. The pooled prevalence of (1) co-existent malaria and bacteremia among febrile patients, (2) the pooled prevalence of bacteremia among patients with malaria, (3) the probability of co-infection, and (4) the pooled prevalence of deaths were estimated by the random-effects model. Fifty-one studies involving 1583 cases of co-infection were included in the analyses. Typhoidal Salmonella spp. and Staphylococcus aureus were the most common Gram-negative and Gram-positive bacteria, respectively. The prevalence of co-existent malaria and bacteremia among febrile patients was 1.9% (95% confidence interval (CI) = 1.5–2.2%, I² = 96.64%, 31 studies). The prevalence of bacteremia among patients with malaria was 7.6% (95% CI = 6.7–8.7%, and I² = 96.68%, 43 studies). Co-infection by malaria and bacteremia did not occur by chance (p = 0.024, odds ratio = 0.64, 95% CI = 0.43–0.94, and I² = 95.7%, 29 studies). The pooled prevalence of deaths among patients with co-infection was 15.0% (95% CI = 8.0–23.0%, I² = 75.23%, 8 studies). On the basis of this study, we conclude that although the prevalence of co-infection was low, patients with malaria appear at greater risk of bacteremia and death.

The Basophil IL-18 Receptor Precisely Regulates the Host Immune Response and Malaria-Induced Intestinal Permeability and Alters Parasite Transmission to Mosquitoes without Effect on Gametocytemia

We have recently demonstrated that basophils are protective against intestinal permeability during malaria and contribute to reduced parasite transmission to mosquitoes. Given that IL-18 is an early cytokine/alarmin in malaria and has been shown to activate basophils, we sought to determine the role of the basophil IL-18R in this protective phenotype. To address this, we infected control [IL18r ^flox/flox or basoIL-18R (+)] mice and mice with basophils lacking the IL-18R [IL18r ^flox/flox × Basoph8 or basoIL-18R (-)] with Plasmodium yoelii yoelii 17XNL, a nonlethal strain of mouse malaria. Postinfection (PI), intestinal permeability, ileal mastocytosis, bacteremia, and levels of ileal and plasma cytokines and chemokines were measured through 10 d PI. BasoIL-18R (-) mice exhibited greater intestinal permeability relative to basoIL-18R (+) mice, along with increased plasma levels of proinflammatory cytokines at a single time point PI, day 4 PI, a pattern not observed in basoIL-18R (+) mice. Surprisingly, mosquitoes fed on basoIL-18R (-) mice became infected less frequently than mosquitoes fed on basoIL-18R (+) mice, with no difference in gametocytemia, a pattern that was distinct from that observed previously with basophil-depleted mice. These findings suggest that early basophil-dependent protection of the intestinal barrier in malaria is mediated by IL-18, and that basophil IL-18R-dependent signaling differentially regulates the inflammatory response to infection and parasite transmission.

Basophil Depletion Alters Host Immunity, Intestinal Permeability, and Mammalian Host-to-Mosquito Transmission in Malaria

Malaria-induced bacteremia has been shown to result from intestinal mast cell (MC) activation. The appearance of MCs in the ileum and increased intestinal permeability to enteric bacteria are preceded by an early Th2-biased host immune response to infection, characterized by the appearance of IL-4, IL-10, mast cell protease (Mcpt)1 and Mcpt4, and increased circulating basophils and eosinophils. Given the functional similarities of basophils and MCs in the context of allergic inflammation and the capacity of basophils to produce large amounts of IL-4, we sought to define the role of basophils in increased intestinal permeability, in MC influx, and in the development of bacteremia in the context of malaria. Upon infection with nonlethal Plasmodium yoelii yoelii 17XNL, Basoph8 × ROSA-DTα mice or baso (-) mice that lack basophils exhibited increased intestinal permeability and increased ileal MC numbers, without any increase in bacterial 16S ribosomal DNA copy numbers in the blood, relative to baso (+) mice. Analysis of cytokines, chemokines, and MC-associated factors in the ileum revealed significantly increased TNF-α and IL-13 at day 6 postinfection in baso (-) mice compared with baso (+) mice. Moreover, network analysis of significantly correlated host immune factors revealed profound differences between baso (-) and baso (+) mice following infection in both systemic and ileal responses to parasites and translocated bacteria. Finally, basophil depletion was associated with significantly increased gametocytemia and parasite transmission to Anopheles mosquitoes, suggesting that basophils play a previously undescribed role in controlling gametocytemia and, in turn, mammalian host-to-mosquito parasite transmission.

Mast Cell Chymase/Mcpt4 Suppresses the Host Immune Response to Plasmodium yoelii, Limits Malaria-Associated Disruption of Intestinal Barrier Integrity and Reduces Parasite Transmission to Anopheles stephensi

An increase in mast cells (MCs) and MCs mediators has been observed in malaria-associated bacteremia, however, the role of these granulocytes in malarial immunity is poorly understood. Herein, we studied the role of mouse MC protease (Mcpt) 4, an ortholog of human MC chymase, in malaria-induced bacteremia using Mcpt4 knockout (Mcpt4-/-) mice and Mcpt4+/+ C57BL/6J controls, and the non-lethal mouse parasite Plasmodium yoelii yoelii 17XNL. Significantly lower parasitemia was observed in Mcpt4-/- mice compared with Mcpt4+/+ controls by day 10 post infection (PI). Although bacterial 16S DNA levels in blood were not different between groups, increased intestinal permeability to FITC-dextran and altered ileal adherens junction E-cadherin were observed in Mcpt4-/- mice. Relative to infected Mcpt4+/+ mice, ileal MC accumulation in Mcpt4-/- mice occurred two days earlier and IgE levels were higher by days 8-10 PI. Increased levels of circulating myeloperoxidase were observed at 6 and 10 days PI in Mcpt4+/+ but not Mcpt4-/- mice, affirming a role for neutrophil activation that was not predictive of parasitemia or bacterial 16S copies in blood. In contrast, early increased plasma levels of TNF-α, IL-12p40 and IL-3 were observed in Mcpt4-/- mice, while levels of IL-2, IL-10 and MIP1β (CCL4) were increased over the same period in Mcpt4+/+ mice, suggesting that the host response to infection was skewed toward a type-1 immune response in Mcpt4-/- mice and type-2 response in Mcpt4+/+ mice. Spearman analysis revealed an early (day 4 PI) correlation of Mcpt4-/- parasitemia with TNF-α and IFN-γ, inflammatory cytokines known for their roles in pathogen clearance, a pattern that was observed in Mcpt4+/+ mice much later (day 10 PI). Transmission success of P. y. yoelii 17XNL to Anopheles stephensi was significantly higher from infected Mcpt4-/- mice compared with infected Mcpt4+/+ mice, suggesting that Mcpt4 also impacts transmissibility of sexual stage parasites. Together, these results suggest that early MCs activation and release of Mcpt4 suppresses the host immune response to P. y. yoelii 17XNL, perhaps via degradation of TNF-α and promotion of a type-2 immune response that concordantly protects epithelial barrier integrity, while limiting the systemic response to bacteremia and parasite transmissibility.

Intestinal inflammation and increased intestinal permeability in Plasmodium chabaudi AS infected mice

Background: Gastrointestinal symptoms are commonly associated with acute Plasmodium spp infection. Malaria-associated enteritis may provide an opportunity for enteric pathogens to breach the intestinal mucosa, resulting in life-threatening systemic infections. Methods: To investigate whether intestinal pathology also occurs during infection with a murine model of mild and resolving malaria, C57BL/6J mice were inoculated with recently mosquito-transmitted Plasmodium chabaudi AS. At schizogony, intestinal tissues were collected for quantification and localisation of immune mediators and malaria parasites, by PCR and immunohistochemistry. Inflammatory proteins were measured in plasma and faeces and intestinal permeability was assessed by FITC-dextran translocation after oral administration. Results: Parasitaemia peaked at approx. 1.5% at day 9 and resolved by day 14, with mice experiencing significant and transient anaemia but no weight loss. Plasma IFN-γ, TNF-α and IL10 were significantly elevated during peak infection and quantitative RT-PCR of the intestine revealed a significant increase in transcripts for ifng and cxcl10. Histological analysis revealed parasites within blood vessels of both the submucosa and intestinal villi and evidence of mild crypt hyperplasia. In faeces, concentrations of the inflammatory marker lactoferrin were significantly raised on days 9 and 11 and FITC-dextran was detected in plasma on days 7 to 14. At day 11, plasma FITC-dextran concentration was significantly positively correlated with peripheral parasitemia and faecal lactoferrin concentration. Conclusions: In summary, using a relevant, attenuated model of malaria, we have found that acute infection is associated with intestinal inflammation and increased intestinal permeability. This model can now be used to explore the mechanisms of parasite-induced intestinal inflammation and to assess the impact of increased intestinal permeability on translocation of enteropathogens.

Intestinal inflammation and increased intestinal permeability in Plasmodium chabaudi AS infected mice

Background: Gastrointestinal symptoms are commonly associated with acute Plasmodium spp infection. Malaria-associated enteritis may provide an opportunity for enteric pathogens to breach the intestinal mucosa, resulting in life-threatening systemic infections. Methods: To investigate whether intestinal pathology also occurs during infection with a murine model of mild and resolving malaria, C57BL/6J mice were inoculated with recently mosquito-transmitted Plasmodium chabaudi AS. At schizogony, intestinal tissues were collected for quantification and localisation of immune mediators and malaria parasites, by PCR and immunohistochemistry. Inflammatory proteins were measured in plasma and faeces and intestinal permeability was assessed by FITC-dextran translocation after oral administration. Results: Parasitaemia peaked at approx. 1.5% at day 9 and resolved by day 14, with mice experiencing significant and transient anaemia but no weight loss. Plasma IFNγ, TNFα and IL10 were significantly elevated during peak infection and quantitative RT-PCR of the intestine revealed a significant increase in transcripts for ifng and cxcl10. Histological analysis revealed parasites within blood vessels of both the submucosa and intestinal villi and evidence of mild crypt hyperplasia. In faeces, concentrations of the inflammatory marker lactoferrin were significantly raised on days 9 and 11 and FITC-dextran was detected in plasma on days 7 to 14. At day 11, plasma FITC-dextran concentration was significantly positively correlated with peripheral parasitemia and faecal lactoferrin concentration. Conclusions: In summary, using a relevant, attenuated model of malaria, we have found that acute infection is associated with intestinal inflammation and increased intestinal permeability. This model can now be used to explore the mechanisms of parasite-induced intestinal inflammation and to assess the impact of increased intestinal permeability on translocation of enteropathogens.

Malaria-induced bacteremia as a consequence of multiple parasite survival strategies

Globally, malaria continues to be an enormous public health burden, with concomitant parasite-induced damage to the gastrointestinal (GI) barrier resulting in bacteremia-associated morbidity and mortality in both adults and children. Infected red blood cells sequester in and can occlude the GI microvasculature, ultimately leading to disruption of the tight and adherens junctions that would normally serve as a physical barrier to translocating enteric bacteria. Mast cell (MC) activation and translocation to the GI during malaria intensifies damage to the physical barrier and weakens the immunological barrier through the release of enzymes and factors that alter the host response to escaped enteric bacteria. In this context, activated MCs release Th2 cytokines, promoting a balanced Th1/Th2 response that increases local and systemic allergic inflammation while protecting the host from overwhelming Th1-mediated immunopathology. Beyond the mammalian host, recent studies in both the lab and field have revealed an association between a Th2-skewed host response and success of parasite transmission to mosquitoes, biology that is evocative of parasite manipulation of the mammalian host. Collectively, these observations suggest that malaria-induced bacteremia may be, in part, an unintended consequence of a Th2-shifted host response that promotes parasite survival and transmission. Future directions of this work include defining the factors and mechanisms that precede the development of bacteremia, which will enable the development of biomarkers to simplify diagnostics, the identification of therapeutic targets to improve patient outcomes and better understanding of the consequences of clinical interventions to transmission blocking strategies.

Mast cells-derived exosomes worsen the development of experimental cerebral malaria

Cerebral malaria (CM) is the most severe neurological complication caused by Plasmodium falciparum infection. The accumulating evidence demonstrated that mast cells (MCs) and its mediators played a critical role in mediating malaria severity. Earlier studies identified that exosomes were emerging as key mediators of intercellular communication and can be released from several kinds of MCs. However, the potential functions and pathological mechanisms of MCs-derived exosomes (MCs-Exo) impacting on CM pathogenesis remain largely unknown. Herein, we utilized an experimental CM (ECM) model (C57BL/6 mice infected with P. berghei ANKA strain), and then intravenously (i.v.) injected MCs-Exo into P. berghei ANKA-infected mice to unfold this mechanism and investigate the effect of MCs-Exo on ECM pathogenies. We also used an in vitro model by investigating the pathogenesis development of brain microvascular endothelial cells line (bEnd.3 cells) co-cultured with P. berghei ANKA blood-stage soluble antigen (PbAg) after MCs-Exo treatment. The higher numbers of MCs and levels of MCs degranulation were observed in skin, cervical lymph node, and brain of ECM mice than those of the uninfected mice. Exosomes were successfully isolated from culture supernatants of mouse MCs line (P815 cells) and characterized by spherical vesicles with the diameter of 30-150 nm, and expression of typical exosomal markers (e.g., CD9, CD63, and CD81). The i.v. injection of MCs-Exo dramatically elevated incidence of ECM in the P. berghei ANKA-infected mice, exacerbated liver and brain histopathological damage, promoted Th1 cytokine response, aggravated brain vascular endothelial activation and blood brain barrier breakdown in ECM mice. In addition, the treatment of MCs-Exo led to the decrease of cells viability and mRNA levels of Ang-1, ZO-1, and Claudin-5, but increase of mRNA levels of Ang-2, CCL2, CXCL1, and CXCL9 in bEnd.3 cells co-cultured with PbAg in vitro. Taken together, our data indicated that MCs-Exo could worsen pathogenesis of ECM in mice.

Metabolomic Analysis of Diverse Mice Reveals Hepatic Arginase-1 as Source of Plasma Arginase in Plasmodium chabaudi Infection

Infections disrupt host metabolism, but the factors that dictate the nature and magnitude of metabolic change are incompletely characterized. To determine how host metabolism changes in relation to disease severity in murine malaria, we performed plasma metabolomics on eight Plasmodium chabaudi-infected mouse strains with diverse disease phenotypes. We identified plasma metabolic biomarkers for both the nature and severity of different malarial pathologies. A subset of metabolic changes, including plasma arginine depletion, match the plasma metabolomes of human malaria patients, suggesting new connections between pathology and metabolism in human malaria. In our malarial mice, liver damage, which releases hepatic arginase-1 (Arg1) into circulation, correlated with plasma arginine depletion. We confirmed that hepatic Arg1 was the primary source of increased plasma arginase activity in our model, which motivates further investigation of liver damage in human malaria patients. More broadly, our approach shows how leveraging phenotypic diversity can identify and validate relationships between metabolism and the pathophysiology of infectious disease. IMPORTANCE Malaria is a severe and sometimes fatal infectious disease endemic to tropical and subtropical regions. Effective vaccines against malaria-causing Plasmodium parasites remain elusive, and malaria treatments often fail to prevent severe disease. Small molecules that target host metabolism have recently emerged as candidates for therapeutics in malaria and other diseases. However, our limited understanding of how metabolites affect pathophysiology limits our ability to develop new metabolite therapies. By providing a rich data set of metabolite-pathology correlations and by validating one of those correlations, our work is an important step toward harnessing metabolism to mitigate disease. Specifically, we showed that liver damage in P. chabaudi-infected mice releases hepatic arginase-1 into circulation, where it may deplete plasma arginine, a candidate malaria therapeutic that mitigates vascular stress. Our data suggest that liver damage may confound efforts to increase levels of arginine in human malaria patients.

Association of Dengue Virus and Leptospira Co-Infections with Malaria Severity

Plasmodium infections are co-endemic with infections caused by other agents of acute febrile illnesses, such as dengue virus (DENV), chikungunya virus, Leptospira spp., and Orientia tsutsugamushi. However, co-infections may influence disease severity, treatment outcomes, and development of drug resistance. When we analyzed cases of acute febrile illness at the All India Institute of Medical Sciences, New Delhi, India, from July 2017 through September 2018, we found that most patients with malaria harbored co-infections (Plasmodium mixed species and other pathogens). DENV was the most common malaria co-infection (44% of total infections). DENV serotype 4 was associated with mild malaria, and Leptospira was associated with severe malaria. We also found the presence of P. knowlesi in our study population. Therefore, in areas with a large number of severe malaria cases, diagnostic screening for all 4 DENV serotypes, Leptospira, and all Plasmodium species should be performed.

Histamine Ingestion by Anopheles stephensi Alters Important Vector Transmission Behaviors and Infection Success with Diverse Plasmodium Species

An estimated 229 million people worldwide were impacted by malaria in 2019. The vectors of malaria parasites (Plasmodium spp.) are Anopheles mosquitoes, making their behavior, infection success, and ultimately transmission of great importance. Individuals with severe malaria can exhibit significantly increased blood concentrations of histamine, an allergic mediator in humans and an important insect neuromodulator, potentially delivered to mosquitoes during blood-feeding. To determine whether ingested histamine could alter Anopheles stephensi biology, we provisioned histamine at normal blood levels and at levels consistent with severe malaria and monitored blood-feeding behavior, flight activity, antennal and retinal responses to host stimuli and lifespan of adult female Anopheles stephensi. To determine the effects of ingested histamine on parasite infection success, we quantified midgut oocysts and salivary gland sporozoites in mosquitoes infected with Plasmodium yoelii and Plasmodium falciparum. Our data show that provisioning An. stephensi with histamine at levels consistent with severe malaria can enhance mosquito behaviors and parasite infection success in a manner that would be expected to amplify parasite transmission to and from human hosts. Such knowledge could be used to connect clinical interventions by reducing elevated histamine to mitigate human disease pathology with the delivery of novel lures for improved malaria control.

Invited Review: Maintain or Improve Piglet Gut Health around Weanling: The Fundamental Effects of Dietary Amino Acids

Gut health has significant implications for swine nutrient utilization and overall health. The basic gut morphology and its luminal microbiota play determinant roles for maintaining gut health and functions. Amino acids (AA), a group of essential nutrients for pigs, are not only obligatory for maintaining gut mucosal mass and integrity, but also for supporting the growth of luminal microbiota. This review summarized the up-to-date knowledge concerning the effects of dietary AA supplementation on the gut health of weanling piglets. For instance, threonine, arginine, glutamine, methionine and cysteine are beneficial to gut mucosal immunity and barrier function. Glutamine, arginine, threonine, methionine and cysteine can also assist with relieving the post-weaning stress of young piglets by improving gut immunological functions, antioxidant capacity, and/or anti-inflammatory ability. Glutamine, glutamate, glycine and cysteine can assist to reconstruct the gut structure after its damage and reverse its dysfunction. Furthermore, methionine, lysine, threonine, and glutamate play key roles in affecting bacteria growth in the lumen. Overall, the previous studies with different AA showed both similar and different effects on the gut health, but how to take advantages of all these effects for field application is not clear. It is uncertain whether these AA effects are synergetic or antagonistic. The interactions between the effects of non-nutrient feed additives and the fundamental effects of AA warrant further investigation. Considering the global push to minimize the antibiotics and ZnO usage in swine production, a primary effort at present may be made to explore the specific effects of individual AA, and then the concert effects of multiple AA, on the profile and functions of gut microbiota in young pigs.

Events associated with susceptibility to invasive Salmonella enterica serovar Typhi in BALB/c mice previously infected with Plasmodium berghei ANKA

Numerous mechanisms have been proposed to explain why patients with malaria are more susceptible to bloodstream invasions by Salmonella spp., however there are still several unknown critical factors regarding the pathogenesis of coinfection. From a coinfection model, in which an S. enterica serovar Typhi (S_Typhi) was chosen to challenge mice that had been infected 24 h earlier with Plasmodium berghei ANKA (P.b_ANKA), we evaluated the influence of malaria on cytokine levels, the functional activity of femoral bone marrow-derived macrophages and neutrophils, and intestinal permeability. The cytokine profile over eight days of coinfection showed exacerbation in the cytokines MCP-1, IFNγ and TNFα in relation to the increase seen in animals with malaria. The cytokine profile was associated with a considerably reduced neutrophil and macrophage count and a prominent dysfunction, especially in ex vivo neutrophils in coinfected mice, though without bacterial modulation that could influence the invasion capacity of ex vivo S_Typhi obtained from liver macerate in non-phagocyte cells. Finally, irregularities in the integrity of intestinal tissue evidenced ruptures in the enterocyte layer, a presence of mononuclear leukocytes in the enterocyte layer, an increase of goblet cells in the enterocyte layer and a high volume of leukocyte infiltrate in the sub-mucosa were greatly increased in coinfected animals. Increases of mononuclear leukocytes in the enterocyte layer and volume of leukocyte infiltrate in the sub-mucosa were also seen in monoinfected animals with P. berghei ANKA. Our findings suggest malaria causes a disarrangement of intestinal homeostasis, exacerbation of proinflammatory cytokines and dysfunction in neutrophils that render the host susceptible to bacteremia by Salmonella spp.

Disrupted Iron Metabolism and Mortality during Co-infection with Malaria and an Intestinal Gram-Negative Extracellular Pathogen

Individuals with malaria exhibit increased morbidity and mortality when infected with Gram-negative (Gr−) bacteria. To explore this experimentally, we performed co-infection of mice with Plasmodium chabaudi and Citrobacter rodentium, an extracellular Gr− bacterial pathogen that infects the large intestine. While single infections are controlled effectively, co-infection results in enhanced virulence that is characterized by prolonged systemic bacterial persistence and high mortality. Mortality in co-infected mice is associated with disrupted iron metabolism, elevated levels of plasma heme, and increased mitochondrial reactive oxygen species (ROS) production by phagocytes. In addition, iron acquisition by the bacterium plays a key role in pathogenesis because co-infection with a mutant C. rodentium strain lacking a critical iron acquisition pathway does not cause mortality. These results indicate that disrupted iron metabolism may drive mortality during co-infection with C. rodentium and P. chabaudi by both altering host immune responses and facilitating bacterial persistence.

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Co-infection with malaria and a Gram-negative bacterial pathogen leads to high mortality

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Co-infection leads to elevated plasma heme and systemic bacterial persistence

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Iron acquisition is critical for bacterial persistence and mortality

Does Malaria Cause Diarrhoea? A Systematic Review

Malaria is a systemic febrile disease that may progress to prostration, respiratory distress, encephalopathy, anemia, and death. Malaria is also an established risk factor for invasive bacterial disease caused, in the majority of cases, by invasive enteropathogens and in particular by non-Typhoidal Salmonella (NTS). Whilst various malaria-related pathologies have been implicated in the risk of NTS bacteraemia in animal models, including intestinal dysbiosis and loss of gut homeostasis, clinical evidence is lacking. As a first step in gathering such evidence, we conducted a systematic review of clinical and epidemiological studies reporting the prevalence of diarrhoea among malaria cases and vice versa. Database searches for "plasmodium" and "diarrhoea" identified 1,771 articles; a search for "plasmodium" and "gastroenteritis" identified a further 215 articles. After review, 66 articles specified an association between the search terms and referred primarily, but not exclusively, to Plasmodium falciparum infections. Overall, between 1.6 and 44% of patients with acute malaria infection reported symptoms of diarrhoea (812 of 7,267 individuals, 11%) whereas 5-42% of patients presenting to hospital with diarrhoea had an underlying malaria parasite infection (totaling 749 of 2,937 individuals, 26%). However, given the broad range of estimates, a paucity of purposeful case control or longitudinal studies, and varied or poorly specified definitions of diarrhoea, the literature provides limited evidence to draw any firm conclusions. The relationship between malaria and gastrointestinal disturbance thus remains unclear. Carefully designed case-control studies and prospective longitudinal studies are required to confidently assess the prevalence and significance of intestinal manifestations of malaria parasite infection.

Diarrhea in Children with Plasmodium falciparum Malaria: A Case-Control Study on the Prevalence and Response to Antimalarial Treatment

The role of Plasmodium in the etiology of acute diarrhea in developing countries remains controversial, and gastrointestinal (GI) symptoms are inconsistently reported in malaria. In this observational case-control study, we investigated the prevalence and risk factors for GI symptoms in hospitalized malarious children aged 1 month to 5 years in northern Uganda. Children with a diagnosis of Plasmodium falciparum malaria were enrolled as cases, and feverish children in whom malaria was excluded were enrolled as controls. Among 451 malarious children, 46.1% had GI symptoms at admission. Compared with controls, the frequency of diarrhea (24.8% versus 11.2%, P < 0.001) and vomiting (35.5% versus 17.5%, P < 0.001) was significantly higher in children with malaria, who had a higher chance of showing either vomiting (odds ratio [OR]: 3.22; 95% CI: 2.14-4.91) or diarrhea (OR: 3.14; 95% CI: 1.99-5.07) at hospital admission. A subgroup analysis performed in children with severe malaria, severe anemia, or high-grade fever confirmed these results. Diarrhea was more frequent in infants and children younger than 3 years than in older children. The analysis of 71 malarious children with diarrhea who received intravenous artesunate showed that the symptom resolved within the first 24 hours since the beginning of the treatment in 85.9% of cases. The 3-fold higher prevalence of diarrhea and vomiting in malarious children compared with febrile controls may provide rationale for incorporating malaria testing in the symptom-guided diagnostic approach of the young child with diarrhea and vomiting in malaria-endemic settings.

Nonlethal Plasmodium yoelii Infection Drives Complex Patterns of Th2-Type Host Immunity and Mast Cell-Dependent Bacteremia

Malaria strongly predisposes to bacteremia, which is associated with sequestration of parasitized red blood cells and increased gastrointestinal permeability. The mechanisms underlying this disruption are poorly understood. Here, we evaluated the expression of factors associated with mast cell activation and malaria-associated bacteremia in a rodent model. C57BL/6J mice were infected with Plasmodium yoeliiyoelli 17XNL, and blood and tissues were collected over time to assay for circulating levels of bacterial 16S DNA, IgE, mast cell protease 1 (Mcpt-1) and Mcpt-4, Th1 and Th2 cytokines, and patterns of ileal mastocytosis and intestinal permeability.

Malaria strongly predisposes to bacteremia, which is associated with sequestration of parasitized red blood cells and increased gastrointestinal permeability. The mechanisms underlying this disruption are poorly understood. Here, we evaluated the expression of factors associated with mast cell activation and malaria-associated bacteremia in a rodent model. C57BL/6J mice were infected with Plasmodium yoeliiyoelli 17XNL, and blood and tissues were collected over time to assay for circulating levels of bacterial 16S DNA, IgE, mast cell protease 1 (Mcpt-1) and Mcpt-4, Th1 and Th2 cytokines, and patterns of ileal mastocytosis and intestinal permeability. The anti-inflammatory cytokines (interleukin-4 [IL-4], IL-6, and IL-10) and MCP-1/CCL2 were detected early after P. yoeliiyoelii 17XNL infection. This was followed by the appearance of IL-9 and IL-13, cytokines known for their roles in mast cell activation and growth-enhancing activity as well as IgE production. Later increases in circulating IgE, which can induce mast cell degranulation, as well as Mcpt-1 and Mcpt-4, were observed concurrently with bacteremia and increased intestinal permeability. These results suggest that P. yoeliiyoelii 17XNL infection induces the production of early cytokines that activate mast cells and drive IgE production, followed by elevated IgE, IL-9, and IL-13 that maintain and enhance mast cell activation while disrupting the protease/antiprotease balance in the intestine, contributing to epithelial damage and increased permeability.

Parasite-bacteria interrelationship

Parasites and bacteria have co-evolved with humankind, and they interact all the time in a myriad of ways. For example, some bacterial infections result from parasite-dwelling bacteria as in the case of Salmonella infection during schistosomiasis. Other bacteria synergize with parasites in the evolution of human disease as in the case of the interplay between Wolbachia endosymbiont bacteria and filarial nematodes as well as the interaction between Gram-negative bacteria and Schistosoma haematobium in the pathogenesis of urinary bladder cancer. Moreover, secondary bacterial infections may complicate several parasitic diseases such as visceral leishmaniasis and malaria, due to immunosuppression of the host during parasitic infections. Also, bacteria may colonize the parasitic lesions; for example, hydatid cysts and skin lesions of ectoparasites. Remarkably, some parasitic helminths and arthropods exhibit antibacterial activity usually by the release of specific antimicrobial products. Lastly, some parasite-bacteria interactions are induced as when using probiotic bacteria to modulate the outcome of a variety of parasitic infections. In sum, parasite-bacteria interactions involve intricate processes that never cease to intrigue the researchers. However, understanding and exploiting these interactions could have prophylactic and curative potential for infections by both types of pathogens.

Impact of Endemic Infections on HIV Susceptibility in Sub-Saharan Africa

Human immunodeficiency virus (HIV) remains a leading cause of global morbidity with the highest burden in Sub-Saharan Africa (SSA). For reasons that are incompletely understood, the likelihood of HIV transmission is several fold higher in SSA than in higher income countries, and most of these infections are acquired by young women. Residents of SSA are also exposed to a variety of endemic infections, such as malaria and various helminthiases that could influence mucosal and systemic immunology. Since these immune parameters are important determinants of HIV acquisition and progression, this review explores the possible effects of endemic infections on HIV susceptibility and summarizes current knowledge of the epidemiology and underlying immunological mechanisms by which endemic infections could impact HIV acquisition. A better understanding of the interaction between endemic infections and HIV may enhance HIV prevention programs in SSA.

Plasmodium-Salmonella Coinfection Induces Intense Inflammatory Response, Oxidative Stress, and Liver Damage: A Mice Model Study for Therapeutic Strategy

Impairment of host immune response in malaria favors bacteremia caused by typhoidal or nontyphoidal serovars of Salmonella enterica. Ofloxacin and Artesunate are the drugs that are clinically proven for treating typhoid and malaria, respectively. The study evaluates the host responses upon treatment with antibiotic (Ofloxacin) and antimalarial (Artesunate) in a standardized mice model harboring coinfection. BALB/c mice (18-22 g) were simultaneously coinfected with Plasmodium yoelii nigeriensis (Pyn) and S. enterica serovar Typhimurium (STm) and then treated with Ofloxacin or/and Artesunate from day 4 to day 7. The bacterial burden, liver function enzymes, oxidative stress, m-RNA expression of Toll-like receptors (TLR-2 and TLR-4), Th1/Th2 cytokines, hemeoxygenase-1, and NFкB were assessed. Ofloxacin treatment failed to counter the bacterial proliferation in Pyn-STm coinfected mice. However, upon controlling parasitemia with antimalarial, the efficacy of Ofloxacin could be regained. Elevated bacterial burden with malaria induces the expression of TLR-2 and TLR-4 triggering intense inflammatory response (NFκB, Th1/Th2 cytokines) in coinfected mice. This results in critical liver damage (ALT, AST, and ALP), oxidative stress (lipid peroxidation, total GSH, catalase, and super oxide dismutase), and hemeoxygenase-1 (HO-1). The study concludes that malaria infection aggravates the secondary infection of Salmonella serovars and the control of septicemia is critical in recovery of the coinfected subject.

Activation of Mast Cells Promote Plasmodium berghei ANKA Infection in Murine Model

Malaria, a mosquito-borne infectious disease, is a severe health problem worldwide. As reported, some anti-malarial drugs with anti-parasitic properties also block mast cells (MCs) activities. It is hypothesized that MCs activity may be correlated with the pathogenesis of malaria. Thus, the role of MCs on malarial pathogenesis and the involved physiological action and pathways need to be further investigated. This study aimed to investigate the effect of MCs activation on malaria disease severity using KunMing mice with Plasmodium berghei ANKA (PbANKA) infection treated with MCs degranulator (compound 48/80, C48/80) or MCs stabilizer (disodium cromoglycate, DSCG). PbANKA infection caused a dramatic increase in MCs density and level of MCs degranulation in cervical lymph node (CLN) and skin. Compared with infected control, C48/80 treatment had shortened survival time, increased parasitemia, exacerbated liver inflammation and CLN hyperplasia, accompanied with increase in vascular leakage and leukocyte number. The infected mice with C48/80 treatment also elevated the release of CCL2, CXCL1, and MMP-9 from MCs in CLN and skin, and TNF-α, IFN-γ, CCR2, and CXCR2 mRNA expression in CLN and liver. In contrast, the infected mice treated with DSCG showed longer survival time, lower parasitemia, improved liver inflammation and CLN hyperplasia, followed by a decline of vascular leakage and leukocyte number. Decreased MCs-derived CCL2, CXCL1, and MMP-9 from CLN and skin, mRNA expression in CLN and liver (TNF-α, IFN-γ, CCR2, and CXCR2) were also observed in infected mice with DSCG treatment. Our data indicated that MCs activation may facilitate the pathogenesis of PbANKA infection.

In vivo and in vitro protective effect of arginine against intestinal inflammatory response induced by Clostridium perfringens in broiler chickens

Background

Necrotic enteritis is a widespread disease in poultry caused by Clostridium perfringens. We previously reported that dietary arginine supplementation protected the intestinal mucosa of broiler chickens with necrotic enteritis, but the related protective mechanisms remain unclear. The in vivo trial was designed as a 2 × 2 factorial arrangement to evaluated the effects of arginine supplementation on inflammatory responses, arginine transporters, arginine catabolism and JAK-STAT signalling pathway in broiler chickens challenged with C. perfringens or without C. perfringens. Furthermore, we validated the in vivo results using intestinal epithelial cells of chicken embryos.

Results

C. perfringens infection markedly increased gut gross pathological and histopathological lesion scores, promoted liver C. perfringens invasion, reduced serum arginine levels, and elevated jejunal mucosal lysozyme activities (P < 0.05), but these effects were significantly reversed by arginine supplementation in vivo (P < 0.05). The challenge significantly increased serum procalcitonin levels, jejunal mucosal iNOS activities and jejunal IL-6, TGF-β3, cationic amino acid transporter (CAT)-1, and CAT-3 mRNA expression (P < 0.05), whereas arginine supplementation significantly reduced jejunal IFN-γ, IL-1β, IL-6, IL-10, TGF-β3, and CAT-3 mRNA expression (P < 0.05). Arginine supplementation significantly attenuated the C. perfringens challenge-induced increases in jejunal iNOS, arginase 2, arginine decarboxylase, arginine:glycine amidinotransferase, JAK1, JAK3, STAT1, and STAT6 mRNA expression (P < 0.05). The in vitro experiment showed that C. perfringens challenge markedly increased cellular cytotoxicity and the mRNA expression of IL-1β, IL-8, IL-10, CAT-1 and CAT-3 (P < 0.05), which were significantly reversed by 50 μmol/L and/or 400 μmol/L arginine pre-treatment (P < 0.05).

Conclusions

Arginine prevented C. perfringens challenge-induced circulated arginine deficiency, normalized intestinal arginine transport and catabolism, down-regulated JAK-STAT signalling pathway and attenuated the inflammatory response, which exerted protective effects on the intestine of broiler chickens.

Dissemination of non-typhoidal Salmonella during Plasmodium chabaudi infection affects anti-malarial immunity

Malaria-associated bacteremia accounts for up to one-third of deaths from severe malaria, and non-typhoidal Salmonella (NTS) has been reported as a major complication of severe malarial infection. Patients who develop NTS bacteremia during Plasmodium infection show higher mortality rates than individuals with malaria alone. Systemic bacteremia can be caused by a wound or translocation from epithelial or endothelial sites. NTS is an intestinal pathogen, however the contribution of bacterial translocation from the intestinal tract during Plasmodium infection is not well studied. Here, we investigated the integrity of the intestinal barrier function of P. chabaudi-infected mice using large molecules and Salmonella infection. Intestinal histology and the adaptive immune response to malaria were also studied using light microscopy and flow cytometry. P. chabaudi infection compromised intestinal barrier function, which led to increased intestinal cellular infiltration. In addition, we observed increased serum lipopolysaccharide binding protein and leakage of soluble molecules from the intestine into the blood in infected mice. Plasmodium infection also increased intestinal translocation and dissemination of NTS to the liver. The adaptive immune response to P. chabaudi infection was also significantly impacted by NTS translocation. Reduced B and T cell activation were observed in co-infected animals, suggesting interference in the malaria-specific immune responses by bacteremia. These studies demonstrate that P. chabaudi infection induces failure of the barrier function of the intestinal wall and enhanced intestinal bacterial translocation, affecting anti-malarial immunity.

Identifying the Components of Acidosis in Patients With Severe Plasmodium falciparum Malaria Using Metabolomics

BACKGROUND

Acidosis in severe Plasmodium falciparum malaria is associated with high mortality, yet the pathogenesis remains incompletely understood. The aim of this study was to determine the nature and source of metabolic acids contributing to acidosis in patients with severe falciparum malaria.

METHODS

A prospective observational study was conducted to characterize circulating acids in adults with P. falciparum malaria (n = 107) and healthy controls (n = 45) from Bangladesh using high-resolution liquid chromatography-mass spectrometry metabolomics. Additional in vitro P. falciparum culture studies were performed to determine if parasites release the acids detected in plasma from patients with severe malaria acidosis.

RESULTS

We identified previously unmeasured plasma acids strongly associated with acidosis in severe malaria. Metabolomic analysis of P. falciparum parasites in vitro showed no evidence that these acids are released by the parasite during its life cycle. Instead, 10 of the plasma acids could be mapped to a gut microbial origin. Patients with malaria had low L-citrulline levels, a plasma marker indicating reduced gut barrier integrity. Longitudinal data showed the clearance of these newly identified acids was delayed in fatal cases.

CONCLUSIONS

These data suggest that a compromise in intestinal barrier function may contribute significantly to the pathogenesis of life-threatening acidosis in severe falciparum malaria.

CLINICAL TRIALS REGISTRATION

NCT02451904.

Physiological Effects of Dietary Amino Acids on Gut Health and Functions of Swine

Gut health has significant implications for swine overall health status and nutrient utilization, due to its various functions including digestion and absorption of nutrients, secretion of mucins and immunoglobulins, and selective barrier protection against harmful antigens and pathogens. Both the basic anatomical structure of the gut (such as epithelial cells) and its luminal microbiota play important roles for maintaining gut health and functions. The interactions between epithelial cells and luminal microbiota have significant impact on host nutrition and health through the metabolism of dietary components. Amino acids, which are major nutrients for pigs, are not only obligatory for maintaining the intestinal mucosal mass and integrity, but also for supporting the growth of microorganisms in the gut. Dietary amino acids are the major fuel of the small intestinal mucosa. Particularly, glutamate, glutamine, and aspartate are the major oxidative fuel of the intestine. Emerging evidence shows that arginine activates the mTOR signaling pathway in the small intestine. Utilization of glycine by the small intestinal mucosa to synthesize glutathione is a very important physiological pathway, and the role of glycine as a powerful cytoprotectant has also been recognized. The major end products of methionine and cysteine metabolism are glutathione, homocysteine and taurine, which play important roles in the intestinal immune and anti-oxidative responses. Threonine is highly utilized by the gut and is particularly important for mucin synthesis and maintenance of gut barrier integrity. Moreover, either a deficiency or an excess of dietary threonine can reduce the synthesis of intestinal mucosal proteins and mucins in young pigs. Various new functions of amino acids on gut health and functions have been discovered in recent years. Thus, this review is to provide some up-to-date knowledge for industry application of dietary amino acids in order to enhance swine gut health and functions, and also it is to provide a comprehensive reference for further scientific research in this regard.

Malaria, anemia, and invasive bacterial disease: A neutrophil problem?

Invasive bacterial disease is well described in immunocompromised hosts, including those with malaria infection. One bacterial infection frequently observed in children with Plasmodium falciparum infection is nontyphoidal salmonella (NTS) infection, in which a typically intestinal infection becomes systemic with serious, often fatal, consequences. In this review, we consider the role of malaria‐induced immunoregulatory responses in tipping the balance from tissue homeostasis during malaria infection to risk of invasive NTS. Also, neutrophils are crucial in the clearance of NTS but their ability to mount an oxidative burst and kill intracellular Salmonella is severely compromised during, and for some time after, an acute malaria infection. Here, we summarize the evidence linking malaria and invasive NTS infections; describe the role of neutrophils in clearing NTS infections; review evidence for neutrophil dysfunction in malaria infections; and explore roles of heme oxygenase‐1, IL‐10, and complement in mediating this dysfunction. Finally, given the epidemiological evidence that low density, subclinical malaria infections pose a risk for invasive NTS infections, we consider whether the high prevalence of such infections might underlie the very high incidence of invasive bacterial disease across much of sub‐Saharan Africa.

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.

Tissue-specific immunopathology during malaria infection

Systemic inflammation mediated by Plasmodium parasites is central to malaria disease and its complications. Plasmodium parasites reside in erythrocytes and can theoretically reach all host tissues via the circulation. However, actual interactions between parasitized erythrocytes and host tissues, along with the consequent damage and pathological changes, are limited locally to specific tissue sites. Such tissue specificity of the parasite can alter the outcome of malaria disease, determining whether acute or chronic complications occur. Here, we give an overview of the recent progress that has been made in understanding tissue-specific immunopathology during Plasmodium infection. As knowledge on tissue-specific host-parasite interactions accumulates, better treatment modalities and targets may emerge for intervention in malaria disease.

Multi-drug resistant non-typhoidal Salmonella associated with invasive disease in western Kenya

Non-typhoidal Salmonella (NTS) is a leading cause of bloodstream infections in Africa, but the various contributions of host susceptibility versus unique pathogen virulence factors are unclear. We used data from a population-based surveillance platform (population ~25,000) between 2007–2014 and NTS genome-sequencing to compare host and pathogen-specific factors between individuals presenting with NTS bacteremia and those presenting with NTS diarrhea. Salmonella Typhimurium ST313 and Salmonella Enteritidis ST11 were the most common isolates. Multi-drug resistant strains of NTS were more commonly isolated from patients presenting with NTS bacteremia compared to NTS diarrhea. This relationship was observed in patients under age five [aOR = 15.16, 95% CI (2.84–81.05), P = 0.001], in patients five years and older, [aOR = 6.70 95% CI (2.25–19.89), P = 0.001], in HIV-uninfected patients, [aOR = 21.61, 95% CI (2.53–185.0), P = 0.005], and in patients infected with Salmonella serogroup B [aOR = 5.96, 95% CI (2.28–15.56), P < 0.001] and serogroup D [aOR = 14.15, 95% CI (1.10–182.7), P = 0.042]. Thus, multi-drug-resistant NTS was strongly associated with bacteremia compared to diarrhea among children and adults. This association was seen in HIV-uninfected individuals infected with either S. Typhimurium or S. Enteritidis. Risk of developing bacteremia from NTS infection may be driven by virulence properties of the Salmonella pathogen.

Though NTS is normally associated with self-limiting gastroenteritis in humans, it is a leading cause of bloodstream infection in Africa. The biological mechanisms that contribute to invasiveness in NTS in Africa are unclear. In this paper we address which specific host and pathogen risk factors are associated with blood stream infection from non-typhoidal Salmonella in rural Kenya. We found that multi-drug resistant (MDR) strains of NTS were associated with NTS bacteremia, even after controlling for known host-factors including HIV, age, and NTS serogroup (a taxonomic grouping). Our results suggest that multi-drug resistant NTS is associated with blood stream infection even in the immune-competent host. Salmonella Typhimurium sequence type ST313, an emerging genotype in sub-Saharan Africa, was the most common cause of blood stream infection in children and adults, followed by Salmonella Enteritidis sequence type ST11. The increasing prevalence of commonly circulating non-typhoidal Salmonella poses a major challenge to the control of highly pathogenic NTS serovars. The specific biological and epidemiological mechanisms driving invasiveness from infection with drug-resistant NTS warrant further investigation.

Immunological bases of increased susceptibility to invasive nontyphoidal Salmonella infection in children with malaria and anaemia

Malaria and anaemia are key underlying factors for iNTS disease in African children. Knowledge of clinical and epidemiological risk-factors for iNTS disease has not been paralleled by an in-depth knowledge of the immunobiology of the disease. Herein, we review human and animal studies on mechanisms of increased susceptibility to iNTS in children.

Acute undifferentiated fever in India: a multicentre study of aetiology and diagnostic accuracy

Background

The objectives of this study were to determine the proportion of malaria, bacteraemia, scrub typhus, leptospirosis, chikungunya and dengue among hospitalized patients with acute undifferentiated fever in India, and to describe the performance of standard diagnostic methods.

Methods

During April 2011–November 2012, 1564 patients aged ≥5 years with febrile illness for 2–14 days were consecutively included in an observational study at seven community hospitals in six states in India.

Malaria microscopy, blood culture, Dengue rapid NS1 antigen and IgM Combo test, Leptospira IgM ELISA, Scrub typhus IgM ELISA and Chikungunya IgM ELISA were routinely performed at the hospitals.

Second line testing, Dengue IgM capture ELISA (MAC-ELISA), Scrub typhus immunofluorescence (IFA), Leptospira Microscopic Agglutination Test (MAT), malaria PCR and malaria immunochromatographic rapid diagnostic test (RDT) Parahit Total™ were performed at the coordinating centre. Convalescence samples were not available.

Case definitions were as follows: Leptospirosis: Positive ELISA and positive MAT. Scrub typhus: Positive ELISA and positive IFA. Dengue: Positive RDT and/or positive MAC-ELISA. Chikungunya: Positive ELISA. Bacteraemia: Growth in blood culture excluding those defined as contaminants. Malaria: Positive genus-specific PCR.

Results

Malaria was diagnosed in 17% (268/1564) and among these 54% had P. falciparum. Dengue was diagnosed in 16% (244/1564). Bacteraemia was found in 8% (124/1564), and among these Salmonella typhi or S. paratyphi constituted 35%. Scrub typhus was diagnosed in 10%, leptospirosis in 7% and chikungunya in 6%. Fulfilling more than one case definition was common, most frequent in chikungunya where 26% (25/98) also had positive dengue test.

Conclusions

Malaria and dengue were the most common causes of fever in this study. A high overlap between case definitions probably reflects high prevalence of prior infections, cross reactivity and subclinical infections, rather than high prevalence of coinfections. Low accuracy of routine diagnostic tests should be taken into consideration when approaching the patient with acute undifferentiated fever in India.

The Roles of Mast Cells in Parasitic Protozoan Infections

Protozoan parasites such as Plasmodium spp., Leishmania spp., Trypanosoma spp., and Toxoplasma gondii are major causes of parasitic diseases in both humans and animals. The immune system plays a critical role against protozoa, but their immune mechanism remains poorly understood. This highlights the need to investigate the function of immune cells involved in the process of parasite infections and the responses of host immune system to parasite infections. Mast cells (MCs) are known to be central players in allergy and anaphylaxis, and it has been demonstrated that MCs have crucial roles in host defense against a number of different pathogens, including parasites. To date, there are many studies that have examined the interaction of helminth-derived antigens and MCs. As one of the major effector cells, MCs also play an important role in the immune response against some parasitic protozoa, but their role in protozoan infections is, however, less well characterized. Herein, we review the current knowledge about the roles of MCs and their mediators during infections involving highly pathogenic protozoa including Plasmodium spp., Leishmania spp., Trypanosoma spp., and T. gondii. We offer a general review of the data from patients and experimental animal models infected with the aforementioned protozoa, which correlate MCs and MC-derived mediators with exacerbated inflammation and disease progression as well as protection against the parasitic infections in different circumstances. This review updates our current understanding of the roles of MCs during parasitic protozoan infections, and the participation of MCs in parasitic protozoan infections could be of a potential therapeutic target.

The antinociception of oxytocin on colonic hypersensitivity in rats was mediated by inhibition of mast cell degranulation via Ca(2+)-NOS pathway

This study was conducted to investigate the effects of oxytocin (OT) on visceral hypersensitivity/pain and mast cell degranulation and the underlying mechanisms. We found that oxytocin receptor (OTR) was expressed in colonic mast cells in humans and rats, as well as in human mast cell line-1 (HMC-1), rat basophilic leukemia cell line (RBL-2H3) and mouse mastocytoma cell line (P815). OT decreased 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced visceral hypersensitivity, colonic mast cell degranulation and histamine release after mast cell degranulation in rats. Also, OT attenuated the compound 48/80 (C48/80)-evoked histamine release in P815 cells and inward currents, responsible for the mast cell degranulation, in HMC-1, RBL-2H3 and P815 cells. Moreover, these protective effects of OT against visceral hypersensitivity and mast cell degranulation were eliminated by coadministration of OTR antagonist atosiban or a nonselective inhibitor of nitric oxide synthase (NOS), NG-Methyl-L-arginine acetate salt (L-NMMA). Notably, OT evoked a concentration-dependent increase of intracellular Ca²⁺ in HMC-1, RBL-2H3 and P815 cells, which was responsible for the activation of neuronal NOS (NOS1) and endothelial NOS (NOS3). Our findings strongly suggest that OT might exert the antinociception on colonic hypersensitivity through inhibition of mast cell degranulation via Ca²⁺-NOS pathway.

IgE and mast cells in host defense against parasites and venoms

IgE-dependent mast cell activation is a major effector mechanism underlying the pathology associated with allergic disorders. The most dramatic of these IgE-associated disorders is the fatal anaphylaxis which can occur in some people who have developed IgE antibodies to otherwise innocuous antigens, such as those contained in certain foods and medicines. Why would such a highly "maladaptive" immune response develop in evolution and be retained to the present day? Host defense against parasites has long been considered the only beneficial function that might be conferred by IgE and mast cells. However, recent studies have provided evidence that, in addition to participating in host resistance to certain parasites, mast cells and IgE are critical components of innate (mast cells) and adaptive (mast cells and IgE) immune responses that can enhance host defense against the toxicity of certain arthropod and animal venoms, including enhancing the survival of mice injected with such venoms. Yet, in some people, developing IgE antibodies to insect or snake venoms puts them at risk for having a potentially fatal anaphylactic reaction upon subsequent exposure to such venoms. Delineating the mechanisms underlying beneficial versus detrimental innate and adaptive immune responses associated with mast cell activation and IgE is likely to enhance our ability to identify potential therapeutic targets in such settings, not only for reducing the pathology associated with allergic disorders but perhaps also for enhancing immune protection against pathogens and animal venoms.

Local and Long-Distance Calling: Conversations between the Gut Microbiota and Intra- and Extra-Gastrointestinal Tract Infections

Preservation of health from infectious diseases depends upon both mucosal and systemic immunity via the collaborative effort of innate and adaptive immune responses. The proficiency of host immunity stems from robust defense mechanisms—physical barriers and specialized immune cells—and a failure of these mechanisms leads to pathology. Intriguingly, immunocompetence to pathogens can be shaped by the gut microbiome as recent publications highlight a dynamic interplay between the gut microbiome and host susceptibility to infection. Modulation of host immunity to enteric pathogens has long been studied where gut bacteria shape multiple facts of both innate and adaptive immunity. Conversely, the impact of gut commensals on host immunity to extra-gastrointestinal (GI) tract infections has only recently been recognized. In this context, the gut microbiome can augment host immunity to extra-GI tract bacterial, viral, and parasitic pathogens. This review explores the research that affords insight into the role of the gut microbiome in various infectious diseases, with a particular emphasis on extra-GI tract infections. A better understanding of the link between the gut microbiome and infectious disease will be critical for improving global health in the years ahead.

Supplementation with Abscisic Acid Reduces Malaria Disease Severity and Parasite Transmission

Nearly half of the world's population is at risk for malaria. Increasing drug resistance has intensified the need for novel therapeutics, including treatments with intrinsic transmission-blocking properties. In this study, we demonstrate that the isoprenoid abscisic acid (ABA) modulates signaling in the mammalian host to reduce parasitemia and the formation of transmissible gametocytes and in the mosquito host to reduce parasite infection. Oral ABA supplementation in a mouse model of malaria was well tolerated and led to reduced pathology and enhanced gene expression in the liver and spleen consistent with infection recovery. Oral ABA supplementation also increased mouse plasma ABA to levels that can signal in the mosquito midgut upon blood ingestion. Accordingly, we showed that supplementation of a Plasmodium falciparum-infected blood meal with ABA increased expression of mosquito nitric oxide synthase and reduced infection prevalence in a nitric oxide-dependent manner. Identification of the mechanisms whereby ABA reduces parasite growth in mammals and mosquitoes could shed light on the balance of immunity and metabolism across eukaryotes and provide a strong foundation for clinical translation.

Increased adhesion of Plasmodium falciparum infected erythrocytes to ICAM-1 in children with acute intestinal injury

BACKGROUND

Children with severe malaria are at increased risk of invasive bacterial disease particularly infection with enteric gram-negative organisms. These organisms are likely to originate from the gut, however, how and why they breach the intestinal interface in the context of malaria infection remains unclear. One explanation is that accumulation of infected red blood cells (iRBCs) in the intestinal microvasculature contributes to tissue damage and subsequent microbial translocation which can be addressed through investigation of the impact of cytoadhesion in patients with malaria and intestinal damage.

METHODS

Using a static adhesion assay, cytoadhesion of iRBCs was quantified in 48 children with malaria to recombinant proteins constitutively expressed on endothelial cell surfaces. Cytoadhesive phenotypes between children with and without biochemical evidence of intestinal damage [defined as endotoxemia or elevated plasma intestinal fatty acid binding protein (I-FABP)] was compared.

RESULTS

The majority of parasites demonstrated binding to the endothelial receptors CD36 and to a lesser extent to ICAM-1. Reduced adhesion to CD36 but not adhesion to ICAM-1 or rosetting was associated with malarial anaemia (p = 0.004). Increased adhesion of iRBCs to ICAM-1 in children who had evidence of elevated I-FABP (p = 0.022), a marker of intestinal ischaemia was observed. There was no correlation between the presence of endotoxemia and increased adhesion to any of the recombinant proteins.

CONCLUSION

Increased parasite adhesion to ICAM-1 in children with evidence of intestinal ischaemia lends further evidence to a link between the cytoadherence of iRBCs in gut microvasculature and intestinal damage.

Mast cells and histamine alter intestinal permeability during malaria parasite infection

Co-infections with malaria and non-typhoidal Salmonella serotypes (NTS) can present as life-threatening bacteremia, in contrast to self-resolving NTS diarrhea in healthy individuals. In previous work with our mouse model of malaria/NTS co-infection, we showed increased gut mastocytosis and increased ileal and plasma histamine levels that were temporally associated with increased gut permeability and bacterial translocation. Here, we report that gut mastocytosis and elevated plasma histamine are also associated with malaria in an animal model of falciparum malaria, suggesting a broader host distribution of this biology. In support of mast cell function in this phenotype, malaria/NTS co-infection in mast cell-deficient mice was associated with a reduction in gut permeability and bacteremia. Further, antihistamine treatment reduced bacterial translocation and gut permeability in mice with malaria, suggesting a contribution of mast cell-derived histamine to GI pathology and enhanced risk of bacteremia during malaria/NTS co-infection.

Inflammation-associated alterations to the intestinal microbiota reduce colonization resistance against non-typhoidal Salmonella during concurrent malaria parasite infection

Childhood malaria is a risk factor for disseminated infections with non-typhoidal Salmonella (NTS) in sub-Saharan Africa. While hemolytic anemia and an altered cytokine environment have been implicated in increased susceptibility to NTS, it is not known whether malaria affects resistance to intestinal colonization with NTS. To address this question, we utilized a murine model of co-infection. Infection of mice with Plasmodium yoelii elicited infiltration of inflammatory macrophages and T cells into the intestinal mucosa and increased expression of inflammatory cytokines. These mucosal responses were also observed in germ-free mice, showing that they are independent of the resident microbiota. Remarkably, P. yoelii infection reduced colonization resistance of mice against S. enterica serotype Typhimurium. Further, 16S rRNA sequence analysis of the intestinal microbiota revealed marked changes in the community structure. Shifts in the microbiota increased susceptibility to intestinal colonization by S. Typhimurium, as demonstrated by microbiota reconstitution of germ-free mice. These results show that P. yoelii infection, via alterations to the microbial community in the intestine, decreases resistance to intestinal colonization with NTS. Further they raise the possibility that decreased colonization resistance may synergize with effects of malaria on systemic immunity to increase susceptibility to disseminated NTS infections.

Infection-related hemolysis and susceptibility to Gram-negative bacterial co-infection

Increased susceptibility to co-infection with enteric Gram-negative bacteria, particularly non-typhoidal Salmonella, is reported in malaria and Oroya fever (Bartonella bacilliformis infection), and can lead to increased mortality. Accumulating epidemiological evidence indicates a causal association with risk of bacterial co-infection, rather than just co-incidence of common risk factors. Both malaria and Oroya fever are characterized by hemolysis, and observations in humans and animal models suggest that hemolysis causes the susceptibility to bacterial co-infection. Evidence from animal models implicates hemolysis in the impairment of a variety of host defense mechanisms, including macrophage dysfunction, neutrophil dysfunction, and impairment of adaptive immune responses. One mechanism supported by evidence from animal models and human data, is the induction of heme oxygenase-1 in bone marrow, which impairs the ability of developing neutrophils to mount a competent oxidative burst. As a result, dysfunctional neutrophils become a new niche for replication of intracellular bacteria. Here we critically appraise and summarize the key evidence for mechanisms which may contribute to these very specific combinations of co-infections, and propose interventions to ameliorate this risk.