The immune response to malaria in utero

Mature beyond their years: young children who escape detection of parasitemia despite living in settings of intense malaria transmission

Despite having the highest risk of progressing to severe disease due to lack of acquired immunity, the youngest children living in areas of highly intense malaria transmission have long been observed to be infected at lower rates than older children. Whether this observation is due to reduced exposure to infectious mosquito bites from behavioral and biological factors, maternally transferred immunity, genetic factors, or enhanced innate immunity in the young child has intrigued malaria researchers for over half a century. Recent evidence suggests that maternally transferred immunity may be limited to early infancy and that the young child's own immune system may contribute to control of malarial symptoms early in life and prior to the development of more effective adaptive immunity. Prospective studies of active and passive detection of Plasmodium falciparum blood-stage infections have identified young children (<5 years old) who remain uninfected through a defined surveillance period despite living in settings of highly intense malaria transmission. Yet, little is known about the potential immunological basis for this 'aparasitemic' phenotype. In this review, we summarize the observational evidence for this phenotype in field studies and examine potential reasons why these children escape detection of parasitemia, covering factors that are either extrinsic or intrinsic to their developing immune system. We discuss the challenges of distinguishing malaria protection from lack of malaria exposure in field studies. We also identify gaps in our knowledge regarding cellular immunity in the youngest age group and propose directions that researchers may take to address these gaps.

The impact of Plasmodium-driven immunoregulatory networks on immunity to malaria

Malaria, caused by infection with Plasmodium parasites, drives multiple regulatory responses across the immune landscape. These regulatory responses help to protect against inflammatory disease but may in some situations hamper the acquisition of adaptive immune responses that clear parasites. In addition, the regulatory responses that occur during Plasmodium infection may negatively affect malaria vaccine efficacy in the most at-risk populations. Here, we discuss the specific cellular mechanisms of immunoregulatory networks that develop during malaria, with a focus on knowledge gained from human studies and studies that involve the main malaria parasite to affect humans, Plasmodium falciparum. Leveraging this knowledge may lead to the development of new therapeutic approaches to increase protective immunity to malaria during infection or after vaccination.

Plasmodium falciparum alters the trophoblastic barrier and stroma villi organization of human placental villi explants

BACKGROUND

The sequestration of Plasmodium falciparum infected erythrocytes in the placenta, and the resulting inflammatory response affects maternal and child health. Despite existing information, little is known about the direct impact of P. falciparum on the placental barrier formed by trophoblast and villous stroma. This study aimed to assess placental tissue damage caused by P. falciparum in human placental explants (HPEs).

METHODS

HPEs from chorionic villi obtained of human term placentas (n = 9) from normal pregnancies were exposed to P. falciparum-infected erythrocytes (IE) for 24 h. HPEs were embedded in paraffin blocks and used to study tissue damage through histopathological and histochemical analysis and apoptosis using TUNEL staining. Culture supernatants were collected to measure cytokine and angiogenic factors and to determine LDH activity as a marker of cytotoxicity. A subset of archived human term placenta paraffin-embedded blocks from pregnant women with malaria were used to confirm ex vivo findings.

RESULTS

Plasmodium falciparum-IE significantly damages the trophoblast layer and the villous stroma of the chorionic villi. The increased LDH activity and pathological findings such as syncytial knots, fibrin deposits, infarction, trophoblast detachment, and collagen disorganization supported these findings. The specific damage to the trophoblast and the thickening of the subjacent basal lamina were more pronounced in the ex vivo infection. In contrast, apoptosis was higher in the in vivo infection. This disparity could be attributed to the duration of exposure to the infection, which significantly varied between individuals naturally exposed over time and the 24-h exposure in the ex vivo HPE model.

CONCLUSION

Exposure to P. falciparum-IE induces a detachment of the syncytiotrophoblast, disorganization of the stroma villi, and an increase in apoptosis, alterations that may be associated with adverse results such as intrauterine growth restriction and low birth weight.

Associations between prenatal malaria exposure, maternal antibodies at birth, and malaria susceptibility during the first year of life in Burkina Faso

In this study, we investigated how different categories of prenatal malaria exposure (PME) influence levels of maternal antibodies in cord blood samples and the subsequent risk of malaria in early childhood in a birth cohort study (N = 661) nested within the COSMIC clinical trial (NCT01941264) in Burkina Faso. Plasmodium falciparum infections during pregnancy and infants' clinical malaria episodes detected during the first year of life were recorded. The levels of maternal IgG and IgG1-4 to 15 P. falciparum antigens were measured in cord blood by quantitative suspension array technology. Results showed a significant variation in the magnitude of maternal antibody levels in cord blood, depending on the PME category, with past placental malaria (PM) more frequently associated with significant increases of IgG and/or subclass levels across three groups of antigens defined as pre-erythrocytic, erythrocytic, and markers of PM, as compared to those from the cord of non-exposed control infants. High levels of antibodies to certain erythrocytic antigens (i.e., IgG to EBA140 and EBA175, IgG1 to EBA175 and MSP142, and IgG3 to EBA140 and MSP5) were independent predictors of protection from clinical malaria during the first year of life. By contrast, high levels of IgG, IgG1, and IgG2 to the VAR2CSA DBL1-2 and IgG4 to DBL3-4 were significantly associated with an increased risk of clinical malaria. These findings indicate that PME categories have different effects on the levels of maternal-derived antibodies to malaria antigens in children at birth, and this might drive heterogeneity to clinical malaria susceptibility in early childhood.

SHIP1 modulates antimalarial immunity by bridging the crosstalk between type I IFN signaling and autophagy

Stringent control of the type I interferon (IFN-I) signaling is critical for host immune defense against infectious diseases, yet the molecular mechanisms that regulate this pathway remain elusive. Here, we show that Src homology 2 containing inositol phosphatase 1 (SHIP1) suppresses IFN-I signaling by promoting IRF3 degradation during malaria infection. Genetic ablation of Ship1 in mice leads to high levels of IFN-I and confers resistance to Plasmodium yoelii nigeriensis (P.y.) N67 infection. Mechanistically, SHIP1 promotes the selective autophagic degradation of IRF3 by enhancing K63-linked ubiquitination of IRF3 at lysine 313, which serves as a recognition signal for NDP52-mediated selective autophagic degradation. In addition, SHIP1 is downregulated by IFN-I-induced miR-155-5p upon P.y. N67 infection and severs as a feedback loop of the signaling crosstalk. This study reveals a regulatory mechanism between IFN-I signaling and autophagy, and verifies SHIP1 can be a potential target for therapeutic intervention against malaria and other infectious diseases. IMPORTANCE Malaria remains a serious disease affecting millions of people worldwide. Malaria parasite infection triggers tightly controlled type I interferon (IFN-I) signaling that plays a critical role in host innate immunity; however, the molecular mechanisms underlying the immune responses are still elusive. Here, we discover a host gene [Src homology 2-containing inositol phosphatase 1 (SHIP1)] that can regulate IFN-I signaling by modulating NDP52-mediated selective autophagic degradation of IRF3 and significantly affect parasitemia and resistance of Plasmodium-infected mice. This study identifies SHIP1 as a potential target for immunotherapies in malaria and highlights the crosstalk between IFN-I signaling and autophagy in preventing related infectious diseases. SHIP1 functions as a negative regulator during malaria infection by targeting IRF3 for autophagic degradation.

Mild/Asymptomatic Maternal SARS-CoV-2 Infection Leads to Immune Paralysis in Fetal Circulation and Immune Dysregulation in Fetal-Placental Tissues

Few studies have addressed the impact of maternal mild/asymptomatic SARS-CoV-2 infection on the developing neonatal immune system. In this study, we analyzed umbilical cord blood and placental chorionic villi from newborns of unvaccinated mothers with mild/asymptomatic SARSCoV-2 infection during pregnancy using flow cytometry, single-cell transcriptomics, and functional assays. Despite the lack of vertical transmission, levels of inflammatory mediators were altered in cord blood. Maternal infection was also associated with increased memory T, B cells, and non-classical monocytes as well as increased activation. However, ex vivo responses to stimulation were attenuated. Finally, within the placental villi, we report an expansion of fetal Hofbauer cells and infiltrating maternal macrophages and rewiring towards a heightened inflammatory state. In contrast to cord blood monocytes, placental myeloid cells were primed for heightened antiviral responses. Taken together, this study highlights dysregulated fetal immune cell responses in response to mild maternal SARS-CoV-2 infection during pregnancy.

Effects of anti-malarial prophylaxes on maternal transfer of Immunoglobulin-G (IgG) and association to immunity against Plasmodium falciparum infections among children in a Ugandan birth cohort

BACKGROUND

The in-utero transfer of malaria specific IgG to the fetus in Plasmodium falciparum infected pregnant women potentially plays a role in provision of immune protection against malaria in the first birth year. However, the effect of Intermittent Prophylactic Treatment in Pregnancy (IPTp) and placental malaria on the extent of in-utero antibody transfer in malaria endemic regions like Uganda remain unknown. The aim of this study was thus to establish the effect of IPTp on in-utero transfer of malaria specific IgG to the fetus and the associated immune protection against malaria in the first birth year of children born to mothers who had P. falciparum infection during pregnancy in Uganda.

METHODS

We screened a total of 637 cord blood samples from a double blinded randomized clinical trial on Sulfadoxine-Pyrimethamine (SP) and Dihydroartemisinin-Piperaquine (DP) IPTp in a Ugandan birth cohort; study conducted from Busia, Eastern Uganda. Luminex assay was used to measure the cord levels of IgG sub-types (IgG1, IgG2, IgG3 and IgG4) against 15 different P. falciparum specific antigens, with tetanus toxoid (t.t) as a control antigen. Man-Whitney U test (non-parametric) in STATA (ver15) was used in statistical analysis of the samples. In addition, Multivariate cox regression analysis was used to determine the effect of maternal transfer of IgG on the incidence of malaria in the first birth year of children under study.

RESULTS

Mothers on SP expressed higher levels of cord IgG4 against erythrocyte binding antigens (EBA140, EBA175 and EBA181) (p<0.05). Placental malaria did not affect cord levels of IgG sub-types against selected P. falciparum specific antigens (p>0.05). Children who expressed higher levels (75th percentile) of total IgG against the six key P. falciparum antigens (Pf SEA, Rh4.2, AMA1, GLURP, Etramp5Ag1 and EBA 175) had higher risk of malaria in the first birth year; AHRs: 1.092, 95% CI: 1.02-1.17 (Rh4.2); 1.32, 95% CI: 1.00-1.74 (PfSEA); 1.21, 95%CI: 0.97-1.52 (Etramp5Ag1); 1.25, 95%CI: 0.98-1.60 (AMA1); 1.83, 95%CI: 1.15-2.93 (GLURP) (GLURP), and 1.35,; 95%CI: 1.03-1.78 (EBA175). Children born to mothers categorized as poorest had the highest risk of malaria infections in the first birth year (AHR: 1.79, 95% CI: 1.31-2.4). Children born to mothers who had malaria infections during gestation had higher risk of getting malaria in the first birth year (AHR 1.30; 95%CI: 0.97-1.7).

CONCLUSION

Malaria prophylaxis in pregnant mothers using either DP or SP does not affect expression of antibodies against P. falciparum specific antigens in the cord blood. Poverty and malaria infections during pregnancy are key risk factors of malaria infections in the first birth year of growth of children. Antibodies against P. falciparum specific antigens does not protect against parasitemia and malaria infections in the first birth year of children born in malaria endemic areas.

Effector Vγ9Vδ2 T cell response to congenital Toxoplasma gondii infection

A major γδ T cell population in human adult blood are the Vγ9Vδ2 T cells that are activated and expanded in a TCR-dependent manner by microbe-derived and endogenously derived phosphorylated prenyl metabolites (phosphoantigens). Vγ9Vδ2 T cells are also abundant in human fetal peripheral blood, but compared with their adult counterparts they have a distinct developmental origin, are hyporesponsive toward in vitro phosphoantigen exposure, and do not possess a cytotoxic effector phenotype. In order to obtain insight into the role of Vγ9Vδ2 T cells in the human fetus, we investigated their response to in utero infection with the phosphoantigen-producing parasite Toxoplasma gondii (T. gondii). Vγ9Vδ2 T cells expanded strongly when faced with congenital T. gondii infection, which was associated with differentiation toward potent cytotoxic effector cells. The Vγ9Vδ2 T cell expansion in utero resulted in a fetal footprint with public germline-encoded clonotypes in the Vγ9Vδ2 TCR repertoire 2 months after birth. Overall, our data indicate that the human fetus, from early gestation onward, possesses public Vγ9Vδ2 T cells that acquire effector functions following parasite infections.

Effect of Plasmodium berghei infection on fetuses in pregnant BALB/c mice at two periods of pregnancy

Malaria is a disease caused by a protozoan of the genus Plasmodium in humans and vertebrates. It has a high morbidity and mortality rate, especially in pregnant women living in countries with high transmission rates. Murine models have been an excellent tool to evaluate the effects of malarial infection in the mother-fetus relationship. For this reason, we evaluated the effect of malarial infection on fetal development at the beginning and middle of the gestational period in BALB/c mice infected with Plasmodium berghei ANKA. Our results show that malarial infection at the beginning of pregnancy markedly affects the development of the fetus in size, weight, and development of its limbs so that the control of the pregnant mother is relevant at the beginning of gestation

Diagnosis & management of imported malaria in pregnant women in non-endemic countries

Malaria in pregnancy is an important cause of maternal and foetal morbidity and is a potentially life-threatening infection. With ever-growing global exchanges, imported malaria in pregnancy is becoming an issue of concern in non-endemic countries where women, because of low immunity, have higher risk of severe diseases and death. Malaria in pregnancy is a dangerous condition which can be associated with important consequences for both mother and child such as stillbirth, low birth weight, maternal anaemia. In non-endemic-countries it is more frequent in its severe form which can lead to maternal death if not treated adequately. Specific anti-malarial interventions such as the use of repellents and insecticide treated bed nets in addition to chemoprophylaxis should be used by pregnant women if they are travelling to endemic areas. In cases of confirmed infection, specific treatment regimens vary according to gestational age and the presence of complications. Malaria should be considered a global health problem, increasingly involving western countries. Clinicians all over the world need to be prepared for this emerging disease both in terms of prevention and therapy.

Risk of malaria in young children after periconceptional iron supplementation

This study in Burkina Faso investigated whether offspring of young mothers who had received weekly periconceptional iron supplementation in a randomised controlled trial were at increased risk of malaria. A child safety survey was undertaken in the peak month of malaria transmission towards the end of the trial to assess child iron biomarkers, nutritional status, anaemia and malaria outcomes. Antenatal iron biomarkers, preterm birth, fetal growth restriction and placental pathology for malaria and chorioamnionitis were assessed. Data were available for 180 babies surviving to the time of the survey when their median age was 9 months. Prevalence of maternal iron deficiency in the last trimester based on low body iron stores was 16%. Prevalence of active placental malaria infection was 24.8%, past infection 59% and chorioamnionitis 55.6%. Babies of iron supplemented women had lower median gestational age. Four out of five children ≥ 6 months were iron deficient, and 98% were anaemic. At 4 months malaria prevalence was 45%. Child iron biomarkers, anaemia and malaria outcomes did not differ by trial arm. Factors associated with childhood parasitaemia were third trimester C-reactive protein level (OR 2.1; 95% CI 1.1-3.9), active placental malaria (OR 5.8; 1.0-32.5, P = 0.042) and child body iron stores (OR 1.13; 1.04-1.23, P = 0.002). Chorioamnionitis was associated with reduced risk of child parasitaemia (OR 0.4; 0.1-1.0, P = 0.038). Periconceptional iron supplementation of young women did not alter body iron stores of their children. Higher child body iron stores and placental malaria increased risk of childhood parasitaemia.

Innate and adaptive γδ T cells: How, when, and why

γδ T cells comprise the third cell lineage of lymphocytes that use, like αβ T cells and B cells, V(D)J gene rearrangement with the potential to generate a highly diverse T cell receptor (TCR) repertoire. There is no obvious conservation of γδ T cell subsets (based on TCR repertoire and/or function) between mice and human, leading to the notion that human and mouse γδ T cells are highly different. In this review, we focus on human γδ T cells, building on recent studies using high-throughput sequencing to analyze the TCR repertoire in various settings. We make then the comparison with mouse γδ T cell subsets highlighting the similarities and differences and describe the remarkable changes during lifespan of innate and adaptive γδ T cells. Finally, we propose mechanisms contributing to the generation of innate versus adaptive γδ T cells. We conclude that key elements related to the generation of the γδ TCR repertoire and γδ T cell activation/development are conserved between human and mice, highlighting the similarities between these two species.

Deleterious effects of malaria in pregnancy on the developing fetus: a review on prevention and treatment with antimalarial drugs

All malaria infections are harmful to both the pregnant mother and the developing fetus. One in ten maternal deaths in malaria endemic countries are estimated to result from Plasmodium falciparum infection. Malaria is associated with a 3-4 times increased risk of miscarriage and a substantially increased risk of stillbirth. Current treatment and prevention strategies reduce, but do not eliminate, malaria's damaging effects on pregnancy outcomes. Reviewing evidence generated from meta-analyses, systematic reviews, and observational data, the first paper in this Series aims to summarise the adverse effects of malaria in pregnancy on the fetus and how the current drug treatment and prevention strategies can alleviate these effects. Although evidence supports the safety and treatment efficacy of artemisinin-based combination therapies in the first trimester, these therapies have not been recommended by WHO for the treatment of malaria at this stage of pregnancy. Intermittent preventive treatment of malaria in pregnancy with sulfadoxine-pyrimethamine is contraindicated in the first trimester and provides imperfect chemoprevention because of inadequate dosing, poor (few and late) antenatal clinic attendance, increasing antimalarial drug resistance, and decreasing naturally acquired maternal immunity due to the decreased incidence of malaria. Alternative strategies to prevent malaria in pregnancy are needed. The prevention of all malaria infections by providing sustained exposure to effective concentrations of antimalarial drugs is key to reducing the adverse effects of malaria in pregnancy.

Placental Malaria

Purpose of Review

Placental malaria is the primary mechanism through which malaria in pregnancy causes adverse perinatal outcomes. This review summarizes recent work on the significance, pathogenesis, diagnosis, and prevention of placental malaria.

Recent Findings

Placental malaria, characterized by the accumulation of Plasmodium-infected red blood cells in the placental intervillous space, leads to adverse perinatal outcomes such as stillbirth, low birth weight, preterm birth, and small-for-gestational-age neonates. Placental inflammatory responses may be primary drivers of these complications. Associated factors contributing to adverse outcomes include maternal gravidity, timing of perinatal infection, and parasite burden.

Summary

Placental malaria is an important cause of adverse birth outcomes in endemic regions. The main strategy to combat this is intermittent preventative treatment in pregnancy; however, increasing drug resistance threatens the efficacy of this approach. There are studies dissecting the inflammatory response to placental malaria, alternative preventative treatments, and in developing a vaccine for placental malaria.

Using two phases of the CD4 T cell response to blood-stage murine malaria to understand regulation of systemic immunity and placental pathology in Plasmodium falciparum infection

Plasmodium falciparum infection and malaria remain a risk for millions of children and pregnant women. Here, we seek to integrate knowledge of mouse and human T helper cell (Th) responses to blood-stage Plasmodium infection to understand their contribution to protection and pathology. Although there is no complete Th subset differentiation, the adaptive response occurs in two phases in non-lethal rodent Plasmodium infection, coordinated by Th cells. In short, cellular immune responses limit the peak of parasitemia during the first phase; in the second phase, humoral immunity from T cell-dependent germinal centers is critical for complete clearance of rapidly changing parasite. A strong IFN-γ response kills parasite, but an excess of TNF compared with regulatory cytokines (IL-10, TGF-β) can cause immunopathology. This common pathway for pathology is associated with anemia, cerebral malaria, and placental malaria. These two phases can be used to both understand how the host responds to rapidly growing parasite and how it attempts to control immunopathology and variation. This dual nature of T cell immunity to Plasmodium is discussed, with particular reference to the protective nature of the continuous generation of effector T cells, and the unique contribution of effector memory T cells.

Innate immunity to malaria-The role of monocytes

Monocytes are innate immune cells essential for host protection against malaria. Upon activation, monocytes function to help reduce parasite burden through phagocytosis, cytokine production, and antigen presentation. However, monocytes have also been implicated in the pathogenesis of severe disease through production of damaging inflammatory cytokines, resulting in systemic inflammation and vascular dysfunction. Understanding the molecular pathways influencing the balance between protection and pathology is critical. In this review, we discuss recent data regarding the role of monocytes in human malaria, including studies of innate sensing of the parasite, immunometabolism, and innate immune training. Knowledge gained from these studies may guide rational development of novel antimalarial therapies and inform vaccine development.