Placental pathology in Plasmodium berghei-infected rats

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.

Contribution of Murine Models to the Study of Malaria During Pregnancy

Annually, many pregnancies occur in areas of Plasmodium spp. transmission, particularly in underdeveloped countries with widespread poverty. Estimations have suggested that several million women are at risk of developing malaria during pregnancy. In particular cases, systemic infection caused by Plasmodium spp. may extend to the placenta, dysregulating local homeostasis and promoting the onset of placental malaria; these processes are often associated with increased maternal and fetal mortality, intrauterine growth restriction, preterm delivery, and reduced birth weight. The endeavor to understand and characterize the mechanisms underlying disease onset and placental pathology face several ethical and logistical obstacles due to explicit difficulties in assessing human gestation and biological material. Consequently, the advent of murine experimental models for the study of malaria during pregnancy has substantially contributed to our understanding of this complex pathology. Herein, we summarize research conducted during recent decades using murine models of malaria during pregnancy and highlight the most relevant findings, as well as discuss similarities to humans and the translational capacity of achieved results.

Malaria in pregnancy: the relevance of animal models for vaccine development

Malaria during pregnancy due to Plasmodium falciparum or P. vivax is a major public health problem in endemic areas, with P. falciparum causing the greatest burden of disease. Increasing resistance of parasites and mosquitoes to existing tools, such as preventive antimalarial treatments and insecticide-treated bed nets respectively, is eroding the partial protection that they offer to pregnant women. Thus, development of effective vaccines against malaria during pregnancy is an urgent priority. Relevant animal models that recapitulate key features of the pathophysiology and immunology of malaria in pregnant women could be used to accelerate vaccine development. This review summarizes available rodent and nonhuman primate models of malaria in pregnancy, and discusses their suitability for studies of biologics intended to prevent or treat malaria in this vulnerable population.

Effects of artemisinins on reticulocyte count and relationship to possible embryotoxicity in confirmed and unconfirmed malarial patients

Rat studies suggest that artemisinin-induced decreases in reticulocyte count are a marker for embryotoxicity (in one study, r = 0.82; p < 0.05). In clinical studies, therapeutic doses of artemisinins induced decreases in reticulocyte count that were larger in five of six groups of healthy volunteers (mean decreases of 47-75%) than in 12 groups of patients with malaria (mean decreases of 0-34% and incidences of low reticulocyte count of 0.6-18%). Malaria causes hypoferremia and drug concentrates in infected red cells so, among the explanations for the lesser decreases in patients, is that malaria protects against artemisinin-induced decreases in reticulocyte count by reducing the target tissue levels of active drug and/or ferrous iron which activates the drug to toxic free radicals. The disease could also protect against embryotoxicity in which case pregnant women without malaria would be at greater risk of artemisinin-induced embryotoxicity. Malaria protection against artesunate toxicity has been observed in rats. No artemisinin-induced embryotoxicity has been identified in limited numbers of women with confirmed malaria in the first trimester. However, in large parts of tropical Africa, malaria treatment is based on fever rather than confirmation of parasitemia and many pregnant women without malaria are exposed to antimalarials. No clinical studies have been conducted on uninfected women for whom pregnancy was identified and then an artemisinin was administered subsequently. Testing in rats and/or humans is needed to determine if malaria protects against reticulocytopenia and embryotoxicity and whether the parasite is a more or less sensitive target than the embryo and reticulocyte.

Of mice and women: rodent models of placental malaria

Pregnant women are at increased malaria risk. The infections are characterized by placental accumulation of infected erythrocytes (IEs) with adverse consequences for mother and baby. Placental IE sequestration in the intervillous space is mediated by variant surface antigens (VSAs) selectively expressed in placental malaria (PM) and specific for chondroitin sulfate A (CSA). In Plasmodium falciparum, these VSA(PM) appear largely synonymous with the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family variant VAR2CSA. As rodent malaria parasites do not possess PfEMP1 homologs, the usefulness of experimental mouse PM models remains controversial. However, many features of murine and human PM are similar, including involvement of VSAs analogous to PfEMP1. It thus appears that rodent model studies can further the understanding of VSA-dependent malaria pathogenesis and immunity.

Variant-specific immunity to Plasmodium berghei in pregnant mice

We have investigated the immunological basis of pregnancy-related Plasmodium berghei recrudescence in immune mice with substantial preexisting immunity. Specifically, we examined the relevance of this experimental model to the study of pregnancy-associated malaria (PAM) caused by P. falciparum in women with substantial preexisting protective immunity. We used mice with immunity induced prior to pregnancy and employed flow cytometry to assess their levels of immunoglobulin G (IgG) recognizing antigens on the surfaces of infected erythrocytes (IEs) in plasma. After immunization, the mice did not possess IgG specific for antigens on IEs obtained during pregnancy-related recrudescence but they acquired recrudescence-specific IgG over the course of several pregnancies and recrudescences. In contrast, levels of antibodies recognizing IEs from nonpregnant mice did not increase with increasing parity. Furthermore, maternal hemoglobin levels increased and pregnancy-related parasitemia decreased with increasing parity. Finally, parasitemic mice produced smaller litters and pups with lower weights than nonparasitemic mice. Taken together, these observations suggest that levels of antibodies specific for recrudescence-type IEs are related to the protection of pregnant mice from maternal anemia, low birth weight, and decreased litter size. We conclude that the model replicates many of the key parasitological and immunological features of PAM, although the P. berghei genome does not encode proteins homologous to the P. falciparum erythrocyte membrane protein 1 adhesins, which are of key importance in P. falciparum malaria. The study of P. berghei malaria in pregnant, immune mice can be used to gain significant new insights regarding malaria pathogenesis and immunity in general and regarding PAM in particular.

Pregnancy outcome and placenta pathology in Plasmodium berghei ANKA infected mice reproduce the pathogenesis of severe malaria in pregnant women

Pregnancy-associated malaria (PAM) is expressed in a range of clinical complications that include increased disease severity in pregnant women, decreased fetal viability, intra-uterine growth retardation, low birth weight and infant mortality. The physiopathology of malaria in pregnancy is difficult to scrutinize and attempts were made in the past to use animal models for pregnancy malaria studies. Here, we describe a comprehensive mouse experimental model that recapitulates many of the pathological and clinical features typical of human severe malaria in pregnancy. We used P. berghei ANKA-GFP infection during pregnancy to evoke a prominent inflammatory response in the placenta that entails CD11b mononuclear infiltration, up-regulation of MIP-1 alpha chemokine and is associated with marked reduction of placental vascular spaces. Placenta pathology was associated with decreased fetal viability, intra-uterine growth retardation, gross post-natal growth impairment and increased disease severity in pregnant females. Moreover, we provide evidence that CSA and HA, known to mediate P. falciparum adhesion to human placenta, are also involved in mouse placental malaria infection. We propose that reduction of maternal blood flow in the placenta is a key pathogenic factor in murine pregnancy malaria and we hypothesize that exacerbated innate inflammatory responses to Plasmodium infected red blood cells trigger severe placenta pathology. This experimental model provides an opportunity to identify cell and molecular components of severe PAM pathogenesis and to investigate the inflammatory response that leads to the observed fetal and placental blood circulation abnormalities.

Rat spongiotrophoblast-specific protein is predominantly a unique low sulfated chondroitin sulfate proteoglycan

We have previously demonstrated that the human placenta contains a uniquely low sulfated extracellular aggrecan family chondroitin sulfate proteoglycan (CSPG). This CSPG is a major receptor for the adherence of Plasmodium falciparum-infected red blood cells (IRBCs) in placentas, causing pregnancy-specific malaria. However, it is not known whether such low sulfated CSPGs occur in placentas of other animals and, if so, whether IRBCs bind to those CSPGs. In this study, we show that rat placenta contains a uniquely low sulfated extracellular CSPG bearing chondroitin sulfate (CS) chains, which comprise only approximately 2% 4-sulfated and the remainder nonsulfated disaccharides. Surprisingly, the core protein of the rat placental CSPG, unlike that of the human placental CSPG, is a spongiotrophoblast-specific protein (SSP), which is expressed in a pregnancy stage-dependent manner. The majority of rat placental SSP is present in the CSPG form, and only approximately 10% occurs without CS chain substitution. Of the total SSP-CSPG in rat placenta, approximately 57% is modified with a single CS chain, and approximately 43% carries two CS chains. These data together with the previous finding on human placental CSPG suggest that the expression of low sulfated CSPG is a common feature of animal placentas. Our data also show that the unique species-specific difference in the biology of the rat and human placentas is reflected in the occurrence of completely different CSPG core protein types. Furthermore, the rat SSP-CSPG binds P. falciparum IRBCs in a CS chain-dependent manner. Since IRBCs have been reported to accumulate in the placentas of malaria parasite-infected rodents, our results have important implications for exploiting pregnant rats as a model for studying chondroitin 4-sulfate-based therapeutics for human placental malaria.

Quinine distribution in pregnant mice with Plasmodium berghei malaria

Maternal malaria is associated with placental insufficiency that leads to intrauterine growth retardation and reduced birth weight. Malaria may impair the exchange of drugs across the placenta especially the transmission of antimalarial drugs to the foetus. The distribution of quinine and its 3-hydroxymetabolite in blood, tissues and foeto-placental unit was evaluated on day 18 of pregnancy of mice infected or not with Plasmodium berghei. During pregnancy, quinine distribution volume increases gradually with the rise of free quinine concentrations in plasma. Quinine concentrations increase in erythrocytes and most tissues without change in systemic clearance. A maternal-to-foetal gradient of 8:1 limits the exposure of foetus to quinine. During malaria, the systemic clearance of quinine and the 3-hydroxyquinine gradually decrease with the rising parasitaemia. Quinine concentrations increase slightly in most of the tissues. The weight of placentas decreases in a parasitaemia-dependant manner and is strongly related to the low uptake of quinine by placenta. Foetal weights and quinine concentrations in foetus only decrease for the highest parasitaemia. In this experimental model, pregnancy facilitates quinine uptake by erythrocytes and peripheral tissues. Malaria induces a hypotrophy of both placenta and foetus. In placenta, the marked decrease of quinine concentrations may impair the clearance of sequestered parasites.

The immunology and pathogenesis of malaria during pregnancy

Women in endemic areas become highly susceptible to malaria during first and second pregnancies, despite immunity acquired after years of exposure. Recent insights have advanced our understanding of pregnancy malaria caused by Plasmodium falciparum, which is responsible for the bulk of severe disease and death. Accumulation of parasitized erythrocytes in the blood spaces of the placenta is a key feature of maternal infection with P. falciparum. Placental parasites express surface ligands and antigens that differ from those of other P. falciparum variants, facilitating evasion of existing immunity, and mediate adhesion to specific molecules, such as chondroitin sulfate A, in the placenta. The polymorphic and clonally variant P. falciparum erythrocyte membrane protein 1, encoded by var genes, binds to placental receptors in vitro and may be the target of protective antibodies. An intense infiltration of immune cells, including macrophages, into the placental intervillous spaces, and the production of pro-inflammatory cytokines often occur in response to infection, and are associated with low birth weight and maternal anemia. Expression of alpha and beta chemokines may initiate or facilitate this cellular infiltration during placental malaria. Specific immunity against placental-binding parasites may prevent infection or facilitate clearance of parasites prior to the influx of inflammatory cells, thereby avoiding a cascade of events leading to disease and death. Much less is known about pathogenic processes in P. vivax infections, and corresponding immune responses. Emerging knowledge of the pathogenesis and immunology of malaria in pregnancy will increasingly lead to new opportunities for the development of therapeutic and preventive interventions and new tools for diagnosis and monitoring.

Scant parasitemia in BALB/c mice with congenital malaria infection

Balb/c mice were examined to determine whether or not they transmitted rodent malaria, Plasmodium berghei, to their fetuses. On the 15th day of pregnancy, mice were inoculated with approximately 3 x 10(6) P. berghei-infected erythrocytes by peritoneal injection. The blood from 27 adult females and 196 neonates was examined using a sensitive polymerase chain reaction (PCR) method with a detection level of approximately 1 parasite/microl blood. The average parasitemia of females at delivery was 8.1%, ranging from nondetectable to 37.1%. In 12 females, nested PCR established the presence of blood parasite DNA. Malaria parasites were microscopically confirmed in 2 of the 12 neonates. Maternal parasitemia at the time of delivery was not correlated with the incidence of vertical infection (6.1%), which was higher in this study than that found in previous studies. Although the combination of balb/c mice and P. berghei has not been used to examine vertical transmission of malaria, our report showed that this model may be used for this purpose.

Placental pathology in malaria: a histological, immunohistochemical, and quantitative study

To characterize the histological changes in malarial placentas and their relationship with parity and maternal and cord parasitemias, we conducted a histological study on 1,179 placentas from Ifakara, Tanzania, an area with intense and perennial malaria transmission. Immunohistochemical and quantitative studies for CD45, fibrin, and villous area were performed in 60 cases. Four hundred fifteen placentas (35.2%) showed parasites (active infections); in 303 of them, parasites co-existed with pigment covered by fibrin (chronic infections), and in 112 only parasites were detected (acute infections). Four hundred seventy-five cases (40.3%) showed hemozoin deposition without parasites (past infections). Of women with parasitized placentas, 46.3% did not show parasites in the peripheral blood. Basal membrane thickening (P = .002), fibrinoid necrosis (P = .004), and prominence of syncytial knots (P = .031) were associated with active malarial infection. No quantitative differences for perivillous fibrin deposition or villous area were found. The most significant association with active malarial infection was intervillous infiltration by mononuclear inflammatory cells (P < .001). Chronic infections were associated with the most severe changes, particularly intervillous mononuclear inflammation (OR, 28.7; 95% CI = 16.0 to 51.5, P< .001). Past infections showed only minimal differences with noninfected placentas. Primiparas showed chronic infections more frequently than multiparas (52% v 15%, P < .001). They also showed significantly higher placental parasitemias and intervillous inflammatory infiltrate. In conclusion, placental histology is more sensitive than peripheral blood examination in detecting malarial infection during pregnancy. Most malarial infections recover during pregnancy, leaving few residual changes in the placenta. Intervillous inflammation is the most frequent finding associated with malaria and is especially severe in primiparas, suggesting that mechanisms other than immunosuppression are responsible for the high susceptibility in this group.

Massive chronic intervillositis of the placenta associated with malaria infection

Massive chronic intervillositis (MCI) is an infrequently recognized placental lesion thought to be of immunologic origin that has been associated with poor fetal outcome. It is characterized by a prominent inflammatory infiltrate in the intervillous space, composed mainly of monocytes and macrophages that can simulate a maternal malignant disorder involving the placenta. The villi are characteristically spared. We report 74 cases of placental malarial infection with morphologic features of MCI. In all cases, the massive inflammatory infiltrate was limited to the intervillous space, which appeared largely obliterated. Increased fibrin deposition and prominent syncytial knots were frequent associated findings. Inflammatory cells were CD45 and CD68 positive, consistent with a monocyte-macrophage population. Some polymorphonuclear leukocytes and scattered T and B lymphocytes were also present. Villi were not inflamed. Malarial pigment was present in all cases, and parasitized maternal erythrocytes were evident in 73 of 74 patients. The histologic pattern of MCI was observed in 17.6% of placentas with malarial parasites. Malarial MCI affected predominantly primigravida women (77%) and was associated with a reduced birth weight, which in 39 (53%) of the infants was less than 2500 g, and a low gestational age. None of the infants with placentas with MCI died in the early neonatal period. Morphologic changes of MCI are seen in a significant percentage of placentas with malarial infection, especially in primigravida women, and are associated with a low birth weight. Malarial infection should therefore be considered in the differential diagnosis of massive intervillous infiltrates.