Plasmodium falciparum malaria elicits inflammatory responses that dysregulate placental amino acid transport

Abstract

Placental malaria (PM) can lead to poor neonatal outcomes, including low birthweight due to fetal growth restriction (FGR), especially when associated with local inflammation (intervillositis or IV). The pathogenesis of PM-associated FGR is largely unknown, but in idiopathic FGR, impaired transplacental amino acid transport, especially through the system A group of amino acid transporters, has been implicated. We hypothesized that PM-associated FGR could result from impairment of transplacental amino acid transport triggered by IV. In a cohort of Malawian women and their infants, the expression and activity of system A (measured by Na⁺-dependent ¹⁴C-MeAIB uptake) were reduced in PM, especially when associated with IV, compared to uninfected placentas. In an in vitro model of PM with IV, placental cells exposed to monocyte/infected erythrocytes conditioned medium showed decreased system A activity. Amino acid concentrations analyzed by reversed phase ultra performance liquid chromatography in paired maternal and cord plasmas revealed specific alterations of amino acid transport by PM, especially with IV. Overall, our data suggest that the fetoplacental unit responds to PM by altering its placental amino acid transport to maintain adequate fetal growth. However, IV more profoundly compromises placental amino acid transport function, leading to FGR. Our study offers the first pathogenetic explanation for FGR in PM.

Figure 1. SLC38A1 and SLC38A2 mRNA expression in syncytiotrophoblast and relationship to birthweight.

Expression of mRNA for the amino acid transporters SLC38A1 and SLC38A2 was quantified in the laser-captured syncytiotrophoblast of placental biopsies by real time qPCR and expressed normalized to the housekeeping gene YWHAZ. A. SLC38A1 mRNA expression in placental malaria (PM) with intervillositis (IV; n = 21) was lower than in uninfected placentas (n = 21; p = 0.008). SLC38A1 transcript levels in placental malaria without intervillositis (n = 11) were intermediate and similar to that in uninfected placentas (p = 0.46) and to that in infected placentas with IV (p = 0.23). A similar profile was observed for SLC38A2 mRNA expression comparing expression in infected placentas with IV and uninfected placentas. B. Placentas from infants born with a low birthweight (regardless of infection status; n = 7) had lower SLC38A1 (p = 0.017) but similar SLC38A2 (p = 0.39) transcript levels compared to those born with a normal birthweight (n = 46). Data shown were obtained in an experiment run in triplicate.

Figure 2. Amino acid uptake and relationship to placental malaria and birthweight.

MVM was purified from uninfected placentas (n = 18) and infected placentas without (n = 14) or with (n = 22) intervillositis (IV) and Na⁺-dependent MeAIB uptake was quantified in duplicate. A. Na⁺-dependent MeAIB uptake by MVM vesicles from infected placentas both with or without IV was significantly lower compared to that of uninfected placentas. Na⁺-dependent MeAIB uptake by MVM of infected placentas with or without IV was not significantly different (p = 0.65). Data are represented as median (horizontal line), interquartile range (box) and 5th/95^th centiles (whiskers). B. Correlation between Na⁺-dependent MeAIB uptake by MVM vesicles and birthweight (n = 49; Rho = 0.26; p = 0.07). Uninfected women are represented by ○, placental malaria (PM) without IV by □ and PM with IV by Δ.

Figure 3. IL-1β placental plasma concentration and relationship to birthweight.

A. IL-1β concentration in plasma sampled from blood of infected placentas with intervillositis (IV; n = 16) was higher (p = 0.017) than that of uninfected placentas (n = 31). IL-1β concentration in infected placentas without IV (n = 16) was intermediate and similar to concentrations in infected placentas with IV (p = 0.1) and to concentrations in uninfected placentas (p = 0.39). Data are represented as median (horizontal line), interquartile range (box) and 5th/95^th centiles (whiskers). The ELISA was performed in duplicate. B. IL-1β concentration in plasma sampled from blood of infected placentas with intervillositis showed a negative correlation with birthweight (n = 16; Rho = −0.52; p = 0.04).

Figure 4. In vitro model of amino acid uptake by placental cells.

A. BeWo cells incubated for 16 h with the conditioned medium from an IE/monocyte co-culture showed a marked decrease in Na⁺-dependent MeAIB uptake relative to cells incubated with an uninfected erythrocyte (UE)/monocyte conditioned medium (n = 5 independent donors in triplicate). B. BeWo cells were treated for 16 h with either recombinant IL-1β (5 ng/mL) or conditioned media from UE/monocyte or IE/monocyte co-culture in the presence (grey bars) or absence (open bars) of IL-1β blocking antibody (BA) (1.25 µg/mL) and Na⁺-dependent MeAIB uptake measured. Data are expressed as mean + SD (n = 2 independent experiments in triplicate). Effect of the BA was tested using a one-sided two-sample T-test.