Mouse models amenable to genetic dissection of host factors of PM pathogenesis were established by analysing the severity of maternal infection, pregnancy outcome, placental pathology and the expression of inflammatory factors following infection with three P. berghei lines that do not induce cerebral malaria in the C57BL/6 mouse. The results provide evidence that infection of pregnant females with these P. berghei lines induces PM typical features, strongly suggesting that parasite factors determining cerebral malaria are not required to develop placental infection.
Nevertheless, the three parasite strains induced different degrees of placental pathology and impaired pregnancy outcome suggesting that parasite factors could underlie a spectrum of PM manifestations and distinct pathogenesis mechanisms. Thus, NK65 parasites induced the most severe syndrome comprising significantly lower foetal weight and decreased placental vascular area, higher percentage of nonviable foetuses per mother and lower number of live newborns. In addition, NK65 was the only parasite line causing maternal death before delivery (Table
). Despite similar maternal parasitaemia in NK65 and K173-infected pregnant mice, the latter presented milder effects in placental pathology and in foetal weight loss. On the other hand, ANKAΔpm4
infection led to lower stillbirth incidence and increased newborn viability compared to the other strains. These observations support the notion that different P. berghei
lines show distinct patterns of PM. In fact, an heterogeneous and wide range of clinical manifestations is also observed in women that have malaria during pregnancy, including increased levels of parasitaemia
], increased number of abortions, preterm delivery, intrauterine growth retardation, low birth weight, maternal mortality
] and structural placenta alterations such as trophoblast thickening and consequent vascular space reduction
]. Thus, the different P. berghei
lines represent a fine-tuning resource in constructing experimental systems to study different aspects of pregnancy associated malaria pathogenesis.
It is widely accepted that accumulation of IE is a key event in the pathogenesis of severe disease as is the case of respiratory distress, CM and severe PM
]. The experiments here presented confirmed that PM development was associated with parasite accumulation in the placenta. Nevertheless, the parasite burden in the placenta was not a major determinant of PM severity as the distinct pathology patterns observed in mice infected with NK65, K173 and ANKAΔpm4
did not correlate with differences in placenta parasite accumulation. In particular, infection with the ANKAΔpm4
line showed a lower impact on foetal viability despite a similar parasite burden in the placenta. An earlier report shows that, ANKAΔpm4
parasites failed to induce disease in an ECM model but the resistance phenotype was correlated with lower parasite accumulation in the brain compared to wild-type P. berghei
ANKA parasites. This virulence-attenuated effect was also observed in ECM-resistant mouse strains where self-resolving infection was associated to antibody-mediated response
]. Nevertheless, this protective effect was not observed in ANKAΔpm4-
infected pregnant mice although foetal viability was increased and correlated with a strong innate immune response. This raises the possibility that the vigorous local innate response in ANKAΔpm4
infected placentas deterred the progression of placental tissue disorganization at least for a short period warranting an improved pregnancy outcome. Although expression of pro-inflammatory markers was less stimulated in K173- and NK65-infected placentas at G18 it is not ruled out the possibility that gene expression differences are not exclusively parasite line-related but could also be influenced by differences in parasite kinetics as parasite expansion in pregnant ANKAΔpm4
mice was somewhat slower as compared to K173 and NK65. Thus, the observed differences in immune responses might also be influenced by the longer exposure of the maternal immune system to ANKAΔpm4
(G10 to G19) as compared to K173 and NK65 parasite lines (G13 to G18). Nevertheless, the PM protracting effects observed in ANKAΔpm4
infection offer now interesting research perspectives. This experimental model can be used to (1) discriminate between the effects exerted by foetal- and maternal-derived inflammatory factors in PM pathogenesis and (2) to ascertain whether innate immune responses can be used to provide effective foetal protection in PM.