The consensus view is that in utero
sensitization to P. falciparum
antigens is a common phenomenon during PAM. Parasites do not usually cross the placental barrier and such sensitization is most probably caused by the transplacental passage of soluble P. falciparum
]. Accordingly, no parasite-positive smears were detected in CB samples of malaria-infected women in this study population.
Pregnant women declared to receive either SP or CQ as malaria prevention during pregnancy. In contrast to a previous study performed in the same area [29
], no differences were observed for P. falciparum
infection rate according to the type of prophylaxis used by the mothers. However, the number of subject included in this study was low and the project was not designed to examine this matter.
In this study, the foetal BDCA-1+
DC subsets expressed significantly higher levels of MHC class II molecules upon PAM, as indicated by the presence of P. falciparum
MP in placenta, which is in agreement with previous data [23
]. The observation that CD86 expression on foetal DC was unaffected by PAM suggests that P. falciparum
stimulation in utero
induces only partial activation of these cells. Failure to provide DC with a sufficiently strong costimulatory signals can impair the ability to form stable interactions with T-cells, as recently shown in a murine model of malaria [30
]. Partial DC maturation can lead to altered T-cell activation and induction of tolerance [31
], possibly contributing to impaired immune responses that have been observed in the offspring of mothers with PAM [9
The findings presented in this study diverge from those of studies on peripheral blood DC from children with acute malaria, where expression levels of MHC class II on the BDCA-1+
DC are reduced compared to healthy controls [35
]. Also, no increase in foetal BDCA-3+
DC was detected upon maternal malaria infection in this study like others have shown in children with severe malaria [35
]. Circulating APC are continuously exposed to P. falciparum
-infected erythrocytes during malaria episodes in children, which may exert a contact-mediated inhibitory effect on DC functionality, as demonstrated by in vitro
]. Conversely, infected erythrocytes are rarely detected in CB of those born to mothers with PAM [23
]. Thus, foetal APC would rarely if ever encounter parasitized red-blood cells, but would be primarily exposed to and influenced by parasite-derived soluble compounds.
Interestingly, TLR9 stimulation led to increased pro- and anti-inflammatory responses of CBMC of neonates whose mothers had MP accumulating in placentas, and there was a tendency towards increased IFN-γ response upon TLR3 stimulation in the same group. Responses via
other TLR ligands, such as LPS were amplified in CBMC but did not change appreciably as a function of maternal malaria infection. Thus, foetal different TLR responses are independently modulated by in utero
exposure to P. falciparum
, consistent with a recent study [22
In humans only PDC and B cells express TLR9 [39
]. In this study, CpG-A, a TLR9 ligand that specifically stimulates PDC [25
], was employed. In concordance with the findings presented in this study, MP or alternatively plasmodial DNA bound to MP activate the TLR9 pathway in human and murine PDC [27
]. This would suggest a role for MP derived from maternal parasitic infection in inducing foetal BDCA-2+
DC partial maturation and increased sensitization to TLR9 ligands. Nevertheless, only low levels of IL-10 and TNF-α were detected in CBMC cultures upon TLR9 stimulation. This was not unexpected given the low frequency of cells able to specifically respond to such stimulus. The biological significance of a slightly increased release of IL-10 and TNF-α by CBMC upon TLR9 stimulation after in utero
exposure to P. falciparum
Notably, MP was the only indicator of maternal malaria infection that was significantly associated with partial activation of foetal DC and to amplified innate response to TLR9 ligation, while other markers of maternal parasitization at delivery such as the presence of parasites in peripheral and/or placental blood, were unrelated to DC activation in the exposed newborns. It has been recently postulated that accumulation of MP in leukocytes is a good indicator of total parasite burden, including parasite sequestration [43
], and therefore we can consider accumulation of MP in placenta as a marker of high intensity of maternal malaria infection and/or of prolonged parasite exposure. In addition, accumulation of MP in placental leukocytes has been associated with increased monocyte activation and inflammation [44
]. As a hypothesis, accumulation of MP may represent a specific activation stimulus and inflammation at the placental level and this may cause partial and inadequate activation of APC in the foetal compartment.
Additionally, maternal age and parity should be taken into consideration when analysing foetal/neonatal innate immunity. Women of higher parity and increased age delivered babies in whom significantly fewer blood APC were found, but these cells exhibited an enhanced activation status. Maternal age but not parity also influenced the APC cytokine responses upon TLR stimulation, such that CBMC of offspring of younger mothers exhibited an increased ability to respond to TLR3, 4 and 9 ligands. These data are in agreement with published data on African [23
] and Caucasian [45
] women and suggest that maternal age and obstetric history may influence foetal/neonatal immune parameters.
Consequences of increased maternal age and/or multiple parities in terms of neonatal responses to pathogens are poorly understood. Two recent studies indicate that the frequency of malaria episodes is higher among infants of malaria-infected multigravidae as compared to primigravidae [6
]. The intrinsic effect of multiple pregnancies on malaria susceptibility in the offspring may be at least partially explained by our finding of a significantly reduced number of myeloid APC in foetal blood from multigravidae. How maternal age or alternatively parity can affect the number, activation status and cytokine secretion capacity of cord blood APC is presently unknown.
In conclusion, placental parasitization, as indicated by the presence of MP in placental leukocytes, is significantly associated with partial maturation of different DC subsets and to slightly increased responses to a TLR9 ligand in cord blood. As semi-maturation of DC leads to tolerance [46
], such partial foetal APC activation may contribute to the altered T-cell responses often observed in newborns of mothers with PAM [5
These observations advocate a possible mechanism by which PAM may modulate foetal/neonatal innate immunity. Further evaluation of APC activation and downstream T-cell responses is ongoing in a large cohort of newborns and infants from mothers with PAM to assess the impact of altered DC activation on the neonatal cell-mediated immunity.
As it is known that neonatal immune responses are largely dependent on the innate branch of immunity and can be improved through selective TLR stimulation [47
], our results should be considered in the development of effective vaccine strategies for infants living in areas where malaria is endemic.