Maternal infections influence infection susceptibility in childhood

Development of vaccines to prevent malaria in pregnant women: WHO MALVAC meeting report

The major public health consequences of malaria in pregnancy have long been acknowledged. However, further information is still required for development and implementation of a malaria vaccine specifically directed to prevent malaria in pregnant women and improve maternal, fetal and infant outcomes. The WHO Malaria Vaccine Advisory Committee (MALVAC) provides guidance to the WHO on strategic priorities and research needs for development of vaccines to prevent malaria. Here we summarize the discussions and conclusions of a MALVAC scientific forum meeting on considerations in the development of vaccines to prevent malaria in pregnant women. This report includes brief summaries of what is known, and major knowledge gaps in disease burden estimation, pathogenesis and immunity, and the challenges with current preventive strategies for malaria in pregnancy. We conclude with the formulation of a conceptual framework for research and development for vaccines to prevent malaria in pregnant women.

Impact of malaria at the end of pregnancy on infant mortality and morbidity

BACKGROUND

There is some consensus that malaria in pregnancy may negatively affect infant's mortality and malaria morbidity, but there is less evidence concerning the factors involved.

METHODS

A total of 1030 Mozambican pregnant women were enrolled in a randomized, placebo-controlled trial of intermittent preventive treatment with sulfadoxine-pyrimethamine, and their infants were followed up throughout infancy. Overall mortality and malaria morbidity rates were recorded. The association of maternal and fetal risk factors with infant mortality and malaria morbidity was assessed.

RESULTS

There were 58 infant deaths among 997 live-born infants. The risk of dying during infancy was increased among infants born to women with acute placental infection (odds ratio [OR], 5.08 [95% confidence interval (CI), 1.77-14.53)], parasitemia in cord blood (OR, 19.31 [95% CI, 4.44-84.02]), low birth weight (OR, 2.82 [95% CI, 1.27-6.28]) or prematurity (OR, 3.19 [95% CI, 1.14-8.95]). Infants born to women who had clinical malaria during pregnancy (OR, 1.96 [95% CI, 1.13-3.41]) or acute placental infection (OR, 4.63 [95% CI, 2.10-10.24]) had an increased risk of clinical malaria during infancy.

CONCLUSIONS

Malaria infection at the end of pregnancy and maternal clinical malaria negatively impact survival and malaria morbidity in infancy. Effective clinical management and prevention of malaria in pregnancy may improve infant's health and survival.

Congenital malaria: the least known consequence of malaria in pregnancy

Congenital malaria is the least known manifestation of malaria and a very neglected area of research. Most of the existing information is limited to case reports in children born to non-immune women. With the use of molecular techniques, congenital infection is being increasingly detected among infants born to semi-immune women in endemic countries. However, many gaps in knowledge remain. The mechanisms and timing of infection are unclear. Furthermore, there is a lack of information on the impact of congenital malaria infection on the subsequent risk of malaria and general morbidity in the infant. There is also a lack of consensus on the clinical guidelines for its management. More research is needed in order to establish adequate preventive and management recommendations to avoid this consequence of malaria in pregnancy.

Perinatal priming of calves born to Schistosoma mattheei-infected dams

The objective of this study was to elucidate whether calves born to infected dams had been primed against Schistosoma mattheei antigens. Infection-confirmed, pregnant cows were randomly selected for monitoring their offspring. Pre-colostral serum was collected from the neonates for the detection of specific antibodies at birth, as they indicate a transplacental transfer of schistosome-specific antibodies and antigen. At the age of approximately 2 months, peripheral blood mononuclear cells (PBMC) of calves were analysed for specific memory by antigen-specific stimulation in vitro. Twenty-six of the 30 calves demonstrated S. mattheei-specific proliferation. All 12 seropositive-born, as well as 14 of the 18 seronegative-born (before colostrum uptake) calves displayed mattheei-specific proliferation. The results indicate that the calves were primed against S. mattheei and might explain why seropositive-born calves from infected dams are better protected against S. mattheei, and query the impermeability of the damaged ruminant placenta with consequences for antigen transfer.

Immunity to schistosomiasis in animals: an update

The present paper reviews the available literature on the development of immunity to animal Schistosoma infections. The majority of the studies on animal schistosomiasis were performed in cattle and pigs and only Schistosoma mattheei, S. bovis and S. japonicum received particular attention, mainly because of their recognized veterinary significance or zoonotic aspect. Although it is an accepted fact that acquired resistance to Schistosoma is of major importance in the regulation of infection intensity in the field, almost nothing is yet known of either the nature of the antigens or of the immune mechanisms involved. The recent studies on immunity development focus in particular on the occurrence of maternal to foetal transfer of immunological substances related to animal Schistosoma infections and possible effects of these transfers on the immunity development of the foetus/newborn. Since congenital infections for Schistosoma species other than S. japonicum are extremely rare, the most plausible route for foetal contact is the transplacental or postnatal transfer of immunological substances. Prenatal transfers of specific antibodies and antigens via placental lesions and postnatal transfers via the colostrum were observed in cattle and pigs, and subsequent modifications of the immune response of the newborn were observed. Placental lesions induced by Schistosoma eggs could allow other pathogens to cross the placenta.

Malaria during pregnancy: a priority area of malaria research and control

More than 2000 million people live in areas where malaria transmission occurs and are therefore at risk of being infected. It follows that 1000 million people are exposed to the risks of malaria when pregnant. Although the special features of malaria during pregnancy have been recognized for nearly a century(1), it is only recently that it is being considered as a priority for malaria research and control, as discussed here by Clara Menendez.

Parasite-specific antibody and cytokine profiles in newborns from Plasmodium falciparum and Entamoeba histolytica/dispar-infected mothers

Passage of parasites and their antigens across the placenta occurs with metazoan as well as protozoan parasites, and this study addressed to which extent exposure to and infection of mothers with Plasmodium spp. and Entamoeba histolytica/dispar has sensitized their offspring for parasite-specific immune responses. While at delivery none of the mothers presented with an acute malaria attack, 42% were seropositive for P. falciparum. In half of the mothers cysts of E. histolytica/dispar were detected in stool specimen, 51% of them were found seropositive for E. histolytica, and E. histolytica-specific immunoglobulin A (IgA) responses were detected in neonates of seropositive mothers as well. Umbilical cord blood cells (UCBC) from neonates, when activated with the mitogen phytohaemagglutinine (PHA) and bacterial streptolysin O (SL-O), released significantly less interferon (IFN)-gamma, interleukin (IL)-10 and tumor necrosis factor (TNF)-alpha into cell culture supernatants than peripheral blood cells (PBMC) of mothers. In response to Plasmodium- and Entamoeba-specific antigens UCBC and PBMC produced equal amounts of IL-1beta, TNF-alpha, IFN-gamma and IL-5, but PBMC from mothers secreted significantly more IL-10. Parasite-specific production of inflammatory and Th(1)- and Th(2)-type cytokines was similar in newborns of Plasmodium and Entamoeba seropositive and seronegative mothers. In summary, repeated exposure and subclinical infection of mothers with E. histolytica or P. falciparum will suffice to prime in utero their children for inflammatory and both Th(1)- and Th(2)-type cytokine responses, and such broad and mixed cytokine spectrum may be of advantage upon secondary parasite challenge in later life.

Prenatal immune priming with helminth infections: parasite-specific cellular reactivity and Th1 and Th2 cytokine responses in neonates

The present investigation aimed to determine to what extent maternal helminth infection primes parasite-specific cellular responsiveness in neonates. Umbilical cord mononuclear blood cells (UCBC) and peripheral blood mononuclear cells (PBMC) from mothers proliferated in response to mitogenic stimulation with concanavalin A, as well as to bacterial Streptococcus pyogenes-derived (streptolysin O) and helminth-specific antigens of Necator americanus and Onchocerca volvulus. Cellular responses to Echinococcus multilocularis (Em) and Oesophagostomum bifurcum (Oes), helminth parasites not endemic in the study area, were absent (for Em) or very low (for Oes due to antigenic cross-reactivity). Cellular responsiveness to mitogen and antigens was higher in mothers than in their neonates. Several Th1-type (IL-2, IL-12, and IFN-gamma) and Th2-type (IL-5 and IL-10) cytokines were produced by UCBC from neonates and PBMC from mothers. Low levels of IFN-gamma were elicited by UCBC in response to helminth and bacterial antigens, while secretion of IL-2 was pronounced and similarly high in neonates and their mothers. Amounts of IL-5 produced by UCBC in response to bacterial SL-O and mitogenic stimulation (PHA) were low, but equivalent levels of IL-5 were induced by intestinal helminth and filaria-derived antigens in neonates and mothers. A pronounced production of IL-10 and IL-12 by UCBC was observed--spontaneous IL-10 and IL-12 secretion by UCBC was higher in neonates than by PBMC from mothers. Net amounts of IL-10 elicited by helminth antigens were similar, while net IL-12 in response to mitogen, and bacterial and helminth antigens was significantly higher in mothers than their offspring. Our results indicate that human maternal helminth infection does sensitize in utero for parasite-specific cellular responsiveness in offspring, and also activates specific production of several cytokines, and such children do not present a dominant expression of immunity of either Th1 or Th2.

In utero sensitization in Chagas' disease leads to altered lymphocyte phenotypic patterns in the newborn cord blood mononuclear cells

Neonates are sensitized in utero to maternal circulating antigens and idiotypes that eventually cross the placental barrier. We believe that children born of mothers under long lasting antigenic stimulation, as in a chronic infection, would be affected by these maternal influences and show differences in the phenotypic repertoire of lymphocytes. To test this hypothesis, we evaluated flow cytometry studies in cord blood mononuclear cells (CBMC) from children born of chagasic mothers without congenital disease, with special attention to T and B cells and expression of activation markers. We have also evaluated the peripheral blood mononuclear cells (PBMC) of these children 6 months after delivery. We show that CBMC of children born of infected mothers have high proliferative responses to antigenic stimulation, significantly lower mean percentages of CD3+ T cells, CD4+ T cells and diminished expression of the costimulatory molecule CD28 in the CD8+ T cell subset. Interestingly, this subpopulation has an increased expression of the MHC class II gene product as evidenced by the expression of HLADR. It is noteworthy that the patterns observed in CBMC T lymphocyte populations of these children closely resemble earlier findings on lymphocytic profiles of chronic chagasic adult patients and those of their mothers. We also show that, 6 months after delivery, some alterations observed at birth are reversed to levels observed in noninfected individuals.

Inflammatory reactions in placental blood of Plasmodium falciparum-infected women and high concentrations of soluble E-selectin and a circulating P. falciparum protein in the cord sera

To better understand reasons for increased susceptibility to malaria in pregnancy; and the interrelationships between maternal malaria, local immune reactions and the development of the fetus, concentrations of soluble interleukin-10 (IL-10), cytokine receptors, adhesion molecules, a Plasmodium falciparum protein, glutamate-rich protein (GLURP) and antibodies to P. falciparum rhoptry-associated protein-1 were measured among 105 Gambian women and their neonates. Peripheral blood concentrations of IL-10, soluble cytokine receptors and soluble adhesion molecules were found to be different from those concentrations measured in the placenta. Markers of inflammatory reactions: IL-10, sIL-2R, sIL-4R, and soluble tumour necrosis factor receptor I (sTNF-RI) were found in high concentrations in the placenta, indicating that inflammatory reactions take place in the placenta which has been regarded as an immunoprivileged site. Concentrations of soluble vascular cell adhesion molecule-1 (sVCAM-1) and soluble intracellular adhesion molecule-1 (sICAM-1), potential adhesion receptors for malaria parasites, were associated with an active P. falciparum infection in the placenta although the associations did not reach significance. P. falciparum exoantigen, GLURP, was detected in cord blood indicating transplacental passage of malarial antigens. Concentrations of E-selectin were higher in cord blood samples compared with peripheral blood samples. This appeared to be associated with development of cord endothelial cells and not with P. falciparum infection.

Relationships between maternal malaria and malarial immune responses in mothers and neonates

Immune responses of 97 Gambian women and their neonates were studied. New methods distinguished between active and previous placental malaria, were used to examine relationships between maternal malaria and neonatal immune responses. Many placentas (61%) had active or previous malarial infection. Maternal and cord malarial IgG levels correlated (P < 0.001). Malarial IgG was raised in cord blood in active placental malaria; IgM was not detected. Mean lymphoproliferation and the proportion of responders to soluble P. falciparum antigens (F32) and conserved regions of p190 expressed on trophozoites and schizonts (190L and 190N) were higher in neonates than mothers. There was no clear relationship between maternal malaria and neonatal mean lymphoproliferation to malarial antigens, although fewer neonates responded when mothers were actively infected. Matched maternal and neonatal lymphoproliferation responses did not correlate. However, first born neonatal lymphoproliferation to PPD and malarial antigens appeared lower than other neonates, in agreement with lower lymphoproliferation in primigravidae compared with multigravidae. Also in common with mothers, autologous plasma suppressed neonatal lymphoproliferation to PPD and malarial antigens, suggesting common immunoregulation. Higher cortisol or other circulating factors in first pregnancies may be implicated. The relevance of cell-mediated malarial immune responses detected at birth remains to be established.