Pregnancy-associated malaria (PAM) is a major cause of low birth weight (LBW) and maternal anemia among primigravidae in areas of highly endemic parasite transmission (
3,
10,
11,
24). PAM is characterized by placental accumulation of
Plasmodium falciparum-infected erythrocytes (IE) that can adhere to proteoglycans such as chondroitin sulfate A (CSA) in the placental intervillous space (
7,
18,
25). Placental infection frequently occurs in the absence of symptoms and so is unsuspected and undetected.
The ability of IE to inhabit the placenta appears to depend on their expression of a distinct subset (VSA
PAM) (
2,
7,
8,
14,
17,
23) of the parasite-encoded, clonally variant surface antigens (VSA) that mediate IE sequestration in nonpregnant individuals (reviewed in reference
12). Levels of VSA
PAM-specific immunoglobulin G (IgG) increase with parity and are associated with protection against placental parasitemia, LBW, and anemia (
6,
23,
24). Together, these findings can explain the sudden reappearance of susceptibility to
P. falciparum malaria during the first pregnancy of hitherto clinically immune women and the pronounced reduction in the incidence of PAM with increasing parity (
7,
24).
RESULTS AND DISCUSSION
Protective immunity to
P. falciparum malaria is acquired following prolonged exposure in areas of endemic parasite transmission. It is accompanied by increases in plasma levels of IgG with specificity for many
P. falciparum antigens including parasite-encoded VSA expressed on the surface of IE (Table
1). Naturally acquired protection appears to be mediated mainly by parasite-specific IgG (
5,
15), and several studies have pointed to VSA as important targets (
4,
13,
16). The evidence in favor of VSA-specific IgG as mediators of acquired protective immunity is strongest and most direct with respect to PAM, which is a major cause of maternal and perinatal suffering in areas of intense parasite transmission (
6,
24).
IPTp of pregnant women, where women are given antimalarials at specified intervals during pregnancy, effectively protects against the adverse consequences of PAM in malaria-exposed primigravidae (
19,
21). Chemoprophylaxis during a first pregnancy can interfere with acquisition of VSA
PAM-specific IgG (
23) and may therefore compromise acquisition of immunological protection from PAM (
6,
24). However, the impact of IPTp on acquisition of PAM immunity is unknown. We therefore compared the plasma levels of VSA-specific IgG among Kenyan primigravidae participating in a randomized, placebo-controlled trial of IPTp (
21).
We found that the levels of IgG with specificity for two parasite lines (Busua and FCR3) not expressing the PAM-type VSA implicated in the pathogenesis of PAM were not significantly different between women receiving active drug and those receiving a placebo (Fig.
1). Specifically, the median plasma levels of IgG with specificity for the VSA expressed by the Busua line were essentially the same in women who received sulfadoxine-pyrimethamine as in women who received a placebo (level in placebo group minus level in active drug group, −0.4 [95% confidence interval, −2.8 to 2.0] MFI unit). Similarly, the median plasma level of IgG with specificity for the VSA expressed by the FCR3 line was only marginally lower among women who received sulfadoxine-pyrimethamine than among women who received a placebo (1.8 [95% confidence interval, −5.0 to 1.4] MFI units). We did not detect any statistically significant relationships between the number of active or placebo drug doses received and VSA-specific IgG levels for either of the parasite lines expressing non-PAM-type VSA (Table
2). These results indicate that IPTp has no significant impact on plasma levels of IgG with specificity for VSA not related to PAM. This interpretation is in line with our earlier finding that chemoprophylaxis does not significantly affect the levels of IgG with specificity for non-PAM-type VSA in Cameroonian primigravidae (
23).
In marked contrast, plasma levels of IgG with specificity for each of three VSA
PAM-expressing isolates (Busua-CSA, FCR3-CSA, EJ24) were markedly and significantly lower in the women who received active drug (median difference for Busua-CSA, 6.4 [95% confidence interval, 1.8 to 11.9] MFI units; median difference for FCR3-CSA, 10.6 [95% confidence interval, 3.7 to 19.0] MFI units; median difference for EJ24, 3.2 [95% confidence interval, 1.0 to 5.3] MFI units) (Fig.
1). In line with this, we have previously shown that chemoprophylaxis can significantly affect levels of VSA
PAM-specific IgG in Cameroonian primigravidae without affecting levels of IgG with other VSA specificities (
23). Levels of IgG with specificity for the VSA expressed by each of the three VSA
PAM-expressing isolates were significantly and inversely related to the number of sulfadoxine-pyrimethamine doses, whereas the number of placebo doses had no effect (Table
2). Titration analysis indicated that the median level of IgG specific for each of the VSA
PAM-expressing isolates was largely unaffected in women who received only one sulfadoxine-pyrimethamine dose but was reduced two- to eightfold in women receiving multiple doses (data not shown). For the Busua-CSA isolate, the median level of VSA-specific IgG in women who had received three sulfadoxine-pyrimethamine doses were not significantly different from levels in unexposed control women (difference, 2.3 [95% confidence interval, −1.4 to 7.0] MFI units;
P [Mann-Whitney rank sum test] = 0.27). We conclude that multidose IPTp adversely affects acquisition of VSA
PAM-specific IgG in primigravidae, leading to significantly lowered plasma levels of VSA
PAM-specific IgG at term. This is an issue of potential clinical concern, as there is strong evidence in favor of a causal relationship between VSA
PAM-specific IgG levels and protection from untoward consequences of PAM (
6,
24). We acknowledge that the timing of IPTp treatments may have contributed to the dose dependency observed here, as women who received three doses received their first dose earlier than two-dose women, who in turn received their first dose earlier than single-dose women. Studies designed specifically to determine the optimal timing and number of IPTp doses are necessary to resolve this issue.
The impact of chemoprophylaxis during a first pregnancy on a second-pregnancy outcome has previously been studied (
9). The authors of that study concluded that restriction of chemoprophylaxis to primigravidae does not appear to put recipients at increased risk in subsequent pregnancies. However, many of the women in their study were recruited and given chemoprophylaxis only in the third trimester, making it probable that some already had PAM and had acquired VSA
PAM-specific immunity at the time of recruitment. Our data on women receiving only one dose of sulfadoxine-pyrimethamine support this possibility. Secondly, endemicity in Gambia is considerably lower than in our study area, making it likely that a proportion of the Gambian women who received a placebo remained uninfected during their first pregnancy and thus did not acquire VSA
PAM-specific immunity. Both factors tend to obscure a protective effect of VSA
PAM-specific IgG against maternal anemia and LBW.
In conclusion, our data suggest that multidose IPTp interferes with acquisition of protective immunity to PAM in P. falciparum-exposed primigravidae. Once given, it may therefore be necessary to repeat IPTp in all subsequent pregnancies. This conclusion is in agreement with the most recent World Health Organization recommendations that women of all parities in malaria-endemic areas should be offered IPTp.