In a prospective cohort study of newborns residing in a malaria holoendemic area of Kenya, Christopher King and colleagues find a subset of children born to malaria-infected women who acquire a tolerant phenotype, which persists into childhood and is associated with increased susceptibility to malarial infection and anemia.
Malaria in pregnancy can expose the fetus to malaria-infected erythrocytes or their soluble products, thereby stimulating T and B cell immune responses to malaria blood stage antigens. We hypothesized that fetal immune priming, or malaria exposure in the absence of priming (putative tolerance), affects the child's susceptibility to subsequent malaria infections.
Methods and Findings
We conducted a prospective birth cohort study of 586 newborns residing in a malaria-holoendemic area of Kenya who were examined biannually to age 3 years for malaria infection, and whose malaria-specific cellular and humoral immune responses were assessed. Newborns were classified as (i) sensitized (and thus exposed), as demonstrated by IFNγ, IL-2, IL-13, and/or IL-5 production by cord blood mononuclear cells (CBMCs) to malaria blood stage antigens, indicative of in utero priming (n = 246), (ii) exposed not sensitized (mother Plasmodium falciparum [Pf]+ and no CBMC production of IFNγ, IL-2, IL-13, and/or IL-5, n = 120), or (iii) not exposed (mother Pf−, no CBMC reactivity, n = 220). Exposed not sensitized children had evidence for prenatal immune experience demonstrated by increased IL-10 production and partial reversal of malaria antigen-specific hyporesponsiveness with IL-2+IL-15, indicative of immune tolerance. Relative risk data showed that the putatively tolerant children had a 1.61 (95% confidence interval [CI] 1.10–2.43; p = 0.024) and 1.34 (95% CI 0.95–1.87; p = 0.097) greater risk for malaria infection based on light microscopy (LM) or PCR diagnosis, respectively, compared to the not-exposed group, and a 1.41 (95%CI 0.97–2.07, p = 0.074) and 1.39 (95%CI 0.99–2.07, p = 0.053) greater risk of infection based on LM or PCR diagnosis, respectively, compared to the sensitized group. Putatively tolerant children had an average of 0.5 g/dl lower hemoglobin levels (p = 0.01) compared to the other two groups. Exposed not sensitized children also had 2- to 3-fold lower frequency of malaria antigen-driven IFNγ and/or IL-2 production (p<0.001) and higher IL-10 release (p<0.001) at 6-month follow-ups, when compared to sensitized and not-exposed children. Malaria blood stage–specific IgG antibody levels were similar among the three groups.
These results show that a subset of children exposed to malaria in utero acquire a tolerant phenotype to blood-stage antigens that persists into childhood and is associated with an increased susceptibility to malaria infection and anemia. This finding could have important implications for malaria vaccination of children residing in endemic areas.
Please see later in the article for Editors' Summary
Each year, Plasmodium falciparum, a mosquito-borne parasite, causes about 500 million cases of malaria and about one million people die as a result. Most of these deaths occur in young children in sub-Saharan Africa. Indeed, malaria accounts for a fifth of all childhood deaths in Africa, which makes it one of the most important childhood infectious diseases in this region. Very young children—those up to 6 months old—are relatively resistant to high-density parasitaemia and to clinical malaria, but children between 6 and 36 months old have an increased susceptibility to parasitaemia and to clinical malaria. Parasitaemia is the presence of P. falciparum parasites in the blood; a high density of blood-stage parasites causes the symptoms of clinical malaria (including high fever) and life-threatening organ damage and anemia (a lack of red blood cells).
Why Was This Study Done?
The age-dependent pattern of susceptibility to malaria suggests that young babies are protected by antibodies provided by their mothers, but that by 6 months old, when these antibodies have largely disappeared, babies have not yet fully developed their own anti-malaria immunity. However, little is known about the acquisition of anti-malaria immunity in infants, a process that needs to be understood in order to design effective vaccines for this age group. In particular, it is unclear how maternal malaria infection affects the acquisition of anti-malaria immunity. Malaria in pregnancy may expose the unborn child to malaria-infected red blood cells and to soluble malaria antigens (molecules that the immune system recognizes as foreign). This exposure could increase or decrease the child's immune responses to blood-stage malaria antigens and thus affect his/her ability to fight off malaria. In this study, the researchers investigated how prenatal malaria exposure affects anti-malaria immunity in young children and their susceptibility to subsequent malaria infections.
What Did the Researchers Do and Find?
The researchers determined which of 586 newborn babies enrolled into their study in an area of Kenya where malaria is very common had been exposed to P. falciparum before birth by looking for parasites in their mother's blood at delivery. They looked for malaria-specific immune responses in T cells (a type of immune system cell) in the newborn babies' cord blood by measuring the production of cytokines (molecules that either activate or inhibit the immune system) by these cells after exposure to malaria antigens. Finally, they examined the infants twice yearly for 3 years for malaria infection, malaria-specific immune responses, and anemia. The researchers classified the babies into three groups; cord blood cells of “sensitized” babies made activating cytokines in response to malaria antigens; cord blood cells of “exposed, not-sensitized” babies did not make activating cytokines but made an inhibitory cytokine (IL-10); and “not-exposed” babies were born to mothers with no P. falciparum infection at delivery. In their first 3 years of life, the exposed, not-sensitized group had a 60% greater risk of malaria infection (measured by counting parasites in their blood) than the unexposed group and a slightly higher risk of malaria infection than the sensitized group. They also had lower hemoglobulin levels (a sign of anemia) than the other babies. At age 6 months, the T cells of exposed, not-sensitized children were less likely to make activating cytokines in response to malaria antigens but made more IL-10 than the T cells of the other children; malaria-specific antibody levels were similar in the three groups.
What Do These Findings Mean?
These findings suggest that some children who are exposed to malaria before birth become “tolerant” to blood-stage malaria antigens. Exposure to malaria antigens before birth “tricks” their T cells into recognizing these antigens as self antigens. This immune tolerance, which persists into childhood, reduces the ability of the immune system to attack and destroy parasites and increases the susceptibility of these tolerant children to malaria infection. Why some children who are exposed to malaria before birth become tolerant while exposure to malaria antigens “primes” the immune system of other children to respond efficiently to these antigens is not clear. However, these findings could have important implications for the design of malaria vaccines for use in areas where children are often exposed to malaria before birth and for the design of strategies for the prevention of malaria during pregnancy.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000116.
This study is further discussed in a PLoS Medicine Perspective by Lars Hviid
Information is available from the World Health Organization on malaria (in several languages)
The US Centers for Disease Control and Prevention provides information on malaria (in English and Spanish)
Information is available from the Roll Back Malaria Partnership on all aspects of global malaria control, including information on malaria in pregnancy and on children and malaria
MedlinePlus provides links to additional information on malaria (in English and Spanish)