IgG antibodies play a role in malaria immunity, but whether and how IgM protects from malaria and the biology of Plasmodium falciparum (Pf)–specific IgM B cells is unclear. In a Mali cohort spanning infants to adults, we conducted longitudinal analyses of Pf- and influenza-specific B cells. We found that Pf-specific memory B cells (MBCs) are disproportionally IgM+ and only gradually shift to IgG+ with age, in contrast to influenza-specific MBCs that are predominantly IgG+ from infancy to adulthood. B cell receptor analysis showed Pf-specific IgM MBCs are somatically hypermutated at levels comparable to influenza-specific IgG B cells. During acute malaria, Pf-specific IgM B cells expand and upregulate activation/costimulatory markers. Finally, plasma IgM was comparable to IgG in inhibiting Pf growth and enhancing phagocytosis of Pf by monocytes in vitro. Thus, somatically hypermutated Pf-specific IgM MBCs dominate in children, expand and activate during malaria, and produce IgM that inhibits Pf through neutralization and opsonic phagocytosis.

In malaria-endemic areas, Plasmodium falciparum prevalence is often high in young women because of 1) low use of insecticide-treated nets before their first pregnancy and 2) acquired immunity, meaning infections are asymptomatic and thus untreated. Consequently, a common source of malaria in pregnancy (MiP) may be infected women becoming pregnant, rather than pregnant women becoming infected. In this study, prevalence of infection was determined by microscopy at first antenatal care (ANC) visit in primigravidae and secundigravidae in Ghana, Burkina Faso, Mali, and The Gambia, four countries with strong seasonal variations in transmission. Duration of pregnancy spent in the rainy season and other risk factors for infection were evaluated using multivariable Poisson regression. We found that the overall prevalence of malaria at first ANC was generally high and increased with time spent pregnant during the rainy season: prevalence among those with the longest exposure was 59.7% in Ghana, 56.7% in Burkina Faso, 42.2% in Mali, and 16.8% in Gambia. However, the prevalence was substantial even among women whose entire pregnancy before first ANC had occurred in the dry season: 41.3%, 34.4%, 11.5%, and 7.8%, respectively, in the four countries. In multivariable analysis, risk of infection was also higher among primigravidae, younger women, and those of lower socioeconomic status, independent of seasonality. High prevalence among women without exposure to high transmission during their pregnancy suggests that part of the MiP burden results from long-duration infections, including those acquired preconception. Prevention of malaria before pregnancy is needed to reduce the MiP burden.

Several infectious and autoimmune diseases are associated with an expansion of CD21 ⁻ CD27 ⁻ atypical B cells (atBCs) that up-regulate inhibitory receptors and exhibit altered B cell receptor (BCR) signaling. The function of atBCs remains unclear, and few studies have investigated the biology of pathogen-specific atBCs during acute infection. Here, we performed longitudinal flow cytometry analyses and RNA sequencing of Plasmodium falciparum ( Pf )–specific B cells isolated from study participants before and shortly after febrile malaria, with simultaneous analysis of influenza hemagglutinin (HA)–specific B cells as a comparator. At the healthy baseline before the malaria season, individuals had similar frequencies of Pf - and HA-specific atBCs that did not differ proportionally from atBCs within the total B cell population. BCR sequencing identified clonal relationships between Pf -specific atBCs, activated B cells (actBCs), and classical memory B cells (MBCs) and revealed comparable degrees of somatic hypermutation. At the healthy baseline, Pf -specific atBCs were transcriptionally distinct from Pf -specific actBCs and classical MBCs. In response to acute febrile malaria, Pf -specific atBCs and actBCs up-regulated similar intracellular signaling cascades. Pf -specific atBCs showed activation of pathways involved in differentiation into antibody-secreting cells and up-regulation of molecules that mediate B-T cell interactions, suggesting that atBCs respond to T follicular helper (T _FH ) cells. In the presence of T _FH cells and staphylococcal enterotoxin B, atBCs of malaria-exposed individuals differentiated into CD38 ⁺ antibody-secreting cells in vitro, suggesting that atBCs may actively contribute to humoral immunity to infectious pathogens.

As public health efforts seek to eradicate malaria, there has been an emphasis on eliminating low-density parasite reservoirs in asymptomatic carriers. As such, diagnosing submicroscopic Plasmodium infections using PCR-based techniques has become important not only in clinical trials of malaria vaccines and therapeutics, but also in active malaria surveillance campaigns. However, PCR-based quantitative assays that rely on nucleic acid extracted from dried blood spots (DBS) have demonstrated lower sensitivity than assays that use cryopreserved whole blood as source material. The density of Plasmodium falciparum asexual parasites was quantified using genomic DNA extracted from dried blood spots (DBS) and the sensitivity of two approaches was compared: quantitative real-time PCR (qPCR) targeting the P. falciparum 18S ribosomal RNA gene, either with an initial conventional PCR amplification prior to qPCR (nested qPCR), or without an initial amplification (qPCR only). Parasite densities determined by nested qPCR, qPCR only, and light microscopy were compared. Nested qPCR results in 10-fold higher sensitivity (0.5 parasites/μl) when compared to qPCR only (five parasites/ul). Among microscopy-positive samples, parasite densities calculated by nested qPCR correlated strongly with microscopy for both asymptomatic (Pearson’s r = 0.58, P < 0.001) and symptomatic (Pearson’s r = 0.70, P < 0.0001) P. falciparum infections. Nested qPCR improves the sensitivity for the detection of P. falciparum blood-stage infection from clinical DBS samples. This approach may be useful for active malaria surveillance in areas where submicroscopic asymptomatic infections are prevalent.

A functional human IgA response to malaria targets a conserved epitope in the N terminus of the Plasmodium falciparum circumsporozoite protein.

BACKGROUND: Malaria pathogenicity is determined, in part, by the adherence of Plasmodium falciparum –infected erythrocytes to the microvasculature mediated via specific interactions between P . falciparum erythrocyte membrane protein (PfEMP1) variant domains and host endothelial receptors. Naturally acquired antibodies against specific PfEMP1 variants can play an important role in clinical protection against malaria. METHODS: We evaluated IgG responses against a repertoire of PfEMP1 CIDR domain variants to determine the rate and order of variant-specific antibody acquisition and their association with protection against febrile malaria in a prospective cohort study conducted in an area of intense, seasonal malaria transmission. RESULTS: Using longitudinal data, we found that IgG antibodies against the pathogenic domain variants CIDRα1.7 and CIDRα1.8 were acquired the earliest. Furthermore, IgG antibodies against CIDRγ3 were associated with reduced prospective risk of febrile malaria and recurrent malaria episodes. CONCLUSION: This study provides evidence that acquisition of IgG antibodies against PfEMP1 variants is ordered and demonstrates that antibodies against CIDRα1 domains are acquired the earliest in children residing in an area of intense, seasonal malaria transmission. Future studies will need to validate these findings in other transmission settings and determine the functional activity of these naturally acquired CIDR variant–specific antibodies. TRIAL REGISTRATION: ClinicalTrials.gov NCT01322581. FUNDING: Division of Intramural Research, National Institute of Allergy and Infectious Diseases, NIH.