VAR2CSA Duffy Binding like (DBL) Domains and Non-Specific IgM Binding in the Acquisition of Acquired Immunity to Pregnancy-Associated Plasmodium falciparum Malarium

Abstract

BACKGROUND: Plasmodium falciparum malaria throughout history has proved to be a significant menace to human health and pregnancy associated malaria (PAM) has been linked to severe consequences in terms of morbidity and mortality. P. falciparum parasites express members of the Plasmodium falciparum Erythrocyte Membrane Protein-1 (PfEMP1) on the surface of infected erythrocytes (IEs) which act as ligands binding to a number of different human vascular host receptors such as Chondroitin Sulphate A (CSA) in the placenta. The adhesion to CSA is mediated by VAR2CSA, which also allows for antigenic variation and immune evasion. VAR2CSA has been considered an important target of acquired protective immunity mediated mainly by specific IgG antibodies. In addition to being targets of specific IgGs, IEs have been demonstrated to select natural IgM in addition to CSA. AIM: This study was designed to determine if binding of VAR2CSA IEs to nonspecific IgM interferes with specific IgG binding to the VAR2CSA epitopes. METHOD: Plasma samples from 109 pregnant Ghanaian women of varying gestational age and parity were purified by the Dynabeads immunoprecipitation method for IgG and IgM on Mannan Binding Protein and grouped into three (unpurified plasma containing IgG and IgM (IgG+IgM+), purified IgG from plasma (IgG+IgM-) and purified IgM from same plasma(IgG-IgM+). The levels of antibody reactivity to two different domains of VAR2CSA (DBL1 and DBL5) and two domains of IT4var60 not implicated in placental malaria, but mediates rosetting (DBL1α and DBL5ε) were measured using indirect ELISA. RESULTS: The study showed increased susceptibility to malaria infection in the primigravidae (13%) than in the multigravidae (2.3%). The levels of IgG measured in the presence of IgM was significantly higher than IgG levels measured in the absence of IgM to both DBL1 domains (p=0.0001). Also there was a significant increase in the reactivity of IgG to the DBL5 domains (VAR2CSA and IT4VAR60) (p<0.0001) in the presence of IgM when compared to the IgG reactivity to the domains in the absence of IgM. However, the decreased IgM reactivity to VAR2CSA DBL5 in the IgG+IgM+ samples may indicate that IgG inhibits IgM binding to VAR2CSA DBL5 domain. Similarly there was an increased reactivity of the antibodies to the domains in the test samples when compared to the control samples. Also among the multigravidae, there was an increased IgG reactivity to the DBL1 domains and DBL5ε domain during gestation when compared to the primigravidae. CONCLUSION: The results from the study showed that the VAR2CSA and IT4var60 domains harboured IgM epitopes alongside malaria specific IgGs. Moreover since the levels of IgG binding to the domains were higher in plasma than in the absence of IgM, it may be suggestive that IgM binding may not interfere with IgG binding to the domains. Also a strong immune response against the DBL1 and DBL5ε domains elicited by the multigravidae during gestation may be as a result of their previous repeated exposure or pre-existing memory to P. falciparum infections during pregnancy.