The N-terminal region of malaria vaccine candidate asparagine-rich merozoite antigen is immunodominant and targeted by polyreactive antibodies

Abstract

The development of malaria blood-stage vaccines has been hampered by sequence variation in many Plasmodium falciparum proteins involved in erythrocyte invasion. In the past few years, asparagine-rich merozoite antigen (PfARMA) has emerged as a potential vaccine candidate due to its low amino acid sequence diversity and the association between anti-PfARMA antibody levels and protection from malaria. Here, we used samples from P. falciparum-exposed individuals to study naturally acquired B cell and antibody responses to PfARMA. B cell responses to PfARMA were dominated by IgM+ B cells that recognized the N-terminal intrinsically disordered region 1 (IDR1) of PfARMA. A human monoclonal antibody (hmAb) to IDR1 was non-neutralizing, while a second hmAb binding to the folded domain showed weak neutralizing activity. Both PfARMA-specific plasma IgM and IgG responses predominately targeted IDR1, and their levels increased with P. falciparum exposure. However, in contrast to previous reports, these antibody responses did not correlate with protection in age- and exposure-matched children. Interestingly, approximately 30% of unexposed individuals had IgG that also targeted IDR1 and was polyreactive, binding to regions with high asparagine content. Finally, we determined that PfARMA is located in or near micronemes that contain erythrocyte binding antigen 175 (PfEBA-175). These data suggest that while IgG to the folded domain of PfARMA may inhibit parasite growth, antibody responses to PfARMA are primarily directed to IDR1. Consequently, these responses may reflect recent exposure rather than contributing to functional immunity to malaria.