Aotus nancymaae model predicts human immune response to the placental malaria vaccine candidate VAR2CSA

Abstract

Placental malaria vaccines (PMV) are being developed to prevent severe sequelae of placental malaria (PM) in pregnant women and their offspring. The leading candidate vaccine antigen VAR2CSA mediates parasite binding to placental receptor chondroitin sulfate A (CSA). Despite promising results in small animal studies, recent human trials of the first two PMV candidates (PAMVAC and PRIMVAC) generated limited cross-reactivity and cross-inhibitory activity to heterologous parasites. Here, we immunized Aotus nancymaae monkeys with three PMV candidates (PAMVAC, PRIMVAC and ID1-ID2a_M1010) adjuvanted with Alhydrogel(R), and exploited the model to investigate boosting of functional vaccine responses during PM episodes as well as with nanoparticle antigens. PMV candidates induced high levels of antigen-specific IgG with significant cross-reactivity across PMV antigens by ELISA. Conversely, PMV antibodies recognized native VAR2CSA and blocked CSA-adhesion of only homologous parasites and not heterologous parasites. PM episodes did not significantly boost VAR2CSA antibody levels or serum functional activity; nanoparticle and monomer antigens alike boosted serum reactivity but not functional activities. Overall, PMV candidates induced functional antibodies with limited heterologous activity in Aotus monkeys, similar to responses reported in humans. The Aotus model appears suitable for preclinical down-selection of PMV candidates and assessment of antibody boosting by PM episodes. Research in Context Evidence before this studyThe Plasmodium falciparum erythrocyte membrane protein VAR2CSA is the leading vaccine candidate antigen to protect pregnant women against placental malaria (PM), which causes serious adverse pregnancy outcomes particularly in first-time mothers living in malaria-endemic areas. Two VAR2CSA-based vaccines (PAMVAC and PRIMVAC) induced strong heterologous functional antibodies in small animals, but induced antibodies with limited cross-inhibitory functional activity in human clinical trials. These observations highlighted the need to establish new animal models that could better recapitulate human pathogenesis and immunity. In ongoing development of a nonhuman primate model for PM, we established an Aotus nancymaae model susceptible to P. falciparum infection during pregnancy that reproduces all the immunoparasitological and histological features of human PM. In this study, we explore the new Aotus model as a platform for evaluating PM vaccine (PMV) immunogenicity and for boosting of vaccine responses during PM episodes. Added value of this studyIn this manuscript, we demonstrate that PMV (including PAMVAC and PRIMVAC) are immunogenic in Aotus monkeys, inducing antibodies with mainly homologous and little heterologous functional activity, as seen in humans but contrary to preclinical reports on these vaccines in small animals. Implications of all the available evidenceOur findings suggest Aotus is a suitable model to assess immunogenicity of VAR2CSA-derived vaccines, in contrast to small animal models. PMV data from human trials and Aotus monkeys suggest that improvements to current VAR2CSA immunogens and/or adjuvants are needed to enhance protective antibody responses, as are studies that evaluate the potential for natural infection to boost vaccine antibody in pregnancy. Therefore, the Aotus PM model may be useful to assess second-generation PMVs seeking to increase strain-transcending activity and to prioritize these for further clinical development.