Development of human reticulocyte-derived exosomes as a new vaccine delivery platform against Plasmodium vivax malaria

Background: Plasmodium vivax is the most widely distributed human malaria parasite. This parasite preferentially invades reticulocytes, cells that selectively remove obsolete proteins through exosome release in their maturation to erythrocytes. Apart from their essential role in erythropoiesis, reticulocyte-derived exosomes (Rex) were shown to be involved in the modulation of the immune response in a murine reticulocyte-prone malaria model resembling P. vivax. Rex from this murine malaria infection carried parasite antigens and when used in immunizations adjuvanted with CpG, elicited a spleen-dependent long-lasting protective immune response, thus, suggesting the use of Rex from infections as a potential approach for vaccination against P. vivax. Methods: To extrapolate these findings to P. vivax, we initially determined the protein composition of human Rex (HuRex). HuRex were isolated from in vitro cultures of human cord blood reticulocytes and subjected to mass spectrometry-based proteomics. To avoid technological confounding, we used two different isolation methodologies, ultracentrifugation and size-exclusion chromatography (SEC). Next, we studied the capture of HuRex by monocyte-derived dendritic cells (mDCs). In parallel, plasma-derived exosomes isolated by SEC from naturally P. vivax-infected patients (PvEx), which we have shown to contain parasite proteins, were used to study their in vitro interaction with sorted immune cell populations from human spleens. Results: HuRex proteomics rendered a list of 418 proteins, where MHC class I molecules and adhesins were identified among others. The presence of MHC class I molecules in HuRex along with their capacity to be captured by mDCs suggests a role of HuRex in antigen presentation. Furthermore, we observed an active uptake of PvEx by human spleen T cells, a population whose distribution was altered by Rex immunization during the protective antimalarial immune response in the murine model. Summary/Conclusion: Further experimentation is guaranteed to determine the role of Rex in antigen presentation and protection against P. vivax infections as well as their potential as a new vaccine delivery platform against P. vivax.