Malaria relapses expand B-cell memory that provides strain-specific protection against reinfection

Background: Relapse infection biology represents a large gap in the understanding of vivax malaria. Studying relapses in patients is difficult due to reinfections in endemic areas and because infections with a single parasite variant are rare. Non-human primate models of vivax malaria overcome these constraints. This study aimed to improve understanding of relapses and reinfections by (1) evaluating if relapses, homologous reinfections, or heterologous reinfections caused clinical disease and (2) by dissecting humoral responses against the parasite during relapses and reinfections. Materials and Methods: Here, the rhesus macaque –Plasmodium cynomolgi model was used. Six rhesus macaques were infected with P. cynomolgi B strain sporozoites and monitored for 100-days to study relapses. The macaques were then radically cured, rested, and re-challenged with homologous sporozoites to evaluate the ability to cause clinical illness and protective immunity. After radical cure and rest, the animals were re-challenged but with heterologous sporozoites (Ceylon strain) to assess clinical illness and humoral immune responses. Parasitemia and clinical status were evaluated daily, and flow cytometry and ELISA were performed to measure B-cell and antibody responses, respectively. Results: Clinical signs of malaria (e.g. fever, anemia) and high parasitemia were prominent during the primary infection. Relapse parasitemias were significantly lower with minimal symptoms (e.g. no fever, no anemia). During relapses, IgG+ memory B-cell frequencies increased with the rise of parasite-specific IgG. Importantly, animals remained asymptomatic during homologous re-infections and had enhanced memory B-cell and antibody responses similar to relapses. In stark contrast, the heterologous reinfection resulted in clinical symptoms although some clinical protection may have been conferred, and B-cell frequencies were similar as the initial infection. Conclusions: Relapses and homologous reinfections are controlled by a memory B-cell response and parasite-specific IgG that leads to lack of clinical disease. The protective memory is conferred after a single infection and can persist for up to two months. However, the memory is strain-specific, and not protective for heterologous strains. Overall, this study provides insight into relapse infection immunology, demonstrates that protective humoral immunity can be developed after a single infection, and suggests that relapses may be mostly asymptomatic whereas infections with new parasite variants are responsible for disease.