Determining the contribution of Natural Killer Cells to the in vivo clearance of Plasmodium erythrocytic stage parasites

About This Grant

Abstract Malaria causes approximately 600,000 deaths annually, along with 200,000 miscarriages and 20,000 maternal deaths—primarily due to the Plasmodium blood-stage of infection. While recent advances in vaccination and monoclonal antibody therapies targeting the liver stage of infection are promising, individuals not protected by these approaches remain vulnerable to blood-stage infection and its associated morbidity and mortality. Although antibodies are known to confer protection against the blood stage, the mechanisms underlying this protection remain poorly defined. We hypothesize that natural killer (NK) cells act in concert with antibodies to kill merozoites released from infected red blood cells, thereby boosting the neutralization efficacy of the antibody. This hypothesis is supported by our lab and others with human correlations. In vitro studies have demonstrated NK cell activity against infected RBCs and merozoites, and clinical data show that enhanced NK cell antibody-dependent killing correlates with reduced parasitemia and protection from malaria. In preliminary data, we showed using re-engineered neutralizing an anti-MSP1 antibody (IgG2a) reduces parasitemia, and 31% of the reduction is Fc receptor dependent shown by IgG2a LALAPG antibody. This proposal seeks to show definitive in vivo evidence for NK cells contributing to merozoite clearance that has been obscured by differences in NK cell and Fc receptor biology between mice and humans. This proposal addresses these limitations using two complementary, innovative approaches. First, adapting a strategy from cancer immunotherapy, we will inject human NK cells and human anti-merozoite antibodies into immunodeficient mice. The in vivo function of these human NK cells will be correlated with their in vitro activity against RH5—the leading blood-stage vaccine candidate not expressed in mouse malaria. Second, we will employ a novel mouse model with an intact immune system that has been genetically engineered to overcome limitations in mouse NK cell effector function and Fc receptor engagement. Using this model with mouse anti- merozoite antibodies will enable a rigorous test of NK cell contribution to parasite clearance in vivo. Together, these experiments will clarify the role of NK cells in the antibody-mediated clearance of blood-stage Plasmodium parasites. The insights gained may inform new strategies to enhance both NK cell and antibody responses to improve malaria vaccines, immunotherapy, and prevention.