Understanding the Acquisition and Kinetics of Naturally Acquired Immunity to Pfrh5 Complex Proteins in Malaria

Abstract

Plasmodium falciparum causes the severe form of malaria affecting mainly children below the age of five years and pregnant women. In the last decade, there has been a global decline in the reported malaria case incidence. Despite the reduction in malaria-associated morbidity and mortality, the disease is still prevalent in many developing countries in the tropics. With emergence of insecticide resistance and drug resistance to the limited range of effective drugs currently available, malaria vaccines remain a critical component of any strategic plan to eliminate and eventually eradicate malaria. Individuals living in endemic regions do develop immunity to malaria after repeated exposure to the parasite and never achieve sterile immunity. Developing an effective malaria vaccine has been difficult mainly due to the complex parasite life cycle, antigenic variation and polymorphism of parasite ligands. Much of the vaccine studies have focused on the asexual blood stage of the parasite life cycle as the clinical symptoms associated with malaria are due to this stage. In spite of this, there is no blood stage vaccine. P. falciparum reticulocyte binding-like protein homolog 5 (PfRH5) and its interacting proteins P. falciparum cysteine-rich protective antigen (PfCyRPA) and P. falciparum protein 113 (Pf113) have emerged as promising blood stage vaccine candidates due to the essential role of PfRH5 in merozoite invasion, its less polymorphic nature and the strain-transcending inhibitory effect of antibodies induced against the PfRH5 complex proteins. Nevertheless, not much is known about the role of PfRH5, PfCyRPA and Pf113 in naturally acquired immunity. In the present studies, the immunogenicity of PfRH5 complex proteins and the kinetics of antibodies induced against the PfRH5 complex proteins in natural infections were examined. In total, 206 Ghanaian children between the ages of 1-12 years who were either symptomatic, asymptomatic or non-parasitemic and healthy were recruited at the Hohoe municipal hospital. Plasma levels of antigen-specific IgG antibodies among acute malaria patients on day of admission showed seroprevalence of PfRH5, PfCyRPA and Pf113 was low compared to other well-studied merozoite antigens. The predominant IgG subclass response to the PfRH5 complex proteins was IgG1 and IgG3. Plasma IgG levels were found not to correlate with protection from malaria, severity of disease, or parasitemia on day of admission. Following treatment, antibodies to the studied antigens rapidly declined suggesting plasma IgG levels to the PfRH5 complex proteins were markers of recent parasite exposure. To better understand the mechanism of action of anti-PfRH5 and anti-PfCyRPA antibodies in naturally acquired immunity, human monoclonal antibodies were isolated from semi-immune Ghanaian adults and tested for functionality. The antibodies showed poor reactivity to PfRH5 and PfCyRPA recombinant proteins and did not exhibit inhibition against P. falciparum in vitro. In contrast, a panel of human monoclonal antibodies isolated from individuals vaccinated with viral vectored PfRH5 elicited strain-transcending inhibitory effects in vitro. Thus, knowledge gained from the functional and structural characterization of the human anti-PfRH5 monoclonal antibodies would greatly advance the rational design of PfRH5 based vaccines.