1. CD47 modulates the phagocytic clearance and replication of Plasmodium yoelii malaria parasite Rajdeep Banerjee 1, Sanjay Khandelwal 2, Yukiko Kozakai 1, Bikash Sahu 1 and Sanjai Kumar 1 1 Laboratory of Emerging Pathogens, Division of Emerging and Transfusion Transmitted Diseases, OBRR, CBER, FDA, MD and 2 Department of Medicine, Duke University Medical Center, Durham, NC. BACKGROUND OBJECTIVES HumanRBC age Virulence Plasmodium vivaxYoungNon Fatal Plasmodium ovale YoungNon Fatal Plasmodium malariae Mature RBCNon Fatal Plasmodium falciparum Young & mature Fatal Mouse Plasmodium yoelii NLYoung RBC Non Fatal Plasmodium bergheiYoung and mature Fatal  Probable association of age specific markers;  Age based preference for RBC infection;  Association among RBC age, CD47 intensity and parasitemia;  Role of spleenic F4/80+ cells;  Immunological basis for CD47 mediated protection against malaria; Biotin (-3, -2, -1 Day) GFP PyNL Day 0 C57BL6 mice (WT & CD47 KO) CONCLUSION  CD47 expression is associated with the age of RBC.  P. yoelii parasites preferentially infects the young CD47 hi RBC to avoid the phagocytosis clearance by splenic macrophages.  CD47-/- mice are significantly resistant to parasite infection.  The modulation of CD47 level influence the severity of malaria disease.  The low IL-10 level may have contributed to enhance resistance against malaria in CD47-/- mice. RESULTS In vivo biotinylation can differentiate young and aged RBC Young RBC have higher CD47 expression Age based preference for RBC infection CD47/integrin associated protein The correlation between parasitized RBC and higher CD47 expression CD47 hi RBC are more prone to infection CD47 -/- mice are highly resistant to infection Modulation of CD47 level regulates the parasitemia Immunological basis underlying CD47 mediated resistance Enhanced phagocytic activity by splenic F4/80 macrophages in GFP-PyNL infected CD47  /  mice Serum cytokine profile Several Plasmodium species exhibit a strong age-based preference for the red blood cells (RBC), which in turn is a major determinant of disease severity. The molecular basis behind this age constrain on RBC infection is poorly understood. RBC and other cells express CD47 as marker of self to distinguish from being recognized as foreign and, thus avoid clearance by immune system. We investigated the role of CD47 on the replication and clearance of asexual blood stage non-lethal Plasmodium yoelii 17XNL (PyNL) malaria in C57Bl/6 mice. Using the biotin-labeling of RBC and GFP-PyNL parasite, we find that PyNL strongly favors to infect the young RBC expressing high level of CD47. Induction of reticulocytosis with a high level of CD47 by phenylhydrazine injection converted non-lethal PyNL into lethal phenotype. Importantly, CD47 -/- mice were highly resistant to PyNL infection and also had accelerated parasite clearance. Compared to wild type mice, CD47 -/- mice had a higher percentage of splenic F4/80 + cells and having a higher percentage of phagocytized infected RBC. CD47 -/- mice also exhibited a Th2-shift during the acute phase of infection. Furthermore, injection of CD47 neutralizing antibody caused a significant reduction in parasite burden in the WT C57BL/6 mice. Thus, in the absence of CD47, malaria infected RBC may become more susceptible to clearance by splenic phagocytic cells which, at least in part, may be responsible for enhanced immunity observed in CD47 -/- mice. Together our results suggest the novel role of CD47 in the regulation of PyNL parasite replication and disease severity.