Blood cells often contact Macrophages in the spleen, liver, and marrow, but whether such contacts activate the macrophage and promote blood cell clearance is a basic question of broad importance not only to cell survival but also to biocompatibility and nanotechnologies. Some years ago, RBCs from knockout mice lacking CD47 were injected into control mice, and the RBCs were found to be cleared rapidly by splenic macrophages - even though the knockout mice showed no RBC defects or anemia [Oldenborg Science 2000]. Stimulated by this paradox and the findings, we decided to focus on the relevance of CD47 'Self' signaling to humans, and we began by characterizing differences between human vs mouse CD47 on RBCs [Dahl Blood 2003, 2004; Subramanian Blood 2006]. Despite many structural differences, our studies of human RBC phagocytosis in vitro showed that human-CD47 can indeed inhibit eating, with signaling to the cytoskeleton against antibody- driven eating occurring through SIRPa on a human macrophage [Tsai & Discher J Cell Biol 2008]. We have now reduced human-CD47 to a 10-20 amino acid 'Self' peptide that binds hSIRPa, inhibits phagocytosis, and even impedes splenic clearance of nanoparticles from the circulation of NOD/SCID (NSG) mice expressing a human-compatible mSIRPa [Rodriguez Science 2013]. Delayed clearance also enhances nanoparticle delivery of dyes and drugs to tumor xenografts. However, 'Self' signaling is hotly debated [Willingham PNAS 2012, Burger Blood 2012; Wang Mol Ther 2013], and effects of particle source (biological and synthetic), size, properties are all largely unclear. In our Aim-1, pathways will be examined for 'Self'-displaying particles or cells that range from Exosomes or Platelet-like Particles and Lentivirus to Gold or Magnetic Nanoparticles, monomeric 'Self Colloids', and also Rigidified RBC shapes relevant to senescence. Phagocytosis pathways in diverse Macrophages will be compared in molecular detail, based in part on Mass Spec-based proteomics studies designed to elucidate differences in Self signaling in vivo as well as in vitro. 'Self' signaling is perhaps complicated by affinitis of human-CD47 for natural SIRPa variants that span a ~50-fold range in our initial studies. New mutants, polymorphisms, and peptides will be studied in Aim-2 to clarify mechanisms and implications of such wide variation. Kinetics and nano-scale forces of 'Self' recognition will be probed in vitro with circulation-relevant microflows of particles and cells past stationary macrophages, per spleen and liver anatomy. Comparisons will be made to single molecule forces obtained with proteinated AFM tips and also via adhesion to Nano-films of 'Self' relevant to implants.
Aim -3 will focus on the in vivo balance in nanoscale signaling between 'Self' recognition of human cells by NSG mouse Macrophages and Ab-induced Antagonism of the Macrophage. Our ultimate goal is to clarify mechanisms of 'Self' recognition from a perspective of blood and through an array of nanotechnology developments.
To work effectively, drugs, imaging agents, gene therapy vectors, and even cell therapies need to get to the intended diseased site(s). Many research groups are developing particles that help in delivery of synthetics, but everything that is injecte contends with Macrophages that often 'eat' the largest fraction of injected materials or cells. Macrophages have the job of spotting intruders in the blood or at a site of disease or injury and destroying the intruder by phagocytosis. We have recently described a facile way to Stop Macrophages from destroying nanoparticles and probably viruses and cells with implications for broader application. We designed and made a small peptide of 10-20 amino acids from a crucial human membrane protein called CD47, which is recognized by macrophages as being as safe as 'Self'. The peptide binds to a specific counter-receptor on macrophages to signal 'Don't Eat Me!' Thus far we have stuck the peptide to commercially available polystyrene nanobeads and shown that many more of the peptide-tagged beads get past macrophage defenses. In mice with tumors, we see that peptide-tagged fluorescent nanobeads accumulate in tumors and deliver drug better. Recognition limits in terms of particle type (synthetic vs biological, includig cells), particle sizes and properties now need to be carefully mapped with appropriately refined nano-tools. Sequence variations in CD47 and its receptor exist even within man, and impacts on adhesive recognition need to be clarified, especially under blood-flow relevant conditions. Ultimately, we aim to elucidate the nanoscience of 'Self'.
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