Date of Award

2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemical and Biomolecular Engineering

First Advisor

Dennis E. Discher

Abstract

Professional phagocytes are white blood cells of the innate immune system that protect humans and other animals from attacks by foreign pathogens by ingesting potentially harmful circulating particles. However, phagocytes such as macrophages may also attack elements that have been intentionally introduced into the body, such as implants, artificial tissues, artificial organs and vesicles bearing therapeutic agents, reducing their lifetime in the body. Nanoparticles and liposomes are similar in size to viruses, and are frequently decorated with antibodies for targeted therapeutics or imaging purposes. Although such particles are sufficiently small to avoid passive entrapment within capillaries in vivo, macrophages in the spleen and liver are known to clear these particles within hours or days after injection into the circulation, limiting proper delivery to target disease sites. This dissertation describes the engineering of long-circulating nanoscale carriers bearing a universal marker that avoids macrophage clearance by mimicking the process of recognition of ”self”cells in vivo. The membrane protein CD47 is a marker of ”self” that impedes phagocytosis of ”self” cells by signaling through a species-specific, highly polymorphic receptor named SIRPα. Among natural mouse variants, only NOD.SCID gamma chain (NSG) mice express a mouse polymorph of SIRPα that cross-reacts with human CD47, and thus provided an ideal platform for our in vivo assessment of human CD47 on synthetic particles. Based on the co-crystallized structure of the CD47-SIRPα complex, polypeptides were designed by simulation and then synthesized with the further reductionist goal of identifying and then exploiting the most minimal components necessary for CD47-SIRPα complex interactions. This thesis details a reductionist approach that avoids many potentially confounding biological factors, and constitutes a first example of a synthetic, human ligand that is easily attached to synthetic surfaces and can successfully mediate binding and signaling to phagocyte receptors to inhibit phagocytic uptake. By understanding how both the full-length as well as the minimal component of the hCD47-SIRPα interaction works in vivo, this dissertation was able to elucidate potential therapeutic roles for anti-hCD47 antibody targeting in biodistribution of drugs delivered to solid tumors, among other applications.

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