Engineering endogenous inflammatory cells as delivery vehicles

Leukocytes are central in directing host inflammatory and immune processes; therefore, leukocyte response to biomaterials is extremely important. Although several leukocyte-derived molecules are used clinically, the long-term efficacy of treatments involving the systemic administration of these bioactive agents has yet to be demonstrated. Hence, the localized delivery of selected cytokines and growth factors produced by endogenous leukocytes is desirable and may have potential therapeutic values in the fundamental processes of tissue healing, growth regulation, and biocompatibility. The specificity and diversity of ligand-receptor interactions offer an attractive method in manipulating cellular behavior. Therefore, a more detailed understanding of the interplay between ligands and cell membrane receptors must be obtained. We designed interleukin-1-derived biomimetic agonists and antagonists to study and modulate leukocyte function in vitro. Selected agonists increased GM-CSF release by adherent human blood-derived macrophages in the presence of the natural IL1beta antagonist, namely IL1ra. Furthermore, IL1-derived biomimetic antagonists neutralized the ability of IL1beta in increasing the release of GM-CSF by adherent macrophages. We employed similar methodologies to elucidate the molecular mechanisms of integrin and extracellular matrix interaction in regulating leukocyte function. Oligopeptides were designed based on the functional structure of fibronectin and grafted on to a polymer network containing polyethyleneglycols. Macrophage adhesion was independent of the peptide identity that contained sequence RGD, PHSRN, PRRARV, or combinations thereof in an integrin-dependent fashion in vitro. However, integrin-dependent FBGC formation in vitro was highly dependent on both RGD and PHSRN in a single peptide formulation and with a specific orientation. From our intracellular signaling studies in vitro, protein tyrosine and serine/threonine kinases were found important in integrin signaling leading to macrophage adhesion mediated by fibronectin-integrin association. Furthermore, RGD and PHSRN appear to be significant in mediating this receptor-ligand association resulting in the necessary signaling characteristic for macrophage adhesion and the subsequent development. Our in vivo results showed that peptide identity played a minimal role in modulating the host inflammatory response and adherent macrophage density. RGD-containing peptides mediated rapid FBGC formation by 4 days of implantation by significantly increasing both the number of macrophages that participate in the cell fusion process and the rate of cell fusion. Both RGD and PHSRN domains were important in mediating FBGC formation at later implantation periods. These findings represent a mechanistic correlation between the role of protein functional architectures in ligand-receptor recognition and the post-ligation signaling events that control cellular behavior in vitro and in vivo. (C) 2002 Elsevier Science B.V. All rights reserved.