Elucidating the biological performance of decellularized bovine pericardium: Understanding macrophage polarization and functional response

At present, no clinically available heart valve replacement completely replicates the performance, function and remodeling capability of a native valve. As the demand for valve replacements increases, there is a need for innovative valve scaffolds such as decellularized tissue matrices. Unfortunately, the mechanisms responsible for the poor clinical performance of bioprosthetic valves is not understood, and similar mechanisms may plague future scaffolds. Explant studies consistently demonstrate the presence of macrophages on failed valve surfaces, but it is unclear whether the presence of macrophages on these materials is a cause, consequence or coincidence. This thesis explores the functional and morphological interactions of macrophages with decellularized tissue matrices in vitro in an effort to understand and predict the biological performance of these materials.;This thesis provides insights into the functional differences in three human macrophage model systems and thereby increases our understanding of macrophage interactions with collagenous biomaterials. Such knowledge is valuable for the development of better tissue heart valve replacements and for the design of collagen-based tissue-engineering scaffolds in general.;The macrophage response to decellularized bovine pericardium (DBP), a representative tissue-engineering substrate, was compared to two control polymers: medical grade polydimethylsiloxane (PDMS) and tissue culture polystyrene (TCPS). Phorbol myristate acetate-differentiated U937 cells on DBP contained less esterase and acid phosphatase activity compared to controls and were less multinucleated. Human monocyte-derived macrophages (MDMs) (trypsinized after 14 d differentiation on TCPS) that were cultured on DBP responded very similarly, validating the use of U937 cells for predicting MDM interactions with collagen. When human MDMs were differentiated directly onto the materials of interest, further observations were made. In general, MDMs on the DBP surface released low amounts of cytokines and MMP-1, with the exception of IL-6, IL-8 and MCP-1. Additionally, directly differentiated differed from trypsinized MDMs by displaying lower overall extracellular and intracellular esterase and acid phosphatase activity on all three surfaces. Although the characterization of the MDM polarization on each surface was not conclusive, this thesis provides some of the first in vitro evidence that macrophages on decellularized tissue matrices may be stimulated towards a more wound healing phenotype.