Phenotypic dichotomies in the foreign body reaction

To better understand the relationship between macrophage/foreign body giant cell adhesion and activation on surface-modified biomaterials, quantitative assessment of adherent cell density (cells per mm2) and cytokine production (pgs per mL) were determined by ELISA. Further analysis to identify cellular activation was carried out by normalizing the cytokine concentration data to provide a measure of cellular activation. This method of analysis demonstrated that hydrophobic surfaces provided statistically significantly greater adherent cell densities than hydrophilic/neutral surfaces. However, when cell activation parameters were determined by normalization to the adherent cell density, the hydrophilic/neutral surfaces demonstrated statistically significantly greater levels of activation and production of IL-10, IL-1β, IL-6, IL-8, and MIP-1β. With increasing time, production of the anti-inflammatory cytokine IL-10 increased, whereas IL-1β, IL-6, and IL-8 decreased and MIP-1β was relatively constant over the culture time period. This observed dichotomy or disparity between adhesion and activation may be related to surface-induced adherent cell apoptosis. Further evaluation of macrophage activation on biomaterial surfaces indicated that an apparent phenotypic switch in macrophage phenotype occurred over the course of the in vitro culture. Analysis of cytokine/chemokine profiles with surface-modified biomaterials revealed similarities between the classically activated macrophages and the biomaterial-adherent macrophages early (day 3) in culture, while at later timepoints the biomaterial-adherent macrophages produced profiles similar to alternatively activated macrophages. Classically activated macrophages are those commonly activated by lipopolysaccharide (LPS) or interferon-γ (IFN-γ) and alternatively activated macrophages are those activated by IL-4/IL-13 or IL-10. Surface modification of biomaterials offer an opportunity to control cellular activation and cytokine profiles in the phenotypic switch, and may provide a means by which macrophages can be induced to regulate particular secretory proteins that direct inflammation, the foreign body reaction, wound healing, and ultimately biocompatibility.