Implantation of any material into the body initiates a cascade of signaling events often producing encapsulation of the implant by an avascular, collagenous, and highly cellularized matrix. This process — known as the foreign body response — persists throughout the lifetime of the implant. The resulting fibrous network of collagen is thought to be produced by fibroblasts acting on cues from resident macrophages. As the primary modulators of the foreign body response, macrophages secrete many different cytokines at the site of injury. The precise intracellular signaling is largely unknown. Understanding the cooperative nature of these cells is important in gaining further knowledge toward the development and possible control of the foreign body response.

To evaluate the effects that macrophages and fibroblasts have upon one another in the presence of a foreign material, a co-culture system using secondary and primary murine-derived cells was implemented. To identify the level of communication that provides the most significant change, both cell types were treated with conditioned media (one-way signaling) from the other cell type, placed in a non-contacting co-culture system (paracrine signaling), and cultured in contact with one another (juxtacrine signaling). As the time course for triggering the formative components of the foreign body response is approximately three weeks, cells were co-cultured for 21 days on tissue culture polystyrene (TCPS) as previously described.1-4 TCPS may activate macrophages over time and change their cytokine secretion profiles, which may in turn affect the behavior of the fibroblasts. Cell culture media was collected at days 1, 2, 3, 7, 14, and 21 and tested for changes in soluble protein secretion. A cytometric bead array (CBA) assay using flow cytometry was used to determine cytokine profiles over time. All cells were imaged frequently on surfaces in cultures to determine any changes in behavior and morphology.

The co-culture system demonstrated, using one-way signaling, paracrine signaling, or juxtacrine signaling was found to induce an increase in some of the inflammatory cytokine profiles over time in both primary and secondary cells, compared to those from mono-cultured fibroblasts or macrophages. However, the greatest increase was seen during juxtacrine signaling when the cells were in physical contact with one another. Where the presence of a foreign material in vivo would stimulate increased inflammatory cytokines, and co-culture provides an increase of inflammatory cytokines, then perhaps a more representative in vitro model of the FBR should consist of multiple cell types. Additionally, understanding that signaling between macrophages and fibroblasts contribute to an increased inflammatory response in vitro may influence the therapeutic targets against both of these cells simultaneously.

References:

1. M.L. Godek, G.S. Malkov, E.R. Fisher, D.W. Grainger, “Macrophage serum-based adhesion to surface chemistry is distinct from that exhibited by fibroblasts,†Plasma Proc. Polym., 3, 485-497, (2006).

2. M.L. Godek, J.A. Sampson, N.L. Duchsherer, Q. McElwee, D.W. Grainger, “Rho GTPase protein expression and activation in murine monocyte/macrophages is not modulated by model biomaterial biomaterial culture surfaces,†J. Biomat. Sci. Polym.

3. M.L. Godek, R. Michel, L.M. Chamberlain, D.G. Castner, D.W. Grainger, “Adsorbed serum albumin is permissive to macrophage attachment to polymer surfaces in cultureâ€, J. Biomed. Mat. Res., under revision, (2007).

4. L.M. Chamberlain, M.L. Godek, M. Gonzalez-Juarrero, D.W. Grainger, “Comparison of murine macrophage cell models for in vitro assessment of the foreign body responseâ€, submitted to J. Biomed. Mater. Res., 2007.