Biomaterial adherent macrophage apoptosis is increased by hydrophilic and anionic substrates in vivo

Abstract

An in vivo rat cage implant system was used to identify potential surface chemistries that prevent failure of implanted biomedical devices and prostheses by limiting monocyte adhesion and macrophage fusion into foreign-body giant cells while inducing adherent-macrophage apoptosis. Hydrophobic, hydrophilic, anionic, and cationic surfaces were used for implantation. Analysis of the exudate surrounding the materials revealed no differences between surfaces in the types or levels of cells present. Conversely, the proportion of adherent cells undergoing apoptosis was increased significantly on anionic and hydrophilic surfaces (46 +/- 3.7 and 57 +/- 5.0%, respectively) when compared with the polyethylene terephthalate base surface. Additionally, hydrophilic and anionic substrates provided decreased rates of monocyte/macrophage adhesion and fusion. These studies demonstrate that biomaterial-adherent cells undergo material-dependent apoptosis in vivo, rendering potentially harmful macrophages nonfunctional while the surrounding environment of the implant remains unaffected.

Fig 1.

Adherent-macrophage adhesion, apoptosis, and fusion. Cages were explanted on days 7 (black bars), 14 (white bars), and 21 (hatched bars), surfaces were washed with PBS, and adherent cells were stained with May–Grünwald/Giemsa (A and C) or annexin V-FITC in situ (B). (A) Cell densities were determined from ×5–20 objective fields for each sample and are expressed as cells per mm². (B) Samples were viewed by confocal microscopy, and cells staining positive were counted as apoptotic. A total of 200 cells were counted, and the results are expressed as the percentage of the cells that were counted as apoptotic. (C) Percent fusion was calculated by dividing the number of nuclei contained within giant cells by the total number of nuclei in the field of view. This process was repeated for ×3–20 objective fields. The data represent mean ± SEM. *, statistical significance, where P < 0.05 when compared with the base surface by using the Student's t test.

Fig 2.

Morphological properties of macrophages adherent to various surface chemistries. Cages were explanted on day 14, surfaces were washed with PBS, and adherent cells were stained with May–Grünwald/Giemsa and observed by light microscopy (A, C, E, G, and I) or stained with annexin V-FITC in situ and observed by confocal microscopy (B, D, F, H, and J).

Fig 3.

Exudate cell apoptosis. Exudate surrounding the implant was collected on days 7 (black bars), 14 (white bars), and 21 (hatched bars). One half of the exudate was centrifuged, and cells were stained with annexin V-FITC and propidium iodide. Cells were analyzed immediately by flow cytometry. Leukocytes were gated, and a total of 100,000 events were counted. The data are expressed as the number of cells staining positive out of the total number counted. The data represent mean ± SEM. *, statistical significance, where P < 0.05 when compared with the empty cage control using Student's t test.