Cellular mechanisms and environmental factors contributing to wound failure following shock and wound contamination are unclear. The activation of macrophages by exposure to hypoxia (pO2<20) and/or lipopolysacharide (10 μg/ml) in vitro was investigated for its effect on macrophage regulation of fibroblast proliferation.
The effect on fibroblast proliferation of conditioned medium from activated murine macrophages or co-culture with activated macrophages was tested by measuring 3T3 fibroblast incorporation of 3H-thymidine and culture DNA content. Unstimulated macrophages produced growth factors that increase fibroblast proliferation (proliferation index (PI) = 1.4 ± 0.15, p < 0.05 vs. control). Activation by hypoxia alone had little effect on macrophage regulation of fibroplasia (PI = 1.55 ± 0.28, N.S. vs. unstimulated macrophages). LPS activated macrophages suppressed fibroplasia and the combination of hypoxia with LPS augmented the suppression (PI = 0.5 ± 0.11, LPS alone, p < 0.05 and 0.25 ± 0.05, LPS + hypoxia, p < 0.01). In addition, hypoxia + LPS treated co-cultures had reduced DNA contents, suggesting reduced cell numbers (12.5 ± 2.6 μg vs. 8.2 ± 2.0 μg).
We screened several macrophage cytokines for their direct effect on 3T3 proliferation and found that mr-Tumor Necrosis Factor-alpha (150 units) also suppressed proliferation. Conditioned supernatants from LPS activated macrophages contained 12 ± 2 units of mrTNF as measured by L929 cytolysis; however, this was significantly less than required to induce suppression of proliferation by direct addition.
The regulatory role of the macrophage appears to be dependent on its level of activation. Activation by hypoxia and LPS altered macrophage regulation of fibroblast proliferation from stimulation to suppression. These results demonstrate that regulation is determined not only by interaction with soluble mediators present in the wound, but on the conditions present in the surrounding microenvironment.
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