Brief hypoxia conditions monocytes to protect reperfused cardiocytes against cell death via the CD11b receptor

Abstract

Background: Traditionally leukocytes have been viewed as the mediators and effectors of cell injury after tissue ischemia and reperfusion through the indiscriminate release of toxic cytokines and oxygen free radicals. This can be detrimental to functioning of the transplanted heart when reperfused after implantation. Paradoxically, evidence now suggests that certain cytotoxic cytokines and even oxygen free radicals can be cytoprotective in smaller concentrations. This study sought to determine whether cultured human monocytes can be pre-conditioned by brief hypoxia to protect cardiomyocytes from cell death after hypoxia and re-oxygenation. Methods: Cultured human monocytes were exposed to transient hypoxia (10 minutes), after which we determined CD11b expression using flow cytometry. The 3 control groups comprised immunoglobulin G-negative controls, fMLP-positive controls, and virgin monocyte (VM) controls. We studied chick embryonic ventricular myocytes in a flow-through chamber while controlling flow rate, pH, O2, and CO2 tension. We quantified cardiomyocyte viability using propidium iodide (5 μmol/liter). Cell systems of cardiomyocytes alone, cardiomyocytes and VM, human monocytes exposed to transient hypoxia before coculture with cardiomyocytes (PCHM-cardiomyocyte), and PCHM cocultured with anti-CD11b antibodies for 30 minutes before coculture with cardiomyocytes (CDHM-cardiomyocyte) were subjected to 1 hour of hypoxia and 3 hours of re-oxygenation, and cell death was determined. The experiment was repeated and the cell systems fixed, stained, and examined for monocytes adhering to cardiomyocytes. Results: CD11b expression increased significantly with both transient hypoxia and fMLP (18.39% ± 4.116%, n = 5, and 37.04% ± 7.783%, n = 5, respectively, p < 0.05 vs VM 100%, n = 5). Coculture of cardiomyocytes with VM had no effects on cardiocyte death (40.6% ± 6.1%, n = 6) compared with controls (46.5% ± 4.0%, n = 10). The PCHM cocultured with cardiomyocytes significantly decreased cardiomyocyte death (25.2% ± 4.7%, n = 6, p < 0.05). This protection was abrogated by the addition of CD11b-blocking antibodies to PCHM before coculture with cardiomyocytes (51.0% ± 6.1%, n = 6, p < 0.05). The PCHM showed increased adhesion to cardiomyocytes (5.4 ± 0.38/high-power [HP] field vs 0.67 ± 0.24/HP field in VM, p < 0.05). The increased adhesion was abolished by CD11b-blocking antibody (0.78 ± 0.28 vs 0.67 ± 0.24 cells/HP field, p < 0.05). Conclusion: These results suggest that monocytes activated by transient hypoxia protect cardiomyocytes during hypoxia and re-oxygenation through expression of CD11b receptors. These cells seem to adhere to myocytes through this receptor to achieve this effect. The exact mechanism is unclear and requires further study. Autologous recipient monocytes may be pre-conditioned to protect the donor heart during reperfusion.