Alveolar hypoxia produces rapid systemic inflammation in rats. Several lines of evidence suggest that the inflammation is not initiated by low systemic tissue partial pressure of oxygen (Po2) but by a mediator released into the circulation by hypoxic alveolar macrophages. The mediator activates tissue mast cells to initiate inflammation. Monocyte chemoattractant protein–1/Chemokine (C-C motif) ligand 2 (MCP-1/CCL2) is rapidly released by hypoxic alveolar macrophages. This study investigated whether MCP-1 is the mediator of the systemic inflammation of alveolar hypoxia. Experiments in rats and in alveolar macrophages and peritoneal mast cells led to several results. (1) Alveolar hypoxia (10% O2 breathing, 60 minutes) produced a rapid (5-minute) increase in plasma MCP-1 concentrations in conscious intact rats but not in alveolar macrophage–depleted rats. (2) Degranulation occurred when mast cells were immersed in the plasma of hypoxic intact rats but not in the plasma of alveolar macrophage–depleted rats. (3) MCP-1 added to normoxic rat plasma and the supernatant of normoxic alveolar macrophages produced a concentration-dependent degranulation of immersed mast cells. (4) MCP-1 applied to the mesentery of normoxic intact rats replicated the inflammation of alveolar hypoxia. (5) The CCR2b receptor antagonist RS-102895 prevented the mesenteric inflammation of alveolar hypoxia in intact rats. Additional data suggest that a cofactor constitutively generated in alveolar macrophages and present in normoxic body fluids is necessary for MCP-1 to activate mast cells at biologically relevant concentrations. We conclude that alveolar macrophage–borne MCP-1 is a key agent in the initiation of the systemic inflammation of alveolar hypoxia.
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