Alveolar macrophages initiate the systemic microvascular inflammatory response to alveolar hypoxia

Abstract

Alveolar hypoxia occurs as a result of a decrease in the environmental [Formula: see text] , as in altitude, or in clinical conditions associated with a global or regional decrease in alveolar ventilation. Systemic effects, in most of which an inflammatory component has been identified, frequently accompany both acute and chronic forms of alveolar hypoxia. Experimentally, it has been shown that acute exposure to environmental hypoxia causes a widespread systemic inflammatory response in rats and mice. Recent research has demonstrated that alveolar macrophages, in addition to their well known intrapulmonary functions, have systemic, extrapulmonary effects when activated, and indirect evidence suggest these cells may play a role in the systemic consequences of alveolar hypoxia. This article reviews studies showing that the systemic inflammation of acute alveolar hypoxia observed in rats is not initiated by the low systemic tissue [Formula: see text] , but rather by a chemokine, Monocyte Chemoattractant Protein-1 (MCP-1, or CCL2) released by alveolar macrophages stimulated by hypoxia and transported by the circulation. Circulating MCP-1, in turn, activates perivascular mast cells to initiate the microvascular inflammatory cascade. The research reviewed here highlights the extrapulmonary effects of alveolar macrophages and provides a possible mechanism for some of the systemic effects of alveolar hypoxia.

Figure 1

Schematic representation of the inflammatory events taking place in a post-capillary venule after the onset of alveolar hypoxia

Figure 2

Time course of MC degranulation, evidenced by the uptake of Ruthenium Red after the onset of alveolar hypoxia. Microphotographs of a mesenteric post capillary venule were obtained every 2 min. The large black dots are used to align the optical Doppler velocimeter used to measure red blood cell velocity. The bottom row is a magnification of the MC shown immediately to the left of the venule. Adapted from Steiner et al., 2003

Figure 3

Effects of topical administration of the MC secretagogue C4880 and of Angiotensin II (Ang II) on the mesenteric microcirculation of normoxic, non acclimatized rats (Nx) and of rats exposed to hypobaric hypoxia (PB = 380 Torr) for 6 days (6d Hx). The red ovals highlight degranulated MCs (Nx + C4880) and adherent leukocytes (Nx + Ang II).

Figure 4

Schematic representation of the inflammatory cascade initiated by alveolar hypoxia: AMO, alveolar macrophages, MCP-1: Monocyte Chemoattractant Protein-1 (also known as CCL2), Tissue MO: systemic tissue macrophages, MC: mast cells; RAS: local renin-angiotensin system. ROS: reactive O₂ species

Figure 5

Effects of independent changes in alveolar and cremaster muscle P_O2 (Pmo₂) in intact rats. Alveolar P_O2 was altered by changing the inspired P_O2. Cremaster muscle P_O2 (estimated using a phosphorescence-quenching method) was altered by equilibrating the cremaster with either 95 % N₂, 5% CO₂ (cremaster hypoxia), or 10 % O₂, 85% N₂, 5% CO₂ (Cremaster normoxia). Cremaster ischemia was induced by mechanical compression of the cremaster pedicle. Cremaster mast cells were identified using Toluidine Blue staining. Adapted from Dix et al., 2003 and Shah et al., 2003

Figure 6

Representative microphotographs of the cremaster microcirculation showing the effects of alveolar hypoxia (10% O₂ breathing) in rats with normal AMO count (Hypoxia, PBS liposomes) and in AMO-depleted rats (Hypoxia, clodronate liposomes) The top two bright field images of the PBS liposomes rat show the expected effect of hypoxia on MC degranulation (left photograph), and on leukocyte endothelial adherence Right). The bottom photograph shows the increased extravasation of fluorescence-labeled albumin (FITC). The photographs of the clodronate liposomes rat show no MC degranulation or leukocyte-endothelial adherence, and minimal FITC albumin extravasation during hypoxia. Adapted from Gonzalez et al., 2007b

Figure 7

Figure 7A: Changes in the macrophage supernatant concentration of H₂O₂ as a function of time in hypoxia or normoxia. Adapted from Chao et al., 2009b 7B: Representative microphotographs of MCs immersed in culture medium (left) supernatant of AMO equilibrated with P_O2 70 Torr (center) and supernatant of peritoneal macrophages equilibrated with P_O2 5 Torr. MC degranulation is evidenced by uptake of Ruthenium Red. Adapted from Chao et al., 2009b

Figure 8

Cytokine and chemokine concentration in the supernatant of primary AMO cultures during normoxia and after 30 min of equilibration with P_O2 70 Torr.

Figure 9

The left vertical axis represents the plasma concentration of MCP-1 in conscious intact rats (black triangles) and in AMO-depleted rats (gray circles) during normoxia and hypoxia (10% O₂ breathing). The left axis represents the percentage of degranulated MCs immersed in the plasma of the samples obtained in both groups of rats. Black vertical bars: intact rats; gray vertical bars, AMO-depleted rats. Adapted from Chao et al., 2010

Figure 10

MCP-1 concentration-dependence of Mc degranulation: The percentage of degranulated MCs observed after immersion in normoxic rats plasma (NX plasma), supernatant of primary AMO cultures equilibrated in normoxia (Nx AMO supernatant) and serum-free DMEM cell culture medium (DMEM) are plotted against the concentrations of MCP-1 added. Adapted from Chao et al., 2010