Mobilization and Margination of Bone Marrow Gr-1high Monocytes during Subclinical Endotoxemia Predisposes the Lungs toward Acute Injury

Abstract

The specialized role of mouse Gr-1high monocytes in local inflammatory reactions has been well documented, but the trafficking and responsiveness of this subset during systemic inflammation and their contribution to sepsis-related organ injury has not been investigated. Using flow cytometry, we studied monocyte subset margination to the pulmonary microcirculation during sub-clinical endotoxemia in mice, and investigated if marginated monocytes contribute to lung injury in response to further septic stimuli. Sub-clinical low-dose i.v. LPS induced a rapid (within 2h), large scale mobilization of bone marrow Gr-1high monocytes and their prolonged margination to the lungs. With secondary LPS challenge, membrane TNF expression on these pre-marginated monocytes substantially increased, indicating their functional priming in vivo. Zymosan challenge produced small increases in pulmonary vascular permeability, which were markedly enhanced by the pre-administration of low-dose LPS. The LPS-zymosan induced permeability increases were effectively abrogated by pre-treatment (30 min before zymosan challenge) with the platelet-activating factor (PAF) antagonist WEB 2086, in combination with the phosphatidylcholine-phospholipase C inhibitor D609, suggesting the involvement of PAF/ceramide mediated pathways in this model. Depletion of monocytes (at 18h post clodronate-liposome treatment) significantly attenuated the LPS-zymosan induced permeability increase. However, restoration of normal LPS-induced Gr-1high monocyte margination to the lungs (at 48h post clodronate-liposome treatment) resulted in the loss of this protective effect. These results demonstrate that mobilization and margination of Gr-1high monocytes during sub-clinical endotoxemia primes the lungs toward further septic stimuli, and suggest a central role for this monocyte subset in the development of sepsis-related acute lung injury.

Epigenetic modifiers reduce inflammation and modulate macrophage phenotype during endotoxemia-induced acute lung injury

“…That study mainly focuses on alveolar macrophages during normal development, rather than on a diseased tissue model. However, during ALI, the number of residential alveolar macrophages is not high enough to combat inflammation and, therefore, under the influence of colony stimulating factor (CSF), the mobilization of monocytes and/or macrophages from bone marrow to the injured alveolar site would be necessary to compensate for the requirements of macrophages (O'Dea et al, 2009). Thus, studying the mechanism of BMDMs in a mouse model of ALI is relevant.…”

HLA‐DR expression and differential trafficking of monocyte subsets following low to intermediate risk surgery*

“…Our findings are compatible with a model of CD14 high monocytes rapidly marginating to the peripheral microvasculature in response to tissue injury due to surgical stress (the initial postoperative decrease), followed by the mobilisation of similar cells from bone marrow and ⁄ or return of the marginated cells into the bloodstream (resulting in the subsequent increase at 24 h). Such a model has been developed from mouse experiments by ourselves [35,36] and others [19,37,38], demonstrating mobilisation and margination of Gr1 high monocytes (which are phenotypically similar to human CD14 high monocytes) to peripheral microvasculature in response to systemic inflammatory stimuli. In contrast, the numbers of CD14 low CD16+ monocytes further decreased at 24 h after surgery, perhaps reflecting the more irreversible movement of these cells out of the circulation such as extravasation into the tissues, similar to the behaviour of mouse Gr1 low monocytes [39] (phenotypically analogous to the human CD14 low CD16+ monocytes) in certain vascular beds.…”

Role of Lung-marginated Monocytes in an In Vivo Mouse Model of Ventilator-induced Lung Injury

“…We have previously demonstrated using mouse models that monocytes are rapidly recruited to the lung microvasculature during systemic endotoxemia and that these marginated cells contribute to the development of ALI by promoting the activation of pulmonary endothelial cells (12). Furthermore, we have shown a clear role for lung-marginated inflammatory Gr-1 high monocytes in the development of pulmonary edema in a lipopolysaccharide (LPS)/zymosan model of ALI (13).…”

“…At termination, lung cell suspensions were prepared from excised lungs by mechanical disruption for flow cytometry analysis, as described in detail previously (11)(12)(13). Samples were stained with fluorophoreconjugated anti-mouse antibodies for CD11b, Gr-1 (Ly6C/G), F4/80, L-selectin, or appropriate isotype-matched controls, and then analyzed using a FACSCalibur cytometer with CellQuest (Becton Dickinson, Oxford, UK) and FlowJo (Tree Star, Ashland, OR) software.…”

“…Recent evidence has suggested, however, that lung-recruited monocytes may also play a significant role in the pathogenesis of ALI (12,13). Monocytes are a pluripotent and heterogeneous population of cells of the mononuclear phagocytic system, capable of releasing a variety of inflammatory mediators.…”