Molecular Pathways of Monocyte Emigration into the Alveolar Air Space of Intact Mice

Abstract

The adhesive interactions involved in monocyte recruitment to the alveolar space in vivo are only poorly defined. To study these interactions, we used a recently developed mouse model that allowed the separation and quantification of freshly recruited monocytes, resident alveolar macrophages (rAM), neutrophils, and lymphocytes in the bronchoalveolar compartment by fluorescence activated cell sorting technology. In these mice, the combined intratracheal administration of the monocyte chemoattractant JE/monocyte chemotactic protein (MCP)-1 and low dose Escherichia coli lipopolysaccharide (LPS) induces a self-limiting pulmonary inflammatory response, characterized by well-controlled sequelae of both neutrophil and monocyte emigration into the alveolar space. In contrast, challenge with JE/MCP-1 provokes the emigration only of monocytes in the absence of lung inflammation. Using an array of function-blocking monoclonal antibodies (mAb) (anti-CD11a, -CD11b, -CD18, -CD49d, -CD54, and -CD106), we characterized the adhesive interactions underlying the transendothelial and transepithelial leukocyte traffic in intact animals. Alveolar monocyte recruitment elicited by JE/MCP-1 alone was strictly dependent on CD11b/CD18, CD54, and CD49d, and partly dependent on CD11a, but not dependent on CD106. In response to JE/MCP-1 plus E. coli LPS, we observed additional engagement of CD11a and CD106 for enhanced alveolar monocyte transmigration. Comigrating neutrophils were found to primarily utilize CD11b, CD18, and CD54, but not CD49d, CD106, or, surprisingly, CD11a. This contrasted with the effect of CD11a on alveolar challenge with macrophage inflammatory protein (MIP)-1alpha instead of JE/MCP-1. In conclusion, we found that in an intact mouse model allowing detailed phenotyping of leukocyte traffic into the alveolar space, the molecular pathways involved in JE/MCP-1-driven monocyte efflux differed under noninflammatory and inflammatory (presence of LPS) conditions. Moreover, the profile of adhesive interactions underlying the monocyte efflux differed from that characterizing neutrophil trafficking.

Alveolar Epithelial Cells Direct Monocyte Transepithelial Migration upon Influenza Virus Infection: Impact of Chemokines and Adhesion Molecules

“…In the present study, ICAM-1 and VCAM-1 expression was found to be up-regulated on the surface of AEC upon influenza A virus infection. Moreover, our inhibition experiments for the first time revealed that monocytes predominantly use CD49d/VCAM-1 adhesion molecule pathways to transmigrate influenza virus-infected epithelium, thereby supporting the role of VLA-4/VCAM-1 interactions as a central molecular pathway in inflammatory monocyte trafficking both in vitro and in vivo (28,35). Since monocyte transmigration could not be blocked completely by either of the employed Abs, involvement of additional adhesion pathways like monocyte integrin interaction with extracellular matrix proteins cannot be excluded (40).…”

Monocytes Are Potent Facilitators of Alveolar Neutrophil Emigration During Lung Inflammation: Role of the CCL2-CCR2 Axis

“…These groups included WT mice, CCR2-deficient mice, chimeric WT mice (lethally irradiated WT mice reconstituted with bone marrow cells from CCR2-deficient mice), and chimeric CCR2-deficient mice (lethally irradiated CCR2-deficient mice reconstituted with WT bone marrow cells). Mice were challenged by intratracheal instillation of CCL2 (50 g/mouse), LPS (10 ng/mouse), or the combination of CCL2 (50 g/mouse) and LPS (10 ng/mouse) for various times (0, 6, 12, 24, and 48 h), according to protocols previously described in detail (2,5,10).…”

“…Although chemokine release and cell surface display of complementary leukocyte and endothelial/epithelial adhesion molecules are centrally involved in these processes (1)(2)(3), the underlying mechanism(s) that shapes the kinetics and extent of neutrophil and monocyte recruitment is unclear (1,4). In recent investigations in which we used a mouse model of acute lung inflammation provoked by intratracheal application of combined LPS and CCL2 to mimic elevated intra-alveolar levels of CCL2 observed in septic adult respiratory distress syndrome patients (3), the successive waves of early alveolar neutrophil and delayed alveolar monocyte recruitment were resolved in detail (5,6).…”

All‐trans retinoic acid down‐regulates expression and function of β2 integrins by human monocytes: opposite effects on monocytic cell lines

“…The establishment of various homotypic and heterotypic adhesion events by monocytes is known to rely primarily on this subfamily of integrin receptors [14]. While their adhesion to activated EC is mediated to high extent by LFA-1 along with Mac-1 [15,16], Mac-1 and more importantly p150,95 are of primary relevance to cell adhesion and spreading on tissue culture plates, chemotaxis and phagocytosis [17,18].…”