Objective: Establish a reproducible method for the isolation and cultivation of murine pulmonary microvascular endothelium. To this end, we exploited the localized pattern of microvascular endothelial activation induced in vivo by inflammatory stimuli to isolate a subpopulation of endothelium for in vitro study.
Methods: Immunohistochemical analyses of the pulmonary vasculature of mice treated systemically with gram-negative bacterial endotoxin (LPS) demonstrated selective expression of VCAM-1 (CD106) in the endothelial lining of small collecting veins, venules, septal capillaries, and, infrequently, small arteries, which was not observed in control mice. Single cell suspensions prepared by enzymatic dissociation of peripheral lobular tissues dissected from the lungs of LPS-stimulated mice were incubated with a phycoerythrin-conjugated antimouse VCAM-1 monoclonal antibody (MK 1.91). Cells expressing this antigen were isolated by sterile fluorescence-activated cell sorting (FACS). Positive cell populations were collected and cultured for 1-2 weeks. When confluent, these primary cultures were further FACS enriched for endothelium, positively selecting for cells incorporating a fluorescent derivative of acetylated low density lipoprotein (Di-I-Ac-LDL).
Results: The resulting population of cells (mouse lung endothelial cells, MLEC) were uniformly positive for the endothelial markers von Willebrand factor, thrombomodulin, and Dil-Ac-LDL uptake. MLEC readily formed tube-like structures when cultured on Matrigel and spontaneously demonstrated a sprouting phenotype on fibronectin or collagen matrices. MLEC retained responsiveness to cytokines (IL-1 alpha, IL-1 beta, TNF alpha, IFN gamma) up to at least eight passages from primary culture and demonstrated upregulation of E-selectin (CD62E) and P-selectin (CD62P) mRNA as early as 2 hr after LPS stimulation. Characteristic temporal expression patterns of cell surface E-selectin (maximal at 4 hr and declining toward baseline by 24 hr), VCAM-1 (maximal at 6-8 hr and remaining elevated for 24-48 hr), and ICAM-1 (maximal at 6-8 hr and maintained at 24 hr) were observed when cultured MLEC were treated with recombinant murine TNF alpha or recombinant human (rh) IL-1 alpha or rhIL-1 beta. The rolling, adhesion, and transmigration of human polymorphonuclear leukocytes was markedly increased on cytokine-activated MLEC monolayers under defined flow conditions.
Conclusion: The strategy of activation-dependent isolation allows for the reproducible selection of a specific subset of microvascular endothelial cells for in vitro study. This experimental approach should further facilitate study of the functional heterogeneity of endothelium and its pathophysiologic dysfunction.