Constitutive and stimulated expression of ICAM-1 protein on pulmonary endothelial cells in vivo

The expression of intercellular adhesion molecule-1 (ICAM-1) on pulmonary endothelial cells after stimulus and subsequent binding of neutrophils is a first step leading to lung injury. A similar process may dictate the binding of tumor cells to the pulmonary endothelium during metastasis. We report the development of a new technique that allowed us to monitor the location and relative expression of ICAM-1 levels on the luminal surface of the pulmonary microvasculature in vivo. This technique uses intravital microscopy together with a two-step labeling procedure involving fluorescent microspheres. Constitutive expression of ICAM-1 was not detectable to a significant level by our model, but expression was observed after upregulation by the systemic administration of TNF alpha. Sprague-Dawley rats were injected with 0-5.0 micrograms/kg TNF alpha and ICAM-1 expression was monitored through 24 hr. ICAM-1 expression was related to both the dose of TNF alpha administered and the time elapsed between injection of TNF alpha and observation. Injection of 5 micrograms/kg TNF alpha caused upregulation of ICAM-1 protein expression from 0.30 +/- 2.76 binding events/175,000 microns2 to 62.6 +/- 5.48 through 4 hr observation, after which levels returned to near baseline within 24 hr. The delay required for maximal expression is likely related to the time required for the cell to respond to the stimulus and generate ICAM-1 protein. Reductions in the relative numbers of ICAM-1 protein expressed between 4 and 24 hr in vivo are likely a result of protein turnover after the initial stimulus.