Expression of endothelial cell-derived tissue plasminogen activator (tPA) protein has been found to be restricted to the endothelia of a discrete group of vessels. While few or no vessels of the brain, heart, kidney, liver, or thymus are positive for this protein, the bronchial arteries show consistent immunostaining, regardless of size, in both mouse and primate lung.

In the primate, positive staining is never present in the endothelium of the aortic lumen but appears in the bronchial circulation within the mainstem bronchus, suggesting that tPA expression begins at or after the branching points where the intercostal or bronchial arteries form. In the mouse, tPA was also present in a small fraction of the vessels of the pulmonary circulation. These were consistently between 7 and 30 [micro] m in diameter. No capillary or large vessel endothelium stains were positive. Conditions known to induce acute pulmonary inflammation increased the number of pulmonary vessels showing tPA antigen, with 96% between 7 and 30 [micro] m. Again, none of the large vessels or capillary endothelia had tPA. The correlation between vessel size and tPA expression was reinforced by the appearance of tPA antigen in small vessels ([is less than] 30 [micro] m) as they branch from larger pulmonary vessels. The increase in tPA expression preceded morphologic indication of acute inflammation; thickening of the alveolar septa, neutrophil diapedesis, fibrin deposition in alveolar spaces, and pulmonary edema were not observed as the level of tPA expression rose.