PECAM1 and chronic obstructive pulmonary disease: Using immunofluorescence to stain for the endothelial cell marker CD31 (Fig. 2A), we found that COPD and COPD-PH individuals had significantly decreased endothelial cell abundance in small pulmonary arteries (p = 0.008 each) and the microvasculature (p = 0.004 and 0.001, respectively) compared to non-smokers and chronic smokers without COPD (Fig. 2B, C).