KDR and chronic obstructive pulmonary disease: Since the less mature progenitors (CD34+KDR+CD133+) were significantly reduced only in severe COPD, while the CD34+KDR+ cells were highly significant even after adjustment, these findings support the hypothesis that CD34+KDR+ cells were being preferentially drawn from the circulation during mild and moderate COPD (increased utilization) and it is not until severe COPD that the more bone marrow derived cells (CD34+KDR+CD133+) become depleted (decreased supply).