Alveolar capillary dysplasia: absence of CD117 immunoreactivity of putative hemangioblast precursor cells.

Alveolar capillary dysplasia is a rare cause of irreversible persistent pulmonary hypertension in newborns resulting from failure of formation of peripheral capillary loops with consequent reduction in the blood-gas barrier. The basic defect in morphogenesis is unknown, although it is postulated that there is a structural abnormality of the hemangioblast precursors of the primitive lung mesenchyme in the septal regions of the developing lung leading to abnormal vasculogenesis. Two cases of alveolar capillary dysplasia evaluated immunohistochemically showed uniform CD117 negativity in the septal interstitial cells forming the pulmonary capillaries. A series of 12 control cases showed positive immunoreactivity for CD117 in a subset of septal interstitial cells forming the putative hemangioblast precursor cells. This novel finding is a useful adjunctive diagnostic tool and may support the hypothesis that absence of putative CD117-positive hemangioblast precursor cells is a key structural defect. This defect results in abnormal vasculogenesis and consequent failure of formation of an adequate blood-gas barrier.