Heat-stable alkaline phosphatase as a marker for human and monkey type-I pneumocytes

The expression of the heat-stable isoenzyme of alkaline phosphatase in the human and monkey (Macaca mulatta, M. fascicularis) lung was investigated at the light- and electron-microscopic level, using cytochemical techniques and immunocytochemical procedures based on monoclonal and polyclonal antibodies against human term-placental alkaline phosphatase. Both in man and monkey, the enzyme was present in type-I pneumocytes. In the monkey, the enzyme was found in all type-I cells. In man, strong staining was observed only in some type-I cells and in certain cuboidal respiratory bronchiolar cells. Staining was localized on the apical and basal plasma membrane, in apical and basal caveolae, and in the underlying basement membrane. The level of heat-stable alkaline phosphatase expression in the human lung was 10-fold lower than in the monkeys studied. In human fetal lung, the onset of heat-stable alkaline phosphatase expression was associated with the development of the alveolar epithelium from 17-20 weeks gestation onward. It is concluded that: (1) heat-stable alkaline phosphatase is a specific constituent of type-I pneumocytes in man and monkeys; and (2) its subcellular localization may explain its rapid appearance in the circulation under certain conditions.