Alveolar macrophages lavaged from human lungs contain protease activity at an optimum pH of 3.0 and possibly a lesser peak of activity at pH 5.5. Protease activity measured at pH 4.1 is inhibited by purified alpha-1-antitrypsin.

Fluorescent antibody studies of human alveolar macrophages showed that alpha-1-antitrypsin is present in normal alveolar macrophages. In addition, macrophages from a patient with a homozygous deficiency of alpha-1-antitrypsin exhibited less fluorescence when incubated in autologous serum than the same macrophages incubated in normal serum. Macrophages from normal subjects showed maximal fluorescence when removed from the lung and additional incubation with serum did not increase fluorescence.

These results implicate the human alveolar macrophage as a possible source of an enzyme that may cause emphysema in patients deficient in alpha-1-antitrypsin. They also show that alpha-1-antitrypsin has access to the alveolus in normal subjects.