Effect of alveolar lining material-coated silica on rat alveolar macrophages

The disparity between in vitro silica cytotoxicity toward macrophages and their in vivo resistance to injury following inhalation of silica at physiologic concentrations is unresolved. It is probable that inhaled silica particles absorb a variety of biological substances including proteins and alveolar lining material (ALM) thus altering the in vivo response of the macrophage to these particles. Silica (SI) particles coated with rat ALM and uncoated SI particles were studied for their ability to injure rat alveolar macrophages (AM) in vitro. Suspensions of particles were tested at concentrations from 0 to 400 micrograms per 2 X 10(6) cells. Cytotoxicity was assessed by the percent of total cellular lactate dehydrogenase (LDH) released by AM into the culture medium during incubation. Comparable physical association by ALM-coated and uncoated SI particles with AM was shown by scanning electron microscopy combined with X-ray energy spectrometry. These data show that SI coated with ALM is effectively phagocytosed by AM in vitro but is much less cytotoxic than uncoated SI. The surfactant lipids which presumably coat inhaled SI particles in the lung may reduce or delay their toxicity for AM.