Respirable Quartz and Kaolin Alumino-silicate Induction of In Vitro Cytotoxicity and Apoptosis in the Presence of Surfactant or Serum: Caveats to Bioassay Interpretation

Respirable sized quartz and kaolin dusts at concentrations from 0.25 to 1.0 mg/ml were comparably cytotoxic measured as lactate dehydrogenase (LDH) release from lavaged rat lung macrophages challenged for 2 h in serum-free medium. Kaolin was about twice as active as quartz on a mass basis and about half as active on a surface area basis. Use of fetal bovine serum (FBS) in the medium reduced this activity for both dusts in a serum concentration-dependent manner. Using rat alveolar macrophage-derived NR8383 cells in medium containing 10% FBS, quartz dust challenge for 6 h at dust concentrations from 50 to 400 μg/ml induced significant and dust concentration-dependent necrosis, as measured by LDH release, and apoptosis, as measured by the terminal deoxynucleotidyl transferase-mediated dUTP–fluorescein nick end DNA labeling assay. Under these same conditions, kaolin dust was significantly active only at the highest dust concentration. Challenge at an intermediate concentration of 100 μg/ml over time periods of 6 h to 5 days produced significant LDH release with quartz at all times, while kaolin-induced activity was significant only at 3 and 5 days and was not as strong as quartzinduced activity at those times. Pretreatment of quartz with dipalmitoyl phosphatidylcholine (DPPC), to model conditioning of respired dust surfaces by interaction with a primary phospholipid component of the pulmonary surfactant, further suppressed quartz activity in the FBS system over 3 days, with no additional DPPC prophylactic effect seen at 5 days. No additional prophylactic effect of DPPC was seen for kaolin in the FBS system. In vitro assays of respirable particulate necrotic or apoptotic activities can be significantly affected by non-physiologically meaningful surface modifications of dusts occurring in a cellular test system, as observed here for serum in cell culture medium. At the same time, interpretation of in vitro assays may be limited if in vivo physiological modification of particle surfaces, such as adsorption of prophylactic components of lung surfactant as studied here, is not modeled in the experimental design.