IL-1β, but not FXIII-A, is actively secreted from adherent THP-1 cells. Adherent THP-1 cells were cultured with IL-4 for 5 days and then were metabolically labeled over a 12-hour period, with LPS present in the medium for the final 6 hours. Subsequently, cells were chased for 24 hours in complete medium and then challenged for 20 minutes with BzATP (200μM). IL-1β (), FXIII-A (■), and LDH were immunoprecipitated from final cell lysates (C), from the 24-hour chase medium (M) and from the medium after BzATP challenge (MBz). The distributions of radioactivity between the fractions are shown (A-B). (C) The distribution of FXIII-A and IL-1β was corrected for the distribution of immunoprecipitable ³⁵S-LDH. Data are averaged from 4 separate experiments. Error bars indicate 1 SD. Insets show representative immunoprecipitates from a single experiment. Although the bulk of labeled IL-1β was secreted into the medium over the 24-hour chase (A), greater than 85% of labeled FXIII-A remained within the cell (B). BzATP resulted in a low level of release of FXIII-A and this was enriched less than 1.5-fold with respect to LDH.