LPS induces (▧) caspase-1 activity and (▪) mIL-1β release in freshly isolated peripheral blood-derived human monocytes and HUVECs. Cells were isolated and cultured in vitro as described in the Materials and Methods and stimulated with 100 ng/mL of LPS (S minnesota Re 595). (A) Freshly isolated human monocytes exhibited a constitutive level of IL-1β release of approximately 950 pg/mL and a constitutive caspase-1 activity of 8 U/mg that both remained constant over time when cells were not stimulated (not shown). Monocytes were assayed for caspase-1 activity of lysates and mIL-1β release into the culture supernatant at the time points indicated. (B) HUVECs were assayed for caspase-1 activity of lysates and mIL-1β release into the culture supernatant after 24 hours in the absence and presence of LPS. Constitutive levels of IL-1 release and caspase-1 activity did not change over time in the absence of LPS (not shown). Shown are the mean values of three independent experiments ± SD. Asterisks indicate a statistically significant difference compared with controls ([A] were considered significant with .01 < P < .02 and [B] with P < .05, respectively).