Kinetics of TNF-α mRNA induction in THP-1 cells stimulated with Stx1. Differentiated THP-1 cells were incubated with 400 ng/mL of Stx1, 200 ng/mL of LPS, or a combination of both for the time points indicated. The cells were lysed and total RNA was extracted. Northern blot analysis was performed to determine the levels of TNF-α mRNA and 18 S RNA. The ratios of cpm TNF-α mRNA:cpm 18 S RNA were calculated as detailed in the Materials and Methods. The data shown are the mean cpm ± SEM from three separate experiments.