Dose response of TNF-α mRNA production by THP-1 cells stimulated with Stx1. (A) Differentiated THP-1 cells were incubated with the indicated concentrations of Stx1 for 12 hours. The cells were lysed and total RNA was extracted. Northern blot analysis was performed to determine the levels of TNF-α mRNA and 18S RNA. (B) The values shown are the ratios of cpm TNF-α mRNA:cpm 18 S RNA of one representative blot from three separate experiments. All test values are statistically different from unstimulated cells by Student's paired t-test (P < .1).