Epithelial HO-1 regulates leukocyte-epithelial crosstalk. a Model to examine epithelial-leukocyte interactions using polarized Caco-2 intestinal epithelial cells (IECs) in the upper chamber of a Transwell insert with THP-1 monocytes in the lower chamber, activated with Salmonella Typhimurium for 6 h (for mRNA) or 24 h (for protein). b–d Expression of IL-8, CCL20, and CXCL1 mRNA from Caco-2 IECs during co-culture with activated THP-1 cells. e IL-1β secretion from THP-1 cell supernatants after 24 h, as measured by ELISA. f IL-1 β secretion from THP-1 cells co-cultured with control shRNA- and HMOX1 shRNA-transduced Caco-2 IECs after 24 h. Data represent combined results from at least 3 independent experiments. g Model to examine physiologic bacterial-epithelial activation using polarized IECs in the upper chamber of a Transwell insert exposed to enteric bacteria in the presence (or absence) of leukocytes in the lower chamber, evaluating IEC chemokine expression. h Expression of IL-8 mRNA from Caco-2 IECs 6 h after exposure of the apical surface to pathogenic bacte ria in the presence or absence of THP-1 monocytes. Combined results from 3 independent experiments. * p < 0.05; *** p < 0.001; **** p < 0.0001.