Cell-cell contact is required for the production of NO in MΦ/mAo cell co-cultures. MΦ and mAo MSCs were grown separated by a 0.4-μm filter (A) and activated with LPS (100 ng/mL) or with LPS (100 ng/mL) + IFNγ (250 ng/mL) for 24 hours. Supernatant of the mAo cells grown in the well and supernatant from the MΦ cells grown in the transwell were analyzed for NO content separately (B). Data are presented as mean ± standard error of the mean and are representative of three experiments each with n = 4. *Significantly different from both transwell and well compartments. IFNγ, interferon-gamma; LPS, lipopolysaccharide; MΦ, bone marrow-derived macrophage; mAo, mouse aorta-derived mesenchymal progenitor; MSC, mesenchymal stem cell; NO, nitric oxide.