C/EBP beta Mediates Synergistic Upregulation of Gene Expression by Interferon-gamma and Tumor Necrosis Factor-alpha in Bone Marrow-Derived Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are potent immunoregulators and have shown clinical utility in suppressing immunity. MSC function is modulated by cytokines, since in. ammatory cytokines, such as interferon-gamma (IFN gamma) concomitant with tumor necrosis factor-alpha (TNF alpha), induce their immunoregulatory capability. Here, we show that IFN gamma and TNF alpha act synergistically to induce high levels of expression of interleukin-6 (IL-6) and several other immune-related molecules in MSCs in vitro. We further found that, while either IFN gamma or TNF alpha alone induced minor expression of C/EBP beta in MSCs, this transcription factor was dramatically upregulated when these cytokines were added together. A causal relationship between C/EBP beta upregulation and IL-6 expression was demonstrated by small interfering RNA knockdown of C/EBP beta. C/EBP beta knockdown also inhibited the synergistic expression of CXCL1, inducible nitric oxide synthase, and CCL5 in response to concomitant IFN gamma and TNF alpha. We conclude that C/EBP beta is a key transcription factor in synergistic gene upregulation by IFN gamma and TNF alpha. Importantly, C/EBP beta similarly mediated synergistic gene induction in response to IFN gamma accompanied by IL-1 beta or lipopolysaccharide, suggesting that synergy between IFN gamma and other stimuli share C/EBP beta as common mechanism. Furthermore, while STAT1 is critical in IFN gamma signaling, we found that STAT1 knockdown in MSCs did not affect C/EBP beta expression or the synergistic induction of IL-6 and CXCL1 by IFN gamma and TNF alpha. Thus, C/EBP beta is not regulated by STAT1. These results demonstrate the importance of cytokine interactions in MSC immunobiology, a better understanding of which will allow improved clinical application of these cells. STEM CELLS 2009;27:942-948