INTRODUCTION
Regulatory T (T reg) cells expressing the transcription factor forkhead box P3 (FOXP3, human; Foxp3, mouse) play an essential role in controlling immune responses to autoantigens, allergens, tumor antigens, transplantation antigens, and infectious agents (Hori et al., 2003; Akdis, 2006). Foxp3 is a member of the forkhead/winged-helix family of transcriptional regulators, and its expression in T reg cells is essential for their development and function (Fontenot et al., 2003; Williams and Rudensky, 2007). A spontaneous mutation of the X-linked Foxp3 gene in scurfy mice causes an autoimmune-like disease, whereas the mutation in humans leads to immunodysregulation, polyendocrinopathy, enteropathy, and X-linked syndrome that is also a severe multiorgan autoimmune disease with hyper-IgE (Ziegler, 2006). 
Although the essential role of Foxp3 in central and peripheral tolerance has been extensively studied, its regulation, cooperation with other transcription factors, and how it functions in inducible T reg (iT reg) cells to suppress various target genes is mostly not yet understood. It is known that Foxp3 cooperates with the nuclear factor of activated T cells (NFAT) or nuclear factor-kappa B (NF-kappaB) to regulate the transcription of different target genes (Schubert et al., 2001; Bettelli et al., 2005; Wu et al., 2006). The Th2 cytokine IL-4 inhibits FOXP3 expression during T cell priming. GATA3 binds to the FOXP3 promoter and can repress the FOXP3 trans-activation process directly in Th2 cells (Mantel et al., 2007). It was further demonstrated that both Th1 and Th2 transcription factors T-bet and GATA3 oppose peripheral induction of Foxp3+ T reg cells in mice through STAT1-, STAT4-, and STAT6-dependent pathways (Wei et al., 2007). Although natural T reg (nT reg) cells that differentiate in the thymus are characterized by their stable Foxp3 expression, the generation of iT reg cells specific for allergens, alloantigens, and autoantigens in the periphery has been associated with a transient Foxp3+ phenotype (Fontenot et al., 2003; Hori et al., 2003). The crucial role of TGF-beta in their generation has been demonstrated. 
The RUNX gene family (Runt-related transcription factor, acute myeloid leukemia [AML], core-binding factor-alpha [CBFalpha], and polyoma enhancer-binding protein-2alpha [PEBP2alpha]) contains three members, RUNX1 (AML1/CBFA2/PEBP2alphaB), RUNX2 (AML3/CBFA1/ PEBP2alphaA), and RUNX3 (AML2/CBFA3/PEBP2alphaC). They are essential transcriptional regulators of different developmental pathways. RUNX2 is mostly important for bone development and osteoblast differentiation (Komori et al., 1997). RUNX1 plays an important role in hematopoiesis during development, and RUNX3 has important functions in thymogenesis and neurogenesis (Wang et al., 1996; Inoue et al., 2002; Levanon et al., 2002). RUNX1 and RUNX3 also work together in the establishment of lineage specification of T lymphocytes (Taniuchi et al., 2002; Egawa et al., 2007). RUNX1 is a frequent target for chromosomal translocations associated with leukemias (Look, 1997), and RUNX3 methylation and silencing is observed in various human epithelial cancers (Blyth et al., 2005). 
RUNX family members share the Runt domain, which is responsible for DNA binding (Ito, 1999). The Runt domain-containing protein constitutes the alpha-chain partner of the heterodimeric CBF complex. RUNX proteins heterodimerize with the non-DNA-binding partner, CBFbeta, which increases the affinity for DNA binding and stabilizes the complex by preventing ubiquitin-dependent degradation (Wang et al., 1993). The CBF complexes regulate the expression of cellular genes through binding to promoters or enhancer elements. The effects of the RUNX-CBFbeta complex regulation are clearly cell lineage and stage specific. They include the crucial choices between cell-cycle exit and continued proliferation, as well as between cell differentiation and self-renewal (Blyth et al., 2005). 
Because of the involvement of RUNX mutations in different autoimmune diseases and the known interaction with TGF-beta, we investigated the impact of RUNX1 and RUNX3 on the expression of FOXP3 and subsequently on the development and function of iT reg cells. This study demonstrates that RUNX1 and RUNX3 induced by TGF-beta are involved in the development and suppressive function of Foxp3+ iT reg cells. 
