Regulatory T cells (T(reg) cells) are required for peripheral tolerance. Evidence indicates that T(reg) cells can adopt specialized differentiation programs in the periphery that are controlled by transcription factors usually associated with helper T cell differentiation. Here we demonstrate that expression of the transcription factor Blimp-1 defined a population of T(reg) cells that localized mainly to mucosal sites and produced IL-10. Blimp-1 was required for IL-10 production by these cells and for their tissue homeostasis. We provide evidence that the transcription factor IRF4, but not the transcription factor T-bet, was essential for Blimp-1 expression and for the differentiation of all effector T(reg) cells. Thus, our study defines a differentiation pathway that leads to the acquisition of T(reg) cell effector functions and requires both IRF4 and Blimp-1.

During immune responses, T cells are subject to clonal competition, which leads to the predominant expansion of high-affinity clones; however, there is little understanding of how this process is controlled. We found here that the transcription factor IRF4 was induced in a manner dependent on affinity for the T cell antigen receptor (TCR) and acted as a dose-dependent regulator of the metabolic function of activated T cells. IRF4 regulated the expression of key molecules required for the aerobic glycolysis of effector T cells and was essential for the clonal expansion and maintenance of effector function of antigen-specific CD8(+) T cells. Thus, IRF4 is an indispensable molecular 'rheostat' that 'translates' TCR affinity into the appropriate transcriptional programs that link metabolic function with the clonal selection and effector differentiation of T cells.

The covalent attachment of ubiquitin to target proteins influences various cellular processes, including DNA repair, NF-κB signalling and cell survival1. The most common mode of regulation by ubiquitin-conjugation involves specialized ubiquitin-binding proteins that bind to ubiquitylated proteins and link them to downstream biochemical processes. Unravelling how the ubiquitin-message is recognized is essential because aberrant ubiquitin-mediated signalling contributes to tumour formation2. Recent evidence indicates that inhibitor of apoptosis (IAP) proteins are frequently overexpressed in cancer and their expression level is implicated in contributing to tumorigenesis, chemoresistance, disease progression and poor patient-survival 3 . Here, we have identified an evolutionarily conserved ubiquitin-associated (UBA) domain in IAPs, which enables them to bind to Lys 63-linked polyubiquitin. We found that the UBA domain is essential for the oncogenic potential of cIAP1, to maintain endothelial cell survival and to protect cells from TNF-α-induced 11Correspondence should be addressed to P.M. or M.G.-H. (pmeier@icr.ac.uk; mads.gyrd@bric.dk). 2 Current address: Biotech Research and Innovation Centre, University of Copenhagen, Ole Maaloees Vej 5, DK-2200 Copenhagen, Denmark. 10 These authors contributed equally to this work.Note: Supplementary Information is available on the Nature Cell Biology website. COMPETING FINANCIAL INTERESTSThe authors declare no competing financial interests.Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions/ AUTHOR CONTRIBUTIONS M.G.-H. and M.D. performed all experiments, except for those in Fig. 4; M.M. and J.S. planned and performed the cIAP1 −/− MEF reconstitution assay; M.M.S. planned and performed the zebrafish reconstitution assay; L.Z., W.X. and S.L. designed, performed and supervised the cIAP1 mouse tumour assay; T.T. provided various constructs and technical support; P.C.A.F. performed sequence alignments and database searches with structural prediction algorithms; J.M.B and M.Z. performed 3D modelling and sequence analysis; M.G.-H. and P.M. designed and supervised the study and wrote the paper. NIH Public Access Author ManuscriptNat Cell Biol. Author manuscript; available in PMC 2010 February 9. IAPs are characterized by the presence of the baculovirus IAP repeat (BIR) domain 6 . In addition, some IAPs, such as XIAP, cIAP1 and cIAP2, also contain a RING finger that provides them with Ub ligase (E3) activity. Although best known for their ability to regulate caspases and apoptosis, IAPs also influence signalling pathways that lead to activation of the NF-κB pathway7 -13. For example, recent evidence indicates that cIAP1 is required to modulate NF-κB activation and suppress TNF-α-mediated apoptosis7 -9 ,13,14 . Further, reciprocal translocation of cIAP2 and MALT1 generates a cIAP2-MALT1 fusion protein that drives constitutive NF-κB activation and B-cell transformation 11,15 . Currently, little is known about how IAPs contribu...

Peptido-mimetic inhibitor of apoptosis protein (IAP) antagonists (Smac mimetics (SMs)) can kill tumour cells by depleting endogenous IAPs and thereby inducing tumour necrosis factor (TNF) production. We found that interferon-γ (IFNγ) synergises with SMs to kill cancer cells independently of TNF − and other cell death receptor signalling pathways. Surprisingly, CRISPR/Cas9 HT29 cells doubly deficient for caspase-8 and the necroptotic pathway mediators RIPK3 or MLKL were still sensitive to IFNγ/SMinduced killing. Triple CRISPR/Cas9-knockout HT29 cells lacking caspase-10 in addition to caspase-8 and RIPK3 or MLKL were resistant to IFNγ/SM killing. Caspase-8 and RIPK1 deficiency was, however, sufficient to protect cells from IFNγ/SM-induced cell death, implying a role for RIPK1 in the activation of caspase-10. These data show that RIPK1 and caspase-10 mediate cell death in HT29 cells when caspase-8-mediated apoptosis and necroptosis are blocked and help to clarify how SMs operate as chemotherapeutic agents.