Cutting Edge: Core Binding Factor ß Is Required for Group 2 Innate Lymphoid Cell Activation.

Group 2 innate lymphoid cells (ILC2) are tissue-resident, long-lived innate effector cells implicated in allergy and asthma. Upon activation, mature ILC2 rapidly secrete large amounts of type-2 cytokines and other effector molecules. The molecular pathways that drive ILC2 activation are not well understood. In this study, we report that the transcriptional controller core binding factor ß (CBFß) is required for ILC2 activation. Deletion or inhibition of CBFß did not impair the maintenance of ILC2 at homeostasis but abolished ILC2 activation during allergic airway inflammation. Treatment with CBFß inhibitors prevented ILC2-mediated airway hyperresponsiveness in a mouse model of acute Alternaria allergen inhalation. CBFß promoted expression of key ILC2 genes at both transcriptional and translational levels. CBF transcriptional complex directly bound to Il13 and Vegfa promoters and enhancers, and controlled gene transcription. CBFß further promoted ribosome biogenesis and enhanced gene translation in activated ILC2. Together, these data establish an essential role for CBFß in ILC2 activation.

Runx/Cbfß complexes protect group 2 innate lymphoid cells from exhausted-like hyporesponsiveness during allergic airway inflammation.

Group 2 innate lymphoid cells (ILC2s) have tissue-resident competence and contribute to the pathogenesis of allergic diseases. However, the mechanisms regulating prolonged ILC2-mediated TH2 cytokine production under chronic inflammatory conditions are unclear. Here we show that, at homeostasis, Runx deficiency induces excessive ILC2 activation due to overly active GATA-3 functions. By contrast, during allergic inflammation, the absence of Runx impairs the ability of ILC2s to proliferate and produce effector TH2 cytokines and chemokines. Instead, functional deletion of Runx induces the expression of exhaustion markers, such as IL-10 and TIGIT, on ILC2s. Finally, these 'exhausted-like' ILC2s are unable to induce type 2 immune responses to repeated allergen exposures. Thus, Runx confers competence for sustained ILC2 activity at the mucosa, and contributes to allergic pathogenesis.

Cbfß2 controls differentiation of and confers homing capacity to prethymic progenitors.

Multipotent hematopoietic progenitors must acquire thymus-homing capacity to initiate T lymphocyte development. Despite its importance, the transcriptional program underlying this process remains elusive. Cbfß forms transcription factor complexes with Runx proteins, and here we show that Cbfß2, encoded by an RNA splice variant of the Cbfb gene, is essential for extrathymic differentiation of T cell progenitors. Furthermore, Cbfß2 endows extrathymic progenitors with thymus-homing capacity by inducing expression of the principal thymus-homing receptor, Ccr9. This occurs via direct binding of Cbfß2 to cell type-specific enhancers, as is observed in Rorγt induction during differentiation of lymphoid tissue inducer cells by activation of an intronic enhancer. As in mice, an alternative splicing event in zebrafish generates a Cbfß2-specific mRNA, important for ccr9 expression. Thus, despite phylogenetically and ontogenetically variable sites of origin of T cell progenitors, their robust thymus-homing capacity is ensured by an evolutionarily conserved mechanism emerging from functional diversification of Runx transcription factor complexes by acquisition of a novel splice variant.

CBFß and HIV Infection.

In order to achieve a persistent infection, viruses must overcome the host immune system. Host restriction factors dominantly block virus transmission, but are subject to down regulation by viral accessory proteins. HIV encodes several accessory factors that overcome different cellular restriction factors. For example, the HIV-1 protein Vif down regulates the human APOBEC3 family of restriction factors by targeting them for proteolysis by the ubiquitin-proteasome pathway. Recently, this function was shown to require the transcription cofactor CBFß, which acts as a template to assist in Vif folding and allow for assembly of an APOBEC3-targeting E3 ligase complex. In uninfected cells, CBFß is an essential binding partner of RUNX transcription factors. By binding CBFß, Vif has also been shown to perturb transcription of genes regulated by the RUNX proteins, including restrictive APOBEC3 family members. Here we review how the link between CBFß and Vif supports transcriptional and post-transcriptional repression of innate immunity. The ability of a single viral protein to coopt multiple host pathways is an economical strategy for a pathogen with limited protein coding capacity to achieve a productive infection.

Transcription factors RUNX1 and RUNX3 in the induction and suppressive function of Foxp3+ inducible regulatory T cells.

Forkhead box P3 (FOXP3)(+)CD4(+)CD25(+) inducible regulatory T (iT reg) cells play an important role in immune tolerance and homeostasis. In this study, we show that the transforming growth factor-beta (TGF-beta) induces the expression of the Runt-related transcription factors RUNX1 and RUNX3 in CD4(+) T cells. This induction seems to be a prerequisite for the binding of RUNX1 and RUNX3 to three putative RUNX binding sites in the FOXP3 promoter. Inactivation of the gene encoding RUNX cofactor core-binding factor-beta (CBFbeta) in mice and small interfering RNA (siRNA)-mediated suppression of RUNX1 and RUNX3 in human T cells resulted in reduced expression of Foxp3. The in vivo conversion of naive CD4(+) T cells into Foxp3(+) iT reg cells was significantly decreased in adoptively transferred Cbfb(F/F) CD4-cre naive T cells into Rag2(-/-) mice. Both RUNX1 and RUNX3 siRNA silenced human T reg cells and Cbfb(F/F) CD4-cre mouse T reg cells showed diminished suppressive function in vitro. Circulating human CD4(+) CD25(high) CD127(-) T reg cells significantly expressed higher levels of RUNX3, FOXP3, and TGF-beta mRNA compared with CD4(+)CD25(-) cells. Furthermore, FOXP3 and RUNX3 were colocalized in human tonsil T reg cells. These data demonstrate Runx transcription factors as a molecular link in TGF-beta-induced Foxp3 expression in iT reg cell differentiation and function.

Indispensable role of the Runx1-Cbfbeta transcription complex for in vivo-suppressive function of FoxP3+ regulatory T cells.

Naturally arising regulatory T (Treg) cells express the transcription factor FoxP3, which critically controls the development and function of Treg cells. FoxP3 interacts with another transcription factor Runx1 (also known as AML1). Here, we showed that Treg cell-specific deficiency of Cbfbeta, a cofactor for all Runx proteins, or that of Runx1, but not Runx3, induced lymphoproliferation, autoimmune disease, and hyperproduction of IgE. Cbfb-deleted Treg cells exhibited impaired suppressive function in vitro and in vivo, with altered gene expression profiles including attenuated expression of FoxP3 and high expression of interleukin-4. The Runx complex bound to more than 3000 gene loci in Treg cells, including the Foxp3 regulatory regions and the Il4 silencer. In addition, knockdown of RUNX1 showed that RUNX1 is required for the optimal regulation of FoxP3 expression in human T cells. Taken together, our results indicate that the Runx1-Cbfbeta heterodimer is indispensable for in vivo Treg cell function, in particular, suppressive activity and optimal expression of FoxP3.

Runx-CBFbeta complexes control expression of the transcription factor Foxp3 in regulatory T cells.

The transcription factor Foxp3 has an indispensable role in establishing stable transcriptional and functional programs of regulatory T cells (T(reg) cells). Loss of Foxp3 expression in mature T(reg) cells results in a failure of suppressor function, yet the molecular mechanisms that ensure steady, heritable Foxp3 expression in the T(reg) cell lineage remain unknown. Using T(reg) cell-specific gene targeting, we found that complexes of the transcription factors Runx and CBFbeta were required for maintenance of Foxp3 mRNA and protein expression in T(reg) cells. Consequently, mice lacking CBFbetab exclusively in the T(reg) cell lineage had a moderate lymphoproliferative syndrome. Thus, Runx-CBFbeta complexes maintain stable high expression of Foxp3 and serve as an essential determinant of T(reg) cell lineage stability.