Ajithkumar Vasanthakumar

The transcriptional regulators IRF4, BATF and IL-33 orchestrate development and maintenance of adipose tissue-resident regulatory T cells

Foxp3+ regulatory T (Treg) cells in visceral adipose tissue (VAT-Treg cells) are functionally specialized tissue-resident cells that prevent obesity-associated inflammation and preserve insulin sensitivity and glucose tolerance. Their development depends on the transcription factor PPAR-γ; however, the environmental cues required for their differentiation are unknown. Here we show that interleukin 33 (IL-33) signaling through the IL-33 receptor ST2 and myeloid differentiation factor MyD88 is essential for development and maintenance of VAT-Treg cells and sustains their transcriptional signature. Furthermore, the transcriptional regulators BATF and IRF4 were necessary for VAT-Treg differentiation through direct regulation of ST2 and PPAR-γ expression. IL-33 administration induced vigorous population expansion of VAT-Treg cells, which tightly correlated with improvements in metabolic parameters in obese mice. Human omental adipose tissue Treg cells also showed high ST2 expression, suggesting an evolutionarily conserved requirement for IL-33 in VAT-Treg cell homeostasis.

IL-27 and IL-12 oppose pro-inflammatory IL-23 in CD4+ T cells by inducing Blimp1

Central nervous system (CNS) autoimmunity is regulated by the balance of pro-inflammatory cytokines and IL-10. Here we identify the transcriptional regulator Blimp1 as crucial to induce IL-10 in inflammatory T helper cells. Pre-committed Th17 cells respond to IL-27 and IL-12 by upregulating Blimp1 and adopt a Tr-1-like phenotype characterized by IL-10 and IFN-γ production. Accordingly, Blimp1-deficient effector T cells fail to produce IL-10, and deficiency in Tr-1 cell function leads to uncontrolled Th17 cell-driven CNS pathology without the need to stabilize the Th17 phenotype with IL-23. IL-23 counteracts IL-27 and IL-12-mediated effects on Tr-1-development reinforcing the pro-inflammatory phenotype of Th17 cells. Thus, the balance of IL-23 vs IL-12/IL-27 signals into CD4(+) effector T cells determines whether tissue inflammation is perpetuated or resolves.

NF-κB subunit specificity in hemopoiesis

Summary:  Although the diverse functions served by the nuclear factor‐κB (NF‐κB) pathway in virtually all cell types are typically employed to deal with stress responses, NF‐κB transcription factors also play key roles in the development of hemopoietic cells. This review focuses on how NF‐κB transcription factors control various aspects of thymic T‐cell and myeloid cell differentiation that include its roles in hemopoietic precursors, conventional αβ T cells, CD4+ regulatory T cells, natural killer T cells, γδ T cells, macrophages, and dendritic cells.

c-Rel Controls Multiple Discrete Steps in the Thymic Development of Foxp3+ CD4 Regulatory T Cells

The development of natural Foxp3+ CD4 regulatory T cells (nTregs) proceeds via two steps that involve the initial antigen dependent generation of CD25+GITRhiFoxp3−CD4+ nTreg precursors followed by the cytokine induction of Foxp3. Using mutant mouse models that lack c-Rel, the critical NF-κB transcription factor required for nTreg differentiation, we establish that c-Rel regulates both of these developmental steps. c-Rel controls the generation of nTreg precursors via a haplo-insufficient mechanism, indicating that this step is highly sensitive to c-Rel levels. However, maintenance of c-Rel in an inactive state in nTreg precursors demonstrates that it is not required for a constitutive function in these cells. While the subsequent IL-2 induction of Foxp3 in nTreg precursors requires c-Rel, this developmental transition does not coincide with the nuclear expression of c-Rel. Collectively, our results support a model of nTreg differentiation in which c-Rel generates a permissive state for foxp3 transcription during the development of nTreg precursors that influences the subsequent IL-2 dependent induction of Foxp3 without a need for c-Rel reactivation.

Modulating T regulatory cells in cancer: how close are we?

Regulatory T cells (Tregs) are a specialized subset of CD4 T cells that have an indispensable role in maintaining immune homeostasis and tolerance. Although studies in mice and humans have clearly highlighted that the absence of these cells results in severe autoimmunity and inflammation, increased Treg numbers and/or function is not always beneficial. This is best exemplified in certain cancers where increased Tregs promote cancer progression by interfering with immune surveillance. Conversely, in other types of cancers that have an inflammatory component, Tregs can inhibit cancer progression by dampening inflammation. In this review article, we provide a historical perspective of the discovery of Tregs, followed by a summary of the existing literature on the role of Tregs in malignancy.

IL‐27 paves different roads to Tr1

Tr1 cells are non‐Foxp3‐expressing regulatory CD4+ T cells that execute suppressor functions by secreting the anti‐inflammatory cytokine IL‐10. Differentiation of this T‐cell subset is facilitated by the heterodimeric cytokine IL‐27, which can activate transcription factors such as c‐Maf and Ahr to positively regulate the differentiation of Tr1 cells and their IL‐10 production. In this issue of the European Journal of Immunology, an alternate transcriptional network regulated by IL‐27 to induce IL‐10 production in Tr1 cells is reported by Iwasaki et al. [Eur. J. Immunol. 2013. 43: 1063‐1073]. This study shows that IL‐27 initiates tandem activation of the transcription factors STAT3 and Egr‐2 to induce il10 in Tr1 cells in a Blimp1‐dependent fashion. These findings indicate a c‐Maf/Ahr independent mechanism that activates IL‐10 production by Tr1 cells and suggest that Il10 induction may depend on both the cytokine environment and the molecular context. Thus, Tr1 cells may be another example of the remarkable plasticity of CD4+ T cells and indeed may not constitute a separate lineage of CD4+ T cells but rather represent a developmental endpoint of several T helper cell differentiation pathways.

Regulation of daunorubicin biosynthesis in Streptomyces peucetius – feed forward and feedback transcriptional control

Streptomyces are a major group of soil bacteria that produce wide range of bioactive compounds including antibiotics. Daunorubicin is a chemotherapeutic agent for treatment of certain types of cancer, which is produced as a secondary metabolite by S. peucetius. Owing to the significance of this drug in treating cancer, understanding the molecular mechanism of its biosynthesis will assist in the genetic manipulation of this strain for better drug yields. Additionally, the knowledge can also be applied to design hybrid antibiotics that can be made in vivo by transferring genes from one Streptomyces species to another. Biosynthesis of daunorubicin in S. peucetius is accomplished by the function of 30 enzyme‐coding genes in a sequential and coordinated fashion. In addition to these enzymes, three transcriptional regulators DnrO, DnrN and DnrI regulate this multi‐step process by forming a coherent feed forward loop regulatory circuit, consequently controlling the entire enzyme coding genes. Since daunorubicin is a DNA intercalating drug, maintaining an optimal intracellular drug concentration is pivotal to prevent self‐toxicity. Commencement of daunorubicin biosynthesis also activates the feedback mechanisms mediated by the metabolite. At exceeding intracellular concentrations, daunorubicin intercalates into DNA sequences and impedes the binding of these transcription factors. This feedback repression is relieved by a group of self‐resistance genes, which concurrently efflux the excess intracellular daunorubicin. This review will discuss the mechanistic role of each transcription factor and their interplay in initiating and maintaining the biosynthesis of daunorubicin in S. peucetius.

The BCL-2 pro-survival protein A1 is dispensable for T cell homeostasis on viral infection

The physiological role of the pro-survival BCL-2 family member A1 has been debated for a long time. Strong mRNA induction in T cells on T cell receptor (TCR)-engagement suggested a major role of A1 in the survival of activated T cells. However, the investigation of the physiological roles of A1 was complicated by the quadruplication of the A1 gene locus in mice, making A1 gene targeting very difficult. Here, we used the recently generated A1−/− mouse model to examine the role of A1 in T cell immunity. We confirmed rapid and strong induction of A1 protein in response to TCR/CD3 stimulation in CD4+ as well as CD8+ T cells. Surprisingly, on infection with the acute influenza HKx31 or the lymphocytic choriomeningitis virus docile strains mice lacking A1 did not show any impairment in the expansion, survival, or effector function of cytotoxic T cells. Furthermore, the ability of A1−/− mice to generate antigen-specific memory T cells or to provide adequate CD4-dependent help to B cells was not impaired. These results suggest functional redundancy of A1 with other pro-survival BCL-2 family members in the control of T cell-dependent immune responses.

A non‐canonical function of Ezh2 preserves immune homeostasis

Enhancer of zeste 2 (Ezh2) mainly methylates lysine 27 of histone‐H3 (H3K27me3) as part of the polycomb repressive complex 2 (PRC2) together with Suz12 and Eed. However, Ezh2 can also modify non‐histone substrates, although it is unclear whether this mechanism has a role during development. Here, we present evidence for a chromatin‐independent role of Ezh2 during T‐cell development and immune homeostasis. T‐cell‐specific depletion of Ezh2 induces a pronounced expansion of natural killer T (NKT) cells, although Ezh2‐deficient T cells maintain normal levels of H3K27me3. In contrast, removal of Suz12 or Eed destabilizes canonical PRC2 function and ablates NKT cell development completely. We further show that Ezh2 directly methylates the NKT cell lineage defining transcription factor PLZF, leading to its ubiquitination and subsequent degradation. Sustained PLZF expression in Ezh2‐deficient mice is associated with the expansion of a subset of NKT cells that cause immune perturbation. Taken together, we have identified a chromatin‐independent function of Ezh2 that impacts on the development of the immune system.

Development and Function of Effector Regulatory T Cells.

Distinguishing self from nonself is a unique feature of the immune system. Although most self-reactive T cells are eliminated in the thymus, a few rogue cells escape the negative selection process and have the potential to mediate autoimmune disease. Over the last decade, there has been a vast improvement in our understanding of the cellular mechanisms that evolved to dampen the deleterious effects of these self-reactive T cells. In particular, T cells expressing the transcription factor FoxP3, known as regulatory T (Treg) cells, play a central role in maintaining immune homeostasis and suppressing autoimmune responses. In addition, Treg cells are endowed with the ability to suppress diverse inflammatory responses both in lymphoid and in nonlymphoid tissues. This requires Treg cells to undergo a peripheral differentiation and specialization program that results in the emergence of effector Treg (eTreg) cells that are characterized by their ability to produce high amounts of immunosuppressive molecules, including IL-10. This chapter discusses the recent advances in our understanding of the mechanisms governing the differentiation, migration, and maintenance of eTreg cells, in particular in nonlymphoid tissues, in health and disease.