Kamran Ghoreschi

Generation of pathogenic T(H)17 cells in the absence of TGF-β signalling.

CD4+ T-helper cells that selectively produce interleukin (IL)-17 (TH17), are critical for host defence and autoimmunity. Although crucial for TH17 cells in vivo, IL-23 has been thought to be incapable of driving initial differentiation. Rather, IL-6 and transforming growth factor (TGF)-β1 have been proposed to be the factors responsible for initiating specification. Here we show that TH17 differentiation can occur in the absence of TGF-β signalling. Neither IL-6 nor IL-23 alone efficiently generated TH17 cells; however, these cytokines in combination with IL-1β effectively induced IL-17 production in naive precursors, independently of TGF-β. Epigenetic modification of the Il17a, Il17f and Rorc promoters proceeded without TGF-β1, allowing the generation of cells that co-expressed RORγt (encoded by Rorc) and T-bet. T-bet+RORγt+ TH17 cells are generated in vivo during experimental allergic encephalomyelitis, and adoptively transferred TH17 cells generated with IL-23 without TGF-β1 were pathogenic in this disease model. These data indicate an alternative mode for TH17 differentiation. Consistent with genetic data linking IL23R with autoimmunity, our findings re-emphasize the importance of IL-23 and therefore may have therapeutic implications.

Janus kinases in immune cell signaling

Summary:u2002 The Janus family kinases (Jaks), Jak1, Jak2, Jak3, and Tyk2, form one subgroup of the non‐receptor protein tyrosine kinases. They are involved in cell growth, survival, development, and differentiation of a variety of cells but are critically important for immune cells and hematopoietic cells. Data from experimental mice and clinical observations have unraveled multiple signaling events mediated by Jaks in innate and adaptive immunity. Deficiency of Jak3 or Tyk2 results in defined clinical disorders, which are also evident in mouse models. A striking phenotype associated with inactivating Jak3 mutations is severe combined immunodeficiency syndrome, whereas mutation of Tyk2 results in another primary immunodeficiency termed autosomal recessive hyperimmunoglobulin E syndrome. By contrast, complete deletion of Jak1 or Jak2 in the mouse are not compatible with life and, unsurprisingly, do not have counterparts in human disease. However, activating mutations of each of the Jaks are found in association with malignant transformation, the most common being gain‐of‐function mutations of Jak2 in polycythemia vera and other myeloproliferative disorders. Our existing knowledge on Jak signaling pathways and fundamental work on their biochemical structure and intracellular interactions allow us to develop new strategies for controlling autoimmune diseases or malignancies by developing selective Jak inhibitors, which are now coming into clinical use. Despite the fact that Jaks were discovered only a little more than a decade ago, at the time of writing there are 20 clinical trials underway testing the safety and efficacy of Jak inhibitors.

Opposing regulation of the locus encoding IL-17 through direct, reciprocal actions of STAT3 and STAT5

Interleukin 2 (IL-2), a cytokine linked to human autoimmune disease, limits IL-17 production. Here we found that deletion of the gene encoding the transcription factor STAT3 in T cells abrogated IL-17 production and attenuated autoimmunity associated with IL-2 deficiency. Whereas STAT3 induced IL-17 and the transcription factor RORγt and inhibited the transcription factor Foxp3, IL-2 inhibited IL-17 independently of Foxp3 and RORγt. STAT3 and STAT5 bound to multiple common sites across the locus encoding IL-17. The induction of STAT5 binding by IL-2 was associated with less binding of STAT3 at these sites and the inhibition of associated active epigenetic marks. Titration of the relative activation of STAT3 and STAT5 modulated the specification of cells to the IL-17-producing helper T cell (TH17 cell) subset. Thus, the balance rather than the absolute magnitude of these signals determined the propensity of cells to make a key inflammatory cytokine.

Modulation of Innate and Adaptive Immune Responses by Tofacitinib (CP-690,550)

Inhibitors of the JAK family of nonreceptor tyrosine kinases have demonstrated clinical efficacy in rheumatoid arthritis and other inflammatory disorders; however, the precise mechanisms by which JAK inhibition improves inflammatory immune responses remain unclear. In this study, we examined the mode of action of tofacitinib (CP-690,550) on JAK/STAT signaling pathways involved in adaptive and innate immune responses. To determine the extent of inhibition of specific JAK/STAT-dependent pathways, we analyzed cytokine stimulation of mouse and human T cells in vitro. We also investigated the consequences of CP-690,550 treatment on Th cell differentiation of naive murine CD4+ T cells. CP-690,550 inhibited IL-4–dependent Th2 cell differentiation and interestingly also interfered with Th17 cell differentiation. Expression of IL-23 receptor and the Th17 cytokines IL-17A, IL-17F, and IL-22 were blocked when naive Th cells were stimulated with IL-6 and IL-23. In contrast, IL-17A production was enhanced when Th17 cells were differentiated in the presence of TGF-β. Moreover, CP-690,550 also prevented the activation of STAT1, induction of T-bet, and subsequent generation of Th1 cells. In a model of established arthritis, CP-690,550 rapidly improved disease by inhibiting the production of inflammatory mediators and suppressing STAT1-dependent genes in joint tissue. Furthermore, efficacy in this disease model correlated with the inhibition of both JAK1 and JAK3 signaling pathways. CP-690,550 also modulated innate responses to LPS in vivo through a mechanism likely involving the inhibition of STAT1 signaling. Thus, CP-690,550 may improve autoimmune diseases and prevent transplant rejection by suppressing the differentiation of pathogenic Th1 and Th17 cells as well as innate immune cell signaling.

Selectivity and therapeutic inhibition of kinases: to be or not to be?

Protein kinases, which serve critical functions in signaling pathways in all cells, are popular therapeutic targets. At present, eight kinase inhibitors have been approved in the United States, each of which shows nanomolar potency. Although the initial goal was to generate inhibitors with a high degree of selectivity, recent experience has revealed that many of these approved compounds target more than one kinase. Surprisingly, this promiscuity is less problematic than one would have imagined; indeed, it opens new therapeutic opportunities. In this Perspective, we discuss the present status of Janus kinase inhibitors—a new class of immunosuppressive drugs—and the advantages and disadvantages of selectively inhibiting this class of kinase.

Signal transduction pathways and transcriptional regulation in Th17 cell differentiation

Over the last decade, our understanding of helper/effector T cell differentiation has changed dramatically. The discovery of interleukin (IL-)17-producing T cells (Th17) and other subsets has changed our view of T cell-mediated immunity. Characterization of the signaling pathways involved in the Th17 commitment has provided exciting new insights into the differentiation of CD4+ T cells. Importantly, the emerging data on conversion among polarized T helper cells have raised the question how we should view such concepts as T cell lineage commitment, terminal differentiation and plasticity. In this review, we will discuss the current understanding of the signaling pathways, molecular interactions, and transcriptional and epigenetic events that contribute to Th17 differentiation and acquisition of effector functions.

Interleukin-27 Priming of T Cells Controls IL-17 Production In trans via Induction of the Ligand PD-L1

Interleukin-27 (IL-27) is a key immunosuppressive cytokine that counters T helper 17 (Th17) cell-mediated pathology. To identify mechanisms by which IL-27 might exert its immunosuppressive effect, we analyzed genes in Txa0cells rapidly induced by IL-27. We found that IL-27 priming of naive Txa0cells upregulated expression of programmed death ligand 1 (PD-L1) in a signal transducer and activator of transcription 1 (STAT1)-dependent manner. When cocultured with naive CD4(+) Txa0cells, IL-27-primed Txa0cells inhibited the differentiation of Th17 cells in trans throughxa0a PD-1-PD-L1 interaction. Inxa0vivo, coadministration ofxa0naive TCR transgenic Txa0cells (2D2 Txa0cells) with IL-27-primed Txa0cells expressing PD-L1 inhibited thexa0development of Th17 cells and protected from severe autoimmune encephalomyelitis. Thus, these data identify a suppressive activity of IL-27, by which CD4(+) Txa0cells can restrict differentiation of Th17 cells in trans.

T helper 17 cell heterogeneity and pathogenicity in autoimmune disease

T helper (Th)17 cells have been proposed to represent a new CD4(+) T cell lineage that is important for host defense against fungi and extracellular bacteria, and the development of autoimmune diseases. Precisely how these cells arise has been the subject of some debate, with apparent species-specific differences in mice and humans. Here, we describe evolving views of Th17 specification, highlighting the contribution of transforming growth factor-β and the opposing roles of signal transducer and activator of transcription (STAT)3 and STAT5. Increasing evidence points to heterogeneity and inherent phenotypic instability in this subset. Ideally, better understanding of expression and action of key transcription factors and the epigenetic landscape of Th17 can help explain the flexibility and diversity of interleukin-17-producing cells.

Helper T-cell differentiation and plasticity: insights from epigenetics

CD4+ T cells have critical roles in orchestrating immune responses to diverse microbial pathogens. This is accomplished through the differentiation of CD4+ T helper cells to specialized subsets in response to microbial pathogens, which evoke a distinct cytokine milieu. Signal transducer and activator of transcription family transcription factors sense these cytokines and they in turn regulate expression of lineage‐defining master regulators that programme selective gene expression, resulting in distinctive phenotypes. However, phenotype and restricted gene expression are determined not only by the action of transcription factors; chromatin accessibility is required for these factors to exert their effect. Technical advances have greatly expanded our understanding of transcription factor action and dynamic changes in the epigenome that accompany cellular differentiation. In this review, we will discuss recent progress in the understanding of how cytokines influence gene expression and epigenetic modifications, and the impact of these findings on our views of helper cell lineage commitment and plasticity.

Reversal of CD8 T-Cell–Mediated Mucocutaneous Graft-Versus-Host-Like Disease by the JAK Inhibitor Tofacitinib

The utility of allogeneic hematopoietic stem cell transplantation is limited by graft-versus-host disease (GVHD), a significant cause of morbidity and mortality. Patients with GVHD exhibit cutaneous manifestations with histological features of interface dermatitis followed by scleroderma-like changes. JAK inhibitors represent a class of immunomodulatory drugs that inhibit signaling by multiple cytokines. Herein we report the effects of tofacitinib in a murine model of GVHD. Oral administration of tofacitinib prevented GVHD-like disease manifested by weight loss and mucocutaneous lesions. More importantly, tofacitinib was also effective in reversing established disease. Tofacitinib diminished the expansion and activation of murine CD8 T cells in this model, and had similar effects on IL-2-stimulated human CD8 T cells. Tofacitinib also inhibited the expression of IFN-γ-inducible chemoattractants by keratinocytes, and IFN-γ-inducible cell death of keratinocytes. Tofacitinib may be an effective drug for treatment against CD8 T-cell–mediated mucocutaneous diseases in patients with GVHD.