Activation of naive CD8 + T cells with antigen in the absence of skewing cytokines triggers their differentiation into effector CTL, which induces death of target cells. We show that CD8 + T cells activated in the presence of the cytokines IL-6 or IL-21 plus TGF-b similar to CD4 + T cells, develop into IL-17-producing (Tc17) cells. These cells display greatly suppressed cytotoxic function along with low levels of the CTL markers: T-box transcription factor Eomesodermin, granzyme B and IFN-c. Instead, these cells express hallmark molecules of Th17 program including retinoic acid receptor-related orphan receptor (ROR)ct, RORa, IL-21 and IL-23R. The expression of the type 17 master regulator RORct is causally linked to Tc17 generation, because its overexpression stimulates production of IL-17 in the presence of IL-6 or IL-21. Both, upregulation of the type 17 program as well as suppression of CTL differentiation are STAT3 dependent. Furthermore, Tc17 cells producing IL-17 but not granzyme B are also detectable in EAE, a mouse model for multiple sclerosis. Our data point to the existence of mutually exclusive CTL and Tc17 developmental pathways in vitro and in vivo.Key words: CTL . EAE . RORgt . STAT3 . Tc17Supporting Information available online Introduction CTL are important effector cells in the immune response to intracellular pathogens and tumors. They differentiate from naive CD8 + T cells following activation by antigen in the absence of skewing cytokines, and during this process they acquire the ability to destroy their targets by releasing cytotoxic molecules such as perforin and granzymes, from granules into the immunological synapse. In addition, CTL secrete cytokines, mostly IFN-g and TNF-a, which function to induce or augment inflammation [1][2][3].Two T-box transcription factors, Eomesodermin (Eomes) and T-bet, are important for the development of effector and memory CTL [4][5][6]. Studies using deletion, overexpression or dominant negative analogs of these factors have suggested that both of them are involved in the regulation of expression of granzyme B and perforin [4,5,7]. Consistent with these data, CD8 + T cells with combined deletion of the Eomes and Tbx21 (encoding T-bet) genes differentiate into cells with highly impaired cytotoxic activity and IFN-g production [8]. Instead, these cells produce Th17 type cytokines and express the IL-23 receptor (IL-23R) as well as the transcription factor retinoic acid receptor-related orphan receptor (ROR)gt, both of 1716Frontline which are characteristic for the type 17 differentiation program [8]. Thus, the phenotype of CD8 + T cells deficient for both Eomes and T-bet is reminiscent of the newly described Th17-cell subset.Th17 cells produce IL-17A, IL-17F, IL-21 and IL-22, which are highly pro-inflammatory and induce severe autoimmunity, e.g. during EAE, the mouse model for multiple sclerosis [9]. The differentiation of these cells requires TGF-b in combination with 11]. Two additional cytokines, IL-21 and IL-23, are also critically involved in the diffe...

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Regulatory CD4+ T cells are important for the homeostasis of the immune system and their absence correlates with autoimmune disorders. Here, we investigate the capacity of IL-27, a cytokine with pro- and anti-inflammatory properties, to regulate the generation of transforming growth factor beta (TGFbeta)-inducible forkhead box P3 (Foxp3)-positive regulatory T (Treg) cells. Our results demonstrate that IL-27 inhibits the acquisition of the Treg phenotype at the level of Foxp3, CD25 and CTLA-4 (CD152) expression as well as the suppressive function. In contrast to TGFbeta-induced Treg cells, the cells generated after differentiation in the presence of TGFbeta and IL-27 maintained the ability for IL-2 and tumour necrosis factor alpha (TNFalpha) production. The inhibitory effect of IL-27 on Treg generation was at least partially signal transducer and activator of transcription 3 (STAT3) dependent as examined by targeted STAT3 protein inhibition using small interfering RNA (siRNA), while STAT1-dependent signals seemed to oppose the STAT3 signals. In turn, TGFbeta blocked IL-27-induced T(h)1 differentiation. Thus, IL-27 and TGFbeta mutually control their effects on CD4+ T-cell differentiation, whereby IL-27 favours inflammatory conditions through a STAT3-dependent inhibition of Treg generation.

Robust cytotoxic CD8 + T-cell response is important for immunity to intracellular pathogens. Here, we show that the transcription factor IFN Regulatory Factor 4 (IRF4) is crucial for the protective CD8 + T-cell response to the intracellular bacterium Listeria monocytogenes. IRF4-deficient (Irf4 −/− ) mice could not clear L. monocytogenes infection and generated decreased numbers of L. monocytogenesspecific CD8 + T cells with impaired effector phenotype and function. Transfer of wild-type CD8 + T cells into Irf4 −/− mice improved bacterial clearance, suggesting an intrinsic defect of CD8 + T cells in Irf4 −/− mice. Following transfer into wild-type recipients, Irf4 −/− CD8 + T cells became activated and showed initial proliferation upon L. monocytogenes infection. However, these cells could not sustain proliferation, produced reduced amounts of IFN-γ and TNF-α, and failed to acquire cytotoxic function. Forced IRF4 expression in Irf4 −/− CD8 + T cells rescued the defect. During acute infection, Irf4 −/− CD8 + T cells demonstrated diminished expression of B lymphocyte-induced maturation protein-1 (Blimp-1), inhibitor of DNA binding (Id)2, and T-box expressed in T cells (T-bet), transcription factors programming effector-cell generation. IRF4 was essential for expression of Blimp-1, suggesting that altered regulation of Blimp-1 contributes to the defects of Irf4 −/− CD8 + T cells. Despite increased levels of B-cell lymphoma 6 (BCL-6), Eomesodermin, and Id3, Irf4 −/− CD8 + T cells showed impaired memory-cell formation, indicating additional functions for IRF4 in this process. As IRF4 governs B-cell and CD4 + T-cell differentiation, the identification of its decisive role in peripheral CD8 + T-cell differentiation, suggests a common regulatory function for IRF4 in adaptive lymphocytes fate decision.