The Transcription Factor T-bet Limits Amplification of Type I IFN Transcriptome and Circuitry in T Helper 1 Cells

Loss of T-bet confers survival advantage to influenza-bacterial superinfection

The transcription factor, T-bet, regulates type 1 inflammatory responses against a range of infections. Here, we demonstrate a previously unaddressed role of T-bet, to influenza virus and bacterial superinfection. Interestingly, we found that T-bet deficiency did not adversely affect the efficacy of viral clearance or recovery compared to wild-type hosts. Instead, increased infiltration of neutrophils and production of Th17 cytokines (IL-17 and IL-22), in lungs of influenza virus-infected T-bet-/- mice, were correlated with survival advantage against subsequent infection by Streptococcus pneumoniae Neutralization of IL-17, but not IL-22, in T-bet-/- mice increased pulmonary bacterial load, concomitant with decreased neutrophil infiltration and reduced survival of T-bet-/- mice. IL-17 production by CD8+, CD4+ and γδ T cell types was identified to contribute to this protection against bacterial superinfection. We further showed that neutrophil depletion in T-bet-/- lungs increased pulmonary bacterial burden. These results thus indicate that despite the loss of T-bet, immune defences required for influenza viral clearance are fully functional, which in turn enhances protective type 17 immune responses against lethal bacterial superinfections.

RNA-binding protein YTHDF3 suppresses interferon-dependent antiviral responses by promoting FOXO3 translation

IFN-stimulated genes (ISGs) are essential effectors of the IFN-dependent antiviral immune response. Dysregulation of ISG expression can cause dysfunctional antiviral responses and autoimmune disorders. Epitranscriptomic regulation, such as N ⁶-methyladenosine (m⁶A) modification of mRNAs, plays key roles in diverse biological processes. Here, we found that the m⁶A "reader" YT521-B homology domain-containing family 3 (YTHDF3) suppresses ISG expression under basal conditions by promoting translation of the transcription corepressor forkhead box protein O3 (FOXO3). YTHDF3 cooperates with two cofactors, PABP1 and eIF4G2, to promote FOXO3 translation by binding to the translation initiation region of FOXO3 mRNA. Both the YTH and the P/Q/N-rich domains of YTHDF3 were required for FOXO3 RNA-binding capacity, however, METTL3-mediated m⁶A modification was not involved in the process observed. Moreover, YTHDF3^-/- mice had increased ISG levels and were resistant to several viral infections. Our findings uncover the role of YTHDF3 as a negative regulator of antiviral immunity through the translational promotion of FOXO3 mRNA under homeostatic conditions, adding insight into the networks of RNA-binding protein-RNA interactions in homeostatically maintaining host antiviral immune function and preventing inflammatory response.

High Levels of Eomes Promote Exhaustion of Anti-tumor CD8+ T Cells

Eomes, a T-box transcription factor, is known important for both function and homeostasis of effector and memory T cells, but was recently also implicated in CD8⁺ T cell exhaustion. However, whether and how Eomes might regulate effector functions or exhaustion of CD8⁺ T cells, especially in the tumor setting, is unknown. Here we first show, as tumor progressed, Eomes expression was elevated in tumor-infiltrating CD8⁺ T cells, especially in PD-1⁺Tim-3⁺ exhausted CD8⁺ T cells. Complete loss of Eomes in T cells resulted in impaired development of anti-tumor CTLs, whereas deletion of one allele of Eomes in T cells decreased development of exhausted CD8⁺ T cells, which offered better tumor control. Integrative analysis of RNAseq and ChIPseq of Eomes-overexpressing T cells revealed that high levels of Eomes expression directly controlled expression of T cell exhaustion genes, such as Havcr2. In addition, Eomes might compete with T-bet binding to regulatory genomic loci to antagonize T-bet functions. Collectively, Eomes exerts bimodal functions in CD8⁺ T cells in tumor.

T-bet and Eomes govern differentiation and function of mouse and human NK cells and ILC1

T-bet and Eomes are T-box transcription factors that drive the differentiation and function of cytotoxic lymphocytes such as NK cells. Their DNA-binding domains are highly similar, suggesting redundant transcriptional activity. However, while these transcription factors have different patterns of expression, the phenotype of loss-of-function mouse models suggests that they play distinct roles in the development of NK cells and other innate lymphoid cells (ILCs). Recent technological advances using reporter mice and conditional knockouts were fundamental in defining the regulation and function of these factors at steady state and during pathological conditions such as various types of cancer or infection. Here, we review these recent developments, focusing on NK cells as prototypical cytotoxic lymphocytes and their development, and also discuss parallels between NK cells and T cells. We also examine the role of T-bet and Eomes in human NK cells and ILC1s. Considering divergent findings on mouse and human ILC1s, we propose that NK cells are defined by coexpression of T-bet and Eomes, while ILC1s express only one of these factors, either T-bet or Eomes, depending on the tissue or the species.

Breaking self-tolerance during autoimmunity and cancer immunity: Myeloid cells and type I IFN response regulation

The generation and regulation of innate immune signals are key determinants of autoimmune pathogenesis. Emerging evidence suggests that parallel processes operating in the setting of solid tumors can similarly determine the balance between tolerance and immunity and ultimately the effectiveness of the antitumor immune response. In both contexts, self-specific responses start with innate immune cell activation that leads to the initial break in self-tolerance, which can be followed by immune response amplification and maturation through innate-adaptive crosstalk, and finally immune-mediated tissue/tumor destruction that can further potentiate inflammation. Of particular importance for these processes is type I IFN, which is induced in response to endogenous ligands, such as self-nucleic acids, and acts on myeloid cells to promote the expansion of autoreactive or tumor-specific T cells and their influx into the target tissue. Evidence from the study of human disease pathophysiology and genetics and mouse models of disease has revealed an extensive and complex network of negative regulatory pathways that has evolved to restrain type I IFN production and activity. Here, we review the overlapping features of self- and tumor-specific immune responses, including the central role that regulators of the type I IFN response and innate immune cell activation play in maintaining tolerance, and discuss how a better understanding of the pathophysiology of autoimmunity can help to identify new approaches to promote immune-mediated tumor destruction.

STAT4 and T-bet control follicular helper T cell development in viral infections

Follicular helper T (Tfh) cells promote germinal center (GC) B cell survival and proliferation and guide their differentiation and immunoglobulin isotype switching by delivering contact-dependent and soluble factors, including IL-21, IL-4, IL-9, and IFN-γ. IL-21 and IFN-γ are coexpressed by Tfh cells during viral infections, but transcriptional regulation of these cytokines is not completely understood. In this study, we show that the T helper type 1 cell (Th1 cell) transcriptional regulators T-bet and STAT4 are coexpressed with Bcl6 in Tfh cells after acute viral infection, with a temporal decline in T-bet in the waning response. T-bet is important for Tfh cell production of IFN-γ, but not IL-21, and for a robust GC reaction. STAT4, phosphorylated in Tfh cells upon infection, is required for expression of T-bet and Bcl6 and for IFN-γ and IL-21. These data indicate that T-bet is expressed with Bcl6 in Tfh cells and is required alongside STAT4 to coordinate Tfh cell IL-21 and IFN-γ production and for promotion of the GC response after acute viral challenge.

T-bet Runs INTERFERence

In this issue of Immunity, Iwata et al. (2017) report that the transcription factor T-bet acts as a selective repressor of the type I interferon (IFN) transcriptional program in response to IFN-γ signaling.