T-bet-dependent regulation of CD8+ T-cell expansion during experimental Trypanosoma cruzi infection

The transcription factor T-bet promotes the pathogenesis of nonalcoholic fatty liver disease by upregulating intrahepatic inflammation

OBJECTIVE

To investigate the effect of the transcription factor T-bet on the pathogenesis of nonalcoholic fatty liver disease (NAFLD) and the regulation of the intrahepatic immune microenvironment.

METHODS

Wild-type and T-bet knockout NASH mouse models were constructed. The effect of T-bet knockout on the pathogenesis of NAFLD was observed by histochemical staining. The expression of T-bet in immune cells in the liver and the effect of T-bet knockout on the proportion and function of immune cell subsets in the liver were determined by flow analysis.

RESULTS

Flow cytometry results indicated that T-bet expression was increased in immune cells, especially NKT cells, in the livers of NAFLD mice. Knocking out the transcription factor T-bet reduced intrahepatic inflammation, reduced lipid accumulation, and ameliorated the pathogenesis of NAFLD. Based on the analysis of immune cell subsets, knocking out the transcription factor T-bet decreased the proportion, survival, and degree of activation of NK, NKT, and CD8 T cells in NAFLD liver; additionally, it decreased the secretion of IFN-γ by T cells and NKT cells but had no effect on the proportion of Th17 cells and Treg cells. Knocking out the transcription factor T-bet also reduced the proportion of proinflammatory myeloid-derived macrophages (MoMFs) in NAFLD liver, mainly the proportion of proinflammatory Ly6Chigh MoMFs. Furthermore, knocking out the transcription factor T-bet had no significant effect on the secretion of TNF-α from MoMFs but significantly reduced the expression of MHC class II molecules. Further analysis showed that the transcription factor T-bet may directly affect the expression of MHC class II molecules H2-AB1 and H2-Dmb1 through transcriptional regulation.

CONCLUSIONS

Knocking out the transcription factor T-bet reduced the proinflammatory effect of innate immune cells (MoMFs, NK cells, and NKT cells) and T lymphocytes in NAFLD liver, thereby reducing intrahepatic inflammation and delaying the pathogenesis of NAFLD.

Impaired T-cell responses in domestic pigs and wild boar upon infection with a highly virulent African swine fever virus strain

Since African swine fever (ASF) first appeared in the Caucasus region in 2007, it has spread rapidly and is now present in numerous European and Asian countries. In Europe, mainly wild boar populations are affected and pose a risk for domestic pigs. In Asia, domestic pigs are almost exclusively affected. An effective and safe vaccine is not available, and correlates of protection are far from being understood. Therefore, research on immune responses, immune dysfunction and pathogenesis is mandatory. It is acknowledged that T cells play a pivotal role. Thus, we investigated T-cell responses of domestic pigs and wild boar upon infection with the highly virulent ASF virus (ASFV) strain 'Armenia08'. For this purpose, we used a flow cytometry-based multicolour analysis to identify T-cell subtypes (cytotoxic T cells, T-helper cells, γδ T cells) and their functional impairment in ASFV-infected pigs. Domestic pigs showed lymphopaenia, and neither in the blood nor in the lymphoid organs was a proliferation of CD8⁺ effector cells observed. Furthermore, a T-bet-dependent activation of the remaining CD8 T cells did not occur. In contrast, a T-cell response could be observed in wild boar at 5 days post-inoculation in the blood and in tendency also in some organs. However, this cytotoxic response was not beneficial as all wild boars showed a severe acute lethal disease and a higher proportion died spontaneously or was euthanized at the humane endpoint.

The evolving role of T-bet in resistance to infection

The identification of T-bet as a key transcription factor associated with the development of IFNγ-producing CD4⁺ T cells predicted a crucial role for T-bet in cell-mediated immunity and in resistance to many intracellular infections. This idea was reinforced by initial reports showing that T-bet-deficient mice were more susceptible to pathogens that survived within the lysosomal system of macrophages. However, subsequent studies revealed IFNγ-dependent, T-bet-independent pathways of resistance to diverse classes of microorganisms that occupy other intracellular niches. Consequently, a more complex picture has emerged of how T-bet and the related transcription factor eomesodermin (EOMES) coordinate many facets of the immune response to bona fide pathogens as well as commensals. This article provides an overview of the discovery and evolutionary relationship between T-bet and EOMES and highlights the studies that have uncovered broader functions of T-bet in innate and adaptive immunity and in the development of the effector and memory T cell populations that mediate long-term resistance to infection.

T-bet optimizes CD4 T-cell responses against influenza through CXCR3-dependent lung trafficking but not functional programming

Although clearance of many intracellular pathogens requires T-bet-dependent CD4 T cell programming, the extent to which T-bet is needed to direct protective CD4 responses against influenza is not known. Here, we characterize wild-type and T-bet-deficient CD4 cells during murine influenza infection. Surprisingly, although T-bet expression has broad impacts on cytokine production by virus-specific CD4 cells, the protective efficacy of T-bet-deficient effector cells is only marginally reduced. This reduction is due to lower CXCR3 expression, leading to suboptimal accumulation of activated T-bet-deficient cells in the infected lung. However, T-bet-deficient cells outcompete wild-type cells to form lung-resident and circulating memory populations following viral clearance, and primed T-bet-deficient mice efficiently clear supralethal heterosubtypic influenza challenges even when depleted of CD8 T cells. These results are relevant to the identification of more incisive correlates of protective T cells and for vaccines that aim to induce durable cellular immunity against influenza.

Genetic association of ankylosing spondylitis with TBX21 influences T-bet and pro-inflammatory cytokine expression in humans and SKG mice as a model of spondyloarthritis

OBJECTIVES

Ankylosing spondylitis (AS) is a highly heritable immune-mediated arthropathy. Inflammation in AS is poorly understood. TBX21 encodes T-bet, a transcription factor, lying within a locus with genome-wide significant association with AS. T-bet is implicated in innate and adaptive immunity. However, the role of T-bet in AS pathogenesis is unclear.

METHODS

We assessed the importance of T-bet in disease development and progression in peripheral blood mononuclear cells from 172 AS cases and 83 healthy controls carrying either risk or protective alleles of the peak AS-associated TBX21 single nucleotide polymorphism. Kinetics and localisation of T-bet expression in the SKG mouse model of spondyloarthropathy was examined, along with the impact of Tbx21 knockout on arthritis development in SKG mice.

RESULTS

Patients with AS had higher T-bet expression than healthy individuals, driven predominantly by natural killer and CD8+ T cells, with expression levels in CD8+ T cells completely distinguishing AS cases from healthy controls. T-bet expression was increased in AS cases carrying risk compared with protective alleles of rs11657479. In curdlan-treated SKG mice, T-bet expression increased early after disease initiation and persisted throughout the course of disease. There was marked reduction in gut and peripheral joint inflammation, and less IFNγ-producing and IL-17-producing CD8+ T cells, in Tbx21-/- compared with wild-type SKG mice.

CONCLUSIONS

AS-associated variants in TBX21 influence T-bet expression. T-bet+ innate and adaptive immune cells have altered IL-17 and IFNγ, and early activation marker CD69 expression than T-bet cells. This indicates that T-bet is a major component of inflammatory pathways of spondyloarthropathy in humans and mice.

Elevated expression of T-bet in mycobacterial antigen-specific CD4(+) T cells from patients with tuberculosis

T-bet is a T-box transcriptional factor that controls the differentiation and effector functions of CD4 T cells. In this study, we studied the role of T-bet in regulating CD4(+) T cell immunity against tuberculosis (TB). T-bet expression in Mycobacterium tuberculosis antigen-specific CD4(+) T cells was significantly higher in patients with active TB than in individuals with latent TB infection (p<0.0001). Comparison of T-bet expression in TCM and TEM subsets showed that CD4(+)T-bet(+)M. tuberculosis antigen-specific CD4(+) T cells had significantly lower frequency of TCM (p=0.003) and higher frequency of TEM (p=0.003) than CD4(+)T-bet(-) cells. The expression of PD-1 in antigen-specific CD4(+) T cells was significantly higher in patients with TB than in individuals with latent TB infection (p=0.006). CD4(+)CD154(+)T-bet(+) T cells had significantly higher expression of PD-1 than CD4(+)CD154(+)T-bet(-) T cells (p=0.0028). It is concluded that T-bet expression might be associated with differentiation into effector memory cells and PD-1 expression in mycobacterial antigen-specific CD4(+) T cells.

CpG and interleukin-15 synergize to enhance IFN-γ production by activated CD8+ T cells

Interleukin-15 (IL-15) regulates the development and maintenance of memory CD8⁺ T cells. Paradoxically, we previously reported that IL-15 could enhance CD8⁺ T-cell responses to IL-12, a proinflammatory cytokine required for optimal priming of effector CD8⁺ T cells. To expand the physiological relevance of these findings, we tested IL-15 for its ability to enhance T-cell responses to bacterial CpG. Expectedly, CpG enhanced the production of IFN-γ by CD8⁺ T cells polyclonally activated with anti-CD3. However, addition of IL-15 to CpG-stimulated cultures led to a striking increase in IFN-γ production. The effect of CpG and IL-15 was also evident with CD8⁺ T cells recovered from mice infected with the parasite Trypanosoma cruzi (T. cruzi) and restimulated with antigen. The observed synergy between CpG and IL-15 occurred in an IL-12-dependent manner, and this effect could even be demonstrated in cocultures of activated CD8⁺ T cells and CD4⁺CD25⁺ regulatory T cells. Although IFN-γ was not essential for CpG-induced IL-12, the ability of CpG and IL-15 to act on CD8⁺ T cells required expression of the IFN-γ-inducible transcription factor T-bet. These data have important implications for development of vaccines and design of therapies to boost CD8⁺ T-cell responses to infectious agents and tumors.

Regulation of proinflammatory Th17 responses during Trypanosoma cruzi infection by IL-12 family cytokines

Previously, we reported that the transcription factor T-bet (Tbx21) regulates Th17 responses to Trypanosoma cruzi infection in an IFN-γ-independent manner. In an effort to further understand this regulation, we examined the development and plasticity of Th17 cells during T. cruzi infection. Th17 cells recovered from infected Tbx21(-/-) mice were amenable to the inhibitory effects of T-bet, as ectopic expression of T-bet reduced IL-17 expression. We subsequently addressed the role of IL-12 family cytokines IL-12 and IL-27 and report that IL-12p35(-/-) mice infected with T. cruzi exhibited a significant increase in Th17 cells and Th17-associated inflammation. Ex vivo culture of these cells with IL-12 led to a dramatic reduction in IL-17 production and concomitant increase in IFN-γ. Importantly, the ability of IL-12 to suppress IL-17 was independent of IFN-γ. Surprisingly, and contrary to results reported for other pathogens, IL-27 had no inhibitory effect on Th17 development, as Ebi-3(-/-) mice failed to show any increase in their T. cruzi-specific Th17 response. Furthermore, IL-27 could not compensate or synergize with IL-12 to suppress IL-17 production ex vivo. Thus, we have established that IL-12, not IL-27, is critical for regulating Th17 responses to T. cruzi.

Trypanosoma cruzi activates cord blood myeloid dendritic cells independently of cell infection

We recently showed that T. cruzi parasites enhance expression of co-stimulatory surface molecules on cord blood myeloid dendritic cells (mDCs). This study aims to gain insight into the role of live parasites and intracellular infection in mDC activation using CSFE-labelled parasites. First, we observed that only a low proportion of mDCs was infected by T. cruzi after overnight culture of whole blood samples and trypomastigotes, as compared with monocytes and granulocytes. Cord blood mDCs were also less infected than their adult counterpart. Second, expression levels of HLA-DR and co-stimulatory molecules CD80, CD83 and CD86 were similar on infected and uninfected mDCs. Parasite lysate also triggered mDCs phenotypic maturation of both cord and adult blood cells, though in a lower extent than live parasites. These results strongly support a central role for extracellular trypomastigotes in activation of mDCs when parasites are incubated with whole blood cells. However, viability of trypomastigotes was not absolutely required for mDC activation.

Transcriptional regulation of the mucosal immune system mediated by T-bet

The immune system faces the arduous task of defending the mucosal surfaces from invading pathogens, but must simultaneously repress responses against commensal organisms and other inert antigens that are abundant in the external environment, as inappropriate immune activation might expose the host to increased risk of autoimmunity. The behavior of individual immune cells is governed by the expression of transcription factors that are responsible for switching immune response genes on and off. T-bet (T-box expressed in T cells) has emerged as one of the key transcription factors responsible for controlling the fate of both innate and adaptive immune cells, and its expression in different immune cells found at mucosal surfaces is capable of dictating the critical balance between permitting robust host immunity and limiting susceptibility to autoimmunity and allergy.

T-bet-independent effects of IL-12 family cytokines on regulation of Th17 responses to experimental T. cruzi infection

Tbx21 (i.e., T-bet) is an IFN-γ-inducible transcription factor that promotes Th1 differentiation. Previously, we reported that Tbx21(-/-) mice develop a robust Th17 response to the parasite Trypanosoma cruzi, including CD4(+) T cell subsets producing IL-17 and IFN-γ. Because of the known inhibitory effects of IFN-γ on Th17 cells, the purpose of this study was to determine the contribution of IFN-γ to regulation of Th17 differentiation during the course of T. cruzi infection. We observed that infection of IFN-γ(-/-) or Stat-1(-/-) mice generated increased numbers of IL-17-producing cells. In sharp contrast to infected Stat-1(-/-) or Tbx21(-/-) mice, however, IFN-γ(-/-) mice developed a lower overall Th17 response, suggesting that IFN-γ was not required for T-bet-dependent activity, including T-bet-dependent expression of CXCR3. To determine if IFN-γ could influence Th17 responses indirectly by acting on APCs, we neutralized IFN-γ in cultures containing APC and T. cruzi antigens. Although anti-IFN-γ increased IL-17 production modestly, anti-IFN-γ and anti-IL-12 led to a significant enhancement of T. cruzi-specific IL-17 (P<0.01). In contrast to the inhibitory effects of IL-12, IL-23 was able to stimulate Tbx21(-/-) T cells and cause a striking increase in T. cruzi-specific IL-17. These data show that the IL-12 family of cytokines can influence Th17 responses in a T-bet-independent manner and that the effects of IFN-γ are not necessarily related to its ability to induce T-bet expression in T cells.

Shaping successful and unsuccessful CD8 T cell responses following infection

CD8 T cells play a vital role in the immunological protection against intracellular pathogens. Ideally, robust effector responses are induced, which eradicate the pathogen, and durable memory CD8 T cells are also established, which help confer protection against subsequent reinfection. The quality and magnitude of these responses is dictated by multiple factors, including their initial interactions with professional antigen-presenting cells, as well as the cytokine milieu and availability of CD4 T cell help. These factors set the transcriptional landscape of the responding T cells, which in turn influences their phenotypic and functional attributes as well as ultimate fate. Under certain conditions, such as during chronic infections, the development of these usually successful responses becomes subverted. Here we discuss advances in our understanding of the cellular and molecular determinants of T cell quality, and the formation of effector, memory, and exhausted CD8 T cells, during acute and chronic infections.