Steven L. Reiner

Helper T Cell Differentiation Is Controlled by the Cell Cycle

Helper T (Th) cell differentiation is highly regulated by cytokines but initiated by mitogens. By examining gene expression in discrete generations of dividing cells, we have delineated the relationship between proliferation and differentiation. Initial expression of IL-2 is cell cycle-independent, whereas effector cytokine expression is cell cycle-dependent. IFNgamma expression increases in frequency with successive cell cycles, while IL-4 expression requires three cell divisions. Cell cycle progression and cytokine signaling act in concert to relieve epigenetic repression and can be supplanted by agents that hyperacetylate histones and demethylate DNA. Terminally differentiated cells exhibit stable epigenetic modification and cell cycle-independent gene expression. These data reveal a novel mechanism governing Th cell fate that initially integrates proliferative and differentiative signals and subsequently maintains stability of the differentiated state.

Effector and memory CD8 + T cell fate coupled by T-bet and eomesodermin

Two seemingly unrelated hallmarks of memory CD8+ T cells are cytokine-driven proliferative renewal after pathogen clearance and a latent effector program in anticipation of rechallenge. Memory CD8+ T cells and natural killer cells share cytotoxic potential and dependence on the growth factor interleukin 15. We now show that mice with compound mutations of the genes encoding the transcription factors T-bet and eomesodermin were nearly devoid of several lineages dependent on interleukin 15, including memory CD8+ T cells and mature natural killer cells, and that their cells had defective cytotoxic effector programming. Moreover, T-bet and eomesodermin were responsible for inducing enhanced expression of CD122, the receptor specifying interleukin 15 responsiveness. Therefore, these key transcription factors link the long-term renewal of memory CD8+ T cells to their characteristic effector potency.*Note: In the version of this article initially published online, the third sentence of the abstract was incorrect. The correct sentence is as follows: “We now show that mice with compound mutations of the genes encoding the transcription factors T-bet and eomesodermin were nearly devoid of several lineages dependent on interleukin 15, including memory CD8+ T cells and mature natural killer cells, and that their cells had defective cytotoxic effector programming.” The error has been corrected for the HTML and print versions of the article. Additionally, in the print version of this article and the version initially published online, some labels for Tbx21 in Figure 7b are incorrect. This correction has been appended to the PDF version.

Asymmetric T Lymphocyte Division in the Initiation of Adaptive Immune Responses

A hallmark of mammalian immunity is the heterogeneity of cell fate that exists among pathogen-experienced lymphocytes. We show that a dividing T lymphocyte initially responding to a microbe exhibits unequal partitioning of proteins that mediate signaling, cell fate specification, and asymmetric cell division. Asymmetric segregation of determinants appears to be coordinated by prolonged interaction between the T cell and its antigen-presenting cell before division. Additionally, the first two daughter T cells displayed phenotypic and functional indicators of being differentially fated toward effector and memory lineages. These results suggest a mechanism by which a single lymphocyte can apportion diverse cell fates necessary for adaptive immunity.

The Transcription Factors T-bet and Eomes Control Key Checkpoints of Natural Killer Cell Maturation

Natural killer (NK) cells play critical roles defending against tumors and pathogens. We show that mice lacking both transcription factors Eomesodermin (Eomes) and T-bet failed to develop NK cells. Developmental stability of immature NK cells constitutively expressing the death ligand TRAIL depended on T-bet. Conversely, maturation characterized by loss of constitutive TRAIL expression and induction of Ly49 receptor diversity and integrin CD49b (DX5(+)) required Eomes. Mature NK cells from which Eomes was deleted reverted to phenotypic immaturity if T-bet was present or downregulated NK lineage antigens if T-bet was absent, despite retaining expression of Ly49 receptors. Fetal and adult hepatic hematopoiesis restricted Eomes expression and limited NK development to the T-bet-dependent, immature stage, whereas medullary hematopoiesis permitted Eomes-dependent NK maturation in adult mice. These findings reveal two sequential, genetically separable checkpoints of NK cell maturation, the progression of which is metered largely by the anatomic localization of hematopoiesis.

Hlx is induced by and genetically interacts with T-bet to promote heritable T H 1 gene induction

Type 1 helper T (TH1) cells are essential for cellular immunity, but their ontogeny, maturation and durability remain poorly understood. By constructing a dominant-negative form of T-bet, we were able to determine the role played by this lineage-inducing trans-activator in the establishment and maintenance of heritable TH1 gene expression. Optimal induction of interferon-γ (IFN-γ) expression required genetic interaction between T-bet and its target, the homeoprotein Hlx. In fully mature TH1 cells, reiteration of IFN-γ expression and stable chromatin remodeling became relatively independent of T-bet activity and coincided with demethylation of DNA. In contrast, some lineage attributes, such as expression of IL-12Rβ2 (interleukin 12 receptor β2), required ongoing T-bet activity in mature TH1 cells and their progeny. These findings suggest that heritable states of gene expression might be maintained by continued expression of the inducing factor or by a mechanism that confers a stable imprint of the induced state.

Anomalous Type 17 Response to Viral Infection by CD8+ T Cells Lacking T-bet and Eomesodermin

When intracellular pathogens invade mammalian hosts, naïve CD8+ T cells differentiate into cytotoxic killers, which lyse infected target cells and secrete cytokines that activate intracellular microbicides. We show that CD8+ T cells deficient in the transcription factors T-bet and eomesodermin (Eomes) fail to differentiate into functional killers required for defense against lymphocytic choriomeningitis virus. Instead, virus-specific CD8+ T cells lacking both T-bet and Eomes differentiate into an interleukin-17–secreting lineage, reminiscent of the helper T cell fate that has been implicated in autoimmunity and extracellular microbial defense. Upon viral infection, mice with T cells lacking both T-bet and Eomes develop a CD8+ T cell–dependent, progressive inflammatory and wasting syndrome characterized by multi-organ infiltration of neutrophils. T-bet and Eomes, thus, ensure that CD8+ T cells adopt an appropriate course of intracellular rather than extracellular destruction.

Development in motion : Helper T cells at work

In mammals, helper T cells orchestrate defense against diverse pathogens. However, these warriors of the immune system can also result in self-inflicted injury culminating in autoimmune and allergic diseases. Recent findings--such as the discovery of the Th17 lineage--have revealed additional complexity in the fates chosen by helper T cells and have begun to reshape our view of how signaling and transcriptional networks generate appropriate and inappropriate immunity.

Cutting Edge: IL-12 Inversely Regulates T-bet and Eomesodermin Expression during Pathogen-Induced CD8+ T Cell Differentiation

Cytokines are critical determinants for specification of lineage-defining transcription factors of CD4+ T cell subsets. Little is known, however, about how cytokines regulate expression of T-bet and eomesodermin (Eomes) in effector and memory CD8+ T cells. We now report that IL-12, a signature of cell-mediated immunity, represses Eomes while positively regulating T-bet in effector CD8+ T cells during infection with Listeria monocytogenes. After resolution of infection and abatement of IL-12 signaling, Eomes expression rises whereas T-bet expression declines in memory CD8+ T cells. Eomes becomes derepressed in effector cells by ablation of IL-12 signaling. In the absence of IL-12, the dynamics of clonal expansion and contraction are also perturbed. Together, these results reveal how a pathogen-associated signal, such as IL-12, could act as a switch, regulating appropriate clonal growth and decline while, in parallel, shaping a unique pattern of fate-determining transcription factors.

A Role for the Transcriptional Repressor Blimp-1 in CD8+ T Cell Exhaustion during Chronic Viral Infection

T cell exhaustion is common during chronic infections and can prevent optimal immunity. Although recent studies have demonstrated the importance of inhibitory receptors and other pathways in T cell exhaustion, the underlying transcriptional mechanisms are unknown. Here, we define a role for the transcription factor Blimp-1 in CD8(+) T cell exhaustion during chronic viral infection. Blimp-1 repressed key aspects of normal memory CD8(+) T cell differentiation and promoted high expression of inhibitory receptors during chronic infection. These cardinal features of CD8(+) T cell exhaustion were corrected by conditionally deleting Blimp-1. Although high expression of Blimp-1 fostered aspects of CD8(+) T cell exhaustion, haploinsufficiency indicated that moderate Blimp-1 expression sustained some effector function during chronic viral infection. Thus, we identify Blimp-1 as a transcriptional regulator of CD8(+) T cell exhaustion during chronic viral infection and propose that Blimp-1 acts as a transcriptional rheostat balancing effector function and T cell exhaustion.

Progenitor and Terminal Subsets of CD8+ T Cells Cooperate to Contain Chronic Viral Infection

Chronic infections strain the regenerative capacity of antiviral T lymphocyte populations, leading to failure in long-term immunity. The cellular and molecular events controlling this regenerative capacity, however, are unknown. We found that two distinct states of virus-specific CD8+ T cells exist in chronically infected mice and humans. Differential expression of the T-box transcription factors T-bet and Eomesodermin (Eomes) facilitated the cooperative maintenance of the pool of antiviral CD8+ T cells during chronic viral infection. T-bethi cells displayed low intrinsic turnover but proliferated in response to persisting antigen, giving rise to Eomeshi terminal progeny. Genetic elimination of either subset resulted in failure to control chronic infection, which suggests that an imbalance in differentiation and renewal could underlie the collapse of immunity in humans with chronic infections. Chronic viral infections like HIV are kept in check by two functionally distinct types of T lymphocyte. Chronic infections like hepatitis C virus (HCV) or HIV are hard on the immune system. In the face of a constant threat, some immune cells like T lymphocytes become “exhausted”; although present, they can no longer mount responses that are effective enough to eliminate the virus. These responses, however, are still important because in many cases they do keep the virus relatively controlled. The mechanisms underlying the population dynamics of T cell responses during chronic viral infection, however, are not well understood. Paley et al. (p. 1220) now demonstrate that the T-box transcription factors T-bet and Eomesodermin differentially regulate two phenotypically and functionally distinct subsets of antiviral CD8+ T cells in mice. The cooperation of these subsets may be important for antiviral immunity during chronic viral infections in humans.