Kruppel-like factor 2 is required for trafficking but not quiescence in postactivated T cells

The transcription factor Kruppel-like factor 2 (KLF2) was proposed to regulate genes involved in cell cycle entry and T cell trafficking; however, the physiological role of its expression in postactivated T cells is not well defined. Previous studies suggested that the cytokines IL-2 and IL-15 differentially regulate KLF2 re-expression in postactivation T cells and that these cytokines also influence effector versus memory T cell differentiation. Using conditional and inducible KLF2-knockout model systems, we tested the specific role of KLF2 expression in activated CD8(+) T cells cultured with these cytokines. KLF2 was required for effective transcription of sphingosine-1-phosphate receptor-1 (S1P(1)) and CD62L in postactivation T cells. However, although different cytokines dramatically altered the expression of cell-cycle-related genes, endogenous KLF2 had a minimal impact. Correspondingly, KLF2-deficient T cells showed dysregulated trafficking but not altered proliferative characteristics following in vivo responses to Ag. Thus, our data help to define KLF2-dependent and -independent aspects of activated CD8(+) T cell differentiation and argue against a physiological role in cell cycle regulation.

Acute ablation of Langerhans cells enhances skin immune responses

Understanding the function of Langerhans cells (LCs) in vivo has been complicated by conflicting results from LC-deficient mice. Human Langerin-DTA mice constitutively lack LCs and develop exaggerated contact hypersensitivity (CHS) responses. Murine Langerin-diphtheria toxin receptor (DTR) mice allow for the inducible elimination of LCs and Langerin(+) dermal dendritic cells (dDCs) after administration of diphtheria toxin, which results in reduced CHS. When Langerin(+) dDCs have partially repopulated the skin but LCs are still absent, CHS returns to normal. Thus, LCs appear to be suppressive in human Langerin-DTA mice and redundant in murine Langerin-DTR mice. To determine whether inducible versus constitutive LC ablation explains these results, we engineered human Langerin-DTR mice in which diphtheria toxin ablates LCs without affecting Langerin(+) dDCs. The inducible ablation of LCs in human Langerin-DTR mice resulted in increased CHS. Thus, LC-mediated suppression does not require their absence during ontogeny or during the steady-state and is consistent with a model in which LCs actively suppress Ag-specific CHS responses.

Krüppel-like factor 2 regulates trafficking and homeostasis of gammadelta T cells

gammadelta T cells are generated in the thymus and traffic to secondary lymphoid organs and epithelial surfaces, where they regulate immune responses. alphabeta T cells require sphingosine 1-phosphate receptor type 1 (S1P(1)) and CD62L for thymic emigration and circulation through secondary lymphoid organs. Both of these genes are regulated by the transcription factor Krüppel-like factor 2 (KLF2) in conventional alphabeta T cells. It is unclear if gammadelta T cells use similar mechanisms. In this study, we show that thymic gammadelta T cells express S1P(1) and that it is regulated by KLF2. Furthermore, KLF2 and S1P(1)-deficient gammadelta T cells accumulate in the thymus and fail to populate the secondary lymphoid organs or gut, in contrast to the expectation from published work. Interestingly, KLF2 but not S1P(1) deficiency led to the expansion of a usually rare population of CD4(+) promyelocytic leukemia zinc finger(+) "gammadelta NKT" cells. Thus, KLF2 is critically important for the homeostasis and trafficking of gammadelta T cells.

Why CD8+ T cells need diversity when growing up

A thymus-specific subunit of the proteasome, beta5t, was previously described and shown to be essential for positive selection of CD8+ T cells. In this issue of Immunity, Nitta et al. (2010) suggest that beta5t is essential because it alters the specific peptide-MHC complexes displayed by cortical epithelial cells.

Epidermal Langerhans cells are not required for UV-induced immunosuppression

UV light can be highly beneficial in the treatment of skin disorders such as psoriasis. It is thought to cause immunosuppression by depleting or altering the function of epidermal Langerhans cells (LC). Our previous studies identified a novel langerin(+) dendritic cell in the dermis, distinct from LC in phenotype, circulation, and function. In this study, we determined the role of LC and dermal langerin(+) cells in UV suppression. UV light suppressed the CD8 T cell response to both contact hypersensitivity and epicutaneous protein immunization, and resulted in a dramatically altered phenotype of LC. UV light did not alter early CD8 T cell activation in the lymph nodes, but rather reduced CD8 T cell expansion at later time points. We found that dermal langerin(+) cells, but not LC, were essential for the CD8 T cell response. Furthermore, in the selective absence of LC, UV light still caused suppression of both CD8 T cell expansion and contact hypersensitivity.

KLF2 transcription-factor deficiency in T cells results in unrestrained cytokine production and upregulation of bystander chemokine receptors

The transcription factor KLF2 regulates T cell trafficking by promoting expression of the lipid-binding receptor S1P(1) and the selectin CD62L. Recently, it was proposed that KLF2 also represses the expression of chemokine receptors. We confirmed the upregulation of the chemokine receptor CXCR3 on KLF2-deficient T cells. However, we showed that this was a cell-nonautonomous effect, as revealed by CXCR3 upregulation on wild-type bystander cells in mixed bone-marrow chimeras with KLF2-deficient cells. Furthermore, KLF2-deficient T cells overproduced IL-4, leading to the upregulation of CXCR3 through an IL-4-receptor- and eomesodermin-dependent pathway. Consistent with the increased IL-4 production, we found high concentrations of serum IgE in mice with T cell-specific KLF2 deficiency. Our findings support a model where KLF2 regulates T cell trafficking by direct regulation of S1P(1) and CD62L and restrains spontaneous cytokine production in naive T cells.

Positive selection optimizes the number and function of MHCII-restricted CD4+ T cell clones in the naive polyclonal repertoire

T cell receptors (TCRs) on T lymphocytes in an individual bind foreign peptides bound to major histocompatibility complex (MHC) molecules expressed in that individual (designated MHC(A)). Results from radiation bone marrow chimeras and TCR transgenic mice indicate that this complex form of antigen recognition is the result of positive selection of clones with low affinity for self peptide:MHC(A) complexes during development. Here we used a sensitive peptide:MHC tetramer enrichment method to quantify the role of positive selection in the generation of the preimmune polyclonal T cell repertoire in normal individuals. We made the surprising observation that mouse and human naive T cells capable of binding to foreign peptide:MHC(A) were present at the same frequency in hosts that expressed MHC(A) or a different MHC isoform (MHC(B)). However, most of the clones in MHC(B) hosts also recognized self peptide:MHC(A) complexes. When these "alloreactive" T cells were removed from the MHC(B) repertoire via negative selection in an MHC(A) host, the number of foreign peptide:MHC(A)-binding T cells was reduced to one fifth and many of the remaining cells did not respond to the peptide. Therefore, although positive selection on MHC(A) was not required to produce foreign peptide:MHC(A)-binding clones, it had a large effect on selecting responsive clones.

Thymic emigration: sphingosine-1-phosphate receptor-1-dependent models and beyond

The thymus is a primary lymphoid organ supporting the development of self-tolerant T cells. Key events in T-cell development in the thymus include lineage commitment, selection events, and thymic emigration. This review discusses the proposed role of sphingosine-1-phosphate and its receptors in the emigration of both conventional and unconventional T-cell subsets from the thymus, and the molecular machinery currently understood to regulate this process. Furthermore, we highlight a role for chemokines and actin-associated proteins in T-cell motility as recent data suggest that T-cell emigration is regulated by more than just a sphingosine-1-phosphate receptor-1-dependent chemotactic axis.

Langerhans cells are not required for the CD8 T cell response to epidermal self-antigens

Langerhans cells (LC) are APC that reside at the barrier surfaces. Mice expressing an OVA peptide in the epidermis (K14-OVAp) were used to study CD8(+) T cell responses to an epidermal self-Ag. Earlier results suggested that LC were the predominant APC, inducing a robust T cell response and autoimmunity. In this study, we used a whole protein model system, the K14-mOVA mouse, in which a transmembrane form of OVA was expressed in keratinocytes. In contrast to K14-OVAp mice, T cells in K14-mOVA mice were activated, but did not expand and instead died by apoptosis. Furthermore, in double-transgenic mice expressing both mOVA and OVAp, robust OT-I expansion occurred, indicating that tolerance to this Ag is not dominant and was due to lack of activating signals. We sought to identify the relevant APC in K14 mice using bone marrow chimeras and found that radioresistant cells (presumably LC) were able to cross-present the OVA Ag from keratinocytes to naive T cells in the lymph node. However, use of LC-deficient mice indicated that LC were not required for the expansion of OT-I in K14-OVAp or the deletion of OT-I in K14-mOVA mice. These data suggest that radioresistant non-LC present self-Ag in K14-OVAp mice and drive a robust CD8 T cell response.

The role of Kruppel-like factor 2 (KLF2) in thymic emigration and trafficking of non-conventional T cell lineages (82.16)

Immune function is dependent on the proper development of T cells in the thymus and emigration of these cells to the periphery. We recently showed the transcription factor KLF2 is required for emigration via regulation of S1P1 (sphingosine 1-phosphate receptor 1) and CD62L in conventional αβ T cells. However, the role of KLF2 and S1P1 in trafficking of non-conventional T cell subsets (γδ T cells, Treg, NKT, and Gut intraepithelial lymphocytes -IEL) has yet to be described. We show here that KLF2 is differentially expressed within the T cells subsets studied using both mRNA expression via quantitative PCR and/or a novel KLF2-GFP reporter mouse model. While KLF2 and S1P1 can be detected in sorted non-conventional thymic T cells and Gut IEL αβ T cells, we report no detectable KLF2 in Gut IEL γδ T cells. Using several KLF2 deficient mouse models, trafficking defects were observed in Treg, NKT, and Gut-IEL (αβ and γδ) populations. Finally, the data suggest that lack of KLF2 alters both the distribution and the phenotype of γδ T cells in secondary lymphoid organs. Overall, our results suggest that KLF2 is a common means by which different T cell lineages regulate trafficking and/or entry.

Funding provided by the National Institutes of Health (AI038903 to SCJ and AI039560 to KAH).

Central tolerance: what have we learned from mice?

Producing a healthy immune system capable of defending against pathogens, while avoiding autoimmunity, is dependent on thymic selection. Positive selection yields functional T cells that have the potential to recognize both self and foreign antigens. Therefore, negative selection exists to manage potentially self-reactive cells. Negative selection results from the induction of anergy, receptor editing, clonal diversion (agonist selection), and/or clonal deletion (apoptosis) in self-reactive clones. Clonal deletion has been inherently difficult to study because the cells of interest are undergoing apoptosis and being eliminated quickly. Furthermore, analysis of clonal deletion in humans has proved even more difficult due to availability of samples and lack of reagents. Mouse models have thus been instrumental in achieving our current understanding of central tolerance, and the evolution of elegant model systems has led to an explosion of new data to be assimilated. This review will focus on recent advances in the field of clonal deletion with respect to three aspects: the development of physiological model systems, signaling pathways that lead to apoptosis, and antigen presenting cell types involved in the induction of clonal deletion.

Clonal deletion of thymocytes can occur in the cortex with no involvement of the medulla

The thymic medulla is generally held to be a specialized environment for negative selection. However, many self-reactive thymocytes first encounter ubiquitous self-antigens in the cortex. Cortical epithelial cells are vital for positive selection, but whether such cells can also promote negative selection is controversial. We used the HY^cd4 model, where T cell receptor for antigen (TCR) expression is appropriately timed and a ubiquitous self-antigen drives clonal deletion in male mice. We demonstrated unambiguously that this deletion event occurs in the thymic cortex. However, the kinetics in vivo indicated that apoptosis was activated asynchronously relative to TCR activation. We found that radioresistant antigen-presenting cells and, specifically, cortical epithelial cells do not efficiently induce apoptosis, although they do cause TCR activation. Rather, thymocytes undergoing clonal deletion were preferentially associated with rare CD11c⁺ cortical dendritic cells, and elimination of such cells impaired deletion.

Thymic emigration: when and how T cells leave home

The thymus supports the differentiation of multiple distinct T cell subsets that play unique roles in the immune system. CD4 and CD8 alpha/beta T cells, gamma/delta T cells, NKT cells, regulatory T cells, and intraepithelial lymphocytes all develop in the thymus and must leave it to provide their functions elsewhere in the body. This article will review recent research indicating differences in the time and migration patterns of T cell subsets found in the thymus. Additionally, we review current understanding of the molecules involved in thymocyte emigration, including the sphingolipid receptor S1P(1) and its regulation by the Krüppel-like transcription factor KLF2.

Identification of a novel population of Langerin+ dendritic cells

Langerhans cells (LCs) are antigen-presenting cells that reside in the epidermis of the skin and traffic to lymph nodes (LNs). The general role of these cells in skin immune responses is not clear because distinct models of LC depletion resulted in opposite conclusions about their role in contact hypersensitivity (CHS) responses. While comparing these models, we discovered a novel population of LCs that resides in the dermis and does not represent migrating epidermal LCs, as previously thought. Unlike epidermal LCs, dermal Langerin(+) dendritic cells (DCs) were radiosensitive and displayed a distinct cell surface phenotype. Dermal Langerin(+) DCs migrate from the skin to the LNs after inflammation and in the steady state, and represent the majority of Langerin(+) DCs in skin draining LNs. Both epidermal and dermal Langerin(+) DCs were depleted by treatment with diphtheria toxin in Lang-DTREGFP knock-in mice. In contrast, transgenic hLang-DTA mice lack epidermal LCs, but have normal numbers of dermal Langerin(+) DCs. CHS responses were abrogated upon depletion of both epidermal and dermal LCs, but were unaffected in the absence of only epidermal LCs. This suggests that dermal LCs can mediate CHS and provides an explanation for previous differences observed in the two-model systems.

Thymic emigration revisited

Conventional αβ T cell precursors undergo positive selection in the thymic cortex. When this is successful, they migrate to the medulla and are exposed to tissue-specific antigens (TSA) for purposes of central tolerance, and they undergo maturation to become functionally responsive T cells. It is commonly understood that thymocytes spend up to 2 wk in the medulla undergoing these final maturation steps before emigrating to peripheral lymphoid tissues. In addition, emigration is thought to occur via a stochastic mechanism whereby some progenitors leave early and others leave late—a so-called “lucky dip” process. However, recent research has revealed that medullary thymocytes are a heterogeneous mix of naive αβ T cell precursors, memory T cells, natural killer T cells, and regulatory T cells. Given this, we revisited the question of how long it takes naive αβ T cell precursors to emigrate. We combined the following three approaches to study this question: BrdU labeling, intrathymic injection of a cellular tag, and RAG2p-GFP reporter mice. We established that, on average, naive αβ T cell precursors emigrate only 4–5 d after becoming single-positive (SP) thymocytes. Furthermore, emigration occurs via a strict “conveyor belt” mechanism, where the oldest thymocytes leave first.

Transcriptional analysis of clonal deletion in vivo

Engagement of the TCR on CD4(+)CD8(+) thymocytes initiates either a program of survival and differentiation (positive selection) or death (clonal deletion), which is dictated in large part by the affinity of the TCR for self-peptide-MHC complexes. Although much is known about the factors involved in positive selection, little is understood about the molecular mechanism leading to clonal deletion. To gain further insight into this process, we used a highly physiological TCR transgenic mouse model to compare gene expression changes under conditions of nonselection, positive selection, and negative selection. We identified 388 genes that were differentially regulated in negative selection compared with either nonselection or positive selection. These regulated genes fall into many functional categories including cell surface and intracellular signal transduction, survival and apoptosis, transcription and translation, and adhesion and migration. Additionally, we have compared our transcriptional profile to profiles of negative selection in other model systems in an effort to identify those genes with a higher probability of being functionally relevant. These included three up-regulated genes, bim, nur77, and ian1, and one down-regulated gene, lip1. Collectively, these data provide a framework for understanding the molecular basis of clonal deletion.

Tec kinases in T cell development: a clue behind the mask?

In this issue of Immunity, studies from Atherly et al (2006) and Broussard et al. (2006) show that mice deficient in Tec kinases support development of a population of CD8(+) T cells with "innate-immune" properties. The similarity of these cells to NKT and MHC-class-Ib-restricted T cells suggests a common developmental pathway.

Kruppel-like factor 2 regulates thymocyte and T-cell migration

Mammalian Kruppel-like transcription factors are implicated in regulating terminal differentiation of several tissue types. Deficiency in Kruppel-like factor (KLF) 2 (also known as LKLF) leads to a massive loss of the peripheral T-cell pool, suggesting KLF2 regulates T-cell quiescence and survival. Here we show, however, that KLF2 is essential for T-cell trafficking. KLF2-deficient (Klf2-/-) thymocytes show impaired expression of several receptors required for thymocyte emigration and peripheral trafficking, including the sphingosine-1-phosphate (S1P) receptor S1P1, CD62L and beta7 integrin. Furthermore, KLF2 both binds and transactivates the promoter for S1P1--a receptor that is critical for thymocyte egress and recirculation through peripheral lymphoid organs. Our findings suggest that KLF2 serves to license mature T cells for trafficking from the thymus and recirculation through secondary lymphoid tissues.

Conditioning of Langerhans cells induced by a primary CD8 T cell response to self-antigen in vivo

Using a previously described model of autoimmune skin disease, we addressed the question of how CD8 T cell responsiveness to self-Ag is regulated during chronic inflammation. In this model, CD8 T cells expand and induce tissue pathology directed at an epidermal self-Ag. However, we show here that this primary CD8 T cell response prevented subsequent expansion of a second CD8 T cell population with the same specificity. This lack of T cell accumulation was not due to Ag elimination, nor was it due to competition between the two T cell populations. However, skin-specific dendritic cells that present Ag in this model--Langerhans cells--underwent significant phenotypic changes associated with a compromised ability to stimulate naive T cells. Our study suggests that conditioning of dendritic cells may play a role in maintaining unresponsiveness to self-Ag during chronic inflammation.

Central tolerance: learning self-control in the thymus

In the past few years, there has been a flurry of discoveries and advancements in our understanding of how the thymus prepares T cells to exist at peace in normal healthy tissue: that is, to be self-tolerant. In the thymus, one of the main mechanisms of T-cell central tolerance is clonal deletion, although the selection of regulatory T cells is also important and is gaining enormous interest. In this Review, we discuss the emerging consensus about which models of clonal deletion are most physiological, and we review recent data that define the molecular mechanisms of central tolerance.

The timing of TCR alpha expression critically influences T cell development and selection

Sequential rearrangement of the T cell receptor for antigen (TCR) beta and alpha chains is a hallmark of thymocyte development. This temporal control is lost in TCR transgenics because the alpha chain is expressed prematurely at the CD4- CD8- double negative (DN) stage. To test the importance of this, we expressed the HY alpha chain at the physiological CD4+ CD8+ double positive (DP) stage. The reduced DP and increased DN cellularity typically seen in TCR transgenics was not observed when the alpha chain was expressed at the appropriate stage. Surprisingly, antigen-driven selection events were also altered. In male mice, thymocyte deletion now occurred at the single positive or medullary stage. In addition, no expansion of CD8 alpha alpha intestinal intraepithelial lymphocytes (IELs) was observed, despite the fact that HY transgenics have been used to model IEL development. Collectively, these data establish the importance of proper timing of TCR expression in thymic development and selection and emphasize the need to use models that most accurately reflect the physiologic process.

A requirement for sustained ERK signaling during thymocyte positive selection in vivo

It is unknown how the contrasting events of positive and negative selection can lead to the distinct biological outcomes of life or death. An increasing body of evidence suggests that the duration of extracellular signal-regulated kinase (ERK) signaling plays a role in thymocyte selection. However, it remains unclear what the kinetics of ERK activation are during positive selection in vivo. In this study, we examined the magnitude and duration of ERK signaling in intact murine thymic tissues cultured under conditions of negative or positive selection. We found that negative selection induced a rapid and robust ERK activation that is associated with death, whereas positive selection stimulated a lower intensity and brief ERK activation that quickly declined and then gradually increased and was sustained over several days. The expression pattern of Egr-1 (early growth response-1), a downstream ERK effector, correlates with the biphasic kinetics of ERK during positive selection. Id3 (inhibitor of differentiation/DNA binding 3) also exhibits biphasic kinetics but appeared to be independent of ERK signaling. Furthermore, inhibitors of T cell receptor ligation and ERK activation block maturation of CD8 single-positive thymocytes even when added after 24 h. These results demonstrate that the in vivo duration of ERK signaling must be sustained to support positive selection.