Deranged Early T Cell Development in Immunodeficient Strains of Nonobese Diabetic Mice

Genetic Variation in Type 1 Diabetes Reconfigures the 3D Chromatin Organization of T Cells and Alters Gene Expression

Genetics is a major determinant of susceptibility to autoimmune disorders. Here, we examined whether genome organization provides resilience or susceptibility to sequence variations, and how this would contribute to the molecular etiology of an autoimmune disease. We generated high-resolution maps of linear and 3D genome organization in thymocytes of NOD mice, a model of type 1 diabetes (T1D), and the diabetes-resistant C57BL/6 mice. Multi-enhancer interactions formed at genomic regions harboring genes with prominent roles in T cell development in both strains. However, diabetes risk-conferring loci coalesced enhancers and promoters in NOD, but not C57BL/6 thymocytes. 3D genome mapping of NODxC57BL/6 F1 thymocytes revealed that genomic misfolding in NOD mice is mediated in cis. Moreover, immune cells infiltrating the pancreas of humans with T1D exhibited increased expression of genes located on misfolded loci in mice. Thus, genetic variation leads to altered 3D chromatin architecture and associated changes in gene expression that may underlie autoimmune pathology.

Mechanisms of Action of Hematopoietic Transcription Factor PU.1 in Initiation of T-Cell Development

PU.1 is an ETS-family transcription factor that plays a broad range of roles in hematopoiesis. A direct regulator of myeloid, dendritic-cell, and B cell functional programs, and a well-known antagonist of terminal erythroid cell differentiation, it is also expressed in the earliest stages of T-cell development of each cohort of intrathymic pro-T cells. Its expression in this context appears to give T-cell precursors initial, transient access to myeloid and dendritic cell developmental competence and therefore to represent a source of antagonism or delay of T-cell lineage commitment. However, it has remained uncertain until recently why T-cell development is also intensely dependent upon PU.1. Here, we review recent work that sheds light on the molecular biology of PU.1 action across the genome in pro-T cells and identifies the genes that depend on PU.1 for their correct regulation. This work indicates modes of chromatin engagement, pioneering, and cofactor recruitment (“coregulator theft”) by PU.1 as well as gene network interactions that not only affect specific target genes but also have system-wide regulatory consequences, amplifying the impact of PU.1 beyond its own direct binding targets. The genes directly regulated by PU.1 also suggest a far-reaching transformation of cell biology and signaling potential between the early stages of T-cell development when PU.1 is expressed and when it is silenced. These cell-biological functions can be important to distinguish fetal from adult T-cell development and have the potential to illuminate aspects of thymic function that have so far remained the most mysterious.

Cytokines, Transcription Factors, and the Initiation of T-Cell Development

Multipotent blood progenitor cells migrate into the thymus and initiate the T-cell differentiation program. T-cell progenitor cells gradually acquire T-cell characteristics while shedding their multipotentiality for alternative fates. This process is supported by extracellular signaling molecules, including Notch ligands and cytokines, provided by the thymic microenvironment. T-cell development is associated with dynamic change of gene regulatory networks of transcription factors, which interact with these environmental signals. Together with Notch or pre-T-cell-receptor (TCR) signaling, cytokines always control proliferation, survival, and differentiation of early T cells, but little is known regarding their cross talk with transcription factors. However, recent results suggest ways that cytokines expressed in distinct intrathymic niches can specifically modulate key transcription factors. This review discusses how stage-specific roles of cytokines and transcription factors can jointly guide development of early T cells.

Of the multiple mechanisms leading to type 1 diabetes, T cell receptor revision may play a prominent role (is type 1 diabetes more than a single disease?)

A single determinant factor for autoimmunity does not exist; disease development probably involves contributions from genetics, the environment and immune dysfunction. Type 1 diabetes is no exception. Genomewide-associated studies (GWAS) analysis in T1D has proved disappointing in revealing contributors to disease prediction; the only reliable marker has been human leucocyte antigen (HLA). Specific HLAs include DR3/DR4/DQ2/DQ8, for example. Because HLA molecules present antigen to T cells, it is reasonable that certain HLA molecules have a higher affinity to present self-antigen. Recent studies have shown that additional polymorphisms in HLA that are restricted to autoimmune conditions are further contributory. A caveat is that not all individuals with the appropriate 'pro-autoimmune' HLA develop an autoimmune disease. Another crucial component is autoaggressive T cells. Finding a biomarker to discriminate autoaggressive T cells has been elusive. However, a subset of CD4 helper cells that express the CD40 receptor have been described as becoming pathogenic. An interesting function of CD40 on T cells is to induce the recombination-activating gene (RAG)1/RAG2 T cell receptor recombination machinery. This observation is contrary to immunology paradigms that changes in TCR molecules cannot take place outside the thymic microenvironment. Alteration in TCR, called TCR revision, not only occurs, but may help to account for the development of autoaggressive T cells. Another interesting facet is that type 1 diabetes (T1D) may be more than a single disease; that is, multiple cellular components contribute uniquely, but result ultimately in the same clinical outcome, T1D. This review considers the process of T cell maturation and how that could favor auto-aggressive T cell development in T1D. The potential contribution of TCR revision to autoimmunity is also considered.

Forging T-Lymphocyte Identity: Intersecting Networks of Transcriptional Control

T-lymphocyte development branches off from other lymphoid developmental programs through its requirement for sustained environmental signals through the Notch pathway. In the thymus, Notch signaling induces a succession of T-lineage regulatory factors that collectively create the T-cell identity through distinct steps. This process involves both the staged activation of T-cell identity genes and the staged repression of progenitor-cell-inherited regulatory genes once their roles in self-renewal and population expansion are no longer needed. With the recent characterization of innate lymphoid cells (ILCs) that share transcriptional regulation programs extensively with T-cell subsets, T-cell identity can increasingly be seen as defined in modular terms, as the processes selecting and actuating effector function are potentially detachable from the processes generating and selecting clonally unique T-cell receptor structures. The developmental pathways of different classes of T cells and ILCs are distinguished by the numbers of prerequisites of gene rearrangement, selection, and antigen contact before the cells gain access to nearly common regulatory mechanisms for choosing effector function. Here, the major classes of transcription factors that interact with Notch signals during T-lineage specification are discussed in terms of their roles in these programs, the evidence for their spectra of target genes at different stages, and their cross-regulatory and cooperative actions with each other. Specific topics include Notch modulation of PU.1 and GATA-3, PU.1-Notch competition, the relationship between PU.1 and GATA-3, and the roles of E proteins, Bcl11b, and GATA-3 in guiding acquisition of T-cell identity while avoiding redirection to an ILC fate.

Developmental gene networks: a triathlon on the course to T cell identity

Cells acquire their ultimate identities by activating combinations of transcription factors that initiate and sustain expression of the appropriate cell type-specific genes. T cell development depends on the progression of progenitor cells through three major phases, each of which is associated with distinct transcription factor ensembles that control the recruitment of these cells to the thymus, their proliferation, lineage commitment and responsiveness to T cell receptor signals, all before the allocation of cells to particular effector programmes. All three phases are essential for proper T cell development, as are the mechanisms that determine the boundaries between each phase. Cells that fail to shut off one set of regulators before the next gene network phase is activated are predisposed to leukaemic transformation.

Reduced regulatory T cell diversity in NOD mice is linked to early events in the thymus

The thymic natural regulatory T cell (Treg) compartment of NOD mice is unusual in having reduced TCR diversity despite normal cellularity. In this study, we show that this phenotype is attributable to perturbations in early and late stages of thymocyte development and is controlled, at least in part, by the NOD Idd9 region on chromosome 4. Progression from double negative 1 to double negative 2 stage thymocytes in NOD mice is inefficient; however, this defect is compensated by increased proliferation of natural Tregs (nTregs) within the single positive CD4 thymocyte compartment, accounting for recovery of cellularity accompanied by loss of TCR diversity. This region also underlies the known attenuation of ERK-MAPK signaling, which may preferentially disadvantage nTreg selection. Interestingly, the same genetic region also regulates the rate of thymic involution that is accelerated in NOD mice. These findings highlight further complexity in the control of nTreg repertoire diversity.

Loss of T cell progenitor checkpoint control underlies leukemia initiation in Rag1-deficient nonobese diabetic mice

NOD mice exhibit major defects in the earliest stages of T cell development in the thymus. Genome-wide genetic and transcriptome analyses were used to investigate the origins and consequences of an early T cell developmental checkpoint breakthrough in Rag1-deficient NOD mice. Quantitative trait locus analysis mapped the presence of checkpoint breakthrough cells to several known NOD diabetes susceptibility regions, particularly insulin-dependent diabetes susceptibility genes (Idd)9/11 on chromosome 4, suggesting common genetic origins for T cell defects affecting this trait and autoimmunity. Genome-wide RNA deep-sequencing of NOD and B6 Rag1-deficient thymocytes revealed the effects of genetic background prior to breakthrough, as well as the cellular consequences of the breakthrough. Transcriptome comparison between the two strains showed enrichment in differentially expressed signal transduction genes, prominently tyrosine kinase and actin-binding genes, in accord with their divergent sensitivities to activating signals. Emerging NOD breakthrough cells aberrantly expressed both stem cell-associated proto-oncogenes, such as Lmo2, Hhex, Lyl1, and Kit, which are normally repressed at the commitment checkpoint, and post-β-selection checkpoint genes, including Cd2 and Cd5. Coexpression of genes characteristic of multipotent progenitors and more mature T cells persists in the expanding population of thymocytes and in the thymic leukemias that emerge with age in these mice. These results show that Rag1-deficient NOD thymocytes have T cell defects that can collapse regulatory boundaries at two early T cell checkpoints, which may predispose them to both leukemia and autoimmunity.

Downregulation of the transcription factor KLF4 is required for the lineage commitment of T cells

The roles of the reprogramming factors Oct4, Sox2, c-Myc and Klf4 in early T cell development are incompletely defined. Here, we show that Klf4 is the only reprogramming factor whose expression is downregulated when early thymic progenitors (ETPs) differentiate into T cells. Enforced expression of Klf4 in uncommitted progenitors severely impaired T cell development mainly at the DN2-to-DN3 transition when T cell lineage commitment occurs and affected the transcription of a variety of genes with crucial functions in early T cell development, including genes involved in microenvironmental signaling (IL-7Rα), Notch target genes (Deltex1), and essential T cell lineage regulatory or inhibitory genes (Bcl11a, SpiB, and Id1). The survival of thymocytes and the rearrangement at the Tcrb locus were impaired in the presence of enforced Klf4 expression. The defects in the DN1-to-DN2 and DN2-to-DN3 transitions in Klf4 transgenic mice could not be rescued by the introduction of a TCR transgene, but was partially rescued by restoring the expression of IL-7Rα. Thus, our data indicate that the downregulation of Klf4 is a prerequisite for T cell lineage commitment.

αβ versus γδ fate choice: counting the T-cell lineages at the branch point

Both αβ and γδ T cells develop in the thymus from a common progenitor. Historically distinguished by their T-cell receptor (TCR), these lineages are now defined on the basis of distinct molecular programs. Intriguingly, in many transgenic and knockout systems these programs are mismatched with the TCR type, leading to the development of γδ lineage cells driven by αβTCR and vice versa. These puzzling observations were recently explained by the demonstration that TCR signal strength, rather than TCR type per se, instructs lineage fate, with stronger TCR signal favoring γδ and weaker signal favoring αβ lineage fates. These studies also highlighted the ERK (extracellular signal regulated kinase)-Egr (early growth response)-Id3 (inhibitor of differentiation 3) axis as a potential molecular switch downstream of TCR that determines lineage choice. Indeed, removal of Id3 was sufficient to redirect TCRγδ transgenic cells to the αβ lineage, even in the presence of strong TCR signal. However, in TCR non-transgenic Id3 knockout mice the overall number of γδ lineage cells was increased due to an outgrowth of a Vγ1Vδ6.3 subset, suggesting that not all γδ T cells depend on this molecular switch for lineage commitment. Thus, the γδ lineage may in fact be a collection of two or more lineages not sharing a common molecular program and thus equipollent to the αβ lineage. TCR signaling is not the only factor that is required for development of αβ and γδ lineage cells; other pathways, such as signaling from Notch and CXCR4 receptors, cooperate with the TCR in this process.

Lineage divergence at the first TCR-dependent checkpoint: preferential γδ and impaired αβ T cell development in nonobese diabetic mice

The first TCR-dependent checkpoint in the thymus determines αβ versus γδ T lineage fate and sets the stage for later T cell differentiation decisions. We had previously shown that early T cells in NOD mice that are unable to rearrange a TCR exhibit a defect in checkpoint enforcement at this stage. To determine if T cell progenitors from wild-type NOD mice also exhibit cell-autonomous defects in development, we investigated their differentiation in the Notch-ligand-presenting OP9-DL1 coculture system, as well as by analysis of T cell development in vivo. Cultured CD4 and CD8 double-negative cells from NOD mice exhibited major defects in the generation of CD4 and CD8 double-positive αβ T cells, whereas γδ T cell development from bipotent precursors was enhanced. Limiting dilution and single-cell experiments show that the divergent effects on αβ and γδ T cell development did not spring from biased lineage choice but from increased proliferation of γδ T cells and impaired accumulation of αβ T lineage double-positive cells. In vivo, NOD early T cell subsets in the thymus also show characteristics indicative of defective β-selection, and peripheral αβ T cells are poorly established in mixed bone marrow chimeras, contrasting with strong γδ T as well as B cell repopulation. Thus, NOD T cell precursors reveal divergent, lineage-specific differentiation abnormalities in vitro and in vivo from the first TCR-dependent developmental choice point, which may have consequences for subsequent lineage decisions and effector functions.

A transcription factor map as revealed by a genome-wide gene expression analysis of whole-blood mRNA transcriptome in multiple sclerosis

Background

Several lines of evidence suggest that transcription factors are involved in the pathogenesis of Multiple Sclerosis (MS) but complete mapping of the whole network has been elusive. One of the reasons is that there are several clinical subtypes of MS and transcription factors that may be involved in one subtype may not be in others. We investigate the possibility that this network could be mapped using microarray technologies and contemporary bioinformatics methods on a dataset derived from whole blood in 99 untreated MS patients (36 Relapse Remitting MS, 43 Primary Progressive MS, and 20 Secondary Progressive MS) and 45 age-matched healthy controls.

Methodology/Principal Findings

We have used two different analytical methodologies: a non-standard differential expression analysis and a differential co-expression analysis, which have converged on a significant number of regulatory motifs that are statistically overrepresented in genes that are either differentially expressed (or differentially co-expressed) in cases and controls (e.g., V$KROX_Q6, p-value <3.31E-6; V$CREBP1_Q2, p-value <9.93E-6, V$YY1_02, p-value <1.65E-5).

Conclusions/Significance

Our analysis uncovered a network of transcription factors that potentially dysregulate several genes in MS or one or more of its disease subtypes. The most significant transcription factor motifs were for the Early Growth Response EGR/KROX family, ATF2, YY1 (Yin and Yang 1), E2F-1/DP-1 and E2F-4/DP-2 heterodimers, SOX5, and CREB and ATF families. These transcription factors are involved in early T-lymphocyte specification and commitment as well as in oligodendrocyte dedifferentiation and development, both pathways that have significant biological plausibility in MS causation.

Fine-scale staging of T cell lineage commitment in adult mouse thymus

T cell development is marked by the loss of alternative lineage choices accompanying specification and commitment to the T cell lineage. Commitment occurs between the CD4 and CD8 double-negative (DN) 2 and DN3 stages in mouse early T cells. To determine the gene regulatory changes that accompany commitment, we sought to distinguish and characterize the earliest committed wild-type DN adult thymocytes. A transitional cell population, defined by the first downregulation of surface c-Kit expression, was found to have lost the ability to differentiate into dendritic cells and NK cells when cultured without Notch-Delta signals. In the presence of Notch signaling, this subset generates T lineage descendants in an ordered precursor-product relationship between DN2, with the highest levels of surface c-Kit, and c-Kit-low DN3 cells. These earliest committed cells show only a few differences in regulatory gene expression, compared with uncommitted DN2 cells. They have not yet established the full expression of Notch-related and T cell differentiation genes characteristic of DN3 cells before beta selection. Instead, the downregulation of select stem cell and non-T lineage genes appears to be key to the extinction of alternative lineage choices.

Non-obese diabetic mice select a low-diversity repertoire of natural regulatory T cells

Thymus-derived Foxp3(+) natural regulatory CD4 T cells (nTregs) prevent autoimmunity through control of pathogenic, autoreactive T cells and other immune effector cells. Using T cell receptor (TCR) transgenic models, diversity within this lineage has been found to be similar to that of conventional CD4 T cells. To determine whether balanced TCR diversity may be perturbed in autoimmunity, we have analyzed receptor composition in C57BL/6 and autoimmune non-obese diabetic (NOD) mice. The natural regulatory and conventional CD4 repertoires of C57BL/6 had similar diversities. Despite the apparently normal thymic development of the NOD nTreg lineage, TCR diversity within the selected repertoire was markedly restricted. Detailed analysis of TCRalpha and -beta chain composition is consistent with positive selection into the natural regulatory lineage being under stringent audition for interaction with MHC class II/self-peptide. The NOD MHC region, including the unique H2-A(g7) class II molecule, partly accounts for the reduction in diversity, but additional NOD genetic contribution(s) are required for complete repertoire compaction. Mechanistic links between MHC, autoimmunity, and nTreg diversity identified in this study are discussed.

A gene regulatory network armature for T lymphocyte specification

Choice of a T lymphoid fate by hematopoietic progenitor cells depends on sustained Notch-Delta signaling combined with tightly regulated activities of multiple transcription factors. To dissect the regulatory network connections that mediate this process, we have used high-resolution analysis of regulatory gene expression trajectories from the beginning to the end of specification, tests of the short-term Notch dependence of these gene expression changes, and analyses of the effects of overexpression of two essential transcription factors, namely PU.1 and GATA-3. Quantitative expression measurements of >50 transcription factor and marker genes have been used to derive the principal components of regulatory change through which T cell precursors progress from primitive multipotency to T lineage commitment. Our analyses reveal separate contributions of Notch signaling, GATA-3 activity, and down-regulation of PU.1. Using BioTapestry (www.BioTapestry.org), the results have been assembled into a draft gene regulatory network for the specification of T cell precursors and the choice of T as opposed to myeloid/dendritic or mast-cell fates. This network also accommodates effects of E proteins and mutual repression circuits of Gfi1 against Egr-2 and of TCF-1 against PU.1 as proposed elsewhere, but requires additional functions that remain unidentified. Distinctive features of this network structure include the intense dose dependence of GATA-3 effects, the gene-specific modulation of PU.1 activity based on Notch activity, the lack of direct opposition between PU.1 and GATA-3, and the need for a distinct, late-acting repressive function or functions to extinguish stem and progenitor-derived regulatory gene expression.

Transcription factor expression dynamics of early T-lymphocyte specification and commitment

Mammalian T lymphocytes are a prototype for development from adult pluripotent stem cells. While T-cell specification is driven by Notch signaling, T-lineage commitment is only finalized after prolonged Notch activation. However, no T-lineage specific regulatory factor has been reported that mediates commitment. We used a gene-discovery approach to identify additional candidate T-lineage transcription factors and characterized expression of >100 regulatory genes in early T-cell precursors using realtime RT-PCR. These regulatory genes were also monitored in multilineage precursors as they entered T-cell or non-T-cell pathways in vitro; in non-T cells ex vivo; and in later T-cell developmental stages after lineage commitment. At least three major expression patterns were observed. Transcription factors in the largest group are expressed at relatively stable levels throughout T-lineage specification as a legacy from prethymic precursors, with some continuing while others are downregulated after commitment. Another group is highly expressed in the earliest stages only, and is downregulated before or during commitment. Genes in a third group undergo upregulation at one of three distinct transitions, suggesting a positive regulatory cascade. However, the transcription factors induced during commitment are not T-lineage specific. Different members of the same transcription factor family can follow opposite trajectories during specification and commitment, while factors co-expressed early can be expressed in divergent patterns in later T-cell development. Some factors reveal new regulatory distinctions between alphabeta and gammadelta T-lineage differentiation. These results show that T-cell identity has an essentially complex regulatory basis and provide a detailed framework for regulatory network modeling of T-cell specification.

Molecular mechanisms that control mouse and human TCR-alphabeta and TCR-gammadelta T cell development

Following specification of hematopoietic precursor cells into the T cell lineage, several developmental options remain available to the immature thymocytes. The paradigm is that the outcome of the T cell receptor rearrangements and the corresponding T cell receptor signaling events will be predominant to determine the first of these choices: the alphabeta versus gammadelta T cell pathways. Here, we review the thymus-derived environmental signals, the transcriptional mediators, and other molecular mechanisms that are also involved in this decision in both the mouse and human. We discuss the differences in cellular events between the alphabeta and gammadelta developmental pathways and try to correlate these with a corresponding complexity of the molecular mechanisms that support them.

Diabetes-induced atrophy is associated with a muscle-specific alteration in NF-kappaB activation and expression

NF-kappaB is a transcription factor implicated in pathological responses that develop during diabetes mellitus, including skeletal muscle atrophy. Given that NF-kappaB activation, protein composition, and content within diabetic skeletal muscle remain generally uncharacterized, a streptozotocin (STZ) model was used to assess NF-kappaB activation, composition, and content. Sprague-Dawley rats were injected with STZ (55 mg/kg) and after 30 days the soleus (SOL), plantaris (PL), red gastrocnemius (RG), and white gastrocnemius (WG) muscles were assessed by electrophoresis mobility shift assay and western blotting. NF-kappaB activation was detected in all muscles examined, but was reduced in RG muscles from diabetic animals. Supershifts indicated NF-kappaB was composed primarily of p50 in diabetic and control animals. The content of both p65 and p52 was elevated in SOL and PL muscles, while p52 was decreased in RG. The coactivating protein, Bcl-3, was increased in WG and RG, but decreased in PL. Both p50 and RelB remained unchanged in all tissues examined. All muscles from diabetic animals demonstrated reduced mass when compared to controls, but only the gastrocnemius demonstrated atrophy as reflected by a reduced muscle-to-body mass ratio. In conclusion, diabetic alterations to the contents and activation of the NF-kappaB protein were tissue-specific, but did not appear to alter dimer composition of constitutively bound NF-kappaB. These results indicate that diabetes may alter NF-kappaB activity and expression in a muscle-specific manner.

Dietary consumption of Echinacea by mice afflicted with autoimmune (type I) diabetes: effect of consuming the herb on hemopoietic and immune cell dynamics

Epidemiological studies indicate that the incidence of Type 1 diabetes, an autoimmune disease, is rising rapidly. However, none of the current therapies produces life long remission, or can prevent the disease onset. The NOD (non-obese diabetic) mouse is currently regarded as an excellent animal model of human Type 1 diabetes. NKT cells are known to be fundamental in modulating the disease, yet they are numerically and functionally deficient in mammals bearing this disease. Indeed, the role of NK cells in inhibiting autoimmunity in general is well established. Immunoregulatory strategies are currently believed to be the way of the future with respect to modulating autoimmune diseases. Based on this hypothesis, and the fact that the herb, Echinacea, is a well demonstrated immunostimulant of NK cells in normal mice/humans, we aimed to investigate, in NOD mice, the effect of short term (days) and long term (months) daily dietary administration of Echinacea, on the absolute levels of NK cells, and five other classes of hemopoietic and immune cells, in the bone marrow and spleen. The results revealed that, in NOD mice, dietary Echinacea, resulted in a significant increase in the absolute numbers of NK cells, irrespective of feeding duration, in the spleen, and moreover, it actually stimulated NK cell production in their bone marrow birth site. We further found that there were transient, early (days), herb exposure-time-dependent, quantitative changes in several of the other hemopoietic and immune cells populations in both the bone marrow and spleen. We conclude that consumption of this herb by NOD mice, at least, has lead to no negative repercussions with respect to the hemopoietic and immune lineages, and secondly, the consistent, long-lasting immunostimulation only of NK cells, may lead to a possible new approach to the treatment of Type 1 diabetes.

Launching the T-cell-lineage developmental programme

Multipotent blood progenitor cells enter the thymus and begin a protracted differentiation process in which they gradually acquire T-cell characteristics while shedding their legacy of developmental plasticity. Notch signalling and basic helix-loop-helix E-protein transcription factors collaborate repeatedly to trigger and sustain this process throughout the period leading up to T-cell lineage commitment. Nevertheless, the process is discontinuous with separately regulated steps that demand roles for additional collaborating factors. This Review discusses new evidence on the coordination of specification and commitment in the early T-cell pathway; effects of microenvironmental signals; the inheritance of stem-cell regulatory factors; and the ensemble of transcription factors that modulate the effects of Notch and E proteins, to distinguish individual stages and to polarize T-cell-lineage fate determination.

Characterization of the NOD/scid-[Tg]DR1 mouse expressing HLA-DRB1*01 transgene: a model of SCID-hu mouse for vaccine development

OBJECTIVE

We previously showed enhanced engraftment of human T cells in the transgenic NonObese Diabetic/severe combined immunodeficient (NOD/scid)-DR1 mice, compared to NOD/scid mice. We now characterize their immunobiology, innate immunity, and intrahepatic neonatal engraftment of cord blood mononuclear cells (CBMNC), and test immune responses of these chimeric mice to an experimental cancer vaccine.

METHODS

Fluorescence in situ hybridization analysis, blood biochemistry, hematology, and fluorescein-activated cell sorting analyses of cellular subsets were performed on NOD/scid-DR1 mice, in comparison to parental NOD/scid mice. Innate immunity and lifespan were examined. Histology of engrafted tissues and short-term intrahepatic engraftment of CBMNC were performed. Intracellular interferon-gamma (IFN-gamma) production was assessed in mice immunized with cancer vaccine.

RESULTS

The DR1 transgene was located on chromosome 5 and no significant changes were observed in blood chemistry, peripheral blood counts, lymphoid subsets, natural killer cell and lipopolysaccharide response, and antigen presentation in the NOD/scid-DR1 mice, compared to NOD/scid mice. Interestingly, NOD/scid-DR1 mice had a significantly longer lifespan (approximately 14 months) than NOD/scid mice (approximately 8.5 months). Engraftment with human cord blood cells resulted in slight changes in the architecture/structure of spleens. No correlation was found between DR1 genotype of the donor CBMNC and extent of engraftment of human T cells. Enhanced engraftment of human cells was observed with intrahepatic injections of CBMNC in neonatal NOD/scid DR1 mice. Intracellular IFN-gamma was detected in human cells, when chimeric mice were immunized with a cancer vaccine.

CONCLUSION

NOD/scid-DR1 mice were similar in most of the physiological parameters as the NOD/scid mice, with the exception of longer lifespan. Intrahepatic engraftment of neonatal mice is the preferred protocol of xenotransplantation in this model and the engrafted human cells can respond to a cancer vaccine.

Progression of regulatory gene expression states in fetal and adult pro-T-cell development

Precursors entering the T-cell developmental pathway traverse a progression of states characterized by distinctive patterns of gene expression. Of particular interest are regulatory genes, which ultimately control the dwell time of cells in each state and establish the mechanisms that propel them forward to subsequent states. Under particular genetic and developmental circumstances, the transitions between these states occur with different timing, and environmental feedbacks may shift the steady-state accumulations of cells in each state. The fetal transit through pro-T-cell stages is faster than in the adult and subject to somewhat different genetic requirements. To explore causes of such variation, this review presents previously unpublished data on differentiation gene activation in pro-T cells of pre-T-cell receptor-deficient mutant mice and a quantitative comparison of the profiles of transcription factor gene expression in pro-T-cell subsets of fetal and adult wildtype mice. Against a background of consistent gene expression, several regulatory genes show marked differences between fetal and adult expression profiles, including those encoding two basic helix-loop-helix antagonist Id factors, the Ets family factor SpiB and the Notch target gene Deltex1. The results also reveal global differences in regulatory alterations triggered by the first T-cell receptor-dependent selection events in fetal and adult thymopoiesis.

The basic helix-loop-helix transcription factor HEBAlt is expressed in pro-T cells and enhances the generation of T cell precursors

The basic helix-loop-helix (bHLH) transcription factors HEB and E2A are critical mediators of gene regulation during lymphocyte development. We have cloned a new transcription factor, called HEBAlt, from a pro-T cell cDNA library. HEBAlt is generated by alternative transcriptional initiation and splicing from the HEB gene locus, which also encodes the previously characterized E box protein HEBCan. HEBAlt contains a unique N-terminal coding exon (the Alt domain) that replaces the first transactivation domain of HEBCan. Downstream of the Alt domain, HEBAlt is identical to HEBCan, including the DNA binding domain. HEBAlt is induced in early thymocyte precursors and down-regulated permanently at the double negative to double positive (DP) transition, whereas HEBCan mRNA expression peaks at the DP stage of thymocyte development. HEBAlt mRNA is up-regulated synergistically by a combination of HEBCan activity and Delta-Notch signaling. Retroviral transduction of HEBAlt or HEBCan into hemopoietic stem cells followed by OP9-DL1 coculture revealed that HEBAlt-transduced precursors generated more early T lineage precursors and more DP pre-T cells than control transduced cells. By contrast, HEBCan-transduced cells that maintained high level expression of the HEBCan transgene were inhibited in expansion and progression through T cell development. HEB(-/-) fetal liver precursors transduced with HEBAlt were rescued from delayed T cell specification, but HEBCan-transduced HEB(-/-) precursors were not. Therefore, HEBAlt and HEBCan are functionally distinct transcription factors, and HEBAlt is specifically required for the efficient generation of early T cell precursors.

Notch/Delta signaling constrains reengineering of pro-T cells by PU.1

PU.1 is essential for early stages of mouse T cell development but antagonizes it if expressed constitutively. Two separable mechanisms are involved: attenuation and diversion. Dysregulated PU.1 expression inhibits pro-T cell survival, proliferation, and passage through beta-selection by blocking essential T cell transcription factors, signaling molecules, and Rag gene expression, which expression of a rearranged T cell antigen receptor transgene cannot rescue. However, Bcl2 transgenic cells are protected from this attenuation and may even undergo beta-selection, as shown by PU.1 transduction of defined subsets of Bcl2 transgenic fetal thymocytes with differentiation in OP9-DL1 and OP9 control cultures. The outcome of PU.1 expression in these cells depends on Notch/Delta signaling. PU.1 can efficiently divert thymocytes toward a myeloid-like state with multigene regulatory changes, but Notch/Delta signaling vetoes diversion. Gene expression analysis distinguishes sets of critical T lineage regulatory genes with different combinatorial responses to PU.1 and Notch/Delta signals, suggesting particular importance for inhibition of E proteins, Myb, and/or Gfi1 (growth factor independence 1) in diversion. However, Notch signaling only protects against diversion of cells that have undergone T lineage specification after Thy-1 and CD25 up-regulation. The results imply that in T cell precursors, Notch/Delta signaling normally acts to modulate and channel PU.1 transcriptional activities during the stages from T lineage specification until commitment.

Developmental and molecular characterization of emerging beta- and gammadelta-selected pre-T cells in the adult mouse thymus

The first checkpoint in T cell development, beta selection, has remained incompletely characterized for lack of specific surface markers. We show that CD27 is upregulated in DN3 thymocytes initiating beta selection, concomitant with intracellular TCR-beta expression. Clonal analysis determined that CD27high DN3 cells generate CD4+CD8+ progeny with more than 90% efficiency, faster and more efficiently than the CD27low majority. CD27 upregulation also occurs in gammadelta-selected DN3 thymocytes in TCR-beta-/- mice and in IL2-GFP transgenic reporter mice where GFP marks the earliest emerging TCR-gammadelta cells from DN3 thymocytes. With CD27 to distinguish pre- and postselection DN3 cells, a detailed gene expression analysis defined regulatory changes associated with checkpoint arrest, with beta selection, and with gammadelta selection. gammadelta selection induces higher CD5, Egr, and Runx3 expression as compared to beta selection, but it triggers less proliferation. Our results also reveal differences in Notch/Delta dependence at the earliest stages of divergence between developing alphabeta and gammadelta T-lineage cells.

Human lymphoid and myeloid cell development in NOD/LtSz-scid IL2R gamma null mice engrafted with mobilized human hemopoietic stem cells

Ethical considerations constrain the in vivo study of human hemopoietic stem cells (HSC). To overcome this limitation, small animal models of human HSC engraftment have been used. We report the development and characterization of a new genetic stock of IL-2R common gamma-chain deficient NOD/LtSz-scid (NOD-scid IL2Rgamma(null)) mice and document their ability to support human mobilized blood HSC engraftment and multilineage differentiation. NOD-scid IL2Rgamma(null) mice are deficient in mature lymphocytes and NK cells, survive beyond 16 mo of age, and even after sublethal irradiation resist lymphoma development. Engraftment of NOD-scid IL2Rgamma(null) mice with human HSC generate 6-fold higher percentages of human CD45(+) cells in host bone marrow than with similarly treated NOD-scid mice. These human cells include B cells, NK cells, myeloid cells, plasmacytoid dendritic cells, and HSC. Spleens from engrafted NOD-scid IL2Rgamma(null) mice contain human Ig(+) B cells and lower numbers of human CD3(+) T cells. Coadministration of human Fc-IL7 fusion protein results in high percentages of human CD4(+)CD8(+) thymocytes as well human CD4(+)CD8(-) and CD4(-)CD8(+) peripheral blood and splenic T cells. De novo human T cell development in NOD-scid IL2Rgamma(null) mice was validated by 1) high levels of TCR excision circles, 2) complex TCRbeta repertoire diversity, and 3) proliferative responses to PHA and streptococcal superantigen, streptococcal pyrogenic exotoxin. Thus, NOD-scid IL2Rgamma(null) mice engrafted with human mobilized blood stem cells provide a new in vivo long-lived model of robust multilineage human HSC engraftment.

Subversion of T lineage commitment by PU.1 in a clonal cell line system

Specification of mammalian T lymphocytes involves prolonged developmental plasticity even after lineage-specific gene expression begins. Expression of transcription factor PU.1 may maintain some myeloid-like developmental alternatives until commitment. Commitment could reflect PU.1 shutoff, resistance to PU.1 effects, and/or imposition of a suicide penalty for diversion. Here, we describe subclones from the SCID.adh murine thymic lymphoma, adh.2C2 and adh.6D4, that represent a new tool for probing these mechanisms. PU.1 can induce many adh.2C2 cells to undergo diversion to a myeloid-like phenotype, in an all-or-none fashion with multiple, coordinate gene expression changes; adh.6D4 cells resist diversion, and most die. Diversion depends on the PU.1 Ets domain but not on known interactions in the PEST or Q-rich domains, although the Q-rich domain enhances diversion frequency. Protein kinase C/MAP kinase stimulation can make adh.6D4 cells permissive for diversion without protecting from suicide. These results show distinct roles for regulated cell death and another stimulation-sensitive function that establishes a threshold for diversion competence. PU.1 also diverts normal T-cell precursors from wild type or Bcl2-transgenic mice to a myeloid-like phenotype, upon transduction in short-term culture. The adh.2C2 and adh.6D4 clones thus provide an accessible system for defining mechanisms controlling developmental plasticity in early T-cell development.