Requirement for RORgamma in thymocyte survival and lymphoid organ development

Cellular Interactions in Lymph Node Development

The organized accumulation of lymphocytes is a biological phenomenon used to optimize both homeostatic immune surveillance, as well as chronic responses to pathogenic stimuli. During embryonic development, circulating hemopoietic cells gather at predestined sites throughout the body, where they are subsequently arranged in T and B cell-specific areas characteristic of secondary lymphoid organs. In contrast, the body seems to harbor a limited second set of selected sites that support formation of organized lymphoid aggregates. However, these are only revealed at times of local, chronic inflammation, when so-called tertiary lymphoid structures appear. Once thought of as two distinct phenomena, recent insights suggest that highly similar networks of paracrine interactions regulate the formation of both secondary and tertiary lymphoid structures. This review will focus on these cellular interactions between organizing and inducing cell populations leading to the formation of lymph nodes or organized inflammatory infiltrates.

Lymphotoxin-mediated regulation of gammadelta cell differentiation by alphabeta T cell progenitors

The thymus gives rise to two T cell lineages, alphabeta and gammadelta, that are thought to develop independently of one another. Hence, double positive (DP) thymocytes expressing CD4 and CD8 coreceptors are usually viewed simply as progenitors of CD4+ and CD8+ alphabeta T cells. Instead we report that DP cells regulate the differentiation of early thymocyte progenitors and gammadelta cells, by a mechanism dependent on the transcription factor RORgt, and the lymphotoxin (LT) beta receptor (LTbetaR). This finding provokes a revised view of the thymus, in which lymphoid tissue induction-type processes coordinate the developmental and functional integration of the two T cell lineages.

Lymphocyte development and function in the absence of retinoic acid-related orphan receptor alpha

The orphan nuclear receptor, retinoid acid-related orphan receptor (ROR)alpha, is essential for the development of cerebellar Purkinje cells and bone tissue. RORalpha may also play a critical role in lymphocyte development and function because staggerer mice, a natural mutant strain with a disrupted expression of RORalpha, have reduced thymic and splenic cellularity. In this report, we analyzed the role of RORalpha in lymphocyte development by examining lymphoid compartments in RORalpha(-/-) mice and Rag-2(-/-) mice reconstituted with RORalpha(-/-) bone marrow. We found that T and B cell development was severely defective in RORalpha(-/-) mice, but not in Rag-2(-/-)/RORalpha(-/-) chimeric mice. We also analyzed cellular and humoral immune responses in Rag-2(-/-)/RORalpha(-/-) chimeric mice. Our results show that serum IgG levels were elevated in Rag-2(-/-)/RORalpha(-/-) chimeric mice after immunization with a T-dependent Ag compared with control chimeras. IFN-gamma production by RORalpha(-/-) CD8(+) T cells after TCR stimulation was also increased. Furthermore, RORalpha(-/-) mast cells and macrophages produced an increased amount of TNF-alpha and IL-6 upon activation. These results indicate that RORalpha indirectly regulates lymphocyte development by providing an appropriate microenvironment and controls immune responses by negatively regulating cytokine production in innate immune cells and lymphocytes.

General method for the modification of different BAC types and the rapid generation of BAC transgenic mice

Most genome projects have relied on the sequencing of bacterial artificial chromosomes (BACs), which encompass 100-300 kb of genomic DNA. As a consequence, several thousand BAC clones are now mapped to the human and mouse genome. It is therefore possible to identify in silico a BAC clone that carries a particular gene and obtain it commercially. Given the large size of BACs, most if not all regulatory sequences of a gene are present and can be used to direct faithful and tissue-specific expression of heterologous genes in vitro in cell cultures and in vivo in BAC-transgenic mice. We describe here an optimized and comprehensive protocol to select, modify, and purify BACs in order to generate BAC-transgenic mice. Importantly, this protocol includes a method to generate, within 2 days, complex plasmid cassettes required to modify BACs, and to efficiently modify different types of BACs selected from the two major BAC libraries available. Altogether, using a combination of genomic database analysis, overlap PCR cloning, and BAC recombination in bacteria, our approach allows for the rapid and reliable generation of "pseudo knockin" mice. genesis 38:39-50, 2004.

IL-7 receptor signals inhibit expression of transcription factors TCF-1, LEF-1, and RORgammat: impact on thymocyte development

Intrathymic T cell development depends on signals transduced by both T cell receptor and cytokine receptors. Early CD4⁻CD8⁻ (double negative) thymocytes require interleukin (IL)-7 receptor (IL-7R) signals for survival and proliferation, but IL-7R signals are normally extinguished by the immature single positive (ISP) stage of thymocyte development. We now demonstrate that IL-7R signals inhibit expression of transcription factors TCF-1, LEF-1, and RORγt that are required for the ISP to double positive (DP) transition in the thymus. In addition, we demonstrate that IL-7R signals also inhibit TCF-1 and LEF-1 expression in mature peripheral T cells. Thus, the present work has identified several important downstream target genes of IL-7R signaling in T cells and thymocytes that provide a molecular mechanism for the inhibitory influence of IL-7R signaling on DP thymocyte development. We conclude that IL-7R signals down-regulate transcription factors required for the ISP to DP transition and so must be terminated by the ISP stage of thymocyte development.

NALT- versus Peyer's-patch-mediated mucosal immunity

Recent studies indicate that the mechanism of nasopharynx-associated lymphoid tissue (NALT) organogenesis is different from that of other lymphoid tissues. NALT has an important role in the induction of mucosal immune responses, including the generation of T helper 1 and T helper 2 cells, and IgA-committed B cells. Moreover, intranasal immunization can lead to the induction of antigen-specific protective immunity in both the mucosal and systemic immune compartments. Therefore, a greater understanding of the differences between NALT and other organized lymphoid tissues, such as Peyer's patches, should facilitate the development of nasal vaccines.

Sustained early growth response gene 3 expression inhibits the survival of CD4/CD8 double-positive thymocytes

In the absence of selection, CD4+, CD8+ double-positive (DP) thymocytes will die after 3-4 days. The mechanism for regulating the life span of DP cells is unknown. Previously, we demonstrated that the zinc finger transcription factor, early growth response gene 3 (Egr3), promotes proliferation during the transition from double negative (DN) to DP. In this study we demonstrate a novel role for Egr3 in controlling DP thymocyte survival in mice. Constitutive transgenic expression of Egr3 in thymocytes increases apoptosis among DP cells and shortens their survival in vitro. In addition, DP cells in Egr3 transgenic mice have poor expression of TCRalpha, and based on the predominant usage of 3' Valpha and 5' Jalpha gene segments, the low level of TCRalpha expression is a result of DP death soon after the initiation of TCRalpha rearrangements. Constitutive transgenic expression of Egr3 results in poor expression of Bcl-x(L) and the thymic isoform of retinoic acid receptor-related orphan receptor gamma (RORgammat) in DP thymocytes, two molecules that are required in DP cells for normal life span. Egr3 expression induced by pre-TCR signals in nontransgenic mice is transient and returns to background levels before RORgammat or Bcl-x(L) is induced. The data support a model in which Egr3 must be transiently induced in response to pre-TCR signals, so that the expression of the prosurvival molecules, RORgammat and Bcl-x(L), can be elevated only after the proliferative signal provided by Egr3 has subsided.

Role of inducible bronchus associated lymphoid tissue (iBALT) in respiratory immunity

Bronchus-associated lymphoid tissue (BALT) is occasionally found in the lungs of mice and humans; however, its role in respiratory immunity is unknown. Here we show that mice lacking spleen, lymph nodes and Peyer's patches generate unexpectedly robust primary B- and T-cell responses to influenza, which seem to be initiated at sites of induced BALT (iBALT). Areas of iBALT have distinct B-cell follicles and T-cell areas, and support T and B-cell proliferation. The homeostatic chemokines CXCL13 and CCL21 are expressed independently of TNFalpha and lymphotoxin at sites of iBALT formation. In addition, mice with iBALT, but lacking peripheral lymphoid organs, clear influenza infection and survive higher doses of virus than do normal mice, indicating that immune responses generated in iBALT are not only protective, but potentially less pathologic, than systemic immune responses. Thus, iBALT functions as an inducible secondary lymphoid tissue for respiratory immune responses.

Thymic origin of intestinal alphabeta T cells revealed by fate mapping of RORgammat+ cells

Intestinal intraepithelial T lymphocytes (IELs) are likely to play a key role in host mucosal immunity and, unlike other T cells, have been proposed to differentiate from local precursors rather than from thymocytes. We show here that IELs expressing the alphabeta T cell receptor are derived from precursors that express RORgammat, an orphan nuclear hormone receptor detected only in immature CD4+CD8+ thymocytes, fetal lymphoid tissue-inducer (LTi) cells, and LTi-like cells in cryptopatches within the adult intestinal lamina propria. Using cell fate mapping, we found that all intestinal alphabeta T cells are progeny of CD4+CD8+ thymocytes, indicating that the adult intestine is not a significant site for alphabeta T cell development. Our results suggest that intestinal RORgammat+ cells are local organizers of mucosal lymphoid tissue.

Gene expression profiles during human CD4+ T cell differentiation

To develop a comprehensive catalogue of phenotypic and functional parameters of human CD4(+) T cell differentiation stages, we have performed microarray gene expression profiling on subpopulations of human thymocytes and circulating naive CD4(+) T cells, including CD3(-)CD4(+)CD8(-) intrathymic T progenitor cells, CD3(int)CD4(+)CD8(+) 'double positive' thymocytes, CD3(high)CD4(+)CD8(-) 'single positive' thymocytes, CD3(+)CD4(+)CD8(-) CD45RA(+)CD62L(+) naive T cells from cord blood and CD3(+)CD4(+)CD8(-) CD45RA(+)CD62L(+) naive T cells from adult blood. These subpopulations were sort-purified to >98% purity and their expressed RNAs were analyzed on Affymetrix Human Genome U133 arrays. Comparison of gene expression signals between these subpopulations and with early passage fetal thymic stromal cultures identify: (i) transcripts that are preferentially expressed in human CD4(+) T cell subpopulations and not in thymic stromal cells; (ii) major shifts in gene expression as progenitor T cells mature into progeny; (iii) preferential expression of transcripts at the progenitor cell stage with plausible relevance to the regulation of expansion and differentiation of these cells; and (iv) preferential expression of potential markers of recent thymic emigrants in naive-phenotype CD4(+) T cells from cord blood. Further evaluation of these findings may lead to a better definition of human thymopoiesis as well as to improved approaches to monitor and to augment the function of this important organ of T cell production.

Critical functions for c-Myb at three checkpoints during thymocyte development

The transcription factor c-Myb is expressed throughout T cell development in the thymus. However, little is understood about c-Myb function because of the embryonic lethality of traditional Myb-null mutations. Using tissue-specific deletion to abrogate c-Myb expression at distinct stages of T cell development, we identify three points at which c-Myb activity is required for normal T cell differentiation: transition through the double-negative 3 stage, survival of preselection CD4(+)CD8(+) thymocytes, and differentiation of CD4 thymocytes. Thus, c-Myb is essential at several stages during T cell development in the thymus.

Rorgamma (Rorc) is a common integration site in type B leukemogenic virus-induced T-cell lymphomas

The retrovirus type B leukemogenic virus (TBLV) causes T-cell lymphomas in mice. We have identified the Rorgamma locus as an integration site in 19% of TBLV-induced tumors. Overexpression of one or more Rorgamma isoforms in >77% of the tumors tested may complement apoptotic effects of c-myc overexpression.

NuRD complex component Mi-2β binds to and represses RORγ-mediated transcriptional activation

RORgamma is a nuclear receptor that binds to DNA motifs as a monomer to constitutively activate target genes. RORgamma plays an important role in thymocyte development and lymph node organogenesis, while the regulation of RORgamma-mediated transcriptional activation is currently unclear. The purpose of this study was to identify other nuclear proteins that interact with RORgamma. A yeast two-hybrid screen with Y190 yeast cells under stringent conditions resulted in the identification of CHD4, also known as Mi-2beta, as a RORgamma-interacting protein. This interaction was confirmed by GST pull-down assays. This interaction occurred within the middle regulatory region (amino acids 719-1164) of Mi-2beta. Transfection of Gal4-RORgamma into HeLa cells resulted in constitutive transactivation of the MH100-tk-luc reporter. The addition of Mi-2beta resulted in a dramatic 50% decrease in Gal4-RORgamma-mediated transactivation. These data demonstrate that RORgamma forms a protein-protein interaction with the regulatory region of Mi-2beta, resulting in inhibition of RORgamma transcriptional activity. These results may provide evidence as to how RORgamma-mediated transactivation is regulated by other nuclear proteins.

TAL1/SCL induces leukemia by inhibiting the transcriptional activity of E47/HEB

Activation of the basic-helix-loop-helix (bHLH) gene TAL1 (or SCL) is a frequent gain-of-function mutation in T cell acute lymphoblastic leukemia (T-ALL). To provide genetic evidence that tal1/scl induces leukemia by interfering with E47 and HEB, we expressed tal1/scl in an E2A or HEB heterozygous background. These mice exhibit disease acceleration and perturbed thymocyte development due to repression of E47/HEB target genes. In tal1/scl thymocytes, we find the corepressor mSin3A bound to the CD4 enhancer, whereas an E47/HEB/p300 complex is detected in wild-type thymocytes. Furthermore, tal1/scl tumors are sensitive to pharmacologic inhibition of HDAC and undergo apoptosis. These data demonstrate that tal1/scl induces leukemia by repressing E47/HEB and suggest that HDAC inhibitors may prove efficacious in T-ALL patients who express TAL1/SCL.

Selective LXXLL peptides antagonize transcriptional activation by the retinoid-related orphan receptor RORgamma

The retinoid-related orphan receptor gamma (RORgamma) has been shown to function as a positive regulator of transcription in many cell lines. Transcriptional activation by nuclear receptors involves recruitment of co-activators that interact with receptors through their LXXLL motifs (NR box). In this study, we analyze the interaction of RORgamma with the co-activator SRC1 and use a series of LXXLL-containing peptides to probe for changes in the conformation of the co-activator interaction surface of the RORgamma LBD. We demonstrate that the H3-4/H12 co-activator interaction surface of RORgamma displays a selectivity for LXXLL peptides that is distinct from those of other nuclear receptors. LXXLL peptides that interacted with RORgamma efficiently antagonized RORgamma-mediated transcriptional activation. Mutations E502Q and Y500F in H12, and K334A, Q347A, and I348D in H3 and H4 of RORgamma, severely impact the recruitment of LXXLL peptides. The effects of these mutations are consistent with predictions made on the basis of the structure of the RORgamma(LBD) derived through homology modeling. These peptide antagonists provide a useful tool to analyze the conformation changes in the RORgamma(LBD) and to study RORgamma receptor signaling.

An essential function for the nuclear receptor RORgamma(t) in the generation of fetal lymphoid tissue inducer cells

Lymphoid tissue inducer (LTi) cells are associated with early development of lymph nodes and Peyer's patches. We show here that during fetal life the nuclear hormone receptor RORgamma(t) is expressed exclusively in and is required for the generation of LTi cells. RORgamma(t+) LTi cells provide essential factors, among which lymphotoxin-alpha1beta2 is necessary but not sufficient for activation of the mesenchyma in lymph node and Peyer's patch anlagen. This early LTi cell-mediated activation of lymph node and Peyer's patch mesenchyma forms the necessary platform for the subsequent development of mature lymphoid tissues.

The role of the nuclear hormone receptor RORgammat in the development of lymph nodes and Peyer's patches

The nuclear hormone receptor retinoic acid-related orphan receptor (ROR)gammat is required for the development of lymph nodes (LNs) and Peyer's patches (PPs), as these organs are absent in RORgammat-deficient mice. During fetal life, RORgammat is expressed exclusively in lymphoid tissue-inducer (LTi) cells, a cell type that localizes in developing LNs and PPs. LTi cells express surface lymphotoxin alpha1beta2 that activates specialized mesenchymal cells to produce chemokines, upregulate adhesion molecules and induce further maturation of lymphoid organs. RORgammat inhibits nuclear factor of activated T-cell (NFAT) function in cell lines and induces the expression of Bcl-xL and p27kip1 in the adult thymus, suggesting that RORgammat prevents cell activation, cell-cycle progression, and apoptosis. We propose that RORgammat, together with the inhibitor of basic helix-loop-helix transcription factor Id2, ensures generation and survival of fetal LTi cells necessary for the development of LNs and PPs.

The impact of CCR7 and CXCR5 on lymphoid organ development and systemic immunity

The development of secondary lymphoid organs is a complex process dependent on a coordinated interaction of cells of hematopoietic and non-hematopoietic origin. In this context, chemokines and cytokines belonging to the tumor necrosis factor (TNF)/lymphotoxin (LT) family are critical signaling molecules during the initial steps of lymph node and Peyer's patch organogenesis. Homeostatic chemokines, such as CXCL13, CCL21, and CCL19, as well as their corresponding receptors, CXCR5 and CCR7, have now been shown to closely cooperate in the development of lymphoid organs and the maintenance of lymphoid tissue microarchitecture. We summarize recent data on the function of CXCR5 and CCR7 and their intricate connection to the TNF/LT system in order to refine the current model of lymphoid organ development.

Positive and negative selection of T cells

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

Lymphocyte accumulation in the spleen of retinoic acid receptor-related orphan receptor gamma-deficient mice

The hormone nuclear receptor retinoic acid receptor-related orphan receptor gamma (RORgamma) plays important roles in thymocyte development and lymphoid organogenesis. RORgamma and its thymus-specific isoform RORgammat are expressed in the thymus, but not in the spleen and bone marrow (BM). However, RORgamma(-/-) mice have 2- to 3-fold more splenocytes than wild-type controls due to an accumulation of conventional resting B lymphocytes. The increase in B lymphocytes in RORgamma(-/-) mice is caused neither by abnormal B cell development in the BM nor by an obvious defect in the peripheral T cell compartment. Furthermore, analyses of BM chimeras using either RORgamma(-/-) or recombinase-activating gene-2(-/-) mice as recipients and wild-type or RORgamma(-/-) mice as donors, respectively, demonstrate that the splenic microenvironment of RORgamma(-/-) mice is defective, since wild-type T and B lymphocytes accumulated in these chimeric mice. In addition, T lymphocyte homeostasis was altered due to a lowered thymic output in RORgamma(-/-) mice. Collectively, these results suggest that RORgamma regulates lymphocyte homeostasis at multiple levels.

Molecular screening and association studies of retinoid-related orphan receptor gamma (RORC): a positional and functional candidate for type 2 diabetes

The retinoid-related orphan receptor gamma (RORC) is a member of the nuclear hormone superfamily which maps to the 1q21-q23 region. Linkage of type 2 diabetes (T2DM) to this region is well replicated. Several factors argue that RORC is a strong candidate for T2DM susceptibility within this region. RORC may form heterodimers with peroxisome-proliferator activated receptor gamma, it is expressed at high levels in skeletal muscle, and expression is induced in adipocytes during differentiation. To test the hypothesis that sequence variation in RORC is a risk factor for T2DM, we screened approximately 21kb of DNA for sequence variation, including 11 exons of the RORC gene, a region 1-kb upstream (5' flanking region), intronic regions flanking the exons, and the entire 3' untranslated region (UTR). Screening was performed using single strand conformation polymorphism (SSCP) analysis in Caucasian individuals of northern European ancestry and in African American individuals. We detected 11 single nucleotide polymorphisms (SNPs), ranging from the promoter region to intron 10. We also confirmed 2 SNPs from public databases that were in regions not included in our screening. Only 1 SNP was nonsynonymous, resulting in Ala to Gly at residue 464 (exon 10). All other SNPs were noncoding. One SNP (intron 3) was unique to Caucasians, and three SNPs (Ala464Gly, intron 2, intron 6) were specific to African American subjects. We typed 7 SNPs spanning the gene from the promoter to 3' UTR in unrelated cases with T2DM and controls of Northern European ancestry. We also tested linkage of a microsatellite within the RORC gene. Modest evidence for linkage (LOD=1.47) was seen on two-point analysis, but no linkage to the RORC region was found on multipoint analysis. However, transmission of the microsatellite alleles from parents to affected offspring showed a trend to deviate from the expected 50% (p=0.078). No association of any other SNP with T2DM was found, but the Ala454Gly variant was 3-fold more common among African American patients with diabetes than in controls. SNPs 1, 2 and 4 were in strong linkage disequilibrium (D>0.85) and may constitute a haplotype block. Our data suggest that RORC cannot explain the linkage of T2DM in this region. The role of the unusual Ala454Gly variant will require a much larger study size to evaluate.

Biology of the TRANCE axis

As the TNF and TNFR superfamilies have grown to more than two dozen combined members over the past 30 years, their involvement in interactions between immune cells, with regard to the events governing cellular differentiation, activation, and survival have been well established. The recently identified TNF superfamily cytokine, TRANCE (RANKL/OPGL/ODF/TNFSF11), which interacts with two receptors-one functional, TRANCE-R (RANK/TNFRSF11A), and one decoy, OPG (TNFRSF11B)-is a survival factor for activated dendritic cells, and may also be important for the maintenance of immune tolerance. TRANCE is also the key cytokine involved in osteoclast differentiation and activation, making TRANCE signaling crucial for proper bone homeostasis, and a potential therapeutic target in diseases such as osteoporosis, osteolytic metastatic cancer, arthritis, and periodontitis. Importantly, the positive role that TRANCE has in activating the immune system, appears to significantly contribute to pathologic bone loss. These observations have spurred intense study of the various ways in which the immune system can influence bone. Furthermore, TRANCE has also been demonstrated to play essential roles in the developmental processes leading to both lymph node formation, and the expansion and function of mammary glands during pregnancy and lactation. Thus, TRANCE is quickly emerging as a cytokine of significant importance to further understanding unique aspects of mammalian biology.

The role of lymphotoxin in development and maintenance of secondary lymphoid tissues

Secondary lymphoid organs provide the necessary microenvironment for the cooperation of antigen-specific T- and B-lymphocytes and antigen-presenting cells in order to initiate an efficient immune response. Remarkable progress in understanding of the mechanisms of lymphoid organogenesis was achieved due to the analysis of various gene-targeted mice. This review primarily focuses on the role of lymphotoxin (LT) in development, maturation and maintenance of secondary lymphoid organs.

Organogenesis of lymphoid tissues

The development of lymphoid organs depends on the correct expression of several molecules within a defined timeframe during ontogeny. Although this is an extremely complex process, with each secondary lymphoid tissue requiring subtly different signals, a common framework for lymphoid development is beginning to emerge. Drawing on studies of lymph nodes, Peyer's patches and nasal-associated lymphoid tissue, an integrative model of lymphoid-tissue development, involving adhesion molecules, cytokines and chemokines, which emphasizes the role of interactions between CD3-CD4+CD45+ 'inducer' cells and VCAM1+ICAM1+ stromal 'organizer' cells is presented.

Concerted action of the chemokine and lymphotoxin system in secondary lymphoid-organ development

Chemokines are essential regulators of lymphocyte migration throughout the body. The chemokine system controls lymphocyte recirculation in immune-system homeostasis, as well as the activation-dependent and tissue-selective trafficking of effector and memory lymphocytes during immune responses. In addition, there is now substantial evidence that chemokines are critical factors for the development and organization of secondary lymphoid organs and that they are involved in all stages of lymphoid organogenesis.

Different cytokines induce surface lymphotoxin-alphabeta on IL-7 receptor-alpha cells that differentially engender lymph nodes and Peyer's patches

The formation of lymph nodes (LN) and Peyer's patches (PP) can be distinguished by the requirement of RANK for LN but not IL-7R(alpha), which is essential for PP development. However, lymphotoxin-alphabeta (LT(alpha)beta) signaling is required for both organs. The cellular basis underlying this dichotomy was revealed by the finding that the fetal IL-7R(alpha)(+) population responded equally well to IL-7 and RANKL to express LT(alpha)beta. IL-7R(alpha)(+) cells harvested from TRAF6(-/-) embryos expressed LTalphabeta in response to IL-7 but not RANKL, demonstrating that the RANK-TRAF6 signaling pathway regulates LT(alpha)beta expression in LN but not in PP. Soluble IL-7 administered to TRAF6(-/-) embryos was sufficient to restore LN genesis indicating the functional similarities of the IL-7R(alpha)(+) inducer cells for LN and PP genesis.

The role of CD45+CD4+CD3- cells in lymphoid organ development

In the last 10 years the continuing search for gene function has yielded many mutant mice that unexpectedly showed a complete lack of lymph nodes and/or Peyer's patches. With the realization that all these functionally highly diverse genes are involved at some point in the development of lymphoid organs, the challenge now is to assign a function to the molecules involved in lymphoid organ development. It will be important to determine the sequence of molecular events and assign this to the cellular events that lead to an accumulation of hematopoietic cells in one location, ultimately forming an organized lymphoid organ. Here we will focus on CD45+CD4+CD3- cells that are the early colonizing cells in lymph nodes and Peyer's patches and develop a hypothetical model of their contribution to the creation of organized lymphoid structures.

Identification of Reverb(alpha) as a novel ROR(alpha) target gene

The nuclear receptor superfamily comprises a large number of ligand-activated transcription factors that are involved in numerous biological processes such as cell proliferation, differentiation, and homeostasis. ROR(alpha) (NR1F1) and Reverb(alpha) (NR1D1) are two members of this family whose biological functions are largely unknown. In addition, no ligand has been yet identified for these two receptors; therefore, they are referred as orphan receptors. Here, we show that ROR(alpha) and Reverb(alpha) are expressed with a similar tissue distribution and are both induced during the differentiation of rat L6 myoblastic cells. Ectopic expression of ROR(alpha)1 in L6 cells significantly induces Reverb(alpha) expression as demonstrated by Northern blot analysis. Using reverse transcription-PCR to analyze Reverb(alpha) gene expression from staggerer mice, we found that there was a significant reduction of Reverb(alpha) mRNA in the skeletal muscle comparing it with the wild-type mice, which suggests that ROR(alpha) is involved in the regulation of Reverb(alpha) gene expression. Transient transfection assays using the Reverb(alpha) promoter demonstrate that ROR(alpha) regulates the Reverb(alpha) gene at the transcriptional level. Furthermore, mutagenesis experiments indicate that ROR(alpha) regulates Reverb(alpha) transcription via a monomeric ROR response element located in the Reverb(alpha) gene promoter. Electrophoretic mobility shift assays show that ROR(alpha) binds strongly to this site in a specific-manner. Finally, overexpression of GRIP-1/TIF-2, but not SRC-1, potentiates ROR(alpha)-stimulated Reverb(alpha) promoter activity in transient transfection experiments. Together, our results identify Reverb(alpha) as a novel target gene for ROR(alpha).

CD4+CD3- cells induce Peyer's patch development: role of alpha4beta1 integrin activation by CXCR5

CD4+CD3- cells are the predominant hematopoietic cells found in mouse fetal intestine. We prove their role as Peyer's patch (PP)-inducing cells by transfer into neonatal PP-deficient mice. To test the requirement of chemokines and adhesion molecules in induction of PP, we studied mice deficient in CXCR5 and/or alpha4beta1 integrin-mediated adhesion. CXCR5-/- mice have CD4+CD3- cells, which are inefficient in inducing PP formation. We show here that CXCR5/CXCL13 signaling activates alpha4beta1 integrin on CD4+CD3- cells. Blocking of beta1 integrin or VCAM-1, the ligand of alpha4beta1 integrin, inhibits PP formation. This study demonstrates the link between chemokine receptors and adhesion molecules that regulates stromal/hematopoietic cell interaction leading to PP formation.

Initiation of NALT organogenesis is independent of the IL-7R, LTbetaR, and NIK signaling pathways but requires the Id2 gene and CD3(-)CD4(+)CD45(+) cells

Initiation of nasopharyngeal-associated lymphoid tissue (NALT) development is independent of the programmed cytokine cascade necessary for the formation of Peyer's patches (PP) and peripheral lymph nodes (PLN), a cytokine cascade which consists of IL-7R, LTalpha1beta2/LTbetaR, and NIK. However, the subsequent organization of NALT seems to be controlled by these cytokine signaling cascades since the maturation of NALT structure is generally incomplete in those cytokine cascade-deficient mice. NALT as well as PP and PLN are completely absent in Id2(-/-) mice. NALT organogenesis is initiated following the adoptive transfer of CD3(-)CD4(+)CD45(+) cells into Id2(-/-) mice, constituting direct evidence that CD3(-)CD4(+)CD45(+) inducer cells can provide an IL-7R-, LTalpha1beta2/LTbetaR-, and NIK-independent tissue organogenesis pathway for secondary lymphoid tissue development.

Multiple roles for tumor necrosis factor-alpha and lymphotoxin alpha/beta in immunity and autoimmunity

Tumor necrosis factor (TNF)-alpha and lymphotoxin (LT) alpha/beta play multiple roles in the development and function of the immune system. This article focuses on three important aspects of the effects of these cytokines on the immune response and on autoimmunity. In several experimental systems (Jurkat T cells, murine T-cell hybridomas), TNF-alpha appears to cause a downregulation of signaling through the TCR, revealed by changes in calcium flux, activation of p21, p23 and ZAP70, and a decrease in nuclear activation of NF-kappaB. Previous and present results suggest that TNF-alpha interferes in some manner with signaling through the TCR, at a locus yet to be delineated. Transgenic expression of LTbetaR-Fc in nonobese diabetic (NOD) transgenic mice results in prevention of type 1 diabetes in NOD mice as long as the level of expression of the fusion protein (under the control of the cytomegalovirus promoter) remains above a level of 2-3 microg/ml. Once the expression levels of the fusion protein have dropped below this critical level, the diabetic process resumes and the animals become diabetic at 40-50 weeks of age, whereas nontransgenic littermates develop diabetes by 25-30 weeks of age. The paradoxical effects of neonatal TNF-alpha administration in NOD mice in increasing incidence of and hastening onset of type 1 diabetes, while neonatal anti-TNF administration completely prevents all signs of islet cell autoimmunity, are due partly to the low levels of CD4+CD25+ T cells in NOD mice. These low levels are reduced by a further 50% on neonatal administration of nontoxic levels of TNF-alpha. In contrast, neonatal administration of anti-TNF-alpha results in a dramatic increase in the levels of CD4+CD25+ regulatory T cells, to levels beyond those seen in wild-type untreated NOD mice. TNF-alpha and LTalpha/beta thus have pleomorphic regulatory effects on the development and expression of autoimmunity.

Regulation of the TCRalpha repertoire by the survival window of CD4(+)CD8(+) thymocytes

T cell receptor (TCR) alpha alleles undergo primary and secondary rearrangement in double-positive (DP) thymocytes. By analyzing TCRalpha rearrangement in orphan nuclear receptor RORgamma-deficient mice, in which the DP lifespan is shorter, and in Bcl-x(L)-transgenic mice, in which the DP lifespan is extended, we show that the progression of secondary V(alpha) to J(alpha) rearrangements is controlled by DP thymocyte survival. In addition, because Bcl-x(L) induces a bias towards 3' J(alpha) usage in peripheral T cells, we conclude that the programmed cell death of DP thymocytes is not simply a consequence of failed positive selection. Rather, it limits the progression of rearrangement along the J(alpha) locus and the opportunities for positive selection, thereby regulating the TCRalpha repertoire.

Characterization of the retinoid orphan-related receptor-alpha coactivator binding interface: a structural basis for ligand-independent transcription

The retinoid orphan-related receptor-alpha (RORalpha) is a member of the ROR subfamily of orphan receptors and acts as a constitutive activator of transcription in the absence of exogenous ligands. To understand the basis of this activity, we constructed a homology model of RORalpha using the closely related TRbeta as a template. Molecular modeling suggested that bulky hydrophobic side chains occupy the RORalpha ligand cavity leaving a small but distinct cavity that may be involved in receptor stabilization. This model was subject to docking simulation with a receptor-interacting peptide from the steroid receptor coactivator, GR-interacting protein-1, which delineated a coactivator binding surface consisting of the signature motif spanning helices 3-5 and helix 12 [activation function 2 (AF2)]. Probing this surface with scanning alanine mutagenesis showed structural and functional equivalence between homologous residues of RORalpha and TRbeta. This was surprising (given that RORalpha is a ligand-independent activator, whereas TRbeta has an absolute requirement for ligand) and prompted us to use molecular modeling to identify differences between RORalpha and TRbeta in the way that the AF2 helix interacts with the rest of the receptor. Modeling highlighted a nonconserved amino acid in helix 11 of RORalpha (Phe491) and a short-length of 3.10 helix at the N terminus of AF2 which we suggest 1) ensures that AF2 is locked permanently in the holoconformation described for other liganded receptors and thus 2) enables ligand-independent recruitment of coactivators. Consistent with this, mutation of RORalpha Phe491 to either methionine or alanine (methionine is the homologous residue in TRbeta), reduced and ablated transcriptional activation and recruitment of coactivators, respectively. Furthermore, we were able to reconstitute transcriptional activity for both a deletion mutant of RORalpha lacking AF2, and Phe491Met, by overexpression of a GAL-AF2 fusion protein, demonstrating ligand-independent recruitment of AF2 and a role for Phe491 in recruiting AF2.

Nuclear hormone receptors in T lymphocytes

Among the numerous steroid and orphan nuclear receptors encoded within mammalian genomes, several are involved in regulating immune system functions. We review here recent studies on the glucocorticoid receptor and the orphan receptors Nur77 and RORgamma. These molecules play key roles in the development and the effector functions of T lymphocytes.

Cutting edge: organogenesis of nasal-associated lymphoid tissue (NALT) occurs independently of lymphotoxin-alpha (LT alpha) and retinoic acid receptor-related orphan receptor-gamma, but the organization of NALT is LT alpha dependent

Peyer's patch and nasal-associated lymphoid tissue (NALT) are mucosal lymphoid tissues that appear similar in structure and function. Surprisingly, we found that NALT, unlike Peyer's patch, was formed independently of lymphotoxin (LT)alpha. Furthermore, using mice deficient in the retinoic acid receptor-related orphan receptor-gamma, we found that NALT was formed in the absence of CD4+CD3- cells, which are thought to be the embryonic source of LTalpha. However, we also found that NALT of LTalpha-/- animals was disorganized and lymphopenic, suggesting that the organization and recruitment of lymphocytes within NALT remained dependent on LTalpha. Finally, we demonstrated that both the structure and function of NALT were restored in LTalpha-/- animals upon reconstitution with normal bone marrow. These results demonstrate that the organogenesis of NALT occurs through unique mechanisms.

Requirement for the NF-kappaB family member RelA in the development of secondary lymphoid organs

The transcription factor nuclear factor (NF)-kappaB has been suggested to be a key mediator of the development of lymph nodes and Peyer's patches. However, targeted deletion of NF-kappaB/ Rel family members has not yet corroborated such a function. Here we report that when mice lacking the RelA subunit of NF-kappaB are brought to term by breeding onto a tumor necrosis factor receptor (TNFR)1-deficient background, the mice that are born lack lymph nodes, Peyer's patches, and an organized splenic microarchitecture, and have a profound defect in T cell-dependent antigen responses. Analyses of TNFR1/RelA-deficient embryonic tissues and of radiation chimeras suggest that the dependence on RelA is manifest not in hematopoietic cells but rather in radioresistant stromal cells needed for the development of secondary lymphoid organs.

Id and development

During development, it is obvious that enormous multiplication and diversification of cells is required to build a body plan from a single fertilized egg and that these two processes, proliferation and differentiation, must be coordinated properly. Id proteins, negative regulators of basic helix-loop-helix transcription factors, possess the ability to inhibit differentiation and to stimulate proliferation, and are useful molecules for investigating the mechanisms regulating development. In the past few years, our understanding of the roles of Id proteins has been substantially enhanced by the detailed investigation of genetically modified animals. The data have indicated that the functions of Id proteins in vivo are functionally related to those revealed by earlier work in cell culture systems. However, unexpected organs and cell types have also been found to require Id proteins for their normal development. This review looks at the advances made in our understanding of the in vivo functions of Id proteins. The topics discussed include neurogenesis, natural killer cell development, lymphoid organogenesis, mammary gland development and spermatogenesis.

The ROR nuclear orphan receptor subfamily: critical regulators of multiple biological processes

The nuclear receptor superfamily, a group of structurally related, ligand-dependent transcription factors, includes a large number of orphan receptors for which no ligand has yet been identified. These proteins function as key regulators of many physiological processes that occur during embryonic development and in the adult. The retinoid-related orphan receptors (RORs) alpha, beta, and gamma comprise one nuclear orphan receptor gene subfamily. RORs exhibit a modular structure that is characteristic for nuclear receptors; the DNA-binding domain is highly conserved and the ligand-binding domain is moderately conserved among RORs. By a combination of alternative promoter usage and exon splicing, each ROR gene generates several isoforms that differ only in their amino terminus. RORs bind as monomers to specific ROR response elements (ROREs) consisting of the consensus core motif AGGTCA preceded by a 5-bp A/T-rich sequence. RORE-dependent transcriptional activation by RORs is cell type-specific and mediated through interactions with nuclear cofactors. RORs have been shown to interact with certain corepressors as well as coactivators, suggesting that RORs are not constitutively active but that their activity is under some regulatory control. RORs likely can assume at least two different conformations: a repressive state, which allows interaction with corepressor complexes, and an active state, which promotes binding of coactivator complexes. Whether the transition between these two states is regulated by ligand binding and/or by phosphorylation remains to be determined. Ca2+/calmodulin-dependent kinase IV (CaMKIV) can dramatically enhance ROR-mediated transcriptional activation. This stimulation involves CaMKIV-mediated phosphorylation not of RORs, but likely of specific nuclear cofactors that interact with RORs. RORalpha is widely expressed. In the cerebellum, its expression is limited to the Purkinje cells. RORalpha-/- mice and the natural RORalpha-deficient staggerer mice exhibit severe cerebellar ataxia due to a defect in Purkinje cell development. In addition, these mice have thin long bones, suggesting a role for RORalpha in bone metabolism, and develop severe atherosclerosis when placed on a high-fat diet. Expression of RORbeta is very restricted. RORbeta is highly expressed in different parts of the neurophotoendocrine system, the pineal gland, the retina, and suprachiasmatic nuclei, suggesting a role in the control of circadian rhythm. This is supported by observations showing alterations in circadian behavior in RORbeta-/- mice. RORgamma, which is most highly expressed in the thymus, plays an important role in thymopoiesis. Thymocytes from RORgamma-/- mice undergo accelerated apoptosis. The induction of apoptosis is, at least in part, due to a down-regulation of the expression of the antiapoptotic gene Bcl-XL. In addition to the thynic phenotype, RORgamma-/- mice lack lymph nodes, indicating that RORgamma is essential for lymph node organogenesis. Overexpression of RORgamma has been shown to inhibit T cell receptor-mediated apoptosis in T cell hybridomas and to repress the induction of Fas-ligand and interleukin 2. These studies demonstrate that RORs play critical roles in the regulation of a variety of physiological processes. Further characterization of the mechanisms of action of RORs will not only lead to the identification of ROR target genes and provide additional insight into their normal physiological functions, but will also determine their roles in disease.

Formation of Peyer's patches

Formation of Peyer's patches requires complex interactions between the gut epithelium, the mesenchyme, and bone-marrow-derived hematopoietic progenitors. The first Peyer's patches anlage appear around embryonic day 15.5, when the endoderm has undergone transition to a simple epithelium, the lymphatic vessels have reached the intestinal mucosa, and mesenchymal cells have started to form clusters. Recent data using knockout mice provide insight into the molecular nature of the signals that mediate Peyer's patch ontogeny. These include members of the tumor-necrosis factor family and homeostatic chemokines.

Knockout mice: a paradigm shift in modern immunology

In the past decade, advances in genetic engineering and mouse knockout technology have transformed our understanding of the immune system. In particular, new perspectives on T-cell development, co-stimulation and activation have emerged from the study of single and multiple gene-knockout animals, as well as from conditional knockout and 'knock-in' mutants. Analysis of these animals has clarified important intracellular signalling pathways and has shed light on the regulatory mechanisms that govern normal immune responses and autoimmunity.

The role of orphan nuclear receptor in thymocyte differentiation and lymphoid organ development

T lymphocytes differentiate in the thymus through several phenotypically distinct stages that are tightly regulated by multiple nuclear transcription factors. Immature CD4+CD8+ double positive (DP) thymocytes make up a majority of the population in the thymus, and exhibit several phenotypic features distinct from mature T cells. DP thymocytes express only about 10% of surface TCR that are found on mature T cells and do not proliferate and produce IL-2 in response to stimulation. Several critical events of T lymphocyte maturation such as TCRalpha gene recombination, positive and negative selection, and CD4/CD8 lineage commitment occur around the DP stage. Recent studies from our group and others on the orphan nuclear receptor RORgamma and its thymus-specific isoform RORgammat support a critical role for this nuclear receptor in the regulation of DP thymocyte function. In addition, RORgamma is required for the development of lymph nodes and Peyer's patches.

The fetal liver counterpart of adult common lymphoid progenitors gives rise to all lymphoid lineages, CD45+CD4+CD3- cells, as well as macrophages

We identified an IL-7Ralpha(+)Sca-1(low)c-Kit(low) population in E14 fetal liver, which is the phenotypical analog of common lymphoid progenitors (CLP) in adult bone marrow. After transfer into newborn mice, the IL-7Ralpha(+)Sca-1(low)c-Kit(low) population rapidly differentiated into CD45(+)CD4(+)CD3(-) cells, which are candidate cells for initiating lymph node and Peyer's patch formation. In addition, this population also gave rise to B, T, NK, and CD8alpha(+) and CD8alpha(-) dendritic cells. The fetal liver precursors expressed a significantly lower level of the myeloid-suppressing transcription factor Pax-5, than adult CLP, and retained differentiation activity for macrophages in vitro. We propose that the transition from fetal liver IL-7Ralpha(+)Sca-1(low)c-Kit(low) cells to adult CLP involves a regulated restriction of their developmental potential, controlled, at least in part, by Pax-5 expression.

The beta-catenin--TCF-1 pathway ensures CD4(+)CD8(+) thymocyte survival

The association of trans-acting T cell factors (TCFs) or lymphoid enhancer factor 1 (LEF-1) with their coactivator beta-catenin mediates transient transcriptional responses to extracellular Wnt signals. We show here that T cell maturation depends on the presence of the beta-catenin--binding domain in TCF-1. This domain is necessary to mediate the survival of immature CD4(+)CD8(+) double-positive (DP) thymocytes. Accelerated spontaneous thymocyte death in the absence of TCF-1 correlates with aberrantly low expression of the anti-apoptotic protein Bcl-x(L). Increasing anti-apoptotic effectors in thymocytes by the use of a Bcl-2 transgene rescued TCF-1-deficient DP thymocytes from apoptosis. Thus, TCF-1, upon association with beta-catenin, transiently ensures the survival of immature T cells, which enables them to generate and edit T cell receptor (TCR) alpha chains and attempt TCR-mediated positive selection.

Ca(2+)/CaM-dependent kinases: from activation to function

Calmodulin (CaM) is an essential protein that serves as a ubiquitous intracellular receptor for Ca(2+). The Ca(2+)/CaM complex initiates a plethora of signaling cascades that culminate in alteration of cellular functions. Among the many Ca(2+)/CaM-binding proteins to be discovered, the multifunctional protein kinases CaMKI, II, and IV play pivotal roles. Our review focuses on this class of CaM kinases to illustrate the structural and biochemical basis for Ca(2+)/CaM interaction with and regulation of its target enzymes. Gene transcription has been chosen as the functional endpoint to illustrate the recent advances in Ca(2+)/CaM-mediated signal transduction mechanisms.

Molecular determinants of TCR expression and selection

The process of T cell development in the thymus is tightly regulated, being dependent on the integration of signals required for thymocyte maturation and survival. Rearrangements, expression and signaling of TCR genes play an indispensable role in this developmental program. Recent advances have provided insights into the molecular mechanisms that regulate TCR repertoire formation at the level of alphabeta versus gammadelta T cell fate and CD4(+) versus CD8(+) lineage determination.

Chemokines in lymphopoiesis and lymphoid organ development

An important role has emerged for chemokines in regulating the distribution of progenitor cells during hematopoietic cell development. As well as recruiting cells, chemokines promote cell retention and cytokine expression. Furthermore, chemokines have been found to have an inductive function in secondary lymphoid organ development.