Probing immune functions in RAG-deficient mice

Natural Killer Cells Are Present in Rag1-/- Mice and Promote Tissue Damage During the Acute Phase of Ischemic Stroke

Rag1^−/− mice, lacking functional B and T cells, have been extensively used as an adoptive transfer model to evaluate neuroinflammation in stroke research. However, it remains unknown whether natural killer (NK) cell development and functions are altered in Rag1^−/− mice as well. This connection has been rarely discussed in previous studies but might have important implications for data interpretation. In contrast, the NOD-Rag1^nullIL2rg^null (NRG) mouse model is devoid of NK cells and might therefore eliminate this potential shortcoming. Here, we compare immune-cell frequencies as well as phenotype and effector functions of NK cells in Rag1^−/− and wildtype (WT) mice using flow cytometry and functional in vitro assays. Further, we investigate the effect of Rag1^−/− NK cells in the transient middle cerebral artery occlusion (tMCAO) model using antibody-mediated depletion of NK cells and adoptive transfer to NRG mice in vivo. NK cells in Rag1^−/− were comparable in number and function to those in WT mice. Rag1^−/− mice treated with an anti-NK1.1 antibody developed significantly smaller infarctions and improved behavioral scores. Correspondingly, NRG mice supplemented with NK cells were more susceptible to tMCAO, developing infarctions and neurological deficits similar to Rag1^−/− controls. Our results indicate that NK cells from Rag1^−/− mice are fully functional and should therefore be considered in the interpretation of immune-cell transfer models in experimental stroke. Fortunately, we identified the NRG mice, as a potentially better-suited transfer model to characterize individual cell subset-mediated neuroinflammation in stroke.

Alternaria alternata challenge at the nasal mucosa results in eosinophilic inflammation and increased susceptibility to influenza virus infection

BACKGROUND

Eosinophils in the nasal mucosa are an elemental feature of allergic rhinitis.

OBJECTIVE

Our objective was to explore eosinophilic inflammation and its impact on respiratory virus infection at the nasal mucosa.

METHODS

Inflammation in the nasal mucosae of mice was evaluated in response to repetitive stimulation with strict intranasal volumes of a filtrate of Alternaria alternata. Mice were then challenged with influenza virus.

RESULTS

Repetitive stimulation with A. alternata resulted in eosinophil recruitment to the nasal passages in association with elevated levels of IL-5, IL-13 and eotaxin-1; eosinophil recruitment was diminished in eotaxin-1^-/- mice, and abolished in Rag1^-/- mice. A. alternata also resulted in elevated levels of nasal wash IgA in both wild-type and eosinophil-deficient ∆dblGATA mice. Interestingly, A. alternata-treated mice responded to an influenza virus infection with profound weight loss and mortality compared to mice that received diluent alone (0% vs 100% survival, ***P < .001); the lethal response was blunted when A. alternata was heat-inactivated. Minimal differences in virus titre were detected, and eosinophils present in the nasal passages at the time of virus inoculation provided no protection against the lethal sequelae. Interestingly, nasal wash fluids from mice treated with A. alternata included more neutrophils and higher levels of pro-inflammatory mediators in response to virus challenge, among these, IL-6, a biomarker for disease severity in human influenza.

CONCLUSIONS AND CLINICAL RELEVANCE

Repetitive administration of A. alternata resulted in inflammation of the nasal mucosae and unanticipated morbidity and mortality in response to subsequent challenge with influenza virus. Interestingly, and in contrast to findings in the lower airways, eosinophils recruited to the nasal passages provided no protection against lethal infection. As increased susceptibility to influenza virus among individuals with rhinitis has been the subject of several clinical reports, this model may be used for further exploration of these observations.

IL-7-induced phosphorylation of the adaptor Crk-like and other targets

IL-7 is required for T cell differentiation and mature T cell homeostasis and promotes pro-B cell proliferation and survival. Tyrosine phosphorylation plays a central role in IL-7 signaling. We identified by two-dimensional electrophoresis followed by anti-phosphotyrosine immunoblotting and mass spectrometry sixteen tyrosine phosphorylated proteins from the IL-7-dependent cell line D1. IL-7 stimulation induced the phosphorylation of the proteins STI1, ATIC and hnRNPH, involved in pathways related to survival, proliferation and gene expression, respectively, and increased the phosphorylation of CrkL, a member of a family of adaptors including the highly homologous Crk isoforms CrkII and CrkI, important in multiple signaling pathways. We observed an increased phosphorylation of CrkL in murine pro-B cells and in murine and human T cells. In addition, IL-7 increased the association of CrkL with the transcription factor Stat5, essential for IL-7 pro-survival activity. The selective tyrosine kinase inhibitor Imatinib. counteracted the IL-7 pro-survival effect in D1 cells and decreased CrkL phosphorylation. These data suggested that CrkL could play a pro-survival role in IL-7-mediated signaling. We observed that pro-B cells also expressed, in addition to CrkL, the Crk isoforms CrkII and CrkI and therefore utilized pro-B cells conditionally deficient in all three to evaluate the role of these proteins. The observation that the IL-7 pro-survival effect was reduced in Crk/CrkL conditionally-deficient pro-B cells further pointed to a pro-survival role of these adaptors. To further evaluate the role of these proteins, gene expression studies were performed in Crk/CrkL conditionally-deficient pro-B cells. IL-7 decreased the transcription of the receptor LAIR1, which inhibits B cell proliferation, in a Crk/CrkL-dependent manner, suggesting that the Crk family of proteins may promote pro-B cell proliferation. Our data contribute to the understanding of IL-7 signaling and suggest the involvement of Crk family proteins in pathways promoting survival and proliferation.

Humanized Immunoglobulin Mice: Models for HIV Vaccine Testing and Studying the Broadly Neutralizing Antibody Problem

A vaccine that can effectively prevent HIV-1 transmission remains paramount to ending the HIV pandemic, but to do so, will likely need to induce broadly neutralizing antibody (bnAb) responses. A major technical hurdle toward achieving this goal has been a shortage of animal models with the ability to systematically pinpoint roadblocks to bnAb induction and to rank vaccine strategies based on their ability to stimulate bnAb development. Over the past 6 years, immunoglobulin (Ig) knock-in (KI) technology has been leveraged to express bnAbs in mice, an approach that has enabled elucidation of various B-cell tolerance mechanisms limiting bnAb production and evaluation of strategies to circumvent such processes. From these studies, in conjunction with the wealth of information recently obtained regarding the evolutionary pathways and paratopes/epitopes of multiple bnAbs, it has become clear that the very features of bnAbs desired for their function will be problematic to elicit by traditional vaccine paradigms, necessitating more iterative testing of new vaccine concepts. To meet this need, novel bnAb KI models have now been engineered to express either inferred prerearranged V(D)J exons (or unrearranged germline V, D, or J segments that can be assembled into functional rearranged V(D)J exons) encoding predecessors of mature bnAbs. One encouraging approach that has materialized from studies using such newer models is sequential administration of immunogens designed to bind progressively more mature bnAb predecessors. In this review, insights into the regulation and induction of bnAbs based on the use of KI models will be discussed, as will new Ig KI approaches for higher-throughput production and/or altering expression of bnAbs in vivo, so as to further enable vaccine-guided bnAb induction studies.

Identification and Characterization of the V(D)J Recombination Activating Gene 1 in Long-Term Memory of Context Fear Conditioning

An increasing body of evidence suggests that mechanisms related to the introduction and repair of DNA double strand breaks (DSBs) may be associated with long-term memory (LTM) processes. Previous studies from our group suggested that factors known to function in DNA recombination/repair machineries, such as DNA ligases, polymerases, and DNA endonucleases, play a role in LTM. Here we report data using C57BL/6 mice showing that the V(D)J recombination-activating gene 1 (RAG1), which encodes a factor that introduces DSBs in immunoglobulin and T-cell receptor genes, is induced in the amygdala, but not in the hippocampus, after context fear conditioning. Amygdalar induction of RAG1 mRNA, measured by real-time PCR, was not observed in context-only or shock-only controls, suggesting that the context fear conditioning response is related to associative learning processes. Furthermore, double immunofluorescence studies demonstrated the neuronal localization of RAG1 protein in amygdalar sections prepared after perfusion and fixation. In functional studies, intra-amygdalar injections of RAG1 gapmer antisense oligonucleotides, given 1 h prior to conditioning, resulted in amygdalar knockdown of RAG1 mRNA and a significant impairment in LTM, tested 24 h after training. Overall, these findings suggest that the V(D)J recombination-activating gene 1, RAG1, may play a role in LTM consolidation.

Experimental models of leprosy

Leprosy (Hansen’s disease) is a chronic granulomatous bacterial disease which mainly affects skin and peripheral nervous system. Leprosy is caused by the obligate intercellular pathogen known as Mycobacterium leprae. Creating experimental models of leprosy is associated with serious problems due to biological characteristics of the pathogen. Numerous attempts to develop experimental models on different types of animals resulted in a few reproducible models on mice and nine-banded armadillos. Strains of knockout mice with genetic defects caused by site-directed mutagenesis are used as a basis for different leprosy models. Experimental models of leprosy are used for screening of anti-leprosy drugs, detection of drug resistance, studies on the pathogenesis of leprosy, production and evaluation of viability of M. leprae, developing of anti-leprosy vaccines.

Spatial organization of the tenascin-C microenvironment in experimental and human cancer

The extracellular matrix (ECM) molecule tenascin-C (TNC) promotes tumor progression. This has recently been demonstrated in the stochastic murine RIP1-Tag2 insulinoma model, engineered to either express TNC abundantly or to be devoid of TNC. However, our knowledge about organization of the TNC microenvironment is scant. Here we determined the spatial distribution of TNC together with other ECM molecules in murine RIP1-Tag2 insulinoma and human cancer tissue (insulinoma and colorectal carcinoma). We found that TNC is organized in matrix tracks together with other ECM molecules of the AngioMatrix signature, a previously described gene expression profile that characterizes the angiogenic switch. Moreover, stromal cells including endothelial cells, fibroblasts and leukocytes were enriched in the TNC tracks. Thus, TNC tracks may provide niches for stromal cells and regulate their behavior. Given similarities of TNC rich niches for stromal cells in human insulinoma and colon cancer, we propose that the RIP1-Tag2 model may be useful for providing insights into the contribution of the tumor stroma specific ECM as promoter of cancer progression.

Listr1 locus regulates innate immunity against Listeria monocytogenes infection in the mouse liver possibly through Cxcl11 polymorphism

Inbred stains of mice display differential susceptibility to infection with the common foodborne pathogen Listeria monocytogenes (Lm). Previously, Listr1 and Listr2, two genetic loci that control differential sensitivity to Lm infection between BALB/cByJ and C57BL/6ByJ mice, were identified. To analyze the role of Listr1 in innate immune responses, we employed congenic mice (C.B6By-Listr1/Rag2 (-/-) ) bearing the C57BL/6ByJ-derived Listr1 locus on a BALB/c-Rag2 (-/-) background. Consistent with the results of a previous genetic analysis, the congenic mice showed increased susceptibility to Lm infection. The bacterial burden in the liver between the congenic and control lines was significantly different (P < 0.05) from 24 h postinfection with Lm. Analysis of genes within the Listr1 locus identified a frameshift mutation in the Cxcl11 gene of the C57BL/6 strain that prevents production of the mature chemokine CXCL11. No differences in inflammatory cell infiltration or cells expressing CXCR3 and CXCR7 which are the receptors of CXCL11 occurred because of CXCL11 deficiency in the congenic mice spleens. However, these mice lacked a distinct population of CD14(+) positive resident mononuclear cells that express intermediate levels of CXCR3 and CXCR7 in the liver. There were fewer microabscesses in the liver of CXCL11-deficient mice during the early stage of infection, which is consistent with their decreased ability to resist Lm. Our results, when taken together, show that the Listr1 locus plays an important role in early control of Lm infection in the mouse liver and that Cxcl11 is a candidate gene for disease severity within this locus.

Sodium dichloroacetate (DCA) reduces apoptosis in colorectal tumor hypoxia

We examined the effect of hypoxia on apoptosis of human colorectal cancer (CRC) cells in vitro and in vivo. All cell lines tested were susceptible to hypoxia-induced apoptosis. DCA treatment caused significant apoptosis under normoxia in SW480 and Caco-2 cells, but these cells displayed decreased apoptosis when treated with DCA combined with hypoxia, possibly through HIF-1alpha dependent pathways. DCA treatment also induced significantly increased growth of SW480 tumor xenografts, and a decrease in TUNEL positive nuclei in hypoxic but not normoxic regions of treated tumors. Thus DCA is cytoprotective to some CRC cells under hypoxic conditions, highlighting the need for further investigation before DCA can be used as a reliable apoptosis-inducing agent in cancer therapy.

Mouse models with human immunity and their application in biomedical research

Biomedical research in human beings is largely restricted to in vitro studies that lack complexity of a living organism. To overcome this limitation, humanized mouse models are developed based on immunodeficient characteristics of severe combined immunodeficiency (SCID) or recombination activating gene (Rag)(null) mice, which can accept xenografts. Peripheral constitution of human immunity in SCID or Rag(null) mice has been achieved by transplantation of mature human immune cells, foetal human thymus, bone marrow, liver tissues, lymph nodes or a combination of these, although efficiency needs to be improved. These mouse models with constituted human immunity (defined as humanized mice in the present text) have been widely used to investigate the basic principles of human immunobiology as well as complex pathomechanisms and potential therapies of human diseases. Here, elements of an ideal humanized mouse model are highlighted including genetic and non-genetic modification of recipient mice, transplantation strategies and proposals to improve engraftments. The applications of the humanized mice to study the development and response of human immune cells, human autoimmune diseases, virus infections, transplantation biology and tumour biology are reviewed as well.

IL-7 induces myelopoiesis and erythropoiesis

IL-7 administration to mice was previously reported to increase the mobilization of progenitor cells from marrow to peripheral sites. We now report that IL-7 increases the number of mature myeloid and monocytic cells in spleen and peripheral blood. This effect required T cells, and we show that IL-7 treatment in vivo induced GM-CSF and IL-3 production by T cells with memory phenotype. However, additional myelopoietic cytokines were shown to be involved because mice deficient in both GM-CSF and IL-3 also responded to IL-7 with increased myelopoiesis. Candidate cytokines included IFN-gamma and Flt3 ligand, which were also produced in response to IL-7. Because IFN-gamma-deficient mice also increased myelopoiesis, it was suggested that IL-7 induced production of redundant myelopoietic cytokines. In support of this hypothesis, we found that the supernatant from IL-7-treated, purified T cells contained myelopoietic activity that required a combination of Abs against GM-CSF, IL-3, and anti-Flt3 ligand to achieve maximum neutralization. IL-7 administration increased the number of splenic erythroid cells in either normal, Rag1 or GM-CSF-IL-3-deficient mice, suggesting that IL-7 might directly act on erythroid progenitors. In support of this theory, we detected a percentage of TER-119(+) erythroid cells that expressed the IL-7Ralpha-chain and common gamma-chain. Bone marrow cells expressing IL-7R and B220 generated erythroid colonies in vitro in response to IL-7, erythropoietin, and stem cell factor. This study demonstrates that IL-7 can promote nonlymphoid hemopoiesis and production of cytokines active in the host defense system in vivo, supporting its possible clinical utility.

De novo carcinogenesis promoted by chronic inflammation is B lymphocyte dependent

Chronic inflammation predisposes tissue to cancer development; however, regulatory mechanisms underlying recruitment of innate leukocytes toward developing neoplasms are obscure. We report that genetic elimination of mature T and B lymphocytes in a transgenic mouse model of inflammation-associated de novo epithelial carcinogenesis, e.g., K14-HPV16 mice, limits neoplastic progression to development of epithelial hyperplasias that fail to recruit innate immune cells. Adoptive transfer of B lymphocytes or serum from HPV16 mice into T and B cell-deficient/HPV16 mice restores innate immune cell infiltration into premalignant tissue and reinstates necessary parameters for full malignancy, e.g., chronic inflammation, angiogenic vasculature, hyperproliferative epidermis. These findings support a model in which B lymphocytes are required for establishing chronic inflammatory states that promote de novo carcinogenesis.

Influence of switch region length on immunoglobulin class switch recombination

The class and effector functions of antibodies are modulated through the process of Ig heavy chain class switch recombination (CSR). CSR occurs between switch (S) regions that lie upstream of the various Ig heavy chain constant region exons. Molecular analyses of S-region functions have been hampered by their large size and repetitive nature. To test potential relationships between S-region size and efficiency of CSR, we generated normal B lymphocytes in which the 12-kb S region flanking the Cgamma1 exons (Sgamma1) was replaced with synthetic or endogenous S regions of various lengths. Replacement of Sgamma1 with 1- and 2-kb synthetic sequences representing the Sgamma1 core repeats or a 4-kb portion of the core endogenous Sgamma1 region supported CSR frequencies that directly correlated with S-region length. These findings indicate that S-region size is an important factor in determining endogenous CSR efficiency. Moreover, these results also will allow the development of a systematic system to test the function of various S-region motifs by replacing endogenous S regions with synthetic S regions controlled for size effects.

Identification of hematopoietic stem/progenitor cells: strength and drawbacks of functional assays

A major challenge in hematopoiesis is to conceive assays that could bring useful insights into experimental and clinical hematology. This means identifying separately the various classes of hematopoietic progenitors that are produced sequentially during the progression from stem cells to differentiated functional cells. Standardized short-term colony assays easily quantify lineage-committed myeloid precursors, but identification of primitive cells, which have both the ability to repopulate durably myeloid and lymphoid lineages and perhaps to self-renew, still depends on in vivo assays. Whatever the assay, two important requisites have to be solved: one is the definition of appropriate read-outs that will depend solely on the function of these cells, and the second is to evaluate precisely their numbers and proliferative potential in quantitative assays. When evaluating hematopoiesis, three parameters have to be taken into account: (1) the lack of reliable correlation between the phenotype of a given cell and its function. This is especially problematic in post-transplantation situations where cells from transplanted animals are analysed; (2) functionally heterogeneous cells are identified in a single assay; and (3) ontogeny-related changes in hematopoietic cell proliferation and self-renewal that, in human beings, hampers the exploration of adult stem cells. Nevertheless, years of progress in the manipulation of hematopoietic stem cells have recently resulted in the purification of a cell subset that repopulates irradiated recipients with absolute efficiency.

Stromal Matrix Metalloproteinase-9 Regulates the Vascular Architecture in Neuroblastoma by Promoting Pericyte Recruitment

Advanced stages of neuroblastoma show increased expression of matrix metalloproteinases MMP-2 and MMP-9, that have been implicated in many steps of tumor progression, suggesting that they play a contributory role. Using pharmacological and genetic approaches, we have examined the role of these MMPs in progression of SK-N-BE (2).10 human neuroblastoma tumors orthotopically xenotransplanted into immunodeficient mice. Mice treated with Prinomastat, a synthetic inhibitor of MMPs, showed an inhibition of tumor cell proliferation in implanted tumors and a prolonged survival (50 versus 39 days in control group, P < 0.035). Treatment with Prinomastat did not affect formation of liver metastases (P = 0.52) but inhibited intravascular colonization by the tumor cells in the lung by 73.8% (P = 0.03) and angiogenesis in both primary tumors and experimental liver metastases. The primary tumors from Prinomastat-treated mice showed a 39.3% reduction in endothelial area detected by PECAM/CD31 staining in tumor sections (P < 0.001), primarily due to the presence of smaller vessels (P = 0.004). MMP-2 is expressed by neuroblastoma tumor cells and stromal cells, whereas MMP-9 is exclusively expressed by stromal cells, particularly vascular cells. To examine the contribution of MMP-9 to tumor angiogenesis, we generated RAG1/MMP-9 double-deficient mice. We observed a significant inhibition of angiogenesis in the immunodeficient RAG1/MMP-9 double-deficient mice orthotopically implanted with tumor cells (P = 0.043) or implanted s.c. with a mixture of tumor cells and Matrigel (P < 0.001). Using an FITC-labeled lectin, we demonstrated an inhibition in the architecture of the tumor vasculature in MMP-9-deficient mice, resulting in fewer and smaller blood vessels. These changes were associated with a 48% decrease in pericytes present along microvessels. Taken together, the data demonstrate that in neuroblastoma, stromally derived MMP-9 contributes to angiogenesis by promoting blood vessel morphogenesis and pericyte recruitment.

Role of passive and adaptive immunity in influencing enterocyte-specific gene expression

Numerous genes expressed by intestinal epithelial cells are developmentally regulated, and the influence that adaptive (AI) and passive (PI) immunity have in controlling their expression has not been evaluated. In this study, we tested the hypothesis that both PI and AI influenced enterocyte gene expression by developing a breeding scheme that used T and B cell-deficient recombination-activating gene (RAG) mice. RNA was isolated from the liver and proximal/distal small intestine at various ages, and the steady-state levels of six different transcripts were evaluated by RNase protection assay. In wild-type (WT) pups, all transcripts [Fc receptor of the neonate (FcRn), polymeric IgA receptor (pIgR), GLUT5, lactase-phlorizin hydrolase (lactase), apical sodium-dependent bile acid transporter (ASBT), and Na+/glucose cotransporter (SGLT1)] studied were developmentally regulated at the time of weaning, and all transcripts except ASBT had the highest levels of expression in the proximal small intestine. In WT suckling pups reared in the absence of PI, pIgR mRNA levels were increased 100% during the early phase of development. In mice lacking AI, the expression of pIgR and lactase were significantly attenuated, whereas FcRn and GLUT5 levels were higher compared with WT mice. Finally, in the absence of both passive and active immunity, expression levels of pIgR and lactase were significantly lower than similarly aged WT mice. In summary, we report that the adaptive and passive immune status of mice influences steady-state mRNA levels of several important, developmentally regulated enterocyte genes during the suckling and weaning periods of life.

Requirement of Shp-2 tyrosine phosphatase in lymphoid and hematopoietic cell development

Shp-1 and Shp-2 are cytoplasmic phosphotyrosine phosphatases with similar structures. Mice deficient in Shp-2 die at midgestation with defects in mesodermal patterning, and a hypomorphic mutation at the Shp-1 locus results in the moth-eaten viable (me(v)) phenotype. Previously, a critical role of Shp-2 in mediating erythroid/myeloid cell development was demonstrated. By using the RAG-2-deficient blastocyst complementation, the role of Shp-2 in lymphopoiesis has been determined. Chimeric mice generated by injecting Shp-2(-/-) embryonic stem cells into Rag-2-deficient blastocysts had no detectable mature T and B cells, serum immunoglobulin M, or even Thy-1(+) and B220(+) precursor lymphocytes. Collectively, these results suggest a positive role of Shp-2 in the development of all blood cell lineages, in contrast to the negative effect of Shp-1 in this process. To determine whether Shp-1 and Shp-2 interact in hematopoiesis, Shp-2(-/-):me(v)/me(v) double-mutant embryos were generated and the hematopoietic cell development in the yolk sacs was examined. More hematopoietic stem/progenitor cells were detected in Shp-2(-/-):me(v)/me(v) embryos than in Shp-2(-/-) littermates. The partial rescue by Shp-1 deficiency of the defective hematopoiesis caused by the Shp-2 mutation suggests that Shp-1 and Shp-2 have antagonistic effects in hematopoiesis, possibly through a bidirectional modulation of the same signaling pathway(s).

The anti-apoptotic activities of Rel and RelA required during B-cell maturation involve the regulation of Bcl-2 expression

Rel and RelA, individually dispensable for lymphopoiesis, serve unique functions in activated B and T cells. Here their combined roles in lymphocyte development were examined in chimeric mice repopulated with c-rel(-/-) rela(-/-) fetal liver hemopoietic stem cells. Mice engrafted with double-mutant cells lacked mature IgM(lo)IgD(hi) B cells, and numbers of peripheral CD4(+) and CD8(+) T cells were markedly reduced. The absence of mature B cells was associated with impaired survival that coincided with reduced expression of bcl-2 and A1. bcl-2 transgene expression not only prevented apoptosis and increased peripheral B-cell numbers, but also induced further maturation to an IgM(lo)IgD(hi) phenotype. In contrast, the survival of double-mutant T cells was normal and the bcl-2 transgene could not rectify the peripheral T-cell deficit. These findings indicate that Rel and RelA serve essential, albeit redundant, functions during the later antigen-independent stages of B- and T-cell maturation, with these transcription factors promoting the survival of peripheral B cells in part by upregulating Bcl-2.

Functional analysis of the murine TCR beta-chain gene enhancer

The TCR beta-chain gene enhancer activates transcription and V(D)J recombination in immature thymocytes. In this paper we present a systematic analysis of the elements that contribute to the activity of the murine TCR beta enhancer in mature and immature T cell lines. We identified a region containing the beta E4, beta E5, and beta E6 motifs as the essential core of the TCR beta enhancer in pro-T cells. In mature cells, the core enhancer had low activity and required, in addition, either 5' or 3' flanking sequences whose functions may be partially overlapping. Mutation of any of the six protein binding sites located within the beta E4-beta E6 elements essentially abolished enhancer activity, indicating that this core enhancer contained no redundant elements. The beta E4 and beta E6 elements contain binding sites for ETS-domain proteins and the core binding factor. The beta E5 element bound two proteins that could be resolved chromatographically and that were both essential for enhancer activity.

Lung Kruppel-like factor, a zinc finger transcription factor, is essential for normal lung development

Lung Kruppel-like factor (LKLF) is a member of the Kruppel-like factor family of transcription factors and is highly expressed in lung with limited distribution in other tissues. Mice lacking LKLF due to inactivation of LKLF by gene targeting die in utero at midgestation around day 12.5 due to severe hemorrhage, making it difficult to study the role of this transcription factor in lung development and function. However, in vitro organ culture of lung buds removed from 11.5-day-old LKLF(-/-) embryos show normal tracheobronchial tree formation. To examine later stages of lung development, the embryonic lethality due to germ line LKLF null mutation was circumvented by constructing LKLF homozygous null mouse embryonic stem cells, using a two-step gene targeting procedure, and determining whether these cells give rise to lung tissue. The targeted cells were used to produce chimeric animals, and the contribution of LKLF-deficient cells to the formation of various internal organs was analyzed. In chimeric mice that survived after birth, null embryonic stem cells contributed significantly to all of the major organs except the lungs. On the other hand, some highly chimeric animals died at birth, and histopathological examination of their lungs suggested abnormalities in their lung development. These studies show that LKLF plays an important role in normal lung development.

Coordinate regulation of B cell differentiation by the transcription factors EBF and E2A

The transcription factors EBF and E2A are required at a similar step in early B cell differentiation. EBF and E2A synergistically upregulate transcription of endogenous B cell-specific genes in a non-B cell line. Here, we examine a genetic collaboration between these factors in regulating B lymphopoiesis. We find that Ebf+/- E2a+/- mice display a marked defect in pro-B cell differentiation at a stage later than observed in the single homozygous mutant mice. Pro-B cells from Ebf+/- E2a+/- mice show reduced expression of lymphoid-specific transcripts, including Pax5, Rag1, Rag2, and mb-1. We also show that EBF directly binds and activates the Pax5 promoter. Together, these data show collaboration between EBF and E2A and provide insight into the hierarchy of transcription factors that regulate B lymphocyte differentiation.

Dissecting NK Cell Development Using a Novel Alymphoid Mouse Model: Investigating the Role of the c-abl Proto-Oncogene in Murine NK Cell Differentiation

NK lymphocytes participate in both innate and adaptive immunity by their prompt secretion of cytokines including IFN-γ, which activates macrophages, and by their ability to lyse virally infected cells and tumor cells without prior sensitization. Although these characteristics of NK cells are well documented, little is known about the genetic program that orchestrates NK development or about the signaling pathways that trigger NK effector functions. By crossing NK-deficient common γ-chain (γc) and recombinase activating gene (RAG)-2 mutant mice, we have generated a novel alymphoid (B−, T−, and NK−) mouse strain (RAG2/γc) suitable for NK complementation in vivo. The role of the c-abl proto-oncogene in murine NK cell differentiation has been addressed in hemopoietic chimeras generated using RAG2/γc mice reconstituted with c-abl−/− fetal liver cells. The phenotypically mature NK cells that developed in the absence of c-abl were capable of lysing tumor targets, recognizing “missing self,” and performing Ab-dependent cellular cytotoxicity. Taken together, these results exclude any essential role for c-abl in murine NK cell differentiation in vivo. The RAG2/γc model thereby provides a novel approach to establish a genetic map of NK cell development.

High Level Expression of the Xlr Nuclear Protein in Immature Thymocytes and Colocalization with the Matrix-Associated Region-Binding SATB1 Protein

The X-linked lymphocyte-regulated (Xlr) protein is a 30,000 Mr nuclear protein bearing homology with meiosis-specific proteins and expressed in late stage B lymphoid cell lines. In the present study we investigated its expression in the T lymphoid lineage. In adults, a high level of expression was detected in CD4−CD8− thymocytes. Most remarkably, the peak of Xlr expression occurred early during thymus cell ontogeny, precisely on days 14–15 of gestation, and was associated with the first wave of pre-T cell differentiation. Its onset preceded the rearrangement of TCR genes, as Xlr expression was conserved in thymus cells from RAG10/0 mice. The lower expression of Xlr on day 13 of fetal development, the bright Thy1+ phenotype of Xlr-positive cells, their large size, and their absence from subcapsular areas suggest that Xlr expression must be turned on within the thymus and not in prethymic precursors. From day 16 of gestation, Xlr expression decreased markedly. At birth and later, Xlrhigh cells were mostly large cells scattered throughout the cortical area. As shown by confocal microscopy, expression of Xlr closely overlapped that of SATB1, which binds special AT-rich DNA sequences associated with the nuclear matrix and plays an important regulatory role for many genes. The remarkably regulated expression of Xlr in the lymphoid cell lineage and of its homologue Xmr in the germ cell lineage suggests that they might play an important role in chromatin metabolism at critical stages of differentiation during which the genome undergoes irreversible rearrangements.

Expression of heme oxygenase-1 can determine cardiac xenograft survival

The rejection of concordant xenografts, such as mouse-to-rat cardiac xenografts, is very similar to the delayed rejection of porcine-to-primate discordant xenografts. In concordant models, this type of rejection is prevented by brief complement inhibition by cobra venom factor (CVF) and sustained T-cell immunosuppression by cyclosporin A (CyA). Mouse hearts that survive indefinitely in rats treated with CVF plus CyA express the anti-inflammatory gene heme oxygenase-1 (HO-1) in their endothelial cells and smooth muscle cells. The anti-inflammatory properties of HO-1 are thought to rely on the ability of this enzyme to degrade heme and generate bilirubin, free iron and carbon monoxide. Bilirubin is a potent anti-oxidant, free iron upregulates the transcription of the cytoprotective gene, ferritin, and carbon monoxide is thought to be essential in regulating vascular relaxation in a manner similar to nitric oxide. We show here that the expression of the HO-1 gene is functionally associated with xenograft survival, and that rapid expression of HO-1 in cardiac xenografts can be essential to ensure long-term xenograft survival.

No requirement for V(D)J recombination in p53-deficient thymic lymphoma

The p53 tumor suppressor is activated in response to a variety of cellular stress signals, although specific in vivo signals that trigger tumor suppression are unknown. In mouse thymocytes, where p53 inactivation leads to tumorigenesis, several observations suggest that V(D)J recombination of T-cell receptor (TCR) loci could provide a DNA damage signal triggering p53-dependent apoptosis and tumor suppression. Inactivation of p53 would allow V(D)J driven mutation of additional cancer genes, facilitating tumorigenesis. Here, we show that mice with a p53 deficiency in thymocytes and unable to carry out V(D)J recombination are not impaired in the development of thymoma. Recombination-activating gene (RAG) deficiencies were introduced into both p53-/- mice and TgTDeltaN transgenic mice, a strain in which 100% of the mice develop thymoma due to thymocyte-specific inactivation of p53 by a simian virus 40 T-antigen variant. V(D)J recombination was dispensable for tumorigenesis since thymomas developed with or without the RAG-1 or RAG-2 gene, although some delay was observed. When V(D)J recombination was suppressed by expression of rearranged TCR transgenes, 100% of the TgTDeltaN mice developed thymoma, surprisingly with reduced latency. Further introduction of a RAG deficiency into these mice had no impact on the timing or frequency of tumorigenesis. Finally, karyotype and chromosome painting analyses showed no evidence for TCR gene translocations in p53-deficient thymomas, although abundant aneuploidy involving frequent duplication of certain chromosomes was present. Thus, contrary to the current hypothesis, these studies indicate that signals other than V(D)J recombination promote p53 tumor suppression in thymocytes and that the mechanism of tumorigenesis is distinct from TCR translocation oncogene activation.

Marrow stromal cells as a source of progenitor cells for nonhematopoietic tissues in transgenic mice with a phenotype of osteogenesis imperfecta

Marrow stromal cells from wild-type mice were infused into transgenic mice that had a phenotype of fragile bones resembling osteogenesis imperfecta because they expressed a human minigene for type I collagen. In mice that were irradiated with potentially lethal levels (700 cGy) or sublethal levels (350 cGy), DNA from the donor marrow stromal cells was detected consistently in marrow, bone, cartilage, and lung either 1 or 2.5 mo after the infusions. The DNA also was detected but less frequently in the spleen, brain, and skin. There was a small but statistically significant increase in both collagen content and mineral content of bone 1 mo after the infusion. Similar results were obtained with infusion of relatively large amounts of wild-type whole marrow cells into the transgenic mice. In experiments in which male marrow stromal cells were infused into a female osteogenesis imperfecta-transgenic mouse, fluorescense in situ hybridization assays for the Y chromosome indicated that, after 2.5 mo, donor male cells accounted for 4-19% of the fibroblasts or fibroblast-like cells obtained in primary cultures of the lung, calvaria, cartilage, long bone, tail, and skin. In a parallel experiment in which whole marrow cells from a male mouse were infused into a female immunodeficient rag-2 mouse, donor male cells accounted for 4-6% of the fibroblasts or fibroblast-like cells in primary cultures. The results support previous suggestions that marrow stromal cells or related cells in marrow serve as a source for continual renewal of cells in a number of nonhematopoietic tissues.

Biochemical and Functional Analyses of Chromatin Changes at the TCR-β Gene Locus During CD4−CD8− to CD4+CD8+ Thymocyte Differentiation

Allelic exclusion is the process wherein lymphocytes express Ag receptors from only one of two possible alleles, and is effected through a feedback inhibition of further rearrangement of the second allele. The feedback signal is thought to cause chromatin changes that block accessibility of the second allele to the recombinase. To identify the putative chromatin changes associated with allelic exclusion, we assayed for DNase I hypersensitivity, DNA methylation, and transcription in 100 kb of the TCR-β locus. Contrary to current models, we identified chromatin changes indicative of an active and accessible locus associated with the occurrence of allelic exclusion. Of 11 DNase I hypersensitive sites identified, 3 were induced during CD4−CD8− to CD4+CD8+ thymocyte differentiation, and demethylation and increased germline transcription of the locus were evident. We further examined the role of the most prominently induced site near the TCR-β enhancer (Eβ) in allelic exclusion by targeted mutagenesis. Two other sites were also examined in New Zealand White (NZW) mice that have a natural deletion in the TCR-β locus. TCR-β gene recombination and allelic exclusion were normal in both mutant mice, negating dominant roles for the three hypersensitive sites in the control of allelic exclusion. The data suggest that alternative cis-regulatory elements, perhaps contained in the Eβ enhancer and/or in the upstream Vβ region, are involved in the control of TCR-β allelic exclusion.

The transcription factor NF-ATc1 regulates lymphocyte proliferation and Th2 cytokine production

NF-ATc1 is a member of a family of genes that encodes the cytoplasmic component of the nuclear factor of activated T cells (NF-AT). In activated T cells, nuclear NF-AT binds to the promoter regions of multiple cytokine genes and induces their transcription. The role of NF-ATc1 was investigated in recombination activating gene-1 (RAG-1)-deficient blastocyst complementation assays using homozygous NF-ATc1-/- mutant ES cell lines. NF-ATc1-/-/RAG-1-/- chimeric mice showed reduced numbers of thymocytes and impaired proliferation of peripheral lymphocytes, but normal production of IL-2. Induction in vitro of Th2 responses, as demonstrated by a decrease in IL-4 and IL-6 production, was impaired in mutant T cells. These data indicate that NF-ATc1 plays roles in the development of T lymphocytes and in the differentiation of the Th2 response.

LKLF: A transcriptional regulator of single-positive T cell quiescence and survival

Mature single-positive (SP) T lymphocytes enter a "resting" state in which they are proliferatively quiescent and relatively resistant to apoptosis. The molecular mechanisms regulating this quiescent phenotype were unknown. Here it was found that the expression of a Kruppel-like zinc finger transcription factor, lung Kruppel-like factor (LKLF), is developmentally induced during the maturation of SP quiescent T cells and rapidly extinguished after SP T cell activation. LKLF-deficient T cells produced by gene targeting had a spontaneously activated phenotype and died in the spleen and lymph nodes from Fas ligand-induced apoptosis. Thus, LKLF is required to program the quiescent state of SP T cells and to maintain their viability in the peripheral lymphoid organs and blood.

Kinetic Evidence of the Regeneration of Multilineage Hematopoiesis From Primitive Cells in Normal Human Bone Marrow Transplanted Into Immunodeficient Mice

Based on initial observations of human CD34+ Thy-1+ cells and long-term culture-initiating cells (LTC-IC) in the bone marrow of some sublethally irradiated severe combined immunodeficient (SCID) mice transplanted intravenously with normal human marrow cells, and the subsequent finding that the NOD/LtSz-scid/scid (NOD/SCID) mouse supports higher levels of human cell engraftment, we undertook a series of time course experiments to examine posttransplant changes in the number, tissue distribution, cycling activity, and in vivo differentiation pattern of various human hematopoietic progenitor cell populations in this latter mouse model. These studies showed typical rapid posttransplant recovery curves for human CD34− CD19+ (B-lineage) cells, CD34+ granulopoietic, erythroid, and multilineage colony-forming cells (CFC), LTC-IC, and CD34+ Thy-1+ cells from a small initial population representing <0.1% of the original transplant. The most primitive human cell populations reached maximum values at 5 weeks posttransplant, after which they declined. More mature cell types peaked after another 5 weeks and then declined. A 2-week course of thrice weekly injections of human Steel factor, interleukin (IL)-3, granulocyte-macrophage colony-stimulating factor (GM-CSF), and erythropoietin (administered just before the mice were killed for analysis) did not alter the pace of regeneration of either primitive or mature human hematopoietic cells, or their predominantly granulopoietic and B-lymphoid pattern of differentiation, although a significant enhancing effect on the level of human cell engraftment sustained after 3 months was noted. Cycling studies showed the human CFC present at 4 to 5 weeks posttransplant to be rapidly proliferating even in mice not given human growth factors. However, by 10 weeks and thereafter, only quiescent human CFC were detected; interestingly, even in mice that were given the 2-week course of growth factor injections. These studies indicate the use of this model for future analysis of the properties and in vivo regulation of primitive human hematopoietic cells that possess in vivo repopulating ability.

Essential role for complex N-glycans in forming an organized layer of bronchial epithelium

Mice lacking the complex subset of N-glycans due to inactivation of the Mgat1 gene die at mid-gestation, making it difficult to identify specific biological functions for this class of cell surface carbohydrates. To circumvent this embryonic lethality and to uncover tissue-specific functions for complex N-glycans, WW6 embryonic stem cells with inactivated Mgat1 alleles were tracked in chimeric embryos. The Mgat1 gene encodes N-acetylglucosaminyltransferase I (Glc-NAc-TI; EC 2.4.1.101), the transferase that initiates the synthesis of complex N-glycans. WW6 cells carry an inert beta-globin transgene that allows their identification in chimeras by DNA-DNA in situ hybridization. Independent Mgat1-/- and Mgat1+/- mutant WW6 isolates contributed like parent WW6 cells to the tissues of embryonic day (E) 10.5 to E16.5 chimeras. However, a cell type-specific difference was observed in lung. Homozygous null Mgat1-/- WW6 cells did not contribute to the epithelial layer in more than 99% bronchi. This deficiency was corrected by transfection of a Mgat1 transgene. Interestingly, heterozygous Mgat1+/- WW6 cells were also deficient in populating the layer of bronchial epithelium. Furthermore, examination of lung bud in E9.5 Mgat1-/- mutant embryos showed complete absence of an organized epithelial cell layer in the bronchus. Thus, complex N-glycans are required to form a morphologically recognizable bronchial epithelium, revealing an in vivo, cell type-specific function for this class of N-glycans.

Primordial emergence of the recombination activating gene 1 (RAG1): sequence of the complete shark gene indicates homology to microbial integrases

The rearrangement of antibody and T-cell receptor gene segments is indispensable to the vertebrate immune response. All extant jawed vertebrates can rearrange these gene segments. This ability is conferred by the recombination activating genes I and II (RAG I and RAG II). To elucidate their origin and function, the cDNA encoding RAG I from a member of the most ancient class of extant gnathostomes, the Carcharhine sharks, was characterized. Homology domains identified within shark RAG I prompted sequence comparison analyses that suggested similarity of the RAG I and II genes, respectively, to the integrase family genes and integration host factor genes of the bacterial site-specific recombination system. Thus, the apparent explosive evolution (or "big bang") of the ancestral immune system may have been initiated by a transfer of microbial site-specific recombinases.

Cellular and molecular analysis of lymphoid development using Rag-deficient mice

The establishment of a functional immune system with diverse antigen receptors is dependent on the V(D)J recombination activating gene products Rag-1 and Rag-2. These two proteins constitute the key lymphoid components required for the activation of antigen receptor rearrangement. Both Rag-1 and Rag-2 are required for the catalysis of the initial stages of V(D)J recombination. Thus, functional disruption of either the Rag-1 or Rag-2 genes by homologous recombination, leads to immunodeficiency due to lymphoid arrest at a stage prior to the recombination of the antigen receptor loci. In Rag-deficient mice, both B- and T-cell differentiation is eliminated due to the absence of antigen receptors. Lymphoid development can be restored by the introduction of rearranged antigen receptor transgenes that give rise to monoclonal populations of fully mature B- or T-cells. The absence of the major conventional populations of B- and T-cells from the Rag-deficient mice provided an excellent background for studying the molecular and cellular mechanisms of lymphoid differentiation. The Rag-deficient background has been used as a system for: the functional analysis of Rag-1 and Rag-2; studying the developmental functions of antigen receptors and other molecules of the immune system; the molecular analysis of the early stages of the B- and T-cell lineages; the co-development of lymphocytes with stroma cells; the identification of minor subpopulations of the developing immune system; the involvement of lymphoid populations in the onset of pathogenesis. In addition, the development of the "blastocyst complementation assay" methodology, based on the phenotype of the Rag-/- mice, allowed the functional analysis of numerous lymphoid specific components.

Consequences of lack of beta 1 integrin gene expression in mice

beta 1 integrins are cell-surface receptors that mediate cell-cell and cell-matrix interactions. We have generated a null mutation in the gene for the beta 1 integrin subunit in mice and embryonic stem (ES) cells. Heterozygous mice are indistinguishable from normal littermates. Homozygous null embryos develop normally to the blastocyst stage, implant, and invade the uterine basement membrane but die shortly thereafter. Using beta 1 integrin-deficient ES cells we have established chimeric embryos and adult mice. Analysis of the chimeric embryos demonstrated the presence of beta 1 integrin-deficient cells in all germ layers indicating that beta 1-null cells can differentiate and migrate in a context of normal tissue. When evaluated at embryonic day 9.5 (E9.5), embryos with a beta 1-null cell contribution below 25% were developing normally, whereas embryos with a contribution above this threshold were distorted and showed abnormal morphogenesis. In adult chimeric mice beta 1 integrin-deficient cells failed to colonize liver and spleen but were found in all other tissues analyzed at levels from 2%-25%. Immunostaining of chimeric mice showed that in cardiac muscle, there were small, scattered patches of myocytes that were beta 1-null. In contrast, many myotubes showed some beta 1-null contribution as a result of fusion between wild-type and mutant myoblasts to form mixed myotubes. The adult chimeric brain contained beta 1-null cells in all regions analyzed. Also, tissues derived from the neural crest contained beta 1 integrin-deficient cells indicating that migration of neuronal cells as well as neural crest cells can occur in the absence of beta 1 integrins.

Bclx regulates the survival of double-positive thymocytes

The bclx gene has been shown to regulate programmed cell death in vitro. We now show that Bclx expression increases dramatically when T cells differentiate from CD4- CD8- (double negative) thymocytes to CD4+ CD8+ [double positive (DP)] thymocytes. In contrast single-positive (SP) thymocytes express negligible amounts of Bclx protein. This expression pattern contrasts with that of Bcl2, which is present in double-negative thymocytes, down-regulated in DP thymocytes, and reinduced upon maturation to SP thymocytes. Elimination of Bclx by gene targeting dramatically shortens the survival of DP thymocytes but not the survival of SP thymocytes or peripheral SP T cells. These data suggest that the induction of Bclx during thymic maturation plays a critical role in regulating the length of time DP thymocytes survive in the absence of selection.

Influence of immunoglobulin heavy- and light-chain expression on B-cell differentiation

To study the influence of immunoglobulin heavy-chain (HC) and light-chain (LC) expression in promoting B-cell differentiation, we have introduced functional immunoglobulin HC and/or LC transgenes into the recombinase activating gene-2-deficient background (RAG-2-/-). RAG-2-/- mice do not undergo endogenous V(D)J rearrangement events and, therefore, are blocked in B- and T-cell development at the early pro-B- and pro-T-cell stages. Introduction of immunoglobulin HC transgenes into the RAG-2-/- background promotes the development of a B-lineage cell population that phenotypically has the characteristics of pre-B cells. We have shown further that this population has altered growth characteristics as measured by interleukin-7 responsiveness in culture. Bone marrow cells from immunoglobulin HC transgenic RAG-2-/- mice have up-regulated expression of germ-line kappa LC gene transcripts and down-regulated expression of lambda 5 surrogate LCs (SLCs). Although mu HC/SLC complexes are detectable intracellularly in HC/RAG-2-/- pre-B-cell populations, HC expression is not readily detectable on the surface of these cells. lambda LC RAG-2-/- mice had a bone marrow B-lineage cell phenotype indistinguishable from that of RAG-2-/- littermates, indicating that LC expression by itself has no influence on pro-B cell differentiation. Strikingly, simultaneous introduction of mu HC and lambda LC transgenes into RAG-2-/- mice led to the generation of a substantial population of "monoclonal" peripheral B-cells that were functional with regard to immunoglobulin secretion, indicating that T cells or diverse immunoglobulin repertoires are not necessary for peripheral B-cell development.