Lymphopoiesis in transgenic mice over-expressing Artemis.

Artemis is a factor of the non-homologous end joining pathway involved in DNA double-strand break repair that has a critical role in V(D)J recombination. Mutations in DCLRE1C/ARTEMIS gene result in radiosensitive severe combined immunodeficiency in humans owing to a lack of mature T and B cells. Given the known drawbacks of allogeneic hematopoietic stem cell transplantation (HSCT), gene therapy appears as a promising alternative for these patients. However, the safety of an unregulated expression of Artemis has to be established. We developed a transgenic mouse model expressing human Artemis under the control of the strong CMV early enhancer/chicken beta actin promoter through knock-in at the ROSA26 locus to analyze this issue. Transgenic mice present a normal development, maturation and function of T and B cells with no signs of lymphopoietic malignancies for up to 15 months. These results suggest that the over-expression of Artemis in mice (up to 40 times) has no deleterious effects in early and mature lymphoid cells and support the safety of gene therapy as a possible curative treatment for Artemis-deficient patients.

Variable correction of Artemis deficiency by I-Sce1-meganuclease-assisted homologous recombination in murine hematopoietic stem cells.

The correction of genetic mutations by homologous recombination is an attractive approach to gene therapy. We used the DNA double-strand breaks introduced by the site-specific endonuclease I-Sce1 as a means of increasing homologous recombination of an exogenous DNA template in murine hematopoietic stem cells (mHSCs). To develop this approach, we chose an Artemis knockout (Art(-/-)) mouse in which exon 12 of the Artemis gene had been replaced by an I-Sce1 recognition site. The I-Sce1 enzyme and the Artemis correction template were each delivered by a self-inactivating (SIN)-integrase-defective lentiviral vector (SIN-IDLV-CMV-ISce1 and SIN-IDLV-Art, respectively). Transduction of Art(-/-) mHSCs with the two vectors successfully reverted the Art(-/-) phenotype in 2 of our 10 experiments. Even though the potential for genotoxicity has yet to be evaluated, this new approach to gene editing appears to be promising. Improving the efficacy of this approach will require further technical work.

Independent mutations of the human CD3-epsilon gene resulting in a T cell receptor/CD3 complex immunodeficiency.

The T-cell receptor (TCR) is composed of two glycoproteins (alpha and beta or gamma and delta) associated with four invariant polypeptides (CD3-gamma, delta, epsilon and zeta). The majority of TCR/CD3 complexes contain six polypeptide chains, and although there is some flexibility in the complex subunit stoichiometry the CD3-epsilon chain is central to CD3 core assembly and full complex formation. We have described previously defective expression of the TCR/CD3 complex in an immunodeficient child. We now report that two independent CD3-epsilon gene mutations present in the parents have segregated in the patient, leading to defective CD3-epsilon chain synthesis and preventing normal association and membrane expression of the TCR/CD3 complex.

A human severe combined immunodeficiency (SCID) condition with increased sensitivity to ionizing radiations and impaired V(D)J rearrangements defines a new DNA recombination/repair deficiency.

The products of recombination activating gene (RAG)1 and RAG2 initiate the lymphoid-specific phase of the V(D)J recombination by creating a DNA double-strand break (dsb), leaving hairpin-sealed coding ends. The next step uses the general DNA repair machinery of the cells to resolve this dsb. Several genes involved in both V(D)J recombination and DNA repair have been identified through the analysis of in vitro mutants (Chinese hamster ovary cells) and in vivo situations of murine and equine severe combined immunodeficiency (scid). These studies lead to the description of the Ku-DNA-dependent protein kinase complex and the XRCC4 factor. A human SCID condition is characterized by an absence of B and T lymphocytes. One subset of these patients also demonstrates an increased sensitivity to the ionizing radiation of their fibroblasts and bone marrow precursor cells. This phenotype is accompanied by a profound defect in V(D)J recombination with a lack of coding joint formation, whereas signal joints are normal. Functional and genetic analyses distinguish these patients from the other recombination/repair mutants, and thus define a new group of mutants whose affected gene(s) is involved in sensitivity to ionizing radiation and V(D)J recombination.

Demonstration of delta rec-pseudo J alpha rearrangement with deletion of the delta locus in a human stem-cell leukemia.

It has been hypothesized that a rearrangement between the delta recombining element (delta Rec) and a pseudo J alpha gene serves to delete the TCR-delta locus before rearrangement of the TCR-alpha genes. We have now sequenced a direct, site-specific rearrangement between the delta Rec element and a pseudo J alpha gene in a human leukemic stem-cell line. Putative "N-sequence" addition was noted at the site of recombination, suggesting that this event occurred at a time when the enzyme(s) involved in N-region addition were active in this cell. This provides support for the view that deletion of the TCR-delta locus is required before rearrangement of the TCR-alpha chain genes.

Mutations in Fas associated with human lymphoproliferative syndrome and autoimmunity.

Fas (also known as Apo1 and CD95) is a cell surface receptor involved in apoptotic cell death. Fas expression and function were analyzed in three children (including two siblings) with a lymphoproliferative syndrome, two of whom also had autoimmune disorders. A large deletion in the gene encoding Fas and no detectable cell surface expression characterized the most affected patient. Clinical manifestations in the two related patients were less severe: Fas-mediated apoptosis was impaired and a deletion within the intracytoplasmic domain was detected. These findings illustrate the crucial regulatory role of Fas and may provide a molecular basis for some autoimmune diseases in humans.

Highly restricted human T cell repertoire in peripheral blood and tissue-infiltrating lymphocytes in Omenn's syndrome.

Omenn's syndrome is an inherited human combined immunodeficiency condition characterized by the presence of a large population of activated and tissue-infiltrating T cells. Analysis of the TCRB repertoire revealed a highly restricted TCRBV usage in three patients. More strikingly, T cell clones from the three patients expressed TCRB chains with VDJ junction similarities, suggesting a common antigenic specificity. Analysis of the TCRA repertoire in one patient also revealed a restricted TCRAV usage. Finally, analysis of the TCRBV repertoire of tissue-infiltrating T cells in one patient suggested nonrandom tissue migration. These results suggest that the oligoclonal expansion of T cells observed in Omenn's syndrome could be the consequence of autoimmune proliferation generated by a profound defect in lymphocyte development.

Increased radiosensitivity of granulocyte macrophage colony-forming units and skin fibroblasts in human autosomal recessive severe combined immunodeficiency.

We studied the radiosensitivity of granulocyte macrophage colony-forming units (GM-CFU) in patients with a severe combined immunodeficiency (SCID). Three patients lacking both mature T and B cells showed a twofold higher GM-CFU radiosensitivity calculated as the DO value (dose required to reduce survival to 37%), and an identical observation was made with fibroblasts from one of these patients. A patient with an SCID with hypereosinophilia, i.e., Omenn's syndrome characterized by extremely restricted T cell heterogeneity and a lack of B cells, also showed abnormal GM-CFU radiosensitivity. In contrast, GM-CFU from a patient lacking only T cells (X-linked form of SCID) showed normal GM-CFU radiosensitivity. These data further support the similarity between human T(-) B(-) SCID and the murine acid mutation characterized by a defect in T cell receptor and immunoglobulin gene rearrangement, and by an abnormal double-strand DNA break repair function. In addition, they strongly suggest that the Omenn's immunodeficiency syndrome may be a leaky T(-)B(-) SCID phenotype as previously indicated by the coexistence of the two phenotypes in siblings.

Restricted heterogeneity of T lymphocytes in combined immunodeficiency with hypereosinophilia (Omenn's syndrome).

We report the immunological characteristics of five patients with Omenn's syndrome, a rare inherited immunodeficiency also known as combined immunodeficiency with hypereosinophilia. The syndrome is characterized by T cell infiltration of skin, gut, liver, and spleen leading to diffuse erythroderma, protracted diarrhea, failure to thrive, and hepatosplenomegaly. Blood T cells as well as those infiltrating the skin and gut were found to express activation markers and were partially activated by mitogens but not by antigens. Although the lesions resembled those in graft-versus-host disease, the blood T cells were shown by DNA haplotype analysis using probes revealing variable number of tandem repeats to belong to the patients as well as the T cells infiltrating the gut and skin in one patient. A given T cell subset (TCR alpha beta+, CD4+/CD8+, or TCR gamma delta+) was predominant in each patient, with a specific distribution in the skin lesions. Moreover, the study of T cell receptor beta, gamma, and delta gene rearrangements in four patients revealed oligoclonality involving C beta 1, C beta 2, or different V gamma J gamma or V delta J delta genes. This indicates that restricted heterogeneity of the T cell repertoire, previously reported in one case, is a major feature of this syndrome. The occurrence of alymphocytosis-type severe combined immunodeficiency in the brother of one of the patients suggests that the restricted heterogeneity of T cell receptor gene usage in Omenn's syndrome may arise from leakiness, within the context of a genetically determined faulty T cell differentiation.

A case report of a patient with microcephaly, facial dysmorphism, chromosomal radiosensitivity and telomere length alterations closely resembling "Nijmegen breakage syndrome" phenotype.

Genetic heterogeneity in Nijmegen breakage syndrome (NBS) is highlighted by patients showing clinical and cellular features of NBS but with no mutations in NBS1 and normal levels of nibrin. NBS is an autosomal recessive disorder, whose clinical cellular signs include growth and developmental defects, dysmorphic facies, immunodeficiency, cancer predisposition, chromosomal instability and radiosensitivity. NBS is caused by mutations in the NBS1 gene, whose product is part of the MRE11/RAD50/NBS1 complex involved in the DNA double-strand break (DSB) response pathway. Since the identification of the NBS1 gene, patients with NBS clinical signs, particularly severe congenital microcephaly, are screened for mutations in the NBS1 gene. Further analyses include X-ray-induced chromosome aberrations, telomere analysis, kinetics of DSBs repair, levels of a panel of proteins involved in the maintenance of genetic stability, radiation-induced phosphorylation of various substrates and cell cycle analysis. We describe a patient with a NBS clinical phenotype, chromosomal sensitivity to X-rays but without mutations in the whole NBS1 or in the Cernunnos gene. Enhanced response to irradiation was mediated neither by DSBs rejoining defects nor by the NBS/AT-dependent DNA-damage response pathway. Notably, we found that primary fibroblasts from this patient displayed telomere length alterations. Cross-talk between pathways controlling response to DSBs and those involved in maintaining telomeres has been shown in the present patient. Dissecting the cellular phenotype of radiosensitive NBS-like patients represents a useful tool for the research of new genes involved in the cellular response to DSBs.

Combined immunodeficiency associated with increased apoptosis of lymphocytes and radiosensitivity fibroblasts.

Severe immunodeficiency characterized by lymphopenia was found in two siblings, one of whom was examined in detail. The calcium flux, pattern of tyrosine phosphorylation of proteins, and interleukin 2 (IL-2) production and proliferation in response to mitogens suggested that the peripheral blood T cells activated normally. The peripheral blood T cells were shown to have an activated phenotype with increased expression of CD45RO+ and CD95/Fas. Increased spontaneous apoptosis occurred in unstimulated lymphocyte cultures. The elevated apoptosis was not due to alterations in expression or to mutations in Bcl-2, Bcl-X(L), or Flip, nor could the spontaneous apoptosis be prevented by blocking Fas, suggesting that it was independent of Fas signaling. This is the first inherited combined immunodeficiency associated with impaired lymphocyte survival. Fibroblasts derived from the patient showed appreciable radiosensitivity in clonal assays, but apoptosis was not elevated. Our results show that the fibroblasts represent a new radiosensitive phenotype not associated with cell cycle checkpoint defects, V(D)J recombination defects, or elevated chromosome breakage. We suggest that the affected gene plays a role in an undetermined damage response mechanism that results in elevated spontaneous apoptosis in lymphoid cells and radiosensitivity in fibroblasts.

[Gene therapy for immune deficiencies]. / Thérapie génique des déficits immunitaires.

Gene therapy offers an attractive option to the most severe forms of primary immunodeficiency diseases. Identification of disease associated genes as well as advances in the technology of gene transfer into hematopoietic progenitor cells have set the basis for the first clinical trials. Settings characterized by the potential for a selective advantage provided to transduced cells are the first diseases to target. The recent example of successful treatment of Severe Combined Immunodeficiency-X1 (gamma c deficiency) illustrates this potential.

V(D)J and immunoglobulin class switch recombinations: a paradigm to study the regulation of DNA end-joining.

The immune system is the site of intense DNA damage/modification, which occur during the development and maturation of B and T lymphocytes. V(D)J recombination is initiated by the Rag1 and Rag2 proteins and the formation of a DNA double-strand break (DNA dsb). This DNA lesion is repaired through the use of the non-homologous end-joining (NHEJ) pathway, several factors of which have been identified through the survey of immunodeficient conditions in humans and mice. Upon antigenic recognition in secondary lymphoid organs, mature B cells further diversify their repertoire through class switch recombination (CSR). CSR is a region-specific rearrangement process triggered by the activation-induced cytidine deaminase factor and also proceeds through the introduction of DNA dsb. However, unlike V(D)J recombination, CSR does not rely strictly on NHEJ for the repair of the DNA lesion. Instead, CSR, but not V(D)J recombination, requires the major factors of the DNA damage response. V(D)J recombination and CSR thus represent an interesting paradigm to study the regulation among the various DNA repair pathways.

Self-tolerance to host and donor following HLA-mismatched bone marrow transplantation.

The transplantation of T cell-depleted HLA-haploidentical bone marrow can correct the severe combined immunodeficiency disease (SCID) caused by the inherited absence of T lymphocytes. Despite a different environment, no severe graft-vs.-host reaction occurred and engrafted T lymphocytes became functional. We have studied tolerance of engrafted T lymphocytes to donor and host HLA antigens in four SCID patients who have been transplanted with bone marrow from one of their HLA-haploidentical parents. Graft-vs.-host reaction was prevented by T cell depletion of infused bone marrow using E rosetting and by in vivo administration of cyclosporine A. Subsequent to bone marrow transplantation (BMT), the engrafted T lymphocytes were shown to be unresponsive in vitro towards host cells collected prior to BMT. Generally, this tolerance could not be explained by a suppressive mechanism. Nevertheless, in one patient suppressive cells were found transiently. In contrast to the early appearance of a tolerance towards host, a reactivity of engrafted donor cells towards donor was always observed within the first 300 days post-grafting. This autoreactivity was mediated by T cells of donor origin and its targets were HLA class II molecules (at least HLA-DR and DQ). The progressive disappearance of this autoreactivity was correlated with the engraftment of Ia-positive cells (monocytes plus B lymphocytes) of donor origin and the achievement of complete immunological reconstitution. In the patient showing the strongest autoreactivity, a donor-specific T cell line has been grown which was shown to specifically inhibit the proliferative response of donor lymphocytes. Concomittantly, the immunological reconstitution remains poor in this patient. These data suggest that tolerance to HLA class II molecules is dependent on the presence of the relevant HLA class II molecule-expressing cells allowing the elimination or the suppression of T lymphocytes specifically directed at these molecules.

High level transient gene expression in human lymphoid cells by SV40 large T antigen boost.

High level transient gene expression in lymphoid cells has always been challenging because of the difficulty to efficiently transfect such cells. This has precluded any attempt to clone cDNA encoding proteins by means of their specific biological function in lymphoid cells. We have developed a very efficient transient eukaryotic expression system analogous to the well-known expression system in COS cells. Firefly luciferase and human CD2 genes were used as reporter genes and cloned into the eukaryotic shuttle vector pCDM8 which contains the strong cytomegalovirus promoter and the SV40 origin of replication for autonomous plasmid replication in permissive host cells that express the large SV40 T Antigen. Co-transfection of the reporter plasmids together with an SV40 T Ag expressing plasmid resulted in the several fold amplification of either the Luc activity or the cell surface expression of the CD2 marker in a transient assay. The level of amplification was dependent on the strength of the promoter used to drive the SV40 T Ag expression and was correlated with the extent of autonomous replication of the reporter plasmid in transfected cells. This highly efficient transient gene expression by SV40 T Ag boost was suitable to several human cell lines, making this system of general interest for expression cloning strategies or other gene transfer application that need high level expression.

T cell receptor delta gene rearrangement and T early alpha (TEA) expression in immature alpha beta lineage thymocytes: implications for alpha beta/gamma delta lineage commitment.

Mature T cells comprise two mutually exclusive lineages expressing heterodimeric alpha beta or gamma delta antigen receptors. During development, beta, gamma, and delta genes rearrange before alpha, and mature gamma delta cells arise in the thymus prior to alpha beta cells. The mechanism underlying commitment of immature T cells to the alpha beta or gamma delta lineage is controversial. Since the delta locus is located within the alpha locus, rearrangement of alpha genes leads to deletion of delta. We have examined the rearrangement status of the delta locus immediately prior to alpha rearrangement. We find that many thymic precursors of alpha beta cells undergo VDJ delta rearrangements. Furthermore, the same cells frequently coexpress sterile T early alpha (TEA) transcripts originating 3' of C delta and 5' of the most upstream J alpha, thus implying that individual alpha beta lineage cells undergo sequential VDJ delta and VJ alpha rearrangements. Finally, VDJ delta rearrangements in immature alpha beta cells appear to be random, supporting models in which alpha beta lineage commitment is determined independently of the rearrangement status at the TCR delta locus.

Functional characterization of the promoter for the human germ-line T cell receptor J alpha (TEA) transcript.

The T cell receptor (TCR)-alpha and -delta loci are contained on the same chromosomal region, and yet are developmentally and genetically independent. The first element of the J alpha cluster (psi J alpha) is the site of an active rearrangement in the human thymus (delta Rec-psi J alpha rearrangement) and is localized downstream of a region expressed as a germ-line sterile transcript (TEA) in the human developing thymus. We hypothesized that the transcription of TEA could be indicative of (or responsible for) the opening of the J alpha to the V(D)J recombinase and undertook to analyze cis-acting sequences controlling the TEA transcription. The promoter of TEA was characterized. It was part of a region that is highly conserved between human and mouse and contained many sites for the putative binding of T cell-specific transcription factors. The in vitro activity of this promoter was dependent on the association with an enhancer. A strong DNase I hypersensitive site was found in the vicinity of this promoter again suggesting the possible presence of protein-DNA interactions in this region. The implications of these results in the general perspective of TCR-alpha/delta gene regulation is discussed.

T early alpha (TEA) regulates initial TCRVAJA rearrangements and leads to TCRJA coincidence.

Both TCRA alleles are rearranged in mature T lymphocytes, as a result of a lack of allelic exclusion at the TRCA locus. We show in a series of T cell clones that the two TCRJA segments are not randomly, but rather coincidentally, rearranged in a given T cell. The TCRJA coincidence relies, in part, on the presence of "T early alpha" (TEA), a cis-regulatory genetic element located upstream of the TCRJA cluster. TEA promotes specific recombinational accessibility that targets primary TCRVAJA rearrangements on the 5' side of the TCRA locus. In a model of multiple waves of TCRVAJA recombination, this cis-regulatory effect of TEA allows for the scanning of the entire TCRJA cluster, thereby increasing the TCR alpha/beta diversity potential.

RORgammaT, a thymus-specific isoform of the orphan nuclear receptor RORgamma / TOR, is up-regulated by signaling through the pre-T cell receptor and binds to the TEA promoter.

TEA (T early alpha) is a genetic element located upstream of the TCR-Jalpha cluster. Thymocytes from mice carrying a targeted deletion of TEA do not rearrange their TCRalpha locus on a window spanning the first nine Jalpha segments. This led us to the hypothesis of TEA having a "rearrangement focusing" activity on the 5' side of the TCR-Jalpha region. We analyzed DNAseI and "phylogenetic" footprints within the TEA promoter in an attempt to identify trans-acting factors that could account for its regulatory function on DNA accessibility. One of these footprints corresponded to a putative DNA-binding site for an orphan nuclear receptor of the ROR / RZR family. The RORgammaT cDNA clone was isolated from a thymus library using a probe corresponding to the DNA-binding domain of RORgamma / TOR. RORgammaT is a thymus-specific isoform of RORgamma, expressed almost exclusively in immature double-positive thymocytes. RORgammaT binds, to the TEA promoter in vitro. Lastly, the expression of RORgammaT is stimulated in two situations that mimic activation through the pre-TCR and in which the thymocytes have their TCR-alpha locus in an "open", yet unrearranged DNA configuration. We propose that the expression of RORgammaT may be part of the pre-TCR activation cascade leading to the maturation of alpha / beta T cells and may participate in the regulation of DNA accessibility in the TCR-Jalpha locus.