Early expression of endomucin on endothelium of the mouse embryo and on putative hematopoietic clusters in the dorsal aorta

Endomucin is a recently identified sialomucin that is specifically expressed on endothelium of the adult mouse. Here, we have analysed the expression of endomucin during development of the vascular system by immunohistochemistry by using three monoclonal antibodies (mAb). We demonstrate that two of the mAb, V.5C7 and V.1A7, recognize epitopes on the nonglycosylated protein, because they recognize the antigen when it is synthesized as a bacterial fusion protein and when it is in vitro translated in a membrane-free reticulocyte lysate. During in vitro differentiation of embryonic stem cells to endothelial cells, endomucin is expressed at day 6 after onset of differentiation, 1 day later than PECAM-1. During differentiation of the mouse embryo, endomucin is first detected at E8.0 in all embryonic blood vessels detectable at this stage but is absent in blood islands of the yolk sac. Analysing the paraaortic-splanchnopleura (P-SP) region and the aorta-gonad-mesonephros (AGM) region as sites of intraembryonic hematopoiesis, we found that endothelium of the dorsal aorta is brightly positive for endomucin at E8.5-9.0 and at E11.5. At later stages and in the adult aorta, endothelial staining is strongly reduced and confined to focal areas. Cell clusters associated with the luminal surface of the endothelium of the dorsal aorta could be stained for endomucin and for CD34. At a later stage (E15.5) single leukocytes in the lumen of large venules were stained for endomucin. We conclude that endomucin is an early endothelial-specific antigen that is also expressed on putative hematopoietic progenitor cells.

Fetal and adult hematopoietic stem cells require beta1 integrin function for colonizing fetal liver, spleen, and bone marrow

Homing of hematopoietic stem cells (HSCs) into hematopoietic organs is a prerequisite for the establishment of hematopoiesis during embryogenesis and after bone marrow transplantation. We show that beta1 integrin-deficient HSCs from the para-aortic splanchnopleura and the fetal blood had hematolymphoid differentiation potential in vitro and in fetal organ cultures but were unable to seed fetal and adult hematopoietic tissues. Adult beta1 integrin null HSCs isolated from mice carrying loxP-tagged beta1 integrin alleles and ablated for beta1 integrin expression by retroviral cre transduction failed to engraft irradiated recipient mice. Moreover, absence of beta1 integrin resulted in sequestration of HSCs in the circulation and their reduced adhesion to endothelioma cells. These findings define beta1 integrin as an essential adhesion receptor for the homing of HSCs.

Regulation of T cell receptor (TCR) beta gene expression by CD3 complex signaling in immature thymocytes: implications for TCRbeta allelic exclusion

During alphabeta thymocyte development, clonotype-independent CD3 complexes are expressed at the cell surface before the pre-T cell receptor (TCR). Signaling through clonotype-independent CD3 complexes is required for expression of rearranged TCRbeta genes. On expression of a TCRbeta polypeptide chain, the pre-TCR is assembled, and TCRbeta locus allelic exclusion is established. We investigated the putative contribution of clonotype-independent CD3 complex signaling to TCRbeta locus allelic exclusion in mice single-deficient or double-deficient for CD3zeta/eta and/or p56(lck). These mice display defects in the expression of endogenous TCRbeta genes in immature thymocytes, proportional to the severity of CD3 complex malfunction. Exclusion of endogenous TCRbeta VDJ (variable, diversity, joining) rearrangements by a functional TCRbeta transgene was severely compromised in the single-deficient and double-deficient mutant mice. In contrast to wild-type mice, most of the CD25(+) double-negative (DN) thymocytes of the mutant mice failed to express the TCRbeta transgene, suggesting defective expression of the TCRbeta transgene similar to endogenous TCRbeta genes. In the mutant mice, a proportion of CD25(+) DN thymocytes that failed to express the transgene expressed endogenous TCRbeta polypeptide chains. Many double-positive cells of the mutant mice coexpressed endogenous and transgenic TCRbeta chains or more than one endogenous TCRbeta chain. The data suggest that signaling through clonotype-independent CD3 complexes may contribute to allelic exclusion of the TCRbeta locus by inducing the expression of rearranged TCRbeta genes in CD25(+) DN thymocytes.

Requirement of CD3 complex-associated signaling functions for expression of rearranged T cell receptor beta VDJ genes in early thymic development

During alpha beta thymocyte development, the clonotypic alpha beta-T cell receptor (TCR) is preceded by sequentially expressed immature versions of the TCR-CD3 complex: the pre-TCR, containing a clonotypic TCR-beta chain and invariant pre-Talpha, is expressed on pre-T cells before rearrangement of the TCR-alpha locus. Moreover, clonotype-independent CD3 complexes (CIC) appear on pro-T cells before VDJ rearrangements of TCR-beta genes. The pre-TCR is known to mediate TCR-beta selection, the prerequisite for maturation of CD4(-)8(-) double negative (DN) thymocytes to the CD4(+)8(+) double positive stage. A developmental function of CIC has so far not been delineated. In mice single deficient and double deficient for CD3zeta/eta and/or p56(lck), we observe a pronounced reduction in the proportions of CD25(+) DN thymocytes that express intracellular TCR-beta chains. TCR-beta transcripts are reduced in parallel with TCR-beta polypeptide chains whereas no reduction in TCR-beta locus rearrangements could be detected. Wild-type levels of TCR-beta transcripts and of cells expressing TCR-beta polypeptide chains are induced by treatment with anti-CD3epsilon mAb. The data suggest that the initial expression of rearranged TCR-beta VDJ genes in pro-T cell to pre-T cell progression is dependent on CD3 complex signaling, and thus define a putative developmental function for CIC.

Hemato-lymphoid in vivo reconstitution potential of subpopulations derived from in vitro differentiated embryonic stem cells

During differentiation in vitro, embryonic stem (ES) cells generate progenitors for most hemato-lymphoid lineages. We studied the developmental potential of two ES cell subpopulations that share the fetal stem cell antigen AA4.1 but differ in expression of the lymphoid marker B220 (CD45R). Upon transfer into lymphoid deficient mice, the B220+ population generated a single transient wave of IgM+ IgD+ B cells but failed to generate T cells. In contrast, transfer of the B220- fraction achieved long-term repopulation of both T and B lymphoid compartments and restored humoral and cell-mediated immune reactions in the recipients. To assess the hemato-lymphopoietic potential of ES cell subsets in comparison to their physiological counterparts, cotransplantation experiments with phenotypically homologous subsets of fetal liver cells were performed, revealing a more potent developmental capacity of the latter. The results suggest that multipotential and lineage-committed lymphoid precursors are generated during in vitro differentiation of ES cells and that both subsets can undergo complete final maturation in vivo.

Reconstitution of B cell subsets in Rag deficient mice by transplantation of in vitro differentiated embryonic stem cells

In vitro differentiated embryonic stem (ES) cells contain a population which is similar to fetal liver pro/pre-B cells on the basis of cell surface antigens and cytoplasmic expression of immunoglobin heavy chain. This population was purified and transplanted into Rag-1 deficient recipients to characterize its developmental potential in vivo. Following intravenous transfer, these cells rapidly reconstituted the splenic B but not the T cell compartment. Reconstitution was transient, indicating the lack of long-term reconstituting capacity. Similar to fetal liver, B-1 type as well as conventional B cells were generated, accompanied by high serum IgM levels. Intraperitoneal injection generated high numbers of peritoneal B cells, predominately of the B-1a phenotype, with poor splenic repopulation and low serum IgM levels. These observations suggest the emergence of two different B lineage precursor populations during in vitro ES cell differentiation and define a possible role of the microenvironment in directing lymphoid development.

Impaired migration but not differentiation of haematopoietic stem cells in the absence of beta1 integrins

Adhesive interactions mediated by integrins of the beta1 subfamily are thought to be critical in controlling differentiation and migration of blood cell precursors. Here we report that chimaeric mice generated with beta1-integrin-deficient embryonic stem (ES) cells lack beta1(-/-) cells in blood and in haematopoietic organs such as spleen, thymus and bone marrow. Chimaeric embryos contain beta1-null haematopoietic cells in the yolk sac and in fetal blood but not in fetal liver. We show that such beta1(-/-) haematopoietic stem cells derived from yolk sac of 10.5-day-old chimaeric embryos readily generate erythroid and myeloid colonies and that beta1(-/-) ES cells can differentiate into mature B lymphocytes in vitro. Our results indicate that haematopoietic stem cells lacking beta1 integrins can form and differentiate into different lineages but cannot colonize the fetal liver.

Immature T cells in peripheral lymphoid organs of recombinase-activating gene-1/-2-deficient mice. Thymus dependence and responsiveness to anti-CD3 epsilon antibody

Thymocytes of mice deficient in the recombinase-activating gene (RAG)-1 or RAG-2 cannot express and receive signals through the pre-TCR. As a result, thymocyte development in these mice terminates at the CD4/8 double negative (DN), IL-2R-alpha-positive stage. Nevertheless, RAG-deficient DN thymocytes express functional CD3 complexes and can therefore be induced by anti-CD3 epsilon mAb to mature to the CD4+8+ double positive stage. In the present paper we demonstrate that the peripheral lymphoid organs (lymph nodes, spleen) and peripheral blood of RAG-deficient mice harbor an immature T cell population which, similar to RAG-deficient DN thymocytes, contains high levels of cytoplasmic CD3 epsilon and responds to anti-CD3 epsilon mAb in vivo. With respect to surface phenotype (Thy1.2+, PgP-1+, HSA+, Fc gamma RII/III-, IL-2R-alpha-, c-kit-), these cells are similar to intermediate stage RAG-deficient DN thymocytes. Moreover, they express mRNA for pre-TCR-alpha and for the nondeleted RAG. Following injection of anti-CD3 epsilon mAb, these cells proliferate, down-regulate heat stable Ag and PgP-1, and partially differentiate to CD4+ and CD8+ double positive and single positive cells. The induced population displays a mixed phenotype, between that of immature thymocytes and lymph node T cells in normal mice. Induction is successful in thymectomized RAG-deficient mice, suggesting that it occurs in the periphery. However, after thymectomy, inducible cells disappear with an approximate half-life of 10 to 14 days. We suggest that DN thymocytes can emigrate and repopulate peripheral lymphoid organs of RAG-deficient mice. These cells respond to CD3 signaling by aberrant maturation, possibly due to the inappropriate microenvironment of peripheral lymphoid organs.

Immature T cells in peripheral lymphoid organs of recombinase-activating gene-1/-2-deficient mice. Thymus dependence and responsiveness to anti-CD3 epsilon antibody

Thymocytes of mice deficient in the recombinase-activating gene (RAG)-1 or RAG-2 cannot express and receive signals through the pre-TCR. As a result, thymocyte development in these mice terminates at the CD4/8 double negative (DN), IL-2R-alpha-positive stage. Nevertheless, RAG-deficient DN thymocytes express functional CD3 complexes and can therefore be induced by anti-CD3 epsilon mAb to mature to the CD4+8+ double positive stage. In the present paper we demonstrate that the peripheral lymphoid organs (lymph nodes, spleen) and peripheral blood of RAG-deficient mice harbor an immature T cell population which, similar to RAG-deficient DN thymocytes, contains high levels of cytoplasmic CD3 epsilon and responds to anti-CD3 epsilon mAb in vivo. With respect to surface phenotype (Thy1.2+, PgP-1+, HSA+, Fc gamma RII/III-, IL-2R-alpha-, c-kit-), these cells are similar to intermediate stage RAG-deficient DN thymocytes. Moreover, they express mRNA for pre-TCR-alpha and for the nondeleted RAG. Following injection of anti-CD3 epsilon mAb, these cells proliferate, down-regulate heat stable Ag and PgP-1, and partially differentiate to CD4+ and CD8+ double positive and single positive cells. The induced population displays a mixed phenotype, between that of immature thymocytes and lymph node T cells in normal mice. Induction is successful in thymectomized RAG-deficient mice, suggesting that it occurs in the periphery. However, after thymectomy, inducible cells disappear with an approximate half-life of 10 to 14 days. We suggest that DN thymocytes can emigrate and repopulate peripheral lymphoid organs of RAG-deficient mice. These cells respond to CD3 signaling by aberrant maturation, possibly due to the inappropriate microenvironment of peripheral lymphoid organs.

A novel mouse thymocyte antigen (F3Ag): down-regulation during the CD4+CD8+ double-positive stage indicates positive selection

We describe a novel mAb (F3) which reacts with a 65 kDa thymocyte surface protein, expressed on approximately 80% of thymocytes, referred to as F3Ag. In ontogeny, F3Ag expression begins in the CD4(-)CD8(-) double-negative (DN) CD25(+) population and is maintained through approximately 85% of the CD4(+)CD8(+) double-positive (DP) stage. DP cells with high TCR expression and CD4(+) single-positive (SP) cells are predominantly negative for F3Ag, whereas many CD8(+) SP thymocytes express F3Ag. F3Ag-DP thymocytes show a reduced expression of RAG-1 and RAG-2 compared with F3Ag+ DP cells. The shutdown of F3Ag expression during the DP stage is related to positive selection: mice deficient for MHC class I and class II molecules maintain F3Ag expression in almost all DP cells. Transgenic (tg) mice carrying TCR restricted for MHC class II show a more pronounced down-regulation of F3Ag in the DP compartment than normal mice, depending on the presence of a positively selecting MHC. The size of the F3Ag- DP subset is positively correlated with the efficacy of positive selection into the CD4(+) SP compartment. Because some CD8(+) SP cells express F3Ag, the relationship between F3Ag down-regulation and positive selection is less obvious in DP cells of mice carrying MHC class I-restricted tg TCR. However, in reaggregate thymic organ cultures, sorted F3Ag- DP cells differentiate into CD8(+) SP cells more rapidly than do F3Ag+ DP cells. Thus, after down-regulation in the DP stage, a proportion of CD8(+) SP cells appears to re-express F3Ag. In addition, the proportion of F3Ag-CD8(+) SP cells depends on the efficacy of positive selection into the CD8 lineage. Taken together, the regulation of the expression of the F3Ag appears to be associated with signals that control thymic repertoire selection.

In vitro generation of lymphoid precursors from embryonic stem cells

Murine embryonic stem (ES) cells represent a model system for studying certain aspects of hemopoiesis because they can differentiate in vitro into several cell types, including those of the hemopoietic system. We developed cell culture conditions in which ES cells undergo hemopoietic differentiation in a low-oxygen (5% O2) atmosphere without additional exogenous factors. After 15-20 days of culture under these conditions, cells bearing surface markers found on cells of the lymphoid lineage (Thy1+, Pgp-1+, c-kit+ and B-220+) were detected. After 13-15 days, transcripts for the recombinase activating genes (RAG) 1 and 2, interleukin (IL) 7, IL-7 receptor and c-kit were expressed. We also investigated rearrangements of the immunoglobulin (Ig) heavy and light chain and the T cell receptor (TCR) loci. After 15 days of differentiation, we detected DJH gene rearrangement with N-region diversity. Productive VHDJH rearrangements are found after 20 days, paralleled by V Kappa J Kappa recombinations indicating a developmental stage comparable, at least, with that of pre B cells. Rearrangements of TCR gamma as well as delta chain segments were also observed, but no TCR beta chain rearrangement. These results demonstrate that ES cells reproducibly generate lymphoid cells in vitro.

Oligoclonal T cells in rheumatoid arthritis: identification strategy and molecular characterization of a clonal T-cell receptor

Immunodominant antigens in rheumatoid arthritis (RA) should induce an expansion of T cells bearing a corresponding T-cell receptor (TCR). We therefore analysed the TCR repertoire at the site of inflammation using two fundamentally different strategies. The total TCR repertoire was examined by generating 'representative' T-cell clone panels, which were subsequently tested for clonality by restriction mapping of the TCR beta gene locus. No clonality was detected in large T-cell clone panels generated with cells from three patients. However, when we selectively analysed the TCR repertoire of in vivo pre-activated, interleukin-2 (IL-2)-responsive T cells, significant T-cell/TCR clonality was found in 2 out of 4 patients. The clonal T cells represented a minority of the total T-cell population with an estimated frequency of 1 in 300 to 1 in 1000 cells. Molecular characterization of a clonal TCR and the use of a specific TCR V beta MoAb ruled out an over-representation of T cells bearing the same V beta element in the total T-cell population, rendering the involvement of super-antigens in the induction of T-cell clonality in this case unlikely.

Expression of the CD2 activation epitope T11-3 (CD2R) on T cells in rheumatoid arthritis, juvenile rheumatoid arthritis, systemic lupus erythematosus, ankylosing spondylitis, and Lyme disease: phenotypic and functional analysis

CD2R is an activation-associated epitope unmasked by a conformational change of the CD2 cell-surface glycoprotein. In spite of elaborate studies on the role of CD2 and CD2R in adhesion and stimulation of T cells in vitro, no instances of CD2R expression in vivo were known to date. We report high levels of CD2R observed on blood and synovial fluid T cells in rheumatoid arthritis and on peripheral blood T cells in juvenile rheumatoid arthritis, systemic lupus erythematosus, ankylosing spondylitis, and Lyme disease. In vivo, expression of CD2R was restricted to T cells, not limited to a particular T-cell subset and not correlated with the expression of p55 interleukin 2R (IL-2R) (CD25) or major histocompatibility complex (MHC) class II molecules. When stimulated to proliferation via CD2 or CD3, ex vivo CD2R+ T cells showed the same basic activation requirements as CD2R-T cells.

Expression of activation antigens on T cells in rheumatoid arthritis patients

The aim of our study was to identify differences in cell surface marker expression between T cells taken from the peripheral blood (PB) of healthy individuals and T cells recovered from inflamed joints of rheumatoid arthritis (RA) patients. Out of 118 monoclonal antibodies (MoAbs) directed against activation antigens on haematopoietic cells, 12 MoAbs recognizing nine distinct surface molecules were selected after a screening procedure to study the expression of the corresponding antigens on T cells from the PB, synovial fluid and synovial tissue of RA patients, and also on T cells from PB and spleens of controls. Using two-colour flow cytometry and immunohistology we found the molecules B-C5, CD39, CD40, CD45 R0, CD54, CD76 and potentially 1D11 to be substantially up-regulated on T cells from various body compartments in RA patients. We thus could determine that the cell surface of T cells in RA patients not only differs in MHC class II expression, but also in a number of other activation-associated cell surface molecules from T cells in healthy individuals.