Thymic stem cells: their interaction with the thymic stroma and tolerance induction

Functional demonstration of intrathymic binding sites and microvascular gates for prothymocytes in irradiated mice

Quantitative intrathymic (i.t.) and i.v. adoptive transfer assays for prothymocytes show strict log dose saturation kinetics, consistent with a finite number of i.t. binding sites (microenvironmental niches). This inference is supported here by demonstration of competitive antagonism obeying one-on-one receptor occupancy kinetics during the establishment of thymic chimerism in irradiated adult mice. The results of primary and secondary transfer experiments suggested that hematogenous precursors (i) enter specific i.t. niches between 4 and 24 h after injection, (ii) compete reversibly with subsequently introduced precursors, (iii) establish insurmountable competition within 5-7 days, (iv) mature through the initial stages of thymocytopoiesis preceding proliferative expansion, and (v) vacate the niches between 7 and 14 days after entry. The results also suggested that, as in non-irradiated mice, prothymocyte importation in irradiated mice is a gated phenomenon. Gate closure was indicated by the inability of i.v.-, but not i.t.-, injected bone marrow (BM) cells to induce thymic chimerism when administered 7--14 days after a primary injection and gate opening by the ability of i.v.-injected BM cells to induce thymic chimerism in competition with circulating host prothymocytes. Gate closing was log dose-responsive and could be induced in individual thymic lobes by unilateral i.t. injection, whereas gate opening, which occurs bilaterally, was not initiated until most of the niches for prothymocytes had been vacated. We therefore posit the existence of a series of associated microvascular gates and microenvironmental niches that act in concert to regulate prothymocyte importation and early thymocyte differentiation.

Thymic reticulum of mice. III. The connective compartment (innervation, vascularisation, fibrous tissues and myoid cells)

T lymphocytes interact at various levels of differentiation, with cells of the thymic reticulum, forming a peculiar and complex microenvironment. Following earlier descriptions by electron microscopy of three types of epithelial cells and two types of non-epithelial cells (macrophages and interdigitated cells) forming the thymic microenvironment, we report a study on a third compartment, the connective tissue, whose elements occur throughout the organ. The components of the capsule and trabeculae, the vascularisation and the innervation of the thymus and the presence of a few myoid cells are described. This is very rarely studied in ultrastructure. All these cells are completely imbricated and form a network trapping the lymphocytes, playing an essential role in the differentiation, maturation and selection of T cells.

Phenotypically diverse mouse thymic stromal cell lines which induce proliferation and differentiation of hematopoietic cells

The heterogeneity of the thymic stroma has made careful characterization of particular thymic stromal cell types difficult. To this end, we have derived a panel of cloned thymic stromal cell lines from simian virus 40 T antigen (SV40-T antigen) transgenic mice. Based on their analysis with monoclonal antibodies that distinguish among subsets of thymic stroma cells, and on the morphology and ultrastructural features of the different clones, we suggest that our panel includes representatives of the thymic subcapsular cortex or thymic nurse cells (427.1), the deep cortex or cortical reticular cells (1308.1) and the medulla including medullary interdigitating (IDC)-like cells (6.1.1) and medullary epithelial cells (6.1.7). A fifth cell type of undesignated but apparent medullary origin (6.1.11) was also isolated. All of the cell lines constitutively express the SV40 T antigen transgene and the class I antigens of the major histocompatibility complex (MHC), and they can be induced to express MHC class II antigens upon stimulation with recombinant interferon-gamma (IFN-gamma). These cell lines elaborate a factor(s) that induces the proliferation of cells from the fetal liver and bone marrow, but not from the neonatal thymus. A factor(s) elaborated by the 1308.1 cell line also induces the proliferation of fetal thymocytes in the absence of mitogens, phorbol esters or calcium ionophore which is augmented with the addition of recombinant interleukin-2 (IL-2). Analysis by reverse transcription polymerase chain reaction with primers for some mouse cytokines reveals that each of these cell lines contain granulocyte-macrophage colony-stimulating factor (GM-CSF) transcripts and that 1308.1, 6.1.1 and 6.1.7 produce IL-6 mRNA. Cell lines 1308.1 and 6.1.1 also produce IL-7; 6.1.1 produces IL-1 beta and tumor necrosis factor (TNF)-alpha while the 427.1 cell line produces IL-5 and IFN-gamma mRNA. None of the cell lines tested express the IL-2 receptor, IL-2, IL-3, IL-4, TNF-beta or macrophage inflammatory proteins mRNA. Conditioned medium (CM) from 1308.1 and 6.1.11 induced differentiation of cells purified from the mouse fetal liver into granulocytes; 1308.1 CM also induced differentiation of the mouse hematopoietic stem cell line 32DCl3(G) suggesting that the CM contains granulocyte (G)-CSF activity. Each cell line produces GM-CSF but the greatest activity is associated with 1308.1 and 6.1.11 CM. The availability of these well-characterized, functional, cloned thymic stromal cells will allow a more detailed analysis of the role of each cell type in both myeloid and T cell development.

Ontogeny of T cell development in avian scleroderma

UCD line 200 chickens develop an inherited fibrotic disease associated with the production of antinuclear antibodies, antibodies to type II collagen, and early skin lesions characterized by intense T lymphocyte infiltrates. In the present study we have investigated the hypothesis that developmental abnormalities in T lymphocyte differentiation predispose the line 200 chickens to autoimmune disease. The status of the thymic microenvironment in these birds during ontogeny was studied with an extensive panel of monoclonal antibodies (mAbs) reactive with distinct stromal cell subsets including MHC determinants. In addition, their T-cell graft-versus-host reactivity and responses to mitogenic stimulation and interleukin (IL)-2 were also analyzed. Line 200 chickens have profound defects in thymic structure with a virtual complete absence of antigens specific for type I epithelium which lines the thymic subcapsular and perivascular regions. There are excessive levels of MHC class II positive cells, particularly in the cortex, and B cells/subset macrophages identified by mAb MUI 36. These defects are found from the late embryonic period, long before clinical disease is manifest. Furthermore, by FACS analysis, line 200 thymocytes have a major increase in IL-2 receptor density. In addition, line 200 chicken peripheral blood lymphocytes (PBL) respond poorly to mitogenic agents and have a reduced response to IL-2. Finally, it is important to note that line 200 PBL produce a normal graft-versus-host reaction. We propose that these abnormalities in T-cell differentiation are selective, not global, and may be reflective of a defect in thymic education resulting in an inappropriate response to self-antigens.

Intrathymic nurse cell lymphocytes can induce a specific graft-versus-host reaction

Single chicken thymic nurse cells (TNC) placed onto the chorionallantoic membrane (CAM), showed that intra-TNC lymphocytes (TNC-L) possess a strong graft-versus-host reactivity (GVHR) in allogeneic MHC combinations. This reaction shows the morphological, phenotypic, and functional characteristics of a classical GVH reaction (GVHR). The induction of a GVHR was significantly higher for TNC-L as compared with thymocytes or peripheral blood lymphocytes (PBL). The specificity of the GVHR was shown by serial transfer experiments onto appropriate allogeneic and syngeneic secondary embryonic hosts. In immunofluorescence analyses with monoclonal antibodies (mAb) to the chicken alpha/beta and gamma/delta T cell receptors (TCR) and the CD3, CD4, and CD8 equivalents, an enrichment of CD3+/CD4+/CD8- and CD3+/CD-4-/CD8+, TCR-alpha/beta + and TCR- gamma/delta + cells was observed inside TNC as compared with extra-TNC thymocytes. A large proportion of CD4+ and/or CD8+ TCR- gamma/delta + cells were demonstrated inside TNC. A minor population among TCR- gamma/delta extra-TNC thymocytes also expressed CD4 and/or CD8 molecules. Based on functional tests and double staining experiments, we propose that CD4+/CD8+ thymocytes enter the TNC where they may undergo positive selection for MHC restriction and further differentiation to CD4 or CD8 single-positive cells. Taken together these data support the concept that TNC contribute a specialized thymic microenvironment for T cell differentiation and maturation.