The lymphoid past of mouse plasmacytoid cells and thymic dendritic cells

There has been controversy over the possible lymphoid origin of certain dendritic cell (DC) subtypes. To resolve this issue, DC and plasmacytoid pre-DC isolated from normal mouse tissues were analyzed for transient (mRNA) and permanent (DNA rearrangement) markers of early stages of lymphoid development. About 27% of the DNA of CD8(+) DC from thymus, and 22-35% of the DNA of plasmacytoid pre-DC from spleen and thymus, was found to contain IgH gene D-J rearrangements, compared with 40% for T cells. However, the DC DNA did not contain IgH gene V-D-J rearrangements nor T cell Ag receptor beta gene D-J rearrangements. The same DC lineage populations containing IgH D-J rearrangements expressed mRNA for CD3 chains, and for pre-T alpha. In contrast, little of the DNA of the conventional DC derived from spleen, lymph nodes, or skin, whether CD8(+) or CD8(-), contained IgH D-J rearrangements and splenic conventional DC expressed very little CD3 epsilon or pre-T alpha mRNA. Therefore, many plasmacytoid pre-DC and thymic CD8(+) DC have shared early steps of development with the lymphoid lineages, and differ in origin from conventional peripheral DC.     

The myelopoietic inducing potential of mouse thymic stromal cells

The thymus has generally been considered as being solely involved in T cell maturation. In this study we have demonstrated that mouse thymic stroma can also support myelopoiesis. Bone marrow from mice treated with 5-fluorouracil was depleted of cells expressing Mac-1, CD4, and CD8 and incubated on lymphocyte-free monolayer cultures of adherent thymic stromal cells. After 7 days there was a marked increase in nonadherent cells, the majority of which were Mac-1+, FcR+, and HSA+. These proliferating bone marrow cells also expressed markers (MTS 17 and MTS 37) found on thymic stromal cells. Such cells were not found in thymic cultures alone, in bone marrow cultured alone, or on control adherent cell monolayers. Supernatants from the cultured thymic stroma, however, were able to induce these cell types in the bone marrow precursor population. Incubation of normal thymocytes with a monolayer of these in vitro cultivated Mac-1+, MTS 17+, MTS 37+ myeloid cells leads to selective phagocytosis of CD4+ CD8+ cells. Hence, this study demonstrates that the thymic adherent cells can induce myelopoiesis in bone marrow-derived precursor cells and provide a form of self-renewal for at least one population of thymic stromal cells. Furthermore, these induced cells are capable of selective phagocytosis of CD4+ CD8+ thymocytes and may provide one mechanism for the selective removal of such cells from the thymus.     

Response of lymphoid cells to thymic hormonal factors isolated from mouse thymus

Thymic hormonal factors were isolated from mouse thymus by two methods. (1) Thymic cytosols in phosphate buffer saline were filtered through Sephadex G100 with 0.1 M NH4HCO3 (pH 8.0) as buffer and the protein peaks were collected. (2) Protein having thymosin activity (F5) was isolated from thymic cytosols after heat inactivation, salt fractionation and desalting on Sephadex G25. Molecular weights of all the proteins were determined on SDS-PAGE. Biological activity of thymic proteins was studied by in vitro and in vivo assays, using synthetic thymosin alpha 1 as the standard. Thymocytes treated with different thymic proteins showed maximum stimulation at 16 h of incubation period. Preincubation of the thymocytes with the thymic proteins and subsequent incubation with Con-A decreased the stimulation index. Incubation of spleen lymphocytes with thymic proteins increased the percentage of Tdt+ cells. The antitumor effects of thymic proteins carried out on animals having leukemia, showed statistically significant results. Clinically however, the antitumor effects of the thymic proteins alone and in combination chemotherapy were negligible at 1 mg/kg body weight dose level.     

Detection of thymic surface antigens and radioactive labeling of mouse lymphoid cells.

A method for the permanent and simultaneous detection of tissue-specific surface antigens and internal radioactive labeling of mouse lymphoid cells is described. Target cells were first reacted with a mouse-derived "antithymocyte serum", incubated with peroxidase-conjugated rabbit serum against mouse immunoglobulins and placed in a substrate solution that leads to staining of the antigen-positive cells. Radioactive labeling was demonstrated by autoradiography performed after the antigen assay. More than 90% of antigen-positive thymocytes could be specifically stained in the assay without staining of similarly treated antigen-negative cells. Autoradiographic grains could be detected over both antigen-positive and antigen-negative cells.     

Measles virus infection of thymic epithelium in the SCID-hu mouse leads to thymocyte apoptosis.

Mortality from measles is caused mostly by secondary infections associated with the depression of cellular immunity. The mechanism of immune suppression and the role of virus strain differences on the immune system are incompletely understood. SCID-hu mice were used to determine the effects of virulent, wild-type (Chicago-1) and avirulent, vaccine (Moraten) strains of measles virus (MV) on the human thymus in vivo. Chicago-1 replicated rapidly, with a 100-fold decrease in numbers of thymocytes, whereas Moraten replicated slowly, without significant thymocyte death. Productive MV infection occurred not in thymocytes but in thymic epithelial and myelomonocytic cells. Wild-type MV infection of thymic stromata leads to induction of thymocyte apoptosis and may contribute to a long-term alteration of immune responses. The extent of thymic disruption reflects the virulence of the virus, and therefore the SCID-hu mouse may serve as the first small animal model for the study of MV pathogenesis.     

Degradation of DNA and chromatin in cells of mouse lymphoid organs after irradiation and PHA administration

Phytohemagglutinin (PHA) administered to irradiated mice did not influence the postirradiation degradation of DNA and the yield of polydeoxynucleotides (PDN) in cells of thymus, spleen and bone marrow. The degree of degradation of DNA and chromatin was higher in the thymus as compared to other studied organs. A possible mechanism of the radiotherapeutic action of PHA is discussed.     

Consequences of interactions between thymic lymphoid and epithelial cells in vitro

A 24-hour co-cultivation of thymocytes and epithelial cells taken from human thymus results in mutual activation of epitheliocytes and thymocytes, as well as in apoptosis of thymocytes. The apoptosis can also be induced by a cultural supernatant of the thymic-epithelial cells, its level being lower, however, than in the co-culture. Thymocyte death and elimination develop faster in a co-culture with allogeneic thymic epithelial cells.     

Differential expression and regulation of cyclooxygenase isozymes in thymic stromal cells

Prostaglandins (PGs) are lipid-derived mediators of rapid and localized cellular responses. Given the role of PG in supporting thymic T cell development, we investigated the expression of the PG synthases, also known as cyclooxygenases (COX)-1 and -2, in the biosynthesis of PGs in thymic stromal cell lines. The predominant isozyme expressed in cortical thymic epithelial cells was COX-1, while COX-2 predominated in the medulla. IFN-gamma up-regulated expression and activity of COX-2 in medullary cells, in which COX-2 was expressed constitutively. In contrast, IFN-gamma down-regulated COX-1 activity, but not expression, in cortical cells. Stromal cells support T cell development in the thymus, although the mediators of this effect are unknown. Selective inhibition of COX-2, but not COX-1, blocked the adhesion of CD4+CD8+ and CD4+CD8- thymocytes to medullary cell lines. No effect of the inhibitors was observed on the interactions of thymocytes with cortical epithelial lines. These data further support the differential regulation of COX-1 and COX-2 expression and function in thymic stromal cells. PGs produced by COX-2 in the medullary thymic stroma may regulate the development of thymocytes by modulating their interaction with stromal cells.     

Electric field-mediated DNA transfer: transient and stable gene expression in human and mouse lymphoid cells.

The technique of DNA transfer by electroporation was investigated in an effort to evaluate its utility for the identification of developmentally controlled regulatory sequences. Transient and stable gene expression was detected in a variety of lymphoid cell lines subjected to electroporation. No correlation existed between the levels of chloramphenicol acetyltransferase (acetyl-CoA; chloramphenicol 3-O-acetyltransferase, EC 2.3.1.28) expression and stable transfection frequency. In all lymphoid cell lines tested, the simian virus 40 early region was a better promoter than was the Rous sarcoma virus long terminal repeat.     

A T lymphoid cell line responds to a thymic stromal cell line by expression of Thy-1 and CD4

We have cloned both T lymphoid and stromal lines from a single murine thymic tumor that was induced by a retrovirus carrying the v-myc oncogene (M-MuLV(myc]. The T lymphoid line, L4, was cloned by growth in agar. L4 cells were initially negative for Thy-1.2 and CD4 (although they contained rearranged TCR-beta genes), and they remained so if passaged in medium alone. However, cocultivation of these Thy-1.2- CD4- cells with the cloned stromal cell line, St3, resulted in sequential expression of Thy-1.2 and CD4 in subpopulations of cells. Thy-1.2+ CD4- and Thy-1.2+ CD4+ L4 subclones were obtained from the cocultures by subsequent cloning in agar. Derivation of these subclones from the starting Thy-1.2- CD4- clone was verified by Southern blot analyses specific for TCR-beta gene rearrangements and for M-MuLV(myc) proviral integration sites. Continuous cocultivation of Thy-1.2+ CD4+ L4 subclones with the St3 stromal cells was necessary for maintenance of CD4 on the cell surface. Furthermore, CD4 expression which was lost when CD4+ L4 cells were removed from the stroma could be reinduced if they were again cultured on St3 stroma. These cells may provide a model system for studying thymocyte-stromal cell interactions in induction and maintenance of expression of Thy-1 and CD4 molecules.     

Transgenic upregulation of IK1 in the mouse heart leads to multiple abnormalities of cardiac excitability

To assess the functional significance of upregulation of the cardiac current (IK1), we have produced and characterized the first transgenic (TG) mouse model of IK1 upregulation. To increase IK1 density, a pore-forming subunit of the Kir2.1 (green fluorescent protein-tagged) channel was expressed in the heart under control of the alpha-myosin heavy chain promoter. Two lines of TG animals were established with a high level of TG expression in all major parts of the heart: line 1 mice were characterized by 14% heart hypertrophy and a normal life span; line 2 mice displayed an increased mortality rate, and in mice < or =1 mo old, heart weight-to-body weight ratio was increased by >100%. In adult ventricular myocytes expressing the Kir2.1-GFP subunit, IK1 conductance at the reversal potential was increased approximately 9- and approximately 10-fold in lines 1 and 2, respectively. Expression of the Kir2.1 transgene in line 2 ventricular myocytes was heterogeneous when assayed by single-cell analysis of GFP fluorescence. Surface ECG recordings in line 2 mice revealed numerous abnormalities of excitability, including slowed heart rate, premature ventricular contractions, atrioventricular block, and atrial fibrillation. Line 1 mice displayed a less severe phenotype. In both TG lines, action potential duration at 90% repolarization and monophasic action potential at 75-90% repolarization were significantly reduced, leading to neuronlike action potentials, and the slow phase of the T wave was abolished, leading to a short Q-T interval. This study provides a new TG model of IK1 upregulation, confirms the significant role of IK1 in cardiac excitability, and is consistent with adverse effects of IK1 upregulation on cardiac electrical activity.     

Loss of medullary dendritic cells in the thymus after cyclosporine and irradiation

Cyclosporine (CsA) induces a paradoxical graft-vs-host-like disease (GVHD) in syngeneic rat chimeras, providing the rat has a thymus and receives mediastinal irradiation. Here we evaluate the effect of CsA and irradiation on the relative abundance of thymic dendritic cells (DC). DC were identified with an immunoperoxidase stain for ED1 and were quantified by computerized planimetry. Normal young rats have scattered DC in the cortex and numerous DC in the medulla with a concentration at the cortico-medullary junction. Short-term CsA induces a marked loss of medullary DC (798 +/- 126 cells/mm to 88 +/- 103, P less than 0.001) and a modest loss of cortical DC (543 +/- 55 to 330 +/- 130, P less than 0.01). While medullary DC normally recover promptly post-CsA, rats receiving mediastinal irradiation demonstrated minimal recovery post-CsA (P less than 0.0003). The prolonged deficiency of medullary DC could represent an essential early step for loss of self-tolerance and sGVHD.     

Treatment of mice with 2,3,7,8-tetrachlorodibenzo-p-dioxin leads to aryl hydrocarbon receptor-dependent nuclear translocation of NF-kappaB and expression of Fas ligand in thymic stromal cells and consequent apoptosis in T cells

We investigated the role of aryl hydrocarbon receptor (AhR) in the regulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced apoptosis in thymic T cells. AhR knockout (KO) mice were resistant to TCDD-induced thymic atrophy and apoptosis when compared with the AhR wild-type mice. TCDD triggered the expression of several apoptotic genes, including FasL in AhR wild-type but not AhRKO mice. TCDD-induced increase in FasL was seen only in thymic stromal but not thymic T cells. When TCDD-exposed stromal cells were mixed with untreated thymic T cells, increased apoptosis was detected in T cells that involved Fas-FasL interactions. Thus, apoptosis in T cells was not detected when TCDD-treated stromal cells from FasL-defective or AhRKO mice were mixed with wild-type T cells or when TCDD-exposed wild-type stromal cells were mixed with Fas-deficient T cells. TCDD treatment, in vivo and in vitro, led to colocalization and translocation of NF-kappaB subunits (p50, p65) to the nucleus in stromal but not T cells from AhR wild-type mice. NF-kappaB activation was not observed in stromal cells isolated from TCDD-treated AhRKO mice. Mutations in NF-kappaB-binding sites on the FasL promoter showed that TCDD regulates FasL promoter activity through NF-kappaB. TCDD treatment in vivo caused activation of the death receptor and mitochondrial pathways of apoptosis. Cross-talk between the two pathways was not necessary for apoptosis inasmuch as TCDD-treated Bid KO mice showed thymic atrophy and increased apoptosis, similar to the wild-type mice. These findings demonstrate that AhR regulates FasL and NF-kappaB in stromal cells, which in turn plays a critical role in initiating apoptosis in thymic T cells.     

IL-12 administration leads to a transient depletion of T cells,B cells,and APCs and concomitant abrogation of the HLA-A2.1-restricted CTL response in transgenic mice

The injection of a mixture of bona fide T cell epitopes can lead to the occurrence of immunodominance, meaning that the immune response is focused on the recognition of a single epitope or a small portion of the epitopes injected. We have previously demonstrated that the administration of rIL-12 can counteract immunodominance in BALB/c mice. In this study, we show that the administration of rIL-12 to HLA-A2.1 transgenic mice (A2k(b) mice) abrogates specifically the immune response against HLA-A2.1-restricted HIV epitopes in the spleen. This lack of immune response is most probably due to a transient depletion of B cells, T cells, macrophages, and dendritic cells in this organ. Therefore, our study explains the mechanism of immunosuppression by rIL-12 in vivo.