Cell-surface trafficking and release of flt3 ligand from T lymphocytes is induced by common cytokine receptor gamma-chain signaling and inhibited by cyclosporin A.

The flt3 ligand (FL) is a growth and differentiation factor for primitive hematopoietic precursors, dendritic cells, and natural killer cells. Human T lymphocytes express FL constitutively, but the cytokine is retained intracellularly within the Golgi complex. FL is mobilized from the cytoplasmic stores and its serum levels are massively increased during the period of bone marrow aplasia after stem cell transplantation (SCT). Signals that trigger the release of FL by T cells remain unknown. This study shows that interleukin (IL)-2, IL-4, IL-7, and IL-15, acting through a common receptor gamma chain (gammac), but not cytokines interacting with other receptor families, are efficient inducers of cell surface expression of membrane-bound FL (mFL) and secretion of soluble FL (sFL) by human peripheral blood T lymphocytes. The gammac-mediated signaling up-regulated FL in a T-cell receptor-independent manner. IL-2 and IL-7 stimulated both FL messenger RNA (mRNA) expression and translocation of FL protein to the cell surface. Cyclosporin A (CsA) inhibited gammac-mediated trafficking of FL at the level of transition from the Golgi to the trans-Golgi network. Accordingly, serum levels of sFL and expression of mFL by T cells of CsA-treated recipients of stem cell allografts were reduced approximately 2-fold (P <.01) compared to patients receiving autologous grafts. The conclusion is that FL expression is controlled by gammac receptor signaling and that CsA interferes with FL release by T cells. The link between gammac-dependent T-cell activation and FL expression might be important for T-cell effector functions in graft acceptance and antitumor immunity after SCT.

Chronic overexpression of membrane-bound flt3 ligand by T lymphocytes in severe aplastic anaemia.

Aplastic anaemia (AA) is an immune-mediated bone marrow failure associated with high serum levels of flt3 ligand (FL). We examined expression of the membrane-bound isoform of FL in peripheral blood and bone marrow cells from AA patients at diagnosis (n = 16) and after immunosuppressive (IS) treatment (n = 36). Flow cytometry demonstrated strongly increased FL levels on the cell surface of T lymphocytes in AA relative to normal controls (P < 0.0001). T-cell-specific expression of membrane-bound FL was confirmed by confocal microscopy. FL mRNA and total cellular FL protein levels were increased about threefold. Overexpression of FL in AA was observed for up to 20 years after IS treatment. FL levels correlated inversely with CD34+ cell numbers and the colony-forming ability of AA bone marrow (R = -0.68 and -0.85 respectively). Histological examination of spleen specimens and bone marrow biopsies gave no evidence of degeneration or fibrosis due to prolonged exposure to high FL. Levels of membrane-bound FL were not increased in autoimmune diseases (n = 23), including rheumatoid arthritis and lupus erythematosus, nor in graft-versus-host disease (n = 8). Chronic overexpression of FL on the surface of T lymphocytes in AA, but not in other T-cell-mediated disorders, suggests that membrane-bound FL plays a role in cell-cell interactions in bone marrow failure and may be important for long-term haemopoietic recovery.

FLT3 ligand gene expression and protein production in human colorectal cancer cell lines and clinical tumor specimens.

Dendritic cells (DC) are professional antigen presenting cells (APC) whose proliferation and functional differentiation can be induced by hematopoietic growth factors including GM-CSF and FLT3 ligand (FL). Colorectal cancers are known to be infiltrated by dendritic cells (DC) and neoplastic cells have been shown to produce GM-CSF. In this work we investigated FLT3 ligand (FL) gene expression and protein production in human colorectal cancer cell lines and clinical tumor specimens. Using reverse transcription polymerase chain reaction (RT-PCR), 6 out of 6 established tumor lines were found to express to variable extents FL gene. In 1 of them, SW480, FL immunoreactivity could be observed by taking advantage of specific antibodies. In contrast, soluble FL could not be detected in any culture supernatant. FLT3 receptor (FR) gene was not expressed and exogenous addition to the cultures of recombinant FL (rFL) did not affect the proliferation of the tumor lines. FL gene expression was investigated using a densitometry-assisted, semiquantitative RT-PCR in clinical tumor specimens. Specific FL gene transcripts were amplified from 12 of 12 surgical samples. In these cases, FL gene expression of significantly lower intensity was also detected in healthy mucosa sampled in the vicinity (2 cm) or at a distance (10 cm) from neoplastic outgrowth. Immunohistochemical studies identified FL-positive cancer cells in 5 of 5 cases tested. No positivity was detected in healthy mucosa epithelia at a distance from the tumor or in stromal cells. FL content in preoperative sera from colorectal cancer patients (n = 13) did not exceed the levels detected in healthy donors (

High incidence of transiently appearing complement-sensitive bone marrow precursor cells in patients with severe aplastic anemia--A possible role of high endogenous IL-2 in their suppression.

In a prospective long-term study on the incidence of paroxysmal nocturnal hemoglobinuria (PNH), 115 consecutive patients with severe aplastic anemia (SAA), 97 treated with antilymphocyte globulin (ALG) and 18 with bone marrow transplantation (BMT), were observed over a period of 4-18 years and tested for the presence of complement-sensitive hematopoietic precursor cells with the bone marrow (BM) sucrose test. Sixteen (14%) of the ALG-treated patients developed clinical signs of PNH between 0.5 and 8 years after treatment. Complement-sensitive BM precursors were found in 89% of the SAA patients at some time during their disease, but in none of 18 normal donors. At diagnosis, their proportion was significantly higher in patients who later developed PNH than in patients who later achieved disease-free complete remission (CR). After ALG, the abnormal population was found in both groups, but it was gradually replaced by normal precursors in remission patients. After BMT, the complement-sensitive population decreased to very low numbers in patients with a stable graft, but increased again in 3 patients upon graft rejection. Mimicking the PNH defect by enzymatic removal of glycosyl-phosphatidylinositol (GPI)-linked proteins from CD34+ cells resulted in their complement sensitivity, suggesting that the BM sucrose test identifies precursor cells carrying the PNH defect. In 66 patients, white blood cells (WBC) in peripheral blood (PB) were examined for GPI-deficient populations by flow cytometry (FACS). Ten patients with signs of clinical or laboratory PNH had over 25% complement-sensitive precursor cells in the BM and a GPI-deficient WBC population in the PB. Of 56 SAA patients without PNH, 8 had an abnormal population detectable with both tests, 26 only with the BM sucrose test, 4 only with PB FACS analysis, and in 18, no abnormal cells were detected with either test. In search for parameters which might explain why in some patients the abnormal population expands, while it regresses or disappears in others, we tested the release of IL-2 as a parameter of immune competence. At diagnosis, IL-2 release was approximately 50% of normal in patients who later developed PNH, while it was double the normal value in patients who later achieved CR. We conclude that the majority of SAA patients transiently harbor complement-sensitive precursor cells in the BM. Patients with more than 25% abnormal BM precursors and low endogenous IL-2 release are at risk of progression to clinical PNH.

Umbilical cord blood from preterm human fetuses is rich in committed and primitive hematopoietic progenitors with high proliferative and self-renewal capacity.

Human umbilical cord blood (CB) has been recognized as a source of hematopoietic stem cells for transplantation. While hematopoietic properties of neonatal CB from full-term pregnancies have been well characterized, little is known about CB from early gestational ages. We analyzed the content and the growth properties of primitive and committed hematopoietic progenitors in preterm CB from second trimester (week 16-28; n = 17) and early third trimester (week 29-34; n = 17) in comparison with term CB (n = 18). The frequency of CD34+ and CD34+CD38- cells was significantly higher in preterm than in term CB (mean, 2.51% and 0.56% vs 0.88% and 0.13%;p < 0.002). The number of colony forming units (CFU) in preterm CB was about twofold higher (230 +/- 6 vs 133 +/- 14/ 10(5) mononuclear cells; p < 0.05) and correlated with the content of CD34+ progenitors (r = 0.73). Long-term culture initiating cells (LTC-IC) were enriched about 2.5-fold (6.7 +/- 2.9 vs 2.6 +/- 1.2/10(5) cells; p < 0.05). Progenitors from preterm CB could be expanded in stroma-free liquid cultures supplemented with hematopoietic growth factors as efficiently as progenitors from term neonates. In short-term cultures containing erythropoietin (Epo), interleukin (IL)-1, IL-3, and IL-6, or granulocyte- (G-) and granulocyte-macrophage colony-stimulating factor (GM-CSF) together with stem cell factor (SCF) or Flt3 ligand (FL), expansion of CFUs was six- to eightfold at week 1. In long-term cultures containing thrombopoietin (TPO) and FL, an approximately 1000-fold expansion of multilineage progenitors was observed at week 10. In summary, we show that preterm CB compared with term CB is richer in hematopoietic progenitors, and that precursors from preterm CB can be extensively expanded ex vivo. This may have implications for the development of transplantation and gene transfer strategies targeting circulating fetal stem cells.

Mechanism of flt3 ligand expression in bone marrow failure: translocation from intracellular stores to the surface of T lymphocytes after chemotherapy-induced suppression of hematopoiesis.

The flt3 ligand (FL) is a growth factor for primitive hematopoietic cells. Serum levels of FL are inversely related to the number and proliferative capacity of early hematopoietic progenitors. We sought to elucidate the molecular mechanism underlying this regulation. Expression of FL was examined in peripheral blood (PB) and bone marrow (BM) cells under normal steady-state hematopoiesis and during transient BM failure induced by chemoradiotherapy in 16 patients with hematological malignancies. Using anti-FL antibodies in Western analysis, flow cytometry, and confocal microscopy, we detected high levels of preformed FL inside but not on the surface of T lymphocytes in steady-state hematopoiesis. Intracellular FL colocalized with giantin and ERGIC-53, indicating that it is stored within and close to the Golgi apparatus. After chemotherapy-induced hematopoietic failure, FL rapidly translocated to the surface of T lymphocytes and the levels of FL released to serum increased approximately 100-fold. Expression of FL mRNA was enhanced only about sevenfold; a similar, twofold to sixfold increase in mRNA was observed in the thymus and BM of mice with irradiation-induced aplasia. Upregulation of FL mRNA was delayed when compared with the appearance of cell surface-associated and soluble protein isoforms. The described changes in FL expression in response to chemotherapy-induced aplasia were observed in all patients, irrespective of the diagnosis and treatment regimen. Our data demonstrate that mobilization of preformed FL from intracellular stores rather than de novo synthesis is responsible for increased FL levels in BM failure.

Effect of flt3 ligand on in vitro growth and expansion of colony-forming bone marrow cells from patients with aplastic anemia.

To determine the value of flt3 ligand (flt3L) in stimulating hematopoiesis in human hypoproliferative bone marrow disorders, we examined its in vitro effect on bone marrow cells from patients with aplastic anemia (AA). Growth response to flt3L, alone and in combination with other hematopoietic growth factors, was investigated in clonogenic methylcellulose assays, in long-term liquid and stroma cultures. Bone marrow cells were derived from 13 AA patients with persisting in vitro growth defect after immunosuppressive treatment and from nine normal bone marrow donors. In methylcellulose cultures, flt3L stimulated formation of hematopoietic colonies only weakly, whereas it had an additive effect when combined with erythropoietin (Epo), stem cell factor (SCF), interleukin-3 (IL-3), interleukin-11 (IL-11), or granulocyte colony-stimulating factor (G-CSF). Flt3L was less effective than SCF and did not further enhance the number of hematopoietic colonies formed in response to SCF-containing combinations of multiple cytokines. In long-term liquid suspension cultures, flt3L was less mitogenic than SCF but its effect on the maintenance of progenitors was superior that of SCF and of IL-3, IL-11, and G-CSF. The total number of clonogenic AA cells increased as much as four-fold during the first culture week and FACS analysis demonstrated expansion of the CD34+CD38+ progenitor cell subset. Despite this enhancement, survival of AA cells remained significantly poorer than that of normal cells, in which the primitive subset of CD34+CD38- cells was maintained up to 4 weeks when flt3L was used as a single factor. Both in normal and AA cultures, flt3L promoted differentiation of cells of the myeloid lineages. In cultures of bone marrow stroma, flt3L had almost no effect on growth and survival of AA progenitors, while in cultures of normal cells the number of colony-forming cells increased up to 10-fold. Although flt3L does not overcome the proliferative defect of AA precursors, we conclude that the ligand is capable of in vitro stimulation and expansion of the reduced progenitor cell pool in AA, when used in appropriate culture conditions. The in vitro effects of flt3L on AA cells differ in many aspects from those of the structurally related cytokine SCF, suggesting a benefit in use of a combination of these two early-acting growth factors.