IL-7R expression and IL-7 signaling confer a distinct phenotype on developing human B-lineage cells

IL-7 is an important cytokine for lymphocyte differentiation. Similar to what occurs in vivo, human CD19? cells developing in human/murine xenogeneic cultures show differential expression of the IL-7 receptor α (IL-7Rα) chain (CD127). We now describe the relationship between CD127 expression/signaling and Ig gene rearrangement. In the present study, < 10% of CD19?CD127? and CD19?CD127? populations had complete VDJ(H) rearrangements. IGH locus conformation measurements by 3D FISH revealed that CD127? and CD127? cells were less contracted than pediatric BM pro-B cells that actively rearrange the IGH locus. Complete IGH rearrangements in CD127? and CD127? cells had smaller CDR3 lengths and fewer N-nucleotide insertions than pediatric BM B-lineage cells. Despite the paucity of VDJ(H) rearrangements, microarray analysis indicated that CD127? cells resembled large pre-B cells, which is consistent with their low level of Ig light-chain rearrangements. Unexpectedly, CD127? cells showed extensive Ig light-chain rearrangements in the absence of IGH rearrangements and resembled small pre-B cells. Neutralization of IL-7 in xenogeneic cultures led to an increase in Ig light-chain rearrangements in CD127? cells, but no change in complete IGH rearrangements. We conclude that IL-7-mediated suppression of premature Ig light-chain rearrangement is the most definitive function yet described for IL-7 in human B-cell development.     

Interleukin-7 induces the proliferation of normal human B-cell precursors.

In the present study, we investigated the effects of human recombinant interleukin-7 (IL-7) on the proliferation of enriched hematopoietic cells isolated from human adult and fetal bone marrow (BM). In cultures of CD34+ cells, IL-7 was found to induce dose-dependent incorporation of 3H-thymidine (3H-TdR), but had no demonstrable effect on the development of myeloid colony-forming cells. Numbers of B-cell precursors (BCP), initially present within CD34+ populations and which included a CD34+CD20+ subset, were significantly increased when CD34+ BM cells were cultured in the presence of IL-7. This effect was most striking on CD20+ BCP, and resulted at least partly from higher numbers of cycling cells as indicated by Hoechst 33342 fluorescence (Calbiochem, Behring Diagnostics, La Jolla, CA). These results indicate that IL-7 promotes the growth of BCP within the CD34+ compartment. In line with the B-lineage affiliation of CD34+ target cells, committed BCP (CD10+ CD19+ surface IgM-) isolated from BM were also found to proliferate in response to IL-7. Interestingly, this effect of IL-7 was strongly potentiated by the addition of IL-3. Taken together, and in accordance with previous observations on murine cells, our data indicate that IL-7 acts as a growth factor during the ontogeny of human B lymphocytes.     

Insulin-like growth factor-1 potentiates expansion of interleukin-7- dependent pro-B cells

Commitment to B-lymphocyte differentiation is characterized by expression of the B220 form of the common leukocyte antigen (Ly-5) and D-JH rearrangement of the Ig heavy chain gene complex. B-lineage progenitor cells, or pro-B cells, that have initiated Ig gene rearrangement, but do not express detectable Ig heavy or light chain protein, have recently been shown to retain substantial capacity for expansion in vitro in the presence of bone marrow (BM) stromal cells and interleukin-7 (IL-7). Although the potentiating effect of stromal cells on pro-B-cell proliferation can be partially attributed to the ligand for the proto-oncogene receptor c-kit (c-kit ligand [KL] or stem cell factor), several lines of evidence suggest that c-kit-mediated cell signalling is not required for pro-B-cell expansion. Previous studies from this laboratory demonstrated that insulin-like growth factor-1 (IGF-1) potentiated the proliferative effect of IL-7 on nonadherent cells from lymphoid long-term BM cultures in a manner similar to that shown for KL. To further delineate specific cell stages that respond to lymphopoietic cytokines, we derived continuously proliferating pro-B-cell lines from day-14 murine fetal liver in the presence of IL-7 and BM stromal cell clone S10. Initial expansion and continued proliferation of these pro-B-cell lines was absolutely dependent on the presence of both IL-7 and stromal cells. In the absence of KL, IL-7-stimulated proliferation of these cells in short- term cultures and addition of either recombinant IGF-1 or KL significantly potentiated this proliferative response. Although IGF-2 and insulin also potentiated the effect of IL-7, our data suggest that neither IGF-2 nor insulin represent normal regulators of intramyeloid lymphocyte development. IGF-1 and KL activate unique cascades of intracellular signalling events and inclusion of both cytokines in cultures of IL-7-stimulated pro-B cells resulted in additive potentiation of the proliferative response. Taken together, these results suggest that expansion of pro-B cells in vivo is maintained by at least three stromal cell-derived cytokines. IL-7 appears to be unique in delivering the primary proliferative signal for pro-B-cell expansion; however, both KL and IGF-1 potentiate the proliferative effect of IL-7 on these cells. The functional redundancy and additive effects of IGF-1 and KL as amplification signals for developing B- lineage cells underscore the essential nature of clonal expansion and diversification in development of immunocompetent lymphoid cells.     

Dual role of IL-7 in the growth and differentiation of immature thymocytes.

The effects of interleukin 7 (IL-7) on subpopulations of CD4-CD8- thymocytes from young adult mice were tested in vitro. When highly purified CD3-CD4-CD8-thymocytes were cultured in the presence of recombinant IL-7, significant proportions of them became CD4+ and/or CD8+ within a day. CD3+ cells were also detected after 2 days. CD3-CD4-CD8- thymocytes were further subdivided into interleukin 2 receptor (IL-2R)- and IL-2R+ populations. The majority of the IL-2R- cells became CD4+ and/or CD8+ in 1 day in the presence of IL-7, and a substantial proportion of them also became CD3+ in 2-3 days. No significant number of CD4+ or CD8+ cells were generated from the IL-2R+ population under the same conditions. However, a small but significant proportion of them became CD3+ in 3-day cultures with IL-7. Although CD4+/CD8+ cells were also generated from the IL-2R- population in 1-day cultures in the absence of IL-7, the viability of the cells declined rapidly, and no significant numbers of CD3+ cells were generated. In proliferation assays, IL-7 alone vigorously stimulated relatively minor subpopulations of CD4-CD8- thymocytes. The IL-7-responsive cells were CD3+, did not express the IL-2R or the heat-stable antigen M1/69, and included both T-cell receptor (TCR)alpha beta + and TCR alpha beta- populations, the latter most likely TCR gamma delta +. The CD3+CD4-CD8- thymocytes, stimulated with IL-7 for 3 days, remained CD4-CD8-. These results demonstrate important roles of IL-7 in the growth and differentiation of CD4-CD8- thymocytes in vitro. It functions as a survival factor and allows CD3-CD4-CD8- cells to undergo their precommitted differentiation without inducing their proliferation, and it also stimulates CD3+CD4-CD8-thymocytes to proliferate without inducing their differentiation.     

The use of recombinant cytokines for enhancing immunohematopoietic reconstitution following bone marrow transplantation. I. Effects of in vitro culturing with IL-3 and GM-CSF on human and mouse bone marrow cells purged with mafosfamide (ASTA-Z)

Enhanced colony formation (CFU-GM) in vitro was observed in murine and human bone marrow (BM) cells following pre-incubation for 2-3 days with recombinant murine GM-CSF or natural purified murine IL-3, and recombinant human GM-CSF or IL-3, respectively. Pre-incubation in the presence of both GM-CSF and IL-3 produced additive stimulatory effects. BM cells previously treated in vitro with mafosfamide (ASTA-Z) under conditions identical to those used in the purging of autologous BM grafts, also demonstrated an enhanced cumulative response to combinations of GM-CSF and IL-3, with up to 100-fold increase in CFU-GM as compared with controls (p less than 0.001). In mice, the number of CFU-S was also significantly increased (2-20 times) following incubation of unpurged and purged BM cells in murine IL-3 and/or GM-CSF. Interestingly, the frequency of both CFU-GM and CFU-S in BM cells first purged with ASTA-Z and then cultured with both cytokines was significantly higher (p less than 0.01) than that in fresh, intact BM cells. In addition, mice transplanted with unpurged or purged, cytokine cultured syngeneic BM cells exhibited a significantly (p less than 0.01) earlier reconstitution of peripheral white blood cells and of BM CFU-GM, and a significantly enhanced anti-sheep red blood cell plaque-forming cell response. Overall, the data suggest that it might be possible to enhance immunohematopoietic reconstitution in recipients of unmanipulated, as well as ASTA-Z purged autologous BM following short-term culture of BM cells with recombinant colony stimulating factors prior to bone marrow transplantation.     

Interleukin 7 receptor ligation stimulates tyrosine phosphorylation, inositol phospholipid turnover, and clonal proliferation of human B-cell precursors.

Functional interleukin 7 (IL-7) receptors are expressed on the surface of multiphenotypic, biphenotypic, and immature B-lineage human lymphoid precursor cells with germ-line immunoglobulin heavy-chain genes but not on more mature B-lineage lymphoid cells with rearranged and/or expressed immunoglobulin heavy-chain genes. Thus, IL-7 may have an important regulatory role during the earliest stages of human B-cell ontogeny. The engagement of the surface IL-7 receptors on immature B-cell precursor cells with recombinant human IL-7 (rhIL-7) results in enhanced tyrosine phosphorylation of multiple phosphoproteins, stimulates inositol phospholipid turnover and DNA synthesis, and promotes their clonal proliferation. These effects are (i) specific for rhIL-7, since rhIL-3, rhIL-4, rhIL-5, rhIL-6, and recombinant human granulocyte colony-stimulating factor do not elicit similar activities on IL-7 receptor-positive human pro-B cells; and (ii) mediated by IL-7 receptors, since they are not observed in IL-7 receptor-negative B-lineage lymphoid cell populations. rhIL-7-induced tyrosine phosphorylation on the 35-, 53-, 55-, 62-, 69-, 76-, 94-, 150-, 170-, and 190-kDa substrates as well as rhIL-7-induced stimulation of inositol phospholipid turnover are abrogated by the tyrosine kinase inhibitor genistein. These results demonstrate that the IL-7 receptor on immature human B-cell precursor populations is intimately linked to a functional tyrosine kinase pathway and tyrosine phosphorylation is an important and perhaps mandatory step in the generation of the IL-7 receptor-linked transmembrane signal.     

Tumor-targeted IL-2 amplifies T cell-mediated immune response induced by gene therapy with single-chain IL-12.

Induction, maintenance, and amplification of tumor-protective immunity after cytokine gene therapy is essential for the clinical success of immunotherapeutic approaches. We investigated whether this could be achieved by single-chain IL-12 (scIL-12) gene therapy followed by tumor-targeted IL-2 using a fusion protein containing a tumor-specific recombinant anti-ganglioside GD(2) antibody and IL-2 (ch14.18-IL-2) in a poorly immunogenic murine neuroblastoma model. Herein, we demonstrate the absence of liver and bone marrow metastases after a lethal challenge with NXS2 wild-type cells only in mice (five of six animals) vaccinated with scIL-12-producing NXS2 cells and given a booster injection of low-dose ch14.18-IL-2 fusion protein. This tumor-protective immunity was effective 3 months after initial vaccination, in contrast to control animals treated with a nonspecific fusion protein or an equivalent mixture of antibody and IL-2. Only vaccinated mice receiving the tumor-specific ch14.18-IL-2 fusion protein revealed a reactivation of CD8(+) T cells and subsequent MHC class I-restricted tumor target cell lysis in vitro. The sequential increase in the usage of TCR chains Vbeta11 and -13 in mouse CD8(+) T cells after vaccination and amplification with ch14.18-IL-2 suggests that the initial polyclonal CD8(+) T cell response is effectively boosted by targeted IL-2. In conclusion, we demonstrate that a successful boost of a partially protective memory T cell immune response that is induced by scIL-12 gene therapy could be generated by tumor-specific targeting of IL-2 with a ch14.18-IL-2 fusion protein. This approach could increase success rates of clinical cancer vaccine trials.     

IL-2 negatively regulates IL-7 receptor alpha chain expression in activated T lymphocytes

Interleukin (IL)-2 is a type I four-alpha-helical bundle cytokine that plays vital roles in antigen-mediated proliferation of peripheral blood T cells and also is critical for activation-induced cell death. We now demonstrate that IL-2 potently decreases expression of IL-7 receptor alpha chain (IL-7Ralpha) mRNA and protein. The fact that IL-7Ralpha is a component of the receptors for both IL-7 and thymic stromal lymphopoietin (TSLP) suggests that IL-2 can negatively regulate signals by each of these cytokines. Previously it was known that the IL-2 and IL-7 receptors shared the common cytokine receptor gamma chain, gamma(c), which suggested a possible competition between these cytokines for a receptor component. Our findings now suggest a previously unknown type of cross-talk between IL-2 and IL-7 signaling by showing that IL-2 signaling can diminish IL-7Ralpha expression via a phosphatidylinositol 3-kinase/Akt-dependent mechanism.     

The FLT3 ligand is a direct and potent stimulator of the growth of primitive and committed human CD34+ bone marrow progenitor cells in vitro

The present studies investigated the effects of the recently cloned flt3 ligand (FL) on the in vitro growth and differentiation of primitive and committed subsets of human CD34+ bone marrow (BM) progenitor cells. FL alone was a weak growth stimulator of CD34+ BM cells, but synergistically and directly enhanced colony formation in combination with interleukin (IL) 3, granulocyte colony-stimulating factor (G-CSF), CSF-1, granulocyte macrophage (GM) CSF stem cell factor (SCF), and IL-6. FL and SCF were equally effective in stimulating colony formation in combination with IL-3. However, the tri-factor combination of FL + IL-3 + SCF stimulated 2.3-fold and 2.5-fold more colonies than FL + IL-3 and SCF + IL-3, respectively. These additional recruited progenitors appeared to be predominantly located in a primitive (CD71-) subset of the CD34+ progenitors, as 4.5-fold more colonies were formed by CD34+CD71- cells in response to FL + IL-3 + SCF than to FL + IL-3 or SCF + IL-3. Similar findings were observed in serum-containing and serum-deprived cultures. Whereas FL did not enhance burst-forming unit-erythroid (BFU-E) colony formation of CD34+ BM cells in the presence of serum, a low number of BFU-E colonies were formed in response to FL plus erythropoietin (Epo) under serum-deprived conditions. In addition, FL both in serum-containing and serum-deprived cultures stimulated colony formation of more committed myeloid progenitors in CD34+CD71+ BM cells. Thus, FL potently stimulates the growth of primitive and more committed human BM progenitor cells.     

The effects of interleukin 7 (IL-7) on human bone marrow in vitro.

Interleukin 7 (IL-7) stimulates the proliferation of pre-B cells from long-term murine lymphoid cultures and normal bone marrow. In addition, IL-7 stimulates the proliferation of murine T cells, including fetal and adult thymocytes as well as peripheral T cells. Flow cytometry and cell enumeration analyses were carried out on light-density human bone marrow cells incubated in the presence or absence of IL-7. The data showed no evidence for a proliferative effect of IL-7 on B-lineage cells expressing CD24 or on myeloid cells expressing CD15; however, IL-7 did stimulate the growth of T cells expressing CD3. After 16 days of stimulation the number of CD3+ cells in marrow cultures increased 350% in the presence of IL-7. In contrast, cultures incubated in the absence of IL-7 showed a 50% decrease in the number of T cells, with a preponderance of myeloid lineage cells. Flow cytometry indicated that cells from IL-7-stimulated cultures were mature T cells because they also expressed cell surface antigens for either CD4 or CD8. These studies show that in contrast to the murine system, IL-7 does not appear to stimulate the growth of human pre-B cells from adult human bone marrow. This is consistent with other experiments that suggest that human pro-B cells and not human pre-B cells respond to IL-7. It appears that IL-7 preferentially promotes the growth of T cells from human marrow.     

Osteoblasts support B-lymphocyte commitment and differentiation from hematopoietic stem cells

Early B lymphopoiesis in mammals is induced within the bone marrow (BM) microenvironment, but which cells constitute this niche is not known. Previous studies had shown that osteoblasts (OBs) support hematopoietic stem cell (HSC) proliferation and myeloid differentiation. We now find that purified primary murine OBs also support the differentiation of primitive hematopoietic stem cells through lymphoid commitment and subsequent differentiation to all stages of B-cell precursors and mature B cells. Lin(-)Sca-1(+)Rag-2(-) BM cell differentiation to B cells requires their attachment to OBs in vitro, and this developmental process is mediated via VCAM-1, SDF-1, and IL-7 signaling induced by parathyroid hormone (PTH). Addition of cytokines produced by nonosteoblastic stromal cells (c-Kit ligand, IL-6, and IL-3) shifted the cultures toward myelopoiesis. Confirming the role of OBs in B lymphopoiesis, we found that selective elimination of osteoblasts in Col2.3Delta-TK transgenic mice severely depleted pre-pro-B and pro-B cells from BM, preceding any decline in HSCs. Taken together, these results demonstrate that osteoblasts are both necessary and sufficient for murine B-cell commitment and maturation, and thereby constitute the cellular homolog of the avian bursa of Fabricius.     

Functional interleukin-7 receptors (IL-7Rs) are expressed by marrow stromal cells: binding of IL-7 increases levels of IL-6 mRNA and secreted protein

DNA spotted microarrays were used to compare gene expression profiles from 2 functionally distinct human marrow stromal cell lines: HS-27a, which supports cobblestone area formation by early hematopoietic progenitors, and HS-5, which secretes multiple cytokines that support the proliferation of committed progenitors. One unexpected result was the high level of interleukin-7 receptor (IL-7R) gene expression in HS-27a stromal cells. Northern blot analysis confirmed the IL-7R RNA expression, and Western blots for the IL-7R protein detected both a full-length (90-kd) IL-7R and a smaller 30-kd fragment in both HS-27a cells and primary stromal cell cultures, whereas only the 90-kd receptor protein was detected in peripheral blood mononuclear cells. Biotinylated IL-7 was shown to bind to HS-27a cells under physiologic conditions, and this binding was inhibited by blocking anti-IL-7 antibodies. Tyrosine phosphorylation of several proteins (55 kd, 30 kd, and 24 kd) in HS-27a cells was rapidly increased after incubation with recombinant IL-7. One of the phosphorylated proteins proved to be the 30-kd IL-7R fragment. Exposure of HS-27a cells to IL-7 resulted in a 10-fold increase in secretion of IL-6 into culture supernatants but no increase in the cytokines stromal cell-derived factor 1, macrophage inflammatory protein 1 alpha, or IL-1 beta. The up-regulation of IL-6 secretion is associated with a rapid but transient increase in detectable levels of IL-6 messenger RNA. These data suggest that IL-7 may function to regulate the milieu of the microenvironment by modulating IL-6 secretion by the IL-7R-expressing stromal elements.     

In vitro and in vivo cytokine-induced facilitation of immunohematopoietic reconstitution in mice undergoing bone marrow transplantation

The aim of this study was to test whether colony stimulating factors (CSF) and other cytokines facilitate the recovery of a variety of immunohematopoietic functions in lethally irradiated mice undergoing bone marrow transplantation (BMT). Two experimental systems were employed: (a) lethally irradiated mice transplanted with syngeneic or T cell-depleted semi-allogeneic bone marrow (BM) cells (0.1-10 x 10(6)), subsequently treated by multiple doses of cytokines; and (b) lethally irradiated mice transplanted with BM cells that had previously been cultivated with cytokines. The cytokines used were: pure natural mouse interleukin-3 (IL-3); recombinant mouse granulocyte-macrophage CSF (rGM-CSF); recombinant human interleukin-2 (rIL-2); and crude cytokine preparations obtained from the culture supernatants of murine leukemia WEHI-3b cells (containing mainly IL-3), and of phorbol myristate acetate (PMA)-stimulated EL4 leukemia cells and concanavalin A-stimulated rat splenocytes (each containing a multitude of cytokines). For BM cultures (1-9 days), the cytokines were used at a dosage of 1-100 U/ml; for in vivo treatment, 2 x 10(2)-5 x 10(4) units were administered intraperitoneally and subcutaneously at different schedules for varying periods (1-3 weeks). The following parameters were tested 1-10 weeks post-BMT: white blood cell count, colony formation in agar and in the spleen of lethally irradiated mice, proliferative responses to mitogens and alloantigens, allocytotoxicity and antibody production (serum agglutinins and plaque-forming cells) against sheep red blood cells. Under appropriate conditions, cytokine treatment either in vitro or in vivo significantly enhanced (2- to 50-fold compared with controls) most functions tested at 2-8 weeks post-BMT, and shortened the time interval required for full immunohematopoietic recovery by 2-5 weeks. In recipients of semi-allogeneic, T lymphocyte-depleted BM no evidence of graft-versus-host disease was found. It is suggested that judicious application in vitro and/or in vivo of certain pure cytokines (e.g. GM-CSF, IL-3) or cytokine 'cocktails' might be beneficial in enhancing hematopoiesis and in the treatment of immunodeficiency associated with BMT.     

Administration of recombinant human interleukin-7 alters the frequency and number of myeloid progenitor cells in the bone marrow and spleen of mice.

The administration of greater than or equal to 5 micrograms interleukin-7 (IL-7) twice a day to mice for 4 to 7 days increased by twofold to fivefold the total number of splenic and peripheral blood leukocytes, but did not appreciably increase bone marrow (BM) cellularity. This regimen of IL-7 administration also resulted in a greater than 90% reduction in the frequency and total number of single lineage colony-forming unit-culture (CFU-c) and multilineage CFU-granulocyte, erythroid, monocyte, megakaryocyte colonies that could be cultured from the BM, but a fivefold to 15-fold increase in the number of these progenitors that could be cultured from the spleen. All of these effects were reversible with progenitor and white blood cell numbers returning to near normal by day 6. Morphologic analysis of cells obtained from the BM of IL-7-treated mice showed an increase in lymphoid cells. Surface phenotype analysis showed that most of this IL-7-induced increase in lymphocytes was attributable to an increase in immature B cells (B220+, sIg-), while cells expressing the myelomonocytic markers 8C5 and MAC-1 decreased by twofold to threefold. Further studies showed that the administration of IL-7 to mice that had been rendered leukopenic by the injection of cyclophosphamide (Cy) or 5-fluorouracil (5FU) exhibited a more rapid recovery and/or overshoot in their peripheral blood lymphocytes when compared with mice treated with Cy or 5FU alone. These results show that IL-7 can differentially regulate myelopoiesis in the BM and spleen, while stimulating lymphopoiesis.     

Ontogeny-associated changes in the cytokine responses of primitive human haemopoietic cells

Time course studies revealed that the combination of Flt-3 ligand (FL), Steel factor (SF) and interleukin-3 (IL-3) did not elicit as large an amplification of the long-term culture-initiating cell (LTC-IC) population in serum-free cultures of CD34⁺CD38⁻ cord blood (CB) cells as was obtained in similar cultures of adult human CD34⁺CD38⁻ bone marrow (BM) cells (4- v 90-fold maximum increases), even though both total and colony-forming cell (CFC) numbers initially increased more rapidly in CB cultures. Multifactorial analysis of the short-term (10 d) effects of different cytokines identified FL and IL-6 in combination with the soluble IL-6 receptor (sIL-6R) as most important for expanding the CB LTC-IC population. In contrast, their counterparts in adult BM were most effectively stimulated by FL, SF and IL-3. For rapid generation of increased numbers of CFC, SF with either FL or IL-6/sIL-6R were found to be the most important contributors in cultures of CD34⁺CD38⁻ CB cells, whereas, in analogous BM cultures, IL-6/sIL-6R and TPO (in addition to FL, SF and IL-3) were required. These findings reinforce the principle of altered cytokine responsiveness as a hallmark of early haemopoietic cell differentiation and demonstrate how cytokine requirements may change during human ontogeny. Identification of conditions for optimizing the expansion of different subsets of primitive CB cells has additional important implications for clinical transplantation and gene transfer.