B lymphopoiesis: global factors, local control

Senescent B Lymphopoiesis is Balanced in Suppressive Homeostasis: Decrease in Interleukin-7 and Transforming Growth Factor-β Levels in Stromal Cells of Senescence-Accelerated Mice

The suppression of the B cell population during senescence has been considered to be due to the suppression of interleukin-7 (IL-7) production and responsiveness to IL-7; however, the upregulation of transforming growth factor-β (TGF-β) was found to contribute to B cell suppression. To investigate the mechanism of this suppression based on the interrelationship between IL-7 and TGF-β during senescence, senescence-accelerated mice (SAMs), the mouse model of aging, were used in this study to elucidate the mechanisms of B lymphopoietic suppression during aging. Similar to regular senescent mice, SAMs showed a decrease in the number of IL-7–responding B cell progenitors (i.e., colony-forming unit pre-B [CFU-pre-B] cells in the femoral bone marrow [BM]). A co-culture system of B lymphocytes and stromal cells that the authors established showed a significantly lower number of CFU-pre-B cells harvested when BM cells were co-cultured with senescent stromal cells than when they were co-cultured with young stromal cells. Interestingly, cells harvested from a senescent stroma and those from the control culture without stromal cells were higher in number than those harvested from a young stroma, thereby implying that an altered senescent stromal cell is unable to maintain self-renewal of the stem cell compartment. Because TGF-β is supposed to suppress the proliferative capacity of pro-B/pre-B cells, we added a neutralizing anti-TGF-β antibody to the co-culture system with a pro-B/pre-B cell-rich population to determine whether such suppression may be rescued. However, unexpectedly, any rescue was not observed and the number of CFU-pre-B cells remained unchanged when BM cells were co-cultured with senescent stromal cells compared with the co-culture with young stromal cells, which essentially showed an increase in the number of CFU-pre-B cells (P < 0.001 in 5 μg/ml). Furthermore, TGF-β protein level in the supernatant of cultured senescent stroma cells was evaluated by enzyme-linked immunoabsorbent assay, but surprisingly, it was found that TGF-β concentration was significantly lower than that of cultured young stromal cells. Thus, TGF-β activity was assumed to decline particularly in a senescent stroma, which means a distinct difference between the senescent suppression of B lymphopoiesis and secondary B lymphocytopenia. Concerning proliferative signaling, on the other hand, the level of IL-7 gene expression in cells from freshly isolated BM decreased significantly with age. Therefore, the acceleration of proliferative signaling and the deceleration of suppressive signaling may both be altered and weakened in a senescent stroma (i.e., homeosupression).

Modulation of mature B cells in mice following treatment with ouabain

Ouabain (OUA) is an endogenous hormone released by the adrenal gland under stress situations. Steroid hormones and glucocorticoids have been characterized as selective inhibitors of lymphopoiesis. The present report shows in vivo modulation of mature B cells in bone marrow, spleen and peripheral blood by ouabain. Mice injected intraperitonially (i.p.) with ouabain 0.56 mg/kg for 3 consecutive days displayed, 24 h after last injection, a decreased cellularity in the bone marrow with diminution of the mature B cell subpopulation while the other B cell subpopulations were preserved. Percentually, the myeloid lineage in bone marrow was increased by ouabain. Numbers of mature B lymphocytes in spleen and peripheral blood were reduced following in vivo treatment. In vitro, the B cell populations were not affected. The effects appear to be independent of steroid hormones and strain. The presence of stable levels of glucocorticoids seems to be important because the effects could only be observed from the fourth week animal's life, when glucocorticoid levels are stable. These results open new perspectives for a potential use of ouabain as an immunomodulator.

Bone marrow lymphoid and myeloid progenitor cells are suppressed in 7,12-dimethylbenz(a)anthracene (DMBA) treated mice

In this study we used colony forming unit (CFU) assays to demonstrate rapid suppression (within 6h) of lymphoid (CFU-preB) and myeloid (CFU-GM) progenitor cells in DMBA-treated mice. The duration of these changes were consistent with the blood levels of DMBA and its metabolites that were achieved by either IP or oral DMBA administration. CFU-GM and CFU-preB activities returned to control levels by 2 and 7 days after oral DMBA exposure, respectively, but remained suppressed through 7 days after IP DMBA administration. The continued presence of low levels of DMBA in the bloodstream following IP administration was associated with sustained suppression of CFU-preB, total bone marrow lymphoid cells and peripheral blood lymphocytes. The changes noted above were not observed in Cyp1b1 null mice, demonstrating the need for local DMBA metabolism in the bone marrow by Cyp1b1 to impair bone marrow CFU-preB and CFU-GM. Furthermore, these data provide evidence that myeloid-lineage cells are restored more quickly than lymphoid-lineage cells after DMBA exposure.

RANK-Fc: a therapeutic antagonist for RANK-L in myeloma

BACKGROUND

Severe bone destruction due to inappropriate osteoclastogenesis is a prominent feature of multiple myeloma (MM). MM increases bone loss by disrupting the checks that normally control signaling by receptor activator of nuclear factor kappaB ligand (RANK-L, also called TRANCE [tumor necrosis factor-related, activation-induced cytokine], osteoprotegerin ligand [OPG-L], osteoclast differentiation factor [ODF], and tumor necrosis factor superfamily member 11 [TNFSF11]), a TNF-family cytokine required for osteoclast differentiation and activation. RANK-L binds to its functional receptor RANK (TNF receptor superfamily member 11a [TNF RSF11a]) to stimulate osteoclastogenesis. Osteotropic cytokines regulate this process by controlling bone marrow stromal expression of RANK-L. Further control over osteoclastogenesis is maintained by regulated expression of osteoprotegerin (OPG, also called osteoclastogenesis inhibitory factor and TNFRSF11b), a soluble decoy receptor for RANK-L. In normal bone marrow, abundant stores of OPG in stroma, megakaryocytes, and myeloid cells provide a natural buffer against increased RANK-L. MM disrupts these controls by increasing expression of RANK-L and decreasing expression of OPG. Concurrent deregulation of RANK-L and OPG expression is found in bone marrow biopsies from patients with MM but not in specimens from patients with non-MM hematologic malignancies.

METHODS

RANK-Fc is a recombinant RANK-L antagonist that is formed by fusing the extracellular domain of RANK to the Fc portion of human immunoglobulin G(1) (hIgG(1)). In vitro, addition of RANK-Fc virtually eliminates the formation of osteoclasts in cocultures of MM with bone marrow and osteoblast/stromal cells. The severe combined immunodeficiency (SCID)/ARH77 mouse model and the SCID-hu-MM mouse model of human MM were used to assess the ability of RANK-Fc to block the development of MM-induced bone disease in vivo. Mice received either RANK-Fc or hIgG(1) 200 microg intravenously three times per week.

RESULTS

RANK-Fc limited bone destruction in both the SCID/ARH-77 model and the SCID-hu-MM model. Administration of RANK-Fc also caused a marked reduction in tumor burden and serum paraprotein in SCID-hu-MM mice that was associated with the restoration of OPG and a reduction in RANK-L expression in the xenograft.

CONCLUSIONS

MM-induced bone destruction requires increased RANK-L expression and is facilitated by a concurrent reduction in OPG, a natural decoy receptor for RANK-L. Administration of the RANK-L antagonist RANK-Fc limits MM-induced osteoclastogenesis, development of bone disease, and MM tumor progression.

Overexpression of RANKL implicates IFN-beta-mediated elimination of B-cell precursors in the osteopetrotic bone of microphthalmic mice

The microphthalmic (mi) mouse possesses a dominant negative mutation in the microphthalmia-associated transcript factor (MITF) transcription factor. These animals are characterized by reduced numbers of peripheral mast and natural killer (NK) cells, are osteopetrotic because of osteoclast reduction and malfunction, lack functional melanocytes, and are deficient for maturing B-cells within the bone marrow. Granulocyte precursor cells, however, are functionally maintained within the mi bone marrow. A central question has been whether the B-cell deficiency of the mi mouse marrow is caused by the absence of an MITF-controlled gene product or because of the compromised, osteopetrotic environment. In this report, we examined mi marrow by performing transcriptional mapping analyses of candidate genes whose products are instrumental for functional osteoclast and B-cell development. Surprisingly, the expression of a subset of such genes including RANKL, stromal-derived factor (SDF-1), B-cell lymphotactin chemokine (BLC), and RANK was dramatically enhanced in the mi marrow. Normal and mutant marrow were also analyzed by subtractive transcript cloning, which identified a number of known and unknown genes with altered transcriptional activity. One such unknown mouse gene possesses a human counterpart that is interferon-beta (IFN-beta) inducible, suggesting the osteopetrotic marrow is enriched for IFN-beta, a cytokine that is known to eliminate B-cell precursors. A model is proposed suggesting excess RANKL sets off a cascade of cytokine production including IFN-beta that leads to the preferential elimination of B-cell precursors in the marrow of osteopetrotic marrow.

Multiple myeloma disrupts the TRANCE/ osteoprotegerin cytokine axis to trigger bone destruction and promote tumor progression

Bone destruction, caused by aberrant production and activation of osteoclasts, is a prominent feature of multiple myeloma. We demonstrate that myeloma stimulates osteoclastogenesis by triggering a coordinated increase in the tumor necrosis factor-related activation-induced cytokine (TRANCE) and decrease in its decoy receptor, osteoprotegerin (OPG). Immunohistochemistry and in situ hybridization studies of bone marrow specimens indicate that in vivo, deregulation of the TRANCE-OPG cytokine axis occurs in myeloma, but not in the limited plasma cell disorder monoclonal gammopathy of unknown significance or in nonmyeloma hematologic malignancies. In coculture, myeloma cell lines stimulate expression of TRANCE and inhibit expression of OPG by stromal cells. Osteoclastogenesis, the functional consequence of increased TRANCE expression, is counteracted by addition of a recombinant TRANCE inhibitor, RANK-Fc, to marrow/myeloma cocultures. Myeloma-stroma interaction also has been postulated to support progression of the malignant clone. In the SCID-hu murine model of human myeloma, administration of RANK-Fc both prevents myeloma-induced bone destruction and interferes with myeloma progression. Our data identify TRANCE and OPG as key cytokines whose deregulation promotes bone destruction and supports myeloma growth.

Regulation of cell survival during B lymphopoiesis in mouse bone marrow: enhanced pre-B-cell apoptosis in CSF-1-deficient op/op mutant mice

OBJECTIVE

Osteopetrotic (op/op) mice are deficient in macrophages and osteoclasts due to a CSF-1 gene mutation. The aim of this study was to evaluate the effect of these deficiencies and of CSF-1-dependent mechanisms on B lymphopoiesis in bone marrow, with special reference to the apoptotic activity of precursor B cells.

MATERIALS AND METHODS

B-cell development and apoptosis were examined in the bone marrow of op/op mice using immunofluorescence labeling and flow cytometry. Short-term cultures of bone marrow were used to evaluate the effect of recombinant CSF-1 on the rate of B-cell apoptosis.

RESULTS

Bone marrow cellularity was greatly reduced in op/op mice compared with normal littermates. However, precursor B cells were disproportionately decreased, most markedly at the pre-B-cell stage. Precursor B cells, particularly pre-B cells, displayed elevated apoptotic incidences both ex vivo and in short-term culture. Addition of recombinant CSF-1 reduced the incidence of apoptosis among precursor B cells in short-term cultures of whole bone marrow suspensions from normal mice but not in cultures of sorted B220+ B-lineage cells.

CONCLUSIONS

The finding of increased pre-B-cell apoptosis in op/op mice provides evidence that CSF-1-dependent mechanisms can strongly influence the survival of precursor B cells in mouse bone marrow, particularly at the pro-B/pre-B cell transition. It is proposed that the local or systemic levels of CSF-1 during ontogeny may thus play a role in regulating B-cell production within the bone marrow microenvironment.

An essential role for Daxx in the inhibition of B lymphopoiesis by type I interferons

Interferon-alpha and -beta inhibit the interleukin-7-mediated growth and survival of T and B lymphoid progenitors via an unknown, STAT1-independent pathway. Gene expression profile analysis of interferon-beta-treated progenitor B cells revealed enhanced Daxx expression, with concomitant Daxx protein increase and nuclear body translocation. The interferon effects included downregulation of cell cycle regulating genes and cell cycle arrest, followed by Bcl-2 downregulation and apoptosis. Daxx antisense oligonucleotides rescued the interferon-treated pro-B cells from growth arrest and apoptosis in parallel with the reduction of nuclear Daxx. These findings implicate the gene repressor function of Daxx in interferon-induced apoptosis of lymphoid progenitors.

Stat-1 is not essential for inhibition of B lymphopoiesis by type I IFNs

Type I IFNs, IFN-alpha, -beta, and -omega, are cytokine family members with multiple immune response roles, including the promotion of cell growth and differentiation. Conversely, the type I IFNs are potent inhibitors of IL-7-dependent growth of early B lineage progenitors, effectively aborting further B lineage differentiation at the pro-B cell stage. Type I IFNs alpha and beta function via receptor-mediated activation of a Jak/Stat signaling pathway in which Stat-1 is functionally important, because many IFN-induced responses are abrogated in Stat-1-deficient mice. To the contrary, we show here that the inhibition of IL-7-dependent B lymphopoiesis by IFN-alphabeta is unaffected in Stat-1-deficient mice. The present data indicate that the type I IFNs can activate an alternative signaling pathway in which neither Stat-1 nor phosphatidylinositol 3'-kinase are essential components.

Impairment of B lymphopoiesis in precocious aging (klotho) mice

Inactivation of the klotho gene in mice results in multiple disorders that resemble human aging after 3 weeks of age. Because hematopoiesis, especially B lymphopoiesis, is affected in humans and mice by aging, we analyzed the hematopoietic state in homozygous klotho (kl/kl) mice. The kl/kl mice showed thymic atrophy and a reduced number of splenocytes. These mice had almost the normal number of myeloid cells, erythroid cells, IL-3-responsive myeloid precursors and colony forming units in spleen (CFU-S) in bone marrow (BM), but had a substantially decreased number of B cells in BM and peripheral blood as compared with wild-type mice. IL-7-responsive B cell precursors and all of the maturation stages of B cells in BM were also reduced. However, the function of hematopoietic stem cells including their capacity of B lymphopoiesis in vivo and in vitro was normal. Early B cell development was also normal in neonates and young kl/kl mice until 2 weeks old without aging phenotypes. RT-PCR analysis revealed that the level of IL-7 gene expression was significantly reduced in freshly isolated kl/kl BM cells. However, injection of IL-7 in kl/kl mice could not rescue the B lymphopenia. These findings indicate that Klotho protein may regulate B lymphopoiesis via its influence on the hematopoietic microenvironment.

Estrogen influences the differentiation, proliferation, and survival of early B-lineage precursors

B lymphocyte production in murine bone marrow is negatively regulated by sex steroids and the aim of this study was to identify early hormone sensitive checkpoints. Estrogen (E2) treatment reduced cμ+ pre-B cells, a change that occurred concomitantly with decreased Ig gene rearrangements and rag-1 transcripts. Estrogen decreased B lineage precursors in Ig transgenic mice, demonstrating that hormonal regulation is independent of the recombination process. B lineage precursors in Bcl-2 transgenic mice were resistant to estrogen treatment, suggesting that life/death decisions are involved in hormonal regulation. A previously uncharacterized population of CD43−cμ− B lineage precursors was identified in normal, Ig transgenic, and RAG−/− mice after estrogen treatment, revealing that down-regulation of CD43 can occur independent of Ig heavy chain expression. These cells expressed transcripts for both tdt andbcl-2, characteristics of early B-cell precursors. BrdU incorporation analysis revealed that the mitotic activity of early B-lineage cells is reduced in hormone-treated mice. We conclude that sex steroids modulate the production of B-lineage cells by influencing the differentiation, proliferation, and survival of early B-cell precursors. These findings are informative about mechanisms of hormonal regulation, as well as the significance of some differentiation-related events.

A splice variant of human ephrin-A4 encodes a soluble molecule that is secreted by activated human B lymphocytes

Ephrin-A4 is a ligand for the erythropoietin-producing hepatocellular (Eph) receptor family of tyrosine kinases. We have identified a secreted form of ephrin-A4, denoted ephrin-A4 (s), which is encoded by an alternatively spliced mRNA and is produced by in vivo activated B cells in tonsils. Blood B cells secrete ephrin-A4 (s) upon stimulation via the B-cell antigen receptor. A subpopulation of tonsil cells in the crypts with a dendritic cell phenotype was shown to express EphA2, an Eph receptor tyrosine kinase that was found to be capable of binding an ephrin-A4 immunoglobulin chimeric protein. We conclude that ephrin-A4 (s) may play a role in the interaction between activated B lymphocytes and dendritic cells in human tonsils. (Blood. 2000;95:221-230)

A splice variant of human ephrin-A4 encodes a soluble molecule that is secreted by activated human B lymphocytes

Ephrin-A4 is a ligand for the erythropoietin-producing hepatocellular (Eph) receptor family of tyrosine kinases. We have identified a secreted form of ephrin-A4, denoted ephrin-A4 (s), which is encoded by an alternatively spliced mRNA and is produced by in vivo activated B cells in tonsils. Blood B cells secrete ephrin-A4 (s) upon stimulation via the B-cell antigen receptor. A subpopulation of tonsil cells in the crypts with a dendritic cell phenotype was shown to express EphA2, an Eph receptor tyrosine kinase that was found to be capable of binding an ephrin-A4 immunoglobulin chimeric protein. We conclude that ephrin-A4 (s) may play a role in the interaction between activated B lymphocytes and dendritic cells in human tonsils. (Blood. 2000;95:221-230)

Regulation of Cell Survival During B Lymphopoiesis: Apoptosis and Bcl-2/Bax Content of Precursor B Cells in Bone Marrow of Mice with Altered Expression of IL-7 and Recombinase-Activating Gene-2

B cell development in mouse bone marrow depends critically upon IL-7. To examine the possible in vivo trophic role of IL-7, we have quantitated apoptosis and Bcl-2 family proteins in populations of phenotypically defined B lineage cells in IL-7-deficient and IL-7-overexpressing mice. Using immunofluorescence labeling, multiparameter flow cytometry, and a short-term culture assay, we show that the apoptotic rates of precursor B cells, but not of more mature B cells, are enhanced by IL-7 gene deletion, associated with increased intracellular content of Bax and decreased Bcl-2, while, conversely, an IL-7 transgene suppresses precursor B cell apoptosis and produces low Bax and high Bcl-2 levels. During normal B cell development, high Bax/Bcl-2 ratios characterize cells undergoing greatest apoptotic cell death. Pro-B cells in RAG-2−/− mice, all destined to abort, show elevated Bax levels and Bax/Bcl-2 ratios. By comparison with the elevated rate of pro-B cell apoptosis in RAG-2−/− mice, provisional estimates have been made for the fraction of pro-B cells undergoing apoptosis in normal mice (70%), IL-7−/− mice (85%), and IL-7 transgenic mice (35%). The results demonstrate that IL-7 strongly promotes in vivo cell survival and maintains antiapoptotic Bcl-2/Bax ratios during the development of precursor B cells in mouse bone marrow.

The enigmatic role of glutamyl aminopeptidase (BP-1/6C3 antigen) in immune system development

The murine B-lymphocyte differentiation antigen BP-1/6C3, a homodimeric integral membrane protein composed of M, 140,000 subunits, has been identified as glutamyl aminopeptidase (EAP, EC 3.4.11.7). This ecto-enzyme cleaves acidic amino acid residues from the amino terminal of polypeptide substrates such as angiotensin II and cholecystokinin-8. Although BP-1/6C3/EAP is expressed by cells in many tissues, among hematopoietic cell lineages this ecto-enzyme is restricted to immature B-lineage cells where its expression is upregulated by interleukin-7 and viral transformation. BP-1/6C3/EAP thus serves as a valuable marker of progression along the B-cell differentiation pathway, but a corresponding biological role has not yet been established.

Glycosylation provides both stimulatory and inhibitory effects on cell surface and soluble CD44 binding to hyaluronan

Glycosylation has been implicated in the regulation of CD44-mediated cell binding of hyaluronan (HA). However, neither the relative contribution of N- and O-linked glycans nor the oligosaccharide structures that alter CD44 affinity for HA have been elucidated. To determine the effect of selective alteration of CD44 oligosaccharide composition on the affinity of CD44 for HA, we developed a novel strategy based on the use of affinity capillary electrophoresis (ACE). Soluble recombinant CD44-immunoglobulin fusion proteins were overproduced in the mutant CHO cell line ldl-D, which has reversible defects in both N- and O-linked oligosaccharide synthesis. Using this cell line, a panel of recombinant glycosidases, and metabolic glycosidase inhibitors, CD44 glycoforms with defined oligosaccharide structures were generated and tested for HA affinity by ACE. Because ldl-D cells express endogenous cell surface CD44, the effect of any given glycosylation change on the ability of cell surface and soluble CD44 to bind HA could be compared. Four distinct oligosaccharide structures were found to effect CD44-mediated HA binding: (a) the terminal alpha2,3-linked sialic acid on N-linked oligosaccharides inhibited binding; (b) the first N-linked N-acetylglucosamine residue enhanced binding; (c) O-linked glycans on N-deglycosylated CD44 enhanced binding; and (d) N-acetylgalactosamine incorporation into non-N-linked glycans augmented HA binding by cell surface CD44. The first three structures induced up to a 30-fold alteration in the intrinsic CD44 affinity for HA (Kd = 5 to >150 microM). The fourth augmented CD44-mediated cellular HA avidity without changing the intrinsic HA affinity of soluble CD44.

Impairment of T and B cell development by treatment with a type I interferon

Type I interferons alpha and beta, naturally produced regulators of cell growth and differentiation, have been shown to inhibit IL-7-induced growth and survival of B cell precursors in vitro. After confirming an inhibitory effect on B lymphopoiesis in an ex vivo assay, we treated newborn mice with an active IFN-alpha2/alpha1 hybrid molecule to assess its potential for regulating B and T cell development in vivo. Bone marrow and splenic cellularity was greatly reduced in the IFN-alpha2/alpha1-treated mice, and B lineage cells were reduced by >80%. The bone marrow progenitor population of CD43+B220+HSA- cells was unaffected, but development of the CD19+ pro-B cells and their B lineage progeny was severely impaired. Correspondingly, IL-7-responsive cells in the bone marrow were virtually eliminated by the interferon treatment. Thymus cellularity was also reduced by >80% in the treated mice. Phenotypic analysis of the residual thymocytes indicated that the inhibitory effect was exerted during the pro-T cell stage in differentiation. In IFN-alpha/beta receptor-/- mice, T and B cell development were unaffected by the IFN-alpha2/alpha1 treatment. The data suggest that type I interferons can reversibly inhibit early T and B cell development by opposing the essential IL-7 response.

Impaired Ability of Bone Marrow Stromal Cells to Support B-Lymphopoiesis With Age

B-lymphopoiesis decreases with age. We studied how aging affects bone marrow stromal cells, because they provide the growth factors and cell contacts required for B-lymphopoiesis. No differences were noted in the cell-surface phenotype of young and old primary-cultured stromal cells. Fluorescence-activated cell sorter-purified stromal cells from old mice were deficient in the ability to support the proliferation of interleukin-7 (IL-7)–specific B-lymphoid cell lines. The kinetics of this response indicated that IL-7 was not immediately available from stromal cells of either age and was further delayed on aged stromal cells. The levels of IL-7 protein within stromal cells were equivalent between young and old animals, suggesting that the production of IL-7 was not altered by aging. Negligible amounts of IL-7 were found either freely secreted or in the extracellular matrix of cultures of young and old marrow. Contact between the lymphoid cells and the primary stromal cells was required for detectable proliferation, suggesting that cell contact was required for the release of IL-7. We propose that stromal cells regulate B-lymphopoiesis by limiting the amount of IL-7 available to the developing precursors. Therefore, we conclude that the age-related decrease in the function of bone marrow stromal cells is related to the impaired release of IL-7.

Impaired Ability of Bone Marrow Stromal Cells to Support B-Lymphopoiesis With Age

B-lymphopoiesis decreases with age. We studied how aging affects bone marrow stromal cells, because they provide the growth factors and cell contacts required for B-lymphopoiesis. No differences were noted in the cell-surface phenotype of young and old primary-cultured stromal cells. Fluorescence-activated cell sorter-purified stromal cells from old mice were deficient in the ability to support the proliferation of interleukin-7 (IL-7)–specific B-lymphoid cell lines. The kinetics of this response indicated that IL-7 was not immediately available from stromal cells of either age and was further delayed on aged stromal cells. The levels of IL-7 protein within stromal cells were equivalent between young and old animals, suggesting that the production of IL-7 was not altered by aging. Negligible amounts of IL-7 were found either freely secreted or in the extracellular matrix of cultures of young and old marrow. Contact between the lymphoid cells and the primary stromal cells was required for detectable proliferation, suggesting that cell contact was required for the release of IL-7. We propose that stromal cells regulate B-lymphopoiesis by limiting the amount of IL-7 available to the developing precursors. Therefore, we conclude that the age-related decrease in the function of bone marrow stromal cells is related to the impaired release of IL-7.

Regulated Expression of the Eph-Related Receptor Tyrosine Kinase Hek11 in Early Human B Lymphopoiesis

Members of the large Eph family of receptor tyrosine kinases (RTKs) display temporally and spatially restricted expression patterns during embryogenesis, suggesting a role in various developmental processes. We have begun to investigate the expression of members of this receptor family during human hematopoiesis, in particular B lymphopoiesis. Expression of Eph RTKs in cells of the B-lymphoid lineage was assessed by using degenerate oligonucleotide primers based on stretches of conserved nucleic acid sequences in members of the Eph family. First, the content of Eph-family RTKs was assessed in freshly sorted fetal bone marrow pro–B cells. This population was found to harbor transcripts of the Hek8 and Hek11 members of this gene family. Subsequent analysis of expression of these genes in B cells representing various differentiation and ontogenic stages showed that the Hek8 transcript is constitutively present in all fetal and adult B-lineage cells, with high levels of expression in peripheral blood B cells. In contrast, the Hek11 transcript was exclusively found in fetal bone marrow pro–B cells and pre–B cells, but not in more mature fetal B-lineage cells. All adult B-lineage cells, from early pro–B cells to end-stage plasma cells, lacked Hek11 transcripts. The developmentally regulated expression of Hek11 during fetal B lymphopoiesis suggests a role for this gene in pre/pro–B cell expansion and/or differentiation and defines a difference in progenitor B cell populations isolated from fetal versus adult human bone marrow.

Hormones, lymphohemopoietic cytokines and the neuroimmune axis

The classical distinction between hormones and cytokines has become increasingly obscure with the realization that homeostatic responses to infection involve coordinated changes in both the neuroendocrine and immune systems. The hypothesis that these systems communicate with one another is supported by the ever-accruing demonstrations of a shared molecular network of ligands and receptors. For instance, leukocytes express receptors for hormones and these receptors modulate diverse biological activities such as the growth, differentiation and effector functions. Leukocyte lineages also synthesize and secrete hormones, such as insulin-like growth factor-I (IGF-I), in response to both growth hormone (GH) and also to cytokines such as tumor necrosis factor-alpha (TNF-alpha). Since hormones share intracellular signaling substrates and biological activities with classical lymphohemopoietic cytokines, neuroendocrine and immune tissues share a common molecular language. The physiological significance of this shared molecular framework is that these homeostatic systems can intercommunicate. One important example of this interaction is the mechanism by which bacterial lipopolysaccharide, by eliciting a pro-inflammatory cytokine cascade from activated leukocytes, modulate pituitary GH secretion as well as other CNS-controlled behavioral and metabolic events. This article reviews the cellular and molecular basis for this communication system and proposes novel mechanisms by which neuroendocrine-immune interactions converge to modulate disease resistance, metabolism and growth.

Expression of Interleukin-7 Receptor by Lineage-Negative Human Bone Marrow Progenitors With Enhanced Lymphoid Proliferative Potential and B-Lineage Differentiation Capacity

Relatively little is known about the relationship of lymphoid-associated gene expression to the proliferation and differentiation potential of early human bone marrow lymphoid progenitors. Surface expression of interleukin-7 (IL-7) receptor-α (IL-7Rα), a component of the high-affinity receptor for the lymphoid precursor growth factor IL-7, defined a CD34+ progenitor subset lacking the CD19+ pro-B phenotype but demonstrating markedly enhanced lymphoid clonogenic capacity and the ability to differentiate into pro-B cells in short-term culture. These progenitors expressed mRNA for the lymphoid-associated genes Igβ, RAG-1, and PAX-5, and were uniformly TdT-positive (TdT+). In contrast, IL-7Rα−/CD19−/CD34+ progenitors had a 50-fold reduced lymphoid clonogenic capacity and did not differentiate into pro-B cells in short-term culture. Expression of TdT and the lymphoid-associated genes Igβ and RAG-1, but not PAX-5, was detected in this fraction, although at lower levels than in the IL-7Rα+ progenitors. In contrast to IL-7Rα, loss of the stem cell factor receptor c-kit was associated with enhanced lymphoid clonogenic potential and increased B-lineage differentiation potential. These results indicate that IL-7Rα expression defines entry into a developmental stage characterized by upregulation of multiple lymphoid-associated genes and enhanced fitness for B-lymphoid differentiation. The onset of IL-7Rα and PAX-5 expression immediately before acquisition of CD19 is consistent with evidence suggesting upregulation of CD19 through pathways involving PAX-5 and IL-7.

Increased sensitivity to apoptotic stimuli in c-abl-deficient progenitor B-cell lines

The protooncogene c-abl encodes a nonreceptor tyrosine kinase whose cellular function is unknown. To study the possible involvement of c-Abl in proliferation, differentiation, and cell cycle regulation of early B cells, long-term lymphoid bone marrow cultures were established from c-abl-deficient mice and their wild-type littermates. Interleukin 7-dependent progenitor B-cell clones and lines expressing B220 and CD43 could be generated from both mutant and wild-type mice. The mutant and wild-type lines displayed no difference in their proliferative capacity as measured by thymidine incorporation in response to various concentrations of interleukin 7. Similarly, c-abl deficiency did not interfere with the ability of mutant clones to differentiate into surface IgM-positive cells in vitro. Analysis of cultures after growth factor deprivation, however, revealed a strikingly accelerated rate of cell death in c-abl mutant cells, due to apoptosis as confirmed by terminal deoxynucleotidyltransferase-mediated UTP nick end labeling analysis. Furthermore, a greater susceptibility to apoptotic cell death in c-abl mutant cells was also observed after glucocorticoid treatment. These results suggest that mutant c-Abl renders the B-cell progenitors more sensitive to apoptosis, and may account for some of the phenotypes observed in c-abl-deficient animals.

Cytokine regulation of early lymphohematopoietic development

A two-step methylcellulose culture provided a method to study the differentiation of murine lymphohematopoietic progenitors. In the presence of two cytokines, one from a group consisting of Steel factor (SF) and flt3/flk2 ligand (FL) and the other from a group consisting of interleukin 6 (IL-6), G-CSF, IL-11 and IL-12, murine lymphohematopoietic progenitors proliferated and generated not only myeloid lineage cells but also committed B cell progenitors. Although somewhat less effectively than SF and FL, IL-4 also synergized with IL-6 or IL-11 in support of B lymphopoiesis. This early process of B lymphopoiesis appears to proceed through three stages: lymphohematopoietic proliferative stage, commitment stage and early B lymphoid proliferative stage. Surprisingly, IL-3 could neither replace nor act synergistically with SF, IL-4 or FL in maintaining the B lymphoid potential of the cells in the primary culture, although IL-3 was very effective in support of multilineage myeloid colony formation. In addition, when added to permissive cytokine combinations, IL-3 inhibited development of the B cell lineage. After screening available lymphohematopoietic cytokines, it was found that IL-1 (both alpha and beta) also has similar inhibitory effects on early B lymphopoiesis. Studies using in vivo transfer of primary colonies suggested that cytokine regulation of commitment to T cell lineage may also be similar to that of B cell lineage.

Insulin-like growth factor 1 and insulin inhibit HIV type 1 replication in cultured cells

Insulin-like growth factor 1 and insulin, considered primarily as metabolic and growth modulatory hormones, were found to inhibit the replication of HIV-1 in cultured cord blood mononuclear cells and chronically HIV-infected U937 cells. The effect of IGF-1 was seen at physiological concentrations or lower (1.7 x 10(-10) M) while that of insulin was observed at supraphysiological concentrations (8 x 10(-7) M). The EC50 for IGF-1 was found to be in the physiological range (2.5-4.5 x 10(-9) M) while that for insulin was considerably higher (1.1-3.3 x 10(-6) M). Insulin-like growth factor 1 and insulin at the concentrations employed exhibited no toxicity on the cells used in these studies. Furthermore, neither IGF-1 nor insulin exhibited any inhibitory activity on purified reverse transcriptase in vitro. Epidermal growth factor from 1.6 x 10(-10) to 1.6 x 10(-8) M demonstrated no inhibition of HIV-1 replication, while IGF-1 inhibited p24 antigen production 49 and 42% at 1.3 x 10(-9) and 1.3 x 10(-8) M IGF-1, respectively. These results suggest that IGF-1 under certain conditions has significant inhibitory effects on HIV-1 replication at physiological concentrations. This may prove to be of therapeutic value in patients infected with HIV-1.

Molecular cloning and chromosomal mapping of a bone marrow stromal cell surface gene, BST2, that may be involved in pre-B-cell growth

Bone marrow stromal cells regulate B-cell growth and development through their surface molecules and cytokines. In this study, we generated a mAb, RS38, that recognized a novel human membrane protein, BST-2, expressed on bone marrow stromal cell lines and synovial cell lines. We cloned a cDNA encoding BST-2 from a rheumatoid arthritis-derived synovial cell line. BST-2 is a 30- to 36-kDa type II transmembrane protein, consisting of 180 amino acids. The BST-2 gene (HGMW-approved symbol BST2) is located on chromosome 19p13.2. BST-2 is expressed not only on certain bone marrow stromal cell lines but also on various normal tissues, although its expression pattern is different from that of another bone marrow stromal cell surface molecule, BST-1. BST-2 surface expression on fibroblast cell lines facilitated the stromal cell-dependent growth of a murine bone marrow-derived pre-B-cell line, DW34. The results suggest that BST-2 may be involved in pre-B-cell growth.

Long-term culture system for selective growth of human B-cell progenitors

We describe a simple reproducible system for enrichment and long-term culture of human B-cell progenitors. Enriched CD34+ cord blood mononuclear cells are seeded onto a murine stromal cell line to establish a biphasic culture system. These cultures are characterized by transient growth of myeloid cells followed by outgrowth of cells highly enriched for early B-cell progenitors. Cultures consisting of > 90% early B-lineage cells [expressing CD10, CD19, CD38, and CD45 but lacking CD20, CD22, CD23, and surface IgM] are maintained for > 12 weeks without growth factor addition. Cells remain predominantly germ line at the immunoglobulin locus and express only low levels of cytoplasmic mu chain, terminal deoxynucleotidyltransferase, and recombination-activating gene 1 product. They are unresponsive to the pre-B-cell growth factors interleukin 7 or stem cell factor, or both, suggesting that growth support is provided by a cross-reactive murine stromal cell factor. Cultured B-cell progenitors are generated in large numbers ( > 10(8) cells from a typical cord blood specimen) suitable for use in biochemical analysis and gene-transfer studies. This system should be useful for study of normal and abnormal early human B-lymphopoiesis.