Cell cycle control of c-kit+IL-7R+ B precursor cells by two distinct signals derived from IL-7 receptor and c-kit in a fully defined medium

Immunological Phenotyping of Mice with a Point Mutation in Cdk4

Cyclin-dependent kinases (CDKs) play a crucial role in regulation of the mammalian cell cycle. CDK4 and CDK6 control the G1/S restriction checkpoint through their ability to associate with cyclin D proteins in response to growth factor signals. CDK4 deficiency in mice gives rise to a range of endocrine-specific phenotypes including diabetes, infertility, dwarfism, and atrophy of the anterior pituitary. Although CDK6 deficiency can cause thymic atrophy due to a block in the double-negative (DN) to double-positive (DP) stage of T cell development, there are no overt defects in immune cell development reported for CDK4-deficient mice. Here, we examined the impact of a novel N-ethyl-N-nitrosourea-induced point mutation in the gene encoding CDK4 on immune cell development. Mutant mice (Cdk4wnch/wnch) showed normal development and differentiation of major immune cell subsets in the thymus and spleen. Moreover, T cells from Cdk4wnch/wnch mice exhibited normal cytokine production in response to in vitro stimulation. However, analysis of the mixed bone marrow chimeras revealed that Cdk4wnch/wnch-derived T cell subsets and NK cells are at a competitive disadvantage compared to Cdk4+/+-derived cells in the thymus and periphery of recipients. These results suggest a possible role for the CDK4wnch mutation in the development of some immune cells, which only becomes apparent when the Cdk4wnch/wnch mutant cells are in direct competition with wild-type immune cells in the mixed bone marrow chimera.

The miR-15a/16-1 and miR-15b/16-2 clusters regulate early B cell development by limiting IL7R receptor expression

MicroRNAs are small non-coding RNAs that have emerged as post-transcriptional regulators involved in development and function of different types of immune cells, and aberrant miRNA expression has often been linked to cancer. One prominent miRNA family in the latter setting is the miR-15 family, consisting of the three clusters miR-15a/16-1, miR-15b/16-2 and miR-497/195, which is best known for its prominent tumor suppressive role in chronic lymphocytic leukemia (CLL). However, little is known about the physiological role of the miR-15 family. In this study, we provide a comprehensive in vivo analysis of the physiological functions of miR-15a/16-1 and miR-15b/16-2, both of which are highly expressed in immune cells, in early B cell development. In particular, we report a previously unrecognized physiological function of the miR-15 family in restraining progenitor B cell expansion, as loss of both clusters induces an increase of the pro-B as well as pre-B cell compartments. Mechanistically, we find that the miR-15 family mediates its function through repression of at least two different types of target genes: First, we confirm that the miR-15 family suppresses several prominent cell cycle regulators such as Ccne1, Ccnd3 and Cdc25a also in vivo, thereby limiting the proliferation of progenitor B cells. Second, this is complemented by direct repression of the Il7r gene, which encodes the alpha chain of the IL-7 receptor (IL7R), one of the most critical growth factor receptors for early B cell development. In consequence, deletion of the miR-15a/16-1 and miR-15b/16-2 clusters stabilizes Il7r transcripts, resulting in enhanced IL7R surface expression. Consistently, our data show an increased activation of PI3K/AKT, a key signaling pathway downstream of the IL7R, which likely drives the progenitor B cell expansion we describe here. Thus, by deregulating a target gene network of cell cycle and signaling mediators, loss of the miR-15 family establishes a pro-proliferative milieu that manifests in an enlarged progenitor B cell pool.

Antibody therapeutics and immunoregulation in cancer and autoimmune disease

Cancer and autoimmune disease are closely related, and many therapeutic antibodies are widely used in clinics for the treatment of both diseases. Among them, the anti-CD20 antibody has proven to be effective against both lymphoid malignancy and autoimmune disease. Moreover, immune checkpoint blockade using the anti-PD1/PD-L1/CTLA4 antibody has improved the prognosis of patients with refractory solid tumors. At the same time, however, over-enhancement of immunoreaction can induce autoimmune reaction. Although anti-TNF antibody therapies represent a breakthrough in the treatment of autoimmune diseases, optimal management is required to control the serious associated issues, including development and progression of cancer, and it is becoming more and more important to control the immunoreaction. In addition, next-generation antibody therapeutics such as antibody-drug conjugates and bispecific antibodies, are anticipated to treat uncontrolled cancer and autoimmune disease. IL-7R signaling plays an important role in the development and progression of both lymphoid malignancy and autoimmune disease. In addition, abnormal homing activity and steroid resistance caused by IL-7R signaling may worsen prognosis. Therefore, anti-IL-7R targeting antibody therapies that enable suppression of such pathophysiological status have the potential to be beneficial for the treatment of both diseases. In this review, we discuss current antibody therapeutics in cancer and autoimmune disease, and describe a new therapeutic strategy for immunoregulation including IL-7R targeting antibodies.

Immunoregulation by IL-7R-targeting antibody-drug conjugates: overcoming steroid-resistance in cancer and autoimmune disease

Steroid-resistance is a common complication in the treatment of malignancies and autoimmune diseases. IL-7/IL-7R signaling, which regulates lymphocyte growth and survival, has been implicated in the development of malignancies and autoimmune diseases. However, the biological significance of IL-7/IL-7R signaling in steroid treatment is poorly understood. Here, we identified a novel relationship between IL-7R signaling and steroid-resistance, and showed that an anti-IL-7R antibody conjugated with SN-38 (A7R-ADC-SN-38) has strong anti-tumor effects against both parental and steroid-resistant malignant cells. Furthermore, inflammation in the mouse autoimmune arthritis model was suppressed to greater extent by A7R-ADC conjugated to MMAE than by A7R-ADC-SN-38. Given that an increased proportion of IL-7R-positive cells is a common mechanism underlying the pathogenesis of autoimmunity, we found that specific depletion of this cell population abrogated the progression of disease. This suggests that the cytotoxicity and immunosuppressive capacity of A7R-ADC could be modulated to treat specific malignancies or autoimmune diseases through the introduction of different payloads, and represents a novel alternative to steroid therapy.

Lymphocyte development in neonatal and adult c-Kit-deficient (c-KitW/W) mice

Hematopoietic stem cells and lymphocyte progenitors express the receptor tyrosine kinase c-Kit. In fetal and neonatal life, c-Kit plays a redundant role in T, and no apparant role in B cell development. In neonatal mice deficient for both c-Kit and the common gamma chain (gammac), a component of the interleukin-7 (IL-7) receptor, the thymus is alymphoid, and therefore lacks T cell receptor (TCR) beta, gamma, and delta rearrangements. Thus, a critical role for c-Kit in T cell development around birth is well established. More recently, it has become possible to examine the impact of c-Kit deficiency under conditions of steady state lymphopoiesis in adult life. Such analysis has been made possible by the identification of a viable adult c-Kit-deficient (c-KitW/W) variant, termed the Vickid mouse. The Vickid mouse arose by outcrossing c-KitW-bearing mice of the WB strain, in which lack of c-Kit is lethal, to a mixed genetic background. In adult Vickid mice, mainstream alphabeta TCR+ thymocyte development, and B cell development in the bone marrow are severely c-Kit-dependent with progressive age. Analysis of other pathways of developing T cells, i.e. CD4-CD8- (double neagative [DN]) alphabeta TCR+ and DN gammadelta TCR+ thymocytes revealed that the development of both lineages is also severely affected by lack of c-Kit. However, numbers of gammadelta TCR+ T cells decline before numbers of alphabeta TCR+ T cells in the thymus. In contrast to T and B cell development, generation of NK cells is not affected in adult c-KitW/W mice.

Induction of pre-B cell proliferation after de novo synthesis of the pre-B cell receptor

The assembly of a pre-B cell receptor (pre-BCR) composed of an Ig mu heavy chain (mu H-chain), the surrogate light (SL) chain, and the Ig alpha/beta dimer is critical for late pro-B cells to advance to the pre-B cell stage. By using a transgenic mouse model, in which mu H-chain synthesis is solely driven by a tetracycline-controlled transactivator, we show that de novo synthesis of mu H-chain in transgenic pro-B cells not only induces differentiation but also proliferation. This positive effect of mu H-chain synthesis on proliferation requires the presence of SL chain and costimulatory signals provided by stromal cells or IL-7. We conclude that pre-BCR signaling induces clonal expansion of early pre-B cells.

Induction of pre-B cell proliferation after de novo synthesis of the pre-B cell receptor

The assembly of a pre-B cell receptor (pre-BCR) composed of an Ig mu heavy chain (mu H-chain), the surrogate light (SL) chain, and the Ig alpha/beta dimer is critical for late pro-B cells to advance to the pre-B cell stage. By using a transgenic mouse model, in which mu H-chain synthesis is solely driven by a tetracycline-controlled transactivator, we show that de novo synthesis of mu H-chain in transgenic pro-B cells not only induces differentiation but also proliferation. This positive effect of mu H-chain synthesis on proliferation requires the presence of SL chain and costimulatory signals provided by stromal cells or IL-7. We conclude that pre-BCR signaling induces clonal expansion of early pre-B cells.

Cloning of the murine thymic stromal lymphopoietin (TSLP) receptor: Formation of a functional heteromeric complex requires interleukin 7 receptor

The cellular receptor for murine thymic stromal lymphopoietin (TSLP) was detected in a variety of murine, but not human myelomonocytic cell lines by radioligand binding. cDNA clones encoding the receptor were isolated from a murine T helper cell cDNA library. TSLP receptor (TSLPR) is a member of the hematopoietin receptor family. Transfection of TSLPR cDNA resulted in only low affinity binding. Cotransfection of the interleukin 7 (IL-7)Ralpha chain cDNA resulted in conversion to high affinity binding. TSLP did not activate cells from IL-7Ralpha(-/)- mice, but did activate cells from gammac(-/)- mice. Thus, the functional TSLPR requires the IL-7Ralpha chain, but not the gammac chain for signaling.

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.

JAK-STAT signaling activated by Abl oncogenes

The Abl oncoproteins v-Abl and BCR-Abl can activate member of the signal transducers and activators of transcription (STAT) family of signaling proteins. The mechanisms by which these oncoproteins activate STATs appear to differ. In cells transformed by v-Abl, Janus kinase (JAK) tyrosine kinases are constitutively activated. In these cells, the v-Abl oncoprotein and the JAK kinases physically associate. Mapping of the JAK interaction domain in v-Abl demonstrates that amino acids within the carboxyl terminal region of v-Abl bind JAKs through a direct interaction. A mutant of v-Abl lacking this region does not bind or activate JAK 1 in vivo, fails to activate STAT proteins, does not induce cellular proliferation, and is less efficient in cellular transformation. Kinase inactive mutants of JAK 1 inhibit the ability of v-Abl to activate STATs, to induce cytokine-independent proliferation, and to transform bone marrow cells. Interestingly, these effects correlate with defects in the activation of several pathways by v-Abl including Akt, PI3-kinase, STATs, and Ras. These data suggest that Jak kinases may play an important role in v-Abl induced transformation. Oncogene (2000).

Identification of CD19(-)B220(+)c-Kit(+)Flt3/Flk-2(+)cells as early B lymphoid precursors before pre-B-I cells in juvenile mouse bone marrow

The combined analysis of the expression of receptor tyrosine kinases c-Kit and Flt3/Flk-2 and of the human CD25 gene expressed as a transgene under the regulation of the mouse lambda5 promoter in the bone marrow of 1-week-old mice allows us to identify three stages of B lymphocyte development before the CD19(+)c-Kit(+) pre-B-I cells. Single-cell PCR analysis of the rearrangement status of the Ig heavy chain alleles allows us to order these early stages of B cell development as follows: (i) B220(+)CD19(-)c-Kit(lo)Flt3/Flk-2(hi)lambda5(-), (ii) B220(+)CD19(-)c-Kit(lo)Flt3/Flk-2(hi)lambda5(+) and (iii) B220(+)CD19(+)c-Kit(lo)Flt3/Flk-2(lo)lambda5(+) before B220(+)CD19(+)c-Kit(lo)Flt3/Flk-2(-)lambda5(+) pre-B-I cells. All these progenitors are clonable on stromal cells in the presence of IL-7 and can differentiate to CD19(+)c-Kit(-) B-lineage cells. A combination of stem cell factor, Flt3 ligand and IL-7 was also able to support the proliferation and differentiation of the progenitors in a suspension culture. Furthermore, the analyses indicate that the onset of D(H)J(H) rearrangements precedes the expression of the lambda5 gene. These progenitor populations were characteristic of juvenile mice and could not be detected in the bone marrow of adult mice. Hence the expression pattern, and probably the function, of the receptor tyrosine kinases in early B cell differentiation appears to be different in juvenile and adult mice.

Cyclin A/CDK2 regulates V(D)J recombination by coordinating RAG-2 accumulation and DNA repair

Accumulation of the V(D)J recombinase protein RAG-2 is restricted to G0/G1 cells by phosphorylation-mediated degradation at the G1-S boundary. Here cyclin A/CDK2 is shown to oppose RAG-2 accumulation; conversely, RAG-2 is induced by p27Kip1 and related CDK inhibitors. Coinduction of RAG-2 and G1 delay by p27Kip1 is accompanied by strong stimulation of V(D)J recombination. Unexpectedly, induction of RAG-2 accumulation in the absence of G1 delay has no effect on recombination frequency. p27Kip1 may stimulate V(D)J recombination by coordinating accumulation of RAG-2 with prolongation of G1, when nonhomologous end joining is preferentially active. Consistent with this, enforced expression of RAG-2 throughout cell cycle is associated with accumulation of aberrant recombination products reminiscent of those formed in the absence of nonhomologous end joining.

Derivation of melanocytes from embryonic stem cells in culture

We report that embryonic stem (ES) cells were efficiently induced to differentiate to melanocytes in vitro. When undifferentiated ES cells were cocultured with a bone marrow-derived stromal cell line, a very small but significant number of melanocytes were reproducibly generated. This process was greatly enhanced by addition of dexamethasone to the culture and strictly depended on steel factor, the ligand for the c-Kit receptor tyrosine kinase. Expression of c-Kit on the precursor cells was confirmed by using SCL/tal-1-/- ES cells, which are defective for producing hematopoietic cells, which were thus ruled out as possible sources of nonmelanogenic c-Kit-expressing cells. The morphology, reactivity to growth factors, and expression of melanogenic markers of the cells generated all indicated unequivocally that these cells were melanocytes. This culture system may provide a potent tool for the study of development and function of melanocytes.

Regulation of E- and P-cadherin expression correlated with melanocyte migration and diversification

Melanocytes (Mc) and their progenitors melanoblasts (Mb) are derived from the neural crest and migrate along the dorsolateral pathway to colonize the dermis, the epidermis, and finally the hair matrix. To examine the involvement of cadherins in the migration of Mc lineage cells, we combined flow cytometric analysis of dissociated live cells with immunohistochemical staining of tissue sections to quantify the level of cadherin expression on the surface of Mb/Mc. At 11.5 days postcoitum, Mb are in the dermis and are E-cadherin(-)P-cadherin(-) (E-cad(-)P-cad(-)). During the next 48 h, a 200-fold increase of E-cadherin expression is induced on the surface of Mb prior to their entry into the epidermis, thereby forming a homogeneous E-cad(high)P-cad(-/low) population. The cadherin expression pattern then diversifies, giving rise to three populations, an E-cad(-)P-cad(-) dermal population, E-cad(high)P-cad(low) epidermal population, and E-cad(-)P-cad(med-high) follicular population. In all three populations, the patterns of expression are region-specific, being identical with those of surrounding cells such as keratinocytes and fibroblasts, and are preserved before and after pigmentation. While most of the epidermal Mb/Mc disappear after the neonatal stage in normal mice, forced expression of steel factor in the epidermis of transgenic mice promotes survival of epidermal Mb/Mc, maintaining epidermal-type cadherin expression pattern (E-cad(high)P-cad(low)) throughout the postnatal life. These findings indicate the involvement of extrinsic cues in coordinating the cadherin expression pattern of Mb/Mc and suggest a role for E- and P-cadherins in guiding Mc progenitors to their final destinations.

A bone marrow-derived stroma cell line, ST2, can support the differentiation of fetal thymocytes from the CD4+ CD8+ double negative to the CD4+ CD8+ double positive differentiation stage in vitro

T-cell precursors differentiate into mature T cells predominantly in the thymus. However, it has also been reported that T-cell precursors mature in extrathymic organs such as the liver, bone marrow, or intestines. In order to investigate the nature of the extrathymic microenvironment that supports T-cell maturation, we examined the effect of a bone marrow-derived stroma cell line, ST2, on T-cell precursors by using a reaggregate thymic organ culture (RTOC) system. We found that ST2 cells supported the differentiation of fetal thymocytes at day 14.5 of gestation from a CD4- CD8- double negative (DN) to a CD4+ CD8+ double positive (DP) differentiation stage in a manner similar to that observed in thymus. Anti-interleukin-7 receptor (IL-7R) and anti-c-kit antibodies blocked the growth of thymocytes in RTOC with ST2 cells, but did not inhibit the generation of DP thymocytes. These data indicate that a bone marrow-derived stroma cell, ST2, which supports B-cell differentiation, is also able to support T-cell development and may constitute one of the microenvironmental components for extrathymic T-cell development.

The role of cytokine receptor signaling in lymphocyte development

Cytokine receptor signaling plays an essential role in the early stages of lymphocyte development. Signals through various cytokine receptors - such as c-kit, flt3/flk2, CXCR4, the IL-7 receptor and the IL-15 receptor - are known to promote the expansion and survival of uncommitted progenitor cells as well as their migration to the appropriate microenvironment and subsequent differentiation into B, T or natural killer cells. The recent generation of mice deficient in one or more of these signaling pathways has revealed which cytokines play unique and/or redundant roles in each of these lymphocyte lineages during this developmental process.

Establishment and characterization of pro-B cell lines from motheaten mutant mouse defective in SHP-1 protein tyrosine phosphatase

Mice homozygous for the motheaten (Hcph(me)) mutation lack a functional SHP-1 protein tyrosine phosphatase, show severe immunologic dysregulation and die at an early age. Severe pneumonitis in me/me mice is associated with abnormal proliferation of macrophages and granulocytes. Overgrowth of macrophages in long term cultures of me/me bone marrow has prevented analyses of lymphopoiesis in vitro. To establish hematopoietic cell lines from me/me mice, we cultured me/me bone marrow with the PA6 stromal cell line in the presence of antagonistic antibody against the receptor (c-Fms) for macrophage colony stimulating factor (M-CSF). In these cultures, overgrowth of M-CSF-dependent macrophages was suppressed by the antagonistic antibody and other hemopoietic cell lineages were generated efficiently from me/me bone marrow. By using this culture system, we established me/me pro-B cell clones (MEBs) with rearranged DH-JH but not VH-DJH. The growth of MEB clones required IL-7 and c-Kit ligand, corresponding to normal pro-B cells which express SHP-1. MEB cells were sensitive to starvation by either IL-7 or c-Kit ligand, resulting in apoptotic death. The present culture system, which supports hematopoiesis of me/me bone marrow, provides useful tools for the determination of the role of SHP-1 in signal transduction of B lymphopoiesis.

Transgene expression of steel factor in the basal layer of epidermis promotes survival, proliferation, differentiation and migration of melanocyte precursors

Mutations at the murine dominant white spotting (KitW) and steel (MgfSl) loci, encoding c-Kit receptor kinase and its ligand respectively, exert developmental defects on hematopoietic cells, melanocytes, germ cells and interstitial cells of Cajal. The expression patterns of steel factor (SLF) observed in the skin and gonads suggest that SLF mediates a migratory or a chemotactic signal for c-Kit-expressing stem cells (melanocyte precursors and primordial germ cells). By targeting expression of SLF to epidermal keratinocytes in mice, we observed extended distribution of melanocytes in a number of sites including oral epithelium and footpads where neither melanocytes nor their precursors are normally detected. In addition, enlarged pigmented spots of KitW and other spotting mutant mice were observed in the presence of the SLF transgene. These results provide direct evidence that SLF stimulates migration of melanocytes in vivo. We also present data suggesting that SLF does not simply support survival and proliferation of melanocytes but also promotes differentiation of these cells. Unexpectedly, melanocyte stem cells independent of the c-Kit signal were maintained in the skin of the SLF transgenic mice. After the elimination of c-Kit-dependent melanoblasts by function-blocking anti-c-Kit antibody, these stem cells continued to proliferate and differentiate into mature melanocytes. These melanoblasts are able to migrate to cover most of the epidermis after several months. The SLF transgenic mice described in this report will be useful in the study of melanocyte biology.

Stromal cells and cytokines in the induction of recombination activating gene (RAG) expression in a human lymphoid progenitor cell

The activation of recombination activating genes (RAGs) plays critical roles in the V(D)J gene recombination machinery and lymphocyte repertoire formation. However, the regulation of RAG gene expression in humans as well as animals is poorly understood. We show that RAG gene expression is activated in a human lymphoid progenitor cell line (FL8.2.4.4) by coculturing them on a bone marrow-derived stromal cell line (PA6) in the presence of cytokines. The RAG transcripts become detectable in 12 hours after initiation of culture, and the increased level is sustained at 24 hours. Among the cytokines, IL-3, IL-6, and IL-7, but not IL-2, IL-4, SCF, GM-CSF induces RAG activation. IL-3, IL-6, and IL-7 exert their effect synergistically on RAG activation. A cognate interaction between FL8.2.4.4 cells and PA6 stromal cells seems to be prerequisite for RAG activation. RAG transcripts are inducible in FL8.2.4.4 cells when cocultured on paraformaldehyde fixed-PA6 stromal cells in the presence of cytokines. These data indicate that two separate signals are both required for induction of RAG activation in lymphoid progenitors; one from the cell surface molecule(s) on stromal cells, and the other from recombinant cytokine(s). The expression of RAG mRNA in FL8.2.4.4 cells is concomitant with induction of recombinase activity. Thus, this system may provide a useful means for further understanding of the mechanisms controlling RAG activation and lymphocyte development in human system.

Murine cutaneous mastocytosis and epidermal melanocytosis induced by keratinocyte expression of transgenic stem cell factor

The growth and differentiation of mast cells and melanocytes require stem cell factor (SCF), the ligand for the kit receptor tyrosine kinase. SCF may exist as a membrane-bound or soluble molecule. Abnormalities of the SCF-kit signaling pathway, with increased local concentrations of soluble SCF, have been implicated in the pathogenesis of the human disease cutaneous mastocytosis, but have not yet been shown to play a causal role. To investigate both the potential of SCF to cause mastocytosis and its role in epidermal melanocyte homeostasis, we targeted the expression of SCF to epidermal keratinocytes in mice with two different transgenes controlled by the human keratin 14 promoter. The transgenes contained cDNAs that either produced SCF, which can exist in both membrane-bound and soluble forms, or SCF, which remains essentially membrane bound. Murine epidermal keratinocyte expression of membrane-bound/ soluble SCF reproduced the phenotype of human cutaneous mastocytosis, with dermal mast cell infiltrates and epidermal hyperpigmentation, and caused the maintenance of a population of melanocytes in the interadnexal epidermis, an area where melanocytes and melanin are found in human skin but where they are not typically found in murine skin. Expression of membrane-bound SCF alone resulted in epidermal melanocytosis and melanin production, but did not by itself cause mastocytosis. We conclude, first, that a phenotype matching that of human mastocytosis can be produced in mice by keratinocyte overproduction of soluble SCF, suggesting a potential cause of this disease. Second, we conclude that keratinocyte expression of membrane-bound SCF results in the postnatal maintenance of epidermal melanocytes in mice. Since the resulting animals have skin that more closely approximates human skin than do normal mice, their study may be more relevant to human melanocyte biology than the study of skin of normal mice.

Antigen-receptor junctional diversity in growth-factor-receptor mutant mice

Precursor lymphocytes undergo expansion prior to immunoglobulin (Ig) or T cell receptor (TCR) rearrangements. Development of thymocytes, but not B cells, is entirely blocked in mice lacking both the receptor-tyrosine-kinase c-kit and the common cytokine receptor gamma chain (gamma c). In c-kit-gamma c-mice, TCR beta rearrangements are limited to mono- or oligoclonal DJ junctions. Here, effects of lack of c-kit or gamma c, or both, on the junctional diversity of TCR gamma and delta, and Ig VH(DH)JH loci were analyzed. All rearrangements were present in wildtype and mutant mice. However, sequencing of the junctions revealed monoclonal TCR gamma (V gamma 2 J gamma 1) and TCR delta (V delta 1(D delta)J delta 2) joints in c-kit-gamma c-, but not c-kit+ gamma c- or wildtype thymocytes. In contrast to TCR beta, gamma and delta loci, VHDHJH junctions were more diverse in c-kit-gamma c-mice. Thus, the two analyzed growth factor receptors mediate signaling pathways required for progenitor expansion and generation of junctional diversity at TCR loci, but have less influence on the diversity of IgH junctions.

C-kit receptors in childhood malignant lymphoblastic cells

The product of the protooncogene c-kit is the receptor for the hematopoietic cytokine stem cell factor (SCF). C-kit is expressed on leukemic cells of the erythroid, myeloid, and mast-cell lineage and SCF has a proliferative effect on some of these cells. The role of SCF and c-kit in lymphoid malignancies is much less clear. Here we review the role of c-kit in normal lymphopoiesis and summarize its role in lymphoid malignancies. C-kit is expressed in normal lymphopoiesis and its ligand SCF synergizes with IL-7 to enhance the proliferation of B- and T-cell progenitors. In malignant lymphopoiesis, c-kit can also be expressed in B and T-lymphoblastic cells from children with non Hodgkin's lymphoma (NHL) or acute lymphoblastic leukemia (ALL) when analyzed by the highly sensitive reverse transcriptase polymerase chain reaction (RT-PCR). While c-kit receptors were detected by flow cytometric (FCM) analysis on about 40% of fresh T-lymphoblastic biopsy tumor cell preparations or T-lymphoblastic cell lines, no receptors were detected on B-lymphoblastic fresh cells or cell lines from children with B-ALL or Burkitt's lymphoma (BL). Almost all of the lymphoblastic cells expressing c-kit protein responded to recombinant human (rh)SCF with a downregulation of c-kit receptors. A proliferative response was detected only in a minority of these cells. B-ALL or BL cell lines showed no response to rhSCF. Upregulation of c-kit in T-lymphoblastic cells could be demonstrated by the addition of IL-1 beta, TNF-alpha, TGF-beta or A23187, and downregulation by rhSCF or phorbol myristate acetate (PMA). Despite upregulation of c-kit mRNA, protein remained undetectable on B-ALL or BL cells in the presence of A23187. The metabolic state of the cells seemed to influence c-kit expression, since c-kit was upregulated in T-lymphoblastic cells by the addition of new medium. C-kit appears to play a role in the growth of some malignant T-lymphoblastic but not B-lymphoblastic cells.

Interactions Between c-kit and Stem Cell Factor Are Not Required for B-Cell Development In Vivo

The receptor-type tyrosine kinase, c-kit is expressed in hematopoietic stem cells (HSC), myeloid, and lymphoid precursors. In c-kit ligand-deficient mice, absolute numbers of HSC are mildly reduced suggesting that c-kit is not essential for HSC development. However, c-kit− HSC cannot form spleen colonies or reconstitute hematopoietic functions in lethally irradiated recipient mice. Based on in in vitro experiments, a critical role of c-kit in B-cell development was suggested. Here we have investigated the B-cell development of c-kitnull mutant (W/W ) mice in vivo. Furthermore, day 13 fetal liver cells from wild type or W/W mice were transferred into immunodeficient RAG-2−/− mice. Surprisingly, transferred c-kit− cells gave rise to all stages of immature B cells in the bone marrow and subsequently to mature conventional B2, as well as B1, type B cells in the recipients to the same extent as transferred wild type cells. Hence, in contrast to important roles of c-kit in the expansion of HSC and the generation of erythroid and myeloid lineages and T-cell precursors, c-kit− HSC can colonize the recipient bone marrow and differentiate into B cells in the absence of c-kit.

Preferential proliferation of murine colony-forming units in culture in a chemically defined condition with a macrophage colony-stimulating factor-negative stromal cell clone

The establishment of culture conditions that selectively support hematopoietic stem cells is an important goal of hematology. In this study, we investigated the possibility of using for this purpose a defined medium, mSFO2, which was developed for stromal cell-dependent bone marrow cultures. We found that a combination of epidermal growth factor (EGF), the OP9 stromal cell line, which lacks macrophage colony-stimulating factor, recombinant stem cell factor, and the chemically defined medium mSFO2 provides a microenvironment where c-Kit+ Thy-1+/lo Mac-1+/lo B220- TER119- common beta + IL-2R gamma + gp130+ cells are selectively propagated from normal, unfractionated bone marrow cells. This cell population produced an in vitro colony at a very high efficiency (50%), whereas it has only limited proliferative ability in the irradiated recipient. Thus, the cells selected in this culture condition might represent colony-forming units in culture (CFU-c) with short-term reconstituting ability. Transferring this cell population into medium containing differentiation signals resulted in the rapid production of mature myelomonocytic and B cell lineages in vitro and in vivo. The fact that a similar culture condition was created by erb-B2-transduced OP9 in the absence of EGF indicated that EGF exerts its effect by acting on OP9 rather than directly on CFU-c. These results suggested that the balance between self-renewal and differentiation of CFU-c can be regulated by extra-cellular signals.

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.

Identification of novel lymphoid tissues in murine intestinal mucosa where clusters of c-kit+ IL-7R+ Thy1+ lympho-hemopoietic progenitors develop

We have revealed that about one and a half thousand tiny clusters, filled with one thousand closely packed lymphocytes, can be found throughout the murine small and large intestinal mucosa. They are located in crypt lamina propria (cryptopatches; CP) and can be first detected at 14-17 d after birth. A large fraction of lymphocytes in CP expresses c-kit, IL-7R, Thy1 and a lymphocyte function-associated antigen, LFA-1, whereas most of them remain CD3-, TCR alpha beta-, TCR gamma delta-, sIgM-, and B220-. The population size of IL-2R alpha+, HSA+ and Pgp-1+ subsets is variable (20-50%) and the composition of CD8+, Ly-1+, and CD4+ subsets is smaller but also variable (3-20%). In the small intestine, CP do not contain cells undergoing apoptosis nor cells bearing RAG-1 molecules, but do contain dendritic stromal cells bearing CD11c/CD18 molecules. The frequency of DNA replicating cells in CP is higher than that in Peyer's patches (PP), is lower than that in the thymic cortex and is almost comparable with that in the thymic medulla. The numbers of CP remain the same in aged mice (> 114 wk) but double after estrogen treatment even though the thymi are attenuated sharply in both conditions. Thus, with respect to histogenesis, lymphocyte composition and tissue level of cellular behavior, neither PP, isolated lymphoid follicles, peripheral LNs, nor thymus are identical with CP. Finally, CP are virtually absent in lamina propria of IL-7R-deficient mice that display a profound reduction in thymic and peripheral lymphoid cellularity. By contrast, CP are present in germ-free mice and in athymic (nu/nu), SCID, TCR beta x delta-/-, RAG-2-/-, PP-deficient (aly/aly), stem cell factor (Sl/Sld) and c-kit (W/Wv) mutant mice. Taking all of these results together, CP are the first identification of gut-associated murine lymphoid tissues where the generation of IL-7-dependent lympho-hematopoietic progenitors for T and/or B cell descendants may start to take place at the age of commencement of weaning.

Signalling in lymphocyte development

A number of important signal-transduction molecules that regulate lymphocyte maturation and proliferation have been identified. These advances provide a platform for studies on how different signalling events are integrated to generate the required number of lymphocytes with an appropriate antigen receptor repertoire.

Circulation of hematopoietic progenitors in the mouse embryo

We used a sensitive in vitro culture system to follow, in embryonic blood, the number and state of commitment of B cell precursors along ontogeny. We describe a wave of circulating multipotent progenitors, first detectable at day 10 of gestation, and reaching a maximum in absolute numbers at day 12. They are undetectable by day 14 of gestation, when committed B cell precursors can be detected in fetal liver. Embryonic marrow contains B cell progenitors by day 15. We propose that fetal liver, thymus, and bone marrow are colonized by the same wave of multipotent hematopoietic cells and define embryonic blood at day 11 of gestation as an important source of multipotent hematopoietic cells, virtually deprived of committed and mature contaminants.

Involvement of Fyn tyrosine kinase in progression of cytokinesis of B lymphocyte progenitor

We analyzed the role of Fyn tyrosine kinase in cell cycle progression of B lymphocyte progenitor (pro B cell). Whereas there were no substantial defects in the intramarrow B cell genesis in the fyn(-) mouse, and long-term proliferation of fyn(-) pro B cells was maintained in vitro under a serum containing culture condition, the cell cycle was arrested at G2/M upon serum deprivation. Morphological analyses demonstrated that the cytokinesis of fyn(-) pro B cells was retarded in the presence of serum and that the entry of fyn(-) pro B cells into late telophase was completely blocked under the serum-free condition. In contrast, the earlier phases of mitosis of fyn(-) pro B cells proceeded normally without FCS. This failure to initiate late telophase resulted in the accumulation of elliptical binucleated cells that might be the outcome of the nuclear division without cytokinesis. Consistent with this defect in the progression of cytokinesis, Fyn was localized in the midspace of dividing pro B cells at anaphase. These results suggested that Fyn localizes at the midspace of dividing pro B cells and regulates the progression of cytokinesis.