Highly restricted expression of a stromal cell determinant in mouse bone marrow in vivo

Murine Bone Marrow Niches from Hematopoietic Stem Cells to B Cells

After birth, the development of hematopoietic cells occurs in the bone marrow. Hematopoietic differentiation is finely tuned by cell-intrinsic mechanisms and lineage-specific transcription factors. However, it is now clear that the bone marrow microenvironment plays an essential role in the maintenance of hematopoietic stem cells (HSC) and their differentiation into more mature lineages. Mesenchymal and endothelial cells contribute to a protective microenvironment called hematopoietic niches that secrete specific factors and establish a direct contact with developing hematopoietic cells. A number of recent studies have addressed in mouse models the specific molecular events that are involved in the cellular crosstalk between hematopoietic subsets and their niches. This has led to the concept that hematopoietic differentiation and commitment towards a given hematopoietic pathway is a dynamic process controlled at least partially by the bone marrow microenvironment. In this review, we discuss the evolving view of murine hematopoietic–stromal cell crosstalk that is involved in HSC maintenance and commitment towards B cell differentiation.

In vitro Differentiation Potential of Mesenchymal Stem Cells

SUMMARY: Mesenchymal stem cells (MSCs) represent a class of multipotent progenitor cells that have been isolated from multiple tissue sites. Of these, adipose tissue and bone marrow offer advantages in terms of access, abundance, and the extent of their documentation in the literature. This review focuses on the in vitro differentiation capability of cells derived from adult human tissue. Multiple, independent studies have demonstrated that MSCs can commit to mesodermal (adipocyte, chondrocyte, hematopoietic support, myocyte, osteoblast, tenocyte), ectodermal (epithelial, glial, neural), and endodermal (hepatocyte, islet cell) lineages. The limitations and promises of these studies in the context of tissue engineering are discussed.

CD10+ stromal cells form B-lymphocyte maturation niches in the human bone marrow

In adult mammals, early B-lymphopoiesis takes place in the bone marrow in close association with stromal cells. Both the phenotype of the stromal cells and the molecules involved in this essential interaction are as yet inadequately described. In this study, all benign, differentiating B-cells (Pax-5+ lymphoid cells) are shown, by using two-colour immunohistochemistry on biopsies from human bone marrow, to be in close contact with scant dendritic CD10+ stromal cells until they leave via the sinusoids. This CD10+ stromal cell population does not fully overlap with the VCAM-1+ stromal cell population. Furthermore, using a set of B-cell differentiation markers (TdT, Pax-5, and CD20), B-cell development is shown to be spatially oriented, with maturation progressing towards bone marrow sinusoids. In conclusion, CD10+ stromal cells form distinct B-lymphocyte maturation niches in the human bone marrow.

Expression of CD28 by bone marrow stromal cells and its involvement in B lymphopoiesis

Young mice lacking CD28 have normal numbers of peripheral B cells; however, abnormalities exist in the humoral immune response that may result from an intrinsic defect in the B cells. The goal of this study was to assess whether CD28 could be involved in the development of B cells. CD28 mRNA was detected preferentially in the fraction of bone marrow enriched for stromal cells. Flow cytometry and RT-PCR analysis demonstrated that CD28 was also expressed by primary-cultured stromal cells that supported B lymphopoiesis. Confocal microscopy revealed that in the presence of B-lineage cells, CD28 was localized at the contact interface between B cell precursors and stromal cells. In addition, CD80 was detected on 2-6% of freshly isolated pro- and pre-B cells, and IL-7 stimulation led to induction of CD86 on 15-20% of pro- and pre-B cells. We also observed that stromal cell-dependent production of B-lineage cells in vitro was greater on stromal cells that lacked CD28. Finally, the frequencies of B-lineage precursors in the marrow from young (4- to 8-wk-old) CD28(-/-) mice were similar to those in wild-type mice; however, older CD28(-/-) mice (15-19 mo old) exhibited a 30% decrease in pro-B cells and a 50% decrease in pre-B cells vs age-matched controls. Our results suggest that CD28 on bone marrow stromal cells participates in stromal-dependent regulation of B-lineage cells in the bone marrow. The localization of CD28 at the stromal cell:B cell precursor interface suggests that molecules important for T cell:B cell interactions in the periphery may also participate in stromal cell:B cell precursor interactions in the bone marrow.

Pre-pro-B cell growth-stimulating factor (PPBSF) upregulates IL-7Ralpha chain expression and enables pro-B cells to respond to monomeric IL-7

Although pro-B cells are well represented in IL-7 knockout (KO) mice, they express abnormally low concentrations of the interleukin-7 receptor alpha-chain (IL-7Ralpha) and do not generate pre-B cells. Here, we demonstrate that pro-B cells from IL-7 KO mice can be induced to generate pre-B cells and immature B cells by exposure to recombinant IL-7 (rIL-7) in vivo but not in vitro. Experiments in recombinant activation gene-1 (RAG-1) KO mice indicate that the in vitro unresponsiveness of IL-7(-/-) pro-B cells to rIL-7 is unrelated to the absence of a functional pre-B cell receptor (pre-BCR). Rather, it appears to be due to the suboptimal expression of the IL-7Ralpha chain. Thus, IL-7(-/-) pro-B cells readily respond to rIL-7 in vitro if IL-7Ralpha chain expression is first upregulated by exposure to IL-7 in vivo or to IL-7(+/+) bone marrow (BM) stromal cells or conditioned medium (CM) therefrom in vitro. Similar results were obtained when pro-B cells from IL-7 KO mice were cultured on IL-7(-/-) BM stromal cells in the presence of rIL-7. This suggested that the recently described pre-pro-B cell growth-stimulating factor (PPBSF), a self-assembling hybrid cytokine comprising IL-7 and the stromal cell-derived hepatocyte growth factor beta-chain (HGFbeta), is required to stimulate pro-B cells from IL-7 KO mice. This inference was verified by demonstrating that purified PPBSF upregulates IL-7Ralpha chain expression on IL-7(-/-) pro-B cells in vitro and enables them to respond to rIL-7 in a stepwise manner. We, therefore, postulate that PPBSF is the operative form of IL-7 that normally induces IL-7Ralpha(lo) pre-pro-B cells to proliferate and differentiate into IL-7Ralpha(hi) pro-B cells, which then proliferate and differentiate into pre-B cells on stimulation with monomeric IL-7.

Surface protein characterization of human adipose tissue-derived stromal cells

Human bone marrow stromal cells are a multipotent population of cells capable of differentiating into a number of mesodermal lineages as well as supporting hematopoeisis. Their distinct protein and gene expression phenotype is well characterized in the literature. Human adipose tissue presents an alternative source of multipotent stromal cells. In this study, we have defined the phenotype of the human adipose tissue-derived stromal cells in both the differentiated and undifferentiated states. Flow cytometry and immunohistochemistry show that human adipose tissue-derived stromal cells have a protein expression phenotype that is similar to that of human bone marrow stromal cells. Expressed proteins include CD9, CD10, CD13, CD29, CD34, CD44, CD 49(d), CD 49(e), CD54, CD55, CD59, CD105, CD106, CD146, and CD166. Expression of some of these proteins was further confirmed by PCR and immunoblot detection. Unlike human bone marrow-derived stromal cells, we did not detect the STRO-1 antigen on human adipose tissue-derived stromal cells. Cells cultured under adipogenic conditions uniquely expressed C/EBPalpha and PPARdelta, two transcriptional regulators of adipogenesis. Cells cultured under osteogenic conditions were more likely to be in the proliferative phases of the cell cycle based on flow cytometric analysis of PCNA and Ki67. The similarities between the phenotypes of human adipose tissue-derived and human bone marrow-derived stromal cells could have broad implications for human tissue engineering.

The monoclonal antibodies 18d7/91f2 recognize a receptor regulatory protein on mouse bone marrow stromal cells

Two monoclonal antibodies 18D7 and 91F2 were developed by immunizing rats with the mouse bone marrow-derived osteogenic cell line MN7. Hybridomas secreting rat antibodies against MN7 cell surface markers were selected by flow cytometry analysis. Both the monoclonal antibody 18D7 and the monoclonal antibody 91F2 are directed against the same cell surface antigen present on MN7 cells. Here, we report on the immunopurification of the 18D7/91F2 antigen and its identification as the prostaglandin F2 alpha receptor regulatory protein (FPRP). FPRP is expressed as a single messenger RNA (mRNA) of approximately 6 kilobases (kb) in MN7 cells and is differentially expressed in developing osteogenic cultures of bone marrow cells of the mouse. However, addition of the monoclonal antibodies 18D7 and 91F2 to these cultures did not inhibit bone formation in vitro. Both monoclonal antibodies reacted with mouse stromal cell lines established from bone marrow, thymus, spleen, and mandibular condyles. Immunohistochemical analysis of mature tibia of mice using the monoclonal antibody 18D7 revealed the presence of a distinct population of bone marrow cells close to trabecular and endosteal bone surfaces. In the central bone marrow, hardly any positive cells were found. In 17-day-old fetal mouse radius 18D7 immunoreactivity was restricted to cells in the periosteum in close vicinity to the bone collar. Mature osteoblasts, osteoclasts, osteocytes, growth plate chondrocytes, and mature macrophages were all negative. Taken together, these results suggest that FPRP plays a role in the osteogenic differentiation 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.

Characterization and Differentiation of an Early Murine Yolk Sac-Derived IL-7-Independent Pre-Pro-B Cell Line

We describe a unique, stable pre-pro-B cell line (YS-PPB) derived from AA4.1+ yolk sac cells from day 10 mouse embryos. This cell line, discovered fortuitously during the course of studies of in vitro B cell differentiation, is independent of IL-7 supplementation for long term expansion in vitro. YS-PPB cells as well as clonal sublines expressed AA4.1, CD43, B220, Sca-1, CD19, heat stable antigen, MHC class I, IL-7R, and FcγR, but did not express cytoplasmic μ-chain, surface IgM (sIgM), or MHC class II molecules. PCR analysis showed that the cells expressed TdT, λ5, and RAG-1 genes, but that their Ig genes were still in germline configuration. The cell line was dependent on direct contact with S17 stromal cells for growth, but, in contrast to bone marrow stem cells, required no additional growth factors for maintenance and expansion. When stimulated with IL-7 and LPS, YS-PPB cells and cells from all tested clonal sublines differentiated into sIgM+ B cells in vitro. Irradiated mice reconstituted with YS-PPB cells yielded spleens containing 38% sIgM+ donor-derived B cells, demonstrating that YS-PPB cells, although stably arrested in development at the boundary between pre-pro-B and pro-B stages of B cell differentiation, still retain their competence to differentiate into mature, Ig-producing B cells when transferred to a normal host environment. Thus, this new cell line can provide a reproducible source of B cell precursors arrested at that critical time in B cell differentiation when the machinery for Ig gene rearrangement is in place but rearrangement has not yet occurred.

Stromal Cell-Independent Maturation of IL-7-Responsive Pro-B Cells

The proliferation, survival, and differentiation of B cell progenitors in primary hematopoietic tissues depends on extracellular signals produced by stromal cells within the microenvironment. IL-7 is a stromal-derived growth factor that plays a crucial role in B lineage development. We have shown that in the presence of IL-7, pro-B cells proliferate and differentiate to a stage in which they are responsive to stromal cells and LPS, leading to terminally differentiated IgM-secreting plasma cells. In this report, we examine in detail the role of stromal cells in the transition from the IL-7-responsive pro-B cell stage to the mature LPS-responsive B cell stage. We demonstrate that this transition fails to occur, even in the presence of stromal cells and LPS, if constant exposure to IL-7 is maintained. The transition from the large pro-B cell stage to the small cμ+ pre-B cell stage occurs independent of stromal cells. Moreover, the “stromal cell-dependent” maturation that occurs subsequent to the expression of surface IgM leading to responsiveness to B cell mitogens can also be accomplished in the absence of stromal cells if pre-B cells are cultured in proximity to each other or at high cell concentrations. Together these results suggest that stromal cells mediate B cell differentiation by providing the necessary growth requirements (i.e., IL-7) to sustain the development of pre-B cells. The progeny of these pre-B cells can then differentiate through as yet unidentified homotypic interactions, leading to the production of LPS-responsive B cells.

Establishment and characterization of an immortal macrophage-like cell line inducible to differentiate to osteoclasts

Osteoclasts are hematopoietic cells essential for bone remodeling and resorption. To understand the process of osteoclast development, we established a macrophage-like cell line C7 that has the potential to differentiate into functional osteoclasts; multinuclear tartrate-resistant acid phosphatase positive cells capable of forming pits on dentin slices. C7 cells share the characteristics of their cell surface molecules and phagocytic activity with macrophages. Generation of osteoclasts from C7 cells was mostly suppressed by the addition of a function-blocking antibody directed to c-Fms, the receptor for macrophage-colony stimulating factor (M-CSF), or by osteoclastogenesis inhibitory factor (OCIF). These responses correspond well with the osteoclast precursors present in bone marrow and peritoneal cavity. Reagents such as bacterial lipopolysaccharide and granulocyte/macrophage-CSF that are known to act as inducers for other cell lineages rather than osteoclasts abolished the potential of osteoclastogenesis in C7 cells. These phenotypes of C7 cells have been stably maintained for more than 2 years. We believe that the cell line established in this study will provide an important tool for osteoclast biology.

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.

Impaired bone marrow microenvironment and immune function in T cell protein tyrosine phosphatase-deficient mice

The T cell protein tyrosine phosphatase (TC-PTP) is one of the most abundant mammalian tyrosine phosphatases in hematopoietic cells; however, its role in hematopoietic cell function remains unknown. In this report, we investigated the physiological function(s) of TC-PTP by generating TC-PTP-deficient mutant mice. The three genotypes (+/+, +/-, -/-) showed mendelian segregation at birth (1:2:1) demonstrating that the absence of TC-PTP was not lethal in utero, but all homozygous mutant mice died by 3-5 wk of age, displaying runting, splenomegaly, and lymphadenopathy. Homozygous mice exhibited specific defects in bone marrow (BM), B cell lymphopoiesis, and erythropoiesis, as well as impaired T and B cell functions. However, myeloid and macrophage development in the BM and T cell development in the thymus were not significantly affected. BM transplantation experiments showed that hematopoietic failure in TC-PTP -/- animals was not due to a stem cell defect, but rather to a stromal cell deficiency. This study demonstrates that TC-PTP plays a significant role in both hematopoiesis and immune function.

The genesis, tutelage and exodus of B cells in the ileal Peyer's patch of sheep

The ileal Peyer's patch (PP) is a prominent lymphoid organ that extends 1-2 meters along the terminal small intestine of sheep. It is comprised of rapidly proliferating B cells that make major contributions to the animals total B cell system. The characteristics of this tissue in sheep have enabled a variety of novel approaches to studying both the B cell system and the contribution of PP to the mucosal immunity. The sheep ileal PP has characteristics that place it in a category similar to that of the thymus, bone marrow and the avian bursa of Fabricius. The ileal PP develops before birth and involutes while the sheep is still young. It produces B cells that populate the immune system but most of the large numbers of newly-formed B cells are rapidly destroyed by apoptosis. It has been concluded that this death is related to a selection event that examines each newly-formed cell. Antibody diversity in sheep is a post-rearrangement event, generated by the process of somatic hypermutation. A comparison of the lambda light chain gene from surviving and dying B cells indicates that when the PP is at its greatest size the dying cells have the characteristics of cells with high affinity receptors for a selecting ligand/antigen. It is proposed that if B cells proliferate in the continual presence of selecting ligand/antigen that the B cell receptor might develop a sufficient affinity to trigger apoptosis. This process might contribute to elimination of clones with high affinity for either self antigens or continually present environmental antigens.

The role of stromal cell heparan sulphate in regulating haemopoiesis

Intimate contact between haemopoietic progenitor cells and elements of the bone marrow stroma is required for progenitor cell proliferation and differentiation. It is believed that the stroma provides particular niches for the development of haemopoietic cells of different lineages. Cytokines, stromal cell surface molecules and molecules of the stromal extracellular matrix all contribute to defining these microenvironmental niches. Data obtained using an in vitro model of haemopoiesis support the view that progenitor cell adhesion to stroma is mediated by multiple receptor-ligand interactions. The possibility of a tethering step, mediated by the engagement of stromal cell heparan sulphate with its ligands on the progenitor cells, preceding stable cell adhesion is discussed. The role of stromal cell heparan sulphate is likely to include cytokine presentation to progenitors as well as the tethering of progenitors to stroma. It is proposed that intracellular signals induced by progenitor cell adhesion to stroma act in association with cytokine induced signals to regulate progenitor cell proliferation and differentiation.

In-vitro models of stroma-dependent lymphopoiesis

The differentiation of hematopoietic stem cells into lymphocytes can be replicated ex vivo under the inductive influence of the stromal cells that frame the bone marrow and thymus. We summarize hereafter the development of culture systems where lymphopoiesis-supporting cell compartments are maintained in either their normal three-dimensional arrangement, in organotypic culture, or as culture dish-adherent monolayers and review the recent and current uses of those in-vitro models to investigate T- and B-cell differentiation in mouse and man.

A minimal conditioning approach to achieve stable multilineage mouse plus rat chimerism

Transplantation of untreated rat bone marrow into lethally irradiated (950 cGy) mouse recipients results in durable xenogeneic (rat-->mouse) chimerism and confers donor-specific transplantation tolerance for subsequent xenografts. The purpose of the present study was to characterize the minimal dose of total body irradiation (TBI) which would allow engraftment of rat bone marrow in mouse recipients. We report here that durable and stable lymphohaematopoietic cross-species chimerism can be achieved using a less than totally ablative radiation-based conditioning approach. The percentage of B10 mouse recipients which engrafted with rat bone marrow cells correlated with the dose of TBI. Engraftment of rat bone marrow stem cells occurred in all animals receiving 750 cGy prior to bone marrow transplantation, while no engraftment was detected at doses less than 650 cGy. Although most of the recipients were repopulated with mixed mouse and rat multilineage chimerism, some exhibited a predominance of rat cells. Although mixed xenogeneic rat/mouse chimeras prepared by lethal TBI produced only mouse derived RBC (red blood cells), chimeras prepared by sublethal conditioning produced both rat and mouse RBC. Only animals with detectable chimerism exhibited specific functional transplantation tolerance to donor xenoantigens, as assessed in vitro by mixed lymphocyte reaction assay. This model may offer an in vivo approach to study the role of species-specific growth factors in stem cell biology as well as the mechanisms for the induction of tolerance across species barriers.

Murine B cell development: commitment and progression from multipotential progenitors to mature B lymphocytes

B lymphocytes, the cellular source of antibody, are critical components of the immune response. They develop from multipotential stem cells, progressively acquiring the traits that allow them to function as mature B lymphocytes. This developmental program is dependent on appropriate interactions with the surrounding environment. These interactions, mediated by cell-cell and cell-matrix interactions, provide the growth and differentiation signals that promote progression along the developmental pathway. This chapter addresses the properties of developing B lineage cells and the nature of the environmental signals that support B lineage progression.

Apoptosis and macrophage-mediated cell deletion in the regulation of B lymphopoiesis in mouse bone marrow

Studies of cell population dynamics and microenvironmental organization of B lymphopoiesis in the bone marrow of normal mice and in various genetically modified states have shown that cell loss, involving processes of apoptosis and macrophage-mediated cell deletion, is a prominent feature of the primary genesis of B lymphocytes. Balanced against the influence of proliferative stimulants, the programmed death of precursor B cells provides a quantitative control, determining the magnitude of the final output of functional B lymphocytes to the peripheral immune system. The cell loss mechanisms can be readily set in motion by external or systemic influences, making the B-cell output particularly vulnerable to suppression by ionizing irradiation, stress or other systemic mediators. In addition, however, cell loss exerts an important quality control in the formation of the primary B-cell repertoire. The combination of apoptosis and macrophage-mediated deletion, acting at successive stages of B-cell differentiation, efficiently eliminates many precursors having non-productive Ig gene rearrangements, cell cycle dysregulations, and certain autoreactive Ig specificities. Outstanding areas of further work abound. Important questions concern the nature of mechanisms which underlie the processes of B-cell apoptosis and macrophage deletion in bone marrow, the microenvironmental signals involved in B-cell life or death decisions and genetic factors which may override these B-cell culling mechanisms. The answers will be relevant to problems of autoimmune disease, humoral immunodeficiency and B-cell neoplasia.

Recognition of murine integrin beta 1 by a rat anti-stromal cell monoclonal antibody

Previous studies with the rat monoclonal antibody KMI6 had localized its antigen in vivo to a discrete subpopulation of marrow stromal cells. The KMI6 antigen has now been identified as the murine homolog of integrin beta 1 by amino acid sequence analysis and by cross-reactivity with antiserum to the avian integrin beta 1. The relative tissue abundance of murine integrin beta 1 was determined by Western blot. Although immunoperoxidase staining of fixed murine hematopoietic tissues demonstrated an abundance of intracellular beta 1, few primary-derived cells of lymphohematopoietic origin were surface positive as assessed by immunofluorescence and flow cytometry. Fetal erythroblasts provided the only exception. In contrast, the antigen was readily detected on the surface of several cultured cell lines in association with a variety of alpha chains. The biochemical properties of the surface labeled murine integrin beta 1 were similar to those of its human counterpart, exhibiting an altered electrophoretic migration under reduced conditions or following N-glycanase treatment. The antibody recognition of the protein was insensitive to glycosylation state, presence of divalent cations, detergents, or transfer to a nitrocellulose membrane. However, on Western blot, the epitope was lost on reduction of the protein, suggesting that it is conformation dependent. These data indicate that although KMI6 epitope is widely distributed, its surface expression in vivo may be restricted within lymphohemopoietic tissues.

Evidence for a stromal cell-dependent, self-renewing B cell population in lymphoid follicles of the ileal Peyer's patch of sheep

Lymphoid follicles of the ileal Peyer's patch (PP) of young sheep function as the major source of B cells and a site of immunoglobulin (Ig) receptor diversification. However, extensive cell death in culture has restricted investigations of ileal PP follicular (iPf)B cell biology. We investigated the possibility that sustained iPfB cell proliferation may require an interaction with mesenchymal stromal cells (SC). Four SC lines, cloned from lymphoid follicles of the ileal PP, and various sheep and xenogeneic mesenchymal cells were used to characterize the nature of iPfB cell-SC interactions. A sustained proliferative response was unique to iPfB cells, required iPfB cell-SC contact, and SC membranes functioned as intact SC to either enhance or inhibit iPfB cell proliferative responses. The iPfB cell proliferation in SC co-cultures was accompanied by extensive cell death and a slow decline in viable cell number. Flow cytometric analysis confirmed that viable lymphocytes, present in SC co-cultures, were immature B cells that expressed surface IgM, with either lambda or kappa. Ig light chain, and that SC co-culture inhibited iPfB cell differentiation. Finally, addition of soluble anti-sheep Ig to iPfB cell-SC co-cultures did not inhibit SC-dependent iPfB cell proliferation or iPfB cell binding to SC. These data indicate that an interaction between specific SC membrane molecules and non-Ig molecules of iPfB cells either supported or inhibited a self-renewing proliferative response by immature (sIgMLo, BAQ44A-) iPfB cells. Finally, SC-dependent iPfB cell proliferation was independent of T cells and extrinsic antigen which further suggests that a functionally distinct B cell population resides in lymphoid follicles of the ileal PP.

Influence of growth factors and substrates on differentiation of haemopoietic stem cells

During the last few years there has been major progress in our understanding of the mechanisms underlying the regulation of haemopoietic stem cell proliferation and development. In the past 12 months, advances have been made in identifying how growth factor receptor expression is regulated in primitive haemopoietic cells, in determining the transcription factors that are associated with development, and in recognizing some of the specific molecular interactions that occur between the bone marrow stromal cells and the haemopoietic progenitor cells. These and previous studies clearly demonstrate that the environment can influence the survival, the proliferation and the differentiation of haemopoietic cells at every stage of development.

Generation and regeneration of cells of the B-lymphocyte lineage

B lymphopoiesis in the mouse and in man can be divided into two phases, one in which the compartments of the B lineage are filled with cells, and a second in which these compartments are maintained by regeneration, turnover and selection. Once the immune system has been built it contains around 5 x 10(8) and 10(12) cells of the B-lymphocyte lineage in the mouse and in man, respectively, of which nearly 10 per cent are precursors that are active in regeneration, whereas over 90 per cent are resting, mature B cells.

Phenotype and function of stromal cells cloned from the ileal Peyer's patch of sheep

The ileal Peyer's patch (PP) is the major site of B cell production and immunoglobulin diversification in lambs, but the factors which regulate these processes are poorly understood. As a first step toward identifying possible regulatory mechanisms, stable long-term cultures of ileal PP stromal cells were established at the clonal level. Four distinct cell types were identified by their phenotype and growth requirements. Immunohistochemical staining confirmed that all clones were mesenchymal (vimentin+; cytokeratin-) in origin and were negative for T cell, B cell, and macrophage markers. Three cell lines were negative for major histocompatibility complex (MHC) I and II molecules, but one cell line, SCN, expressed MHC I, MHC II and CD44 molecules, and a subpopulation of SCN cells expressed BAQ44A, a B cell differentiation molecule. The four cell lines produced different types and amounts of extracellular matrix proteins, and their growth was not influenced by exogenous human interleukin 1 (IL-1), IL-2, transforming growth factor beta 1 (TGF beta 1), or bovine fibroblast growth factor (FGF) but was influenced by serum. When tested for their capacity to support lymphocyte growth, all clones produced a soluble factor(s) that was mitogenic for ileal and jejunal PP cells and thymocytes. Similar growth promoting activity was observed with culture supernatants of murine, human and bovine fibroblasts but could not be reproduced using recombinant human cytokines. Furthermore, coculture of stromal cells with ileal PP follicular B cells elicited a proliferative response unique to each stromal cell line. Coculture with increasing numbers of SCN cells inhibited B cell proliferative responses, whereas coculture with SCG2 and SCF32 cells enhanced B cell proliferative response at both low and high stromal cell densities. Ileal PP follicular B cells rapidly bound to the surface of all stromal cell clones, and this interaction was specific when compared with thymocytes or peripheral blood lymphocytes. These results suggest that ileal PP stromal cells are a phenotypically and functionally heterogeneous population that may enhance or inhibit B lymphopoiesis in the ileal PP.