Regulation and localization of lymphocyte production in the bone marrow

The homeostatic mechanisms which control B lymphocyte renewal in the bone marrow are unknown. Mouse bone marrow produces many small lymphocytes which develop surface IgM and other B lymphocyte properties. Putative precursors show cytoplasmic mu chains but earlier progenitors have been characterized. Some marrow small lymphocytes are long-lived recirculating B and T cells. [3H]Thymidine and IgM labelling in femoral marrow sections suggest that recirculating lymphocytes migrate mainly through the marrow periphery while indigenous lymphocytes may be formed peripherally and migrate centrally as they mature. Thus, the localization of lymphocytes appears to be non-random. The effects of possible regulatory factors on bone marrow small lymphocytes production have been examined by [3H]thymidine labelling and radioautography. Administration of anti-IgM antibodies in vivo eliminates all B lymphocytes but the marrow lymphocyte production rate remains unchanged. After sublethal X-irradiation the marrow shows an over-shoot B lymphocyte production, while the lymphocyte numbers in shielded marrow remain stable. In neonatally thymectomized or congenitally athymic mice marrow lymphocyte production is unaffected. Studies in germ-free and antigen-stimulated mice reveal a basal level of marrow lymphocyte production, normally stimulated non-specifically by environmental factors. Thus, marrow lymphocyte production appears to be basically independent of feedback control from the peripheral B lymphocyte pool or of specific humoral factors, but fluctuates widely after perturbation or when amplified by exogenous stimuli. These findings suggest the importance of microenvironmental factors, as yet undefined, in the regulation of bone marrow lymphocytes.

Basement membrane of mouse bone marrow sinusoids shows distinctive structure and proteoglycan composition: a high resolution ultrastructural study

Venous sinusoids in bone marrow are the site of a large-scale traffic of cells between the extravascular hemopoietic compartment and the blood stream. The wall of the sinusoids consists solely of a basement membrane interposed between a layer of endothelial cells and an incomplete covering of adventitial cells. To examine its possible structural specialization, the basement membrane of bone marrow sinusoids has now been examined by high resolution electron microscopy of perfusion-fixed mouse bone marrow. The basement membrane layer was discontinuous, consisting of irregular masses of amorphous material within a uniform 60-nm-wide space between apposing endothelial cells and adventitial cell processes. At maximal magnifications, the material was resolved as a random arrangement of components lacking the "cord network" formation seen in basement membranes elsewhere. Individual components exhibited distinctive ultrastructural features whose molecular identity has previously been established. By these morphological criteria, the basement membrane contained unusually abundant chondroitin sulfate proteoglycan (CSPG) revealed by 3-nm-wide "double tracks," and moderate amounts of both laminin as dense irregular coils and type IV collagen as 1-1.5-nm-wide filaments, together with less conspicuous amounts of amyloid P forming pentagonal frames. In contrast, 4.5-5-nm-wide "double tracks" characteristic of heparan sulfate proteoglycan (HSPG) were absent. The findings demonstrate that, in comparison with "typical" basement membranes in other tissues, the bone marrow sinusoidal basement membrane is uniquely specialized in several respects. Its discontinuous nature, lack of network organization, and absence of HSPG, a molecule that normally helps to maintain membrane integrity, may facilitate disassembly and reassembly of basement membrane material in concert with movements of adventitial cell processes as maturing hemopoietic cells pass through the sinusoidal wall: the exceptionally large quantity of CSPG may represent a reservoir of CD44 receptor for use in hemopoiesis.

Suppressed apoptosis of pre-B cells in bone marrow of pre-leukemic p190bcr/abl transgenic mice

Mice transgenic for a p190bcr/abl construct develop pre-B cell leukemia/lymphoma, providing a model of Ph+ ALL. To investigate events in tumorigenesis, immunofluorescence labeling, flow cytometry and a short-term culture assay were used to quantitate precursor B cells and their apoptotic rates in bone marrow of p190bcr/abl transgenic mice over a wide age range. Malignancies appeared rapidly at 8-12 weeks of age, followed by slower tumor onset. At 8-12 weeks in normal mice, the apoptotic rate fell among pro-B cells but increased steeply among pre-B cells, while the total number of B lineage cells declined. In contrast, in p190bcr/abl transgenic mice over the same time period, while pro-B cells remained normal in apoptotic rate and number, apoptosis of pre-B cells was markedly inhibited and the number of B lymphocytes increased. At later ages (14-30 weeks), B cell precursors in control mice remained constant in apoptotic activity and number, while in the few surviving transgenic mice B cell populations were expanded. The results reveal characteristic changes in apoptotic activity among B cell precursors in bone marrow during early life, severely perturbed in preleukemic p190bcr/abl transgenic mice by a preferential suppression of pre-B cell apoptosis. p190bcr/abl may thus promote leukemogenesis by permitting aberrant cells generated during early B cell development to evade a normal quality checkpoint and negative selection.

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.

Isolation of amelogenin-positive ameloblasts from rat mandibular incisor enamel organs by flow cytometry and fluorescence activated cell sorting

The purpose of this study was to use amelogenin as a marker to examine the feasibility of isolating ameloblasts from enamel organ cell populations by fluorescence activated cell sorting. After treating dissected rat enamel organs with proteolytic enzymes to loosen cell attachments and labial connective tissues, dissociated cell suspensions were fixed, then immunostained with rabbit anti-rM179 recombinant amelogenin antibody and FITC-conjugated goat anti-rabbit Ig G antibody. Flow cytometry indicated that about 70% of the total cell sample and virtually all the larger cells therein were amelogenin-positive. Fluorescence activated cell sorting yielded a sample of amelogenin-positive cells at 97% purity. Immunofluorescence microscopy indicated that these isolated amelogenin-positive cells varied widely in size and morphology. This was attributed to loss of intercellular support for ameloblasts once they were dissociated from each other, and to some fragmentation caused when the cells were initially physically removed from the teeth. The results demonstrate that viable ameloblast cell fractions, especially representing cells at the secretory stage, can be purified from enzymic digests of rat enamel organ by sorting on the basis of cell size alone. From these fractions, subpopulations of ameloblasts may be identified when differentiation specific cell surface markers become available.

Melatonin inhibits apoptosis during early B-cell development in mouse bone marrow

The pineal secretory product, melatonin, exerts a variety of effects on the immune system. Administration of melatonin stimulates cell-mediated immunity, particularly by inhibiting apoptosis among T lymphocytes in the thymus and inducing production of T-cell-derived cytokines. However, its possible effects on the humoral immune system are unclear. In the present study, we have examined whether melatonin may influence the in vivo development of B lymphocytes in mouse bone marrow, a process in which apoptosis is normally a prominent feature. Double immunofluorescence labeling and flow cytometry were used to quantitate phenotypically defined precursor B-cell and mature B-cell populations and their apoptotic rates in bone marrow of mice fed either melatonin-containing or control diet for 16 days from 9 wk of age. In short-term bone marrow cultures, the incidence of apoptosis among large pre-B cells, including cells expressing the lambda5 component of pre-B-cell receptor, was markedly reduced in melatonin-treated mice, associated with an increase in the absolute number of large pre-B cells in bone marrow. In contrast, apoptosis of earlier precursor B cells and mature B lymphocytes did not differ from control values. The results indicate that orally administered melatonin can substantially promote the survival of precursor B cells in mouse bone marrow. Melatonin treatment may thus boost the survival of newly formed B cells mediating humoral immunity.

Regulation of cell survival during B lymphopoiesis: increased pre-B cell apoptosis in CD24-transgenic mouse bone marrow

CD24 (heat-stable antigen) is expressed in a developmentally regulated fashion by B cell precursors in mouse bone marrow (BM), but its role in B lymphopoiesis remains obscure. A slight overexpression of CD24 in transgenic (Tg) mice leads to depletion of B lymphoid cells in BM. The present study examines whether CD24 is involved in apoptotic selection of B lineage cells under normal microenvironmental conditions in vivo. Double immunofluorescence labeling and flow cytometry have been used to quantitate the apoptotic rates of phenotypically defined B cell populations in BM of CD24-Tg mice. Apoptosis of pre-B cells expressing cytoplasmic mu heavy chains of IgM but lacking surface (s)IgM was increased both ex vivo and in short-term culture, while the number of pre-B cells was halved compared to BM of normal mice. In contrast, B220+mu- pro-B cells and sIgM+ B lymphocytes showed no significant change in either apoptosis or number. The findings provide evidence that CD24 can play a role in vivo in modulating pre-B cell apoptosis, a quality control checkpoint in B cell development.

Apoptosis and its modulation during B lymphopoiesis in mouse bone marrow

Studies in normal, gene-deleted, transgenic and mutant mice have examined apoptotic cell death and its role in B lymphopoiesis in bone marrow. Apoptotic activity has been quantitated among phenotypically defined populations of precursor B cells using flow cytometry of apoptotic cells and an established model of B-cell development. In normal mice, the frequencies of apoptotic cells (apoptotic index) and accumulation of apoptotic cells during short-term culture (apoptotic rate) are maximal at around the pro/pre-B-cell transition and among immature B lymphocytes. The brief period between onset of apoptosis and clearance by macrophages (apoptotic transit time) is similar for most precursor B-cells. Apoptosis-modulating factors produce substantial changes in apoptotic activity among pro-B and pre-B cells, associated with altered expression of bcl-2 family proteins. Pro-B-cell apoptosis, normally extensive, is markedly suppressed in the absence of p53. Complete pro-B-cell abortion in RAG-2 deletion provides an assay for apoptotic fractions in other experimental systems. Pre-B-cell apoptosis is enhanced by deficiencies of interleukin (IL)-7, Abl protooncogene or colony-stimulating factor (CSF)-1 and overexpression of heat-stable antigen, and is inhibited by IL-7 and p190bcr/abl transgenes. CSF-1 and melatonin administration inhibit pre-B-cell apoptosis, probably via stromal cell stimulation. Such apoptotic modulation has implications for B-cell homeostasis, quality control, immunodeficiency and neoplasia.

Myelointegration of titanium implants: B lymphopoiesis and hemopoietic cell proliferation in mouse bone marrow exposed to titanium implants

Multinucleated giant cells have been observed at interfaces between bone marrow and titanium implants in mouse femurs. This raises concern that macrophage-derived factors might perturb local lymphohemopoiesis, possibly even predisposing to neoplasia in the B lymphocyte lineage. It has been found that an implant-marrow interface with associated giant cells persists for at least 1.5 years. Precursor B cells show early increases in number and proliferative activity. At later intervals, however, they do not differ significantly from controls, and there are no perturbations in spatial localization of either B lineage cells or DNA-synthesizing hemopoietic cells. The results of this investigation in mice demonstrate that, following initial marrow regeneration and fluctuating precursor B cell activity, and despite the presence of giant cells, titanium implants apparently become well-tolerated by directly apposed bone marrow cells in a lasting state of "myelointegration."

Regulation of cell survival during B lymphopoiesis: suppressed apoptosis of pro-B cells in P53-deficient mouse bone marrow

B cell development in mouse bone marrow (BM) is subject to quality controls that eliminate aberrant cells by apoptosis, but the intrinsic cellular mechanisms that mediate this negative B cell selection remain unclear. The p53 tumor suppressor transduces signals resulting in apoptosis and cell cycle arrest in cells that sustain DNA damage. Faulty V(D)J recombination in scid lymphocyte precursors activates a p53-dependent DNA damage checkpoint. In the present study, we have examined whether p53 is involved in apoptotic selection of normally developing B cells in BM. Double immunofluorescence labeling and flow cytometry were used to quantitate phenotypically defined B cell populations and their apoptotic rates in BM of homozygous p53-deficient mice. B220(+) mu(-) and terminal deoxynucleotidyl transferase (TdT)(+) pro-B cells were increased in both incidence and absolute number to controls. In contrast, pre-B cells were only slightly increased and the sIgM(+) B lymphocyte compartment remained essentially normal. The incidence of apoptosis among p53(-/-) pro-B cells was greatly reduced, both ex vivo and in short-term culture, whereas, apoptosis of pre-B cells and B lymphocytes was not significantly different from normal. The results indicate that p53 is actively involved as an apoptosis inducer at an early quality control checkpoint in B lymphopoiesis.

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.

Effect of a bcl-2 transgene on production and localization of precursor B cells in mouse bone marrow

Many B cell precursors die while differentiating in mouse bone marrow. To ascertain the mechanisms involved in this process, populations of B lineage cells and their tissue localization were analyzed in bone marrow of transgenic mice overexpressing the apoptosis inhibitor, Bcl-2. Immunofluorescence labeling and mitotic arrest were used to quantitate the number and proliferative activity of mu- pro-B cells (terminal deoxynucleotidyl transferase [TdT]+B220-, TdT+B220+, and TdT-B220+); pre-B cells (cmu+); and B cells (smu+). Mature B cells (IgM+IgD+) were increased 16- to 20-fold. In addition, immature B lymphocytes (IgM+IgD-/low), representing newly formed cells, were increased three- to sixfold, whereas pre-B cells and late pro-B cells were increased 30 to 60% in production rate. Earlier pro-B cells expressing TdT were unaffected. In spleen, both mature and immature B cells were greatly increased, but cells of precursor phenotype were few and TdT+ cells were absent. The in vivo location of B cells was examined by autoradiography using light and electron microscopy after intravenous injection of 125I-labeled antibodies. B lineage cells (B220+) were increased throughout bone marrow, often within dilated venous sinusoids, particularly in subosteal regions. Many intravascular and perisinusoidal cells were IgDhigh mature B lymphocytes. In contrast, many other IgM+ and IgDlow immature B lymphocytes clustered extravascularly around the central venous sinus. Plasma cells with distended endoplasmic reticulum were numerous. These findings provide evidence that, in addition to expanding the recirculating pool of B cells entering bone marrow from the blood stream, high levels of Bcl-2 can inhibit some of the apoptosis occurring during B cell differentiation, thereby expanding populations of B lymphopoietic precursor cells within the bone marrow parenchyma.

Exogenous interleukin 7 as a proliferative stimulant of early precursor B cells in mouse bone marrow: efficacy of IL-7 injection, IL-7 infusion and IL-7-anti-IL-7 antibody complexes

B lymphocyte development in mouse bone marrow (BM) occurs in association with stromal cells which provide essential growth factors, notably interleukin 7 (IL-7). The ability of recombinant IL-7 given systemically by several modes of administration to stimulate proliferation of precursor B cells at successive stages of development was examined. Using immunofluorescence labelling and mitotic arrest, the in vivo proliferative cell dynamics of phenotypically defined B lineage cells was quantitated. Single injections of murine IL-7 (muIL-7) in low and intermediate doses stimulated production both of pro-B cells preceeding the expression of mu heavy chains and large pre-B cells expressing cytoplasmic mu. In contrast, higher doses were not stimulatory. Infusion of muIL-7 from subcutaneous micro-osmotic pumps produced a proliferative wave of TdT+ pro-B cells and pre-B cells followed by elevated numbers of postmitotic small pre-B cells and B cells. Stimulation tended to be followed by secondary oscillations of B lymphopoiesis or an IL-7 refractory state. Pronounced increases in TdT+ pro-B and pre-B cell production resulted from injecting small doses of either muIL-7 or human IL-7 (hIL-7), complexed with anti-IL-7 antibody as carrier protein. The results indicate that exogenous IL-7 at optimal dose can markedly stimulate in vivo B lymphopoiesis from the earliest detectable TdT+ pro-B cell stage onward, and is effective when delivered as a cytokine-anticytokine complex.

Two models of murine B lymphopoiesis: a correlation

During B cell genesis in mouse bone marrow (BM), precursor B cells pass through a series of developmental stages that have been defined by changes in expression of various marker molecules. The use of dissimilar phenotypic criteria in different laboratories, however, has led to the formulation of disparate models of B lymphopoiesis not fully reconciled with one another. We have directly compared two such models, one based on expression of intracellular mu heavy chain of IgM (c mu) and terminal deoxynucleotidyl transferase (TdT), the other monitoring cell surface leukosialin (CD43), heat-stable antigen (HSA; CD24) and the ectopeptidase BP-1. Each model uses cell surface B220 glycoprotein (CD45RA) to denote the B cell lineage. We have examined the cellular composition of four sorted BM fractions by immunofluorescent labeling of CD43, HSA and BP-1, using immunofluorescence microscopy of cytocentrifuged fractions to quantitate precursor B cell populations expressing either c mu or TdT. The results reveal a range of B cell differentiation stages within individual sorted BM fractions, providing a cross-reference between these two analytical methods and contributing to a unified model of B cell development in mouse BM.

Apoptosis during B lymphopoiesis in mouse bone marrow

To evaluate the magnitude of cell death and the critical stages at which it occurs during B lymphopoiesis in mouse bone marrow (BM), we have examined the kinetics of apoptosis at defined stages of B cell differentiation. FACS-sorted B220+ BM cells exhibited a low incidence of morphologically apoptotic cells by electron microscopy. In freshly prepared BM suspensions, the incidence of hypodiploid cells detected by multiparameter flow cytometry was greater among large dividing B220+ surface IgM- (sIgM-) precursor B cells and sIgM(low) immature B lymphocytes than among terminal deoxynucleotidyl transferase+ (TdT+) pro-B cells, small nondividing B220+ sIgM- precursors, and surface IgD+ mature B lymphocytes. During short-term culture, apoptotic cells, identified by both DNA content and in situ DNA strand break labeling, increased linearly with time without macrophage ingestion, providing an assay for the rate of entry into apoptosis. B220+ B lineage cells accumulated in apoptosis more rapidly than cells of other lineages. The apoptotic rate was greater among B220+ sIgM- precursor cells than sIgM+ B cells, and was highest among B220+ mu- pro-B cells. Coculture with stromal cells reduced the apoptotic rate of B220+ sIgM- precursors to a greater extent than that of sIgM+ B lymphocytes. The results lead to estimates of the actual number of B lineage cells undergoing apoptosis per unit time in successive differentiation compartments. The findings indicate that, although influenced by local microenvironmental factors, apoptotic cell death occurs most markedly at two developmental stages associated with Ig heavy chain gene rearrangement and Ag receptor expression, respectively.

Apoptosis during B lymphopoiesis in mouse bone marrow

To evaluate the magnitude of cell death and the critical stages at which it occurs during B lymphopoiesis in mouse bone marrow (BM), we have examined the kinetics of apoptosis at defined stages of B cell differentiation. FACS-sorted B220+ BM cells exhibited a low incidence of morphologically apoptotic cells by electron microscopy. In freshly prepared BM suspensions, the incidence of hypodiploid cells detected by multiparameter flow cytometry was greater among large dividing B220+ surface IgM- (sIgM-) precursor B cells and sIgM(low) immature B lymphocytes than among terminal deoxynucleotidyl transferase+ (TdT+) pro-B cells, small nondividing B220+ sIgM- precursors, and surface IgD+ mature B lymphocytes. During short-term culture, apoptotic cells, identified by both DNA content and in situ DNA strand break labeling, increased linearly with time without macrophage ingestion, providing an assay for the rate of entry into apoptosis. B220+ B lineage cells accumulated in apoptosis more rapidly than cells of other lineages. The apoptotic rate was greater among B220+ sIgM- precursor cells than sIgM+ B cells, and was highest among B220+ mu- pro-B cells. Coculture with stromal cells reduced the apoptotic rate of B220+ sIgM- precursors to a greater extent than that of sIgM+ B lymphocytes. The results lead to estimates of the actual number of B lineage cells undergoing apoptosis per unit time in successive differentiation compartments. The findings indicate that, although influenced by local microenvironmental factors, apoptotic cell death occurs most markedly at two developmental stages associated with Ig heavy chain gene rearrangement and Ag receptor expression, respectively.

Upregulation of endogenous p53 and induction of in vivo apoptosis in B-lineage lymphomas of E(mu)-myc transgenic mice by deregulated c-myc transgene

E(mu)-myc transgenic mice carry a constitutively overexpressed c-myc oncogene and develop B-lineage lymphomas. Previous studies have shown that c-myc overexpression can lead to in vitro apoptosis. Here, we investigated the in vivo effects of altered c-myc expression on cell proliferation versus death in spontaneously arising E(mu)-myc tumors. E(mu)-myc tumors display extensive in vivo apoptosis confined to small clusters of cells with greatly increased expression of both the c-myc transgene and the endogenous p53 gene as compared with that in normal, pretumor, or surrounding tumor tissue. This restricted overexpression of both the c-myc transgene and the endogenous p53 gene in small clusters of apoptotic tumor cells indicates that overexpression of these genes and apoptosis are not obligatory or uniform during tumor development and suggests that further somatic mutations or microenvironmental influences may be responsible for these properties. Nevertheless, the clear ability of tumor cells to undergo apoptosis in vivo may be exploitable for therapeutic purposes.

Prelymphomatous B cell hyperplasia in the bone marrow of interleukin-7 transgenic mice: precursor B cell dynamics, microenvironmental organization and osteolysis

Transgenic mice carrying mouse interleukin-7 (IL-7) cDNA under the control of MHC class II (E alpha) promoter develop B lymphoid tumors. We have analyzed population dynamics of early precursor B cells and electron microscopic organization of bone marrow (BM) during the prelymphomatous phase. Immunofluorescence labeling of terminal deoxynucleotidyl transferase (TdT), B220 glycoprotein, and mu heavy chains have been used to quantitate three populations of pro-B cells lacking mu chains, cytoplasmic mu-bearing pre-B cells, and surface mu-bearing B lymphocytes. Proliferative activity was assayed by metaphase arrest. In BM of IL-7 transgenic mice, the number and proliferative activity of cells in each of the pro-B and pre-B cell populations were markedly increased. B lymphocytes increased to a lesser extent. The BM cavity was considerably expanded and cortical bone showed focal osteolysis. Immature lymphoid cells compressed the venous sinusoids and exuded through eroded bone. Apoptotic bodies, macrophages, and plasma cells were unusually prominent. B lymphocytes and cells of B precursor phenotype were also much increased in the spleen. These results demonstrate that overexpression of IL-7 causes excessive proliferation of a wide range of precursor B cells in BM. Such prolonged stimulation at early stages of B cell development, prone to genetic errors, may predispose to neoplasia. The bone resorption in these transgenic mice provides a model for bone lesions in BM malignancies.

IL-1 as systemic modifier of B lymphopoiesis. Recombinant IL-1 alpha binds to stromal cells and sinusoid endothelium in bone marrow and precursor B cell dynamics

B lymphopoiesis in mouse bone marrow (BM) can be stimulated by circulating products derived from activated macrophages in the spleen. To examine whether IL-1 could mediate this effect, we have administered murine rIL-1alpha in a range of doses, determining its effect on precursor B cells and its capacity to bind to stromal cells in BM. Immunofluorescence labeling of terminal deoxynucleotidyl transferase (TdT), B220 glycoprotein, and mu-chains has been used to quantitate pro-B cells lacking mu (TdT+; 13220+mu-), pre-B cells expressing cytoplasmic mu, and B lymphocytes bearing surface mu. Proliferative activity was measured by mitotic arrest. Single i.p. injections of rIL-1alpha produced a proliferative stimulation of pro-B cells and pre-B cells at optimal doses, whereas other doses were suppressive. Infusion of rIL-1alpha from s.c. osmotic pumps depressed B lymphopoiesis at high dose rates, but stimulated precursor B cell proliferation at lower dose rates. Intravenous 125I-labeled rIL-1alpha bound strongly to a subset of stromal reticular cells and sinusoidal endothelium in BM, as detected by light and electron microscope radio-autography. Computer-aided analysis located rIL-1alpha-binding stromal cells mainly in the outer zones of BM, sites of proliferating precursor B cells, rather than the more central zone. The results demonstrate that IL-1 can act systemically at various dose levels as either a positive or negative modifier of B lymphopoiesis in BM, probably acting indirectly via stromal reticular cells and endothelial cells. Thus, inflammatory processes associated with macrophage activation and IL-1 secretion may have pronounced effects on B cell genesis in BM.

IL-1 as systemic modifier of B lymphopoiesis. Recombinant IL-1 alpha binds to stromal cells and sinusoid endothelium in bone marrow and precursor B cell dynamics

B lymphopoiesis in mouse bone marrow (BM) can be stimulated by circulating products derived from activated macrophages in the spleen. To examine whether IL-1 could mediate this effect, we have administered murine rIL-1alpha in a range of doses, determining its effect on precursor B cells and its capacity to bind to stromal cells in BM. Immunofluorescence labeling of terminal deoxynucleotidyl transferase (TdT), B220 glycoprotein, and mu-chains has been used to quantitate pro-B cells lacking mu (TdT+; 13220+mu-), pre-B cells expressing cytoplasmic mu, and B lymphocytes bearing surface mu. Proliferative activity was measured by mitotic arrest. Single i.p. injections of rIL-1alpha produced a proliferative stimulation of pro-B cells and pre-B cells at optimal doses, whereas other doses were suppressive. Infusion of rIL-1alpha from s.c. osmotic pumps depressed B lymphopoiesis at high dose rates, but stimulated precursor B cell proliferation at lower dose rates. Intravenous 125I-labeled rIL-1alpha bound strongly to a subset of stromal reticular cells and sinusoidal endothelium in BM, as detected by light and electron microscope radio-autography. Computer-aided analysis located rIL-1alpha-binding stromal cells mainly in the outer zones of BM, sites of proliferating precursor B cells, rather than the more central zone. The results demonstrate that IL-1 can act systemically at various dose levels as either a positive or negative modifier of B lymphopoiesis in BM, probably acting indirectly via stromal reticular cells and endothelial cells. Thus, inflammatory processes associated with macrophage activation and IL-1 secretion may have pronounced effects on B cell genesis in BM.

Adhesion receptors on bone marrow stromal cells: in vivo expression of vascular cell adhesion molecule-1 by reticular cells and sinusoidal endothelium in normal and gamma-irradiated mice

Cell adhesion molecules (CAMs) play a key role in interactions between stromal and hematopoietic cells in bone marrow (BM) and in cell traffic through vascular endothelium. To examine the identity of CAMs involved in these processes in mouse BM, we have investigated the in vivo expression of vascular cell adhesion molecule-1 (VCAM-1) and its counter-receptor, very late antigen-4 (VLA-4). Radioiodinated monoclonal antibodies (MoAbs) detecting VLA-4 and VCAM-1 were injected intravenously. Antibody binding was detected in BM by light and electron microscope radioautography. VCAM-1 labeling was restricted to stromal reticular cells and endothelial cells lining BM sinusoids. VCAM-1+ reticular cells formed patchy concentrations, especially in subosteal regions, associated with lymphoid, granulocytic, and erythroid cells. After gamma-irradiation to deplete hematopoietic cells, reticular cells and endothelial cells all showed VCAM-1 labeling in apparently increased intensity. VLA-4 labeling was shown by undifferentiated blast cells and lymphohematopoietic cells both in BM cell suspensions and in vivo, especially at reticular cell contact points. The results demonstrate that VCAM-1 is expressed in vivo by certain BM reticular cells, suggesting that the molecule mediates adhesion to multiple lineages of lymphohematopoietic cells. The finding that VCAM-1 is also expressed constitutively by BM sinusoidal endothelium, unlike its inductive expression by endothelia elsewhere, suggests that VCAM-1 and VLA-4 may be involved in regulating the normal cell traffic between BM and the blood stream.

Perturbation of B cell genesis in the bone marrow of pristane-treated mice. Implications for plasmacytoma induction

A single injection of pristane was given i.p. to plasmacytoma-susceptible BALB/cAn mice. At intervals up to 6 mo thereafter, immunofluorescence labeling of intranuclear terminal deoxynucleotidyl transferase (TdT), cell surface B220 glycoprotein, cytoplasmic mu-chains of IgM (c mu), and surface mu-chains (s mu), together with mitotic arrest techniques, were used to quantitate the in vivo population dynamics of precursor B cells in the bone marrow. TdT-expressing pro-B cells (TdT+B220-, TdT+B220+), before the expression of mu-chains, showed sustained increases in both population size and the number of cells flowing through mitosis per unit time. In contrast, populations of pre-B cells (c mu + s mu -) and B cells (s mu +) were consistently depressed for long periods of time, including the phase of plasmacytoma formation. Precursor B cells in DBA/2 mice, a plasmacytoma-resistant strain, showed similar responses to pristane treatment. The results demonstrate that a single injection of pristane, which greatly increases the demand for macrophage activity in the peritoneal space, causes sustained distant alterations in B cell lymphopoiesis in the bone marrow; specifically, a prolonged increased proliferation of pro-B cells coupled with a depression and a exaggerated loss of pre-B cells and B cells. The protracted stress on B cell lymphopoiesis may be a predisposing factor in the subsequent development of c-myc-activating chromosomal rearrangements that play a critical role in plasmacytomagenesis.

Perturbation of B cell genesis in the bone marrow of pristane-treated mice. Implications for plasmacytoma induction

A single injection of pristane was given i.p. to plasmacytoma-susceptible BALB/cAn mice. At intervals up to 6 mo thereafter, immunofluorescence labeling of intranuclear terminal deoxynucleotidyl transferase (TdT), cell surface B220 glycoprotein, cytoplasmic mu-chains of IgM (c mu), and surface mu-chains (s mu), together with mitotic arrest techniques, were used to quantitate the in vivo population dynamics of precursor B cells in the bone marrow. TdT-expressing pro-B cells (TdT+B220-, TdT+B220+), before the expression of mu-chains, showed sustained increases in both population size and the number of cells flowing through mitosis per unit time. In contrast, populations of pre-B cells (c mu + s mu -) and B cells (s mu +) were consistently depressed for long periods of time, including the phase of plasmacytoma formation. Precursor B cells in DBA/2 mice, a plasmacytoma-resistant strain, showed similar responses to pristane treatment. The results demonstrate that a single injection of pristane, which greatly increases the demand for macrophage activity in the peritoneal space, causes sustained distant alterations in B cell lymphopoiesis in the bone marrow; specifically, a prolonged increased proliferation of pro-B cells coupled with a depression and a exaggerated loss of pre-B cells and B cells. The protracted stress on B cell lymphopoiesis may be a predisposing factor in the subsequent development of c-myc-activating chromosomal rearrangements that play a critical role in plasmacytomagenesis.

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.

A donor-derived asialo-GM1+ cell induces depression of B-cell genesis during systemic graft-versus-host disease

The nature of the effector cell(s) responsible for the depression of B-cell genesis in the bone marrow of mice undergoing systemic graft-versus-host disease (GVHD) has been examined. Donor C57BL/6 (B6) mice were treated in vivo with either a single injection of anti-asialo GM1 antibody (anti-ASGM1) to eliminate naturally occurring (endogenous) ASGM1+ cells or B6xAF1 (B6AF1) lymphoid cells followed by anti-ASGM1 to eliminate both endogenous and "induced" ASGM1+ cells. Lymphoid cells from donor mice after the elimination of endogenous ASGM1+ cells produced severe GVHD and concomitant depression of B-cell genesis when injected into B6AF1 recipients. In contrast, cells from donors depleted of both the endogenous and inducible ASGM1+ populations did not cause GVHD or depletion of B lineage cells in B6AF1 recipients but did depress B-cell genesis in B6C3F1 mice. The "induced" ASGM1+ cells were Thy 1+, but their elimination did not significantly alter either overall T-cell function or specific cytotoxic T-cell (CTL) reactivity against the sensitizing (B6AF1) strain. The results suggest that the effector cell responsible for the depression of B-cell genesis during systemic GVHD can be induced to express ASGM1, is strain-specific and Thy 1+; but is not a conventional CTL.

Apoptosis and macrophage-mediated deletion of precursor B cells in the bone marrow of E mu-myc transgenic mice

Transgenic mice expressing the c-myc proto-oncogene under the control of the Ig heavy chain enhancer (E mu-myc) all eventually develop clonal pre-B- or B-cell tumors. The preneoplastic period is characterized by increased polyclonal proliferation of pro-B and pre-B cells in the bone marrow (BM) associated with a reduced number of B cells, suggesting a high degree of B-cell loss. To examine the mechanisms of this cell loss, we have identified B220+ B-lineage cells within the BM of pretumorous E mu-myc transgenic mice by in vivo radiolabeling and electron microscope radioautography. Large mitotic B220(+)-labeled cells form prominent clusters in the extravascular compartment of the BM. Some B220+ small lymphocytes, as well as large lymphoid cells, enter BM sinusoids. However, in addition, large numbers of B220+ cells exhibit nuclear chromatin condensation, fragmentation, and other morphologic features characteristic of apoptotic cell death. Propidium iodide staining and flow cytometry of BM cells from pretumorous E mu-myc transgenic mice, as well as agarose gel electrophoresis of DNA, confirm extensive apoptosis. Many B220+ apoptotic cells are closely associated with the extensive processes of prominent macrophages that contain numerous B220+ apoptotic bodies and complex lysosomal systems. These results suggest that the constitutive expression of c-myc oncogene in BM B-lineage cells, which increases the proliferation of precursor B cells, also leads to increased apoptotic cell death and rapid elimination by resident macrophages. Further mutations may be needed to block these protective mechanisms and permit surviving c-myc-dysregulated cells to leave the BM and to initiate tumorigenesis.

c-kit expression by B cell precursors in mouse bone marrow. Stimulation of B cell genesis by in vivo treatment with anti-c-kit antibody

To examine the in vivo role of c-kit receptor in B lymphopoiesis we have evaluated precursor B cell populations expressing c-kit in mouse bone marrow and the effects on B cell genesis of administering a neutralizing anti-c-kit mAb, ACK2. Double immunofluorescence labeling and mitotic arrest were used to examine bone marrow cells from BALB/c mice. Almost one-half of TdT+ cells and one-quarter of B220+ cells coexpressed c-kit, mainly at low intensities, and were actively proliferating in vivo. c-kit+ cells that lacked B lineage markers expressed c-kit in high intensities and entered mitosis at an exceptionally rapid rate. In ACK2-treated mice, erythroid and granulocytic cells were almost completely absent from the bone marrow, whereas, in contrast, B lymphopoiesis was stimulated. Pre-B cells expressing cytoplasmic mu-chains; as well as TdT+B220+ cells before mu expression, were increased two- to fourfold in number and production rate. IgM-bearing B lymphocytes were increased in bone marrow and spleen. The results demonstrate that many early precursor B cells in mouse bone marrow constitutively express c-kit receptor. The failure of ACK2 treatment to block B lymphopoiesis, however, suggests that c-kit receptor function is not essential for precursor B cell development in vivo, but can be replaced by alternative signaling systems. The stimulation of B cell genesis by ACK2 treatment may reflect a conferred advantage in the competition for microenvironmental factors which underlies the balance between B lymphopoiesis and other hemopoietic lineages in vivo.

c-kit expression by B cell precursors in mouse bone marrow. Stimulation of B cell genesis by in vivo treatment with anti-c-kit antibody

To examine the in vivo role of c-kit receptor in B lymphopoiesis we have evaluated precursor B cell populations expressing c-kit in mouse bone marrow and the effects on B cell genesis of administering a neutralizing anti-c-kit mAb, ACK2. Double immunofluorescence labeling and mitotic arrest were used to examine bone marrow cells from BALB/c mice. Almost one-half of TdT+ cells and one-quarter of B220+ cells coexpressed c-kit, mainly at low intensities, and were actively proliferating in vivo. c-kit+ cells that lacked B lineage markers expressed c-kit in high intensities and entered mitosis at an exceptionally rapid rate. In ACK2-treated mice, erythroid and granulocytic cells were almost completely absent from the bone marrow, whereas, in contrast, B lymphopoiesis was stimulated. Pre-B cells expressing cytoplasmic mu-chains; as well as TdT+B220+ cells before mu expression, were increased two- to fourfold in number and production rate. IgM-bearing B lymphocytes were increased in bone marrow and spleen. The results demonstrate that many early precursor B cells in mouse bone marrow constitutively express c-kit receptor. The failure of ACK2 treatment to block B lymphopoiesis, however, suggests that c-kit receptor function is not essential for precursor B cell development in vivo, but can be replaced by alternative signaling systems. The stimulation of B cell genesis by ACK2 treatment may reflect a conferred advantage in the competition for microenvironmental factors which underlies the balance between B lymphopoiesis and other hemopoietic lineages in vivo.

Cell populations during tumorigenesis in Eu-myc transgenic mice

Transgenic mice bearing a c-myc oncogene under control of the immunoglobulin heavy chain (Igh) enhancer (Eu-myc mice) (1, reviewed in 2) undergo a reproducible series of developmental stages and die from malignancies of the B lymphocyte lineage. To investigate the cellular events underlying tumorigenesis in this model, we quantified B lymphoid subpopulations and turnover at various stages of this process. An early stage was characterized by the presence in the blood of many large proliferating B lineage cells marked by surface antigen phenotype IgM+l-, B220low, CD5-, Mac-1low. During a prolonged intermediate 'remission' phase of different duration in each mouse, B lymphocytes in the periphery were non-proliferative, few, and of conventional phenotype (IgM+, B220+, CD5-, Mac-1-), while subsets of precursor B cells were both numerous and highly proliferative in the bone marrow. In the final stage of tumorigenesis, large proliferating cells similar in phenotype to those of the early period reappeared and increased rapidly in numbers. This B cell tumorigenic progression occurred independently of interactions with T lymphocytes. Evidence of massive cell death in the bone marrow during the intermediate phase, plus molecular characterization of the final tumors, suggested that the end of the peripheral 'remission' period and entry into the terminal stage of tumorigenesis may be due to a clone of cells acquiring the ability to circumvent normal processes of cell death and elimination that usually regulate the egress of B cells from the bone marrow to the periphery.

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

B lymphocyte precursor cells in mouse bone marrow develop in close association with stromal cells which provide essential growth signals. To identify molecules that may normally play a role in this interaction we have examined the in vivo binding of a new monoclonal antibody (mAb) (KMI6) that recognizes a determinant on a bone marrow stromal cell line (BMS2) in vitro. Flow cytometric and radioautographic evaluations revealed that the antigen recognized by KMI6 is represented on the surface of an extremely small number of cells in bone marrow cell suspensions from adult mice. An apparent molecular mass of 110 kD was obtained by surface labeling of a stromal cell clone and immunoprecipitation. Purified mAb KMI6 labeled with 125I was then given intravenously to young C3H/HeJ mice. Unbound mAb was washed out by cardiac perfusion and femoral bone marrow was examined by light and electron microscope radioautography. KMI6 labeling was heavy on the plasma membrane of many stromal cells, especially those located towards the outer subosteal region. The KMI6-labeled stromal cells were usually associated with cells of lymphoid morphology which they often completely surrounded. The labeling was restricted to areas of stromal cell plasma membranes in contact with lymphoid cells. The lymphoid cells themselves, as well as macrophages and other hemopoietic cells, failed to bind mAb KMI6 significantly. Stromal cells in bone marrow depleted of hemopoietic cells by gamma-irradiation (9,5 Gy) bound mAb KMI6 at reduced intensity. The results demonstrate that the KMI6 determinant, a 110-kD protein, is expressed on bone marrow stromal cells in vivo. Its restriction to areas of interaction with lymphoid cells suggests a role in forming microenvironmental niches of B lymphopoiesis. The surface membrane of individual stromal cells may thus be functionally polarized towards interacting B cell precursors and other hemopoietic cells.

Dynamics and localization of early B-lymphocyte precursor cells (pro-B cells) in the bone marrow of scid mice

Mice homozygous for the scid (severe combined immunodeficiency) mutation are generally unable to produce B lymphocytes, a condition attributed to defective rearrangement of immunoglobulin genes in precursor B cells. Some early B-lineage cells are present in the bone marrow (BM), however. In scid mice, we defined three subsets of early progenitor B cells lacking mu heavy chains (pro-B cells) based on the expression of terminal deoxynucleotidyl transferase (TdT) and B220 glycoprotein: (a) early pro-B cells (TdT+B220-), (b) intermediate pro-B cells (TdT+B220+), and (c) late pro-B cells (TdT-B220+). Double immunofluorescence labeling of BM cell suspensions has shown normal numbers of early and intermediate pro-B cells, substantially reduced numbers of late pro-B cells, and an absence of pre-B cells and B cells. Early and intermediate pro-B cells accumulated in metaphase in near-normal numbers after intraperitoneal (IP) vincristine administration. B220+ pro-B cells have been localized in BM sections by the binding of intravenously (IV) administered 125I monoclonal antibody (MoAb) 14.8, detected by light and electron microscope radioautography. Many B220+ cells were located peripherally in the bone-lining cell layers associated with stromal reticular cells. More centrally located B220+ cells were frequently associated with macrophages containing prominent cytoplasmic inclusions. Occasional B220+ cells were present in venous sinusoids. These results demonstrate that many pro-B cells in scid mice occupy microenvironments in the BM near the surrounding bone. The pro-B cells maintain normal rates of production during stages of presumptive mu heavy-chain gene rearrangement, apparently unaffected by the absence of a mature B cell pool. Nearly all defective cells then abort at the late pro-B cell stage and are deleted, apparently by macrophages. The findings contribute to models of in vivo differentiation, regulation, localization, and selection of early B-lineage cells in the BM.

Changes in the populations of null, NK1.1+, and Thy1lo lymphocytes in the bone marrow of tumor-bearing mice: Effect of indomethacin treatment

Mouse bone marrow produces many "null" lymphocytes which lack B and T lineage markers (B220-Thy1-). A subset of these cells expresses the natural killer (NK) cell marker, NK1.1. In addition, some rapidly renewed bone marrow lymphocytes express low intensities of Thy1 (Thy1lo). In view of their possible implication in tumor-host interactions these various cell populations have now been examined in mice injected with either the nonmetastatic Ehrlich ascites (EA) tumor or the Lewis lung carcinoma (LLc), a highly metastatic solid tumor. In each case, the number of null lymphocytes, as defined by a lack of radioautographic labeling of either B220 glycoprotein or Thy1, increased markedly in both the bone marrow and spleen. Treatment with the prostaglandin inhibitor, indomethacin, enhanced the increase in null cells in the bone marrow and spleen of LLc-bearing mice. The number of null small lymphocytes expressing NK1.1, as detected by combined radioautographic and immunoperoxidase techniques, increased almost 30-fold in LLc-bearing mice. The number of Thy1lo small lymphocytes increased in parallel with null cells during EA tumor growth. The findings accord with the hypothesis that the null lymphocyte population produced in mouse bone marrow includes newly formed NK lineage cells which sequentially express NK1.1 and Thy1lo. The present work demonstrates that the populations of null, NK1.1+, and Thy1lo lymphocytes in mouse bone marrow expand rapidly during the early growth of transplanted tumors, the initial increase in null lymphocytes apparently being curtailed by prostaglandin production. The results suggest that the production of null lymphocytes in mouse bone marrow is responsive to tumor development, possibly providing cells to be involved in tumor-host interactions.

Regulation of early precursor B cell proliferation in mouse bone marrow: stimulation by exogenous agents mediated by macrophages in the spleen

An increase in pre-B cell proliferation and B lymphocyte production in mouse bone marrow has previously been shown to follow the administration of various foreign agents in vivo. The responses of early precursor B cells before the expression of mu chains (pro-B cells) have now been examined, using double immunofluorescence labeling for terminal deoxynucleotidyl transferase (TdT) and B220 glycoprotein as detected by monoclonal antibody 14.8. A single injection of sheep red blood cells (SRBC) was followed by an increase in the number of cells in three defined populations of early precursor B cells lacking mu chains (TdT+ 14.8- cells, TdT+ 14.8+ cells, and 14.8+ mu- cells) as well as cytoplasmic mu-bearing pre-B cells and surface mu-bearing B lymphocytes. An accompanying increase in proliferative activity was indicated by the numbers of 14.8+ mu- cells and pre-B cells which accumulated in metaphase after inducing mitotic arrest with vincristine. These effects were all abrogated either by treating mice with silica to depress macrophage function or by splenectomy. In mice given multiple injections of SRBC for 4 weeks the elevated levels of early precursor B cell production and B cell genesis were sustained. The work demonstrates that the in vivo production of early precursor B cells, putatively including those at the stage of Ig heavy chain gene rearrangement, can be stimulated by exposure to external agents acting indirectly by a silica-sensitive, spleen-dependent mechanism. The findings suggest that the level of pro-B cell proliferation and primary B cell genesis normally taking place in mouse bone marrow may reflect the level of exposure to potential stimulants in the external environment mediated by activation of splenic macrophages. The possibility that abnormally high levels of macrophage activation could predispose to dysregulations of the B cell lineage is raised.

The effect of the graft-versus-host reaction on B lymphocyte production in bone marrow of mice. Depressed genesis of early progenitors prior to mu heavy chain expression

The effects of systemic graft-versus-host (GVH) reactions on the early precursor cell populations involved in primary B lymphocyte genesis have been examined in the bone marrow of (C57BL/6xA)F1 mice injected with lymphoid cells from A strain mice. Double immunofluorescence labeling techniques for the intranuclear enzyme, terminal deoxynucleotidyl transferase (TdT), the B220 cell surface glycoprotein detected by monoclonal antibody, 14.8, and surface or cytoplasmic mu chains of IgM (s mu, c mu) were used to quantitate 3 putative early B lineage progenitors preceding mu chain expression (TdT+14.8-mu-, TdT+14.8+mu- and TdT-14.8+mu-), pre-B cells (c mu+, s mu-) and B lymphocytes (s mu+). After initiating GVH reactions, the early B precursor cells, pre-B cells, and B lymphocytes in the bone marrow all fell rapidly in numbers, being almost completely absent from 10-15 days to the end of the 30-day assay period. The decline of some of the early progenitors started at a later time and was less complete than that of the more differentiated B lineage cells. In the spleen, B lymphocytes declined rapidly in numbers after 8 days to less than 5% of normal values from 12 days onward. The results demonstrate that systemic GVH reactions in mice almost completely eliminate the B cell lineage, including early precursor cells apparently undergoing mu chain rearrangement in the bone marrow. The pattern of depletion suggests that a range of B lineage progenitor cells may be directly susceptible to GVH reactions. The findings contribute to a model for the pathogenesis of the humoral immunodeficiency of systemic GVH disease.

Population dynamics of "null" and Thy1lo lymphocytes in mouse bone marrow: genesis of cells with natural killer cell lineage characteristics

The population dynamics of "null" small lymphocytes lacking B and T lineage markers in mouse bone marrow have been examined using a combination of immunolabeling and hydroxyurea (HU) deletion techniques. The binding of the B lineage-associated mAb, 14.8, and anti-Thy1.2 to bone marrow cells has been detected radioautographically. Null cells lacking 14.8 and Thy1.2 determinants (14.8- Thy1-) formed a substantial subset (12-14%) of bone marrow small lymphocytes, representing 0.5 x 10(6) cells per femur (2-3% of nucleated cells). HU treatment revealed an exceptionally rapid turnover of the null small lymphocyte population (T1/2, 7.5 hr) compared with 14.8+ cells (T1/2, 20.5 hr) and Thy1+ cells (T1/2, 53 hr). Small lymphocytes bearing low intensities of Thy1 (Thy1lo) were also rapidly renewed (T1/2, 28 hr) whereas those with high intensities of Thy1 (Thy1hi) were renewed only slowly (T1/2, 123 hr). During ontogeny, null small lymphocytes first appeared in the fetal liver by Day 11 and the fetal spleen by Day 16, but increased rapidly in the bone marrow in early postnatal life. Double immunolabeling techniques demonstrated that 10% of null small lymphocytes in the bone marrow expressed NK1.1 antigen, while larger proportions bound to tumor (YAC.1) cells in vitro and displayed Fc receptors. The NK1.1-bearing fraction of null small lymphocytes in bone marrow was depleted by HU treatment only after an initial delay. NK1.1 was also expressed on subsets of Thy1lo cells and Thy1hi cells. The results have revealed the continuous production in mouse bone marrow of null and Thy1lo small lymphocytes, totaling 1-3 x 10(7) cells/day and 1.2 x 10(6) cells/day, respectively. The findings suggest that the large-scale production of null lymphocytes in mouse bone marrow includes the genesis of NK lineage cells which express NK1.1 and Thy1lo during a period of terminal maturation.

To B, or not to B: that is the question

Irrespective of the prevailing fashion, Germinal Centre Conferences (GCC) have treated the immune system in a determinedly holistic manner. At the recent 10th GCC* the advantages of this approach were demonstrated in elegant new schema for lymphocyte development, accessory cell function, tolerance and autoimmunity, and for the organization and function of mucosal defence. This report concentrates on only one aspect of the meeting - the development of an integrated view of the natural history of B cells.

Microenvironmental organization and stromal cell associations of B lymphocyte precursor cells in mouse bone marrow

B lymphocyte precursor cells expressing B220 glycoprotein have been examined in mouse bone marrow (BM) by the in vivo binding of monoclonal antibody (mAb) 14.8 visualized by light and electron microscope radio autography. Young mice were injected intravenously with 125I-labeled mAb 14.8 and then perfused to remove unbound antibody. Quantitative analysis of radioauto graphic sections of femoral BM revealed many labeled mAb 14.8-binding cells which were situated both singly and in groups throughout the extravascular BM parenchyma. Groups of large 14.8+ cells were located in patchy areas in the peripheral regions of the BM near the endosteum. These cells were shown to include proliferating precursor B cells by using mice given vincristine sulfate to stop cells in metaphase and mice treated from birth with anti-IgM antibodies to delete mature B lymphocytes. Electron microscopy revealed clusters of 14.8+ cells intimately associated with the processes of stromal reticular cells. Other 14.8+ cells were in close contact with macrophages; in some instances the intervening cell membranes were indistinct and the macrophages contained 14.8+ material in their cytoplasm. In addition, 14.8+ small lymphocytes were highly concentrated within the lumen of some sinusoids. The present method of detecting B lineage precursor cells in situ has led to a working model of the microenvironmental organization of primary B cell genesis in vivo. The model proposes (a) a centrally directed sequence of differentiation initiated by early precursor cells situated peripherally near the surrounding bone; (b) close associations between precursor B cells and stromal reticular cells; (c) deletion of ineffective B cells by macrophages, and (d) an intravascular maturation phase before B lymphocytes are finally delivered into the blood stream.

Early B-lymphocyte precursor cells in mouse bone marrow: subosteal localization of B220+ cells during postirradiation regeneration

The localization of early B-lymphocyte precursor cells in the bone marrow of young mice has been studied during recovery from sublethal whole body gamma-irradiation (150 rad). Initial studies by double immunofluorescence labeling of the B-lineage-associated cell surface glycoprotein, B220, and of mu heavy chains in bone marrow cell suspensions, demonstrated a sequential wave of regeneration of early B precursor cells, pre-B cells, and B cells. Early B precursor cells expressing B220 but not mu chains were enriched at 1-3 days following irradiation. After in vivo administration of 125I-labeled monoclonal antibody 14.8 to detect B220+ cells in situ, light and electron microscope radioautography of femoral bone marrow sections revealed concentrations of labeled B220+ cells located peripherally near the cortical bone at 1-3 days following irradiation, increasing in numbers in more central areas by 5-7 days. Proliferative B220+ precursor cells were found within layers of bone-lining cells and in a subosteal area characterized by a prominent electron-dense extracellular matrix, often associated with stromal reticular cells. The results demonstrate that the precursor cells that are active in the bone marrow early in the recovery of B lymphopoiesis after gamma-irradiation are located both within and near the endosteum of the surrounding bone. The distinctive extracellular matrix and stromal cell associations noted in this region may contribute to a supportive local microenvironment for early hemopoietic progenitor cells.

B cell development in the bone marrow

Recent years have produced considerable progress in defining stages in the development of B cells in vivo, and in revealing interactions with regulatory molecules and cells. Studies of the phenotype and population dynamics of precursor B cells in mouse bone marrow have quantitated cell production at sequential steps of differentiation and have also indicated a substantial cell death. The proliferation of precursor B cells is influenced both by systemic factors and by cytokines derived from bone marrow stromal cells. In situ immunolabeling has revealed that early precursor B cells are closely associated with subosteal stromal cells, aberrant B lineage cells appear to be deleted by macrophages and terminal B cells mature within the lumen of vascular sinusoids before being released. The findings lead to working models of the in vivo differentiation, regulation and microenvironmental organization of B cell genesis in the bone marrow.

Proliferation of B cell precursors in bone marrow of pristane-conditioned and malaria-infected mice: implications for B cell oncogenesis

Two widely different agents implicated in the etiology of neoplasias of the B cell lineage, pristane and malaria, have both been found to produce a prolonged increase in the level of proliferative activity and cell production by early B lymphocyte precursor cells in mouse bone marrow. This apparently leads to an elevated level of cell loss, suggesting the production of many aberrant early cells. The mechanism and significance of this effect remain to be determined. However, the present findings focus attention on the early stages of B cell genesis in the bone marrow as possible target cells for the initiation of genetic events leading to neoplasia. Together with previous work, the results suggest that pathologically elevated levels of macrophage activation may play a role in predisposing to various B cell neoplasias.

Dynamics of early B lymphocyte precursor cells in mouse bone marrow: proliferation of cells containing terminal deoxynucleotidyl transferase

Three populations of early B lymphocyte precursor cells lacking mu heavy chains have been defined in mouse bone marrow, based on immunofluorescence labeling for terminal deoxynucleotidyl transferase (TdT) and B220 glycoprotein, as detected by monoclonal antibody 14.8 (TdT+14.8- cells; TdT+14.8+ cells; TdT-14.8+ cells). We have now analyzed the frequency, size distribution, proliferation and production rates of TdT+ cells in mouse bone marrow. These formed well-defined populations of medium-sized cells, the TdT+14.8+ cells tending to be larger than TdT+14.8- cells (modal cell diameters in cytocentrifuge preparations; 10.0 microns and 9.0 microns, respectively). Some TdT+ cells (1%-2%) were normally in metaphase, the TdT being dispersed through the cytoplasm. After inducing mitotic arrest with vincristine, the incidence of TdT+ cells in metaphase increased linearly from 2 to 4 h, indicating a turnover of 5.1%/h for TdT+14.8- cells and 9.0%/h for TdT+14.8+ cells. Subtraction of turnover data for TdT+14.8+ cells from those previously obtained for 14.8+ mu- cells gave values for the population of TdT-14.8+ cells. The calculated daily turnover of cells in the three compartments increased progressively (TdT+14.8-, 2.5 x 10(6) cells; TdT+14.8+, 5.0 x 10(6) cells; TdT-14.8+, 36.0 x 10(6) cells), accompanied by a shortening of the average apparent cell cycle time (TdT+14.8-, 20 h; TdT+14.8+, 11 h; TdT-14.8+, 8 h). The results demonstrate a progressive expansion of cell production at three putatively successive stages of early B lymphocyte development before the expression of mu chains. The findings contribute to a kinetic model of primary B cell genesis in mouse bone marrow.

Post-irradiation regeneration of early B-lymphocyte precursor cells in mouse bone marrow

To examine the sequential development of early B-cell precursors in mouse bone marrow, B-lineage cells have been examined during a wave of post-irradiation regeneration. Cell phenotypes have been defined using double-immunofluorescence labelling techniques for (i) terminal deoxynucleotidyl transferase (TdT); (ii) B220 glycoprotein, detected by the binding of mAb 14.8; (iii); mu heavy chains in the cytoplasm (c mu) and at the cell surface (s mu). Three populations of mu- cells (TdT+14.8-; TdT+14.8+; TdT-14.8+) have been proposed to be early B-cell precursors which would give rise to c mu+s mu- pre-B cells and thus to s mu+ B lymphocytes. From 3 to 7 days after a sublethal dose (150 rads) of whole body gamma-irradiation, the B-lineage cells recovered rapidly to exceed normal numbers. The early B-cell precursors increased to peaks of 1.8-2.5 times normal numbers, preceding by 1 day a comparable increase and overshoot of c mu+s mu- pre-B cells, followed by recovery of s mu+ B lymphocytes. The TdT+14.8- cells peaked first at 5 days, subsiding again at 7-10 days with a shift from large- to medium-sized cells. The TdT+14.8+ cells showed a later peak (6 days), a more sustained wave in cell numbers and a delayed shift in cell size. The substantial population of 14.8+ mu- cells reached maximal observed values at 7 days and still maintained a predominantly large cell size profile at 10 days. The timing, cell-size shifts and progressive amplification of the waves of regeneration accord with a dynamic model in which the TdT+14.8-,TdT+14.8+ and TdT-14.8+ cells form three successive stages in B-cell differentiation before the expression of mu chains, presumptively including the stage of mu chain gene rearrangement. In addition, the results provide an experimental system for the enrichment of early B-cell precursors in mouse bone marrow.

Regulation of B-lymphocyte production in the bone marrow: mediation of the effects of exogenous stimulants by adoptively transferred spleen cells

The cellular mechanism by which an injection of sheep red blood cells (SRBC) results in an increased production of B lymphocytes in mouse bone marrow has been studied by adoptive cell transfer and the use of two in vivo assays of bone marrow B-cell genesis. Proliferation of B progenitor cells was examined by immunofluorescent labeling combined with mitotic arrest, while small lymphocyte renewal was measured by [3H]thymidine labeling and radioautography. In C3H/HeJ mice the administration of SRBC resulted in increased proliferation among bone marrow pre-B cells which contained cytoplasmic mu heavy chains but lacked kappa light chains and surface mu chains. The turnover of small lymphocytes also increased. These stimulatory effects were transferred to naive recipient mice by organ fragments and by cell suspensions harvested from the spleens of donor mice injected with SRBC. In contrast, spleen cells and thymus cells from saline-injected donors and thymus cells from SRBC-injected donors had no such stimulatory effects. The results demonstrate that spleen cells mediate the stimulatory effect of SRBC on bone marrow B-lymphocyte production. Spleen cell transfer provides a system to study further the cells and factors involved in the regulation by external environmental agents of the rate of primary B-cell genesis in vivo.