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

Elucidation of the Binding Orientation in α2,3- and α2,6-Linked Neu5Ac-Gal Epitopes toward a Hydrophilic Molecularly Imprinted Monolith

N-Acetylneuraminic acid and its α2,3/α2,6-glycosidic linkages with galactose (Neu5Ac-Gal) are major carbohydrate antigen epitopes expressed in various pathological processes, such as cancer, influenza, and SARS-CoV-2. We here report a strategy for the synthesis and binding investigation of molecularly imprinted polymers (MIPs) toward α2,3 and α2,6 conformations of Neu5Ac-Gal antigens. Hydrophilic imprinted monoliths were synthesized from melamine monomer in the presence of four different templates, namely, N-acetylneuraminic acid (Neu5Ac), N-acetylneuraminic acid methyl ester (Neu5Ac-M), 3'-sialyllactose (3SL), and 6'-sialyllactose (6SL), in a tertiary solvent mixture at temperatures varying from -20 to +80 °C. The MIPs prepared at cryotemperatures showed a preferential affinity for the α2,6 linkage sequence of 6SL, with an imprinting factor of 2.21, whereas the α2,3 linkage sequence of 3SL resulted in nonspecific binding to the polymer scaffold. The preferable affinity for the α2,6 conformation of Neu5Ac-Gal was evident also when challenged by a mixture of other mono- and disaccharides in an aqueous test mixture. The use of saturation transfer difference nuclear magnetic resonance (STD-NMR) on suspensions of crushed monoliths allowed for directional interactions between the α2,3/α2,6 linkage sequences on their corresponding MIPs to be revealed. The Neu5Ac epitope, containing acetyl and polyalcohol moieties, was the major contributor to the sequence recognition for Neu5Ac(α2,6)Gal(β1,4)Glc, whereas contributions from the Gal and Glc segments were substantially lower.

The mitochondrial iron transporter ABCB7 is required for B cell development, proliferation, and class switch recombination in mice

Iron-sulfur (Fe-S) clusters are cofactors essential for the activity of numerous enzymes including DNA polymerases, helicases, and glycosylases. They are synthesized in the mitochondria as Fe-S intermediates and are exported to the cytoplasm for maturation by the mitochondrial transporter ABCB7. Here, we demonstrate that ABCB7 is required for bone marrow B cell development, proliferation, and class switch recombination, but is dispensable for peripheral B cell homeostasis in mice. Conditional deletion of ABCB7 using Mb1-cre resulted in a severe block in bone marrow B cell development at the pro-B cell stage. The loss of ABCB7 did not alter expression of transcription factors required for B cell specification or commitment. While increased intracellular iron was observed in ABCB7-deficient pro-B cells, this did not lead to increased cellular or mitochondrial reactive oxygen species, ferroptosis, or apoptosis. Interestingly, loss of ABCB7 led to replication-induced DNA damage in pro-B cells, independent of VDJ recombination, and these cells had evidence of slowed DNA replication. Stimulated ABCB7-deficient splenic B cells from CD23-cre mice also had a striking loss of proliferation and a defect in class switching. Thus, ABCB7 is essential for early B cell development, proliferation, and class switch recombination.

How should we sequence and combine novel therapies in CLL?

With the recent approval of several effective and well-tolerated novel agents (NAs), including ibrutinib, idelalisib, venetoclax, and obinutuzumab, patients with chronic lymphocytic leukemia (CLL) have more therapeutic options than ever before. The availability of these agents is both an important advance for patients but also a challenge for practicing hematologist/oncologists to learn how best to sequence NAs, both with respect to chemoimmunotherapy (CIT) and to other NAs. The sequencing of NAs in clinical practice should be guided both by an individual patient's prognostic markers, such as FISH and immunoglobulin heavy chain variable region (IGHV)-mutation status, as well as the patient's medical comorbidities and goals of care. For older, frailer patients with lower-risk CLL prognostic markers, NA monotherapy may remain a mainstay of CLL treatment for years to come. For younger, fitter patients and those with higher-risk CLL, such as del(17p) or unmutated IGHV, combination approaches may prove to be more valuable than NA monotherapy. Trials are currently evaluating the efficacy of several such combination approaches, including NA plus anti-CD20 monoclonal antibody, NA plus NA (with or without anti-CD20 monoclonal antibody), and NA plus CIT. Given the tremendous efficacy of the already approved NAs, as well as the promising data for next generation NAs, the development of well-tolerated, highly effective combination strategies with curative potential for patients with CLL has become a realistic goal.

Bone Marrow-Derived Stem Cell Populations Are Differentially Regulated by Thyroid or/and Ovarian Hormone Loss

Bone marrow-derived stem cells (BMDSCs) play an essential role in organ repair and regeneration. The molecular mechanisms by which hormones control BMDSCs proliferation and differentiation are unclear. Our aim in this study was to investigate how a lack of ovarian or/and thyroid hormones affects stem cell number in bone marrow lineage. To examine the effect of thyroid or/and ovarian hormones on the proliferative activity of BMDSCs, we removed the thyroid or/and the ovaries of adult female rats. An absence of ovarian and thyroid hormones was confirmed by Pap staining and Thyroid Stimulating Hormone (TSH) measurement, respectively. To obtain the stem cells from the bone marrow, we punctured the iliac crest, and aspirated and isolated cells by using a density gradient. Specific markers were used by cytometry to identify the different BMDSCs types: endothelial progenitor cells (EPCs), precursor B cells/pro-B cells, and mesenchymal stem cells (MSCs). Interestingly, our results showed that hypothyroidism caused a significant increase in the percentage of EPCs, whereas a lack of ovarian hormones significantly increased the precursor B cells/pro-B cells. Moreover, the removal of both glands led to increased MSCs. In conclusion, both ovarian and thyroid hormones appear to have key and diverse roles in regulating the proliferation of cells populations of the bone marrow.

Phosphatase Wip1 controls antigen-independent B-cell development in a p53-dependent manner

Wild-type p53-induced phosphatase 1 (Wip1), a phosphatase previously considered as an oncogene, has been implicated in the regulation of thymus homeostasis and neutrophil maturation. However, the role of Wip1 in B-cell development is unknown. We show that Wip1-deficient mice exhibit a significant reduction of B-cell numbers in the bone marrow, peripheral blood, and spleen. A reciprocal transplantation approach revealed a cell-intrinsic defect in early B-cell precursors caused by Wip1 deficiency. Further experiments revealed that Wip1 deficiency led to a sustained activation of p53 in B cells, which led to increased level of apoptosis in the pre-B-cell compartment. Notably, the impairment of B-cell development in Wip1-deficient mice was completely rescued by genetic ablation of p53, but not p21. Therefore, loss of Wip1 phosphatase induces a p53-dependent, but p21-independent, mechanism that impairs B-cell development by enhancing apoptosis in early B-cell precursors. Moreover, Wip1 deficiency exacerbated a decline in B-cell development caused by aging as evidenced in mice with aging and mouse models with serial competitive bone marrow transplantation, respectively. Our present data indicate that Wip1 plays a critical role in maintaining antigen-independent B-cell development in the bone marrow and preventing an aging-related decline in B-cell development.

B-cell tolerance in transplantation: is repertoire remodeling the answer?

T lymphocytes are the primary targets of immunotherapy in clinical transplantation; however, B lymphocytes and their secreted alloantibodies are also highly detrimental to the allograft. Therefore, the achievement of sustained organ transplant survival will likely require the induction of B-lymphocyte tolerance. During development, acquisition of B-cell tolerance to self-antigens relies on clonal deletion in the early stages of B-cell compartment ontogeny. We contend that this mechanism should be recapitulated in the setting of alloantigens and organ transplantation to eliminate the alloreactive B-cell subset from the recipient. Clinically feasible targets of B-cell-directed immunotherapy, such as CD20 and B-lymphocyte stimulator (BLyS), should drive upcoming clinical trials aimed at remodeling the recipient B-cell repertoire.

Stromal cells modulate TCL1 expression, interacting AP-1 components and TCL1-targeting micro-RNAs in chronic lymphocytic leukemia

The tissue microenvironment in chronic lymphocytic leukemia (CLL) has an increasingly recognized role in disease progression, but the molecular mechanisms of cross talk between CLL cells and their microenvironment remain incompletely defined. Bone marrow stromal cells (BMSC) protect CLL cells from apoptosis in a contact-dependent fashion, and have been used for the identification of key pathways such as the CXCR4-CXCL12 axis. To further dissect the molecular impact of BMSC on survival and the molecular activation signature of CLL cells, we co-cultured CLL cells with different BMSC. Gene expression profiling of CLL cells revealed that the lymphoid proto-oncogene TCL1 was among the top genes upregulated in CLL cells by BMSC. TCL1 mRNA and protein upregulation by BMSC was paralleled by decreases of TCL1-interacting FOS/JUN, and confirmed by qRT-PCR, immunoblotting, immunoprecipitations, and flow cytometry. Stroma mediated increases in TCL1 were also associated with decreased levels of TCL1-regulatory micro-RNAs (miR-29b, miR-181b, miR-34b). These findings demonstrate that the microenvironment has a proactive role in the regulation of the known signaling enhancer and pro-survival molecule TCL1 in CLL. This provides a further rationale for therapeutically targeting the cross talk between CLL and BMSC.

Annexin A1 regulates neutrophil clearance by macrophages in the mouse bone marrow

Under homeostatic conditions, a proportion of senescent CXCR4(hi) neutrophils home from the circulation back to the bone marrow, where they are phagocytosed by bone marrow macrophages. In this study, we have identified an unexpected role for the anti-inflammatory molecule annexin A1 (AnxA1) as a critical regulator of this process. We first observed that AnxA1(-/-) mice have significantly increased neutrophil numbers in their bone marrow while having normal levels of GM and G colony-forming units, monocytes, and macrophages. Although AnxA1(-/-) mice have more neutrophils in the bone marrow, a greater proportion of these cells are senescent, as determined by their higher levels of CXCR4 expression and annexin V binding. Consequently, bone marrow neutrophils from AnxA1(-/-) mice exhibit a reduced migratory capacity in vitro. Studies conducted in vitro also show that expression of AnxA1 is required for bone marrow macrophages, but not peritoneal macrophages, to phagocytose apoptotic neutrophils. Moreover, in vivo experiments indicate a defect in clearance of wild-type neutrophils in the bone marrow of AnxA1(-/-) mice. Thus, we conclude that expression of AnxA1 by resident macrophages is a critical determinant for neutrophil clearance in the bone marrow.

Diverse marrow stromal cells protect CLL cells from spontaneous and drug-induced apoptosis: development of a reliable and reproducible system to assess stromal cell adhesion-mediated drug resistance

Marrow stromal cells (MSCs) provide important survival and drug resistance signals to chronic lymphocytic leukemia (CLL) cells, but current models to analyze CLL-MSC interactions are heterogeneous. Therefore, we tested different human and murine MSC lines and primary human MSCs for their ability to protect CLL cells from spontaneous and drug-induced apoptosis. Our results show that both human and murine MSCs are equally effective in protecting CLL cells from fludarabine-induced apoptosis. This protective effect was sustained over a wide range of CLL-MSC ratios (5:1 to 100:1), and the levels of protection were reproducible in 4 different laboratories. Human and murine MSCs also protected CLL cells from dexamethasone- and cyclophosphamide-induced apoptosis. This protection required cell-cell contact and was virtually absent when CLL cells were separated from the MSCs by micropore filters. Furthermore, MSCs maintained Mcl-1 and protected CLL cells from spontaneous and fludarabine-induced Mcl-1 and PARP cleavage. Collectively, these studies define common denominators for CLL cocultures with MSCs. They also provide a reliable, validated tool for future investigations into the mechanism of MSC-CLL cross talk and for drug testing in a more relevant fashion than the commonly used suspension cultures.

Neutrophil mobilization and clearance in the bone marrow

The bone marrow is the site of neutrophil production, a process that is regulated by the cytokine granulocyte colony-stimulating factor (G-CSF). Mature neutrophils are continually released into the circulation, with an estimated 10(11) neutrophils exiting the bone marrow daily under basal conditions. These leucocytes have a short half-life in the blood of approximately 6.5 hr, and are subsequently destroyed in the spleen, liver and indeed the bone marrow itself. Additionally, mature neutrophils are retained in the bone marrow by the stromal cell-derived factor (SDF-1alpha)/chemokine (C-X-C motif) receptor 4 (CXCR4) chemokine axis and form the bone marrow reserve. Following infection or inflammatory insult, neutrophil release from the bone marrow reserve is substantially elevated and this process is mediated by the co-ordinated actions of cytokines and chemokines. In this review we discuss the factors and molecular mechanisms regulating the neutrophil mobilization and consider the mechanisms and functional significance of neutrophil clearance via the bone marrow.

BLNK suppresses pre-B-cell leukemogenesis through inhibition of JAK3

Pre-B-cell leukemia spontaneously develops in BLNK-deficient mice, and pre-B-cell acute lymphoblastic leukemia cells in children often lack BLNK protein expression, demonstrating that BLNK functions as a tumor suppressor. However, the mechanism by which BLNK suppresses pre-B-cell leukemia, as well as the identification of other genetic alterations that collaborate with BLNK deficiency to cause leukemogenesis, are still unknown. Here, we demonstrate that the JAK3/STAT5 signaling pathway is constitutively activated in pre-B leukemia cells derived from BLNK(-/-) mice, mostly due to autocrine production of IL-7. Inhibition of IL-7R signaling or JAK3/STAT5 activity resulted in the induction of p27(kip1) expression and cell-cycle arrest, accompanied by apoptosis in the leukemia cells. Transgene-derived constitutively active STAT5 (STAT5b-CA) strongly synergized with the loss of BLNK to initiate leukemia in vivo. In the leukemia cells, exogenously expressed BLNK inhibited autocrine JAK3/STAT5 signaling, resulting in p27(kip1) induction, cell-cycle arrest, and apoptosis. BLNK-inhibition of JAK3 was dependent on the binding of BLNK to JAK3. These data indicate that BLNK normally regulates IL-7-dependent proliferation and survival of pre-B cells through direct inhibition of JAK3. Thus, somatic loss of BLNK and concomitant mutations leading to constitutive activation of Jak/STAT5 pathway result in the generation of pre-B-cell leukemia.

The bone marrow perisinusoidal niche for recirculating B cells and the positive selection of bone marrow-derived B lymphocytes

A unique 'second' niche for follicular B cells has been described in the extravascular compartment of the bone marrow surrounding vascular sinusoids. The occupancy of this niche by B cells presumably evolved to facilitate humoral immune responses to blood-borne pathogens. B cells appear to be sustained in this niche by bone marrow dendritic cells and are lost from this compartment in certain mutant mice. We discuss here what is known regarding the mechanisms of entry and egress of B cells from the perisinusoidal niche and also consider the function of the bone marrow as a secondary lymphoid organ. Although immature B cells can mature into follicular B cells in this niche as well as in the spleen, the lineage commitment event that accompanies positive selection of B cells occurs only in the spleen.

The role of the bone marrow in neutrophil clearance under homeostatic conditions in the mouse

In humans, 10(11) neutrophils are released from the bone marrow per day, and these cells have a half-life in the blood of only approximately 6.5 h. Although it is generally believed that neutrophils are cleared from the circulation via the liver and spleen, in this study using (111)In-labeled senescent neutrophils, we show that in mice, 32% of neutrophils are cleared from the circulation via the bone marrow. We have previously shown that senescent neutrophils home to the bone marrow in a CXCR4-dependent manner, and we show here that pretreatment of neutrophils with pertussis toxin significantly inhibits neutrophil clearance via the bone marrow (75%), consistent with a role for chemokines in this process. By labeling senescent neutrophils with inert fluorescent microspheres, we have tracked their fate and shown that in vivo, they are ultimately phagocytosed by bone marrow stromal macrophages. Finally, we show that under noninflammatory conditions, circulating levels of neutrophils are regulated by granulocyte-colony stimulating factor (G-CSF), but not interleukin-17. Interestingly, we report that the uptake of apoptotic neutrophils by bone marrow macrophages stimulates their production of G-CSF in vitro. Taken together, these data provide evidence that the bone marrow represents a major site of neutrophil clearance in mice.

The role of melatonin in immuno-enhancement: potential application in cancer

Melatonin, a neurohormone produced mainly by the pineal gland, is a modulator of haemopoiesis and of immune cell production and function, both in vivo and in vitro. Physiologically, melatonin is associated with T-helper 1 (Th1) cytokines, and its administration favours Th1 priming. In both normal and leukaemic mice, melatonin administration results in quantitative and functional enhancement of natural killer (NK) cells, whose role is to mediate defenses against virus-infected and cancer cells. Melatonin appears to regulate cell dynamics, including the proliferative and maturational stages of virtually all haemopoietic and immune cells lineages involved in host defense - not only NK cells but also T and B lymphocytes, granulocytes and monocytes - in both bone marrow and tissues. In particular, melatonin is a powerful antiapoptotic signal promoting the survival of normal granulocytes and B lymphocytes. In mice bearing mid-stage leukaemia, daily administration of melatonin results in a survival index of 30-40% vs. 0% in untreated mice. Thus, melatonin seems to have a fundamental role as a system regulator in haemopoiesis and immuno-enhancement, appears to be closely involved in several fundamental aspects of host defense and has the potential to be useful as an adjuvant tumour immunotherapeutic agent.

Notch1 enhances B-cell receptor-induced apoptosis in mature activated B cells without affecting cell cycle progression and surface IgM expression

The transmembrane receptor Notch1 plays a crucial role in differentiation and apoptosis of hematopoietic cells. To investigate the influence of Notch1 on apoptosis and cell growth of mature murine B cells, we transduced the murine B-lymphoma line NYC 31.1 with a constitutively active, intracellular form of human Notch1 (Notch1-ICT). NYC cells represent mature activated B cells that can be induced to undergo apoptosis by crosslinking of the B-cell receptor (BCR). In contrast to investigations in immature chicken B-cell lines, transduced Notch1-ICT did not affect cell cycle progression, cell growth or surface IgM levels in NYC cells and resulted only in a slight induction of apoptosis. However, BCR-crosslinking enhanced apoptosis, but did not influence cell cycle progression in Notch1-ICT-positive NYC cells. These data imply a distinct function of Notch1 in mature murine B-cells as compared to immature chicken B cells and provide further evidence for Notch1's involvement in B-cell differentiation and development.

The life and death of a B cell

Regulation of apoptosis in the B cell lineage has implications for homeostasis, quality control of the antibody response, and tolerance. In this chapter we examine the different checkpoints that control life and death decisions of B cells during the antigen-independent and antigen-dependent phases of their development. We discuss the cell death mechanism involved in elimination of unwanted B cells at different stages of their development as well as the signals that trigger or repress the apoptotic process. At the steady state, before or after development of an immune response, B cell apoptosis ensures that the antigen receptor (BCR) on newly produced B cells is functional and does not recognize self-antigens with high avidity. It also ensures that the size of the peripheral B cell compartment remains constant in spite of the continuous input of B cells from the bone marrow. All these processes are controlled by the mitochondrial death pathway and are thus perturbed by overexpression of the antiapoptotic members of the bcl-2 gene family. By contrast, the death receptor pathway plays a prominent role during the antigen-dependent phase of B cell development. Three sets of membrane molecules stand as crucial regulators of B cell survival. First, the BCR which plays a central but ambiguous role. On the one hand, it triggers death of B cells that recognize self-antigens or have been exposed to repeated antigenic stimulations. On the other hand, it promotes survival of the peripheral mature B cell pool and protects activated B cells from CD95-induced killing. Second, the death receptor Fas/CD95 which is instrumental in censoring B cells activated in a bystander fashion at the initiation of the response to T-dependent antigens. It also drives elimination of low-affinity and self-reactive B cell clones that arise through the process of somatic mutations during the germinal center reaction. As such, it contributes to the affinity maturation of the antibody response. Finally, three membrane receptors (TACI, BCMA, and BAFF-R) which bind a newly discovered member of the tumor necrosis factor family named BAFF. BAFF acts specifically on peripheral B cells but its cellular targets seem to be restricted to two splenic B cell populations: (i) transitional immature B cells and (ii) marginal zone B cells, known to be responsible for the response to thymus-independent type 2 antigens. This suggests its possible implication in positive selection of peripheral B cells and in the antibacterial B cell responses.

Glucocorticoid-induced apoptosis in early B cells from human bone marrow

The sensitivity of normal human lymphoid precursor cells to glucocorticoid-induced apoptosis is a subject of controversy. The in vitro response of cells of the B lineage (CD19(+)) from the marrow of 22 adult subjects to glucocorticoids was evaluated herein using both natural steroids and dexamethasone (Dex). When exposed to 1 micro M Dex, 32% of the subjects exhibited high losses of CD19(+) B cells in the range of 45%. The remaining subjects exhibited more modest losses in CD19(+) cells of 26%-40%. Surprisingly, cortisol, a naturally produced glucocorticoid, produced B lineage losses nearly equivalent to Dex, which reached maximum by 12 hr. It was subsequently noted that the variances in losses of CD19(+) cells among the subjects correlated closely with the proportion of early CD10(+) CD19(+) B cells present in the initial population. The latter cells exhibited a high degree of sensitivity to glucocorticoids, with losses of 60%-80% noted. Mature B cells bearing IgD, on the other hand, were fairly resistant to glucocorticoids. Merocyanine 540, a membrane dye that fluoresces in the disordered membrane of apoptotic cells, confirmed that early or progenitor B cells in human bone marrow were indeed undergoing glucocorticoid-induced apoptosis, which could be blocked by the glucocorticoid antagonist RU38486. These data provide evidence that human marrow B cells, especially early B-cell progenitors, are quite sensitive to glucocorticoids and readily undergo apoptosis within a few hours of exposure to the steroids.

In vitro cytotoxicity assay to screen compounds for apoptosis-inducing potential on lymphocytes and neutrophils

In vitro cytotoxicity assay to screen compounds for apoptosis-inducing potential on lymphocytes and neutrophils was investigated. Mouse, rat, dog, and human whole blood were incubated for 4 and 6 hr with actinomycin D, camptothecin, cortisone acetate, cycloheximide, doxorubicin, etoposide, 5-FU, mitomycin C and puromycin. Apoptotic lymphocytes and neutrophils were counted. All test compounds induced in vitro apoptosis of lymphocytes and/or neutrophils, but there were different potencies among the test compounds and there were also species differences in susceptibility. To investigate the in vivo effects of etoposide and cycloheximide which induced apoptosis of rat lymphocytes and that of rat lymphocytes and neutrophils, respectively, in in vitro assay, rats were intravenously administered either etoposide at 12.5, 25 or 50 mg/kg or cycloheximide at 1.25, 2.5 or 5 mg/kg. Etoposide caused decreases of circulating lymphocytes at 3 hr after administration in a dose-dependent manner, -16, -25 and -51%. Although cycloheximide caused neither decreased lymphocyte nor neutrophil counts, apoptosis in 30% of neutrophils was observed in rats receiving 5 mg/kg at 3 hr after administration. Etoposide at 50 mg/kg and cycloheximide at 5 mg/kg caused lymphocyte apoptosis in the spleen, thymus, mesenteric lymph nodes, bone marrow, and Peyer's patch from 1 to 6 hr after administration, with the maximum changes at 3 hr. In addition to apoptosis of these organs, cycloheximide at 5 mg/kg caused apoptosis of polymorphonuclear cells in the lamina propria of the small intestine. Therefore, it was found that the changes seen in the in vivo experiments considerably reflected the changes seen in the in vitro experiments. From these results, apoptosis is probably one of the major mechanisms for leukocyte toxicity induced by cytotoxic compounds, and the in vitro assay to screen compounds for acute apoptosis-inducing potential on lymphocytes and neutrophils would be useful as a primary screening method for animal toxicity studies. It may also be useful for risk assessments in advance of clinical trials.

Modulatory role of thymosin-alpha-1 in normal bone-marrow haematopoiesis and its effect on myelosuppression in T-cell lymphoma bearing mice

In continuation with the earlier and ongoing studies on Thymosin-alpha-1 (Talpha1) exerting its immunomodulatory effects on various components of the immune system including T-cells, NK-cells, blood lymphocytes and macrophages, the role of Talpha1 in normal bone-marrow haematopoiesis has been investigated in the present study. The haematopoietic alterations associated with the growth of murine T-cell lymphoma, Dalton's Lymphoma (DL) and subsequently its restoration by Talpha1 was also investigated. It is observed that the non-adherent bone-marrow cells from normal mice (N-BMCs) exhibited enhanced proliferation on in vitro treatment with Talpha1 (dose range of 1-100 ng/ml) with maximal response at 100 ng/ml of Talpha1. In vitro stimulation with 100 ng/ml of Talpha1 also resulted in increased myeloid colony formation, as manifested by the rise in total number of colonies, frequency of the individual colony types and their size. This response was further upregulated in the presence of various colony stimulating factors (CSFs) like MCSF, GMCSF, GCSF and IL-3. Similarly, in vivo administration of Talpha1 (a single intraperitoneal injection of 10 microg per mouse) to normal mice also resulted in enhanced proliferation and colony formation by BMCs as compared with BMCs obtained from untreated mice. On the contrary, the progressive growth of T-cell lymphoma in mice led to suppressed myelogenesis, with marked reduction in the total colony numbers and their size. The BMCs from DL-bearing mice (DL-BMCs) displayed a preferential lineage-restricted differentiation towards the granulocytic-type colonies with maximum numbers of CFU-Gs and CFU-GMs, followed by CFU-Ms. However, incubation of DL-BMCs with 100 ng/ml of Talpha1, in vitro restored their suppressed proliferation and colony forming ability (CFA) with significantly enhanced total number of colonies and individual colony types, which further increased in the presence of CSFs. In vivo studies with BMCs from DL-bearing mice treated with single intraperitoneal injection of 10 microg Talpha1/mouse also resulted in significant enhancement in their proliferative as well as colony forming ability in comparison to that of untreated DL-mice. The present observations suggest that Talpha1 can positively modulate the haematopoietic functions of normal murine BMCs, in addition to its myelorestorative role in tumour-bearing mice showing suppressed myelopoiesis.

Chemokine receptors and stromal cells in the homing and homeostasis of chronic lymphocytic leukemia B cells

Chronic lymphocytic leukemia (CLL) is a disease characterized by an accumulation, of mature, functionally incompetent B lymphocytes in the blood, secondary lymphoid tissues, and marrow. Lymphocyte trafficking and homing to specialized microenvironments is an active process that depends on the sequential engagement of adhesion molecules and activation through chemokine receptors. CLL B cells express functional CXCR3, CXCR4, and CXCR5 chemokine receptors that can direct leukemia cell chemotaxis in vitro. Marrow stromal cells, blood-derived "nurse-like cells", and extramedullary stromal cells of mesenchymal origin secrete high amounts of stromal cell-derived factor-1 (SDF-1) and thereby can attract CLL B cells via CXCR4. In vitro, CLL cells are rescued from apoptosis by cell-cell contact with such cells. Moreover, we found that the capacity of these cells to protect leukemia cells from apoptosis in vitro is mediated, at least in part, by the SDF-1 chemokine. Taken together, these findings suggest that chemokines and their receptors on CLL B cells can govern the homing and survival of leukemia B cells in vivo and therefore may contribute to their noted resistance to chemotherapy-induced apoptosis. Conceivably, CXCR4, and possibly other chemokine receptors, may represent a novel target for the development of effective treatment of this disease.

Neutrophil maturation and activation determine anatomic site of clearance from circulation

The long-term disposition of circulating neutrophils and the site of disappearance from circulation remain unclear. We investigated neutrophil localization in mice using (111)In-labeled murine peripheral blood neutrophils, mature bone marrow neutrophils, and peritoneal exudate neutrophils to track in vivo localization of these different cell populations. Infused peripheral neutrophils were found to localize equally between liver and marrow sites by 4 h (31.2 +/- 1.9 vs. 31.9 +/- 1.8%), whereas exudate neutrophils predominantly localized to liver (42.0 +/- 1.1%) and marrow-derived neutrophils to the marrow (65.9 +/- 6.6%) where they were found to localize predominantly in the hematopoietic cords. Stimulation of marrow neutrophils before infusion caused a shift in localization from marrow to liver, and subsequent induction of an inflammatory site after infusion and marrow sequestration led to remobilization of infused marrow neutrophils but not of peripheral neutrophils. These results indicate that the marrow participates in removing neutrophils from circulation, with evidence supporting both storage and perhaps disposal functions. Furthermore, models for circulating neutrophil homeostasis should consider that the site of retention is governed by the maturation and activation states of the cell.

Pre-B cell receptor signaling mediates selective response to IL-7 at the pro-B to pre-B cell transition via an ERK/MAP kinase-dependent pathway

B lymphocyte development is regulated at multiple checkpoints, mediated by signals originating both inside and outside the cell. Two signaling pathways known to be essential in this process are interleukin-7 (IL-7) and the pre-B cell receptor (pBCR). We have shown previously that these signaling pathways intersect functionally. Specifically, response to low concentrations of IL-7 requires pBCR expression. In this report, we identify the ERK/MAP kinase pathway as a key regulatory component of this response. We propose a molecular mechanism for the selective expansion of pBCR(+) precursors and for the culling of inappropriately rearranged pro-B cells.

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.

Characterization of human sialoadhesin, a sialic acid binding receptor expressed by resident and inflammatory macrophage populations

Sialoadhesin is a macrophage-restricted cellular interaction molecule and a prototypic member of the Siglec family of sialic acid binding immunoglobulin (Ig)-like lectins. So far, it has only been characterized in rodents. Here, we report the molecular cloning, binding properties, and expression pattern of human sialoadhesin. The predicted protein sequences of human and mouse sialoadhesin are about 72% identical, with the greatest similarity in the extracellular region, which comprises 17 Ig domains in both species. A recombinant protein consisting of the first 4 N-terminal domains of human sialoadhesin fused to the Fc region of human IgG1 mediated sialic acid-dependent binding with a specificity similar to its mouse counterpart, preferring sialic acid in the alpha2,3 glycosidic linkage over the alpha2,6 linkage. By flow cytometry with peripheral blood leukocytes, recombinant sialoadhesin bound strongly to granulocytes with intermediate binding to monocytes, natural killer cells, B cells, and a subset of CD8 T cells. Using antibodies raised to the recombinant protein, sialoadhesin was immunoprecipitated from the THP-1 human monocytic cell line as an approximate 200-kd glycoprotein. The expression pattern of human sialoadhesin was found to be similar to that of the mouse receptor, being absent from monocytes and other peripheral blood leukocytes, but expressed strongly by tissue macrophages in the spleen, lymph node, bone marrow, liver, colon, and lungs. High expression was also found on inflammatory macrophages present in affected tissues from patients with rheumatoid arthritis.

Blood-derived nurse-like cells protect chronic lymphocytic leukemia B cells from spontaneous apoptosis through stromal cell–derived factor-1

A subset of blood cells from patients with B-cell chronic lymphocytic leukemia (CLL) spontaneously differentiates in vitro into large, round, or fibroblast-like adherent cells that display stromal cell markers, namely vimentin and STRO-1. These cells also express stromal cell–derived factor-1 (SDF-1), a CXC chemokine that ordinarily is secreted by marrow stromal cells. Leukemia B cells attach to these blood-derived adherent cells, down-modulate their receptors for SDF-1 (CXCR4), and are protected from undergoing spontaneous apoptosis in vitro. Neutralizing antibodies to SDF-1 inhibit this effect. Moreover, the rapid deterioration in the survival of CLL B cells, when separated from such cells, is mitigated by exogenous SDF-1. This chemokine also results in the rapid down-modulation of CXCR4 and activation of p44/42 mitogen-activated protein-kinase (ERK 1/2) by CLL B cells in vitro. It is concluded that the blood of patients with CLL contains cells that can differentiate into adherent nurse-like cells that protect leukemia cells from undergoing spontaneous apoptosis through an SDF-1–dependent mechanism. In addition to its recently recognized role in CLL B-cell migration, SDF-1–mediated CLL B-cell activation has to be considered a new mechanism involved in the microenvironmental regulation of CLL B-cell survival.

Blood-derived nurse-like cells protect chronic lymphocytic leukemia B cells from spontaneous apoptosis through stromal cell–derived factor-1

A subset of blood cells from patients with B-cell chronic lymphocytic leukemia (CLL) spontaneously differentiates in vitro into large, round, or fibroblast-like adherent cells that display stromal cell markers, namely vimentin and STRO-1. These cells also express stromal cell–derived factor-1 (SDF-1), a CXC chemokine that ordinarily is secreted by marrow stromal cells. Leukemia B cells attach to these blood-derived adherent cells, down-modulate their receptors for SDF-1 (CXCR4), and are protected from undergoing spontaneous apoptosis in vitro. Neutralizing antibodies to SDF-1 inhibit this effect. Moreover, the rapid deterioration in the survival of CLL B cells, when separated from such cells, is mitigated by exogenous SDF-1. This chemokine also results in the rapid down-modulation of CXCR4 and activation of p44/42 mitogen-activated protein-kinase (ERK 1/2) by CLL B cells in vitro. It is concluded that the blood of patients with CLL contains cells that can differentiate into adherent nurse-like cells that protect leukemia cells from undergoing spontaneous apoptosis through an SDF-1–dependent mechanism. In addition to its recently recognized role in CLL B-cell migration, SDF-1–mediated CLL B-cell activation has to be considered a new mechanism involved in the microenvironmental regulation of CLL B-cell survival.

Immature B cells in bone marrow express Fas/FasL

Negative selection is a process by which autoreactive lymphocytes are eliminated from the developing antigen receptor repertoire. The mechanisms regulating negative selection of immature B lymphocytes in the bone marrow are poorly elucidated. Human bone marrow cells were examined in order to investigate the presence of the members of the Fas (APO-1/CD95) system. Here we demonstrate the expression of Fas in immature B lymphocytes (CD10/CD19+/CD40+/sIg+), and the presence of Fas natural ligand (FasL) in CD19+ bone marrow cells. The observed expression of apoptosis-related molecules might indicate how negative selection of autoreactive B cells can occur in human bone marrow.

A novel activation induced lymphocyte surface antigen, 90.12, is also expressed on apoptotic cells

We describe a monoclonal antibody, mAb 90.12, which recognizes a novel activation induced lymphocyte surface antigen. Flow cytometric analysis of normal tissues shows the antigen to be expressed on higher percentages of B lymphocytes in the bone marrow than in the spleen and the lymph node. Similarly, the 90.12 antigen is expressed on higher percentages of thymocytes than peripheral T cells. MAb 90. 12 immunoprecipitates three proteins with a molecular weight of 12-18 kDa which are not linked to the membrane by phosphotidylinositol. Expression of the 90.12 antigen is increased on activated B cells and the extent of upregulation varies with the stimulus. Lipopolysaccharide (LPS) stimulation results in expression on most B cells, while expression is upregulated on only a subset of B cells stimulated with anti-immunoglobulin M (IgM), interleukin(IL)4 and IL5. Finally, we show that 90.12 antigen expression is also increased on apoptotic cells.

Effects of T3R alpha 1 and T3R alpha 2 gene deletion on T and B lymphocyte development

Thyroid hormones bind to several nuclear receptors encoded by T3R alpha and T3R beta genes. There is now accumulating evidence that thyroid hormones act on the immune system. Indeed, mice deficient for thyroid hormones show a reduction in lymphocyte production. However, the mechanisms involved and, in particular, the role of the different thyroid hormone receptors in lymphocyte development have not been investigated. To address that question, we have studied lymphocyte development in mice deficient for the T3R alpha 1 and T3R alpha 2 gene products. A strong decrease in spleen cell numbers was found compared with wild-type littermates, B lymphocytes being more severely affected than T lymphocytes. A significant decrease in splenic macrophage and granulocyte numbers was also found. In bone marrow, a reduction in CD45+/IgM- pro/pre-B cell numbers was found in these mice compared with wild-type littermates. This decrease seems to result from a proliferation defect, as CD45+/IgM- cells incorporate less 5-bromo-2'-deoxyuridine in vivo. To define the origin of the bone marrow development defect, chimeric animals between T3R alpha-/- and Rag1-/- mice were generated. Results indicate that for B cells the control of the population size by T3R alpha 1 and T3R alpha 2 is intrinsic. Altogether, these results show that T3R alpha 1 or T3R alpha 2 gene products are implicated in the control of the B cell pool size.

Proliferation of Bone Marrow Pro-B Cells Is Dependent on Stimulation by the Pituitary/Thyroid Axis

The frequency and absolute number of pro-B, pre-B, and B cells in the bone marrow of the hypothyroid strain of mice are significantly reduced compared with those of their normal littermates. To investigate why this is the case, various B cell developmental processes were examined in the thyroid hormone-deficient mice. These studies revealed that the frequency of pro-B cells in the S-G2/M phase of the cell cycle was significantly reduced in hypothyroid mice. That thyroid hormone deficiency was responsible for this proliferation defect was established by demonstrating that treatment of hypothyroid mice with thyroxine resulted in a specific increase in the frequency and total number of cycling pro-B cells. The latter effect was paralleled by increases in the frequency and number of bone marrow B lineage cells. Additional in vitro experiments revealed that at least some thyroid hormone effects were directly mediated on the bone marrow. Taken together, these data demonstrate that thyroid hormones are required for normal B cell production in the bone marrow through regulation of pro-B cell proliferation and establish a role for the pituitary/thyroid axis in B cell development.

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.

Identification of CD22 ligands on bone marrow sinusoidal endothelium implicated in CD22-dependent homing of recirculating B cells

CD22 is a B cell-specific transmembrane protein known to function as a negative regulator of B cell signaling. It has also been implicated in cell adhesion through recognition of alpha2,6-linked sialic acids on glycans of target cells. Previous studies showed that CD22-deficient mice had a strongly reduced population of mature recirculating B cells in the bone marrow despite normal B cell development. Using a soluble recombinant form of the receptor (CD22-Fc), we demonstrate here that sialylated ligands for CD22 are expressed on sinusoidal endothelial cells of murine bone marrow but not on endothelial cells in other tissues examined. Injection of CD22-Fc revealed that the CD22 ligands in the bone marrow were accessible to the circulation. Treatment of mice with either CD22-Fc or affinity-purified anti-CD22 antibody led to an approximately 50% reduction in mature recirculating B cells in the bone marrow without affecting numbers in the spleen. Finally, consistent with the notion that CD22 is a homing receptor, we show that compared with wild-type mice, CD22-deficient animals have a lower number of immunoglobulin M-secreting plasma cells in the bone marrow.

Negative selection of immature B cells by receptor editing or deletion is determined by site of antigen encounter

Immature B cells that encounter self-antigen are eliminated from the immune repertoire by negative selection. Negative selection has been proposed to take place by two distinct mechanisms: deletion by apoptosis or alteration of the antigen receptor specificity by receptor editing. While convincing evidence exists for each, the two models are inherently contradictory. In this paper, we propose a resolution to this contradiction by demonstrating that the site of first antigen encounter dictates which mechanism of negative selection is utilized. We demonstrate that the bone marrow microenvironment provides signals that block antigen-induced deletion and promote RAG reinduction. In the periphery, the absence of these signals allows the immature B cell to default to apoptosis as a result of BCR engagement.

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.

Type I IFN sets the stringency of B cell repertoire selection in the bone marrow

Locally produced type I interferon (IFN-I) enhances the sensitivity of bone marrow B cell to IgM receptor ligation. The establishment of B cell repertoires, on the other hand, seems to involve selective processes that are critically dependent on B cell receptor (BCR) ligation. In order to assess the importance of BCR triggering thresholds on the selection of polyclonal unmanipulated B cell populations, we compared VH gene expression and reactivity repertoires in various B cell compartments of wild-type and IFN-I receptor-deficient mice (IFN-I-R-/-). These analyses demonstrate that increased B cell sensitivity to BCR ligation mediated by IFN-I in the bone marrow (BM) has consequences on the stringency of B cell repertoire selection. Thus, the normal counter-selection of both VH7183 gene family expression and multireactivity was impaired among immature BM B cells from mutant mice. Furthermore, as a result of reduced efficiency of BCR ligation-dependent inhibition of terminal differentiation, IFN-I-R-/- animals produce, in BM and thymus, higher numbers of plasma cells secreting antibodies that are more multireactive than wild-type animals. Finally, mutant serum IgM natural antibodies display a more reactive repertoire than controls, a likely reflection of the BM resident plasma cell repertoire. The present observations demonstrate, therefore, that local modulation of BCR triggering thresholds leads to important modifications in the generation and/or selection of normal B cell populations.

In vivo incidence of apoptosis evaluated with the TdT FragEL DNA fragmentation detection kit in cartilage and bone cells of the rat tibia

Previous studies have shown the occurrence of cell death by apoptosis in cartilage and bone cells, and have suggested a functional relationship between bone growth and remodelling on one hand, and numbers of apoptotic cells on the other. At present, no in vivo studies are available on the frequency of the apoptotic process measured at one time and in one place using the cartilage and bone cells of single specimens. The aim of the present investigation was to measure the in vivo incidence of apoptosis in cartilage and bone cells of the upper epiphysis and secondary ossification metaphyseal bone of the tibia in normal young adult rats. Apoptotic cells were visualized with the terminal deoxynucleotidyl transferase (TdT) FragEL DNA fragmentation detection kit, which is analogous to the TdT-mediated nick end-labelling (TUNEL) method. In the growth cartilage, only a few TUNEL-positive terminal hypertrophic chondrocytes were found; they were 1.32 +/- 0.70% of the total hypertrophic chondrocytes counted along the chondro-osseous junction. There were only a few apoptotic osteoblastic cells and osteocytes (0.22 +/- 0.22% and 0.15 +/- 0.16% of total osteoblasts and osteocytes respectively). TUNEL-positive osteoclasts were 1.03 +/- 0.57% of the total of osteoclastic cells; they usually showed only one or two apoptotic nuclei. The total number of TUNEL-positive bone marrow cells were also counted (56.78 +/- 10.29/mm2 of bone marrow spaces). Our results confirm that apoptosis does occur in hypertrophic chondrocytes and bone cells, and show that its frequency is very low. However, chiefly because of its short lifespan, the frequency of apoptosis in cartilage and bone may be higher than that shown by the TUNEL method. The static estimate that can be obtained with this method might lead to misleading conclusions on the physiological significance of such a dynamic, rapid and asynchronous process, whose precise importance in bone growth and remodelling remains to be determined.

Apoptosis and antiphospholipid antibodies

OBJECTIVE

To analyze the potential links between antiphospholipid antibodies (aPL) and apoptosis in the pathogenesis of the antiphospholipid antibody syndrome (APS).

METHODS

A review was undertaken of the most relevant scientific literature on apoptosis and autoimmune phenomena. Experimental and human pathology were reviewed to substantiate the hypothesis that apoptosis is involved in the generation of aPL.

RESULTS

Several considerations suggest that exposure of phospholipids (PL) during apoptosis may be a driving antigenic stimulus to the production of aPL. Furthermore, the molecular PL-protein complexes formed during apoptosis are targeted by "pathogenic" aPL. The binding and the clearance of apoptotic cells by these autoantibodies likely further enhances the aPL immune response. Experimental models and human pathology suggest that a restricted genetic background is key to the development of this immune response.

CONCLUSIONS

Abnormalities of apoptosis observed in the course of autoimmune conditions likely provide an antigenic stimulus to the production of aPL.

Regulation of anti-DNA B cells in recombination-activating gene-deficient mice

Anti-DNA antibodies are regulated in normal individuals but are found in high concentration in the serum of systemic lupus erythematosus (SLE) patients and the MRL lpr/lpr mouse model of SLE. We previously studied the regulation of anti-double-stranded (ds)DNA and anti-single-stranded (ss)DNA B cells in a nonautoimmune background by generating mice carrying immunoglobulin transgenes coding for anti-DNAs derived from MRL lpr/lpr. Anti-dsDNA B cells undergo receptor editing, but anti-ssDNA B cells seem to be functionally silenced. Here we have investigated how anti-DNA B cells are regulated in recombination- activating gene (RAG)-2-/- mice. In this setting, anti-dsDNA B cells are eliminated by apoptosis in the bone marrow and anti-ssDNA B cells are partially activated.

The Evolution of B Precursor Leukemia in the Eμ-ret Mouse

Eμ-ret mice carrying an RFP/RET fusion gene under the transcriptional control of the immunoglobulin heavy chain enhancer develop B lineage leukemias/lymphomas. We have characterized B-cell development in these mice before the onset of clinical disease to determine the steps involved in leukemogenesis. Flow cytometry reveals that the CD45R+CD43+CD24+BP-1+late pro–B-cell population is markedly expanded in the bone marrow of 3- to 5-week-old Eμ-ret mice. Compared with late pro–B cells from transgene-negative mice, Eμ-ret late pro–B cells have a limited capacity to differentiate in interleukin (IL)-7 and a higher incidence of VDJ rearrangements, but a similar cell cycle profile. In contrast, CD45R+CD43+CD24+BP-1−early pro–B cells from 3- to 5-week-old Eμ-ret mice, which also express the RFP/RET transgene, differentiate in IL-7 similarly to their normal counterparts. Furthermore, early pro–B cells from Eμ-ret and transgene-negative mice have an identical pattern of growth inhibition when exposed to interferons (IFNs)-α/β and -γ, whereas, pro–B-cell leukemia lines derived from Eμ-ret mice are markedly less sensitive to growth inhibition by these IFNs. In 13-week-old well-appearing Eμ-ret mice, late pro–B cells upregulate CYCLIN D1 expression and downregulate CASPASE-1 expression in a pattern that correlates with the emergence of B precursor cells in the peripheral blood and the loss of other B lineage subsets in the bone marrow. Taken together, these results suggest that the expression of the RFP/RET transgene initially prevents the normal elimination of late pro–B cells with nonproductive rearrangements. Secondary events that simultaneously disturb the normal transcriptional regulation of genes involved in the control of the cell cycle and apoptosis may allow for subsequent malignant transformation within the expanded late pro–B-cell population.

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.

Variance in the resistance of murine early bone marrow B cells to a deficiency in zinc

Little is known of the effects of nutritional deficiencies on lymphopoietic processes. Nevertheless, deficiencies in zinc adversely affect immune function causing thymic atrophy and lymphopenia in both humans and animals. Previous studies of the effects of zinc deficiency (ZD) on lymphopoiesis in adult mice indicated that a suboptimal intake of zinc caused a 50% or more depletion of the marrow of developing B cells. Thus, interference in the production of lymphocytes by ZD appeared to be a significant factor in the loss of host defence capacity. In the current study three-colour immunofluorescence phenotyping of early bone marrow B lymphocytes (B220+ immunoglobulin-) using flow cytometry demonstrated that a 27-day period of ZD caused a 50-70% decline in pre-B cells (B220+ CD43- immunoglobulin M (IgM)-) for moderate and severely zinc-deficient mice, respectively. Conversely, early pro-B cells (B220+ CD43+ 6C3-) and late pro-B cells (B220+ CD43+ 6C3+) exhibited little or no change in their distribution within the marrow. Indeed, the greater resistance of pro-B cells resulted in a 50% increase in the proportion of this subset within the B-cell compartment of the marrow as the deficiency in zinc advanced. Collectively, the data indicate that the B-cell compartment of the marrow is substantially altered by ZD and the stage specific sensitivity noted among early B cells may be related to chronically elevated levels of glucocorticoids present during ZD or other parameters that affect their survival and resistance to apoptosis.

Endogenous activation of apoptosis in bursal lymphocytes: inhibition by phorbol esters and protein synthesis inhibitors

The bursa of Fabricius represents the primary immune organ where immature B cells undergo maturational changes in avian species. Isolation of bursal lymphocytes for analysis in cell culture results in the rapid endogenous activation of apoptosis. After 2 h of incubation, over 45% of the lymphocytes were shown to be undergoing apoptosis and by 6 h 80% were undergoing apoptosis as demonstrated by a terminal deoxynucleotidyltransferase-mediated dUTP-fluorescein isothiocynate nick end-labeling flow-cytometric analysis. These results were corroborated by a propidium iodide-staining flow-cytometric assay and by an agarose gel electrophoresis DNA fragmentation assay that demonstrated internucleosomal DNA cleavage of genomic DNA in apoptotic bursal lymphocytes. Endogenous activation of apoptosis in bursal lymphocytes could be inhibited in a dose-dependent fashion with the phorbol ester, phorbol 12-myristate 13-acetate, but not the phorbol ester antagonist 4 alpha-phorbol 12,13-didecanoate. In addition, apoptosis could be inhibited in a dose-dependent fashion with inhibitors of protein translation, cycloheximide, and puromycin, as well as the transcriptional inhibitor actinomycin D. These results suggest that endogenous activation of bursal lymphocyte apoptosis may be mediated by the protein kinase C signal transduction pathway and activation of this process appears to be dependent upon de novo protein biosynthesis.

The Fas antigen and Fas-mediated apoptosis in B-cell differentiation

In the B-cell lineage, Fas, a type 1 membrane protein belonging to the tumor necrosis factor receptor (TNF) family, is expressed on B-cells at a restricted developmental stage and on activated B-cells, but not on naive mature B-cells. Apoptosis mediated by Fas-Fas ligand interactions may be involved in the peripheral elimination of autoreactive B-cells and in the regulation of the immune response through deletion of B-cells activated by foreign antigens, as for the T-cell lineage. Fas-mediated apoptosis associated with B-cell activation is affected by costimulation through other accessory signaling molecules like CD40, whose ligands are on T-cells.

Control of B cell development by Ras-mediated activation of Raf

Cell fate commitment in a variety of lineages requires signals conveyed via p21ras. To examine the role of p21ras in the development of B lymphocytes, we generated transgenic mice expressing a dominant-negative form of Ras in B lymphocyte progenitors, using a novel transcriptional element consisting of the Emu enhancer and the lck proximal promoter. Expression of dominant-negative Ras arrests B cell development at a very early stage, prior to formation of the pre-B cell receptor. Furthermore, an activated form of Raf expressed in the same experimental system could both drive the maturation of normal pro-B cells and rescue development of progenitors expressing dominant-negative Ras. Hence p21ras normally regulates early development of B lymphocytes by a mechanism that involves activation of the serine/threonine kinase Raf.