Origin of CD5+ B cells and natural IgM-secreting cells: reconstitution potential of adult bone marrow, spleen and peritoneal cells

Induction of B-cell development in adult mice reveals the ability of bone marrow to produce B-1a cells

To study B-cell development from bone marrow (BM), we generated recombination-activating gene 1 (Rag1)–targeted mice lacking mature lymphocytes. B-cell development can be induced in such mice by B cell–specific restoration of a functional Rag1 transcription unit. Follicular and marginal zone B cells populated the spleen when Rag1 expression was permitted. Notably, the peritoneal cavity was dominated by bona fide B-1a cells, as judged by surface markers and functional properties. These BM-derived B-1a cells exhibited a polyclonal VDJ repertoire with substantial N nucleotide insertions. Nevertheless, physiologic frequencies of phosphatidylcholine-specific B cells were detected. Importantly, the BM of young and 5-month-old mice was indistinguishable with regard to the potential to generate B-1a cells.

Signals from a self-antigen induce positive selection in early B cell ontogeny but are tolerogenic in adults

Positive and negative signals from self-Ags shape the B cell repertoire and the development of distinct B cell subsets, but little is known about what distinguishes these signals. To address this question, we have studied the development of anti-hen egg lysozyme MD4 Ig transgene B cells while systematically varying the level, distribution, and timing of exposure to different forms of hen egg lysozyme as a self-Ag. This process has allowed us to explore the effects of Ag independent of BCR specificity. Our findings show how the selection of autoreactive B cells is a competitive process involving immunogenic and tolerogenic forms of self-Ags. Due to a developmental switch during B cell ontogeny, autoreactive anti-hen egg lysozyme MD4 Ig transgene B cells are negatively selected by self-Ags in adult bone marrow but susceptible to positive selection by some of the same self-Ags in fetal and neonatal life. However, the persistence of B1 cells and IgM autoantibodies from early ontogeny enables autoreactive B cells from the adult bone marrow to escape negative selection. Our data suggest that this rescue may be due to the clearance or masking of self-Ag by IgM autoantibody. We discuss the implications of these findings in terms of B cell selection and the maintenance of self-tolerance during early and adult life.

Differential Role for Cyclic AMP Response Element Binding Protein-1 in Multiple Stages of B Cell Development, Differentiation, and Survival

CREB-1 is expressed in the bone marrow and in developing B cells. To determine the role of CREB-1 in developing B cells in the bone marrow, several lines of transgenic (Tg) mice overexpressing a dominant-negative Ser(119-ala) phosphomutant CREB-1 in the bone marrow were generated. Analysis of RNA and protein revealed expression of the transgene in the bone marrow. Flow cytometric analysis of bone marrow cells from Tg mice revealed approximately 70% increase in pre-B1 (CD43(+)B220(+)CD24(+(int))) and approximately 60% decreased pre-BII (CD43(+)B220(+)CD24(++(high))) cells, indicating a developmental block in pre-BI to pre-BII transition. Consistent with this, the Tg mice showed approximately 4-fold decrease in immature and mature B cells in the bone marrow. RT-PCR analysis of RNA from Tg mice revealed increased JunB and c-Jun in pre-BII cells associated with decreased S-phase entry. Adoptive transfer of bone marrow cells into RAG-2(-/-) mice resulted in reconstitution of non-Tg but not Tg bone marrow-derived CD43(+)B220(+)CD24(high) population that is normally absent in RAG-2(-/-) mice. In the periphery, the Tg mice exhibited decreased CD21(dim)CD23(high)IgM(+) follicular B cells in the spleen and increased B1a and B1b B cells in the peritoneum. While exhibiting normal Ab responses to T-independent Ags and primary response to the T-dependent Ag DNP-keyhole limpet hemocyanin, the Tg mice exhibited severely impaired secondary Ab responses. These studies provide the first evidence for a differential role for CRE-binding proteins in multiple stages of B cell development, functional maturation, and B1 and B2 B cells.

Ly49E expression points toward overlapping, but distinct, natural killer (NK) cell differentiation kinetics and potential of fetal versus adult lymphoid progenitors

Using a new antibody, we found previously that contrary to adult natural killer (NK) cells, fetal NK cells have a unique phenotype, as they exclusively express Ly49E. This can be explained by an intrinsic different NK differentiation potential of fetal versus adult lymphoid progenitors, by immaturity of fetal NK cells or by instability of Ly49E expression. Here, we show that adult progenitor cells were still capable of differentiating into Ly49E-expressing NK cells but at a much lower frequency. Surprisingly, Ly49E expression in vitro did not require stromal cells. Kinetic analysis in vivo showed that Ly49E was expressed early, together with CD94/NKG2 and Ly49G2, followed by Ly49C, and finally Ly49D. Transfer of sorted Ly49E-positive fetal NK cells showed stable Ly49E expression, and later, part of these cells up-regulated other Ly49 members. These data indicate that although there are intrinsic differences, there is no strict fetal and adult wave of NK cell differentiation.

Regeneration of natural antibody repertoire after massive ablation of lymphoid system: robust selection mechanisms preserve antigen binding specificities

Natural Abs (NAbs) are Igs present in the serum and body fluids of healthy vertebrate animals, without any previous intentional immunization. NAbs often exhibit autoreactivity but also play an essential role in immunity, being a first line of defense against infectious microorganisms. We have previously analyzed the natural serum IgM Ab repertoire of normal mice, characterizing their reactivity with hundreds/thousands of self Ags; a significant similarity among different individuals was observed, and it was found that many reactivities of NAbs stably kept during adulthood were established early in life, implicating that period as a critical time window in the physiology of NAb repertoire selection. In the work reported here, experiments were conducted to address the role of normal lymphocyte ontogeny to the formation and stability of adult NAb repertoire. The massive destruction of the lymphoid system was promoted in adult mice with gamma-irradiation, and regeneration of hemopoietic tissues was granted by bone marrow or fetal liver inoculum. NAb repertoire regeneration was followed for 60 days after gamma-irradiation in bone marrow or fetal liver chimeric animals. The analysis of serum IgM reactivity with hundreds/thousands of self Ags showed that the NAb repertoire regenerated most of its original format after massive destruction of lymphoid compartments, characterizing autoreactive repertoire selection as a robust biological process. The data also show that regeneration of the NAb repertoire occurred similarly in fetal liver and bone marrow chimeras, although the latter animals poorly reconstituted their CD5(+) B1 cell compartment, suggesting that B1 cells are not essential for natural Ab regeneration.

Origins and functions of B-1 cells with notes on the role of CD5

Whether B-1a (CD5+) cells are a distinct lineage derived from committed fetal/neonatal precursors or arise from follicular B-2 cells in response to BCR ligation and other, unknown signals remains controversial. Recent evidence indicates that B-1a cells can derive from adult precursors expressing an appropriate specificity when the (self-) antigen is present. Antibody specificity determines whether a B cell expressing immunoglobulin transgenes has a B-2, B-1a or marginal zone (MZ) phenotype. MZ cells share many phenotypic characteristics of B-1 cells and, like them, appear to develop in response to T independent type 2 antigens. Because fetal-derived B cell progenitors fail to express terminal deoxynucleotidyl transferase (TdT) and for other reasons, they are likely to express a repertoire that allows selection into the B-1a population. As it is selected by self-antigen, the B-1 repertoire tends to be autoreactive. This potentially dangerous repertoire is also useful, as B-1 cells are essential for resistance to several pathogens and they play an important role in mucosal immunity. The CD5 molecule can function as a negative regulator of BCR signaling that may help prevent inappropriate activation of autoreactive B-1a cells.

Cutting edge commentary: origins of B-1 cells

The origin of B-1a cells, a minority population of B cells that express CD5, are abundant in coelomic cavities, and often produce autoantibodies, has been the subject of study for many years. Accumulating evidence demonstrates that the hypothesis that only B cells arising in fetal or neonatal tissues have the potential to become B-1a cells cannot be true. Rather, B cell receptor-mediated signaling initiated by ligation of autoantigen has now been shown to be required for induction of the B-1a phenotype. Furthermore, cells with a functional B-1a phenotype can be induced from adult precursors by appropriate Ag. At the same time, microenvironment-specific events may determine the likelihood that a given B cell, either adult or fetal derived, enters this pathway. CD5 expression and possibly localization to the peritoneum appear to provide some protection to autoreactive cells otherwise slated for elimination.

Peritoneal B-cell development depends on strain, radiation, and time

OBJECTIVE

B-1a, B-1b, and B-2 cells represent the three B-cell subsets in mice. Previous studies have demonstrated that peritoneal B-1a cell development is absent, or nearly so, from adult bone marrow transfers into irradiated adult hosts. The majority of these studies have been performed under a limited set of conditions with irradiated host mice. Here we examined that under a variety of conditions, peritoneal B-1a cells can develop in significant numbers from adult bone marrow transfers into severe combined immunodeficient (SCID) and recombination activation gene 2(-) (RAG-2(-)) mice.

MATERIALS AND METHODS

Adult bone marrow was transferred into various strains of irradiated and nonirradiated adult immunodeficient RAG-2(-) and SCID mice. Peritoneal B-cell engraftment was examined by fluorescein-activated cell sorting analysis and unpaired t-tests were used to determine significant differences of B-cell engraftment among the various conditions of cell transfer.

RESULTS

The level of B-1a cell engraftment was variously affected by the type of host immunodeficiency, the combination of donor and host strains, and the time allowed for engraftment. Irradiation of SCID, but not RAG-2(-), host mice inhibited B-1a-cell engraftment. Additionally, decreasing the number of bone marrow progenitor cells transferred was not found to preferentially affect B-1a cell development in irradiated RAG-2(-) hosts.

CONCLUSION

In the context of these strains, we conclude that adult murine bone marrow contains progenitors that have the capacity to reconstitute peritoneal B-1a cell populations to donor levels.

Adult bone marrow contains precursors for CD5+ B cells

To determine directly whether B cell precursors of adult origin are capable of generating CD5+ B cells, we reconstituted neonatal C3H. SCID mice with adult C57BL/6 bone marrow and analyzed splenic B cells 10 months later. Surface staining and flow cytometry revealed that the B cells were of donor origin and that 30% were CD5+. This confirms that in vivo generated CD5+ B cells can be adult derived. After anti-IgM (but not lipopolysaccharide) stimulation in vitro, virtually all of the B cells from the bone marrow-reconstituted mice expressed surface CD5. Sequence analysis of expressed VHDJH genes from the CD5+ B cells present after anti-IgM stimulation revealed a high frequency of N nucleotide addition in CDR3 regions. The presence of N nucleotides indicates that these sequences were derived from CD5+ B cells of adult origin rather than from long-lived fetal precursor B cells present in either the adult bone marrow at the time of transfer or adult spleen. These experiments demonstrate conclusively that adult bone marrow contains precursors for CD5+ B cells and that unlike fetal liver-derived precursors these express terminal deoxynucleotidyl transferase.

Peritoneal defences and peritoneum-associated lymphoid tissue

The peritoneum is mainly protected by the innate immune system. This consists of mechanical clearance of the peritoneal cavity, activation of complement, and the actions of polymorphonuclear neutrophils and macrophages. The specific immune system, which is mediated by the activity of lymphocytes, provides a secondary amplification system that may be of great importance for patients with intraperitoneal sepsis. This review provides an overview of the relevant innate immune mechanisms and explores the possible role of peritoneum-associated lymphoid tissue.

Ability of spleen, peritoneal cavity, and lymph node B cells to reconstitute serum immunoglobulin in SCID mice

The impact of intrinsic B lymphocyte heterogeneity and of microenvironmental influences on serum immunoglobulin production by B cells was examined by intravenous (i.v.) and intraperitoneal (i.p.) transfer of BALB/c and BALB.xid (X-chromosome-linked immunedefective; XID) lymph node (LN), splenic (SP) and peritoneal cavity (PerC) cells into severe-combined immune-defective (SCID) mice. The results indicate that each B-cell source restores all immunoglobulin classes within 5 weeks of transfer, the rates for each isotype, however, differ between the B-cell sources. Serum IgM levels were restored most rapidly by PerC cell transfer, followed by SP and LN cell transfer. In addition, normal immunoglobulin levels were reached in the absence of complete lymphoid reconstitution. Serum immunoglobulin phenotypes characteristic of the donor strain, e.g. reduced IgM and IgG3 production by XID B cells, were maintained after transfer into the SCID recipient. Microenvironmental influences were indicated by reduced immunoglobulin production after i.p. transfer and after i.v. transfer into irradiated SCID recipients. The data show that both B-cell type and microenvironment play significant roles in generating the heterogeneous pool of B cells required for humoral immunity.

Thymic epithelium induces full tolerance to skin and heart but not to B lymphocyte grafts

Athymic nude mice reconstituted at birth with allogeneic thymic epithelia (TE) from day 10 embryos (E10), show life-long specific tolerance to skin and heart grafts, but eliminate B lymphocytes of the TE donor haplotype, nearly as well as those from a third strain. Previous immunizations with B cells do not alter the state of tolerance to skin grafts, but specifically accelerate elimination of lymphocytes. In contrast, transplantation of E15 allogeneic thymuses already seeded by hematopoietic cells resulted in chimeras tolerant to both skin and B lymphocytes. In vitro reactivities towards stimulator spleen cells of the haplotype of the thymus were observed in both E10 TE and E15 thymus chimeras. We conclude that induction of full in vivo tolerance to B cells requires hematopoietic cells, while this is not the case for induction of tolerance to skin and heart tissues; furthermore, in vitro reactivity to stimulator spleen cells of the tolerized haplotype is independent of in vivo tolerance.

Immunophenotyping of murine peritoneal macrophages and lymphocytes by flow cytometry

Peritoneal cells include mainly macrophages and both T and B lymphocytes. We describe a simple procedure to analyze by flow cytometry the antigens expressed on macrophage and/or lymphocyte membranes. F4/80 and CR3 macrophage antigens were detected with anti-F4/80 and anti-Mac-1 monoclonal antibodies (mAb) respectively, CD4 and THY1.2 T lymphocyte antigens with GK1.5 and J1j mAbs respectively, B CD5 lymphocytes were identified with LY1.2 mAb and Fc gamma RII/Fc gamma RIII were detected with 2.4G2 mAb.

Isolation of peritoneal precursors of B-1 cells in the adult mouse

Two weeks of daily peritoneopheresis of adult mice result in the selective depletion of B-1 cells, followed by the appearance of a population of B220+IgM-lymphocytes in the peritoneal cavity. These cells share with bone marrow (BM) pre-B cells expression of lambda 5, VpreB, and RAG-1 genes and a higher fraction of unrearranged V to DJ heavy (H) chain immunoglobulin (Ig) gene segments, when compared with mature B lymphocytes. Upon transfer to SCID recipients, sorted peritoneal B220+IgM- cells fail to colonize the BM, repopulate very few B cells in the spleen, but entirely reconstitute the B-1 cell compartment in the peritoneal and pleuropericardial cavities. In contrast, parallel transfers of sorted BM and pleuropericardial cavities. In contrast, parallel transfers of sorted BM B220+IgM- cells result in reconstitution of the BM and spleen B lineage cell compartments, but in no coelomic B cell repopulation. Both types of pre-B cells reconstitute splenic plasma cells of donor origin, but with markedly distinct efficiencies: the ratio of IgM-plasma cell/B cell numbers in the spleens of peritoneal pre-B cell recipients is more than 500-fold higher than that of recipients reconstituted by BM pre-B cells. We take these data to indicate that (1) differentiative commitment to the B-1 cell population occurs before selection events on mature cells; (2) B-1 precursors exist or may be locally produced in the adult mouse; (3) there is a lineage-related differential ability of mature B cells to undergo terminal differentiation to high-rate Ig secretion.

Mouse peritoneal macrophages: characterization of functional subsets following Percoll density gradients

Mouse resident peritoneal cells were separated into twelve fractions on Percoll gradients according to their specific density and were thoroughly characterized by Giemsa staining, some biochemical assays, immunophenotyping and phagocytic tests. Among these fractions, the macrophages were mainly represented in 7 subsets of 1.073 to 1.104 g/ml densities. The results of this study emphasize that resident peritoneal macrophages of primo-explantation can be divided into two distinct subpopulations with separate functions, related to the stage of cell maturity. In fact, our results show that one macrophage subpopulation is rich in immature cells, characterized by their peroxidative activity, the expression of F4/80 antigen, Mac-1 and Fc receptors, in correlation with their high specific density; the second subpopulation contains mature macrophages (lower percentage of peroxidase-positive cells) with lower densities and a lower level of expression of the above-mentioned molecules. Antibody-dependent and antibody-independent bacterial phagocytosis, the phagocytic index and Fc gamma RII rosetting increased together with the cell density, and were elevated in the immature cell subpopulation. T and B lymphocytes were also identified in all the macrophage subsets, but in a low proportion.