Selection at multiple checkpoints focuses V(H)12 B cell differentiation toward a single B-1 cell specificity

Receptor editing constrains development of phosphatidyl choline-specific B cells in VH12-transgenic mice

B1 B cells reactive to phosphatidyl choline (PtC) exhibit restricted immunoglobulin heavy chain (HC) and light chain (LC) combinations, exemplified by V_H12/Vκ4/5H. Two checkpoints are thought to focus PtC⁺ B cell maturation in V_H12-transgenic mice (VH12 mice): V-J rearrangements encoding a "permissive" LC capable of V_H12 HC pairing are selected first, followed by positive selection based on PtC binding, often requiring LC receptor editing to salvage PtC^- B cells and acquire PtC reactivity. However, evidence obtained from breeding VH12 mice to editing-defective dnRAG1 mice and analyzing LC sequences from PtC⁺ and PtC^- B cell subsets instead suggests that receptor editing functions after initial positive selection to remove PtC⁺ B cells in VH12 mice. This offers a mechanism to constrain natural, polyreactive B cells to limit their frequency. Sequencing also reveals occasional in-frame hybrid LC genes, reminiscent of type 2 gene replacement, that, testing suggests, arise via a recombination-activating gene (RAG)-independent mechanism.

Innate B Cells: the Archetype of Protective Immune Cells

The innate B cell (IBC) population is heterogeneous and involved in the primary immune response. IBC functions include a high ability to produce natural antibodies with IgM isotype, the elimination of apoptotic cells, and a capacity to be cognate help to T cells. Among IBC subsets, B-1 cells and marginal zone B cells are the main producers of IgM, act as rapid immune responders that may relocate to follicular lymphoid and differentiate to cytokine and antibody-secreting cells shortly after infection. IBCs functions are highly dependent on their localization site and the nature of their B cell receptor repertoire, suggesting a high plasticity range of different immune responses. In this review, we will describe the nature and functions of the different innate-like B cell subsets, first in mice and then in humans. Besides this, we will emphasize the strong ability of these cells to undertake different protective functions from the first line of defense against pathogens to the regulatory role of the broader immune response.

Expression of plasma cell alloantigen 1 defines layered development of B-1a B-cell subsets with distinct innate-like functions

Innate-like B-1a cells contribute significantly to circulating natural antibodies and mucosal immunity as well as to immunoregulation. Here we show that these classic functions of B-1a cells segregate between two unique subsets defined by expression of plasma cell alloantigen 1 (PC1), also known as ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1). These subsets, designated B-1a.PC1(lo) and B-1a.PC1(hi), differ significantly in IgH chain utilization. Adoptively transferred PC1(lo) cells secreted significantly more circulating natural IgM and intestinal IgA than PC1(hi) cells. In contrast, PC1(hi) cells produced more IL-10 than PC1(lo) cells when stimulated with LPS and phorbol 12-myristate 13-acetate (PMA). PC1(hi) cells were also more efficient than PC1(lo) cells in regulating Th1 cell differentiation, even though both B-1a subsets were comparably active in stimulating T-cell proliferation. Furthermore, PC1(lo) cells generated antigen-specific IgM responses to pneumococcal polysaccharide antigens, whereas PC1(hi) cells do not. We found that PC1(lo) cells develop from an early wave of B-1a progenitors in fetal life, whereas PC1(hi) cells are generated from a later wave after birth. We conclude that identification of B-1a.PC1(lo) and B-1a.PC1(hi) cells extends the concept of a layered immune system with important implications for developing effective vaccines and promoting the generation of immunoregulatory B cells.

Analyses of recombinant stereotypic IGHV3-21-encoded antibodies expressed in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) cells that use IgH encoded by IGHV3-21 and that have a particular stereotypic third CDR (HCDR3), DANGMDV (motif-1), almost invariably express Ig L chains (IgL) encoded by IGLV3-21, whereas CLL that use IGHV3-21-encoded IgH with another stereotypic HCDR3, DPSFYSSSWTLFDY (motif-2), invariably express κ-IgL encoded by IGKV3-20. This nonstochastic pairing could reflect steric factors that preclude these IgH from pairing with other IgL or selection for an Ig with a particular Ag-binding activity. We generated rIg with IGHV3-21-encoded IgH with HCDR3 motif-1 or -2 and IgL encoded by IGKV3-20 or IGLV3-21. Each IgH paired equally well with matched or mismatched κ- or λ-IgL to form functional Ig, which we screened for binding to an array of different Ags. Ig with IGLV3-21-encoded λ-IgL could bind with an affinity of ∼ 2 × 10(-6) M to protein L, a cell-wall protein of Peptostreptococcus magnus, independent of the IgH, indicating that protein L is a superantigen for IGLV3-21-encoded λ-IgL. We also detected Ig binding to cofilin, a highly conserved actin-binding protein. However, cofilin binding was independent of native pairing of IgH and IgL and was not specific for Ig with IgH encoded by IGHV3-21. We conclude that steric factors or the binding activity for protein L or cofilin cannot account for the nonstochastic pairing of IgH and IgL observed for the stereotypic Ig made by CLL cells that express IGHV3-21.

A new site-directed transgenic rheumatoid factor mouse model demonstrates extrafollicular class switch and plasmablast formation

The AM14 rheumatoid factor (RF) transgenic (Tg) mouse has been valuable for studying how self-reactive B cells are regulated beyond central tolerance, because they remain ignorant in normal mice. AM14 B-cell activation can be studied on autoimmune-prone strains or by inducing activation with IgG2a anti-chromatin antibodies (Abs). Despite the utility of conventional Ig-Tg mice, site-directed Ig-Tg (sd-Tg) mice provide a more physiological model for B-cell responses, allowing class switch and somatic hypermutation. We report here the creation of an AM14 sd-Tg mouse and describe its phenotype on both normal and autoimmune-prone backgrounds. AM14 sd-Tg B cells develop normally but remain unactivated in the BALB/c background, even after significant aging. In contrast, in the autoimmune-prone strain MRL/lpr, AM14 sd-Tg B cells become activated and secrete large amounts of IgG RF Ab into the serum. Class-switched Ab-forming cells were found in the spleen and bone marrow. IgG RF plasmablasts were also observed in extrafollicular clusters in the spleens of aged AM14 sd-Tg MRL/lpr mice. Class switch and Ab secretion were observed additionally in AM14 sd-Tg BALB/c B cells activated in vivo using IgG2a anti-chromatin Abs. Development of IgG auto-Abs is a hallmark of severe autoimmunity and is related to pathogenesis. Using the AM14 sd-Tg, we now show that switched auto-Ab-forming cells develop robustly outside germinal centers, further confirming the extrafollicular expression of activation induced cytidine deaminase (AID). This model will allow more physiological studies of B-cell biology in the future, including memory responses marked by class switch.

Microanatomy of the liver immune system

The critical metabolic functions of the liver often eclipse any perception of its role as an immune organ. However, the liver as a mediator of systemic and local innate immunity and an important site of immune regulation is now an accepted concept. Complex repertoires of lymphoid and non-lymphoid cells are key to hepatic defense and immunoregulation. Hepatic cells of myeloid lineage include Kupffer cells and dendritic cells. Intrahepatic lymphocytes are distinct both in phenotype and function from their counterparts in any other organ and include both conventional (CD4+ and CD8+ alphabeta T cell receptor (TCR)+ T cells, B cells, natural killer (NK) cells) and nonconventional lymphoid cells (natural killer T (NKT) cells, gamma delta TCR+ T cells, CD4- CD8- T cells). Many hepatic T cells express the TCR at an intermediate level and the great majority of them either coexpress NK cell markers (NKT cells) or they are apoptosing peripheral T cells. The percentage of activated (CD69+) and memory (CD45RB low+) lymphocytes is much higher while naive (CD62L high) and resting T cells as well as B lymphocytes are underrepresented in the liver. The discovery of major populations of lymphoid cells in the liver that differ phenotypically, functionally and even perhaps developmentally from populations in other regions has been key to the evolving perception of the liver as a regulatory lymphoid organ. This chapter will focus on these populations and how they contribute to immune surveillance against malignant, infectious and autoimmune disease of the liver.

Characterization of the light chain-restricted clonal B cells in peripheral blood of HCV-positive patients

To investigate the association between hepatitis C virus (HCV) and B cell proliferation, we searched for the clonal B cells by flow cytometric analysis of the surface immunoglobulin kappa (kappa):lambda (lambda) light chain ratios of the circulating B (CD19+) cells in 240 HCV-positive patients and 150 negative controls with liver diseases. Clonal B cells with light chain restriction (kappa:lambda ratio >3:1 or <1:2) were analyzed for CD5 expression and the presence of monoclonal immunoglobulin heavy-chain (IGH) gene rearrangements and the t(14;18) chromosomal translocation. Clonal B cells were detected in 7 cases with HCV (2.9%), but was never detected in the controls (p < 0.05). Of the 7 cases, all had monoclonal IGH gene rearrangements and one had the t(14;18) chromosomal translocation. These HCV-related clonal B cells are not uniform in the intensity of CD5 expression and showed no increase in the frequencies of CD5+ population compared with non-clonal B cells. No "chronic lymphocytic leukemia-phenotype" cells were found. The loss of clonality was observed in 2 cases treated with interferon and in one case treated with splenectomy. The longitudinal study is required to determine whether these circulating clonal B cells progress to lymphoproliferative disorders in future or not.

Autoantigen-B cell antigen receptor interactions that regulate expression of B cell antigen receptor Loci

Levels of AgR (BCR) expression are regulated during B cell development, activation, and induction of tolerance. The mechanisms responsible for and consequences of this regulation are poorly understood. We have described a class of DNA-based autoantigen-reactive B cell that down-regulates BCR expression during development to mature follicular phenotype. In this study, we show that at immature stages of primary differentiation, individual B cells of this type can dynamically modulate levels of expression of BCR in inverse proportion to degree of autoantigen engagement and induced BCR signaling. These adjustments in BCR expression are not associated with cell death, BCR revision, or altered development, and do not require TLR 9. Strikingly, modulation of BCR subunit gene RNA levels and transcription parallels these changes in BCR expression, indicating a direct link between autoantigen-BCR interactions of this type and regulation of transcription of BCR-encoding loci. We propose that this adaptive process allows this class of autoreactive B cell to avoid conventional tolerance pathways and promotes development to mature phenotype.

Btk regulates multiple stages in the development and survival of B-1 cells

B-1 cells are important players in the first line of defense against pathogens. According to current models for the origin of B-1 cells, they either represent a separate lineage from conventional B-2 cells or differentiate from conventional B-2 cells via an intermediate, B-1(int), in response to positive selection by antigen. Here we show that Btk, a Tec family kinase that mediates B cell antigen receptor (BCR) signaling, is required at multiple stages of B-1 cell development. VH12 anti-phosphatidylcholine (PtC) IgH transgenic mice provide a model for the induced differentiation of B-1 cells. This transgene selects for PtC-reactive cells and induces them to adopt a B-1 phenotype. Both processes have been shown to depend on Btk. To determine whether this is secondary to a requirement for Btk in the development of mature B-2 cells, we crossed VH12 transgenic mice to mice expressing low levels of Btk. B-2 cell development occurs normally in Btk(lo) mice despite reduced responsiveness to BCR crosslinking. Analysis of VH12.Btk(lo) mice reveals that Btk regulates the B-1(int) to B-1 transition and/or the survival of splenic B-1 cells, in part via a mechanism independent of its role in BCR signaling. We also show that Btk mediates the survival of, and expression of IL-10 by, those B-1 cells that do develop and migrate to the peritoneum. Multiple roles for Btk in B-1 cell development and maintenance may explain the particular sensitivity of this population to mutations in components of Btk signaling pathways.

Analysis of the Individual Contributions of Igα (CD79a)- and Igβ (CD79b)-Mediated Tonic Signaling for Bone Marrow B Cell Development and Peripheral B Cell Maturation

The individual contribution of Igalpha and Igbeta for BCR-triggered fates is unclear. Prior evidence supports conflicting ideas concerning unique as well as redundant functions for these proteins in the context of BCR/pre-BCR signaling. Part of this ambiguity may reflect the recent appreciation that Igalpha and Igbeta participate in both Ag-independent (tonic) and Ag-dependent signaling. The present study undertook defining the individual requirement for Igalpha and Igbeta under conditions where only ligand-independent tonic signaling was operative. In this regard, we have constructed chimeric proteins containing one or two copies of the cytoplasmic domains of either Igalpha or Igbeta and Igalpha/Igbeta heterodimers with targeted Tyr-->Phe modifications. The ability of these proteins to act as surrogate receptors and trigger early bone marrow and peripheral B cell maturation was tested in RAG2(-/-) primary pro-B cell lines and in gene transfer experiments in the muMT mouse model. We considered that the threshold for a functional activity mediated by the pre-BCR/BCR might only be reached when two functional copies of the Igalpha/Igbeta ITAM domain are expressed together, and therefore the specificity conferred by these proteins can only be observed in these conditions. We found that the ligand-independent tonic signal is sufficient to drive development into mature follicular B cells and both Igalpha and Igbeta chains supported formation of this population. In contrast, neither marginal zone nor B1 mature B cell subsets develop from bone marrow precursors under conditions where only tonic signals are generated.

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.

An antibody VH gene that promotes marginal zone B cell development and heavy chain allelic inclusion

The Ig heavy (H) chain plays a pivotal role in the regulation of primary B cell development through its association with a variety of other proteins including Igalpha and Igbeta, the surrogate light chain components and bona fide L chains, to form transmembrane signaling complexes. Little is known about how alterations in the structure of the H chain variable region influence association with these proteins, or the signaling capacity of the complexes that form. Here we describe a line of VH 'knockin' mice in which the transgene-encoded VH region differs by eight amino acid residues from the VH region in a VH knockin line we previously constructed and characterized. The transgenic H chain locus in the line of mice we characterized earlier efficiently promotes H chain allelic exclusion and all phases of primary B cell development, resulting in the generation of mature B1, marginal zone (MZ) and follicular (FO) B cell compartments. In contrast, the transgenic H chain locus in the new line fails to enforce allelic exclusion, as evidenced by the majority of peripheral B cells expressing two H chains on their surfaces. Moreover, this locus inefficiently drives bone marrow B lymphopoiesis and FO B cell development. However, this H chain locus does promote MZ B cell development, from precursors that appear to be generated during fetal and neonatal life. We discuss these data in the context of previous findings on the influence of Ig H chain structure on primary B cell development.

Association of B-1 B cells with follicular dendritic cells in spleen

Although CD5(+) B-1 B cells have been recognized as an infrequent B cell subset in mice for many years, attempts to identify their histologic location in normal mouse spleen have proven difficult due to both their paucity and low level expression of CD5. In this study we have studied V(H)11/D(H)/J(H) gene-targeted mice, V(H)11t, that develop elevated numbers of CD5(+) V(H)11/V(k)9 B cells with an anti-phosphatidylcholine (anti-PtC) autoreactive specificity, allowing B-1 B cell detection by anti-PtC Id-specific Abs in spleen section staining. Using this approach we found that anti-PtC B-1 cells first appear within the white pulp in neonates, expand in association with follicular dendritic cells (FDC), and localize more centrally than other (non-B-1) IgD(high) follicular B cells in adults. Among neonatal B cells, CD5(+) B-1 cells in both normal and V(H)11t mouse spleen and peritoneal cavity express the highest levels of CXCR5, which is important for FDC development. Injection of purified spleen or peritoneal B-1 cells into RAG knockout mice resulted in B-1 cell follicle formation in spleen, inducing FDC development and plasma cell generation. These results indicate that B-1 B cells are the first B cells to express fully mature levels of CXCR5, thereby promoting the development of FDC.

The role of natural antibodies in atherogenesis

Atherosclerosis is now widely recognized as a chronic inflammatory disease that involves innate and adaptive immune responses. Both cellular and humoral components of the immune system have been implicated in atherogenesis. Natural antibodies can be considered humoral factors of innate immunity, and their functional role in health and disease has been reexamined in recent years. Natural antibodies exhibit a remarkably conserved repertoire that includes a broad specificity for self-antigens. For this reason, they are believed to be a product of natural selection and have been suggested to play an important role in "housekeeping" functions. Recent evidence has revealed that oxidation-specific epitopes are important and maybe immunodominant targets of natural antibodies, suggesting an important function for these antibodies in the host response to consequences of oxidative stress, for example, to the oxidative events that occur when cells undergo apoptosis. This review will focus on these recent findings and discuss the emerging evidence for an important role of natural antibodies in atherogenesis.

Natural antibodies and the autoimmunity of atherosclerosis

In recent years, the subject of natural antibodies has been revisited and the immunobiological roles of these humoral factors are being better defined. These antibodies are secreted by distinct sets of innate-like B cells, B-1 cells and marginal zone B cells, which arise early in development to become the sources of "natural immune memory". Due to their interactions with a variety of self-determinants, natural antibodies have previously been postulated to play roles in the maintenance of host homeostasis. A central paradigm has recently been developed from the demonstration that oxidation derived epitopes on apoptotic cells and oxidized low-density lipoproteins are recognized by the phosphorylcholine-specific germline encoded B-1 cell natural antibody, T15, which has provided important insights into possible "house-keeping" functions under both normal and pathological conditions. In this review, the potential functions of natural antibodies in the pathogenesis and progression of the chronic inflammatory condition of atherosclerosis are discussed, as well as their capacities for apoptotic cell binding and clearance. These interactions of natural antibodies and oxidation-epitopes from phospholipids appear to provide a dynamic immunobiological connection linking host responses in infection, autoimmunity and atherosclerosis.

Regulation of B-cell development by antibody specificity

Although autoantigen-induced negative selection plays an important role in shaping the mature B-cell repertoire, studies in recent years have suggested that differentiation into any of the three mature B-cell subsets (marginal zone B cells, follicular B cells and B-1 B cells) is not a passive product of autoreactive B-cell elimination. Instead, evidence suggests that entry into a mature subset involves active B-cell receptor signaling and self-antigen-mediated positive selection.

Selection during development of VH11+ B cells: a model for natural autoantibody-producing CD5+ B cells

Natural autoantibodies constitute a large portion of serum immunoglobulin M (IgM) and bridge the adaptive and innate immune systems, serving as a rapid response to common pathogens. Many arise from a distinctive subset of B cells, termed B-1, that express CD5. Here, we describe our studies with a representative CD5+ B-cell-derived natural autoantibody, the VH11Vkappa9 B-cell receptor (BCR) that binds a determinant on senescent erythrocytes. This specificity represents 5-10% of the CD5+ B-cell subset, with a large portion accounted for by two novel BCRs, VH11Vkappa9 and VH12Vkappa4. We have found that the development of B-lineage cells with a VH11 rearrangement is surprisingly restricted at several crucial bottlenecks: (i). one of the most common VH11 rearrangements generates a heavy-chain protein that only inefficiently assembles a pre-BCR, key for recombinase-activating gene downregulation/allelic exclusion and pre-B-clonal expansion; (ii). cells containing VH11- micro chains lacking N-addition are favored for progression to the B-cell stage, eliminating most bone marrow VH11 rearrangements; and (iii). only a subset of Vkappa-light chains combine with VH11 heavy chain to foster progression to the mature B-cell stage. Together, these constrain VH11 generation to fetal development and may favor production of B cells with the prototype VH11Vkappa9 BCR.

Evidence for a ligand-mediated positive selection signal in differentiation to a mature B cell

Positive selection is required for B cell differentiation, as indicated by the requirement for expression of the pre-B cell receptor (pre-BCR) and the BCR at the pre-B and immature B cell stages, respectively. Positive selection mediated by a tonic signal from these receptors is sufficient to drive B cell differentiation beyond the pre-B and immature B cell stages, but it is unclear whether additional positive selection signals are required for differentiation to a mature B-2 cell. We have identified a population of Ig transgenic B cells that differentiatively arrest at a transitional B cell stage in the spleen. They exhibit no evidence of Ag encounter or negative selection and can differentiate to mature B-2 cells in vivo upon weak BCR stimulation or adoptive transfer to irradiated hosts. These data are consistent with a requirement for a ligand-mediated BCR signal for differentiation to a mature B-2 cell.

Positive selection focuses the VH12 B-cell repertoire towards a single B1 specificity with survival function

B cells of varying antigen specificities are consistently present in the unmanipulated repertoire. These B cells appear to belong to the marginal zone (MZ) and B1 B-cell subsets and provide protection to the blood and lymph, respectively. Some are specific for self-antigens, suggesting that they are selected based on specificity for self but have a protective role against foreign pathogens. One of these specificities is for phosphatidylcholine (PtC). Anti-PtC B cells comprise 5-8% of the B1 repertoire and are protective against bacterial pathogens. In general, they are restricted to the expression of two VH/Vkappa combinations, VH11/Vkappa9 and VH12/Vkappa4/5H. This review focuses on the differentiation of VH12 anti-PtC B cells. They undergo a series of positive selection events beginning at the pre-B-cell stage that enriches for those with a VHCDR3 and L chain required for PtC binding and eliminating the majority of VH12 B cells that lack the ability to bind PtC. Thus, positive selection focuses the VH12 repertoire toward PtC, ensuring that anti-PtC VH12 B cells are a significant component of the B1-cell repertoire in all individuals.

Expansion of innate CD5pos B cells expressing high levels of CD81 in hepatitis C virus infected liver

BACKGROUND/AIMS

Association of hepatitis C virus (HCV) with increased autoantibodies, mixed cryoglobulinaemia, non-Hodgkin's B-cell lymphoma and increased peripheral innate (CD5(pos)) B cells suggests a role for B-lymphocytes in the pathogenesis of HCV-infection.

METHODS

Flow cytometry was used to estimate CD5(pos) B cell levels and CD81 co-expression in chronic HCV infection. Viral load was assessed using PCR.

RESULTS

We demonstrate expansion of innate B cells in HCV-infected liver from patients with fibrosis score less than stage II (39%, % of total B cells, P=0.002) and end stage HCV cirrhosis (20%, P<0.05) compared with normal liver (8%). Expression of CD81, a signal transducing molecule and putative HCV receptor, was significantly increased on peripheral blood CD5(pos) B cells compared with conventional B cells (P=0.0001). Higher levels of CD81 on CD5(pos) B cells were more dramatic in the liver of HCV-infected individuals. However, no significant difference was observed in the viral load of CD5(pos)CD81(High) B cells and CD5(neg)CD81(Low) B cells.

CONCLUSIONS

Increased expression of CD81 on innate B cells, a population that is expanded in the livers and peripheral blood of chronic HCV-infected patients, suggests a role in viral specific activation and clonal proliferation in chronic HCV infection.

Expression of human complement receptor type 2 (CD21) in mice during early B cell development results in a reduction in mature B cells and hypogammaglobulinemia

Complement receptor (CR) type 2 (CR2/CD21) is normally expressed only during the immature and mature stages of B cell development. In association with CD19, CR2 plays an important role in enhancing mature B cell responses to foreign Ag. We used a murine Vlambda2 promoter/Vlambda2-4 enhancer minigene to develop transgenic mice that initiate expression of human CR2 (hCR2) during the CD43(+)CD25(-) late pro-B cell stage of development. We found peripheral blood B cell numbers reduced by 60% in mice expressing high levels of hCR2 and by 15% in mice with intermediate receptor expression. Splenic B cell populations were altered with an expansion of marginal zone cells, and basal serum IgG levels as well as T-dependent immune responses were also significantly decreased in transgenic mice. Mice expressing the highest levels of hCR2 demonstrated in the bone marrow a slight increase in B220(int)CD43(+)CD25(-) B cells in association with a substantial decrease in immature and mature B cells, indicative of a developmental block in the pro-B cell stage. These data demonstrate that stage-specific expression of CR2 is necessary for normal B cell development, as premature receptor expression substantially alters this process. Alterations in B cell development are most likely due to engagement of pre-B cell receptor-mediated or other regulatory pathways by hCR2 in a CD19- and possibly C3 ligand-dependent manner.

Mice deficient in complement receptors 1 and 2 lack a tissue injury-inducing subset of the natural antibody repertoire

Intestinal ischemia-reperfusion (IR) injury is initiated when natural Abs recognize neoantigens that are revealed on ischemic cells. Cr2(-/-) mice, deficient in complement receptors (CR)1 and CR2, demonstrate defects in T-dependent B-2 B cell responses to foreign Ags and have also been suggested to manifest abnormalities of the B-1 subset of B lymphocytes. To determine whether these CRs might play a role in the generation of the natural Abs that initiate intestinal IR injury, we performed experiments in Cr2(-/-) and control Cr2(+/+) mice. We found that Cr2(-/-) mice did not demonstrate severe intestinal injury that was readily observed in control Cr2(+/+) mice following IR, despite having identical serum levels of IgM and IgG. Pretreatment of Cr2(-/-) mice before the ischemic phase with IgM and IgG purified from the serum of wild-type C57BL/6 mice reconstituted all key features of IR injury, demonstrating that the defect involves the failure to develop this subset of natural Abs. Pretreatment with IgM and IgG individually demonstrates that each contributes to unique features of IR injury. In sum, CR2/CR1 play an unanticipated but critical role in the development of a subset of the natural Ab repertoire that has particular importance in the pathogenesis of IR injury.

Level of B cell antigen receptor surface expression influences both positive and negative selection of B cells during primary development

To examine the effect of B cell Ag receptor (BCR) surface density on B cell development, we studied multiple lines of mice containing various copy numbers of an IgH micro delta transgene. The V(H) gene in this transgene encodes multireactive BCRs with low affinity for self Ags. These BCRs promote differentiation to a B cell subpopulation that shares some, but not all of the properties of marginal zone (MZ) B cells. Surface BCR level was found to be related to transgene gene copy number in these mice. In mice containing 1-15 copies of the transgene, elevated surface BCR levels were correlated with increased numbers of B cells in the MZ-like subset. However, in mice containing 20-30 copies of the transgene, massive clonal deletion of B cells was observed in the bone marrow, few B cells populated the spleen, and B cells were essentially absent from the lymph nodes. These data support the idea that autoantigens mediate not only negative, but positive selection of developing B cells as well. More importantly, they illustrate the profound influence of BCR surface density on the extent to which either of these selective processes take place.

Autoreactivity by design: innate B and T lymphocytes

Innate B and T lymphocytes are a subset of lymphocytes that express a restricted set of semi-invariant, germ-line-encoded, autoreactive antigen receptors. Although they have long been set apart from mainstream immunological thought, they now seem to represent a distinct immune-recognition strategy that targets conserved stress-induced self-structures, rather than variable foreign antigens. Innate lymphocytes regulate a range of infectious, tumour and autoimmune conditions. New studies have shed light on the principles and mechanisms that drive their unique development and function, and show their resemblance to another subset of innate lymphocytes, the natural killer cells.

B cell development pathways

B cell development is a highly regulated process whereby functional peripheral subsets are produced from hematopoietic stem cells, in the fetal liver before birth and in the bone marrow afterward. Here we review progress in understanding some aspects of this process in the mouse bone marrow, focusing on delineation of the earliest stages of commitment, on pre-B cell receptor selection, and B cell tolerance during the immature-to-mature B cell transition. Then we note some of the distinctions in hematopoiesis and pre-B selection between fetal liver and adult bone marrow, drawing a connection from fetal development to B-1/CD5(+) B cells. Finally, focusing on CD5(+) cells, we consider the forces that influence the generation and maintenance of this distinctive peripheral B cell population, enriched for natural autoreactive specificities that are encoded by particular germline V(H)-V(L) combinations.

A VH12 transgenic mouse exhibits defects in pre-B cell development and is unable to make IgM+ B cells

V(H)12 B cells undergo stringent selection at multiple checkpoints to favor development of B-1 cells that bind phosphatidylcholine. Selection begins with the V(H) third complementarity-determining region (CDR3) at the pre-B cell stage, in which most V(H)12 pre-B cells are selectively eliminated, enriching for those with V(H)CDR3s of 10 aa and a fourth position Gly (designated 10/G4). To understand this selection, we compared B cell differentiation in mice of two V(H)12 transgenic lines, one with the favored 10/G4 V(H)CDR3 and one with a non-10/G4 V(H)CDR3 of 8 aa and no Gly (8/G0). Both H chains drive B cell differentiation to the small pre-BII cell stage, and induce allelic exclusion and L chain gene rearrangement. However, unlike 10/G4 pre-B cells, 8/G0 pre-B cells are deficient in cell division and unable to differentiate to B cells. We suggest that this is due to poor 8/G0 pre-B cell receptor expression and to an inability to form an 8/G0 B cell receptor. Our findings also suggest that V(H)12 H chains have evolved such that association with surrogate and conventional L chains is most efficient with a 10/G4 CDR3. Thus, selection for phosphatidylcholine-binding B-1 cells is most likely the underlying evolutionary basis for the loss of non-10/G4 pre-B cells.

The mouse CD1d-restricted repertoire is dominated by a few autoreactive T cell receptor families

To define the phenotype and T cell receptor (TCR) repertoire of CD1d-dependent T cells, we compared the populations of T cells that persisted in major histocompatibility complex (MHC)-deficient mice, which lack mainstream T cells, with those from MHC/CD1d doubly deficient mice, which lack both mainstream and CD1d-dependent T cells. Surprisingly, up to 80% of the CD1d-dependent T cells were stained by tetramers of CD1d/alpha-galactosylceramide, which specifically identify the previously described CD1d autoreactive Valpha14-Jalpha18/Vbeta8 natural killer (NK) T cells. Furthermore, zooming in on the CD1d-dependent non-Valpha14 T cells, we found that, like Valpha14 NK T cells, they mainly expressed recurrent, CD1d autoreactive TCR families and had a natural memory phenotype. Thus, CD1d-restricted T cells differ profoundly from MHC-peptide-specific T cells by their predominant use of autoreactive and semiinvariant, rather than naive and diverse, TCRs. They more closely resemble other lineages of innate lymphocytes such as B-1 B cells, gammadelta T cells, and NK cells, which express invariant or semiinvariant autoreactive receptors. Finally, we demonstrate that the MHC-restricted TCR repertoire is essentially non-cross-reactive to CD1d. Altogether, these findings imply that lipid recognition by CD1d-restricted T cells may have largely evolved as an innate rather than an adaptive arm of the mouse immune system.

Anti-Sm B cell differentiation in Ig transgenic MRL/Mp-lpr/lpr mice: altered differentiation and an accelerated response

To determine the regulation of B cells specific for the ribonucleoprotein Sm, a target of the immune system in human and mouse lupus, we have generated mice carrying an anti-Sm H chain transgene (2-12H). Anti-Sm B cells in nonautoimmune 2-12H-transgenic (Tg) mice are functional, but, in the absence of immunization, circulating anti-Sm Ab levels are not different from those of non-Tg mice. In this report, we compare the regulation of anti-Sm B cells in nonautoimmune and autoimmune MRL/Mp-lpr/lpr (MRL/lpr) and bcl-2-22-Tg mice. Activation markers are elevated on splenic and peritoneal anti-Sm B cells of both nonautoimmune and autoimmune genetic backgrounds indicating Ag encounter. Although tolerance to Sm is maintained in 2-12H/bcl-2-22-Tg mice, it is lost in 2-12H-Tg MRL/lpr mice, as the transgene accelerates and increases the prevalence of the anti-Sm response. The 2-12H-Tg MRL/lpr mice have transitional anti-Sm B cells in the spleen similar to nonautoimmune mice. However, in contrast to nonautoimmune mice, there are few if any peritoneal anti-Sm B-1 cells. These data suggest that a defect in B-1 differentiation may be a factor in the loss of tolerance to Sm and provide insight into the low prevalence of the anti-Sm response in lupus.

B1 cells: similarities and differences with other B cell subsets

Since their discovery, B1 B cells' origins and developmental pathways have eluded characterization. In the past year, focus on B1 B cells has shifted dramatically from developmental to functional aspects of these cells. Most advances have been made in describing the physiological activities of B1 cells, including their migration, activation by antigen and role in both autoimmunity and malignancy.

Selective maturation of VH12 B cells in the spleen enriches for anti-phosphatidyl choline B cells: evidence for receptor editing

PtC-specific B-1 cells originate from conventional B-2 (B-0) cells as a result of antigen activation. VH12 B cells specific for PtC are enriched at two developmental checkpoints in the bone marrow; first at the pre-BI to pre-BII transition where VH12 pre-B cells with anti-PtC VHCDR3 are enriched, and second at the pre-BII to immature B cell transition where L chain diversity is restricted. This restriction is due to the inability of most L chains to associate with VH12 H chains. We present evidence here of a third developmental checkpoint that enriches for PtC-specific B cells, at the transitional to mature B-2 (B-0) cell stage. Most VH12 transitional B cells do not differentiate to a mature B-2 cell and, of those that do, most have undergone receptor editing. The V kappa 4/5H L chain appears to be one of the few L chains that can support differentiation to the mature B-2 cell stage, providing an explanation for its dominance among VH12 B cells in the spleen. Once cells reach this stage, those that bind PtC are induced to differentiate to B-1. Thus, through selection at multiple differentiative stages and the induction of extensive secondary V kappa rearrangement and receptor editing, VH12 B cell differentiation is focused toward specificity for PtC and selection to the B-1 subset.

Germline diversity of the expressed BALB/c VhJ558 gene family

Although the mouse immunoglobulin heavy chain (Igh) locus contains 15 heavy chain V (Vh) gene families, at least half of the Vh gene segments are members of the VhJ558 family. This large Vh gene family represents the least characterized germline coding regions of any of the mouse antigen receptor loci and the contribution of individual VhJ558 genes to the preimmune repertoire is poorly understood. In fact, relatively few germline VhJ558 sequences have been reported for BALB/c, the foundation strain for mouse immunoglobulin genetics and the prototypic strain of the Igh(a) haplotype. Here we present a database consisting of 66 sequences estimated to represent one-half of the total number of functional BALB/c VhJ558 genes. Our results indicate that a subset of the VhJ558 genes is highly expressed in the preimmune repertoire, with just nine Vh sequences accounting for nearly 50% of the VhJ558 heavy chains expressed by splenic B cells. We show that this disparity in the expressed Vh gene repertoire is not due to the position of the Vh genes relative to the Dh cluster or to multiple germline copies of the highly expressed VhJ558 genes. Together, these data constitute the first detailed analysis of functional BALB/c VhJ558 genes, demonstrate a striking bias in the use of particular VhJ558 genes in the preimmune repertoire, and provide sufficient information to study the regulation of the Dh-distal region of the Igh-V locus at the level of individual genes.

CD1d-restricted mouse V alpha 14 and human V alpha 24 T cells: lymphocytes of innate immunity

Mouse V alpha 14 T cells and their human homologs, V alpha 24 T cells, are prominent subsets of CD1d-restricted T cells. Here we discuss their striking similarities to B-1 B cells and gammadelta T cells and propose that these immune cells mediate various innate strategies in response to endogenous or exogenous danger signals.

The level of expression of mu heavy chain modifies the composition of peripheral B cell subpopulations

The B cell receptor (BCR) has a decisive role in transducing signals required for the development of B cells and their survival in the periphery. However, the processes that initiate these signals remain unclear and concepts of constitutive and ligand-dependent signaling have been proposed. Using a mu-transgenic mouse model, we have analyzed the impact of high surface IgM expression on the composition of the splenic B cell population. kappa-deficient mice homozygous for the H3-mu transgene have B cells with a higher BCR surface density than H3 heterozygous mice. This higher BCR expression is associated with an increase in the percentage and the total number of splenic B cells. In addition, an important proportion of CD23(-)CD21(+) marginal zone (MZ) B cells can be observed in H3 homozygous mice. However, these modifications operate in the absence of impairment of the positive selection process of the H3-mu/lambda1 combination over the H3-mu/lambda2 + 3 ones. These results suggest that (i) a constitutive BCR signaling directly correlated with BCR surface density is responsible for the efficient B cell colonization of the periphery with an accumulation of B cells in the MZ and (ii) a ligand-dependent BCR signal is responsible for the clonotype composition of the mature B cell repertoire.

CD1-restricted T-cell responses and microbial infection

CD1, a conserved family of major histocompatibility (MHC)-like glycoproteins in mammals, specializes in capturing lipid rather than peptide antigen for presentation to T lymphocytes. The principles and mechanisms of this newly discovered immune strategy differ markedly from those governing classical MHC-peptide presentation. They might be exploited for the design of new lipid-based microbial vaccines and adjuvants.

Underutilization of the V kappa 10C gene in the B cell repertoire is due to the loss of productive VJ rearrangements during B cell development

The V kappa10 family of murine light chain Ig genes is composed of three members, two of which (V kappa 10A and V kappa 10B) are well used. V kappa 10C, the third member of this family, is not detected in any expressed Abs. Our previous work showed that V kappa 10C is structurally functional and can recombine, but mRNA levels in spleen were extremely low relative to those of V kappa 10A and V kappa 10B. Furthermore, while the V kappa 10C promoter was efficient in B cells, it was shown to work inefficiently in pre-B cell lines. Here, we extend our analysis of the V kappa 10 family and examine V kappa 10 gene accessibility, their representation in V kappa cDNA phage libraries, and the frequency and nature of rearrangements during different stages of B cell development. We demonstrate that V kappa 10C is under-represented in V kappa cDNA libraries, but that the frequency of its sterile transcripts in pre-B cells surpasses both V kappa 10A and V kappa 10B, indicating that the gene is as accessible as V kappa 10A and V kappa 10B to the recombination machinery. We also demonstrate that V kappa 10C recombines at a frequency equal to that of V kappa 10A in pre-B cells and has a normal nonproductive to productive recombination ratio. As B cells develop, however, both the frequency of V kappa 10C rearrangements and the presence of productive rearrangements decline, indicating that these cells are in some fashion being eliminated.

Development and function of B-1 cells

Results from immunoglobulin-transgenic mice and BCR-mutant mice have been widely interpreted in recent years as supporting a simple 'activation' model for the origin of CD5+/B-1 B cells. However cell transfer experiments over 10 years ago and recent work investigating pre-BCR signaling suggest striking differences between B cell development in fetal liver and adult bone marrow, lending support for a 'lineage' model that we favor. Recent progress has been made relating to the development and function of the CD5+/B-1 B cell subpopulation in mice; the data can be viewed in the context of the generation of this subpopulation by a distinctive fetal B cell developmental process.

B-1 cells: orthodox or conformist?

B-1 cells differ from conventional peripheral B cells (B-2) by anatomical location, surface marker expression, antibody repertoire and growth properties. The lineage hypothesis of B-1 cell development attributes the properties of B-1 cells to their unique origin. The induced differentiation hypothesis suggests the surface-immunoglobulin-driven development of B-1 cells from common B-1/B-2 cell progenitors. In both models self-antigen-induced signalling plays the central role in positive selection of B-1 cells. The ability of B-1 cells to be positively selected by self-antigens raises questions about the mechanism of this phenomenon.

Identification of a precursor to phosphatidyl choline-specific B-1 cells suggesting that B-1 cells differentiate from splenic conventional B cells in vivo: cyclosporin A blocks differentiation to B-1

The origin of B-1 cells is controversial. The initial paradigm posited that B-1 and B-2 cells derive from separate lineages. More recently it has been argued that B-1 cells derive from conventional B cells as a result of T-independent Ag activation. To understand B-1 cell differentiation, we have generated Ig transgenic (Tg) mice using the H and L chain genes (VH12 and Vkappa4) of anti-phosphatidyl choline (anti-PtC) B cells. In normal mice anti-PtC B cells segregate to B-1. Segregation is intact in VH12 (6-1) and VH12/Vkappa4 (double) Tg mice that develop large numbers of PtC-specific B cells. However, if B-1 cell differentiation is blocked, anti-PtC B cells in these Tg mice are B-2-like in phenotype, suggesting the existence of an Ag-driven differentiative pathway from B-2 to B-1. In this study, we show that double Tg mice have a population of anti-PtC B cells that have the phenotypic characteristics of both B-2 and B-1 cells and that have the potential to differentiate to B-1 (B-1a and B-1b). Cyclosporin A blocks this differentiation and induces a more B-2-like phenotype in these cells. These findings indicate that these cells are intermediate between B-2 and B-1, further evidence of a B-2 to B-1 differentiative pathway.