Major histocompatibility complex class II expression distinguishes two distinct B cell developmental pathways during ontogeny

Single-cell genomics identifies distinct B1 cell developmental pathways and reveals aging-related changes in the B-cell receptor repertoire

Background

B1 cells are self-renewing innate-like B lymphocytes that provide the first line of defense against pathogens. B1 cells primarily reside in the peritoneal cavity and are known to originate from various fetal tissues, yet their developmental pathways and the mechanisms underlying maintenance of B1 cells throughout adulthood remain unclear.

Results

We performed high-throughput single-cell analysis of the transcriptomes and B-cell receptor repertoires of peritoneal B cells of neonates, young adults, and elderly mice. Gene expression analysis of 31,718 peritoneal B cells showed that the neonate peritoneal cavity contained many B1 progenitors, and neonate B cell specific clustering revealed two trajectories of peritoneal B1 cell development, including pre-BCR dependent and pre-BCR independent pathways. We also detected profound age-related changes in B1 cell transcriptomes: clear difference in senescence genetic program was evident in differentially aged B1 cells, and we found an example that a B1 subset only present in the oldest mice was marked by expression of the fatty-acid receptor CD36. We also performed antibody gene sequencing of 15,967 peritoneal B cells from the three age groups and discovered that B1 cell aging was associated with clonal expansion and two B1 cell clones expanded in the aged mice had the same CDR-H3 sequence (AGDYDGYWYFDV) as a pathogenically linked cell type from a recent study of an atherosclerosis mouse model.

Conclusions

Beyond offering an unprecedent data resource to explore the cell-to-cell variation in B cells, our study has revealed that B1 precursor subsets are present in the neonate peritoneal cavity and dissected the developmental pathway of the precursor cells. Besides, this study has found the expression of CD36 on the B1 cells in the aged mice. And the single-cell B-cell receptor sequencing reveals B1 cell aging is associated with clonal expansion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00795-6.

Crucial Role of Increased Arid3a at the Pre-B and Immature B Cell Stages for B1a Cell Generation

The Lin28b⁺Let7⁻ axis in fetal/neonatal development plays a role in promoting CD5⁺ B1a cell generation as a B-1 B cell developmental outcome. Here we identify the Let7 target, Arid3a, as a crucial molecular effector of the B-1 cell developmental program. Arid3a expression is increased at pro-B cell stage and markedly increased at pre-B and immature B cell stages in the fetal/neonatal liver B-1 development relative to that in the Lin28b⁻Let7⁺ adult bone marrow (BM) B-2 cell development. Analysis of B-lineage restricted Lin28b transgenic (Tg) mice, Arid3a knockout and Arid3a Tg mice, confirmed that increased Arid3a allows B cell generation without requiring surrogate light chain (SLC) associated pre-BCR stage, and prevents MHC class II cell expression at the pre-B and newly generated immature B cell stages, distinct from pre-BCR dependent B development with MHC class II in adult BM. Moreover, Arid3a plays a crucial role in supporting B1a cell generation. The increased Arid3a leads higher Myc and Bhlhe41, and lower Siglec-G and CD72 at the pre-B and immature B cell stages than normal adult BM, to allow BCR signaling induced B1a cell generation. Arid3a-deficiency selectively blocks the development of B1a cells, while having no detectable effect on CD5⁻ B1b, MZ B, and FO B cell generation resembling B-2 development outcome. Conversely, enforced expression of Arid3a by transgene is sufficient to promote the development of B1a cells from adult BM. Under the environment change between birth to adult, altered BCR repertoire in increased B1a cells occurred generated from adult BM. However, crossed with B1a-restricted V_H/D/J IgH knock-in mice allowed to confirm that SLC-unassociated B1a cell increase and CLL/lymphoma generation can occur in aged from Arid3a increased adult BM. These results confirmed that in fetal/neonatal normal mice, increased Arid3a at the pre-B cell and immature B cell stages is crucial for generating B1a cells together with the environment for self-ligand reactive BCR selection, B1a cell maintenance, and potential for development of CLL/Lymphoma in aged mice.

Fetal Lymphoid Progenitors Become Restricted to B-1 Fates Coincident with IL-7Rα Expression

B-1 cells represent a sub-fraction of B lymphocytes that participate in T cell-independent antibody production and contribute to innate immunity. While the production of B-1 cells is favored during the fetal waves of lymphopoiesis, it has been unclear when and how that differentiation option is specified. To clarify this, lymphoid and hematopoietic progenitors of fetal liver (FL) and adult bone marrow (ABM) were examined for the B cell differentiation potential. Mouse common lymphoid progenitors (CLPs) and more primitive KSL fraction of FL and ABM were transferred to SCID mice and donor-derived B cell subsets were analyzed 4 weeks later. CLPs were also cultured on ST2 stromal cells for 6 days prior to transplantation. While Lin^- IL-7Rα⁺ CLPs from ABM differentiated to B-1, B-2 and marginal zone B (MZB) cells, equivalent cells from d15 FL differentiated mostly to B-1a cells. We found that fetal CLPs had less ability to colonize the bone marrow than adult CLPs. However, the fetal/adult difference was already present when progenitors were cultured in an identical condition before transplantation. More primitive KSL fraction of FL could generate the same broad spectrum of B cells typical of adults, including splenic MZB cells. In conclusion, we argue that FL and ABM-CLPs are intrinsically different regarding B-1/B-2 fates and the difference is acquired just before or coincident with the acquisition of IL-7Rα expression.

Inflammatory response: role of B1 cells

Inflammation is powerful response to destroy invading organisms, and an exaggerated response can lead to death of the host. Macrophages secrete mediators that activated circulating neutrophils leading to its migration into infectious site. Recently, it has been shown that lymphocytes have an action modulating the early phase of inflammatory response. In this article, we analyze the role of B1 in the inflammatory response of different origins and finally focus attention on sepsis. B lymphocyte deficiency has been linked to acute infection presumably owing to the lack of an adaptive immune response to effectively clear pathogens. Individuals with X-linked agammaglobulinemia (XLA) present B1 lymphocyte deficiency caused by mutations in the Bruton tyrosine kinase (Btk). Some data show that B1 cells might contribute to susceptibility in experimental paracoccidioidomycosis. On the other hand, B1 cells are shown to be detrimental in other mouse models of microbial infection, such as experimental Chagas disease, leishmaniasis, and Staphylococcus aureus-induced arthritis. B1 cell plays a protective role in the host of the effects of endotoxemia. In a murine model of endotoxemia by lipopolysaccharide, B1 cell participates in both interleukin 10 and immunoglobulin M secretion with a consequent reduction in mortality.

Reduced production of B-1-specified common lymphoid progenitors results in diminished potential of adult marrow to generate B-1 cells

B-1 B cells have been proposed to be preferentially generated from fetal progenitors, but this view is challenged by studies concluding that B-1 production is sustained throughout adult life. To address this controversy, we compared the efficiency with which hematopoietic stem cells (HSCs) and common lymphoid progenitors (CLPs) from neonates and adults generated B-1 cells in vivo and developed a clonal in vitro assay to quantify B-1 progenitor production from CLPs. Adult HSCs and CLPs generated fewer B-1 cells in vivo compared with their neonatal counterparts, a finding corroborated by the clonal studies that showed that the CLP compartment includes B-1- and B-2-specified subpopulations and that the former cells decrease in number after birth. Together, these data indicate that B-1 lymphopoiesis is not sustained at constant levels throughout life and define a heretofore unappreciated developmental heterogeneity within the CLP compartment.

Genetic control of the B cell response to LPS: opposing effects in peritoneal versus splenic B cell populations

Lipopolysaccharide (LPS) from gram-negative bacteria activates B cells, enabling them to proliferate and differentiate into plasma cells. This response is critically dependent on the expression of TLR4; but other genes, such as RP105 and MHC class II, have also been shown to contribute to B cell LPS response. Here, we have evaluated the role of genetic control of the B cell response to LPS at the single cell level. We compared the response to LPS of peritoneal cavity (PEC) and splenic B cells on the BALB/c genetic background (LPS-low responder) to those on the C57BL/6J background (LPS-high responder) and their F1 progeny (CB6F1). Both PEC and splenic B cells from B6 exhibited 100% clonal growth in the presence of LPS; whereas, BALB/c PEC and splenic B cells achieved only 50% and 23% clonal growth, respectively. Adding CpG to the LPS stimulus pushed PEC B cell clonal growth in the low responder strain BALB/c up to 90%, showing that the nonresponse to LPS is a specific effect. Surprisingly, PEC B cells on the F1 background behaved as high responders, while splenic B cells behaved as low responders to LPS. The data presented here reveals a previous unsuspected behavior in the genetic control of the B cell response to LPS with an opposing impact in splenic versus peritoneal cavity B cells. These results suggest the existence of an, as yet, unidentified genetic factor exclusively expressed by coelomic B cells that contributes to the control of the LPS signaling pathway in the B lymphocyte.

A population of CD19highCD45R-/lowCD21low B lymphocytes poised for spontaneous secretion of IgG and IgA antibodies

Ab responses to selected Ags are produced by discrete B cell populations whose presence and functional relevance vary along the ontogeny. The earliest B lineage-restricted precursors in gestational day 11 mouse embryos display the CD19(+)CD45R/B220(-) phenotype. Phenotypically identical cells persist throughout gestation and in postnatal life, in parallel to the later-arising, CD19(+)CD45R(+) B cells. Very early after birth, the CD19(+)CD45R(-) B cell subset included high frequencies of spontaneously Ig-secreting cells. In the adult spleen, a small subset of CD19(high)CD45R(-/low)IgM(+/-)IgD(-)CD21/Cr2(-/low) cells, which was detected in perifollicular areas, displayed genetic and phenotypical traits of highly differentiated B cells, and was enriched in IgG- and IgA-secreting plasma cells. In vitro differentiation and in vivo adoptive transfer experiments of multipotent hemopoietic progenitors revealed that these CD19(high)CD45R(-/low) B cells were preferentially regenerated by embryo-, but not by adult bone marrow-, derived progenitors, except when the latter were inoculated into newborn mice. Both the early ontogenical emergence and the natural production of serum Igs, are shared features of this CD19(high)CD45R(-/low) B cell population with innate-like B lymphocytes such as B1 and marginal zone B cells, and suggest that the new population might be related to that category.

Unraveling B-1 progenitors

B-1 cells comprise a small percentage of the B lymphocytes that reside in multiple tissues in the mouse, including the peritoneal and pleural cavities. Functionally, B-1 cells participate in innate immunity by producing the majority of the natural IgM in serum, which protects against invading pathogens before the onset of the adaptive immune response. B-1 cells arise from fetal and neonatal progenitors and are distinct from the adult bone marrow progenitors that give rise to follicular and marginal zone B-2 cells. Recent studies have attempted to delineate the progenitors of B-1 cells from those of B-2 cells. Notably, the identification of CD45R(-/lo)CD19(+) B-1 progenitors and expression of two surface determinants, CD138 and major histocompatibility class II antigens, distinguish developing B-1 cells from B-2 cells.

Expression Patterns of H2-O in Mouse B Cells and Dendritic Cells Correlate with Cell Function

In the endosomes of APCs, the MHC class II-like molecule H2-M catalyzes the exchange of class II-associated invariant chain peptides (CLIP) for antigenic peptides. H2-O is another class II-like molecule that modulates the peptide exchange activity of H2-M. Although the expression pattern of H2-O in mice has not been fully evaluated, H2-O is expressed by thymic epithelial cells, B cells, and dendritic cells (DCs). In this study, we investigated H2-O, H2-M, and I-A(b)-CLIP expression patterns in B cell subsets during B cell development and activation. H2-O was first detected in the transitional 1 B cell subset and high levels were maintained in marginal zone and follicular B cells. H2-O levels were down-regulated specifically in germinal center B cells. Unexpectedly, we found that mouse B cells may have a pool of H2-O that is not associated with H2-M. Additionally, we further evaluate H2-O and H2-M interactions in mouse DCs, as well as H2-O expression in bone marrow-derived DCs. We also evaluated H2-O, H2-M, I-A(b), and I-A(b)-CLIP expression in splenic DC subsets, in which H2-O expression levels varied among the splenic DC subsets. Although it has previously been shown that H2-O modifies the peptide repertoire, H2-O expression did not alter DC presentation of a number of endogenous and exogenous Ags. Our further characterization of H2-O expression in DCs, as well as the identification of a potential free pool of H2-O in mouse splenic B cells, suggest that H2-O may have a yet to be elucidated role in immune responses.

Functional Requirements for the Lysosomal Thiol Reductase GILT in MHC Class II-Restricted Antigen Processing

Ag processing and presentation via MHC class II is essential for activation of CD4(+) T lymphocytes. gamma-IFN-inducible lysosomal thiol reductase (GILT) is present in the MHC class II loading compartment and has been shown to facilitate class II Ag processing and recall responses to Ags containing disulfide bonds such as hen egg lysozyme (HEL). Reduction of proteins within the MHC class II loading compartment is hypothesized to expose residues for class II binding and protease trimming. In vitro analysis has shown that the active site of GILT involves Cys(46) and Cys(49), present in a CXXC motif that shares similarity with the thioredoxin family. To define the functional requirements for GILT in MHC class II Ag processing, a GILT-deficient murine B cell lymphoma line was generated and stably transduced with wild-type and cysteine mutants of GILT. Intracellular flow cytometric, immunoblotting, and immunofluorescence analyses demonstrated that wild-type and mutant GILT were expressed and maintained lysosomal localization. Transduction with wild-type GILT reconstituted MHC class II processing of a GILT-dependent HEL epitope. Mutation of either Cys(46) or Cys(49) abrogated MHC class II processing of a GILT-dependent HEL epitope. In addition, biochemical analysis of these mutants suggested that the active site facilitates processing of precursor GILT to the mature form. Precursor forms of GILT-bearing mutations in Cys(200) or Cys(211), previously found to display thiol reductase activity in vitro, could not mediate Ag processing. These studies demonstrate that the thiol reductase activity of GILT is its essential function in MHC class II-restricted Ag processing.

B-1 B cell development

CD5+ B cells have attracted considerable interest because of their association with self-reactivity, autoimmunity, and leukemia. In mice, CD5+ B cells are readily generated from fetal/neonatal precursors, but inefficiently from precursors in adult. One model proposed to explain this difference is that their production occurs through a distinctive developmental process, termed B-1, that enriches pre-B cells with novel germline VDJs and that requires positive selection of newly formed B cells by self-Ag. In contrast, follicular B cells are generated throughout adult life in a developmental process termed B-2, selecting VDJs that pair well with surrogate L chain, and whose maturation appears relatively independent of antigenic selection. In the present study, I focus on processes that shape the repertoire of mouse CD5+ B cells, describing the differences between B-1 and B-2 development, and propose a model encompassing both in the generation of functional B cell subpopulations.

Phenotypically distinct B cell development pathways map to the three B cell lineages in the mouse

A recent article by Montecino-Rodriguez et al. [Montecino-Rodriguez, E., Leathers, H. & Dorshkind, K. (2006) Nat. Immunol.7, 293-301] has distinguished the early progenitors for B-1 cells, which principally develop in neonates, from early progenitors for B-2 cells, which principally develop in adult bone marrow. Here we introduce syndecan-1 (CD138) and MHC class II (I-A) as markers of early B cell development [Hardy, R. R., Carmack, C. E., Shinton, S. A., Kemp, J. D. & Hayakawa, K. (1991) J. Exp. Med. 173, 1213-1225; Hardy fractions B-D] and show that the expression of these markers distinguishes the predominant B cell development pathway in neonates from the corresponding predominant pathway in adults (both progenitors are present but differently represented in each case). We show that pre-B cells (Hardy fraction D) in the predominant adult pathway express high levels of CD138 and intermediate levels of I-A, whereas the corresponding pre-B cells in the pathway that predominates in neonates do not express either of these markers. As expected, because most of the pre-B cells in adults express CD138, we find that sorted CD138+ adult pre-B cells differentiate to IgM+ B cells in vitro. Sorted CD138- pre-B cells from neonates, the majority subset at this age, also mature to IgM+ cells (without passing through a CD138+ stage). Importantly, our studies here confirm the differential representation of adult and neonatal progenitor populations and further demonstrate that CD138 expression subdivides the adult CD19+, B220-6B2-/low population shown to contain B-1 progenitors in a way consistent with the predominance of B-1b progenitors in adults. Thus, CD138 expression provides a key route to distinguishing early B cell development pathway for what now are clearly three B cell lineages.

Right place, right time, right peptide: DO keeps DM focused

Peptide loading of major histocompatibility class II molecules is catalyzed in late endosomal and lysosomal compartments of cells by the catalytic action of human leukocyte antigen (HLA)-DM (H-2M in mice). In B cells, dendritic cells and thymic epithelial cells, the peptide loading of class II molecules is modified by the expression of the non-classical class II molecule, HLA-DO (H-2O in mice). Collectively, studies to date support that DO/H-2O expression inhibits the presentation of antigens acquired by cells via fluid phase endocytosis. However, in B cells, the expression of H-2O promotes the presentation of antigens internalized by the B-cell receptor. In this review, we summarize the literature pertaining to DO assembly, transport, and function, with an emphasis on the function of DO/H-2O.

Transcriptional control of B cell development and function

The generation, development, maturation and selection of mammalian B lymphocytes is a complex process that is initiated in the embryo and proceeds throughout life to provide the organism an essential part of the immune system it requires to cope with pathogens. Transcriptional regulation of this highly complex series of events is a major control mechanism, although control is also exerted on all other layers, including splicing, translation and protein stability. This review summarizes our current understanding of transcriptional control of the well-studied murine B cell development, which bears strong similarity to its human counterpart. Animal and cell models with loss of function (gene "knock outs") or gain of function (often transgenes) have significantly contributed to our knowledge about the role of specific transcription factors during B lymphopoiesis. In particular, a large number of different transcriptional regulators have been linked to distinct stages of the life of B lymphocytes such as: differentiation in the bone marrow, migration to the peripheral organs and antigen-induced activation.

Regulatory mechanisms that determine the development and function of plasma cells

Plasma cells are terminally differentiated final effectors of the humoral immune response. Plasma cells that result from antigen activation of B-1 and marginal zone B cells provide the first, rapid response to antigen. Plasma cells that develop after a germinal center reaction provide higher-affinity antibody and often survive many months in the bone marrow. Transcription factors Bcl-6 and Pax5, which are required for germinal center B cells, block plasmacytic differentiation and repress Blimp-1 and XBP-1, respectively. When Bcl-6-dependent repression of Blimp-1 is relieved, Blimp-1 ensures that plasmacytic development is irreversible by repressing BCL-6 and PAX5. In plasma cells, Blimp-1, XBP-1, IRF4, and other regulators cause cessation of cell cycle, decrease signaling from the B cell receptor and communication with T cells, inhibit isotype switching and somatic hypermutation, downregulate CXCR5, and induce copious immunoglobulin synthesis and secretion. Thus, commitment to plasmacytic differentiation involves inhibition of activities associated with earlier B cell developmental stages as well as expression of the plasma cell phenotype.

DNA alkylating agents alleviate silencing of class II transactivator gene expression in L1210 lymphoma cells

MHC class II (Ia) Ag expression is inversely correlated with tumorigenicity and directly correlated with immunogenicity in clones of the mouse L1210 lymphoma (1 ). Understanding the mechanisms by which class II Ag expression is regulated in L1210 lymphoma may facilitate the development of immunotherapeutic approaches for the treatment of some types of lymphoma and leukemia. This study demonstrates that the variation in MHC class II Ag expression among clones of L1210 lymphoma is due to differences in the expression of the class II transactivator (CIITA). Analysis of stable hybrids suggests that CIITA expression is repressed by a dominant mechanism in class II-negative L1210 clones. DNA-alkylating agents such as ethyl methanesulfonate and the chemotherapeutic drug melphalan activate CIITA and class II expression in class II negative L1210 cells, and this effect appears to be restricted to transformed cell lines derived from the early stages of B cell ontogeny. Transient transfection assays demonstrated that the CIITA type III promoter is active in class II(-) L1210 cells, despite the fact that the endogenous gene is not expressed, which suggests that these cells have all of the transacting factors necessary for CIITA transcription. An inverse correlation between methylation of the CIITA transcriptional regulatory region and CIITA expression was observed among L1210 clones. Furthermore, 5-azacytidine treatment activated CIITA expression in class II-negative L1210 cells. Collectively, our results suggest that 1) CIITA gene expression is repressed in class II(-) L1210 cells by methylation of the CIITA upstream regulatory region, and 2) treatment with DNA-alkylating agents overcomes methylation-based silencing of the CIITA gene in L1210 cells.

Nature or nurture? Steady-state lymphocyte formation in adults does not recapitulate ontogeny

Substantial progress has been made in determining developmental relationships between lymphocyte precursors and those corresponding to other blood cell lineages. Indeed, exploitation of RAG1/GFP knock-in mice has recently made it possible to chart the entire sequence of lymphocyte differentiation events in adult bone marrow and thymus. However, the differentiation pathways proposed for fetal life are very different from this model. We review many examples where the results of gene targeting experiments are substantially dependent on developmental age. In mice, adult patterns of gene expression and corresponding properties of lymphocyte precursors are not fully established until several weeks after birth, and the same might be true for humans. Furthermore, examples are cited where fetal hematopoietic cells did not efficiently acquire those properties when transplanted to an adult environment. There are several important implications of these findings. Cognizance of developmental age-related changes might resolve apparent conflicts in the literature. Hematopoietic stem cells and their lymphoid lineage progeny appear in waves, and a direct connection is yet to be established between fetal stem cells and ones that sustain adult blood cell formation. There is the possibility that adult stem cells derive from founders with an unknown origin.

Delayed cellular maturation and decreased immunoglobulin kappa light chain production in immature B lymphocytes lacking B cell linker protein

B cell linker (BLNK) protein is a component of the B cell receptor (BCR) signaling pathway and BLNK(-/-) mice have a block in B lymphopoiesis at the pro-B/pre-B cell stage. To study the effect of BLNK mutation at later stages of B cell development, we introduce an innocuous transgenic BCR into BLNK(-/-) mice and show that two populations of immature B cells distinguishable by their IgM(low (lo)) and IgM(high (hi)) phenotypes are found in the bone marrow of these mice in contrast to a single population of IgM(hi) cells found in control BCR-transgenic BLNK(+/+) mice. The mutant IgM(lo) and IgM(hi) cells are at an earlier developmental stage compared with the control IgM(hi) cells as indicated by their differential expression of CD43, B220, and major histocompatibility complex class II antigens and their timing of generation in culture. Thus, in the absence of BLNK the differentiation of immature B cells is delayed. Furthermore, mutant IgM(lo) cells produce equivalent level of immunoglobulin (Ig) mu but less Ig kappa proteins than control and mutant IgM(hi) cells and this defect is attributed to a decrease in the amount of kappa transcripts being generated. Finally, splenic B cells in BCR-transgenic BLNK(-/-) mice are predominantly of the transitional B cell phenotype and are rapidly lost from the peripheral B cell pool. Taken together, the data suggest a role for BLNK and perhaps BCR signaling, in the regulation of kappa light chain expression and continued immature B cell differentiation.

Regulated expression of human histocompatibility leukocyte antigen (HLA)-DO during antigen-dependent and antigen-independent phases of B cell development

Human histocompatibility leukocyte antigen (HLA)-DO, a lysosomal resident major histocompatibility complex class II molecule expressed in B cells, has previously been shown to be a negative regulator of HLA-DM peptide loading function. We analyze the expression of DO in human peripheral blood, lymph node, tonsil, and bone marrow to determine if DO expression is modulated in the physiological setting. B cells, but not monocytes or monocyte-derived dendritic cells, are observed to express this protein. Preclearing experiments demonstrate that approximately 50% of HLA-DM is bound to DO in peripheral blood B cells. HLA-DM and HLA-DR expression is demonstrated early in B cell development, beginning at the pro-B stage in adult human bone marrow. In contrast, DO expression is initiated only after B cell development is complete. In all situations, there is a striking correlation between intracellular DO expression and cell surface class II-associated invariant chain peptide expression, which suggests that DO substantially inhibits DM function in primary human B cells. We report that the expression of DO is markedly downmodulated in human germinal center B cells. Modulation of DO expression may provide a mechanism to regulate peptide loading activity and antigen presentation by B cells during the development of humoral immune responses.

Identification of a germ-line pro-B cell subset that distinguishes the fetal/neonatal from the adult B cell development pathway

Studies presented here show that the expression of CD4, MHC class II (Ia,) and B220 cleanly resolves a major and a minor subset within the earliest pro-B cell population (germ-line pro-B) in adult bone marrow (BM). The major subset expresses intermediate B220 and low CD4 levels. The minor subset, which constitutes roughly 20% of the adult germ-line pro-B, expresses very low B220 levels and does not express CD4. Ia is clearly detectable at low levels on the major germ-line pro-B subset, both in wild-type adult mice and in gene-targeted mice (RAG2-/- and microMT), in which B cell development terminates before the pre-B cell stage. A small proportion of cells in the more mature pro-B cell subsets (Hardy Fractions B and C) also express Ia at this level. In contrast, Ia levels on the minor subset are barely above (or equal to) background. Surprisingly, the major germ-line pro-B cell subset found in adults is missing in fetal and neonatal animals. All of the germ-line pro-B in these immature animals express a phenotype (very low B220, no CD4, or Ia) similar to that of the minor pro-B cell subset in adult BM. Because B cell development in fetal/neonatal animals principally results in B-1 cells, these findings demonstrate that the B-1 development pathway does not include the major germ-line pro-B subset found in adult BM and hence identify a very early difference between the B-1 and -2 development pathways.

Long-lived polyclonal B-cell lines derived from midgestation mouse embryo lymphohematopoietic progenitors reconstitute adult immunodeficient mice

Lymphohematopoietic progenitors derived from midgestation mouse embryos were established in long-term cultures with stromal cell monolayers and interleukin 7 (IL-7), giving rise to B-lineage cell lines. The initial emergence and in vitro establishment of these early embryo cell lines were highly sensitive to IL-7-mediated signals, in comparison to cell lines similarly obtained using precursors from late fetal liver (> 13 days postcoitum) and adult bone marrow. The early embryo-derived progenitors spontaneously differentiated in vitro to CD19(+)IgM(+) immature B cells in the presence of optimal concentrations of IL-7, in contrast to those progenitors obtained from late gestation and adult mice, whose differentiation only occurred in the absence of IL-7. The newly in vitro-generated B cells of the early embryo cell lines repopulated adult immunodeficient severe combined immunodeficient mice on their adoptive transfer in vivo and generated specific humoral immune responses after immunization.

A VH11V kappa 9 B cell antigen receptor drives generation of CD5+ B cells both in vivo and in vitro

B lymphocytes can be divided into different subpopulations, some with distinctive activation requirements and probably mediating specialized functions, based on surface phenotype and/or anatomical location, but the origins of most of these populations remain poorly understood. B cells constrained by transgenesis to produce an Ag receptor derived from a conventional (B-2) type cell develop a B-2 phenotype, whereas cells from mice carrying a B-1-derived receptor acquire the B-1 phenotype. In this study transgenic enforced expression of a B cell receptor (mu/kappa) originally isolated from a CD5+ (B-1a) B cell generates B-1 phenotype cells in bone marrow cultures that show a distinctive B-1 function, survival in culture. Despite their autoreactivity, we find no evidence for receptor editing or that the paucity of B-2 cells is the result of tolerance-induced selection. Finally, Ca2+ mobilization studies reveal a difference between transgenic B-1 cells in spleen and peritoneal cavity, with cells in spleen much more responsive to anti-B cell receptor cross-linking. We discuss these results in terms of specificity vs lineage models for generation of distinctive B cell subpopulations.

Identification of CD19(-)B220(+)c-Kit(+)Flt3/Flk-2(+)cells as early B lymphoid precursors before pre-B-I cells in juvenile mouse bone marrow

The combined analysis of the expression of receptor tyrosine kinases c-Kit and Flt3/Flk-2 and of the human CD25 gene expressed as a transgene under the regulation of the mouse lambda5 promoter in the bone marrow of 1-week-old mice allows us to identify three stages of B lymphocyte development before the CD19(+)c-Kit(+) pre-B-I cells. Single-cell PCR analysis of the rearrangement status of the Ig heavy chain alleles allows us to order these early stages of B cell development as follows: (i) B220(+)CD19(-)c-Kit(lo)Flt3/Flk-2(hi)lambda5(-), (ii) B220(+)CD19(-)c-Kit(lo)Flt3/Flk-2(hi)lambda5(+) and (iii) B220(+)CD19(+)c-Kit(lo)Flt3/Flk-2(lo)lambda5(+) before B220(+)CD19(+)c-Kit(lo)Flt3/Flk-2(-)lambda5(+) pre-B-I cells. All these progenitors are clonable on stromal cells in the presence of IL-7 and can differentiate to CD19(+)c-Kit(-) B-lineage cells. A combination of stem cell factor, Flt3 ligand and IL-7 was also able to support the proliferation and differentiation of the progenitors in a suspension culture. Furthermore, the analyses indicate that the onset of D(H)J(H) rearrangements precedes the expression of the lambda5 gene. These progenitor populations were characteristic of juvenile mice and could not be detected in the bone marrow of adult mice. Hence the expression pattern, and probably the function, of the receptor tyrosine kinases in early B cell differentiation appears to be different in juvenile and adult mice.

B cell development and activation defects resulting in xid-like immunodeficiency in BLNK/SLP-65-deficient mice

Engagement of the B cell receptor (BCR) leads to the activation of tyrosine kinases and other signaling molecules that ultimately determine the type and magnitude of the B lymphocyte's cellular response. The adaptor protein BLNK/SLP-65 plays a pivotal role in BCR signal transduction by coupling Syk activation to downstream elements such as Grb2, phospholipase C-gamma, Vav and Nck. We have generated BLNK(-/-) mice to determine the physiological role of this protein in B cell development and activation. BLNK(-/-) mice exhibit an incomplete block in B cell development with a severe inhibition of pro-B to pre-B cell differentiation. BLNK(-/-) sIgM(+) cells can develop, seed the peripheral lymphoid tissues and accumulate in numbers overtime. However, these mutant B cells failed to mature and are non-responsive to BCR cross-linking in terms of proliferation and up-regulation of activation markers such as CD69 and CD86 (B7-2). In addition, the CD5(+) subset of B cells is absent. The immune response to T cell-independent antigen but not T cell-dependent antigen is also impaired. Overall, the phenotype of BLNK(-/-) mice bears a striking resemblance to that of xid mice which is the murine model of human XLA that has a mutation in Bruton's tyrosine kinase. This raises the interesting possibility that mutation in BLNK/SLP-65 may be responsible for certain human immunodeficiencies.

B cell antigen receptor specificity and surface density together determine B-1 versus B-2 cell development

Mice expressing the immunoglobulin (Ig) heavy (H) chain variable (V) region from a rearranged V(H)12 gene inserted into the IgH locus generate predominantly B-1 cells, whereas expression of two other V(H) region transgenes (V(H)B1-8 and V(H)glD42) leads to the almost exclusive generation of conventional, or B-2, cells. To determine the developmental potential of B cells bearing two distinct B cell antigen receptors (BCRs), one favoring B-1 and the other favoring B-2 cell development, we crossed V(H)12 insertion mice with mice bearing either V(H)B1-8 or V(H)glD42. B cells coexpressing V(H)12 and one of the other V(H) genes are readily detected in the double IgH insertion mice, and are of the B-2 phenotype. In mice coexpressing V(H)12, V(H)B1-8 and a transgenic kappa chain able to pair with both H chains, double H chain-expressing B-2 cells, and B-1 cells that have lost V(H)B1-8 are generated, whereas V(H)B1-8 single producers are undetectable. These data suggest that B-1 but not B-2 cells are selected by antigenic stimuli in whose delivery BCR specificity and surface density are of critical importance.

c-Myb is essential for early T cell development

The c-Myb transcription factor is important for fetal hematopoiesis and has been proposed to mediate later stages of lymphocyte development. Using homozygous null c-Myb/Rag1 chimeric mice, we have determined that c-Myb plays an important role in the differentiation of macrophages and lymphocytes from precursor stem cells. We also determine that deletion of c-Myb leads to a complete block in early T cell development just before the oligopotent thymocyte matures into the definitive T cell precursor. These data indicate that c-Myb plays an important role at multiple stages of hematopoiesis and is required at an early stage of T cell development.

Somatic Mutation in the Neonatal Mouse

Several mechanisms that diversify the adult immune repertoire, such as terminal deoxynucleotidyl transferase-dependent N region addition, are not available to the neonatal mouse. One important process that contributes to protective immunity in the adult is somatic mutation, which plays a major role in the generation of high affinity memory B cells. It is not clear whether B cells in the neonatal mouse can activate the somatic mutation machinery. To investigate this, we immunized neonates with poly(l-Tyr,l-Glu)-poly-d, l-Ala–poly-l-Lys complexed with methylated BSA, or (4-hydroxy-3-nitrophenyl)acetyl coupled to chicken γ-globulin. Eight to fourteen days after priming, V(D)J rearrangements of known VH genes (VHSM7 family) were screened for mutations using a temperature-melt hybridization assay and oligonucleotide probes specific for complementarity-determining regions I and II; possible mutations were confirmed by sequence analysis. More mutations per sequence were found in heavy chains from neonates immunized with (4-hydroxy-3-nitrophenyl)acetyl coupled to chicken γ-globulin than in those from neonates immunized with poly(l-Tyr, l-Glu)-poly-d,l-Ala-poly-l-Lys complexed with methylated BSA. Mutations were found in heavy chains lacking N regions, suggesting that B cells of the putative fetal lineage can somatically mutate and diversify an initially limited repertoire. Since neonates immunized as early as 1 or 2 days after birth had mutations, the somatic mutation machinery can be activated soon after birth, suggesting that early vaccination should result in affinity maturation and protective immunity in the neonate.

Rapid elimination of mature autoreactive B cells demonstrated by Cre-induced change in B cell antigen receptor specificity in vivo

Developing autoreactive B cells edit their B cell antigen receptor (BCR) in the bone marrow and are clonally deleted when they fail to reexpress an innocent BCR. Here, inducible Cre-loxP-mediated gene inversion is used to change the specificity of the BCR on mature IgM+ IgD+ B cells in vivo to address the fate of lymphocytes encountering self-antigens at this developmental stage. Expression of an autoreactive BCR on mature B cells leads to their rapid elimination from the periphery, a process that is inhibited by constitutive bcl-2 transgene expression in an antigen dose-dependent manner. Thus, selection of mature B cells into the long-lived peripheral pool does not prevent their deletion upon encounter of self-antigens.

Expression of CD3 epsilon, CD3 zeta, and RAG-1/RAG-2 in decidual CD56+ NK cells

In early pregnancy the uterine mucosa, the decidua, is infiltrated by numerous CD56bright, CD16- natural killer (NK) cells. In this paper we have shown that unlike adult peripheral blood NK cells these uterine NK cells express cytoplasmic CD3 and CD3 zeta, but CD3 gamma and CD3 delta were not found in the majority of cells. In contrast to previous reports, there was no evidence of RAG-1 nor RAG-2 expression in decidual NK cells. These findings have implications for the ontogeny and development of decidual NK cells.

MHC Class II Expression in Double Mutant Mice Lacking Invariant Chain and DM Functions

Invariant (Ii) chain and DM functions are required at distinct stages during class II maturation to promote occupancy by diverse peptide ligands. The class II molecules expressed by mutant mouse strains lacking Ii chain or DM activities display discrete structural and functional abnormalities. The present report describes the cellular and biochemical characteristics of Ii−DM− doubly deficient mice. As for Ii chain mutants, their mature AαbAβb dimers similarly exhibit reduced mobilities in SDS-PAGE, and in functional assays these molecules behave as if empty or occupied by an easily displaced peptide. Additionally, the present experiments demonstrate that the production of floppy AαbAβb dimers is TAP independent. In comparison with Ii chain mutants, Ii−DM− doubly deficient cell populations exhibit increased peptide binding activities and consistently greater presentation abilities in T cell stimulation assays. These functional differences appear to reflect higher class II surface expression associated with their increased representation of B lymphocytes. We also observe defective B cell maturation in mice lacking Ii chain or DM expression, and interestingly, B cell development appears more severely compromised in Ii−DM− double mutants. These mutant mice lacking both Ii chain and DM activities should prove useful for analyzing nonconventional class II Ag presentation under normal physiological conditions in the intact animal.

Regulated Expression of the Eph-Related Receptor Tyrosine Kinase Hek11 in Early Human B Lymphopoiesis

Members of the large Eph family of receptor tyrosine kinases (RTKs) display temporally and spatially restricted expression patterns during embryogenesis, suggesting a role in various developmental processes. We have begun to investigate the expression of members of this receptor family during human hematopoiesis, in particular B lymphopoiesis. Expression of Eph RTKs in cells of the B-lymphoid lineage was assessed by using degenerate oligonucleotide primers based on stretches of conserved nucleic acid sequences in members of the Eph family. First, the content of Eph-family RTKs was assessed in freshly sorted fetal bone marrow pro–B cells. This population was found to harbor transcripts of the Hek8 and Hek11 members of this gene family. Subsequent analysis of expression of these genes in B cells representing various differentiation and ontogenic stages showed that the Hek8 transcript is constitutively present in all fetal and adult B-lineage cells, with high levels of expression in peripheral blood B cells. In contrast, the Hek11 transcript was exclusively found in fetal bone marrow pro–B cells and pre–B cells, but not in more mature fetal B-lineage cells. All adult B-lineage cells, from early pro–B cells to end-stage plasma cells, lacked Hek11 transcripts. The developmentally regulated expression of Hek11 during fetal B lymphopoiesis suggests a role for this gene in pre/pro–B cell expansion and/or differentiation and defines a difference in progenitor B cell populations isolated from fetal versus adult human bone marrow.

The BCL-6 proto-oncogene controls germinal-centre formation and Th2-type inflammation

Structural alterations of the promoter region of the BCL-6 proto-oncogene represent the most frequent genetic alteration associated with non-Hodgkin lymphoma, a malignancy often deriving from germinal-centre B cells. The BCL-6 gene encodes a zinc-finger transcriptional repressor normally expressed in both B cells and CD4+ T cells within germinal centres, but its precise function is unknown. We show that mice deficient in BCL-6 displayed normal B-cell, T-cell and lymphoid-organ development but have a selective defect in T-cell-dependent antibody responses. This defect included a complete lack of affinity maturation and was due to the inability of follicular B cells to proliferate and form germinal centres. In addition, BCL-6-deficient mice developed an inflammatory response in multiple organs characterized by infiltrations of eosinophils and IgE-bearing B lymphocytes typical of a Th2-mediated hyperimmune response. Thus, BCL-6 functions as a transcriptional switch that controls germinal centre formation and may also modulate specific T-cell-mediated responses. Altered expression of BCL-6 in lymphoma represents a deregulation of the pathway normally leading to B cell proliferation and germinal centre formation.

B cell development in mice

The development and establishment of the B Cell Repertoire is the net result of both genetic and environmental forces. The primary event at the genetic level is Ig gene rearrangement resulting in numerous possible combination of genes which can be further modified by somatic events such as N segment addition and somatic mutation. Environmental forces in the form of self and exogenous Ags also shape the repertoire by positively or negatively selecting B cells according to the specificity of their Ig receptors. These are dynamic processes beginning with the earliest expression of immunoglobulins in fetal life and continuing throughout life. In this review we discuss the genetic and selective mechanisms responsible for differences in the early immune system compared to that of the adult.

Divergent evolution in the mechanisms controlling major histocompatibility complex class II gene transcription in mouse and human

The expression of the major histocompatibility complex (MHC) class II gene family is developmentally regulated and, in general, in a coordinate manner. In this study, we show that the expression of the entire repertoire of human class II genes, otherwise transcriptionally silent in the bare lymphocyte syndrome-derived BLS1 cell line, can be rescued by somatic cell hybridization with normal mouse spleen cells. The analysis of the interspecies cell hybrids revealed a particularly important and unprecedented aspect. A return to the BLS1-like, human MHC class II-negative phenotype due to segregation of mouse chromosomes was accompanied in certain hybrids by loss of IE, but not IA cell surface antigen expression. At the molecular level, this was the result of lack of E alpha-specific mRNA in the presence of E beta-, A alpha- and A beta-specific mRNA. Thus, the mouse trans-acting function operating across species barriers and able to complement the defect of human BLS1 cells diverged in mice to control Ea, but not Eb, Aa and Ab gene expression. These findings suggest that evolutionary pressure has maintained the expression of the MHC class II multigene family under the control of quite distinct species-specific transcriptional mechanisms.

Development of the mouse B-cell repertoire

The mouse B-cell repertoire early in ontogeny contains B cells with receptor immunoglobulins exhibiting high connectivity, multi/self-reactivity, and generally low affinity. This is due structurally to extensive restriction in the germline components used to generate the B-cell receptor. The selective pressure acting on the nascent repetoire has both negative and positive components as seen in our in vivo models. VH81X-bearing B cells from the VH81X transgenic mice (and probably from normal mice) are subject to self-selective pressure with two components: a positive one favoring a certain (self-reactive) specificity in the CD23-IgM+ population and a negative one preventing the entry of B cells with this specificity into the CD23+IgM+ compartment.