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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.

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The proto-oncogene c-abl encodes a tyrosine kinase that is hypothesized to function in proliferation-stimulatory signaling pathways. Previous work on mice homozygous for targeted mutations in the c-abl gene (ablml and abl2 mutant strains) has demonstrated multiple defects, including a susceptibility to infections that results in a high mortality rate after weaning. FACS analysis of the hemopoietic system of c-abl mutants demonstrated variable reductions in B and T lymphocytes in adult bone marrow, thymus, spleen, and peripheral blood. In addition, bone marrow from mutants showed a decreased ability to respond to interleukin-7. We further found that B cells from ablm1 mice had a reduced ability to respond to lipopolysaccharide (decreased to 10% of control response) that was dependent on the culture conditions and the tissue of origin of B cells. Peripheral blood from the mutants also had a reduced response to the T cell mitogen concanavalin A. Immune response in ablm1 mice as determined by the mixed lymphocyte response and the sheep red blood cell plaque-forming assay was grossly normal. These findings suggest that although specific signaling pathways in lymphocytes may involve c-Abl, the immune system can function in the absence of a normal c-abl gene product.

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B-1 cells represent a distinct population of B lymphocytes with unique phenotypic, developmental and functional characteristics. We present evidence that the expression of MHC class II antigens differentiates two distinct developmental pathways which define the fetal-type (FT) and adult-type B-cell lineages. Further in-vivo and in-vitro analyses suggest that B-1 cells are derived primarily if not exclusively from the FT lineage. Combining these results with other studies suggesting that signalling plays a role in the development of B-1 cells, we propose a modified dual lineage model for the generation of B-1 cells. In this model fetal-type B cells are uniquely 'born' with the capacity to be B-1 cells, however, they must be properly educated (i.e. receive the appropriate signals) before they can be 'made' functional, phenotypic B-1 cells. With respect to the function of B-1 cells, we have used PerC/BM allotype chimeric animals to investigate the capacity of B-1 cells to participate in the formation of germinal centers. In these mice we were unable to detect any significant involvement of B-1 cells in the generation of germinal centers. These results are discussed in the context of the known characteristics of B-1 responses including the low frequency of hypermutation and isotype class switching.

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Mice homozygous for a mutation in the c-abl tyrosine kinase gene have multiple defects including high postnatal mortality, runting, morphological abnormalities, susceptibility to infections, and reductions in lymphocytes and their precursors. FACS analysis of bone marrow from mutant mice demonstrates variable reductions in pro-B and pre-B cells. While the numbers of cells in these populations are profoundly reduced in some mutants (16 and 1.2% of control pro-B and pre-B cells, respectively), normal levels are found in other individuals. In the affected mutants, some reductions are observed in many stages of B cell development. The response of B cell precursors to the cytokine interleukin-7 is variably affected while that of several other cytokines (stem cell factor, interleukin-3, GM-CSF, G-CSF, and erythropoietin) is normal in c-abl mutants. The population defects caused by the c-abl mutation can be recreated in normal mice by the transfer of adult bone marrow but, surprisingly, not fetal liver. These studies demonstrate that c-Abl signaling pathways may play a role in the earliest stages of B cell development in a developmental stage-specific manner. In spite of these variable abnormalities, however, the hemopoietic system of c-abl mutant animals is surprisingly intact.

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Analysis of B cell development in three strains of gamma 2b transgenic mice shows that the gamma 2b H chain can replace the microH chain in promoting B cell differentiation. The 348C line produces 90% gamma 2b-only B cells and 10% B cells; which co-express gamma 2b and endogenous sIgM and sIgD. These IgG2b+ B cells develop into mature, recirculating CD23+ B cells. The 343-1 and gamma 2b-T15 transgenic mice produce sIgMhigh:sIgDlow:CD23- B cells that generally co-express the gamma 2b transgene-encoded H chain. Such B cells are either developmentally arrested immature B cells or arise from B-1 (CD5) progenitors. The gamma 2b-T15 mice can produce gamma 2b-only CD23+ B cells following inactivation of the endogenous mu locus, whereas 343-1 mice fail to develop B cells. Thus, gamma 2b H chains: 1) can act alone to promote the development of mature B cells, 2) synergize with microH chains for allelic exclusion, and 3) vary in their influence on B cell development in different transgenic mouse strains.

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Previous studies distinguished two murine B cell lineages: the conventional lineage, which comprises the majority of B cells, and the Ly-1 B lineage (B-1a), which represents a small percentage of total adult B cells. A third subset, B-1b cells, shares many properties with B-1a cells, including the characteristic ability to self-replenish, but does not express Ly-1 (CD5). Reconstitution studies presented here show that (i) although the B220- population in adult spleen and bone marrow contains very little progenitor activity for B-1a cells, it can reconstitute roughly half the normal number of B-1b cells; (ii) B-1 progenitors present in adult bone marrow and spleen function at low levels in adult animals; (iii) peritoneal B-1 cells (principally B-1b) that develop following bone marrow transfer, like B-1 cells from normal animals, are capable of substantial self-replenishment; and (iv) conventional B cells do not expand (self-replenish) in adoptive recipients, although they can persist for long periods. Collectively, these progenitor and self-replenishment characteristics provide a developmental base for distinguishing B-1a, B-1b and conventional B cells.

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CD43 (leukosialin) expression has previously been demonstrated on the surface of developing B cells in mouse bone marrow and on plasma cells induced in vitro, but not on peripheral B cells in spleen. Here we show that CD43, as recognized by mAb S7, is indeed expressed on a small population of splenic B cells. Flow cytometric phenotyping of normal mice and radiation chimeras reveals that CD43/S7 is expressed on virtually all (> 90 to 95%) splenic B-1 cells and the majority of peritoneal B-1 cells, but not on conventional B cells. The expression of CD43/S7, in conjunction with other cell surface markers, clearly distinguishes B-1 cells from follicular, marginal zone, and immature B cells in the unstimulated adult spleen and permits further phenotyping of these subsets. The phenotype of splenic and peritoneal B-1 cells in normal BALB/c and BAB/25 mice is essentially identical with the exception that all peritoneal B-1 cells express CD11b (Mac-1) and some lack CD43/S7 and heat stable Ag (as detected by the mAb 53-10) expression. Although splenic B-1, marginal zone, and immature B cells share many phenotypic characteristics, these studies show that, in addition to CD43, they differ with respect to the expression levels of a variety of Ags including heat stable Ag, B220, and the B cell activation Ag B7.

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All mature B cells coexpress major histocompatibility complex (MHC) class II molecules, I-A and I-E, which are restriction elements required for antigen presentation to CD4+ T cells. However, the expression of class II during the early stages of B cell development has been unclear. We demonstrate here that there is a difference in the expression of class II during murine B cell development in the fetal liver and adult bone marrow (BM). These differences define two distinct B cell developmental pathways. The Fetal-type (FT) pathway is characterized by pre-B and immature IgM+ B cells generated in the fetal liver which initially lack all class II expression. In contrast, the Adult-type (AT) pathway is typified by B cells developing in the adult BM which express class II molecules from the pre-B cell stage. In vitro stromal cell cultures of sorted fetal liver and adult BM pro-B cells indicated that the difference in I-A expression during B cell development is intrinsic to the progenitors. In addition, we show that FT B cell development is not restricted to the fetal liver but occurs in the peritoneal cavities, spleens, liver, and BM of young mice up to at least 1 mo of age. The AT B cell development begins to emerge after birth but is, however, restricted to the BM environment. These findings indicate that there are two distinct B cell developmental pathways during ontogeny, each of which could contribute differentially to the immune repertoire and thus the functions of B cell subsets and lineages.

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Primary immunization of BALB/c mice with alpha(1-->6)dextran (DEX), a native bacterial polysaccharide, induces an unexpected pattern of splenic B-cell responses. After a peak of antibody-secreting B-cell response at day 4, deposition of dextran-anti-dextran immune complexes, as revealed by staining with both dextran and antibodies to dextran, occurs and persists in splenic follicles until at least the fourth week after immunization. Antigen-specific B cells appear and proliferate in such follicles, leading by day 11 to development of DEX-specific germinal centers as characterized by the presence of distinct regions of DEX+ peanut agglutinin-positive (PNA+) cells. At this time, fluorescence-activated cell sorter analysis also reveals the appearance of a distinct population of DEX+ PNA+ splenic B cells. In contrast, DEX+ PNA- cells, characterized by intense cytoplasmic staining, are present outside of splenic follicles, peak at day 4 to day 5, and persist until at least day 28. The frequency of these cells correlates with DEX-specific antibody-secreting cells, as detected by the ELISA-spot assay. Thus, in addition to the expected plasma cellular response, the typical T-cell-independent type II antigen, DEX, surprisingly also elicits the formation of antigen-specific germinal centers. These observations raise fundamental questions about the roles of germinal centers in T-cell-independent immune responses.

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The M54 transgenic mouse line, which carries the 17.2.25 Ig mu heavy chain gene, rearranges Ig heavy chains and expresses both transgenic and endogenous mu. B cell lineage development is selectively impaired in these mice and cells that simultaneously express transgenic and endogenous mu ('double-producers') are common amongst the B cells and plasma cells that do develop. Weaver, Imanishi-kari, Baltimore and colleagues failed to obtain double-producing hybridomas from M54 mice; however, molecular and serologic studies presented here show that such hybridomas are readily generated. These hybridomas are extremely unstable and rapidly yield variants producing either transgenic or endogenous mu. Therefore the stable cloned lines we obtained, like Weaver et al., were almost all single or non-producers. We also found that the VH gene usage in our hybridomas was skewed towards the JH proximal (VHQ52, VH81X) families, supporting the idea that the expression of the M54 transgene alters the endogenous Ig repertoire.

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In this paper we have outlined the evidence for two distinct branches of the B-1 cell lineage. The data show that phenotypically B-1a and B-1b cells are essentially identical, distinguished only by the presence or absence of the CD5 antigen. Functionally no differences between the two populations have yet been identified. Both produce anti-PtC antibodies, a specificity not observed in conventional B cells. Both produced high levels of IgM as measured in adoptive transfer experiments. Developmentally, B-1a and B-1b cells are indistinguishable with respect to generation from progenitors present in fetal liver and omentum, feedback regulation of new B-1a and B-1b cells from bone marrow, self-replenishment from Ig+ cells following adoptive transfer, and the generation of clonal populations. The major difference in the two populations is seen in the development of B-1a and B-1b cells from B220- progenitors in the adult bone marrow. Although B220- B-1a progenitors are rare in adult (greater than 6 weeks) bone marrow, the progenitors for B-1b cells persist well into adulthood. Our understanding of B-1b cell ontogeny is at a stage similar to that of B-1a cells five years ago. We have evidence from transfer experiments that strongly suggests the existence of two distinct progenitors for B-1a and B-1b, but we have yet to physically separate these progenitors as Solvansen et al. have done for B-1 and conventional B cells. Furthermore we must determine whether the B-1b cells that develop from fetal liver and bone marrow are functionally and developmentally equivalent to those that develop from adult bone marrow. As with B-1a cells, the role of B-1b cells in the immune system is unclear. Although we have not yet discerned functional differences between B-1a and B-1b, given the recent identification of CD72 (Lyb-2) as the ligand for CD5, it is tempting to speculate that B-1a cells are more involved in B-B cell interactions such as idiotype-anti-idiotype regulation of the early B-cell repertoire and that B-1b cells are more involved in B-T cell interactions. Whatever their function, it is clear that in trying to understand the role of the B-1 lineage it is important to consider both the B-1a and B-1b lineages.

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Cell-transfer studies presented here distinguish three murine B cell lineages: conventional B cells, which develop late and are continually replenished from progenitors in adult bone marrow; Ly-1 B cells (B-1a), which develop early and maintain their numbers by self-replenishment; and Ly-1B "sister" (B-1b) cells, which share many of the properties of Ly-1 B cells, including self-replenishment and feedback regulation of development but can also readily develop from progenitors in adult bone marrow. The sequential emergence of these lineages, the time at which their progenitors function during ontogeny, and the distinctions among their repertoires and functions suggest that evolution has created a layered immune system in which the immune response potential of each successive lineage is adapted to its particular niche.

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We have generated mice that carry a germline mutation in which a large portion of the RAG-2 coding region is deleted. Homozygous mutants are viable but fail to produce mature B or T lymphocytes. Very immature lymphoid cells were present in primary lymphoid organs of mutant animals as defined by surface marker analyses and Abelson murine leukemia virus (A-MuLV) transformation assays. However, these cells did not rearrange their immunoglobulin or T cell receptor loci. Lack of V(D)J recombination activity in mutant pre-B cell lines could be restored by introduction of a functional RAG-2 expression vector. Therefore, loss of RAG-2 function in vivo results in total inability to initiate V(D)J rearrangement, leading to a novel severe combined immune deficient (SCID) phenotype. Because the SCID phenotype was the only obvious abnormality detected in RAG-2 mutant mice, RAG-2 function and V(D)J recombinase activity, per se, are not required for development of cells other than lymphocytes.

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We demonstrate that, on average, greater than 90% of B lymphocytes in fetal spleen express CD5 at gestational ages of 17-23 weeks. Similarly, CD5+ B cells (B-1 cells) are the major B cell subset in umbilical cord blood. These findings depend on the optimization of fluorochrome conjugated anti-CD5 reagents for multiparameter fluorescent-activated cell sorter (FACS) analysis. From infancy through childhood the percentage of B-1 cells gradually diminishes in both spleen and peripheral blood. Stable adult levels, 25-35% of the total B cell population, are reached in late adolescence. The decrease in the percentage of B-1 cells in spleen is accompanied by an increase in conventional (CD5-) B cells, keeping the percentage of total B cells per mononuclear cells relatively constant. In contrast, in peripheral blood, the concentration of both B-1 cells and total B cells decreases, while T cells increase. At the functional level, we show that polyreactive IgM autoantibodies are produced by FACS-sorted CD5high B cells, but not by CD5- B cells from adolescent spleen. In contrast, fetal splenic CD5high and CD5- B cells appear functionally uniform, both producing IgM autoantibodies that are typical of B-1 cells. The apparent level of CD5- B cells in fetal spleen, on average 10% of total B cells, may still result from limitations of our reagent. The prominence of B-1 cells in fetal spleen and cord blood, the gradual reduction of B-1 cells with increasing age, and its characteristic repertoire, all suggest a role for this cell type in immunologically immature hosts.

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Recombinant tumor necrosis factor alpha (TNF-alpha) administration significantly delayed the development of lupuslike nephritis in the New Zealand black x New Zealand white (NZB x NZW)F1 and to a lesser extent in the MRL-lpr/lpr model systems. TNF-alpha treatment was effective when treatment was initiated at 2, 3, or 4 months of age but was ineffective if initiated as late as 6.5 months of age. Treatment of (NZB x NZW)F1 mice for 3 months was more effective than treatment continued for 6 months. Anti-TNF-alpha antibodies did not develop in these mice. Flow microfluorometry analysis showed no major effects on B, T, or monocyte cell population in cells from the peritoneum, spleen, lymph node, and thymus. A decrease in class II Ia expression on macrophages in the peritoneum of TNF-alpha-treated mice was noticed. A correlation between the level of TNF-alpha inducibility in vitro and the effect of TNF-alpha administration in vivo could be shown. Although a limited polymorphism could be shown by restriction fragment length polymorphism, using an amplified (AC)n microsatellite located in the 5' regulatory region of TNF-alpha, a much more extensive interallelic polymorphism was found. The AC microsatellite allele found in NZW mice was unique and different from other lupus strains and nonautoimmune strains. These results have possible implications to the pathogenesis of systemic lupus erythematosus.

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The c-abl gene, originally identified as the cellular homolog of the transforming gene of the Abelson murine leukemia virus, encodes a protein-tyrosine kinase of unknown function that is expressed in all mammalian tissues. We have previously described the introduction of a mutation in the c-abl gene into the mouse germline via targeted gene disruption of embryonic stem cells. We now show that mice homozygous for this mutation are severely affected, displaying increased perinatal mortality, runtedness, and abnormal spleen, head, and eye development. We have examined components of the immune system and have found major reductions in B cell progenitors in the adult bone marrow, with less dramatic reductions in developing T cell compartments.

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The combined expression of the M167 mu/kappa anstiphosphocholine (PC) transgenes with the x-linked immunodeficiency gene, xid, results in an almost total failure to develop B cells in the peripheral lymphoid organs of such mice. Although there is no significant difference between the normal transgene positive (TG+) female offspring and the immunodeficient TG+ xid males with respect to the number of B220+ pre-B cells and IgM+B220+B cells that develop in their bone marrow, the hemizygous xid males have 85% fewer B cells in their spleens than the phenotypically normal heterozygous F1 females. In xid M167-mu-transgenic mice, PC-specific B cells also fail to develop in the spleen; however, numerous B cells bearing the mua+VH1(+)-transgene product associated with endogenous kappa L chains that do not give rise PC-specific antibodies are present. In the phenotypically normal TG+ (B6.CBA/N x mu 243-4)F1 female mice, PC-specific B cells represent almost 10% of the total B cell population, and these B cells express an M167-Id that has been produced by association of the VH1 transgene product with an endogenous V kappa 24L chain. B cells expressing the normally dominant T15-Id are not detectable in the spleens of these M167 mu-transgenic mice. Furthermore, M167-Id+ B cells are present at a fivefold lower level in the bone marrow of mu-TG+ normal mice than in their spleens. These data suggest that the PC-specific B cells that develop in TG+ xid mice are either clonally deleted via some "IgR-directed" mechanism or they fail to receive the appropriate signals to exit the bone marrow or to enter the peripheral lymphoid tissues. This hypothesis is supported by the finding that TNP-specific B cells develop normally and do not undergo clonal deletion in xid mice carrying the Sp6 mu/kappa anti-TNP transgenes.

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An IgG1 mouse monoclonal antibody (MAb) specific for a mouse IgM allotypic determinant in the a, c, f, g, h, and j haplotypes was derived from a fusion of SP2/O-Ag14 mouse myeloma cells with C57BL/6 mouse spleen cells (Igh-Cb) immune to TC31, a MAb of the IgMa allotype. MAb from one hybridoma derived from this fusion (designated DS1) was demonstrated to bind in an ELISA to immunoglobulin bearing the IgMa allotype (TC31, MOPC104E), but not to immunoglobulin bearing the IgMb allotype (C.BPC112). Fluorescein-conjugated DS1 was shown to bind to the surface of BALB/cByJ splenic B cells, but was shown to have negligible binding on C57BL/6J cells. Similarly, DS1-conjugated Sepharose beads were able to stimulate in vitro proliferation of BALB/c, but not C57BL/6 splenic B cells. DS1 was unable to bind to spleen cells from BALB/c allotype congenic strains, BAB/14 (Igh-Cb) and C.AL-20 (Igh-Co), demonstrating that DS1 recognizes a determinant under the control of a gene linked to the Igh-C gene complex. Using sera from recombinant inbred lines, the determinant defined by DS1 was shown to be linked to the Igh-1 locus. Furthermore, the determinant was localized to the CH1 domain of the mu heavy chain. Sera from BALB/cByJ, NMRI, CBA/J, SEA/GnJ, RIIIs/J, and CE/J mouse strains were shown to bind to DS1 in an ELISA, while sera from A/J, SJL/J, NZB/B1NJ, AKR/J, C57BL/6J, and C57BL/10SnJ mouse strains did not bind to DS1. From these data we propose that DS1 is reactive with specificity Igh-6.1, which was originally defined by an allotypic antiserum developed by Black et al. (Immunogenetics 7:213, 1978).

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