Terminal deoxynucleotidyl transferase and repertoire development

In mice, the absence of terminal deoxynucleotidyl transferase (Tdt) expression during fetal and neonatal life provides a window in development where clones of lymphocytes are generated that provide protective immunity. Introducing premature Tdt activity interferes with the development of these clones and results in an impaired ability to make protective antibodies. Conversely, gene-targeted disruption of Tdt prevents N additions at all stages of T and B-lymphocyte development and promotes the development of fetal-like T and B-cell clones into adulthood, with accompanying alterations in repertoire. The alternative splice forms of Tdt may be necessary to provide regulatory mechanisms to restrict N addition to appropriate stages of the developmental pathways, the details of which are being revealed. The evidence continues to build that Tdt is a key player in influencing the outcome of V(D)J recombination during lymphocyte and repertoire development.

The Arthus reaction in rodents: species-specific requirement of complement

We induced reverse passive Arthus (RPA) reactions in the skin of rodents and found that the contribution of complement to immune complex-mediated inflammation is species specific. Complement was found to be necessary in rats and guinea pigs but not in C57BL/6J mice. In rats, within 4 h after initiation of an RPA reaction, serum alternative pathway hemolytic titers decreased significantly below basal levels, whereas classical pathway titers were unchanged. Thus the dermal reaction proceeds coincident with systemic activation of complement. The serine protease inhibitor BCX 1470, which blocks the esterolytic and hemolytic activities of the complement enzymes Cls and factor D in vitro, also blocked development of RPA-induced edema in the rat. These data support the proposal that complement-mediated processes are of major importance in the Arthus reaction in rats and guinea pigs, and suggest that BCX 1470 will be useful as an anti-inflammatory agent in diseases where complement activation is known to be detrimental.

Positive selection from newly formed to marginal zone B cells depends on the rate of clonal production, CD19, and btk

Using immunoglobulin heavy chain transgenic mice, we show that B cell clones reaching the long-lived pool are heterogeneous: some are enriched in the CD21(high) compartment (mostly marginal zone [MZ]), others reside primarily in the follicles (FO). Altering the composition of the B cell receptor through N region additions decreases the rate of clonal production and the MZ enrichment. This process can be recapitulated by purified CD21(low) B cells and is due to a preferential clonal survival that requires a functional btk tyrosine kinase. We also show that generation of the MZ population is dependent on CD19. These findings suggest that the MZ B cell repertoire is positively selected and have functional implications for antigenic responses effected by B cells from this microenvironment.

Partial IgA-Deficiency with Increased Th2-Type Cytokines in TGF-β1 Knockout Mice

Though it has been shown that TGF-β1 directs B cells to switch to IgA in vitro, no studies have assessed TGF-β1 effects on mucosal vs systemic immunity in vivo. When the B cell functions of TGF-β1 gene-disrupted (TGF-β1−/−) mice were analyzed, significantly decreased IgA levels and increased IgG and IgM levels in serum and external secretions were observed. Further, analysis of Ab forming cells (AFC) isolated from both mucosal and systemic lymphoid tissue showed elevated IgM, IgG, and IgE, with decreased IgA AFC. A lack of IgA-committed B cells was seen in TGF-β1−/− mice, especially in the gastrointestinal (GI) tract. Splenic T cells triggered via the TCR expressed elevated Th2-type cytokines and, consistent with this observation, a 31-fold increase in serum IgE was seen in TGF-β1−/− mice. Thus, uncontrolled B cell responses, which include elevated IgE levels, a lack of antiinflammatory IgA, and an excess of complement-binding IgG and IgM Abs, will promote inflammation at mucosal surfaces in TGF-β1−/− mice and likely contribute to pulmonary and GI tract lesions, ultimately leading to the early death of these mice.

Partial IgA-deficiency with increased Th2-type cytokines in TGF-beta 1 knockout mice

Though it has been shown that TGF-beta 1 directs B cells to switch to IgA in vitro, no studies have assessed TGF-beta 1 effects on mucosal vs systemic immunity in vivo. When the B cell functions of TGF-beta 1 gene-disrupted (TGF-beta 1-/-) mice were analyzed, significantly decreased IgA levels and increased IgG and IgM levels in serum and external secretions were observed. Further, analysis of Ab forming cells (AFC) isolated from both mucosal and systemic lymphoid tissue showed elevated IgM, IgG, and IgE, with decreased IgA AFC. A lack of IgA-committed B cells was seen in TGF-beta 1-/- mice, especially in the gastrointestinal (GI) tract. Splenic T cells triggered via the TCR expressed elevated Th2-type cytokines and, consistent with this observation, a 31-fold increase in serum IgE was seen in TGF-beta 1-/- mice. Thus, uncontrolled B cell responses, which include elevated IgE levels, a lack of antiinflammatory IgA, and an excess of complement-binding IgG and IgM Abs, will promote inflammation at mucosal surfaces in TGF-beta 1-/- mice and likely contribute to pulmonary and GI tract lesions, ultimately leading to the early death of these mice.

CD21high IgMhigh splenic B cells enriched in the marginal zone: distinct phenotypes and functions

Collectively, these findings suggest that MZ B cells have unique signaling and subsequent differentiative capabilities that permit them to react much more vigorously than the majority of splenic B cells (FO) in the earliest stages of an in vivo immune response. This is particularly evident with limiting T cell help, low concentration of thymus-independent mitogens or low amounts of particulate blood-borne antigen in the spleen (Fig. 4). They are uniquely situated adjacent to the marginal sinuses and a rich array of antigen trapping macrophages. Because of this location the MZ B cells are ideally positioned for immediate exposure to blood-borne antigens. In contrast, the FO B cells are in juxtaposition to the PALS which may expedite the interactions of FO B cells with T cells and antigen presenting cells. Collectively these properties point to a role for FO B cells in antibody responses to T dependent antigens generated in germinal centers. These responses occur temporally later in immune responses and may be involved principally in the response to protein antigens. [figure: see text].

IgMhighCD21high Lymphocytes Enriched in the Splenic Marginal Zone Generate Effector Cells More Rapidly Than the Bulk of Follicular B Cells

Ag encounter will recruit Ag-specific cells from the pool of mature B lymphocytes in the spleen and activate them to perform effector functions: generation of Ab-forming cells (plasma cells) and presentation of Ag to T cells. We have compared the ability of mature follicular and marginal zone cells to develop into effector B cells. The generation of marginal zone B cells and their localization in the marginal sinus area are T cell and CD40 ligand independent, suggesting that they do not represent a postgerminal center population. Compared with mature recirculating follicular B cells, they express several characteristics of previous antigenic experience, including higher levels of B7.1 (CD80) and B7.2 (CD86) when freshly isolated and following in vitro stimulation. After a brief 6- to 8-h in vitro stimulation with LPS or anti-CD40 Abs, marginal zone B cells become potent APCs. In addition, their ability to proliferate and differentiate into plasma cells in response to low doses of T-independent polyclonal stimuli (LPS) is far greater than that of follicular B cells. These findings indicate a functional heterogeneity within splenic mature B lymphocytes, with marginal zone B cells having the capacity to generate effector cells in early stages of the immune response against particulate Ags scavenged efficiently in this special anatomical site.

IgMhighCD21high lymphocytes enriched in the splenic marginal zone generate effector cells more rapidly than the bulk of follicular B cells

Ag encounter will recruit Ag-specific cells from the pool of mature B lymphocytes in the spleen and activate them to perform effector functions: generation of Ab-forming cells (plasma cells) and presentation of Ag to T cells. We have compared the ability of mature follicular and marginal zone cells to develop into effector B cells. The generation of marginal zone B cells and their localization in the marginal sinus area are T cell and CD40 ligand independent, suggesting that they do not represent a postgerminal center population. Compared with mature recirculating follicular B cells, they express several characteristics of previous antigenic experience, including higher levels of B7.1 (CD80) and B7.2 (CD86) when freshly isolated and following in vitro stimulation. After a brief 6- to 8-h in vitro stimulation with LPS or anti-CD40 Abs, marginal zone B cells become potent APCs. In addition, their ability to proliferate and differentiate into plasma cells in response to low doses of T-independent polyclonal stimuli (LPS) is far greater than that of follicular B cells. These findings indicate a functional heterogeneity within splenic mature B lymphocytes, with marginal zone B cells having the capacity to generate effector cells in early stages of the immune response against particulate Ags scavenged efficiently in this special anatomical site.

Increased junctional diversity in fetal B cells results in a loss of protective anti-phosphorylcholine antibodies in adult mice

Fetal Igs are less diverse than adult Igs, largely because of the lack of N addition in the absence of Tdt. To test whether the absence of Tdt is essential, we generated Tg mice that express Tdt and add N regions in fetal B cells. When challenged as adults with PC-containing Streptococcus pneumoniae, these mice fail to make the hallmark T15 anti-PC Ab encoded by canonical rearrangements of Ig H and L chain genes. The anti-PC Abs from these mice are altered by premature N addition and do not protect against death from virulent pneumococcal infection. These results show that maintenance of lower Ig diversity in early life is essential for the acquisition of a complete functional adult repertoire.

Independently ligating CD38 and Fc gammaRIIB relays a dominant negative signal to B cells

CD38 is expressed on a variety of hematopoietic cells and has a unique enzymatic activity that converts nicotinamide adenine dinucleotide (NAD) into cyclic ADP-ribose (cADPR) and then into ADPR. CD38 is expressed at increasingly higher levels on B cells at each stage of B cell differentiation, and is then down-regulated on germinal center B cells and mature plasma cells. Crosslinking of CD38 on the surface of mature, resting B cells induces B-cell proliferation, which is enhanced by co-signals such as IL-4 and LPS. CD38-induced proliferation is abrogated by Fc gammaRIIB ligation and this inhibition can be effected by the addition of anti-Fc gammaRII Ab midway through a 48 h in vitro culture indicating that it delivers a potent negative signal to CD38 activated B cells. The suppressive signal was shown to occur through the Fc gammaRIIB because CD38-induced B-cell activation was not inhibited by the ligation of Fc gammaRIIB in Fc gammaRII-deficient B cells. These results indicate that Fc gammaRIIB can act as a regulatory molecule that modulates CD38 signals in vivo.

A developmental bias in reading frame usage by human fetal thymic TCRBDJ transcripts is not present in genomic TCRBDJ rearrangements

We have previously reported that reading-frame usage and functional diversification is developmentally regulated, with virtually all TCRB DJ mRNA transcripts using a single reading frame at 8 weeks of gestational age, tapering to 50% by adult life. We used the polymerase chain reaction to create genomic libraries of DJ rearrangements in the TCRB locus from thymuses at 7.7, 10, and 16 weeks of gestational age, and from adult thymuses. Clones were randomly picked and sequenced to determine junctional sequences and reading-frame utilization. The resulting data address the hypothesis that cells bearing genomic joints in reading frame one are preferentially selected during fetal life. This hypothesis predicts that reading-frame bias would also be observed among genomic DJ joints. Instead, we observed random utilization of the three possible D-region reading frames among genomic D1s1 ==> J1s1 joints during fetal life. Similar results were obtained at 7.7 weeks of gestational age in a second thymus in which both RNA and DNA were simultaneously isolated and used to create libraries of TCRBDJ transcripts or rearrangements. We conclude that reading-frame utilization is random among genomic D1s1-JB1s1 rearrangements and that the preferential usage of a single reading frame among mRNA transcripts of TCRB DJ transcripts is the result of preferential transcription of genomic TCRB DJ joints in a single reading frame, or that TCRB DJ transcripts have a longer half-life than transcripts in reading frames two or three.

T and B cell development in BP-1/6C3/aminopeptidase A-deficient mice

Stage-restricted expression of cell surface molecules serves to delineate B lineage cells during their progressive differentiation within the bone marrow. The BP-1/6C3 Ag, aminopeptidase A (APA), is selectively expressed by the pre-B and immature B cells. This ectoenzyme, which is also present on bone marrow-derived stromal cells, thymic cortical epithelial cells, renal proximal tubular cells, intestinal enterocytes, and endothelial cells, cleaves acidic glutamyl and aspartyl residues from the N-terminus of angiotensin and other biologically active peptides to quench their functional activity. BP-1/6C3/APA expression by early B lineage cells is up-regulated by IL-7, an important growth factor for pre-B cells and T cells. To explore the physiologic role of this peptidase, we generated a mouse model of BP-1 deficiency by gene targeting in embryonal stem cells. While mice homozygous for the BP-1 mutation did not express detectable BP-1 protein or enzyme activity, they developed normally, generated normal numbers of T and B cells, exhibited integrity of Ab responses to both thymus-dependent and -independent Ags, and produced normal serum Ig levels. Phenotypic analysis of bone marrow and thymic lymphocytes indicated a normal pattern of B and T lineage differentiation. B lymphopoiesis in fetal liver cultures and the proliferative responses of bone marrow cells to IL-7 and LPS were also unimpaired. These findings indicate that BP-1 ectoenzyme activity is not essential for normal B and T cell development.

T and B Cell Development in BP-1/6C3/Aminopeptidase A-Deficient Mice

Stage-restricted expression of cell surface molecules serves to delineate B lineage cells during their progressive differentiation within the bone marrow. The BP-1/6C3 Ag, aminopeptidase A (APA), is selectively expressed by the pre-B and immature B cells. This ectoenzyme, which is also present on bone marrow-derived stromal cells, thymic cortical epithelial cells, renal proximal tubular cells, intestinal enterocytes, and endothelial cells, cleaves acidic glutamyl and aspartyl residues from the N-terminus of angiotensin and other biologically active peptides to quench their functional activity. BP-1/6C3/APA expression by early B lineage cells is up-regulated by IL-7, an important growth factor for pre-B cells and T cells. To explore the physiologic role of this peptidase, we generated a mouse model of BP-1 deficiency by gene targeting in embryonal stem cells. While mice homozygous for the BP-1 mutation did not express detectable BP-1 protein or enzyme activity, they developed normally, generated normal numbers of T and B cells, exhibited integrity of Ab responses to both thymus-dependent and -independent Ags, and produced normal serum Ig levels. Phenotypic analysis of bone marrow and thymic lymphocytes indicated a normal pattern of B and T lineage differentiation. B lymphopoiesis in fetal liver cultures and the proliferative responses of bone marrow cells to IL-7 and LPS were also unimpaired. These findings indicate that BP-1 ectoenzyme activity is not essential for normal B and T cell development.

Generation of the germline peripheral B cell repertoire: VH81X-lambda B cells are unable to complete all developmental programs

The generation of VH81X heavy chain lambda-light chain-expressing B cells (VH81X-lambda+ B cells) was studied in VH81X heavy chain transgenic mice as well as in VH81X JH (-/-) and VH81X JH (-/-) Ck (-/-) mice, in which competition resulting from expression of heavy and light chains from the endogenous heavy and kappa light chain loci was prevented. We show that although lambda light chain gene rearrangements occur normally and give rise to light chains that associate with the transgenic heavy chain to form surface and soluble IgM molecules, further B cell development is almost totally blocked. The few VH81X-lambda+ B cells that are generated progress into a mature compartment (expressing surface CD21, CD22, CD23, and low CD24 and having a relatively long life span) but they also have reduced levels of surface Ig receptor and express higher amounts of Fas Ag than VH81X-kappa+ B cells. These VH81X-lambda+ B cells reach the peripheral lymphoid organs and accumulate in the periarteriolar lymphoid sheath but are unable to generate primary B cell follicles. In other heavy chain transgenic mice (MD2, M167, and M54), lambda+ B cells are generated. However, they seem to be preferentially selected in the peripheral repertoire of some transgenic heavy chain mice (M54) but not in others (MD2, M167). These studies show that a crucial selection step is necessary for B cell survival and maintenance in which B cells, similar to T cells, receive signals depending on their clonal receptors.

Generation of the Germline Peripheral B Cell Repertoire: VH81X-λ B Cells Are Unable to Complete All Developmental Programs

The generation of VH81X heavy chain λ-light chain-expressing B cells (VH81X-λ+ B cells) was studied in VH81X heavy chain transgenic mice as well as in VH81X JH −/− and VH81X JH −/− Ck −/− mice, in which competition resulting from expression of heavy and light chains from the endogenous heavy and κ light chain loci was prevented. We show that although λ light chain gene rearrangements occur normally and give rise to light chains that associate with the transgenic heavy chain to form surface and soluble IgM molecules, further B cell development is almost totally blocked. The few VH81X-λ+ B cells that are generated progress into a mature compartment (expressing surface CD21, CD22, CD23, and low CD24 and having a relatively long life span) but they also have reduced levels of surface Ig receptor and express higher amounts of Fas Ag than VH81X-κ+ B cells. These VH81X-λ+ B cells reach the peripheral lymphoid organs and accumulate in the periarteriolar lymphoid sheath but are unable to generate primary B cell follicles. In other heavy chain transgenic mice (MD2, M167, and M54), λ+ B cells are generated. However, they seem to be preferentially selected in the peripheral repertoire of some transgenic heavy chain mice (M54) but not in others (MD2, M167). These studies show that a crucial selection step is necessary for B cell survival and maintenance in which B cells, similar to T cells, receive signals depending on their clonal receptors.

Marginal zone B cells exhibit unique activation, proliferative and immunoglobulin secretory responses

Mouse splenic B cells can be separated based on their distinctive expression of cell surface antigens. Marginal zone (MZ) B cells are CD38high CD21high CD23low/-, while follicular (FO) B cells are CD21int CD23int and newly formed (NF) B cells are CD21dim/- CD23-. Exploiting these phenotypic distinctions, we isolated the three B cell subsets and assessed their other phenotypic differences and functional capabilities in vitro. FO B cells proliferate more than the other B cell subsets in response to either IgM or CD38 cross-linking. MZ B cells proliferate better than FO B cells when stimulated with lipopolysaccharide (LPS), sub-optimal levels of LPS and CD38 cross-linking or CD40 ligation. When NF, FO and MZ B cells were stimulated with either LPS or LPS and interleukin-4, MZ B cells secreted more IgM and IgG3 than the other two subsets. Similarly, calcium fluxes measured within MZ B cells are larger in amplitude and more sustained after B cell receptor cross-linking than those found in the other two subsets. Collectively, these results indicate that CD38high CD21high CD23low/- MZ B cells are specially suited to play a unique role in response to antigens delivered to the MZ area.

Development of VH81X transgene-bearing B cells in fetus and adult: sites for expansion and deletion in conventional and CD5/B1 cells

The most D-proximal functional VH gene, VH81X, is preferentially expressed in the mouse fetal B cell repertoire; however, it is expressed in few B cells in the adult. To determine when VH81X gene expression affects size and phenotype of particular stages in B cell differentiation, transgenic mice have been developed expressing a germline fetal liver-derived VH81X-mu rearrangement. Comparative analysis of B lymphopoiesis reveals similarities and differences between fetal liver and adult bone marrow which pinpoint developmental stages in mice during which VH81X-expressing B cell progenitors expand or deplete compartment sizes. These include a similar reduction in c-kitR+ and establishment of a predominant CD43low/HSAhigh phenotype within the B220+ CD43+ compartment which is dependent on the association of the transgene with lambda 5. In contrast, the CD43- pre-B and immature B cell compartments are expanded in the fetus but not in the adult. In addition, there are other factors that later disfavor the survival of VH81X-expressing B1 and B2 cells. Thus the failure to detect VH81X-bearing B cells in the adult is the result of a multistep selection process occurring at all stages during B repertoire expansion.

Isolation and purification of CD14-negative mucosal macrophages from normal human small intestine

Mucosal macrophages play a fundamental role in the regulation of immunological events and inflammation in the small intestine. Because no information is available on normal small intestinal macrophages, we developed a technique for the isolation and purification of jejunal lamina propria macrophages in order to study their phenotype and activity. From sections of normal human jejunum, lamina propria mononuclear cells were isolated by neutral protease digestion and then subjected to counterflow centrifugal elutriation to purify the macrophages. The cells isolated by this procedure contained < 1% CD3+ lymphocytes and displayed the size distribution, morphological features, ultrastructure and phagocytic activity of mononuclear phagocytes. In contrast to blood monocytes, however, mucosal macrophages from the jejunum did not exhibit adherence properties or express CD14, a receptor for the lipopolysaccharide-binding protein. The purification of large numbers of lamina propria macrophages by this procedure offers the opportunity to define the role of this cell in the physiological inflammation characteristic of normal intestinal mucosa and the pathological inflammation associated with small intestinal diseases.

Mouse CD38 is down-regulated on germinal center B cells and mature plasma cells

Germinal center formation is the result of antigenic stimulation of B cells in a T cell-rich area. B cells cycle through the germinal centers, and a small percentage survive to become plasma cells or memory B cells. The transformation from a mature B cell into a germinal center B cell and finally into a terminally differentiated B cell is not well understood. Human CD38 is highly expressed on both germinal center B cells and plasma cells, and is useful in delineating these B cell subsets and in understanding the signaling events involved in the development of these B cells. To determine whether CD38 expression on activated germinal center B cells and postgerminal center B cells influences germinal center differentiation, we studied the expression of CD38 in the mouse. CD38 is expressed on follicular B cells in the Peyer's patches but is down-regulated on germinal center B cells located within the Peyer's patches. CD38dim/-B220+ germinal center B cells are also found in the spleens of immunized but not control mice, suggesting that Ag-stimulated germinal center formation is involved in the production of CD38dim/-B220+ B cells. Furthermore, mature plasma cells isolated from in vitro LPS cultures do not express CD38, but do contain high levels of cytoplasmic Ig. These results are in contrast to studies in humans in which CD38 is not found on follicular B cells but is highly expressed on germinal center B cells and plasma cells.

Mouse CD38 is down-regulated on germinal center B cells and mature plasma cells

Germinal center formation is the result of antigenic stimulation of B cells in a T cell-rich area. B cells cycle through the germinal centers, and a small percentage survive to become plasma cells or memory B cells. The transformation from a mature B cell into a germinal center B cell and finally into a terminally differentiated B cell is not well understood. Human CD38 is highly expressed on both germinal center B cells and plasma cells, and is useful in delineating these B cell subsets and in understanding the signaling events involved in the development of these B cells. To determine whether CD38 expression on activated germinal center B cells and postgerminal center B cells influences germinal center differentiation, we studied the expression of CD38 in the mouse. CD38 is expressed on follicular B cells in the Peyer's patches but is down-regulated on germinal center B cells located within the Peyer's patches. CD38dim/-B220+ germinal center B cells are also found in the spleens of immunized but not control mice, suggesting that Ag-stimulated germinal center formation is involved in the production of CD38dim/-B220+ B cells. Furthermore, mature plasma cells isolated from in vitro LPS cultures do not express CD38, but do contain high levels of cytoplasmic Ig. These results are in contrast to studies in humans in which CD38 is not found on follicular B cells but is highly expressed on germinal center B cells and plasma cells.

Evidence for selection of a population of multi-reactive B cells into the splenic marginal zone

Antibody reactivity to self-antigens is a normal component of the immune system. To study the mechanism by which self-reactive B cells are generated and maintained, we analyzed B cell development in transgenic mice that express a rearranged VH81X heavy chain from the pre-immune repertoire. In these mice, > 95% of B cells express the transgene in association with a variety of kappa light chains but V kappa 1 C being the dominant light chain. These transgenic B cells with identical V kappa 1C-J kappa 5 joins do not normally secrete IgM in vivo, but antibodies derived from these B cells, through LPS activation in vitro or after hybridoma immortalization, are self-reactive and recognize an ubiquitous epitope(s) on intracytoplasmic proteins from different tissues. They have the phenotype and localization pattern of long-lived marginal zone B cells and their development in vivo is blocked by injection of soluble VH81X-V kappa 1CJ kappa 5 IgM antibody. The observations in this transgenic mouse provide evidence for positive selection of a population of self-reactive B cells. These B cells enter the peripheral pool of B cells where they localize in the marginal zone of the spleen and, in contrast to other transgene-expressing B cells, do not secrete IgM antibody.

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.

Generation and function of natural self-reactive B lymphocytes

To study the significance of developmentally restricted VH gene expression, and natural self-reactive B lymphocytes, VH81X-mu-heavy chain transgenic mice were constructed and the expression of this gene analysed at different stages of development and in different B-cell subsets. It was found that the majority of B cells expressed the transgenic heavy chain associated with different kappa but not lambda light chains. These B cells were predominantly self reactive and distributed normally to peripheral lymphoid organs. Among the CD23-/IgMhi B cells of transgenic mice there was a population of clonally unrelated relatively self-reactive B cells that expressed identical antigen receptors but did not secrete antibody. Continued administration of antibody of the same specificity during post-natal development reduced the generation of these B cells to very low levels. These studies show that although natural self-reactive B cells are continuously generated as a result of positive selection by self-antigen, their maturation may be blocked at the CD23-/IgMhi stage. Their failure to enter the CD23+ IgMlo B cell pool may prevent them from receiving T-cell help and production of high affinity potentially pathogenic autoreactive IgG antibodies.

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.

Sequence analysis and antigen binding characteristics of Ig SCID Ig+ mice

SCID mice as they age may develop a limited number of functional B and T cell clones. VH and VL gene sequences of hybridoma antibodies derived from the spleens of five adult SCID Ig+ mice revealed nine clonally unrelated, i.e. derived from different precursor lymphocytes, groups of Ig. Of these, four used the VH7183 family, one Q52, three J558 and one VH3609. VH7183 was seen in excess over J558 despite a 5-fold larger number of J558 family members in the mouse genome. Preferential use of D proximal VH genes along with reduced N region addition in the heavy chain VDJ joins of these antibodies suggest derivation from a population of lymphocytes with characteristics of the perinatal repertoire. All of the hybridomas produced kappa light chains of several different VL family groups. Sequences of 23 VH and 22 VL genes of hybridomas derived from one mouse (no. 45) showed that they were all clonally related, differing only in somatic mutations. Autoreactivity of these antibodies to cellular antigens [including (U1)RNP-specific A protein and SmB] was analyzed, and shown to correlate with the geneology and specific mutations within members of the clone. The geneological tree derived from sequence data permitted a study of the microevolution of a clone of B cells and showed that as it developed there was a tendency toward increased strength of self-reactive binding as well as alterations in specificity toward these autoantigens. Analysis of replacement:silent mutation ratios within members of this clone reflected limited affinity maturation in mouse 45 and none for another mouse, no. 58. These data are consistent with the progressive differentiation of B cells in an immunodeficient state in the absence of clonal competition with B cells of similar antigen specificities as they populate the spleen.