Ordered rearrangement of immunoglobulin heavy chain variable region segments

Mice triallelic for the Ig heavy chain locus: implications for VHDJH recombination

V(H)DJ(H) recombination has been extensively studied in mice carrying an Ig heavy chain rearranged transgene. In most models, inhibition of endogenous Ig rearrangement occurs, consistently with the feedback model of IgH recombination. Nonetheless, an incomplete IgH allelic exclusion is a recurrent observation in these animals. Furthermore, transgene expression in ontogeny is likely to start before somatic recombination, thus limiting the use of Ig-transgenic mice to access the dynamics of V(H)DJ(H) recombination. As an alternative approach, we challenged the regulation of somatic recombination with the introduction of an extra IgH locus in germline configuration. This was achieved by reconstitution of RAG2(-/-) mice with fetal liver cells trisomic for chromosome 12 (Ts12). We found that all three alleles can recombine and that the ratio of Ig allotype-expressing B cells follows the allotypic ratio in trisomic cells. Although these cells are able to rearrange the three alleles, the levels of Ig phenotypic allelic exclusion are not altered when compared with euploid cells. Likewise, we find that most VDJ rearrangements of the silenced allele are unable to encode a functional mu-chain, indicating that the majority of these cells are also genetically excluded. These results provide additional support for the feedback model of allelic exclusion.

Bruton's tyrosine kinase is required for signaling the CD79b-mediated pro-B to pre-B cell transition

Formation of the pre-BCR complex is a critical check point during B cell development and induces the transition of pro-B to pre-B cells. CD79b (Igbeta) is a signaling component in the pre-BCR complex, since differentiation to the pre-B phenotype is induced by cross-linking the CD79b expressed on developmentally arrested pro-B cells from recombination-activating gene (RAG)-2-deficient mice. Bruton's tyrosine kinase (BTK) plays important roles in B cell development. However, its molecular mechanisms in early B cell development are not fully understood. To examine whether BTK functions in CD79b-mediated signaling for the pro-B/pre-B transition, we utilized RAG2/BTK double-knockout (DKO) mice. Pro-B cells from RAG2/BTK-DKO mice did not differentiate into pre-B cells following CD79b cross-linking, although tyrosine phosphorylation of cellular proteins including Erk1/2 and phospholipase C-gamma2 was induced in the same manner as RAG2-KO mice. BTK is phosphorylated after cross-linking of CD79b on RAG2-deficient pro-B cells. These findings suggest that BTK-dependent pathways downstream of CD79b are critical for the pro-B/pre-B transition and BTK-independent signaling pathways are also activated via the pre-BCR complex.

Induction of a diverse T cell receptor gamma/delta repertoire by the helix-loop-helix proteins E2A and HEB in nonlymphoid cells

During specific stages of thymocyte development, the T cell receptor (TCR) locus is assembled from variable (V), diversity (D), and joining (J) gene segments. Proper TCR gamma and delta V(D)J rearrangement during thymocyte development requires the presence of the E2A proteins. Here we show that E2A and a closely related protein, HEB, in the presence of recombination activating gene (RAG)1 and RAG2, each have the ability to activate TCR gamma and delta rearrangement in human kidney cells. The coding joints are diverse, contain nucleotide deletions, and occasionally show the presence of P nucleotides. Interestingly, only a subset of V, D, and J segments are targeted by the E2A and HEB proteins. Thus, E2A and HEB permit localized accessibility of the TCR gamma and delta loci to the recombination machinery. These data indicate that a distinct but diverse TCR repertoire can be induced in nonlymphoid cells by the mere presence of the V(D)J recombinase and the transcriptional regulators, E2A and HEB.

CD40 expression is induced by the introduction of IgM receptor on the surface of pro-B cell line NFS70

To examine the molecular mechanism of B cell differentiation, we introduced rearranged immunoglobulin (Ig) mu- and kappa-chain genes into the NFS70 pro-B cell line and observed their maturation. The IgR(+)-transfectants had characteristics of mature surface IgM (sIgM)+ B220high CD40+ CD38+ CD25+ B cells. CD40 expression levels were regulated by stimulation via the IgR. In comparison to wild type NFS70 cells, NF-kappaB activity was up-regulated in the IgR transfectants. Anti-IgR crosslinking of IgR+ cells induced down-regulation of CD40 expression that correlated with down-regulation of NF-kappaB activity in the IgR(+)-transfectants. Immature CD19+ sIgD- B cells from bone marrow also showed an alteration of CD40 expression in response to anti-IgR crosslinking. The results suggest that expression of IgR on the surface is one of the factors responsible for further maturation of B cells.

Ordered recombination of immunoglobulin light chain genes occurs at the IGK locus but seems less strict at the IGL locus

Regulation of allelic and isotypic exclusion of human immunoglobulin (Ig) light-chain genes was studied in 113 chronic B-cell leukemias as a "single-cell" model that allowed complete analysis of each light chain allele. Our data show that monospecific Ig light chain expression is in about 90% of cases determined by ordered recombination: Igkappa gene (IGK) rearrangements, followed by IGK deletions and Iglambda gene (IGL) rearrangements, resulting in the presence of only one functional Ig light chain rearrangement. In about 10% (10 cases), 2 functional Ig light chain rearrangements (IGK/IGL or IGL/IGL, but not IGK/IGK) were identified. This might be explained by the fact that regulation of the ordered recombination process is not fully strict, particularly when the IGL locus is involved. Unfavorable somatic mutations followed by receptor editing might have contributed to this finding. Eight of these 10 cases indeed contained somatic mutations. In cases with 2 functional Ig light chain rearrangements, both alleles were transcribed, but monospecific Ig expression was still maintained. This suggests that in these cases allelelic exclusion is not regulated at the messenger RNA level but either at the level of translation or protein stability or via preferential pairing of Ig light and Ig heavy chains. Nevertheless, ordered rearrangement processes are the main determinant for monospecific Ig light chain expression.

Induction of pre-B cell proliferation after de novo synthesis of the pre-B cell receptor

The assembly of a pre-B cell receptor (pre-BCR) composed of an Ig mu heavy chain (mu H-chain), the surrogate light (SL) chain, and the Ig alpha/beta dimer is critical for late pro-B cells to advance to the pre-B cell stage. By using a transgenic mouse model, in which mu H-chain synthesis is solely driven by a tetracycline-controlled transactivator, we show that de novo synthesis of mu H-chain in transgenic pro-B cells not only induces differentiation but also proliferation. This positive effect of mu H-chain synthesis on proliferation requires the presence of SL chain and costimulatory signals provided by stromal cells or IL-7. We conclude that pre-BCR signaling induces clonal expansion of early pre-B cells.

VH gene replacement in thymocytes

The quasi-monoclonal (QM) mouse has a functionally rearranged H chain gene inserted into its natural position in the IgH locus. In this position, the H chain gene is subject to many of the same activities as normally arranged H chain genes, including somatic hypermutation, V(H) gene replacement, and class switch recombination. Here, we have used this mouse strain to determine some of the rules that govern the V(D)J recombination activity of the IgH locus in thymus. We focused on the requirements for V(H) gene replacement. In normal mice, thymic DJ(H) rearrangements are common, but VDJ(H) rearrangements are not. We found intermediate products of V(H) replacement in double-positive CD4(+)CD8(+) cells of the QM thymus, demonstrating that the inserted V(H) gene was accessible and ruling out the possibility that a V(H) gene per se cannot be rearranged in the thymus. We found transcripts from the knocked-in H chain gene of QM, but no mu H chain protein was detectable in thymocytes. Cloning and sequencing of these transcripts revealed that some had been generated by V(H) gene replacement. Corresponding signal joints could also be identified. These results suggest that neither a B cell-specific signal nor an Ig protein are necessary to activate V(H)-to-VDJ(H) joining in thymocytes. Possible mechanisms remaining to account for overcoming the barrier to V(H) joining in thymocytes include the insertion of a transcriptionally active gene segment and/or the inactivation of a silencer.

Biased V beta usage in immature thymocytes is independent of DJ beta proximity and pT alpha pairing

During thymus development, the TCR beta locus rearranges before the TCR alpha locus. Pairing of productively rearranged TCR beta-chains with an invariant pT alpha chain leads to the formation of a pre-TCR and subsequent expansion of immature pre-T cells. Essentially nothing is known about the TCR V beta repertoire in pre-T cells before or after the expression of a pre-TCR. Using intracellular staining, we show here that the TCR V beta repertoire is significantly biased at the earliest developmental stage in which VDJ beta rearrangement has occurred. Moreover (and in contrast to the V(H) repertoire in immature B cells), V beta repertoire biases in immature T cells do not reflect proximity of V beta gene segments to the DJ beta cluster, nor do they depend upon preferential V beta pairing with the pT alpha chain. We conclude that V gene repertoires in developing T and B cells are controlled by partially distinct mechanisms.

Antibody regulation of B cell development

Antibodies on the surface of B lymphocytes trigger adaptive immune responses and control a series of antigen-independent checkpoints during B cell development. These physiologic processes are regulated by a complex of membrane immunoglobulin and two signal transducing proteins known as Ig alpha and Ig beta. Here we focus on the role of antibodies in governing the maturation of B cells from early antigen-independent through the final antigen-dependent stages.

Genetic modulation of T cell receptor gene segment usage during somatic recombination

Lymphocyte antigen receptors are not encoded by germline genes, but rather are produced by combinatorial joining between clusters of gene segments in somatic cells. Within a given cluster, gene segment usage during recombination is thought to be largely random, with biased representation in mature T lymphocytes resulting from protein-mediated selection of a subset of the total repertoire. Here we show that T cell receptor D beta and J beta gene segment usage is not random, but is patterned at the time of recombination. The hierarchy of gene segment usage is independent of gene segment proximity, but rather is influenced by the ability of the flanking recombination signal sequences (RSS) to bind the recombinase and/or to form a paired synaptic complex. Importantly, the relative frequency of gene segment usage established during recombination is very similar to that found after protein-mediated selection, suggesting that in addition to targeting recombinase activity, the RSS may have evolved to bias the naive repertoire in favor of useful gene products.

Role of B cell antigen receptor in regulation of V(D)J recombination and cell survival

B lymphocytes learn through the interaction of the B cell receptor with antigens in the context of B cell developmental stage and environmental cues. B cells can respond by proliferation and antibody secretion, programmed cell death, or modification of the antibody genes themselves through secondary immunoglobulin gene rearrangements or somatic point mutation. A critical learning process is that of self/nonself-discrimination. We have shown that one potent mechanism for immune self-tolerance in B cells is ongoing antibody light chain gene rearrangements, which can result in "receptor editing" that changes antigen receptor specificity. This process appears to be developmentally regulated, because it is confined to cells at an immature stage of development. Cells at later stages of development can be tolerized by apoptosis, but probably not by receptor editing.

VHD rearrangements in human immunoglobulin heavy chain minilocus transgenic mice

Typically, immunoglobulin VHDJH recombination is performed in two steps with D to JH rearrangement preceding VH to DJH rearrangement. Using a human immunoglobulin heavy chain transgenic minilocus, we previously demonstrated that a non-conventional human D gene segment termed DIR2 could be recombined to a VH gene segment to form VHD rearrangements. Here, we demonstrate that VHD rearrangements involve conventional D gene segments as well. VHD rearrangements are easily detected and are diverse. Similarly to DJH rearrangements, VHD rearrangements occur by deletion and inversion. They occur approximately 1000 times less frequently than DJH rearrangements. VHD rearrangements can constitute intermediates for the formation of VHDDJH rearrangements.

Mechanisms that direct ordered assembly of T cell receptor beta locus V, D, and J gene segments

T cell receptor (TCR) beta variable region genes are assembled in progenitor T cells from germ-line Vbeta, Dbeta, and Jbeta segments via an ordered two-step process in which Dbeta to Jbeta rearrangements occur on both alleles before appendage of a Vbeta to a preexisting DJbeta complex. Direct joining of Vbeta segments to nonrearranged Dbeta or Jbeta segments, while compatible with known restrictions on the V(D)J recombination mechanism, are infrequent within the endogenous TCRbeta locus. We have analyzed mechanisms that mediate ordered Vbeta, Dbeta, and Jbeta assembly via an approach in which TCRbeta minilocus recombination substrates were introduced into embryonic stem cells and then analyzed for rearrangement in normal thymocytes by recombinase-activating gene 2-deficient blastocyst complementation. These analyses demonstrated that Vbeta segments are preferentially targeted for rearrangement to Dbeta as opposed to Jbeta segments. In addition, we further demonstrated that Vbeta segments can be appended to nonrearranged endogenous Dbeta segments in which we have eliminated the ability of Dbeta segments to join to Jbeta segments. Our findings are discussed in the context of the mechanisms that regulate the ordered assembly and utilization of V, D, and J segments.

Different sensitivity to receptor editing of B cells from mice hemizygous or homozygous for targeted Ig transgenes

Ig knock-in mice have been used to study the relative contribution of receptor selection versus clonal selection in the control of autoreactive B cells. The anti-MHC class I 3-83Ig knock-in (3-83Igi) mice manifest extensive receptor editing in the presence of H-2(b). However, receptor editing is also observed on the H-2(d) background, although reactivity toward this antigen is below detection and its presence does not affect the generation of 3-83Ig(+) mature B cells in classical 3-83Ig transgenic mice. In this study we have analyzed the contribution of genetic background, B cell receptor signaling, and transgene copy number on the initiation and extent of receptor editing in the 3-83Igi;H-2(d) mice. Crossing the 3-83Ig insertion into either CD45-deficient H-2(d) mice or onto the BALB/c background reduces the extent of receptor editing and increases the fraction of 3-83Ig-expressing B cells, indicating that in the original line editing depends on B cell receptor signaling induced by cross-reacting antigen(s). However, receptor editing is still detectable in hemizygous 3-83Igi mice even on the BALB/c background, on which the 3-83 antibody was originally raised, whereas it is abrogated in homozygous 3-83Igi;H-2(d) animals. This latter observation indicates that immature B cells expressing immunoglobulin from single heavy and light chain loci, as they do physiologically, utilize receptor editing for an exquisite quality control of their antigen receptor that may only partly be based on self-reactivity.

Unexpected rearrangement and expression of the immunoglobulin lambda1 locus in scid mice

In severe combined immunodeficient (scid) mice, V(D)J recombination is severely impaired due to a recessive mutation (scid). Thus, we were surprised to find in this study that Vlambda1-Jlambda1 rearrangement is routinely detectable in scid fetal liver, adult bone marrow, and spleen in the apparent absence of completed VH-DJH and Vkappa-Jkappa rearrangements. Particularly surprising, we found the level of Vlambda1-Jlambda1 rearrangement in scid fetal liver to be comparable to that in fetal liver of wild-type mice. The majority of scid Vlambda1-Jlambda1 rearrangements contained abnormal deletions at the VJ junction, consistent with the known effect of scid. However, approximately 15% of Vlambda1-Jlambda1 rearrangements lacked abnormal deletions. Productive lambda1 transcripts resulting from in-frame rearrangements were readily detectable in scid adult bone marrow and spleen, consistent with our ability to detect lambda1-expressing cells by flow cytometry in the spleens of bcl-2-transgenic scid mice. Strikingly, lambda1 transcripts from individual scid mice often showed VJ junctional sequences with the same recurring palindromic (P) additions of three, four, or five nucleotides. To account for these findings, we suggest that (a) nonhomologous end joining of Vlambda1 and Jlambda1 coding ends in fetal B lineage cells may not be (severely) impaired by scid; (b) recurring P additions in scid lambda1 transcripts may reflect certain molecular constraints imposed by scid on the resolution of Vlambda1 and Jlambda1 hairpin coding ends; and (c), scid lymphocytes with productively rearranged Vlambda1 and Jlambda1 elements may differentiate into recombinase-inactive cells and emigrate from bone marrow to spleen.

Recombination signal sequences restrict chromosomal V(D)J recombination beyond the 12/23 rule

The genes encoding the variable regions of lymphocyte antigen receptors are assembled from variable (V), diversity (D) and joining (J) gene segments. V(D)J recombination is initiated by the recombinase activating gene (RAG)-1 and -2 proteins, which introduce DNA double-strand breaks between the V, D and J segments and their flanking recombination signal sequences (RSSs). Generally expressed DNA repair proteins then carry out the joining reaction. The conserved heptamer and nonamer sequences of the RSSs are separated by non-conserved spacers of 12 or 23 base pairs (forming 12-RSSs and 23-RSSs). The 12/23 rule, which is mediated at the level of RAG-1/2 recognition and cutting, specifies that V(D)J recombination occurs only between a gene segment flanked by a 12-RSS and one flanked by a 23-RSS. Vbeta segments are appended to DJbeta rearrangements, with little or no direct Vbeta to Jbeta joining, despite 12/23 compatibility of Vbeta 23-RSSs and Jbeta12-RSSs. Here we use embryonic stem cells and mice with a modified T-cell receptor (TCR)beta locus containing only one Dbeta (Dbeta1) gene segment and one Jbeta (Jbeta1) gene cluster to show that the 5' Dbeta1 12-RSS, but not the Jbeta1 12-RSSs, targets rearrangement of a diverse Vbeta repertoire. This targeting is precise and position-independent. This additional restriction on V(D)J recombination has important implications for the regulation of variable region gene assembly and repertoire development.

Development of the murine and human immune system: differential effects of immunotoxicants depend on time of exposure

Fetal and early postnatal life represent critical periods in vertebrate immune system development. Disruption of such development by perinatal immunotoxic chemical exposure has been widely described in experimental animal models. The resultant inhibited postnatal immune responses in such animals are often more dramatic and persistent than those after exposure during adult life. Further, recent reports suggest that prenatal exposure to immunotoxicants may exacerbate postnatal aberrant immune responses (e.g., hypersensitivity disorders and autoimmune disease) in genetically predisposed rodents. Limited information is available regarding the possibility of inhibited postnatal immune capacity in humans as a result of developmental immunotoxicant exposure. The multifactorial nature of hypersensitivity and autoimmune responses will further complicate the elucidation of possible relationships between chemical exposure during ontogeny of the human immune system and immune-mediated disease later in life. Taken together, however, the available animal data suggest the potential for altered postnatal immune function in humans exposed to immunotoxicants (e.g., environmental chemicals and therapeutic agents) during fetal and/or early postnatal life.

Development of the murine and human immune system: differential effects of immunotoxicants depend on time of exposure

Fetal and early postnatal life represent critical periods in vertebrate immune system development. Disruption of such development by perinatal immunotoxic chemical exposure has been widely described in experimental animal models. The resultant inhibited postnatal immune responses in such animals are often more dramatic and persistent than those after exposure during adult life. Further, recent reports suggest that prenatal exposure to immunotoxicants may exacerbate postnatal aberrant immune responses (e.g., hypersensitivity disorders and autoimmune disease) in genetically predisposed rodents. Limited information is available regarding the possibility of inhibited postnatal immune capacity in humans as a result of developmental immunotoxicant exposure. The multifactorial nature of hypersensitivity and autoimmune responses will further complicate the elucidation of possible relationships between chemical exposure during ontogeny of the human immune system and immune-mediated disease later in life. Taken together, however, the available animal data suggest the potential for altered postnatal immune function in humans exposed to immunotoxicants (e.g., environmental chemicals and therapeutic agents) during fetal and/or early postnatal life.

Frequency and characterization of phenotypic Ig heavy chain allelically included IgM-expressing B cells in mice

Ig H chain (IgH) allelic exclusion remains a puzzling topic. Here, we address the following question: Do phenotypic IgH allelically included cells exist in normal mice and, if so, at what frequency? Sorted cells from heterozygous mice were evaluated for the expression of both IgM allotypes by double intracytoplasmic stainings. Dual expressors were found at a frequency of 1 in 104 splenic B cells. These data were confirmed by direct sequencing of IgH-rearranged alleles obtained after single cell (or clone) PCR on dual expressors. Typically, these cells have one rearranged J558 VH whereas, in the other allele, a D-proximal VH gene is used. Interestingly, dual expressors have rearranged IgH alleles with similar CDR3 lengths. These results show that, in contrast to the kappa L chain and the TCR beta-chain, IgH allelic exclusion is the result of an extremely stringent mechanism. We discuss two non-mutually exclusive scenarios for the origin of IgH dual expressors: 1) IgH allelically included cells arise when the first allele to rearrange productively is unable to form a pre-BCR; dual expressors could be a subset of this population in which, upon conventional L chain rearrangement, both IgH are expressed at the surface; and 2) synchronous rearrangement of the IgH alleles.

Individual V(H) promoters vary in strength, but the frequency of rearrangement of those V(H) genes does not correlate with promoter strength nor enhancer-independence

The process of V(D)J recombination is highly regulated. Germline transcription of unrearranged gene segments precedes V(D)J rearrangement, and the correlation between germline transcription and accessibility for recombination is strong; thus it has been suggested that germline transcription may be required for rearrangement. If germline transcription is essential for rearrangement, then the level of transcription of individual gene segments might affect the relative frequency of recombination of those genes. Also, since the intronic enhancer, E(mu), is very distant from V(H) genes before they rearrange, then any promoters which were enhancer dependent might have a transcriptional advantage. Here we study in luciferase vectors the promoters of three functional genes of the V(H)S107 family, and compare them to that of the most frequently rearranging gene in the mouse I(g)H locus, V(H)81X, and to a V(H)J558 gene. Within the V(H)S107 family, the three V(H) genes rearrange with very different relative frequencies, with V1 rearranging the most, and V13 seldom rearranging. We show that only the strong V(H)J558 promoter has significant luciferase reporter gene activity in the absence of E(mu). V1 has only 20% as much activity as J558 in the absence of E(mu), and the other promoters have less than 8% of the activity of J558. Notably, the 81X promoter has essentially no enhancer-independent activity. In the presence of E(mu), V1 has equivalent activity to J558, while the other promoters show much less activity. Again, 81X is the weakest promoter of all, despite being the most frequently rearranging gene. Finally, we show that the steady state level of V(H)S107 and V(H)7183 germline transcripts in vivo is very low. Thus, these data show little correlation between the strength or enhancer-independence of these V(H) promoters and the relative frequency of recombination of the corresponding V(H) genes. In addition, the data show that individual V(H) promoters have different strengths even in the presence of E(mu), demonstrating that even promoters within a single V(H) family can be quite heterogeneous.

Receptor selection in B and T lymphocytes

The process of clonal selection is a central feature of the immune system, but immune specificity is also regulated by receptor selection, in which the fate of a lymphocyte's antigen receptor is uncoupled from that of the cell itself. Whereas clonal selection controls cell death or survival in response to antigen receptor signaling, receptor selection regulates the process of V(D)J recombination, which can alter or fix antigen receptor specificity. Receptor selection is carried out in both T and B cells and can occur at different stages of lymphocyte differentiation, in which it plays a key role in allelic exclusion, positive selection, receptor editing, and the diversification of the antigen receptor repertoire. Thus, the immune system takes advantage of its control of V(D)J recombination to modify antigen receptors in such a way that self/non-self discrimination is enhanced. New information about receptor editing in T cells and B-1 B cells is also discussed.

Proliferation of Bone Marrow Pro-B Cells Is Dependent on Stimulation by the Pituitary/Thyroid Axis

The frequency and absolute number of pro-B, pre-B, and B cells in the bone marrow of the hypothyroid strain of mice are significantly reduced compared with those of their normal littermates. To investigate why this is the case, various B cell developmental processes were examined in the thyroid hormone-deficient mice. These studies revealed that the frequency of pro-B cells in the S-G2/M phase of the cell cycle was significantly reduced in hypothyroid mice. That thyroid hormone deficiency was responsible for this proliferation defect was established by demonstrating that treatment of hypothyroid mice with thyroxine resulted in a specific increase in the frequency and total number of cycling pro-B cells. The latter effect was paralleled by increases in the frequency and number of bone marrow B lineage cells. Additional in vitro experiments revealed that at least some thyroid hormone effects were directly mediated on the bone marrow. Taken together, these data demonstrate that thyroid hormones are required for normal B cell production in the bone marrow through regulation of pro-B cell proliferation and establish a role for the pituitary/thyroid axis in B cell development.

B-cell-receptor-dependent positive and negative selection in immature B cells

This review touches on only a small part of the complex biology of B cells, but serves to illustrate the point that the antigen receptor is the most important of many cell-surface receptors affecting cell-fate decisions. Receptor expression is necessary, but not sufficient, for cell survival. It is also essential that a B cell's antigen-receptor specificity be appropriate for its environment. The need to balance reactivity with self tolerance has resulted in an intricate feedback control (affected by both the recombinase and cell survival) that regulates independent selection events at the level of the receptor and the cell.

Allelic exclusion of the T cell receptor beta locus requires the SH2 domain-containing leukocyte protein (SLP)-76 adaptor protein

Signaling via the pre-T cell receptor (TCR) is required for the proliferative expansion and maturation of CD4(-)CD8(-) double-negative (DN) thymocytes into CD4(+)CD8(+) double-positive (DP) cells and for TCR-beta allelic exclusion. The adaptor protein SH2 domain-containing leukocyte protein (SLP)-76 has been shown to play a crucial role in thymic development, because thymocytes of SLP-76(-/-) mice are arrested at the CD25(+)CD44(-) DN stage. Here we show that SLP-76(-/-) DN thymocytes express the pre-TCR on their surfaces and that introduction of a TCR-alpha/beta transgene into the SLP-76(-/-) background fails to cause expansion of DN thymocytes or developmental progression to the DP stage. Moreover, analysis of TCR-beta rearrangement in SLP-76(-/-) TCR-transgenic mice or in single CD25(+)CD44(-) DN cells from SLP-76(-/-) mice indicates an essential role of SLP-76 in TCR-beta allelic exclusion.

The roles of preB and B cell receptors in the stepwise allelic exclusion of mouse IgH and L chain gene loci

Membrane-bound preBCR of wild-type mice, and probably also preBCR-like V(preB) muH chain complexes in lambda5-deficient mice, signal allelic exclusion so that < 0.1% of all preB-II cells and all subsequent B lineage cells express two muH chains on their surface. On the other hand a large number of muH chains which are originally generated at the transition of preB-I to preB-II cells cannot pair with surrogate L chains, cannot form a preBCR on the surface and, hence, allow two H chain alleles to be productively rearranged in one B-lineage cell. By contrast membrane-bound BCR on immature B cells does not signal allelic or isotypic exclusion Of Ig kappaL and lambdaL chain gene loci. This allows the rearrangement machinery to remain active, and secondary L chain rearrangements on one kappaL chain allele are frequently observed. Rapid selection of fitting H/L chain pairs, forming BCR on the surface, allows B-lineage cells to enter the mature B cell pool where the rearrangement machinery is shut off, securing allelic exclusion of L chain loci in most B cells.

Developmental neurobiology: a genetic Cheshire cat?

In the wake of evidence that essential neurogenic processes might involve aspects of DNA rearrangement, recent discoveries about the unusual arrangement of genes encoding neuronal adhesion molecules known as protocadherins are very intriguing. But is this just a coincidence?

Biased VH Gene Usage in Early Lineage Human B Cells: Evidence for Preferential Ig Gene Rearrangement in the Absence of Selection

Certain VH genes are predominantly expressed in mature B cells. We hypothesized that several, mutually nonexclusive VH-dependent mechanisms operating at distinct stages during B cell development may be responsible for overrepresentation of these VH genes. In the present study, we have assessed whether one of the mechanisms involves preferential rearrangement at the pro-B cell stage. The frequency of individual VH4 and VH3 genes in rearrangement libraries from FACS-purified human CD34+/CD19+ pro-B and CD34−/CD19+ pre-B cells was assessed. The in-frame and out-of-frame rearrangements from both cell populations were analyzed using a high resolution PAGE system. The frequencies of individual VH gene segments among out-of-frame rearrangements from pro-B cells were determined, because these frequencies should reflect only processes before the translation of the μ-heavy chain and should not be biased by selection mechanisms. Our results demonstrate that, at the pro-B cell stage, the V4–34, V4–39, and V4–59 gene segments are the most frequently rearranged VH4 family genes, and the V3–23 and V3–30 gene segments are the most frequently rearranged VH3 family genes. This finding suggests that the predominant expression of these VH genes in peripheral mature B cells is determined to a significant degree by their preferential rearrangement during V-DJ recombination.

Clonal evolution as judged by immunoglobulin heavy chain gene rearrangements in relapsing precursor-B acute lymphoblastic leukemia

Oligoclonality and ongoing clonal evolution are common features in patients with precursor-B (pre-B) acute lymphoblastic leukemia (ALL), as judged by immunoglobulin heavy chain (IgH) gene rearrangement analysis. These features are considered to be results of secondary rearrangements after malignant transformation or emergence of new tumor clones. In the present study we analyzed the IgH gene rearrangement status in 18 cases with relapsing pre-B ALL using variable heavy chain (V(H)) gene family specific polymerase chain reaction (PCR) amplification and single stranded conformation polymorphism (SSCP) analysis. Clonal IgH rearrangements were displayed in all leukemias but one, and altered rearrangement patterns occurred in five cases (29%), which were selected for detailed nucleotide sequence analysis. In one case, multiple subclones at diagnosis were suggested to be derived from a progenitor clone through joining of different V(H) germline gene segments to a pre-existing D-J(H) complex (V(H) to D-J(H) joining). Evidence for V(H) gene replacement with identical N-sequences at the V(H)-D junction and a common D-J(H) region was observed in one case. Diversification at the V(H)-D junction consisting of heterogeneous N-sequences were observed in one case. This molecular modification of the V(H)-D region could fit a hypothesized "open-and-shut" mechanism. Nevertheless, despite these ongoing events at least one IgH rearrangement remained unchanged throughout the disease in most patients, indicating that the immunoglobulin heavy chain locus can be a suitable marker for detection of minimal residual disease (MRD).

The role of components of recombination signal sequences in immunoglobulin gene segment usage: a V81x model

It has long been appreciated that some immunoglobulin (and T-cell receptor) gene segments are used much more frequently than others. The VHsegment V81x is a particularly striking case of overusage. Its usage varies with the stage of B-cell development and with the strain of mice, but it is always high in B cell progenitors. We have found that the coding sequence and the recombination signal sequences (RSS) are identical in five mouse strains, including CAST/Ei, a strain derived from the species Mus castaneus. Thus, the strain differences cannot be attributed to sequences within V81x itself. V81x RSS mediated recombination at rates significantly higher than another VHRSS. Although the V81x nonamer differs at one base pair from the consensus sequence, an RSS with this nonamer and a consensus heptamer recombines as well as the consensus RSS. When the V81x spacer is replaced by that of VA1, the frequency of recombination decreases by approximately 5-fold; thus, the contribution of variation in natural spacers to variability in VHusage in vivo is likely to be more than has been previously appreciated. Furthermore, the contribution of the heptamer and nonamer to differential VHusage in our assay is correlated inversely with their conservation throughout the VHlocus.

Immature thymocytes employ distinct signaling pathways for allelic exclusion versus differentiation and expansion

T cell receptor (TCR) beta chain allelic exclusion occurs at the thymocyte CD4- 8- (double-negative, or DN) to CD4+ 8+ (double-positive, or DP) transition, concurrently with differentiation and cellular expansion, and is imposed by a negative feedback loop in which a product of the first rearranged TCRbeta allele arrests further recombination in the TCRbeta locus. All of the major events associated with the development of DP cells can be induced by the introduction of TCRbeta or activated Lck transgenes. Here, we present evidence that the signaling pathways that promote thymocyte differentiation and expansion of RAG-deficient DN cells but not those that suppress rearrangements of endogenous TCRbeta genes in normal DN cells are engaged by activated Ras. We propose that TCRbeta allelic exclusion is mediated by effector pathways downstream of Lck but independent of Ras.

Differential regulation of transcription termination occurring at two different sites on the micro-delta gene complex

The progression of polymerases across the micro-delta Ig heavy chain gene complex is characterized by two termination events occurring at different sites on the transcription unit and at different times during B cell differentiation. We have utilized two mouse strains to analyze the regulatory determinants for these events in primary B cells. In the transgenic pmicro.microdeltaRatt strain a 1160 bp intervening DNA segment (the att site) has been inverted. This mutation results in the abrogation of transcription termination that occurs in early B cells. Using a novel method that takes advantage of an internal ribosome entry site we have further restricted the size of the segment that is needed for inducing transcription termination in transfectants. This 200 bp termination-inducing sequence operates in tumor equivalents of early but not mature B cells and the activity is correlated with differential binding of nuclear proteins. To explore the regulatory basis for the change in site of transcription termination upon B cell activation we have examined the microS-/- deletion mutant strain in which the microS poly(A) site has been eliminated. The results suggest that polyadenylation at the microS site plays a dominant but not exclusive role in regulating transcription termination in activated B cells.

Extended duration of DH-JH rearrangement in immunoglobulin heavy chain transgenic mice: implications for regulation of allelic exclusion

Here we show that suppression of VH-DJH rearrangement in mice bearing a mu heavy (H) chain transgene (mu-tg mice) is associated with an extended period of DH-JH rearrangement, the first step of Immunoglobulin H chain gene rearrangement. Whereas DH-JH rearrangement is normally initiated and completed at the pro-B cell stage, in mu-tg mice it continues beyond this stage and occurs most frequently at the small (late) pre-B stage. Despite ongoing DH-JH rearrangement in late pre-B cells of mu-tg mice, VH-DJH rearrangement is not detectable in these cells. We infer that the lack of VH-DJH rearrangement primarily reflects tg-induced acceleration of B cell differentiation past the stage at which rearrangement of VH elements is permissible. In support of this inference, we find that the normal representation of early B lineage subsets is markedly altered in mu-tg mice. We suggest that the effect of a productive VH-DJH rearrangement at an endogenous H chain allele may be similar to that of a mu-tg; i.e., cells that make a productive VH-DJH rearrangement on the first attempt rapidly progress to a developmental stage that precludes VH-DJH rearrangement at the other allele (allelic exclusion).

Pleiotropic changes controlled by the pre-T-cell receptor

The construction of various gene-deficient mice has facilitated the understanding of the role of various receptors and signaling pathways that control the generation of alphabeta lineage cells. A predominant role is occupied by the pre-TCR, which not only generates large numbers of alphabeta lineage cells but also controls TCRbeta allelic exclusion as well as commitment to the gammadelta lineage versus the alphabeta lineage.

Distinct factors regulate the murine RAG-2 promoter in B- and T-cell lines

The recombination activating genes RAG-1 and RAG-2 are expressed in a lymphoid-cell-specific and developmentally regulated fashion. To understand the transcriptional basis for this regulation, we have cloned and characterized the murine RAG-2 promoter. The promoter was lymphoid cell specific, showing activity in various B- and T-cell lines but little activity in nonlymphoid cells. To our surprise, however, the promoter was regulated differently in B and T cells. Using nuclear extracts from B-cell lines, we found that the B-cell-specific transcription factor BSAP (Pax-5) could bind to a conserved sequence critical for promoter activity. BSAP activated the promoter in transfected cells, and the BSAP site was occupied in a tissue-specific manner in vivo. An overlapping DNA sequence binding to a distinct factor was necessary for promoter activity in T cells. Full promoter activity in T cells was also dependent on a more distal DNA sequence whose disruption had no effect on B-cell activity. The unexpected finding that a B-cell-specific factor regulates the RAG-2 promoter may explain some of the recently observed differences in the regulation of RAG transcription between B and T cells.

Positive and negative regulation of V(D)J recombination by the E2A proteins

A key feature of B and T lymphocyte development is the generation of antigen receptors through the rearrangement and assembly of the germline variable (V), diversity (D), and joining (J) gene segments. However, the mechanisms responsible for regulating developmentally ordered gene rearrangements are largely unknown. Here we show that the E2A gene products are essential for the proper coordinated temporal regulation of V(D)J rearrangements within the T cell receptor (TCR) gamma and delta loci. Specifically, we show that E2A is required during adult thymocyte development to inhibit rearrangements to the gamma and delta V regions that normally recombine almost exclusively during fetal thymocyte development. The continued rearrangement of the fetal Vgamma3 gene segment in E2A-deficient adult thymocytes correlates with increased levels of Vgamma3 germline transcripts and increased levels of double-stranded DNA breaks at the recombination signal sequence bordering Vgamma3. Additionally, rearrangements to a number of Vgamma and Vdelta gene segments used predominantly during adult development are significantly reduced in E2A-deficient thymocytes. Interestingly, at distinct stages of T lineage development, both the increased and decreased rearrangement of particular Vdelta gene segments is highly sensitive to the dosage of the E2A gene products, suggesting that the concentration of the E2A proteins is rate limiting for the recombination reaction involving these Vdelta regions.

CC Chemokine Receptors, CCR-1 and CCR-3, Are Potentially Involved in Antigen-Presenting Cell Function of Human Peripheral Blood Monocyte-Derived Dendritic Cells

We examined the potential involvement of two CC chemokine receptors (CCRs), CCR-1 and CCR-3, in the functional activation of granulocyte-macrophage colony-stimulating factor (GM-CSF) plus interleukin-4 (IL-4)–generated human peripheral blood monocyte-derived immature dendritic cells (DCs). Flow cytometric analysis showed that CCR-1, CCR-3, CCR-5, and CXC chemokine receptor (CXCR)-4 were expressed on the cell surface of monocyte-derived DCs. Treatment with a monoclonal antibody (MoAb) to either CCR-1 or CCR-3 but not MoAbs to CCR-5 and CXCR-4 abolished chemotactic migration of monocyte-derived DCs. The DCs treated with either the anti–CCR-1 MoAb or anti–CCR-3 MoAb were less efficient than untreated DCs in proliferation of allogeneic T cells (TCs) and TC-derived secretion of interferon-γ (IFN-γ). The homotypic aggregation of DCs and heterotypic aggregation of DCs with TCs were suppressed by the anti–CCR-1 MoAb or anti–CCR-3 MoAb. These results indicate that CCR-1 and CCR-3 specifically regulate interaction of TCs and DCs in the process of antigen presentation.

CC Chemokine Receptors, CCR-1 and CCR-3, Are Potentially Involved in Antigen-Presenting Cell Function of Human Peripheral Blood Monocyte-Derived Dendritic Cells

We examined the potential involvement of two CC chemokine receptors (CCRs), CCR-1 and CCR-3, in the functional activation of granulocyte-macrophage colony-stimulating factor (GM-CSF) plus interleukin-4 (IL-4)–generated human peripheral blood monocyte-derived immature dendritic cells (DCs). Flow cytometric analysis showed that CCR-1, CCR-3, CCR-5, and CXC chemokine receptor (CXCR)-4 were expressed on the cell surface of monocyte-derived DCs. Treatment with a monoclonal antibody (MoAb) to either CCR-1 or CCR-3 but not MoAbs to CCR-5 and CXCR-4 abolished chemotactic migration of monocyte-derived DCs. The DCs treated with either the anti–CCR-1 MoAb or anti–CCR-3 MoAb were less efficient than untreated DCs in proliferation of allogeneic T cells (TCs) and TC-derived secretion of interferon-γ (IFN-γ). The homotypic aggregation of DCs and heterotypic aggregation of DCs with TCs were suppressed by the anti–CCR-1 MoAb or anti–CCR-3 MoAb. These results indicate that CCR-1 and CCR-3 specifically regulate interaction of TCs and DCs in the process of antigen presentation.

Terminal Deoxynucleotidyl Transferase Expression During Neonatal Life Alters DH Reading Frame Usage and Ig-Receptor-Dependent Selection of V Regions

During neonatal life, Ig diversity is limited in many respects. The absence of terminal deoxynucleotidyl transferase (TdT) expression with the consequent lack of nontemplated addition during the neonatal period, coupled with the predominant usage of a single DH reading frame (RF), leads to severe limitations of diversity in the CDR3 region of Ig heavy (H) chains. The neonatal Ig H chain repertoire is also characterized by restricted VH usage, with predominant expression of certain VH segments, such as VH81x, that are rarely evident during adult life. In this report, we examine the effect of enforced TdT expression on the neonatal repertoire of VH81xDJH rearrangements. We find that TdT synthesis abrogates DH RF bias during the fetal/neonatal period through a Ig-receptor-independent mechanism. These findings suggest that DH RF bias during neonatal life is determined largely by homology-directed joining. We also find that TdT synthesis alters the selection of productively rearranged VH81xDJH alleles in the neonatal spleen through a Ig-receptor-dependent mechanism. Analysis of predicted CDR3 amino acid sequences indicates that positive selection of VH81x-encoded H chains is correlated with the presence of a consensus sequence immediately adjacent to the VH segment. These data support the hypothesis that the CDR3 region is critical in determining the ability of VH81x-encoded H chains to form functional receptors that support positive selection of B lymphocytes. Together, our results demonstrate that TdT can indirectly influence the Ig repertoire by influencing both receptor-dependent and receptor-independent selection processes.

Re-expression of RAG-1 and RAG-2 genes and evidence for secondary rearrangements in human germinal center B lymphocytes

V(D)J recombination occurs in immature B cells within primary lymphoid organs. However, recent evidence demonstrated that the recombination activating genes RAG-1 and RAG-2 can also be expressed in murine germinal centers (GC) where they can mediate secondary rearrangements. This finding raises a number of interesting questions, the most important of which is what is the physiological role, if any, of secondary immunoglobulin (Ig) gene rearrangements. In the present report, we provide evidence that human GC B cells that have lost surface immunoglobulin re-express RAG-1 and RAG-2, suggesting that they may be able to undergo Ig rearrangement. Furthermore, we describe two mature B cell clones in which secondary rearrangements have possibly occurred, resulting in light chain replacement. The two clones carry both kappa and lambda light chains productively rearranged, but fail to express the x chain on the cell surface due to a stop codon acquired by somatic mutation. Interestingly, the analysis of the extent of somatic mutations accumulated by the two light chains might suggest that the lambda chain could have been acquired through a secondary rearrangement. Taken together, these data suggest that secondary Ig gene rearrangements leading to replacement may occur in human GC and may contribute to the peripheral B cell repertoire.

Haemophilus influenzae Type b Polysaccharides-Protein Conjugate Vaccine Elicits a More Diverse Antibody Repertoire in Infants Than in Adults

Conjugation of bacterial polysaccharides (PS) to protein carriers confers the ability to elicit protective serum Ab in infants, who respond poorly to plain PS. The serum Ab of young children immunized with Haemophilus influenzae type b (Hib) PS conjugate vaccine varies with age and Ag formulation. To understand these age-related changes in human anti-Hib PS immune responses we determined the variable region gene sequences encoding anti-Hib PS mAbs of infants immunized with Hib oligosaccharide-diphtheria toxin vaccine. The anti-Hib PS repertoire of children differs from that of adults. A smaller proportion of mAbs from children have high affinity for Hib PS, and the overall variable region gene repertoire of infants is more diverse than that in adults. Variable region genes encoding high affinity mAbs of infants are similar to the restricted repertoire described in adults. Low affinity anti-Hib PS mAbs of infants are encoded by a heterogeneous group of genes that are uncommonly observed in the adult repertoire. Abs with high affinity for Hib PS from infants, like most mAbs from adults, react only with Hib PS and the structurally similar PS of Escherichia coli K100, whereas low affinity mAbs of infants are polyreactive. The low affinity anti-Hib PS mAbs of infants immunized with Hib oligosaccharide-diphtheria toxin vaccine vaccine are not reflected in serum Ab. However, the differences between the variable region gene repertoires of adults and infants may account for the distinct immunologic characteristics of the anti-Hib PS responses in young children immunized with other vaccine formulations.

Differential Usage of VH Gene Segments Is Mediated by cis Elements

Ig diversity is generated in large part by the combinatorial joining of the Ig gene segments, VH, D, and JH, that together encode the variable domain of Ig. The final Ig repertoire, however, not only reflects the diversity generated through V(D)J recombinatorial joining, but it is also the product of a number of developmental restraints and selections. To avoid such restrictions and assess the recombination potential of individual Ig gene segments, we constructed Ig heavy (H) chain microlocus plasmids, each of which contain germline coding, recombination signal, and flanking sequences of a VH, D, and JH gene segment. These plasmids allow us to assess the recombination potential of the segments in the context of their natural flanking DNA sequences, but in the absence of any higher order chromatin structure or cellular selection. We found that the frequency and extent of deletions and additions at the recombination breakpoints are similar to those observed at rearranged Ig H chain loci in intact animals. The relative frequencies of the types of rearrangements—VD-J, V-DJ, VinvD-J (invD = inverted D), and VDJ—however, differ strongly. Moreover, V81x, the most used VH gene segment in intact mice, also is overused in this plasmid assay, 15 to 30 times that of another VH segment. This result indicates that the overuse of V81x in the early B cell repertoire can be a consequence of its DNA sequence and not of cellular activities.

Negative regulation of Ig gene rearrangement by a 150-bp transcriptional silencer

We previously showed that the V-J intervening sequence of the chicken lambda immunoglobulin locus contains a strong silencer that acts both on transcription and rearrangement. We show here that the transcriptional silencer activity can be ascribed to a minimal 150-bp fragment. The rearrangement silencing activity was previously shown by the replacement of the V-J intervening sequence with a neutral DNA fragment that dramatically increased the rate of rearrangement of the transgene. Insertion of the minimal silencer in this neutral fragment is shown here to result in a marked decrease in rearrangement of the transgenic construct. Strikingly, deletion of 28 bp from the 150-bp fragment abolished most of the transcriptional silencing activity and had a similar effect on rearrangement. These results conclusively correlate the silencing activity on both rearrangement and transcription.

Activated murine B lymphocytes and dendritic cells produce a novel CC chemokine which acts selectively on activated T cells

Genes were isolated using the suppression subtractive hybridization method by stimulation of pro/pre B cells with anti-CD40 and interleukin (IL)-4 to mature S mu-Sepsilon-switched cells. One of the strongly upregulated genes encodes a novel murine CC chemokine we have named ABCD-1. The ABCD-1 gene has three exons separated by 1. 2- and 2.7-kb introns. It gives rise to a 2.2-kb transcript containing an open reading frame of 276 nucleotides. Two polyadenylation sites are used, giving rise to cDNAs with either 1550 or 1850 bp of 3' untranslated regions. The open reading frame encodes a 24 amino acid-long leader peptide and a 68 amino acid-long mature protein with a predicted molecular mass of 7.8 kD. ABCD-1 mRNA is found in highest quantities in activated splenic B lymphocytes and dendritic cells. Little chemokine mRNA is present in lung, in unstimulated splenic cells, in thymocytes, and in lymph node cells. No ABCD-1 mRNA is detected in bone marrow, liver, kidney, or brain, in peritoneal exudate cells as well as in the majority of all unstimulated B lineage cells tested. It is also undetectable in Concanavalin A-activated/IL-2-restimulated splenic T cells, and in bone marrow-derived IL-2-induced natural killer cells and IL-3-activated macrophages. Recombinant ABCD-1 revealed a concentration-dependent and specific migration of activated splenic T lymphoblasts in chemotaxis assays. FACS(R) analyses of migrated cells showed no preferential difference in migration of CD4(+) versus CD8(+) T cell blasts. Murine as well as human T cells responded to ABCD-1. Freshly isolated cells from bone marrow, thymus, spleen, and lymph node, IL-2-activated NK cells, and LPS-stimulated splenic cells, all did not show any chemotactic response. Thus, ABCD-1 is the first chemokine produced in large amounts by activated B cells and acting selectively on activated T lymphocytes. Therefore, ABCD-1 is expected to play an important role in the collaboration of dendritic cells and B lymphocytes with T cells in immune responses.

Regulation of promoter and intron enhancer activity in immunoglobulin heavy-chain genes during B-cell differentiation

Chloramphenicol acetyltransferase (CAT) transgenic mice, in which the transgene is regulated by the VH promoter and heavy-chain intron enhancer (E mu), were examined to investigate the variation of activity of these cis-acting elements during the late stage of B-cell development. CAT enzyme activity decreased when resting B cells were stimulated through B-cell receptors (BCR) with goat anti-mouse IgM antibodies in vitro. On the other hand, when these B cells were stimulated by lipopolysaccharide (LPS) in vitro, they showed enhanced CAT activity, accompanied by an increase in the number of CD43+ B220+ cells (pro-plasma and plasma cells). In addition, the CAT activities in CD43+ B220+ and PNAhi B220+ cells from immunized mice were higher than those in CD43- B220+ and PNAlo B220+ cells, respectively. These results suggest that the activity of E mu in the context of VH promoter was transiently down-regulated by stimulation through the BCR but enhanced at the pro-plasma and plasma stages.

Establishment of MAIDS-defective virus-infected B cell lines and their characterization

Mice inoculated with the murine AIDS (MAIDS)-defective virus develop severe B and T cell dysfunctions. The primary event in the development of this disease is the infection and polyclonal expansion of the target cells of this defective virus, which have been reported to belong to the B cell lineage. To further study the central role that these cells play in the development of MAIDS, we attempted to establish MAIDS-defective virus-infected B cell lines in vitro. We succeeded in establishing two cell lines, SD1 and CSTB5, from the enlarged organs of C57BL/6 mice inoculated with helper-free stocks of the MAIDS-defective virus. Both cell lines are not transplantable in syngeneic C57BL/6 mice or in nude or CD8-/- mice and are apparently not malignant. They both belong to the B lineage, as their immunoglobulin (Ig) genes, but not the T cell receptor (TcR) beta locus, are rearranged, suggesting that they are relatively mature B cells. However, analysis of cell surface marker expression by FACS revealed a surface phenotype similar to that of pre-B cells (MHC I+, MHC II+, B7.2+, sIgM-, sIgG-, kappa-, B220-, CD5-, Thy1.2-, TcR-, CD3-, CD4-, CD8-, Mac-1-, 33D1-). Additionally, the CSTB5 cells express CD40 and the SD1 cells express CD43. Both cell lines contain the MAIDS-defective provirus and express the expected 4.2-kb viral RNA and the corresponding Pr60gag protein. The CSTB5 cells are nonproducer, while the SD1 cell line produces what appears to be an endogenous MuLV. The phenotype of these cell lines is very similar to what is known about the target B cells of this virus in vivo. These new established cell lines are likely to be useful in elucidating the mechanism(s) by which the MAIDS-defective virus causes its target B cells to proliferate and induce T cell anergy in infected animals.

B-1 cell development: evidence for an uncommitted immunoglobulin (Ig)M+ B cell precursor in B-1 cell differentiation

Murine phosphatidyl choline (PtC)-specific B cells in normal mice belong exclusively to the B-1 subset. Analysis of anti-PtC (VH12 and VH12/Vkappa4) transgenic (Tg) mice indicates that exclusion from B-0 (also known as B-2) occurs after immunoglobulin gene rearrangement. This predicts that PtC-specific B-0 cells are generated, but subsequently eliminated by either apoptosis or differentiation to B-1. To investigate the mechanism of exclusion, PtC-specific B cell differentiation was examined in mice expressing the X-linked immunodeficiency (xid) mutation. xid mice lack functional Bruton's tyrosine kinase (Btk), a component of the B cell receptor signal transduction pathway, and are deficient in B-1 cell development. We find in C57BL/ 6.xid mice that VH12 pre-BII cell selection is normal and that PtC-specific B cells undergo modest clonal expansion. However, the majority of splenic PtC-specific B cells in anti-PtC Tg/xid mice are B-0, rather than B-1 as in their non-xid counterparts. These data indicate that PtC-specific B-0 cell generation precedes segregation as predicted, and that Btk function is required for efficient segregation to B-1. Since xid mice exhibit defective B cell differentiation, not programmed cell death, these data are most consistent with an inability of PtC-specific B-0 cells to convert to B-1 and a single B cell lineage.