Ordering of human bone marrow B lymphocyte precursors by single-cell polymerase chain reaction analyses of the rearrangement status of the immunoglobulin H and L chain gene loci

Molecular features responsible for the absence of immunoglobulin heavy chain protein synthesis in an IgH− subgroup of multiple myeloma

This study involved 12 patients with multiple myeloma (MM), in whom malignant plasma cells did not contain immunoglobulin heavy chain (IgH) protein chains. Southern blot analysis revealed monoallelic Jh gene rearrangements in 10 patients, biallelic rearrangement in 1 patient, and biallelic deletion of the Jh and Cμ regions in 1 patient. Heteroduplex polymerase chain reaction analysis enabled the identification and sequencing of 9 clonal Jhgene rearrangements. Only 4 of the joinings were complete Vh-(D)-Jhrearrangements, including 3 in-frame rearrangements with evidence of somatic hypermutation. Five rearrangements concerned incomplete Dh-Jh joinings, mainly associated with deletion of the other allele. Curiously, in at least 1 of these 5 cases the second allele seemed to be in germline configuration, whereas the in-frame Vκ-Jκgene rearrangements contained somatic mutations. The configuration of the IGH genes was further investigated by use of Ch probes. In 5 patients the rearrangements in the Jh and Ch regions were not concordant, probably caused by illegitimate IGH class switch recombination (chromosomal translocations to 14q32.3). These data indicate that in many IgH− MM patients illegitimateIGH class switch rearrangement or illegitimate deletion of the functional Vh-(Dh)-Jhallele are responsible for IgH negativity. For example, the exclusive presence ofDh-Jhrearrangements in combination with mutated IGK genes can only be explained in terms of normal B-cell development, if the second (functional) IGH allele is deleted, which was probably the case in most patients. Therefore, defects at the DNA level are responsible for the lack of IgH protein production in most IgH− MM patients.

Fates of human B-cell precursors

Development of mammalian B-lineage cells is characterized by progression through a series of checkpoints defined primarily by rearrangement and expression of immunoglobulin genes. Progression through these checkpoints is also influenced by stromal cells in the microenvironment of the primary tissues wherein B-cell development occurs, ie, fetal liver and bone marrow and adult bone marrow. This review focuses on the developmental biology of human bone marrow B-lineage cells, including perturbations that contribute to the origin and evolution of B-lineage acute lymphoblastic leukemia and primary immunodeficiency diseases characterized by agammaglobulinemia. Recently described in vitro and in vivo models that support development and expansion of human B-lineage cells through multiple checkpoints provide new tools for identifying the bone marrow stromal cell–derived molecules necessary for survival and proliferation. Mutations in genes encoding subunits of the pre-B cell receptor and molecules involved in pre-B cell receptor signaling culminate in X-linked and non–X-linked agammaglobulinemia. A cardinal feature of these immunodeficiencies is an apparent apoptotic sensitivity of B-lineage cells at the pro-B to pre-B transition. On the other end of the spectrum is the apoptotic resistance that accompanies the development of B-lineage acute lymphoblastic leukemia, potentially a reflection of genetic abnormalities that subvert normal apoptotic programs. The triad of laboratory models that mimic the bone marrow microenvironment, immunodeficiency diseases with specific defects in B-cell development, and B-lineage acute lymphoblastic leukemia can now be integrated to deepen our understanding of human B-cell development.

Fates of human B-cell precursors

Development of mammalian B-lineage cells is characterized by progression through a series of checkpoints defined primarily by rearrangement and expression of immunoglobulin genes. Progression through these checkpoints is also influenced by stromal cells in the microenvironment of the primary tissues wherein B-cell development occurs, ie, fetal liver and bone marrow and adult bone marrow. This review focuses on the developmental biology of human bone marrow B-lineage cells, including perturbations that contribute to the origin and evolution of B-lineage acute lymphoblastic leukemia and primary immunodeficiency diseases characterized by agammaglobulinemia. Recently described in vitro and in vivo models that support development and expansion of human B-lineage cells through multiple checkpoints provide new tools for identifying the bone marrow stromal cell–derived molecules necessary for survival and proliferation. Mutations in genes encoding subunits of the pre-B cell receptor and molecules involved in pre-B cell receptor signaling culminate in X-linked and non–X-linked agammaglobulinemia. A cardinal feature of these immunodeficiencies is an apparent apoptotic sensitivity of B-lineage cells at the pro-B to pre-B transition. On the other end of the spectrum is the apoptotic resistance that accompanies the development of B-lineage acute lymphoblastic leukemia, potentially a reflection of genetic abnormalities that subvert normal apoptotic programs. The triad of laboratory models that mimic the bone marrow microenvironment, immunodeficiency diseases with specific defects in B-cell development, and B-lineage acute lymphoblastic leukemia can now be integrated to deepen our understanding of human B-cell development.

Age-dependent changes in B lymphocyte development in man and mouse

In recent years, detailed analyses of B cell development in both humans and mice have revealed similar subsets of precursors along the same pathway of differentiation. From these studies it also became clear that both species undergo age related changes in this B lymphocyte development program. In this review we summarize these findings and discuss, potential mechanisms underlying these age related changes, and possible causative correlations between these changes and age related B cell abnormalities.

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

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

Composition of the GABA(A) receptors of retinal dopaminergic neurons

Transgenic technology, single-cell RT-PCR, and immunocytochemistry were combined to investigate the composition of the GABA(A) receptors of dopaminergic (interplexiform) amacrine (DA) cells. A mouse line was used in which these neurons were labeled with human placental alkaline phosphatase and could therefore be identified in vitro after dissociation of the retina. We performed single-cell RT-PCR on the isolated cells and showed that (1) DA cells contained the messages for alpha1, alpha3, alpha4, beta1, beta3, gamma1, gamma2(S), and gamma2(L) subunits; (2) this transcript repertory did not change on dissociation of the retina and throughout the time required for cell harvesting; and (3) all DA cells contained the entire transcript repertory. Immunocytochemistry with subunit-specific antibodies showed that all subunits were expressed and appeared homogeneously distributed throughout the cell membrane at a low concentration. In addition, with the exception of alpha4, the subunits formed clusters at the surface of the dendrites and on the inner pole of the cell body. Because of their size, shape, and topographic coincidence with GABAergic endings, the clusters were interpreted as postsynaptic active zones containing GABA(A) receptors. The composition of the synaptic receptors was not uniform: clusters distributed throughout the dendritic tree contained alpha3, beta3, and, less frequently, beta1 subunits, whereas clusters containing the alpha1 subunit were confined to large dendrites. Therefore, DA cells possess at least two types of GABA(A) receptors localized in different synapses. Furthermore, they exhibit multiple extrasynaptic GABA(A) receptors.

A human early B-cell factor-like protein participates in the regulation of the human CD19 promoter

CD19 is a functional component of the B-cell receptor complex where it acts as a modulator of the cellular response to surface immunoglobulin signaling. The gene is expressed from early B-cell developmental stages until the mature B-cell stage, but not in the plasma cell. The human CD19 promoter has been suggested to be regulated by the B-cell specific activator protein. BSAP, the Erg transcription factor and unidentified factors interacting with a GC rich binding site denoted PyG. In this report we present data suggesting that one of the PyG interacting factors is related to mouse early B-cell factor (EBF). Recombinant mouse EBF binds to the PyG site with an affinity about 3-fold lower than to the EBF binding site from the mouse mb-1 promoter in electrophoretic mobility shift assays. Furthermore, the PyG box binds to a factor in nuclear extracts from human B-cell lines that also interact with the mouse mb-1 promoter EBF binding site. Mutation of the PyG box impaired binding of the factor and the function of a minimal CD19 promoter in human cells of the B lineage, but not in Jurkat T or non-lymphoid HeLa cells. In addition to this, murine EBF was able to activate a wild type but not a PyG mutant human CD19 promoter 7-fold upon transient co-transfection in HeLa cells. Thus, we suggest that a human homologue of mouse EBF participate in transcriptional regulation of the human CD19 promoter.

Composition of the GABA(A) receptors of retinal dopaminergic neurons

Transgenic technology, single-cell RT-PCR, and immunocytochemistry were combined to investigate the composition of the GABA(A) receptors of dopaminergic (interplexiform) amacrine (DA) cells. A mouse line was used in which these neurons were labeled with human placental alkaline phosphatase and could therefore be identified in vitro after dissociation of the retina. We performed single-cell RT-PCR on the isolated cells and showed that (1) DA cells contained the messages for alpha1, alpha3, alpha4, beta1, beta3, gamma1, gamma2(S), and gamma2(L) subunits; (2) this transcript repertory did not change on dissociation of the retina and throughout the time required for cell harvesting; and (3) all DA cells contained the entire transcript repertory. Immunocytochemistry with subunit-specific antibodies showed that all subunits were expressed and appeared homogeneously distributed throughout the cell membrane at a low concentration. In addition, with the exception of alpha4, the subunits formed clusters at the surface of the dendrites and on the inner pole of the cell body. Because of their size, shape, and topographic coincidence with GABAergic endings, the clusters were interpreted as postsynaptic active zones containing GABA(A) receptors. The composition of the synaptic receptors was not uniform: clusters distributed throughout the dendritic tree contained alpha3, beta3, and, less frequently, beta1 subunits, whereas clusters containing the alpha1 subunit were confined to large dendrites. Therefore, DA cells possess at least two types of GABA(A) receptors localized in different synapses. Furthermore, they exhibit multiple extrasynaptic GABA(A) receptors.

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.

Comparative studies of different stromal cell microenvironments in support of human B-cell development

This study compared human murine stromal cells for their capacity to support human hematopoietic stem cell (HSC) development into the B lineage. FACS sorted human fetal bone marrow (BM) HSC (CD34+CD19- or CD34+/CD10-/CD19-/CD45RA) were cultured on human fetal BM stromal cells, human skin fibroblasts, or murine S17 stromal cells and analyzed by flow cytometry or reverse transcriptase polymerase chain reaction. CD34+CD19- HSC on human BM stromal cells or fibroblasts differentiated into B-lineage cells with a continuum in density of surface CD19 expression, and some cells expressing micro/kappa or micro/lambda B-cell receptors. In contrast, CD19+ cells from S17 cultures had two- to fourfold higher levels of CD19, but no cells expressing B-cell receptors. The number and percentage of CD19+ cells was high, intermediate, or low in the human BM, human fibroblast, or murine S17 stromal cell cultures, respectively. Reverse transcriptase polymerase chain reaction analysis showed that TdT, CD19, and DHQ52-J(H) rearrangements were expressed at comparable levels when CD34+/CD19- HSC were plated on human or murine stromal cells. In contrast, CD34+/CD10-/CD19-/CD45RA HSC plated on human or murine stromal cells expressed CD19 in both cultures, but TdT was only expressed in human stromal cell cultures. We conclude that human BM stromal cell, human skin fibroblasts, and murine S17 stromal cell cultures can provide complementary and comparative tools for identification of stromal cell ligands with potentially unique functions in regulating human B-cell development.

Phenotypic and molecular characterization of human peripheral blood B-cell subsets with special reference to N-region addition and J kappa-usage in V kappa J kappa-joints and kappa/lambda-ratios in naive versus memory B-cell subsets to identify traces of receptor editing processes

We identified a population of IgM+IgD+ B-cells in the peripheral blood (PB) of humans that express somatically mutated V-region genes like classical class switched or IgM-only memory B-cells and comprise around 15% of PB B-cells in adults. Mutated IgM+IgD+ cells differ from unmutated naive IgM+IgD+ cells in that they express the CD27 cell surface antigen. In addition, a very small subset of IgD-only B-cells was identified in the PB that carried rearranged VH-genes with an extremely high load of somatic mutations (up to 60 mutations per gene). A common characteristic of the four somatically mutated subsets, which altogether comprise 40% of PB B-lymphocytes in adults, is the surface expression of CD27. This antigen may thus represent a general marker for memory B-cells in the human. Somatically mutated and unmutated PB B-cell subsets were analyzed for N-region addition and J kappa-usage in V kappa J kappa-joints, and in addition for the respective kappa/lambda-ratios: N-nucleotides could be identified in a large fraction of V kappa-regions of all B-cell subsets, indicating that N-region insertion already occurs in the pre-germinal center (GC) phase of B-cell development. Both the J kappa-usage in expressed V kappa J kappa-joints and the kappa/lambda-ratio from somatically mutated B-cells do not differ substantially from those of the unmutated cells, so that in terms of these parameters, a contribution of secondary V kappa J kappa-rearrangements in shaping the memory B-cell repertoire is not detectable.

Ig Heavy Chain Gene Rearrangements in T-Cell Acute Lymphoblastic Leukemia Exhibit Predominant Dh6-19 and Dh7-27 Gene Usage, Can Result in Complete V-D-J Rearrangements, and Are Rare in T-Cell Receptor β Lineage

Rearranged IGH genes were detected by Southern blotting in 22% of 118 cases of T-cell acute lymphoblastic leukemia (ALL) and involved monoallelic and biallelic rearrangements in 69% (18/26) and 31% (8/26) of these cases, respectively. IGH gene rearrangements were found in 19% (13/69) of CD3− T-ALL and in 50% of TCRγδ+ T-ALL (12/24), whereas only a single TCRβ+ T-ALL (1/25) displayed a monoallelicIGH gene rearrangement. The association with the T-cell receptor (TCR) phenotype was further supported by the striking relationship between IGH and TCR delta (TCRD) gene rearrangements, ie, 32% of T-ALL (23/72) with monoallelic or biallelicTCRD gene rearrangements had IGH gene rearrangements, whereas only 1 of 26 T-ALL with biallelic TCRD gene deletions contained a monoallelic IGH gene rearrangement. Heteroduplex polymerase chain reaction (PCR) analysis with Vh and Dh family-specific primers in combination with a Jhconsensus primer showed a total of 39 clonal products, representing 7 (18%) Vh-(Dh-)Jh joinings and 32 (82%) Dh-Jh rearrangements. Whereas the usage of Vh gene segments was seemingly random, preferential usage of Dh6-19 (45%) and Dh7-27 (21%) gene segments was observed. Although the Jh4 and Jh6 gene segments were used most frequently (33% and 21%, respectively), a significant proportion of joinings (28%) used the most upstream Jh1 and Jh2 gene segments, which are rarely used in precursor-B-ALL and normal B cells (1% to 4%). In conclusion, the high frequency of incomplete Dh-Jh rearrangements, the frequent usage of the more downstream Dh6-19 and Dh7-27 gene segments, and the most upstream Jh1 and Jh2 gene segments suggests a predominance of immature IGH rearrangements in immature (non-TCRβ+) T-ALL as a result of continuing V(D)J recombinase activity. More mature β-lineage T-ALL with biallelic TCRD gene deletions apparently have switched off their recombination machinery and are less prone to cross-lineageIGH gene rearrangements. The combined results indicate thatIGH gene rearrangements in T-ALL are postoncogenic processes, which are absent in T-ALL with deleted TCRD genes and completed TCR alpha (TCRA) gene rearrangements.

The Human (ΨL+μ−) proB Complex: Cell Surface Expression and Biochemical Structure of a Putative Transducing Receptor

The surrogate light chain (ΨL) associates with μ and Ig-Igβ chains to form the preB-cell receptor that plays a critical role in early B-cell differentiation. Discrepancies exist in human concerning the existence of ΨL+μ− proB cells and the biochemical structure of such a proB-cell complex remains elusive. Among new antihuman VpreB monoclonal antibodies (MoAbs), 5 of the γκ isotype bound to recombinant and native VpreB protein with high affinity. They recognized 4 discrete epitopes, upon which 2 were in the extra-loop fragment. Such MoAbs detected the ΨL at the cell surface of either preB or on both proB and preB cells. The previously reported SLC1/SLC2 MoAbs recognize a conformational epitope specific for the μ/ΨL association in accordance with their preB-cell reactivity. Using the proB/preB 4G7 MoAb, ΨL cell surface expression was detected on normal bone marrow, not only on CD34−CD19+ preB but also on CD34+CD19+ proB cells. Futhermore, this MoAb identified ΨL+μ− fresh proB leukemic cells of the TEL/AML1 type. Biochemical studies showed that, at the proB stage, the ΨL is associated noncovalently with two proteins of 105 and 130 kD. Triggering of this complex induces intracellular Ca2+ flux, suggesting that the ΨL may be involved in a new receptor at this early step of the B-cell differentiation.

Ig Heavy Chain Gene Rearrangements in T-Cell Acute Lymphoblastic Leukemia Exhibit Predominant Dh6-19 and Dh7-27 Gene Usage, Can Result in Complete V-D-J Rearrangements, and Are Rare in T-Cell Receptor β Lineage

Rearranged IGH genes were detected by Southern blotting in 22% of 118 cases of T-cell acute lymphoblastic leukemia (ALL) and involved monoallelic and biallelic rearrangements in 69% (18/26) and 31% (8/26) of these cases, respectively. IGH gene rearrangements were found in 19% (13/69) of CD3− T-ALL and in 50% of TCRγδ+ T-ALL (12/24), whereas only a single TCRβ+ T-ALL (1/25) displayed a monoallelicIGH gene rearrangement. The association with the T-cell receptor (TCR) phenotype was further supported by the striking relationship between IGH and TCR delta (TCRD) gene rearrangements, ie, 32% of T-ALL (23/72) with monoallelic or biallelicTCRD gene rearrangements had IGH gene rearrangements, whereas only 1 of 26 T-ALL with biallelic TCRD gene deletions contained a monoallelic IGH gene rearrangement. Heteroduplex polymerase chain reaction (PCR) analysis with Vh and Dh family-specific primers in combination with a Jhconsensus primer showed a total of 39 clonal products, representing 7 (18%) Vh-(Dh-)Jh joinings and 32 (82%) Dh-Jh rearrangements. Whereas the usage of Vh gene segments was seemingly random, preferential usage of Dh6-19 (45%) and Dh7-27 (21%) gene segments was observed. Although the Jh4 and Jh6 gene segments were used most frequently (33% and 21%, respectively), a significant proportion of joinings (28%) used the most upstream Jh1 and Jh2 gene segments, which are rarely used in precursor-B-ALL and normal B cells (1% to 4%). In conclusion, the high frequency of incomplete Dh-Jh rearrangements, the frequent usage of the more downstream Dh6-19 and Dh7-27 gene segments, and the most upstream Jh1 and Jh2 gene segments suggests a predominance of immature IGH rearrangements in immature (non-TCRβ+) T-ALL as a result of continuing V(D)J recombinase activity. More mature β-lineage T-ALL with biallelic TCRD gene deletions apparently have switched off their recombination machinery and are less prone to cross-lineageIGH gene rearrangements. The combined results indicate thatIGH gene rearrangements in T-ALL are postoncogenic processes, which are absent in T-ALL with deleted TCRD genes and completed TCR alpha (TCRA) gene rearrangements.

The Human (ΨL+μ−) proB Complex: Cell Surface Expression and Biochemical Structure of a Putative Transducing Receptor

The surrogate light chain (ΨL) associates with μ and Ig-Igβ chains to form the preB-cell receptor that plays a critical role in early B-cell differentiation. Discrepancies exist in human concerning the existence of ΨL+μ− proB cells and the biochemical structure of such a proB-cell complex remains elusive. Among new antihuman VpreB monoclonal antibodies (MoAbs), 5 of the γκ isotype bound to recombinant and native VpreB protein with high affinity. They recognized 4 discrete epitopes, upon which 2 were in the extra-loop fragment. Such MoAbs detected the ΨL at the cell surface of either preB or on both proB and preB cells. The previously reported SLC1/SLC2 MoAbs recognize a conformational epitope specific for the μ/ΨL association in accordance with their preB-cell reactivity. Using the proB/preB 4G7 MoAb, ΨL cell surface expression was detected on normal bone marrow, not only on CD34−CD19+ preB but also on CD34+CD19+ proB cells. Futhermore, this MoAb identified ΨL+μ− fresh proB leukemic cells of the TEL/AML1 type. Biochemical studies showed that, at the proB stage, the ΨL is associated noncovalently with two proteins of 105 and 130 kD. Triggering of this complex induces intracellular Ca2+ flux, suggesting that the ΨL may be involved in a new receptor at this early step of the B-cell differentiation.

Independent regulation of the two Pax5 alleles during B-cell development

The developmental control genes of the Pax family are frequently associated with mouse mutants and human disease syndromes. The function of these transcription factors is sensitive to gene dosage, as mutation of one allele or a modest increase in gene number results in phenotypic abnormalities. Pax5 has an important role in B-cell and midbrain development. By following the expression of individual Pax5 alleles at the single-cell level, we demonstrate here that Pax5 is subject to allele-specific regulation during B-lymphopoiesis. Pax5 is predominantly transcribed from only one allele in early progenitors and mature B cells, whereas it switches to a biallelic transcription mode in immature B cells. The allele-specific regulation of Pax5 is stochastic, reversible, independent of parental origin and correlates with synchronous replication, in contrast with imprinted and other monoallelically expressed genes. As a consequence, B-lymphoid tissues are mosaics with respect to the transcribed Pax5 allele, and thus mutation of one allele in heterozygous mice results in deletion of the cell population expressing the mutant allele due to loss of Pax5 function at the single-cell level. Similar allele-specific regulation may be a common mechanism causing the haploinsufficiency and frequent association of other Pax genes with human disease.

μ-Surrogate Light Chain Physicochemical Interactions of the Human PreB Cell Receptor: Implications for VH Repertoire Selection and Cell Signaling at the PreB Cell Stage

The surrogate light chain (SL) composed of the λ-like and VpreB polypeptides is organized as two Ig domains and an extra-loop structure. It associates to the μ-chain in preB cells. We have produced human VpreB, SL, two Fdμ (VH-CH1), and the two corresponding Fab-like (Fdμ-SL) recombinant proteins in baculovirus. The correctness of the general conformation of the proteins was assessed by epitope mapping and affinity measurements using a new batch of anti-VpreB mAbs. Plasmon resonance analysis showed that both VpreB and the entire SL associated with the Fdμ fragments, with Kd values of 3 × 10−8 M for VpreB-Fdμ and of 10−9 to 10−10 M, depending upon the VH, for SL-Fdμ. These results indicate that the λ-like chain, in addition to be covalently bound to the Cμ1 domain, also interacts with the VH domain. Therefore, a dual role of the SL emerges: 1) interaction of the C-domain of λ-like would release the μ-chain from its interaction with binding protein in the endoplasmic reticulum, and 2) interaction of a part of λ-like and most of VpreB would bind to VH, ensuring a “quality control” of the native heavy chain that represents the first step of selection of the B cell repertoire. We also demonstrated that two Fab-like fragments did not interact with each other, suggesting that activation of the cell surface preB receptor does not involve aggregation neither in cis nor in trans of the Fab-like structures.

Flow Cytometric Diagnosis of the Cell Lineage and Developmental Stage of Acute Lymphoblastic Leukemia by Novel Monoclonal Antibodies Specific to Human Pre–B-Cell Receptor

Three novel monoclonal antibodies (MoAbs) have been established that recognize distinct epitopes of a human pre–B-cell receptor (pre-BCR) composed of a μ heavy (μH) chain and a λ5/VpreB surrogate light (SL) chain. HSL11 reacts with λ5 whereas HSL96 reacts with VpreB. Intriguingly, HSL2 does not bind to each component of the pre-BCR but does bind to the completely assembled pre-BCR complex. Flow cytometric analyses with cytoplasmic staining of a panel of human cell lines showed that HSL11 and HSL96 specifically stained cell lines derived from the pro–B and pre–B-cell stages of B-cell development. In contrast, HSL2 stained exclusively cell lines derived from the pre–B-cell stage. These results prompted us to explore the possibility of clinical application of these MoAbs for the determination of the cell lineage and developmental stage of acute lymphoblastic leukemia (ALL). Whereas none of mature B-lineage ALLs (B-ALLs), T-lineage ALLs (T-ALLs), and acute myeloid leukemias analyzed were stained in the cytoplasm with these three MoAbs, the vast majority of non–B- and non–T-ALLs (53 out of 56 cases) were found positive for either λ5, Vpre-B, or both in their cytoplasm. Among these 53 cytoplasmic SL chain-positive ALLs, 19 cases were also positive for cytoplasmic μH chain, indicative of pre–B-cell origin. Interestingly, 6 out of these 19 pre–B-ALL cases were found negative for cytoplasmic staining with HSL2. From these results, we propose a novel classification of B-ALL in which five subtypes are defined on the basis of the differential expression of SL chain, μH chain, pre-BCR, and light chain along the B-cell development.

Flow Cytometric Diagnosis of the Cell Lineage and Developmental Stage of Acute Lymphoblastic Leukemia by Novel Monoclonal Antibodies Specific to Human Pre–B-Cell Receptor

Three novel monoclonal antibodies (MoAbs) have been established that recognize distinct epitopes of a human pre–B-cell receptor (pre-BCR) composed of a μ heavy (μH) chain and a λ5/VpreB surrogate light (SL) chain. HSL11 reacts with λ5 whereas HSL96 reacts with VpreB. Intriguingly, HSL2 does not bind to each component of the pre-BCR but does bind to the completely assembled pre-BCR complex. Flow cytometric analyses with cytoplasmic staining of a panel of human cell lines showed that HSL11 and HSL96 specifically stained cell lines derived from the pro–B and pre–B-cell stages of B-cell development. In contrast, HSL2 stained exclusively cell lines derived from the pre–B-cell stage. These results prompted us to explore the possibility of clinical application of these MoAbs for the determination of the cell lineage and developmental stage of acute lymphoblastic leukemia (ALL). Whereas none of mature B-lineage ALLs (B-ALLs), T-lineage ALLs (T-ALLs), and acute myeloid leukemias analyzed were stained in the cytoplasm with these three MoAbs, the vast majority of non–B- and non–T-ALLs (53 out of 56 cases) were found positive for either λ5, Vpre-B, or both in their cytoplasm. Among these 53 cytoplasmic SL chain-positive ALLs, 19 cases were also positive for cytoplasmic μH chain, indicative of pre–B-cell origin. Interestingly, 6 out of these 19 pre–B-ALL cases were found negative for cytoplasmic staining with HSL2. From these results, we propose a novel classification of B-ALL in which five subtypes are defined on the basis of the differential expression of SL chain, μH chain, pre-BCR, and light chain along the B-cell development.

Four-Color Flow Cytometric Investigation of Terminal Deoxynucleotidyl Transferase–Positive Lymphoid Precursors in Pediatric Bone Marrow: CD79a Expression Precedes CD19 in Early B-Cell Ontogeny

Terminal deoxynucleotidyl transferase (TdT)-positive cells in human bone marrow (BM) are a phenotypically inhomogeneous population of precursor cells. In their majority, these TdT+ cells are unambiguously committed to the B lineage, as evidenced by CD19 expression. However, TdT+ precursors that lack CD19 also exist and these may encompass a differentiation potential for the B as well as for other lineages. Because recent data suggested that CD19 expression is not the earliest differentiation event in B-cell ontogeny, we sought to reevaluate TdT+ lymphoid precursors in pediatric BM to define the phenotypic denominator of B-lineage affiliation upstream of CD19. Using four-color flow cytometry, we focused on the assessment of the CD79a antigen, which is highly B-cell specific and which may also be expressed very early in B-cell ontogeny. We found that a majority of TdT+ cells coexpressed CD19 and CD79a in addition to CD10 and CD34, whereas, in all investigated samples, some TdT+ precursors lacked CD19 but expressed CD79a, which suggestively indicates also their B-lineage affiliation. In contrast to the CD19+precursors, which were usually CD10hi and CD79b+, these CD19−CD79a+putative B-cell precursors preferentially expressed CD10 at low levels and were CD79b+ in only 41%. About 17% of these TdT+CD19−CD79a+ precursors also coexpressed CD33 and CD7, but not myeloperoxidase, CD14, or cytoplasmic CD3, which is discussed in the light of cellular activation rather than lineage promiscuity. Our data confirm that the earliest differentiation stages of B cells can be dissected upon expression of the lineage antigens CD79a and CD19 and imply that CD79a is earlier expressed than CD19. This raises the chance to follow the sequential events heralding B-cell commitment in the most immature precursors by correlating phenotypic and genetic differentiation markers.

© 1998 by The American Society of Hematology.

Four-Color Flow Cytometric Investigation of Terminal Deoxynucleotidyl Transferase–Positive Lymphoid Precursors in Pediatric Bone Marrow: CD79a Expression Precedes CD19 in Early B-Cell Ontogeny

Terminal deoxynucleotidyl transferase (TdT)-positive cells in human bone marrow (BM) are a phenotypically inhomogeneous population of precursor cells. In their majority, these TdT+ cells are unambiguously committed to the B lineage, as evidenced by CD19 expression. However, TdT+ precursors that lack CD19 also exist and these may encompass a differentiation potential for the B as well as for other lineages. Because recent data suggested that CD19 expression is not the earliest differentiation event in B-cell ontogeny, we sought to reevaluate TdT+ lymphoid precursors in pediatric BM to define the phenotypic denominator of B-lineage affiliation upstream of CD19. Using four-color flow cytometry, we focused on the assessment of the CD79a antigen, which is highly B-cell specific and which may also be expressed very early in B-cell ontogeny. We found that a majority of TdT+ cells coexpressed CD19 and CD79a in addition to CD10 and CD34, whereas, in all investigated samples, some TdT+ precursors lacked CD19 but expressed CD79a, which suggestively indicates also their B-lineage affiliation. In contrast to the CD19+precursors, which were usually CD10hi and CD79b+, these CD19−CD79a+putative B-cell precursors preferentially expressed CD10 at low levels and were CD79b+ in only 41%. About 17% of these TdT+CD19−CD79a+ precursors also coexpressed CD33 and CD7, but not myeloperoxidase, CD14, or cytoplasmic CD3, which is discussed in the light of cellular activation rather than lineage promiscuity. Our data confirm that the earliest differentiation stages of B cells can be dissected upon expression of the lineage antigens CD79a and CD19 and imply that CD79a is earlier expressed than CD19. This raises the chance to follow the sequential events heralding B-cell commitment in the most immature precursors by correlating phenotypic and genetic differentiation markers.

© 1998 by The American Society of Hematology.

Identification of a late stage of small noncycling pTalpha- pre-T cells as immediate precursors of T cell receptor alpha/beta+ thymocytes

During thymocyte development, progression from T cell receptor (TCR)beta to TCRalpha rearrangement is mediated by a CD3-associated pre-TCR composed of the TCRbeta chain paired with pre-TCRalpha (pTalpha). A major issue is how surface expression of the pre-TCR is regulated during normal thymocyte development to control transition through this checkpoint. Here, we show that developmental expression of pTalpha is time- and stage-specific, and is confined in vivo to a limited subset of large cycling human pre-T cells that coexpress low density CD3. This restricted expression pattern allowed the identification of a novel subset of small CD3(-) thymocytes lacking surface pTalpha, but expressing cytoplasmic TCRbeta, that represent late noncycling pre-T cells in which recombination activating gene reexpression and downregulation of T early alpha transcription are coincident events associated with cell cycle arrest, and immediately preceding TCRalpha gene expression. Importantly, thymocytes at this late pre-T cell stage are shown to be functional intermediates between large pTalpha+ pre-T cells and TCRalpha/beta+ thymocytes. The results support a developmental model in which pre-TCR-expressing pre-T cells are brought into cycle, rapidly downregulate surface pre-TCR, and finally become small resting pre-T cells, before the onset of TCRalpha gene expression.

B-cell development: a comparison between mouse and man

A common variable immunodeficiency (CVID) patient, who carries mutations on both alleles of the gene encoding the surrogate light chain component lambda 5/14.1, shows a similar phenotype of B-cell deficiency as the lambda 5-deficient mutant mouse. As discussed here by Paolo Ghia and colleagues, this points to a remarkably similar developmental pathway of B cells in humans and mice.

Early stages in the development of human T, natural killer and thymic dendritic cells

T-cell development is initiated when CD34+ pluripotent stem cells or their immediate progeny leave the bone marrow to migrate to the thymus. Upon arrival in the thymus the stem cell progeny is not yet committed to the T-cell lineage as it has the capability to develop into T, natural killer (NK) and dendritic cells (DC). Primitive hematopoietic progenitor cells in the human thymus express CD34 and lack CD1a. When these progenitor cells develop into T cells they traverse a number of checkpoints. One early checkpoint is the induction of T-cell commitment, which correlates with appearance of CD1a and involves the loss of capacity to develop into NK cells and DC and the initiation of T-cell receptor (TCR) gene rearrangements. Basic helix-loop-helix transcription factors play a role in induction of T-cell commitment. CD1a+CD34+ cells develop into CD4+CD8 alpha+ beta+ cells by upregulating first CD4, followed by CD8 alpha and then CD8 beta. Selection for productive TCR beta gene rearrangements (beta selection) likely occurs in the CD4+CD8 alpha+ beta- and CD4+CD8 alpha+ beta+ populations. Although the T and NK-cell lineages are closely related to each other, NK cells can develop independently of the thymus. The fetal thymus is most likely one site of NK-cell development.

Development of the early B cell population in Xenopus

Like mammals, the amphibian Xenopus uses combinatorial joining of the immunoglobulin V, D and J elements and multiple rearrangements to generate its B cell repertoire. Xenopus larvae hatch 2 days after fertilization and individuals are under pressure to develop an immune repertoire when the number of available cells is small (approximately 5 and 200 IgM-positive cells on days 5 and 11 after fertilization, respectively). In the liver, in a first phase of differentiation spanning days 5-12 after fertilization before immunological competence, the heavy (H) chain locus starts rearranging followed by the light (L) chain locus 3 days later. By immunohistology the first B cells expressing H and L chain are detectable on day 10. Despite the small number of cells available and the lack of external antigen selection at these early stages, the repertoire is heterogeneous. The VH families are used stepwise, although their genes are interspersed in the genome. The earliest family used (VH1) is homologous to the VH3 family of human and to the VH7183 of the mouse which are also overrepresented in early mammalian development. In the second phase, from day 12-13 onwards, the spleen differentiates and the animal becomes immunologically competent. The V, D and J usage is similar to that of adults although VDJ junctions lack N nucleotides until metamorphosis. A preferential reading frame for D and one specific DJ junction are overrepresented during this second phase. The visible bias toward homology-based junction results in fact from selection after rearrangement.

Antigen receptor engagement turns off the V(D)J recombination machinery in human tonsil B cells

The germinal center (GC) is an anatomic compartment found in peripheral lymphoid organs, wherein B cells undergo clonal expansion, somatic mutation, switch recombination, and reactivate immunoglobulin gene V(D)J recombination. As a result of somatic mutation, some GC B cells develop higher affinity antibodies, whereas others suffer mutations that decrease affinity, and still others may become self-reactive. It has been proposed that secondary V(D)J rearrangements in GCs might rescue B cells whose receptors are damaged by somatic mutations. Here we present evidence that mature human tonsil B cells coexpress conventional light chains and recombination associated genes, and that they extinguish recombination activating gene and terminal deoxynucleotidyl transferase expression when their receptors are cross-linked. Thus, the response of the recombinase to receptor engagement in peripheral B cells is the opposite of the response in developing B cells to the same stimulus. These observations suggest that receptor revision is a mechanism for receptor diversification that is turned off when antigen receptors are cross-linked by the cognate antigen.

Surrogate Light Chain Production During B Cell Differentiation: Differential Intracellular Versus Cell Surface Expression

Expression of the surrogate light (ψL) chain genes encoding the VpreB and λ5/14.1 proteins is restricted to B-lineage cells. Pro-B and pre-B cells produce ψL chains, but whether both employ these as cell surface receptor components remains enigmatic. Recombinant human VpreB protein was used to generate a large panel of monoclonal anti-VpreB Abs to examine this issue. Native ψL chain proteins within pro-B cells as well as those serving as receptor components on pre-B cells were precipitated by 16 of the 26 anti-VpreB Abs. Surrogate light chains were easily detected on pre-B cell lines, whereas these anti-VpreB Abs reacted with pro-B cell lines only after plasma membrane permeabilization. The subpopulation of normal bone marrow cells bearing pre-B receptors included large and small pre-B cells exclusively, although pro-B cells also contained intracellular VpreB. VpreB proteins were not detected on or within B cells in bone marrow or the circulation, but a subpopulation of B cells in germinal centers was found to express the VpreB proteins intracellularly. Surrogate L chains are thus intermittently produced during human B-lineage differentiation, while their role as receptor components appears limited to the pre-B cell stage.

Cross-lineage expression of Ig-beta (B29) in thymocytes: positive and negative gene regulation to establish T cell identity

Developmental commitment involves activation of lineage-specific genes, stabilization of a lineage-specific gene expression program, and permanent inhibition of inappropriate characteristics. To determine how these processes are coordinated in early T cell development, the expression of T and B lineage-specific genes was assessed in staged subsets of immature thymocytes. T lineage characteristics are acquired sequentially, with germ-line T cell antigen receptor-beta transcripts detected very early, followed by CD3epsilon and terminal deoxynucleotidyl transferase, then pTalpha, and finally RAG1. Only RAG1 expression coincides with commitment. Thus, much T lineage gene expression precedes commitment and does not depend on it. Early in the course of commitment to the T lineage, thymocytes lose the ability to develop into B cells. To understand how this occurs, we also examined expression of well defined B lineage-specific genes. Although lambda5 and Ig-alpha are not expressed, the mu 0 and I mu transcripts from the unrearranged IgH locus are expressed early, in distinct patterns, then repressed just before RAG1 expression. By contrast, RNA encoding the B cell receptor component Ig-beta was found to be transcribed in all immature thymocyte subpopulations and throughout most thymocyte differentiation. Ig-beta expression is down-regulated only during positive selection of CD4(+)CD8(-) cells. Thus several key participants in the B cell developmental program are expressed in non-B lineage-committed cells, and one is maintained even through commitment to an alternative lineage, and repressed only after extensive T lineage differentiation. The results show that transcriptional activation of "lymphocyte-specific" genes can occur in uncommitted precursors, and that T lineage commitment is a composite of distinct positive and negative regulatory events.

The Ig Heavy Chain 3′ End Confers a Posttranscriptional Processing Advantage to Bcl-2–IgH Fusion RNA in t(14;18) Lymphoma

The chromosomal translocation t(14;18) in lymphoma leads to an overproduction of the Bcl-2 protein on the basis of increased Bcl-2 mRNA levels. Whereas the juxtaposition of Bcl-2 with the Ig heavy chain locus causes a transcriptional activation, 70% of the lymphomas also produce Bcl-2–Ig fusion RNAs with Ig 3′ ends. Using S1 nuclease protection assays that can discriminate between nuclear RNA precursors and spliced mRNA, we found that the fusion RNAs in t(14;18) cell lines exhibit an additional posttranscriptional processing advantage. Transfection experiments with artificial genes containing various Bcl-2 or Ig 3′ ends show that this effect is (1) related to RNA splicing and/or nucleocytoplasmic transport; (2) independent of transcriptional activation by the heavy chain enhancer; (3) dependent on the presence of the JH-CH and C-γ1 Ig introns; and (4) tissue specific for B cells. This constitutes a novel mechanism of oncogene deregulation unrelated to transcriptional activation or half-life prolongation. The data further support the existence of a tissue-specific posttranscriptional pathway of Ig regulation in B cells.

The Ig Heavy Chain 3′ End Confers a Posttranscriptional Processing Advantage to Bcl-2–IgH Fusion RNA in t(14;18) Lymphoma

The chromosomal translocation t(14;18) in lymphoma leads to an overproduction of the Bcl-2 protein on the basis of increased Bcl-2 mRNA levels. Whereas the juxtaposition of Bcl-2 with the Ig heavy chain locus causes a transcriptional activation, 70% of the lymphomas also produce Bcl-2–Ig fusion RNAs with Ig 3′ ends. Using S1 nuclease protection assays that can discriminate between nuclear RNA precursors and spliced mRNA, we found that the fusion RNAs in t(14;18) cell lines exhibit an additional posttranscriptional processing advantage. Transfection experiments with artificial genes containing various Bcl-2 or Ig 3′ ends show that this effect is (1) related to RNA splicing and/or nucleocytoplasmic transport; (2) independent of transcriptional activation by the heavy chain enhancer; (3) dependent on the presence of the JH-CH and C-γ1 Ig introns; and (4) tissue specific for B cells. This constitutes a novel mechanism of oncogene deregulation unrelated to transcriptional activation or half-life prolongation. The data further support the existence of a tissue-specific posttranscriptional pathway of Ig regulation in B cells.

B-cell lymphopoiesis in mouse and man

Functional immunoglobulin gene rearrangement is a sine qua non for successful B cell development in mammalian bone marrow, but other factors are also important. Studies reported during the past year have contributed new insight into the surface receptor complexes and signaling outcomes that influence the fate of B cell precursors. Identification and characterization of secreted and membrane-associated stromal cell products, and their actions on B-cell precursors, was a parallel area of ongoing investigation.

Unbalanced Expression of Bcl-2 Family Proteins in Follicular Lymphoma: Contribution of CD40 Signaling in Promoting Survival

Although highly responsive, advanced stage follicular lymphoma (FL) is not curable with conventional treatment. This relative resistance is thought to be due to the t(14;18) that results in the constitutive overexpression of the death-inhibiting protein bcl-2. However, the observation that FL cells are sensitive to treatment in vivo and prone to apoptosis on in vitro culture questions whether bcl-2 alone is responsible for the pathogenesis and clinical behavior of this disease. Therefore, multiple genes are likely to be involved in both the lymphomagenesis and the clinical course of FL. We examined whether expression of other bcl-2 family genes might also be operative. Here, we show that FL cells display a different pattern of expression of bcl-2 family proteins from normal germinal center (GC) B cells that are thought to be their normal counterpart. FL cells express the death-suppressor proteins bcl-2, bcl-xL, and mcl-1; whereas GC B cells express bcl-xL and mcl-1 but also the proapoptotic proteins bax-α and bad. Although maintaining constitutive levels of bcl-2 and mcl-1, FL cells are not protected from apoptosis when cultured in vitro. Their propensity to undergo apoptosis is temporally associated with downregulation of bcl-xL. More importantly, activation of FL cells via CD40 not only prevents downregulation but increases the level of bcl-xL expression and results in promotion of survival. These results support the hypothesis that the overexpression of bcl-2 is not the only antiapoptotic mechanism responsible for the pathogenesis of FL. Survival of FL cells is determined by a number of death-inhibiting proteins, among which bcl-xL appears to have the most critical role. Moreover, these findings are consistent with the hypothesis that, although FL cells are malignant, they respond to microenvironmental signals such as CD40L that appear to contribute to their survival through the upregulation of death-inhibiting proteins.

Unbalanced Expression of Bcl-2 Family Proteins in Follicular Lymphoma: Contribution of CD40 Signaling in Promoting Survival

Although highly responsive, advanced stage follicular lymphoma (FL) is not curable with conventional treatment. This relative resistance is thought to be due to the t(14;18) that results in the constitutive overexpression of the death-inhibiting protein bcl-2. However, the observation that FL cells are sensitive to treatment in vivo and prone to apoptosis on in vitro culture questions whether bcl-2 alone is responsible for the pathogenesis and clinical behavior of this disease. Therefore, multiple genes are likely to be involved in both the lymphomagenesis and the clinical course of FL. We examined whether expression of other bcl-2 family genes might also be operative. Here, we show that FL cells display a different pattern of expression of bcl-2 family proteins from normal germinal center (GC) B cells that are thought to be their normal counterpart. FL cells express the death-suppressor proteins bcl-2, bcl-xL, and mcl-1; whereas GC B cells express bcl-xL and mcl-1 but also the proapoptotic proteins bax-α and bad. Although maintaining constitutive levels of bcl-2 and mcl-1, FL cells are not protected from apoptosis when cultured in vitro. Their propensity to undergo apoptosis is temporally associated with downregulation of bcl-xL. More importantly, activation of FL cells via CD40 not only prevents downregulation but increases the level of bcl-xL expression and results in promotion of survival. These results support the hypothesis that the overexpression of bcl-2 is not the only antiapoptotic mechanism responsible for the pathogenesis of FL. Survival of FL cells is determined by a number of death-inhibiting proteins, among which bcl-xL appears to have the most critical role. Moreover, these findings are consistent with the hypothesis that, although FL cells are malignant, they respond to microenvironmental signals such as CD40L that appear to contribute to their survival through the upregulation of death-inhibiting proteins.

Early Onset of Immunoglobulin Heavy Chain Gene Rearrangements in Normal Human Bone Marrow CD34+ Cells

To characterize early B-cell precursors in humans, we correlated immunoglobulin heavy chain (IgH) gene rearrangement status with the CD34, CD19, and CD10 cell surface markers. Highly purified adult bone marrow (BM) cell fractions were obtained by two successive rounds of flow cytometric cell sorting, and IgH rearrangements were analyzed by polymerase chain reaction (PCR) amplification. Complete VDJH rearrangements were observed in the CD34+ CD19+ fraction, but not in the more immature CD34+ CD19− fraction. About one quarter of these rearrangements had an open reading frame, thus potentially permitting the synthesis of a μ chain. Partial DJH rearrangements were detected in both CD34+ CD19+ and CD34+ CD19− subsets, although they were less abundant in the latter. When triple labeling was used to better characterize the CD34+ CD19− population, DJH rearrangements were found to be present in the CD34+ CD10+ CD19− fraction, but not in the more primitive CD34+ CD10− CD19−. These results indicate that IgH gene rearrangements occur in CD34+ BM cells and that they initiate in immature progenitors expressing the CD10, but not yet the CD19 surface antigen. Finally, the presence of IgH gene rearrangements in CD34+ BM cells provides a useful marker of clonality to evaluate the possible involvement of these cells in various B-cell lymphoid malignancies.

A non-XLA primary deficiency causes the earliest known defect of B cell differentiation in humans: a comparison with an XLA case

We report a detailed comparison of B cell defects in two patients, one XLA and one non-XLA. Both had severe agammaglobulinemia with a total absence of CD19+ cells in the periphery. In the non-XLA case, CD19 expression was also highly impaired in the bone marrow, resulting in the absence of both B and preB compartments. Early proB cells were present since CD34+CD10+ and some CD19+CD10+ mostly CD34+ were identified, although diminished. By contrast, in the XLA patient the CD34+CD19+ proB cells were increased whereas the CD34-CD19+ preB cell population was low. Semi-quantitative RT-PCR analysis performed on mononuclear bone marrow cells from the non-XLA patient indicated that lambda-like, VpreB, Rag-1, Rag-2 and TdT transcripts expressed during proB cell stages were found at normal levels whereas E2A, CD10, Syk, Pax-5, CD19, Ig alpha, Ig beta, VH-C mu and V kappa-C kappa transcripts characteristic of later stages were severely depressed. By contrast in the XLA patient most of these transcripts were observed in normal amounts. The phenotype of the non-XLA patient resembles that of Pax-5 or Ig beta knock-out mice, but since the coding sequence of both cDNAs were shown to be normal, the blockage might rather result from an altered regulation of one of these genes or from defect of other genes. All these data indicate that the non-XLA patient suffers from a new genetic defect that results in an arrest of differentiation within the proB cell compartment, before the onset of Ig gene rearrangements. From all agammaglobulinemias reported so far, including XLA cases and those resulting from C mu gene defects, the non-XLA patient exhibits the earliest blockage in the B cell differentiation pathway.