Antigen-stimulated dissociation of BCR mIg from Ig-alpha/Ig-beta: implications for receptor desensitization. (1/221)

B cell antigen receptor (BCR) ligation leads to receptor desensitization wherein BCR remain competent to bind antigen and yet fail to transduce signals. Desensitized BCR exhibit a defect at the most proximal level of signal transduction, consistent with failed transmission of signals through the receptor complex. We report that antigen stimulation leads to dissociation or destabilization of the BCR reflected by inability to coimmunoprecipitate Ig-alpha/Ig-beta with mIg. This destabilization is temporally correlated with desensitization and occurs in BCR containing mIgM and mIgD. Induction of BCR destabilization requires tyrosine kinase activation but is not induced by phosphatase inhibitors. BCR destabilization occurs at the cell surface and "dissociated" Ig-alpha/Ig-beta complexes remain responsive to anti-Ig-beta stimulation, suggesting that mIg-transducer uncoupling may mediate receptor desensitization.  (+info)

An alternatively spliced form of CD79b gene may account for altered B-cell receptor expression in B-chronic lymphocytic leukemia. (2/221)

Several functional anomalies of B-chronic lymphocytic leukemia (B-CLL) cells may be explained by abnormalities of the B-cell receptor (BCR), a multimeric complex formed by the sIg homodimer and the noncovalently bound heterodimer Igalpha/Igbeta (CD79a/CD79b). Because the expression of the extracellular Ig-like domain of CD79b has been reported to be absent in the cells of most CLL cases, we have investigated the molecular mechanisms that may account for this defect. Peripheral blood lymphocytes (PBL) from 50 patients and two cell lines (MEC1, MEC2) obtained from the PBL of one of them were studied. MEC1, MEC2, and 75% of CLL cases did not express detectable levels of the extracellular Ig-like domain of CD79b, which was nevertheless present in greater than 80% CD19(+) cells from normal donors. In healthy subjects the expression of CD79b was equally distributed in CD5(+) and CD5(-) B-cell subsets. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of CD79b RNA from all patients and from MEC1 and MEC2 cell lines consistently yielded two fragments of different size (709 bp and 397 bp). The 709-bp band corresponds to CD79b entire transcript; the 397-bp band corresponds to an alternatively spliced form lacking exon 3 that encodes the extracellular Ig-like domain. Both fragments were also visible in normal PBL. The expression of the 397-bp fragment was increased in normal activated B cells, while no difference was seen between CD5(+) and CD5(-) B cells. To obtain a more accurate estimate of the relative proportions of the two spliced forms, a radioactive PCR was performed in 13 normal and 22 B-CLL samples and the results analyzed using a digital imager. The mean value of the CD79b to the CD79b internally deleted ratio was 0.64 +/- 0.20 SD in normal donors and 0.44 +/- 0.27 SD in B-CLL (P =.01). Direct sequencing of 397-bp RT-PCR products and of genomic DNA corresponding to exon 3 from MEC1, MEC2, their parental cells, and five fresh B-CLL samples did not show any causal mutation. Single-strand conformation polymorphism analysis of exon 3 performed in 18 additional B-CLL cases showed a single abnormal shift corresponding to a TGT --> TGC polymorphic change at amino acid 122. We propose a role for the alternative splicing of CD79b gene in causing the reduced expression of BCR on the surface of B-CLL cells. As normal B cells also present this variant, the mechanism of CD79b posttranscriptional regulation might reflect the activation stage of the normal B cell from which B-CLL derives.  (+info)

Quantitative analysis of CD79b, CD5 and CD19 in mature B-cell lymphoproliferative disorders. (3/221)

BACKGROUND AND OBJECTIVE: Distinction between B-cell chronic leukemias can be difficult due to overlap in cell morphology and immunologic features. We investigated, by quantitative flow cytometry, the expression of CD79b, CD5 and CD19 in cells from a variety of B-cell disorders to see whether this analysis adds further information useful to the diagnosis and characterization of these diseases. DESIGN AND METHODS: Peripheral blood cells from 6 normal individuals were used as reference controls. The diseases of the 63 patients investigated comprised: 29 chronic lymphocytic leukemia (CLL), six of them with atypical morphology, 6 B-cell prolymphocytic leukemia (PLL), 12 splenic lymphoma with villous lymphocytes (SLVL) and 16 mantle-cell (Mc) lymphoma in leukemic phase. The study was carried out by triple immunostaining with directly conjugated monoclonal antibodies (MoAb) against CD79b, CD5 and CD19 and quantitative estimation of the antigens per cell assessed with standard microbeads (Quantum Simply Cellular). RESULTS: Compared to normal B-cells, the number of CD19 molecules was significantly lower in cells from all of the B-cell disorders except PLL. The intensity of CD5 in leukemic B-cells was significantly higher in CLL cells, including atypical cases, and Mc lymphoma than in normal B-cells, whilst PLL and SLVL had values similar to those of normal B-lymphocytes. CD79b was expressed at lower levels in all types of leukemic cells compared to normal B-lymphocytes but differences were statistically significant in CLL, Mc lymphoma and SLVL. The number of CD79b molecules per cell was significantly lower in typical CLL than in the remaining B-cell diseases whilst the comparison of CD5 and CD19 intensity between CLL and non-CLL samples failed to show any statistically significant difference. INTERPRETATION AND CONCLUSIONS: Distinct antigen density patterns for the various conditions emerged from this analysis: Typical CLL was characterized by moderate CD5 and weak or negative CD79b expression. Mc lymphoma showed an homogeneous pattern, characterized by similar expression of CD5 than CLL but significantly stronger expression of CD79b whilst PLL and SLVL had weak CD5 and moderate CD79b expression. Atypical CLL had an intermediate pattern of CD79b antigen expression ranging from weak to moderate with bright CD5. Unlike CD5 and CD79b, CD19 did not discriminate the various B-cell disorders but only between normal and leukemic cells.  (+info)

Ig alpha and Ig beta are required for efficient trafficking to late endosomes and to enhance antigen presentation. (4/221)

The B cell Ag receptor (BCR) is a multimeric complex, containing Ig alpha and Ig beta, capable of internalizing and delivering specific Ags to specialized late endosomes, where they are processed into peptides for loading onto MHC class II molecules. By this mechanism, the presentation of receptor-selected epitopes to T cells is enhanced by several orders of magnitude. Previously, it has been reported that, under some circumstances, either Ig alpha or Ig beta can facilitate the presentation of Ags. However, we now demonstrate that if these Ags are at low concentrations and temporally restricted, both Ig alpha and Ig beta are required. When compared with the BCR, chimeric complexes containing either chain alone were internalized but failed to access the MHC class II-enriched compartment (MIIC) or induce the aggregation and fusion of its constituent vesicles. Furthermore, Ig alpha/Ig beta complexes in which the immunoreceptor tyrosine-based activation motif tyrosines of Ig alpha were mutated were also incapable of accessing the MIIC or of facilitating the presentation of Ag. These data indicate that both Ig alpha and Ig beta contribute signaling, and possibly other functions, to the BCR that are necessary and sufficient to reconstitute the trafficking and Ag-processing enhancing capacities of the intact receptor complex.  (+info)

Modifications of Igalpha and Igbeta expression as a function of B lineage differentiation. (5/221)

Transcription of the mb1 and B29 genes is initiated when lymphoid progenitors enter the B cell differentiation pathway, and their transmembrane Igalpha and Igbeta products constitute essential signaling components of pre-B and B cell antigen receptors. We analyzed Igalpha/Igbeta biosynthesis, heterogeneity, and molecular interactions as a function of human B lineage differentiation in cell lines representative of the pro-B, pre-B, and B cell stages. All B lineage representatives produced a 36-kDa Igbeta form and three principal Igalpha forms, transient 33/40-kDa species and a mature 44-kDa glycoprotein. Deglycosylation revealed a major Igalpha core protein of 25 kDa and a minor 21-kDa Igalpha protein, apparently the product of an alternatively spliced mRNA. In pro-B cells, the Igalpha and Igbeta molecules existed primarily in separate unassembled pools, exhibited an immature glycosylation pattern, did not associate with surrogate light chain proteins, and were retained intracellularly. Their unanticipated association with the Lyn protein-tyrosine kinase nevertheless suggests functional potential for the Igalpha/Igbeta molecules in pro-B cells. Greater heterogeneity of the Igalpha and Igbeta molecules in pre-B and B cell lines was attributable to increased glycosylation complexity. Finally, the Igalpha/Igbeta heterodimers associated with fully assembled IgM molecules as a terminal event in B cell receptor assembly.  (+info)

Mutations in Igalpha (CD79a) result in a complete block in B-cell development. (6/221)

Mutations in Btk, mu heavy chain, or the surrogate light chain account for 85-90% of patients with early onset hypogammaglobulinemia and absent B cells. The nature of the defect in the remaining patients is unknown. We screened 25 such patients for mutations in genes encoding components of the pre-B-cell receptor (pre-BCR) complex. A 2-year-old girl was found to have a homozygous splice defect in Igalpha, a transmembrane protein that forms part of the Igalpha/Igbeta signal-transduction module of the pre-BCR. Studies in mice suggest that the Igbeta component of the pre-BCR influences V-DJ rearrangement before cell-surface expression of mu heavy chain. To determine whether Igalpha plays a similar role, we compared B-cell development in an Igalpha-deficient patient with that seen in a mu heavy chain-deficient patient. By immunofluorescence, both patients had a complete block in B-cell development at the pro-B to pre-B transition; both patients also had an equivalent number and diversity of rearranged V-DJ sequences. These results indicate that mutations in Igalpha can be a cause of agammaglobulinemia. Furthermore, they suggest that Igalpha does not play a critical role in B-cell development until it is expressed, along with mu heavy chain, as part of the pre-BCR.  (+info)

Analysis of the B-cell receptor B29 (CD79b) gene in familial chronic lymphocytic leukemia. (7/221)

The B-cell antigen receptor (BCR) comprises membrane Igs (mIgs) and a heterodimer of Igalpha (CD79a) and Igbeta (CD79b) transmembrane proteins, encoded by the mb-1 and B29 genes, respectively. These accessory proteins are required for surface expression of mIg and BCR signaling. B cells from chronic lymphocytic leukemia (B-CLL) frequently express low to undetectable surface Ig, as well as CD79b protein. Recent work described genetic aberrations affecting B29 expression and/or function in B-CLL. Because the prevalence of CLL is increased among first degree relatives, we analyzed the B29 gene in 10 families including 2 affected members each. A few silent or replacement mutations were observed at the genomic level, which never lead to truncated CD79b protein. Both members of the same family did not harbor the same mutations. However, a single silent base change in the B29 extracellular domain, corresponding to a polymorphism, was detected on 1 allele of most patients. These results indicate that the few mutations observed in the B29 gene in these patients do not induce structural abnormalities of the CD79b protein and thus do not account for its low surface expression in B-CLL. Furthermore, genetic factors were not implicated, because identical mutations were not observed among 2 members of the same family.  (+info)

A negative regulatory role for Ig-alpha during B cell development. (8/221)

The development of B cells requires the expression of an antigen receptor at distinct points during maturation. The Ig-alpha/beta heterodimer signals for these receptors, and mice harboring a truncation of the Ig-alpha intracellular domain (mb-1(delta(c)/delta(c)) have severely reduced peripheral B cell numbers. Here we report that immature mb-1(delta(c)/delta(c) B cells are activated despite lacking a critical Ig-alpha-positive signaling motif. As a consequence of abnormal activation, transitional immature IgMhighIgDlow B cells are largely absent in mb-1delta(c)/delta(c) mutants, accounting for the paucity of mature B cells. Thus, Ig-alpha cytoplasmic tail truncation yields an antigen receptor complex on immature B cells that signals constitutively. These data illustrate a role for Ig-alpha in negatively regulating antigen receptor signaling during B cell development.  (+info)

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