B-Cell Antigen Receptor–Induced Apoptosis Requires Both Igα and Igβ

Therapeutic targeting of the BCR-associated protein CD79b in a TCR-based approach is hampered by aberrant expression of CD79b

B-cell malignancies were efficiently recognized by T cells expressing high-affinity alloHLA-restricted TCRs specific for CD79b. Aberrant expression of CD79b in non–B cells caused unwanted reactivity, rendering CD79b unsuitable for TCR-based immunotherapies.

Rotavirus infection alters peripheral T-cell homeostasis in children with acute diarrhea

The patterns of gene expression and the phenotypes of lymphocytes in peripheral blood mononuclear cells (PBMC) from children with diarrhea caused by rotavirus and healthy children were compared by using DNA microarray, quantitative PCR, and flow cytometry. We observed increased expression of a number of genes encoding proinflammatory cytokines and interferon or interferon-stimulated proteins and demonstrated activation of some genes involved in the differentiation, maturation, activation, and survival of B lymphocytes in PBMC of patients with rotavirus infection. In contrast, we observed a consistent pattern of lower mRNA levels for an array of genes involved in the various stages of T-cell development and demonstrated a reduction in total lymphocyte populations and in the proportions of CD4 and CD8 T lymphocytes from PBMC of patients. This decreased frequency of T lymphocytes was transient, since the proportions of T lymphocytes recovered to almost normal levels in convalescent-phase PBMC from most patients. Finally, rotavirus infection induced the activation and expression of the early activation markers CD83 and CD69 on a fraction of CD19 B cells and the remaining CD4 and CD8 T lymphocytes in acute-phase PBMC of patients; the expression of CD83 continued to be elevated and was predominantly exhibited on CD4 T lymphocytes in convalescent-phase PBMC. On the basis of these findings at the molecular, phenotypic, and physiologic levels in acute-phase PBMC, we conclude that rotavirus infection induces robust proinflammatory and antiviral responses and B-cell activation but alters peripheral T-cell homeostasis in children.

IgA Fc receptor I signals apoptosis through the FcRγ ITAM and affects tumor growth

The IgA Fc receptor (FcαRI) has dual proinflammatory and anti-inflammatory functions that are transmitted through the immunoreceptor tyrosine-based activation motifs (ITAMs) of the associated FcRγ subunit. Whereas the involvement of FcαRI in inflammation is well documented, little is known of its anti-inflammatory mechanisms. Here we show that monomeric targeting of FcαRI by anti-FcαRI Fab or serum IgA triggers apoptosis in human monocytes, monocytic cell lines, and FcαRI+ transfectants. However, the physiologic ligand IgA induced apoptosis only when cells were cultured in low serum conditions, indicating differences with induction of anti-inflammatory signaling. Apoptosis signaling required the FcRγ ITAM, as cells transfected with FcαRI or with a chimeric FcαRI-FcRγ responded to death-activating signals, whereas cells expressing a mutated FcαRIR209L unable to associate with FcRγ, or an ITAM-mutated chimeric FcαRI-FcRγ, did not respond. FcαRI-mediated apoptosis signals were blocked by treatment with the pan-caspase inhibitor zVAD-fmk, involved proteolysis of procaspase-3, and correlated negatively with SHP-1 concentration. Anti-FcαRI Fab treatment of nude mice injected subcutaneously with FcαRI+ mast-cell transfectants prevented tumor development and halted the growth of established tumors. These findings demonstrate that, on monomeric targeting, FcαRI functions as an FcRγ ITAM-dependent apoptotic module that may be fundamental for controlling inflammation and tumor growth.

Epigenetic processes play a major role in B-cell-specific gene silencing in classical Hodgkin lymphoma

Many B-lineage-specific genes are down-regulated in Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (cHL). We investigated the involvement of epigenetic modifications in gene silencing in cHL cell lines and in microdissected primary HRS cells. We assessed the expression and methylation status of CD19, CD20, CD79B, SYK, PU.1, BOB.1/OBF.1, BCMA, and LCK, all of which are typically down-regulated in cHL. We could reactivate gene expression in cHL cell lines with the DNA demethylating agent 5-aza-deoxycytidine (5-aza-dC). Using methylation-specific polymerase chain reaction (MSP), bisulfite genomic sequencing, and digestion with methylation-sensitive endonuclease followed by polymerase chain reaction (PCR), we determined the methylation status of promoter regions of PU.1, BOB.1/OBF.1, CD19, SYK, and CD79B. Down-regulation of transcription typically correlated with hypermethylation. Using bisulfite genomic sequencing we found that in microdissected HRS cells of primary cHL SYK, BOB.1/OBF.1, and CD79B promoters were also hypermethylated. Ectopic expression of both Oct2 and PU.1 in a cHL cell line potentiated endogenous PU.1 and SYK expression after 5-aza-dC treatment. These observations indicate that silencing of the B-cell-specific genes in cHL may be the consequence of a compromised regulatory network where down-regulation of a few master transcription factors results in silencing of numerous genes.

The alternative transcript of CD79b is overexpressed in B-CLL and inhibits signaling for apoptosis

The B-cell receptor (BCR) for antigen is composed of surface immunoglobulin (sIg), which provides antigen specificity, and a noncovalently associated signaling unit, the CD79a/b heterodimer. Defects in CD79 can influence both BCR expression and signaling and may explain why cells from certain malignancies, such as B-chronic lymphocytic leukemia (B-CLL), often express diminished and inactive BCR. Recently, an alternative transcript of CD79b (DeltaCD79b) has been reported that is up-regulated in B-CLL and may explain this diminished BCR expression. Here we assess the expression of DeltaCD79b in B-CLL and other lymphoid malignancies and investigate its function. High relative expression of DeltaCD79b was confirmed in most cases of B-CLL and found in 6 of 6 cases of splenic lymphomas with villous lymphocytes (SLVLs) and hairy cell leukemia. In a range of Burkitt lymphoma cell lines, expression of DeltaCD79b was relatively low but correlated inversely with the ability of the BCR to signal apoptosis when cross-linked by antibody (Ab). Interestingly, when Ramos-EHRB cells, which express low DeltaCD79b, were transfected with this transcript, they were transformed from being sensitive to anti-Fcmu-induced apoptosis to being highly resistant. Although DeltaCD79b was expressed as protein, its overexpression did not reduce the level of cell surface BCR. Finally, we showed that the inhibitory activity of DeltaCD79b depended on an intact leader sequence to ensure endoplasmic reticulum (ER) trafficking and a functional signaling immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic tail. These results point to DeltaCD79b being a powerful modulator of BCR signaling that may play an important role in normal and malignant B cells.

The CGM1a (CEACAM3/CD66d)-mediated phagocytic pathway of Neisseria gonorrhoeae expressing opacity proteins is also the pathway to cell death

Phagocytosis of Opa+ Neisseria gonorrhoeae (gonococcus, GC) by neutrophils is in part dependent on the interaction of Opa proteins with CGM1a (CEACAM3/CD66d) antigens, a neutrophil-specific receptor. However, the signaling pathways leading to phagocytosis have not been characterized. Here we show that interaction of OpaI bacteria with neutrophils or CGM1a-transfected DT40 cells induces calcium flux, which correlates with phagocytosis of bacteria. We identified an immunoreceptor tyrosine-based activation motif (ITAM) in CGM1a, and showed that the ability of CGM1a to transduce signals and mediate phagocytosis was abolished by mutation of the ITAM tyrosines. We also demonstrated that CGM1a-ITAM-mediated bacterial phagocytosis is dependent on Syk and phospholipase C activity in DT40 cells. Unexpectedly, the activation of the CGM1a-ITAM phagocytic pathway by Opa+ GC results in induction of cell death.

Igα and Igβ Are Required for Efficient Trafficking to Late Endosomes and to Enhance Antigen Presentation

The B cell Ag receptor (BCR) is a multimeric complex, containing Igα and Igβ, 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 Igα or Igβ can facilitate the presentation of Ags. However, we now demonstrate that if these Ags are at low concentrations and temporally restricted, both Igα and Igβ 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, Igα/Igβ complexes in which the immunoreceptor tyrosine-based activation motif tyrosines of Igα were mutated were also incapable of accessing the MIIC or of facilitating the presentation of Ag. These data indicate that both Igα and Igβ 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.

An Alternatively Spliced Form of CD79b Gene May Account for Altered B-Cell Receptor Expression in B-Chronic Lymphocytic Leukemia

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 Ig/Igβ (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.

Aberrations of the B-Cell Receptor B29 (CD79b) Gene in Chronic Lymphocytic Leukemia

Leukemic B cells in chronic lymphocytic leukemia (B-CLL) typically exhibit low or undetectable surface Ig. Because the B29 (CD79b and Igβ) and mb-1 (CD79a and Igα) gene products are required for surface Ig display in the B-cell receptor complex (BCR), we analyzed the expression of these genes in B-CLL cells. The majority (83%) of the randomly selected B-CLL patient samples analyzed exhibited low or undetectable surface BCR measured by μ heavy chain and B29 expression. Levels of mb-1 mRNA in these B-CLL samples with low surface BCR were similar to those in normal B cells. Among those with decreased surface expression, B29 mRNA was not detected in half of these B-CLL samples. The remaining B-CLL samples with diminished surface BCR contained normal levels of B29 mRNA. Further analysis of cDNA clones from the majority of these latter samples contained point mutations, insertions, or deletions that were largely located in the B29 transmembrane and cytoplasmic domains. These results indicate the occurrence of somatic mutations predicted to affect B29 expression and/or function in the majority of B-CLL and suggest that these aberrations underlie the diminished surface BCR display and loss of BCR signaling characteristic of this leukemia.