Differential mRNA stabilities affect mRNA levels in mutant mouse myeloma cells

RNA Splicing in the Transition from B Cells to Antibody-Secreting Cells: The Influences of ELL2, Small Nuclear RNA, and Endoplasmic Reticulum Stress

In the transition from B cells to Ab-secreting cells (ASCs) many genes are induced, such as ELL2, Irf4, Prdm1, Xbp1, whereas other mRNAs do not change in abundance. Nonetheless, using splicing array technology and mouse splenic B cells plus or minus LPS, we found that induced and "uninduced" genes can show large differences in splicing patterns between the cell stages, which could influence ASC development. We found that ∼55% of these splicing changes depend on ELL2, a transcription elongation factor that influences expression levels and splicing patterns of ASC signature genes, genes in the cell-cycle and N-glycan biosynthesis and processing pathways, and the secretory versus membrane forms of the IgH mRNA. Some of these changes occur when ELL2 binds directly to the genes encoding those mRNAs, whereas some of the changes are indirect. To attempt to account for the changes that occur in RNA splicing before or without ELL2 induction, we examined the amount of the small nuclear RNA molecules and found that they were significantly decreased within 18 h of LPS stimulation and stayed low until 72 h. Correlating with this, at 18 h after LPS, endoplasmic reticulum stress and Ire1 phosphorylation are induced. Inhibiting the regulated Ire1-dependent mRNA decay with 4u8C correlates with the reduction in small nuclear RNA and changes in the normal splicing patterns at 18 h. Thus, we conclude that the RNA splicing patterns in ASCs are shaped early by endoplasmic reticulum stress and Ire1 phosphorylation and later by ELL2 induction.

A Lentiviral Vector Allowing Physiologically Regulated Membrane-anchored and Secreted Antibody Expression Depending on B-cell Maturation Status

The development of lentiviral vectors (LVs) for expression of a specific antibody can be achieved through the transduction of mature B-cells. This approach would provide a versatile tool for active immunotherapy strategies for infectious diseases or cancer, as well as for protein engineering. Here, we created a lentiviral expression system mimicking the natural production of these two distinct immunoglobulin isoforms. We designed a LV (FAM2-LV) expressing an anti-HCV-E2 surface glycoprotein antibody (AR3A) as a membrane-anchored Ig form or a soluble Ig form, depending on the B-cell maturation status. FAM2-LV induced high-level and functional membrane expression of the transgenic antibody in a nonsecretory B-cell line. In contrast, a plasma cell (PC) line transduced with FAM2-LV preferentially produced the secreted transgenic antibody. Similar results were obtained with primary B-cells transduced ex vivo. Most importantly, FAM2-LV transduced primary B-cells efficiently differentiated into PCs, which secreted the neutralizing anti-HCV E2 antibody upon adoptive transfer into immunodeficient NSG (NOD/SCIDγc(-/-)) recipient mice. Altogether, these results demonstrate that the conditional FAM2-LV allows preferential expression of the membrane-anchored form of an antiviral neutralizing antibody in B-cells and permits secretion of a soluble antibody following B-cell maturation into PCs in vivo.

Transcription elongation factor ELL2 drives Ig secretory-specific mRNA production and the unfolded protein response

Differentiation of B cells into Ab-secreting cells induces changes in gene transcription, IgH RNA processing, the unfolded protein response (UPR), and cell architecture. The transcription elongation factor eleven nineteen lysine-rich leukemia gene (ELL2) stimulates the processing of the secreted form of the IgH mRNA from the H chain gene. Mice (mus musculus) with the ELL2 gene floxed in either exon 1 or exon 3 were constructed and crossed to CD19-driven cre/CD19(+). The B cell-specific ELL2 conditional knockouts (cKOs; ell2(loxp/loxp) CD19(cre/+)) exhibit curtailed humoral responses both in 4-hydroxy-3-nitrophenyl acetyl-Ficoll and in 4-hydroxy-3-nitrophenyl acetyl-keyhole limpet hemocyanin immunized animals; recall responses were also diminished. The number of immature and recirculating B cells in the bone marrow is increased in the cKOs, whereas plasma cells in spleen are reduced relative to control animals. There are fewer IgG1 Ab-producing cells in the bone marrow of cKOs. LPS ex vivo-stimulated B220(lo)CD138(+) cells from ELL2-deficient mouse spleens are 4-fold less abundant than from control splenic B cells; have a paucity of secreted IgH; and have distended, abnormal-appearing endoplasmic reticulum. IRE1α is efficiently phosphorylated, but the amounts of Ig κ, ATF6, BiP, Cyclin B2, OcaB (BOB1, Pou2af1), and XBP1 mRNAs, unspliced and spliced, are severely reduced in ELL2-deficient cells. ELL2 enhances the expression of BCMA (also known as Tnfrsf17), which is important for long-term survival. Transcription yields from the cyclin B2 and the canonical UPR promoter elements are upregulated by ELL2 cDNA. Thus, ELL2 is important for many aspects of Ab secretion, XBP1 expression, and the UPR.

Engineered cell surface expression of membrane immunoglobulin as a means to identify monoclonal antibody-secreting hybridomas

Monoclonal antibodies (mAbs) have proven to be effective biological reagents in the form of therapeutic drugs and diagnostics for many pathologies, as well as valuable research tools. Existing methods for isolating mAb-producing hybridomas are tedious and time consuming. Herein we describe a novel system in which mAb-secreting hybridoma cells were induced to co-express significant amounts of the membrane form of the secreted immunoglobulin (Ig) on their surfaces and are efficiently recovered by fluorescent activated cell sorting (FACS). Fusion of a novel myeloma parent, SP2ab, expressing transgenic Igalpha and Igbeta of the B-cell receptor complex (BCR) with spleen cells resulted in hybridomas demonstrating order of magnitude increases in BCR surface expression. Surface Ig levels correlated with transgenic Igalpha expression, and these cells also secreted normal levels of mAb. Hundreds of hybridoma lines producing mAbs specific for a variety of antigens were rapidly isolated as single cell-derived clones after FACS. Significant improvements using the Direct Selection of Hybridomas (DiSH) by FACS include reduced time and labor, improved capability of isolating positive hybridomas, and the ease of manipulating cloned cell lines relative to previously existing approaches that require Limiting Dilution Subcloning (LDS).

Expression of the thyroid hormone receptor gene, erbAalpha, in B lymphocytes: alternative mRNA processing is independent of differentiation but correlates with antisense RNA levels

The erbAalpha gene encodes two alpha-thyroid hormone receptor isoforms, TRalpha1 and TRalpha2, which arise from alternatively processed mRNAs, erbAalpha1 (alpha1) and erb alpha2 (alpha2). The splicing and alternative polyadenylation patterns of these mRNAs resemble that of mRNAs encoding different forms of immunoglobulin heavy chains, which are regulated at the level of alternative processing during B cell differentiation. This study examines the levels of erbAalpha mRNA in eight B cell lines representing four stages of differentiation in order to determine whether regulation of the alternatively processed alpha1 and alpha2 mRNAs parallels the processing of immunoglobulin heavy chain mRNAs. Results show that the pattern of alpha1 and alpha2 mRNA expression is clearly different from that observed for immunoglobulin heavy chain mRNAs. B cell lines display characteristic ratios of alpha1/alpha2 mRNA at distinct stages of differentiation. Furthermore, expression of an overlapping gene, Rev-ErbAalpha (RevErb), was found to correlate strongly with an increase in the ratio of alpha1/alpha2 mRNA. These results suggest that alternative processing of erbAalpha mRNAs is regulated by a mechanism which is distinct from that regulating immunoglobulin mRNA. The correlation between RevErb and erbAalpha mRNA is consistent with negative regulation of alpha2 via antisense interactions with the complementary RevErb mRNA.

Alternative poly(A) site selection in complex transcription units: means to an end?

Many genes have been described and characterized which result in alternative polyadenylation site use at the 3'-end of their mRNAs based on the cellular environment. In this survey and summary article 95 genes are discussed in which alternative polyadenylation is a consequence of tandem arrays of poly(A) signals within a single 3'-untranslated region. An additional 31 genes are described in which polyadenylation at a promoter-proximal site competes with a splicing reaction to influence expression of multiple mRNAs. Some have a composite internal/terminal exon which can be differentially processed. Others contain alternative 3'-terminal exons, the first of which can be skipped in some cells. In some cases the mRNAs formed from these three classes of genes are differentially processed from the primary transcript during the cell cycle or in a tissue-specific or developmentally specific pattern. Immunoglobulin heavy chain genes have composite exons; regulated production of two different Ig mRNAs has been shown to involve B cell stage-specific changes in trans -acting factors involved in formation of the active polyadenylation complex. Changes in the activity of some of these same factors occur during viral infection and take-over of the cellular machinery, suggesting the potential applicability of at least some aspects of the Ig model. The differential expression of a number of genes that undergo alternative poly(A) site choice or polyadenylation/splicing competition could be regulated at the level of amounts and activities of either generic or tissue-specific polyadenylation factors and/or splicing factors.

B-lineage regulated polyadenylation occurs on weak poly(A) sites regardless of sequence composition at the cleavage and downstream regions

Early/memory and plasma B-cell lines and fibroblasts were analyzed for their ability to use a 5' proximal (variant) versus a 3' distal (constant) poly(A) site, in the absence of a competing splice, from a set of related constructs. The proximal:distal poly(A) site use (P:D ratio) of the resulting cytoplasmic poly(A)+ mRNA is a measure of poly(A) site strength. In this context the immunoglobulin gamma2b secretory-specific poly(A) site showed a P:D ratio of 1:1 in plasma cells, 0.43:1 in early/memory B-cells and an intermediate value in fibroblasts. Meanwhile, a construct with a proximal SV40 early-like poly(A) site produced mRNA with a P:D ratio of >>50:1 in all cell types. Alterations in the region downstream of the proximal poly(A) addition site and at the site itself resulted in changes in the P:D ratio. However, these poly(A) sites, all with a P:D ratio of < or = 5:1, were used most efficiently in plasma cells. Constructs totally devoid of immunoglobulin sequences, but containing heterologous poly(A) sites producing mRNA with P:D ratios of < or = 5:1, were also used more efficiently in plasma cells. We therefore conclude that weak poly(A) sites, regardless of sequence composition, are used more efficiently in plasma cells than in the other cell types.

Altered expression of the retinoblastoma susceptibility gene in chronic lymphocytic leukaemia

The pathogenesis of chronic lymphocytic leukaemia (CLL) is unknown. One of the most frequent cytogenetic abnormalities in CLL is a deletion within the long arm of chromosome 13, the region to which the retinoblastoma (Rb) gene has been mapped. Lack of Rb expression has been linked to the carcinogenic process in many human tumours. We therefore sought to investigate the role of Rb gene inactivation in CLL using differential polymerase chain reaction on reverse transcribed RNA. The result of the PCR was quantitated using HPLC. 5/39 patients revealed a lack or significantly impaired expression of the Rb gene upon differential PCR analysis. In addition, immunocytochemical studies were performed using the Rb-specific monoclonal antibody PMG245. 10/56 patients showed a weak or absent expression upon immunocytochemical analysis compared to monocytes or granulocytes. The samples lacking Rb were from both early and late stage CLL. Our results indicate that inactivation of the Rb protein occurs in a fraction of CLL cases and can be found in early and late stages of the disease.