Regulated expression of immunoglobulin trans-mRNA consisting of the variable region of a transgenic mu chain and constant regions of endogenous isotypes

In-cell generation of antibody single-chain Fv transcripts by targeted RNA trans-splicing

The humoral immune response propels the production of a diversified pool of antibodies with high affinity and selectivity for the eliciting antigen. Their isolation entails either B-cell cloning or the linking of authentic pairs of variable region genes encoding them. We hypothesized that targeted RNA trans-splicing (TS) inside the B-cell nucleus could be harnessed as a novel means to link both variable region genes and reconstitute genuine immune B-cell specificities. This could be accomplished by a special targeting gene harboring a peptide linker exon flanked by sequences capable of targeting both heavy (HC) and light chain (LC) transcripts. Following sequential trans-splicing reactions, the resulting RNA in each cell would encode the two variable regions, joined by the peptide linker. In this study, we examined genetic components and configurations required for the separate trans-splicing steps and for the combined two-step reactions. Using a model antibody, we show that in transiently transfected cells, we can target variable region exons through both their acceptor and donor splice sites, precisely joining an exon encoding a synthetic linker and the complementary variable region so as to form a single-chain Fv. We also demonstrate the accurate formation of single-chain Fv transcript as a result of trans-splicing of RNA synthesized from two chromosomal genes expressed by a stably transfected B-cell hybridoma. Our attempts to link the two variable region genes via a synthetic linker exon through sequential trans-splicing events were only successful with regard to both ends of the linker and to the 3' end of the light chain, but repeatedly resulted in a deletion at the 5' end of the joined heavy chain transcript. The implications of our findings on the potential application of trans-splicing for the isolation of useful antibodies are discussed.

IL-5 Induces IgG1 Isotype Switch Recombination in Mouse CD38-Activated sIgD-Positive B Lymphocytes

Mouse B cells express CD38, whose ligation by anti-CD38 Ab induces their proliferation and protection from apoptosis. We previously showed that stimulation of mouse splenic B cells with IL-5 together with CS/2, an anti-mouse CD38 mAb, induces production of IgG1 and IgM. Here we examined the role of IL-5 and CS/2 in the expression of germline γ1 transcripts and the generation of reciprocal products forming DNA circles as byproducts of μ-γ1 switch recombination. By itself, CS/2 induced significant expression of germline γ1 transcripts in splenic naive B cells, whereas IL-5 neither induced nor enhanced germline γ1 expression. Increased cellular content of reciprocal product, which is characteristic of μ-γ1 recombination, was not observed after culturing B cells with CS/2, but increased reciprocal product, along with high levels of lgG1 secretion, was found when B cells were cultured with CS/2 plus IL-5. Although IL-4 did not, by itself, induce μ-γ1 recombination in B cells stimulated with CS/2, in conjunction with CS/2 plus IL-5, IL-4 dramatically enhanced sterile γ1 transcription and IgG1 production. These results demonstrate that CD38 ligation induces only germline γ1 transcription and that IL-5 promotes both μ-γ1 switch recombination and lgG1 secretion in an IL-4-independent manner.

IgA class switch in I alpha exon-deficient mice. Role of germline transcription in class switch recombination

Studies have implicated defective Ig class switch in the pathogenesis of IgA deficiency. To understand better the molecular events that regulate IgA class switch, a 1.4-kb region of the IgA locus containing the I alpha exon was replaced with a human hypoxanthine phosphoribosyltransferase minigene by gene targeting in murine embryonic stem cells. The I alpha exon-deficient mice derived from these embryonic stem cells had normal IgA levels in serum and secretions and normal numbers of IgA B cells in Peyer's patches and spleen. Further, I alpha exon-deficient B cells efficiently underwent IgA class switch in vitro, despite the absence of I alpha exon-containing germline transcripts. Notably, I alpha exon-deficient B cells did not require TGF-beta for IgA class switch since stimulation with LPS alone led to IgA expression. Nonetheless, whereas I alpha exon-deficient B cells constitutively expressed human hypoxanthine phosphoribosyltransferase transcripts, they did not produce IgA in the absence of LPS stimulation. These results demonstrate that the I alpha exon or transcripts containing the I alpha exon are not required for IgA class switch. Further, the effects of TGF-beta on I alpha locus transcription can be supplanted by expression of a heterologous minigene at that locus, but a second signal is required for the induction of IgA class switch.

Alternatively spliced, germline J alpha 11-2-C alpha mRNAs are the predominant T cell receptor alpha transcripts in mouse kidney

We recently reported the expression of a truncated T cell receptor (TCR) alpha mRNA in kidney and brain of normal mice. In the kidney, the truncated TCR alpha transcript was expressed by bone marrow-dependent, non-T large interstitial cells located predominantly in the medulla. Here, we report the molecular characterization of the truncated TCR alpha transcript from kidney. Using a modified anchored-PCR (A-PCR) technique and directional cloning, 37 cDNA clones extending 5' of the C alpha region were generated. cDNA sequencing showed that 29 of the clones (78%) originated in the J alpha 11-2 region. Of these clones, 17 started upstream or in the J alpha 11-2 exon and contained the entire J alpha 11-2 sequence correctly spliced to the first C alpha exon. Analysis of the sequence revealed the presence of multiple stop codons in all three reading frames. The other 12 clones originated further upstream of the J alpha 11-2 exon and did not include the J alpha 11-2 exon, but rather arose from the joining of a cryptic splice donor signal to the usual TCR alpha C splice acceptor. This alternatively spliced transcript contained an open reading frame extending from the upstream J alpha 11-2 region to 82 nucleotides downstream of the beginning of the TCR C alpha region, and potentially encoded a 36 amino acid polypeptide. The remaining eight clones all contained the J alpha TA61 region correctly spliced to C alpha with two of these extending upstream of the J alpha TA61 exon. The predominance of J alpha 11-2-C alpha containing clones was confirmed by RNase protection assay using total RNA from kidney and spleen of scid mice. The 3' region of the transcript contained a fully conserved, correctly spliced TCR alpha C region which was polyadenylated at the 3' end. The truncated TCR alpha mRNA could be detected in preparations of cytoplasmic RNA, indicating that this transcript follows a normal RNA processing pathway. Our results demonstrate that the truncated TCR alpha mRNA expressed in normal mouse kidney is a germline J-C transcript resulting from transcription initiated predominantly upstream of the J alpha 11-2 region. This germline transcript in the kidney is undergoing alternative splicing leading to the appearance of an open reading frame coding for a short polypeptide. These results suggest that the product of this transcript may be functionally relevant.

DNA rearrangement can account for in vitro switching to IgG1

During immune responses, B lymphocytes may switch from the expression of immunoglobulin M (IgM) to the expression of another isotype (e.g., IgG, IgE, IgA). In stable hybridomas and myelomas expressing a "switched" (S) isotype, DNA deletions between S mu and a "downstream" S region (S region recombination) have been found. In primary B cells, studies of the molecular basis of switching have been limited by the ability to sensitively quantitate the amount of DNA deletion; such studies would be of interest because other nondeletional mechanisms (trans-splicing, alternative processing of a long transcript) have been proposed to account for isotype switching in certain circumstances. We have applied the digestion-circularization polymerase chain reaction (DC-PCR) technique to measure the amount of S region recombination that occurs in the course of class switching in primary B lymphocytes. Resting B cells were cultured in lipopolysaccharide (LPS) and interleukin 4 (IL-4) to stimulate switching to IgG1. These cells begin to express membrane IgG1 at day 2.5 of culture and reach maximum expression by day 4.5. DNA was prepared from cultured cells and analyzed for S mu-S gamma 1 rearrangement by DC-PCR. Chimeric switch regions, indicating S mu-S gamma 1 recombination, were detected in amounts that, in most cases, correlated with surface expression. Furthermore, when cells were sorted on the basis of surface IgG1 expression, a mean of at least one S mu-S gamma 1 rearrangement per cell was seen in five out of seven experiments. In general, the IgG1+ cells obtained at 4.5 and 5.5 d of culture had close to 2 S mu-S gamma 1 rearrangements per cell. In IgG1- cells, S mu-S gamma 1 rearrangements were detectable, but at frequencies substantially lower that in IgG1+ cells. Thus, these results indicate that DNA deletion accompanies class switching in normal B cells stimulated with LPS and IL-4.

Isolation of cDNA encoding a binding protein specific to 5'-phosphorylated single-stranded DNA with G-rich sequences

We have isolated the cDNA encoding a binding protein to the sequence motif of the immunoglobulin S mu region by the southwestern method. The binding protein designated S mu bp-2 specifically binds to 5'-phosphorylated single-stranded DNA containing 5'-G and GGGG stretches. The amino acid sequence deduced from the cDNA sequence showed that the S mu bp-2 belongs to the putative helicase superfamily which is involved in replication, recombination and repair. Expression of S mu bp-2 mRNA is ubiquitous and augmented in spleen cells stimulated with lipopolysaccharide and interleukin 4 which also induce class switching. The S mu bp-2 gene is conserved among vertebrates. Possible involvement of S mu bp-2 in class switching is discussed.

Biased distribution of recombination sites within S regions upon immunoglobulin class switch recombination induced by transforming growth factor beta and lipopolysaccharide

We have characterized extrachromosomal circular DNAs from adult mouse spleen cells that were induced to switch to immunoglobulin A (IgA) with bacterial lipopolysaccharide (LPS) and transforming growth factor beta (TGF-beta), and identified breakpoints of S mu/S gamma 3, S mu/S gamma 2, S mu/S alpha, S gamma 3/S alpha, and S gamma 2/S alpha recombinants. The S mu recombination donor sites clustered in the 3' half of the S mu region, while the S alpha recombination acceptor sites clustered in the 5' half of the S alpha region. In addition, donor and acceptor sites of S gamma regions also clustered in the 3' and 5' parts, respectively. These site preferences are in sharp contrast to the dispersed distribution of S mu/S gamma 1 breakpoints within both S mu and S gamma 1 regions upon IgG1 switch induced by LPS and interleukin 4. Our results support the hypotheses that TGF-beta increases the frequency of switch recombination events to IgA and that the switch recombination to IgA often proceeds by successive recombination of S mu/S gamma and S gamma/S alpha.