Membrane isoforms of human immunoglobulins of the A1 and A2 isotypes: structural and functional study

Unique repetitive nucleic acid structures mirror switch regions in the human IgH locus

Immunoglobulin (Ig) genes carry the unique ability to be reshaped in peripheral B lymphocytes after these cells encounter a specific antigen. B cells can then further improve their affinity, acquire new functions as memory cells and eventually end up as antibody-secreting cells. Ig class switching is an important change that occurs in this context, thanks to local DNA lesions initiated by the enzyme activation-induced deaminase (AID). Several cis-acting elements of the Ig heavy (IgH) chain locus make it accessible to the AID-mediated lesions that promote class switch recombination (CSR). DNA repeats, with a non-template strand rich in G-quadruplexes (G4)-DNA, are prominent cis-targets of AID and define the so-called "switch" (S) regions specifically targeted for CSR. By analyzing the structure of the human IgH locus, we uncover that abundant DNA repeats, some with a putative G4-rich template strand, are additionally present in downstream portions of the IgH coding genes. These like-S (LS) regions stand as 3' mirror-images of S regions and also show analogies to some previously reported repeats associated with the IgH locus 3' super-enhancer. A regulatory role of LS repeats is strongly suggested by their specific localization close to exons encoding the membrane form of Ig molecules, and by their conservation during mammalian evolution.

AID-induced remodeling of immunoglobulin genes and B cell fate

Survival and phenotype of normal and malignant B lymphocytes are critically dependent on constitutive signals by the B cell receptor (BCR) for antigen. In addition, either antigen ligation of the BCR or various mitogenic stimuli result in B cell activation and induction of activation-induced deaminase (AID). AID activity can in turn mediate somatic hypermutation (SHM) of immunoglobulin (Ig) V regions and also deeply remodel the Ig heavy chain locus through class switch recombination (CSR) or locus suicide recombination (LSR). In addition to changes linked to affinity for antigen, modifying the class/isotype (i.e. the structure and function) of the BCR or suddenly deleting BCR expression also modulates the fate of antigen-experienced B cells.

Deletion of the α immunoglobulin chain membrane-anchoring region reduces but does not abolish IgA secretion

Class switching and plasma cell differentiation occur at a high level within all mucosa-associated lymphoid tissues. The different classes of membrane immunoglobulin heavy chains are associated with the Igα/Igβ heterodimer within the B-cell receptor (BCR). Whether BCR isotypes convey specific signals adapted to the corresponding differentiation stages remains debated but IgG and IgA membranes have been suggested to promote plasma cell differentiation. We investigated the impact of blocking expression of the IgA-class BCR through a 'αΔtail' targeted mutation, deleting the Cα immunoglobulin gene membrane exon. This allowed us to evaluate to what extent class switching and plasma cell differentiation can be concurrent processes, allowing some αΔtail(+/+) B cells with an IgM BCR to directly differentiate into IgA plasma cells and yield serum secreted IgA in spite of the absence of membrane IgA(+) B lymphocytes. By contrast, in secretions the secretory IgA was very low, indicating that J-chain-positive plasma cells producing secretory IgA overwhelmingly differentiate from previously class-switched membrane IgA(+) memory B cells. In addition, although mucosa-associated lymphoid tissues are a major site for plasma cell accumulation, αΔtail(+/+) mice showed that the gut B-cell lineage homeostasis is not polarized toward plasma cell differentiation through a specific influence of the membrane IgA BCR.

Lipid rafts hinder binding of antibodies to the extracellular segment of the membrane-anchor peptide of mIgA

Membrane-bound IgA (mIgA) is associated with Igα/Igβ as the B cell receptor (BCR) complex on mIgA-expressing B cells. The α chain of mIgA (mα) contains a C-terminal membrane-anchor peptide, which encompasses extracellular, transmembrane and intracellular segments. The extracellular segment, referred to as the mIg isotype-specific (migis-α) segment or the extracellular membrane proximal domain of mα, has been proposed to be a specific antigenic site suitable for isotype-specific targeting of mIgA-expressing B cells by antibodies. In this study, we developed several anti-migis-α monoclonal antibodies (mAbs), such as mAb 29C11, specific to a segment towards the N-terminus of the 26 amino acid long migis-α. The mAbs bound strongly to synthetic peptides of migis-α and to various recombinant proteins containing migis-α as revealed by ELISA. On B cells, however, flow cytometric analysis suggested that these mAbs did not bind strongly to mIgA. After lipid rafts of B cells were disrupted by cholesterol extraction, the mAbs were able to bind strongly to the treated B cells. Moreover, immunoprecipitation analysis of these mAbs indicated that mIgA could only be pulled down by the mAbs when mIgA-expressing B cells were solubilized by strong detergents, such as sodium dodecyl sulfate (SDS), or when lipid rafts were disrupted. Together, these results suggest that the migis-α region of mIgA in the BCR is associated with lipid rafts, which hinder binding of migis-α-specific antibodies to mIgA on the cell surface. Further studies are in progress to evaluate the suitability of 29C11 or its affinity-improved variants for targeting mIgA-expressing B cells.

Genetic variations in the C epsilon mX domain of human membrane-bound IgE

The epsilon chain of membrane-bound IgE (mIgE) is expressed predominantly as a "long" isoform, containing an extra segment of 52 amino acid (a.a.) residues, referred to as C epsilon mX, between the CH4 domain and the C-terminal membrane-anchoring transmembrane peptide. C epsilon mX results from an alternative splicing of the epsilon RNA transcript at 156-bp upstream of the splicing acceptor site used by the "short" isoform. Here, based on an analysis of the C epsilon mX genomic DNA sequences of 320 subjects residing in Taiwan, we report that single-nucleotide polymorphisms have been found at two positions, namely, G/T at #46 and A/G at #93 (along the 156 bp of C epsilon mX), with the former creating an amino acid change from Val to Leu at #16 (along the 52 a.a. of C epsilon mX) and the latter resulting in no change (Gly). Among the 640 C epsilon mX sequences identified, the previously known 46G93A allelic form appeared 293 times, the newly discovered 46T93A allelic form (GeneBank accession no. GU208817) 26 times, and the 46G93G allelic form (GU208818) 321 times. No 46T93G allelic form was found. Serum IgE measurements showed that the polymorphisms did not correlate with the levels of serum IgE. The anti-C epsilon mX monoclonal antibody, 4B12, could bind equally well to mIgE.Fc(L)(16V) and mIgE.Fc(L)(16L). While genetic variation of C epsilon mX of broader populations should also be investigated, these newly discovered genetic variants of C epsilon mX in the Taiwanese population do not seem to affect the feasibility of using an anti-C epsilon mX mAb, such as 4B12, to target mIgE-expressing B cells.

Premature replacement of mu with alpha immunoglobulin chains impairs lymphopoiesis and mucosal homing but promotes plasma cell maturation

Sequentially along B cell differentiation, the different classes of membrane Ig heavy chains associate with the Ig alpha/Ig beta heterodimer within the B cell receptor (BCR). Whether each Ig class conveys specific signals adapted to the corresponding differentiation stage remains debated. We investigated the impact of the forced expression of an IgA-class receptor throughout murine B cell differentiation by knocking in the human C alpha Ig gene in place of the S mu region. Despite expression of a functional BCR, homozygous mutant mice showed a partial developmental blockade at the pro-B/pre-BI and large pre-BII cell stages, with decreased numbers of small pre-BII cells. Beyond this stage, peripheral B cell compartments of reduced size developed and allowed specific antibody responses, whereas mature cells showed constitutive activation and a strong commitment to plasma cell differentiation. Secreted IgA correctly assembled into polymers, associated with the murine J chain, and was transported into secretions. In heterozygous mutants, cells expressing the IgA allele competed poorly with those expressing IgM from the wild-type allele and were almost undetectable among peripheral B lymphocytes, notably in gut-associated lymphoid tissues. Our data indicate that the IgM BCR is more efficient in driving early B cell education and in mucosal site targeting, whereas the IgA BCR appears particularly suited to promoting activation and differentiation of effector plasma cells.

Alleles and isoforms of human membrane-bound IgA1

In humans, IgA exists as two subclasses, IgA1 and IgA2, which contain distinct alpha1 and alpha2 heavy chains, respectively. Both subclasses also have membrane-bound forms (mIgA1 and mIgA2) containing the corresponding malpha1 and malpha2 heavy chains, which differ from alpha1 and alpha2 by an additional "membrane-anchor" peptide segment extending from the CH3 domain of alpha1 and alpha2. The membrane-anchor segment has three parts: an extracellular, a transmembrane, and an intracellular segment. The heavy chain malpha1 exists in short and long isoforms, referred to as malpha1S and malpha1L, with the latter containing extra 6 amino acid residues, GSCSVA, at the N-terminus of the extracellular segment (residues 453-458). By studying the genomic and mRNA sequences of malpha1 and malpha2 from 30 individuals residing in Taiwan, we have found that, in addition to the known malpha1 allele, referred to as malpha1(456S), malpha1 also has a previously unknown allele, referred to as malpha1(456C) (GenBank accession no. EU431191). This newly identified allele is present in the donor population at a similar proportion to malpha1(456S), and appears to exist only as the long isoform, i.e. malpha1L, rather than the short isoform, malpha1S. Furthermore, we confirmed that malpha2 exists only as the short isoform. Future studies will examine whether these mIgA1 variations affect the regulation of IgA synthesis and whether mIgA1 can provide an antigenic site for the immunological targeting of IgA-expressing B cells.

Tracking membrane and secretory immunoglobulin alpha heavy chain mRNA variation during B-cell differentiation by real-time quantitative polymerase chain reaction

Primary transcripts for all Ig heavy chain isotypes are alternatively processed to encode either secreted or membrane forms of the same antibody and, in plasma cells, a shift towards the secreted form occurs. In principle, measuring the relative quantities of secreted and membrane forms for a particular isotype could monitor B-cell plasmacytoid differentiation. Ratios of alpha heavy chain mRNA secreted (alphas) to membrane (alpham) form were assessed by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR; TaqMan) using an IgA plasma cell line (NCI-H929), a surface IgA+ line (Dakiki) and human tonsillar B cells. While NCI-H929 cells showed the highest alphas: alpham ratio as expected, alphas mRNA predominated for all unstimulated B cells and Dakiki cells. Treatment of B cells and Dakiki cells with IL-2 and IL-10 resulted in a further progression towards the alphas form, correlating with increased human plasma cell antigen-1 (HPC1) mRNA levels. However, alpha mRNA processing and HPC1 expression were independently regulated, as IFN-gamma treatment suppressed HPC1 levels while increasing alphas: alpham ratios. Cytokine-mediated increases in the alphas: alpham ratio resulted from strongly enhanced levels of alphas with relatively constant alpham values. Differentiation-related changes in mRNA processing can thus be tracked by automated quantitative PCR.