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/Igß 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.

Fanconi's syndrome induced by a monoclonal Vkappa3 light chain in Waldenstrom's macroglobulinemia.

Fanconi's syndrome (FS) is a disorder of sodium-dependent proximal tubule reabsorption, which may complicate plasma cell disorders producing a free monoclonal light chain (LC). FS often occurs in the setting of smoldering myeloma and features cytoplasmic crystalline inclusions of monoclonal kappa LC in proximal tubular cells and malignant plasma cells. Although the clinical and pathological presentation may vary, including lack of crystal formation, monoclonal kappa LCs that underlie FS show a striking genetic and biochemical homogeneity: they almost always belong to the Vkappa1 subgroup of variability and originate from 2 germline genes, O2/O12 or O8/O18. Their variable domain sequences present unusual hydrophobic residues, responsible for the resistance to proteolysis, which leads to LC accumulation in the endocytic compartment of proximal tubule cells. We report a patient with slowly progressive Waldenstrom's macroglobulinemia and full-blown FS with accumulation of a monoclonal kappa LC within proximal tubules, but no detectable crystalline organization. This LC, which belonged to the unusual Vkappa3 subgroup and derived from the L2/L16 germline gene, showed no common substitution with previously described FS kappaI LC and was sensitive to trypsin digestion. These data show that molecular and biochemical characteristics of kappa LCs in patients with FS are more heterogeneous than initially suspected. Mechanisms other than resistance of LCs to endosomal proteolysis probably are involved in the pathogenesis of FS-associated plasma cell dyscrasias.

Immunologic basis for the rare occurrence of true nonsecretory plasma cell dyscrasias.

Lymphocytes and plasma cells are major actors of the adaptive immune response and can rightly be considered as human health keepers. However, recombination and mutation events occurring at high rate in the B cell lineage also expose these cells to gene alterations, potentially resulting in uncontrolled and life-threatening cell proliferation. Although in cultured cell lines, such gene alterations frequently generate nonsecretory variants, most immunoproliferative B cell disorders feature in vivo immunoglobulin (Ig) secretion. In this paper, we review the molecular mechanisms involved in various instances of the rare, nonsecretory myelomas, in light of current notions about the molecular control of Ig production, assembly, and secretion in normal B cells. We finally document the attractive hypothesis that B cell clones, which retain nonsecretable, intracellular Igs, may be ideal, in vivo targets for efficient anti-idiotypic immune responses, and clones featuring an abundant secretion may by contrast easily induce T cell anergy and escape the anti-tumoral immune surveillance.

The immunoglobulin heavy-chain locus hs3b and hs4 3' enhancers are dispensable for VDJ assembly and somatic hypermutation.

The more distal enhancers of the immunoglobulin heavy-chain 3' regulatory region, hs3b and hs4, were recently demonstrated as master control elements of germline transcription and class switch recombination to most immunoglobulin constant genes. In addition, they were shown to enhance the accumulation of somatic mutations on linked transgenes. Since somatic hypermutation and class switch recombination are tightly linked processes, their common dependency on the endogenous locus 3' enhancers could be an attractive hypothesis. VDJ structure and somatic hypermutation were analyzed in B cells from mice carrying either a heterozygous or a homozygous deletion of these enhancers. We find that hs3b and hs4 are dispensable both for VDJ assembly and for the occurrence of mutations at a physiologic frequency in the endogenous locus. In addition, we show that cells functionally expressing the immunoglobulin M (IgM) class B-cell receptor encoded by an hs3b/hs4-deficient locus were fully able to enter germinal centers, undergo affinity maturation, and yield specific antibody responses in homozygous mutant mice, where IgG1 antibodies compensated for the defect in other IgG isotypes. By contrast, analysis of Peyer patches from heterozygous animals showed that peanut agglutinin (PNAhigh) B cells functionally expressing the hs3b/hs4-deficient allele were dramatically outclassed by B cells expressing the wild-type locus and normally switching to IgA. This study thus also highlights the role of germinal centers in the competition between B cells for affinity maturation and suggests that membrane IgA may promote recruitment in an activated B-cell compartment, or proliferation of activated B cells, more efficiently than IgM in Peyer patches.

A monoclonal V kappa l light chain responsible for incomplete proximal tubulopathy.

Calcium and phosphate metabolism abnormalities are frequent in myeloma patients and the role of renal lesions in such ionic perturbations may have been overlooked. The authors herein report the complete primary structure of a Bence Jones Vkappal light chain responsible for myeloma-associated proximal tubulopathy with increased phosphaturia. Plasma and serum biochemical evaluations indicated a proximal tubular dysfunction mainly manifested as tubular acidosis and phosphate loss. The study of a kidney biopsy showed interstitial and tubular lesions with numerous myeloma casts and peculiar features of the proximal tubular cells, which carried numerous phagolysosomal inclusions with occasional crystalline periodic striation. The nephrotoxic light chain primary structure was deduced from the bone marrow monoclonal plasma cells RNA. The kappal sequence was highly homologous to kappa chains previously characterized in patients with Fanconi syndrome. It was related to the Vkappal subgroup and was composed of a variable segment encoded by the O8/O18 germline gene rearranged to Jkappa4. The primary sequence presented unusual features restricted to the variable region, including substitutions of residues 28 and 31 in the complementary determining region 1 (CDR1) by amino acids of different charge. An unusual conformation of the kappal domain, likely resulting from somatic hypermutation, could alter the catabolism of the protein after its internalization and result in the tubular cell dysfunction. Comparison with Fanconi syndrome studies suggests that Vkappal Bence Jones proteins may damage proximal tubular cells to an extent varying according to light chain (LC) sequence and structure, either leading to crystal formation and Fanconi syndrome or inducing partial inhibition of proximal tubule function.

B cell development arrest upon insertion of a neo gene between JH and Emu: promoter competition results in transcriptional silencing of germline JH and complete VDJ rearrangements.

Previous targeting experiments within the IgH locus have shown that V(D)J recombination was affected by an insertion of a neo gene within E(mu) upstream of the core enhancer, but not by insertions downstream of the enhancer. Similarly, class switch recombination to a given (C) gene was affected only by interposition of neo in between that gene and the 3' IgH enhancers. Here we show that insertion of neo upstream E(mu) only marginally impairs V(D)J recombination, but results in an altered D and J(H) gene usage and completely blocks transcription of the germline J(H) region and the rearranged VDJ segments. Although transcriptional silencing of J(H) occurs upstream of the insertion and results in the lack of mature B cells in homozygous mutant animals, IgH transcription is maintained downstream of the insertion together with neo transcription and can be up-regulated by LPS stimulation or upon fusion with plasmacytoma cells. Altogether these data argue for a polarized "neo effect" involving promoter competition and further show that V(D)J rearrangement can be uncoupled from transcription.