ABSTRACT
The VpreB3 gene product was first characterized as an immunoglobulin (Ig) mu heavy chain-binding protein in mouse precursor B (pre-B) cells. Although its function is unknown, it has been proposed to participate in the assembly and transport of the pre-B cell receptor. We have identified a VpreB3 orthologous gene in chicken that is located close to the immunoglobulin light chain (LC) gene cluster and specifically expressed in the bursa of Fabricius. By overexpressing VpreB3 in the DT40 IgM(+) immature chicken B cell line, we have characterized VpreB3 as an endoplasmic reticulum-resident glycoprotein that binds preferentially to free IgLC. However, binding to IgHC is observed in IgLC-deficient DT40 cells. Interaction of VpreB3 with free IgLC is partly covalent and induces retention of free IgLC in the endoplasmic reticulum, preventing their secretion without affecting IgM surface expression. Our results demonstrate that this evolutionarily conserved molecule may play a role in the regulation of the maturation and secretion of free IgLC in B cells. We discuss possible implications in the regulation of the immune response.
Subject(s)
Immunoglobulin Light Chains/chemistry , Membrane Glycoproteins/chemistry , Amino Acid Sequence , Animals , B-Lymphocytes/metabolism , Cell Line , Chickens , DNA, Complementary/metabolism , Dimerization , Electrophoresis, Gel, Two-Dimensional , Electrophoresis, Polyacrylamide Gel , Endoplasmic Reticulum , Flow Cytometry , Glycoside Hydrolases/pharmacology , Golgi Apparatus , Humans , Immunoblotting , Immunoglobulin M/chemistry , Kinetics , Membrane Glycoproteins/metabolism , Mice , Microscopy, Confocal , Models, Genetic , Molecular Sequence Data , Multigene Family , Pre-B Cell Receptors , Precipitin Tests , Protein Binding , RNA, Messenger/metabolism , Receptors, Antigen, B-Cell , Reverse Transcriptase Polymerase Chain Reaction , Sequence Homology, Amino Acid , Time Factors , Tissue DistributionABSTRACT
Immunodeficiency, centromeric region instability, and facial anomalies (ICF) syndrome is a rare autosomal recessive disease. Mutations in the DNA methyltransferase 3B (DNMT3B) gene are responsible for most ICF cases reported. We investigated the B-cell defects associated with agammaglobulinemia in this syndrome by analyzing primary B cells from 4 ICF patients. ICF peripheral blood (PB) contains only naive B cells; memory and gut plasma cells are absent. Naive ICF B cells bear potentially autoreactive long heavy chain variable regions complementarity determining region 3's (V(H)CDR3's) enriched with positively charged residues, in contrast to normal PB transitional and mature B cells, indicating that negative selection is impaired in patients. Like anergic B cells in transgenic models, newly generated and immature B cells accumulate in PB. Moreover, these cells secrete immunoglobulins and exhibit increased apoptosis following in vitro activation. However, they are able to up-regulate CD86, indicating that mechanisms other than anergy participate in silencing of ICF B cells. One patient without DNMT3B mutations shows differences in immunoglobulin E (IgE) switch induction, suggesting that immunodeficiency could vary with the genetic origin of the syndrome. In this study, we determined that negative selection breakdown and peripheral B-cell maturation blockage contribute to agammaglobulinemia in the ICF syndrome.
Subject(s)
B-Lymphocytes/immunology , Chromosomal Instability/immunology , DNA (Cytosine-5-)-Methyltransferases/genetics , Face/abnormalities , Immunologic Deficiency Syndromes/immunology , Mutation , Adolescent , Antigens, CD/blood , B-Lymphocytes/cytology , B-Lymphocytes/pathology , Base Sequence , Cell Differentiation , Child , Child, Preschool , Chromosomal Instability/genetics , DNA Primers , Female , Flow Cytometry , Humans , Immunoglobulin A/blood , Immunoglobulin G/blood , Immunoglobulin M/blood , Immunologic Deficiency Syndromes/genetics , Lymphocyte Count , Male , Reverse Transcriptase Polymerase Chain Reaction , Syndrome , DNA Methyltransferase 3BABSTRACT
UNLABELLED: Males with X-linked agammaglobulinaemia (XLA) due to mutations in the Bruton tyrosine kinase gene constitute the major group of congenital hypogammaglobulinaemia with absence of peripheral B cells. In these cases, blockages between the pro-B and pre-B cell stage in the bone marrow are found. The remaining male and female cases clinically similar to XLA represent a genotypically heterogeneous group of diseases. In these patients, various autosomal recessive disorders have been identified such as mutations affecting IGHM, CD79A, IGLL1 genes involved in the composition of the pre-B cell receptor (pre-BCR) or the BLNK gene implicated in pre-BCR signal transduction. In this paper, we report on a young female patient characterised by a severe non-XLA agammaglobulinaemia that represents a new case of Igmu defect. We show that the B cell blockage at the pro-B to pre-B cell transition is due to a large homologous deletion in the IGH locus encompassing the IGHM gene leading to the inability to form a functional pre-BCR. The deletion extends from the beginning of the diversity (D) region to the IGHG2 gene, with all JH segments and IGHM, IGHD, IGHG3 and IGHG1 genes missing. CONCLUSION: alteration in Igmu expression seems to be relatively frequent and could account for most of the reported cases of autosomal recessive agammaglobulinaemia.
