Biomedical subjects
Publications and source records attributed to Y Laâbi.
Recent studies on BAFF, a member of the tumor necrosis factor family, and the discovery of a new BAFF receptor have revealed that this ligand-receptor pair is essential for B-cell survival and differentiation, holding promise for a better understanding and treatment of some autoimmune diseases and lymphomas.
A novel dendritic cell (DC) surface molecule termed F4/80-like-receptor (FIRE) has been selected based on its differential expression between DC subsets. The gene encoding FIRE has been cloned and sequenced, and mAbs specific for FIRE have been produced. FIRE is a seven-transmembrane-spanning molecule with two epidermal growth factor-like domains in the extracellular region. It is a novel member of the epidermal growth factor/transmembrane-7 protein subfamily and shows similarity to the macrophage marker F4/80. FIRE is expressed by CD8- DC, but not by CD8+ DC, and it is down-regulated on DC activation. It is expressed by blood monocytes and by some tissue macrophages, but not by most macrophage cell lines or by lymphoid cells. FIRE is a useful marker of myeloid cells with a DC developmental potential.
Dendritic cells (DC) are potent antigen presenting cells that activate naive T cells. It is becoming increasingly clear that DC are not a homogeneous cell population, but comprise different subpopulations that differ in ontogeny and function. To further the molecular characterisation of DC, we screened for genes that were differentially expressed amongst DC subsets and could therefore give insight into their varying biological functions. Using Representational Difference Analysis (RDA) we identified a gene (CIRE) that is expressed at higher levels in the myeloid-related CD8alpha(-) DC than in the lymphoid-related CD8alpha(+) DC. CIRE is a 238 amino acid type II membrane protein, of approximately 33 kDa in size, whose extracellular region contains a C-type lectin domain. Northern blot analysis revealed that CIRE is almost exclusively expressed in DC and was not detected in organs such as heart, brain, kidney, liver, and thymus. T cells failed to express message for CIRE, whilst B cells expressed very low levels. These data here further substantiated by Northern blot analysis of 18 cell lines of various origins (myeloid, macrophage, B and T cell) where only one cell line, which was of myeloid origin and could give rise to DC, expressed mRNA for CIRE. Semi-quantitative RT-PCR suggested that CIRE is down-regulated upon activation. CIRE shares 57% identity with human DC-SIGN, a molecule that has been shown to be the ligand of ICAM-3 and that is also a receptor that binds HIV and facilitates trans-infection of T cells.
Analysis of the critical cellular processes of self-generation, commitment, and maturation induction ideally requires the use of clonal cultures using cells with a capacity to undergo all three processes. Preprogenitor cells from normal mouse marrow are proving useful cells for such studies. Cells of a newly established cloned leukemic cell line, the GB2, are providing a useful analogous leukemic system because GB2 cells form stratified subpopulations of clonogenic cells able to be clonally analyzed in vitro in which self-renewal is demonstrable, but in which near-normal maturation can be induced by a wide range of hematopoietic regulators.
As if the TNF receptor and ligand superfamily was not big enough, two new receptors and their ligands have now been added to it. As Laâbi and Strasser explain in their Perspective, the receptors BCMA and TACI and their ligands BAFF/BLys and APRIL, respectively, are important for B lymphocyte survival, proliferation, and differentiation.
The cloned pro-B-lymphocyte murine leukemic cell line GB2, was established from a leukemic Max41 x Emu-myc double transgenic mouse. Its Igh alleles are rearranged and its surface markers are primarily B-lymphoid, but a small proportion of the cells also express surface Gr-1 and some cells develop the morphology of maturing granulocytes. The cell line grows continuously in suspension culture without the addition of growth factors, but expresses mRNA for M-CSF, TPO and Flt-3-ligand. When stimulated in agar cultures by GM-CSF, G-CSF, M-CSF, IL-3, SCF, IL-6, leukemia inhibitory factor (LIF), IL-5 or IFNgamma, GB2 cells generated blast colonies or colonies of maturing granulocytes and macrophages. There was a striking similarity in colony types, relative colony numbers and maturation of colony cells to those formed by normal bone marrow cells in response to the same stimuli. GB2 blast colony-forming cells exhibited self-renewal as well as an ability to form granulocyte-macrophage colony-forming progeny, with evidence that a hierarchical sequence of clonogenic cells is generated in the cell line even after subcloning. Factor-specific maturation was clearly initiated by the action of the added growth factors. In contrast, FACS-sorting experiments showed that commitment to various types of colony-forming cell occurs in maintenance suspension cultures in the apparent absence of potentially relevant growth factors.
BCMA is a human gene expressed preferentially in mature B lymphocytes as a 1.2 kb mRNA, which encodes a 184 amino acid peptide (BCMAp). The study of BCMA mRNA expression, using human malignant B cell lines characteristic of different stages of B lymphocyte differentiation, demonstrated that the BCMA mRNA is absent in the pro-B lymphocyte stage. It is expressed faintly at the pre-B cell stage and its expression increases with B lymphocyte maturation. Polyclonal antibodies were used to show, by cellular fractionation and immunoprecipitation, that BCMAp is a non-glycosylated integral membrane protein. Furthermore, BCMAp inserts, in vitro, into canine microsomes, as a type I integral membrane protein. Cell surface labeling showed that BCMAp is not expressed in the plasma membrane of mature B lymphocytes. Immunofluorescence studies revealed that BCMAp lies in a cap-like structure near the nucleus, that was identified as the Golgi apparatus by co-localization of BCMAp with CTR433, a marker of the medial cisternae of the Golgi apparatus. Confocal scanning laser microscopy of U266 plasma cells labeled with markers of various Golgi apparatus subcompartments strongly suggests that BCMAp is located in the cis part of the Golgi apparatus. Thus, BCMAp is the first Golgi resident protein with a tissue specificity and whose expression is linked to the stage of differentiation of B lymphocytes. The location of BCMAp in the Golgi apparatus and its high expression in plasmocytes (secreting large amounts of Ig) suggest that BCMAp is implicated in the intracellular traffic of Ig.
A t(4;16)(q26;p13.1) chromosome translocation found in tumour cells from a patient with a T cell lymphoma was shown to rearrange the interleukin 2 gene, normally located on chromosome band 4q26, with sequences from chromosome band 16p13.1. A cDNA library of tumour cells was screened with an interleukin 2 gene-specific probe. Three clones were isolated, which consisted, from 5' to 3', of the three first exons of the interleukin 2 gene followed by a 16p13 in-frame sequence encoding 181 amino acids. A probe derived from this sequence detected a 1.2 kb transcript in various cell lines exhibiting mature B lymphoid cell features, but this was not detected in other cell lines representative of other haematopoietic lineages, or in other organs. For this reason, the novel gene was termed BCM for B cell maturation. The open reading frame of BCM normal cDNA predicted a 184 amino acid protein with a single transmembrane domain which had no homology with any protein sequence stored in data banks. Our data indicate that BCM is a new gene whose expression coincides with B cell terminal maturation.