Selection of membrane IgM- variants from a mIgM+ murine B lymphoma cell: problems and solutions

Efficient selection of mu m-mutants from mu m-expressing myeloma cells by treatment with ricin A-conjugated anti-mu antibody

We have developed an efficient system for obtaining myeloma mutants defective in trans-acting factors required for immunoglobulin (Ig) gene expression. The system consists of a myeloma cell line designed for this purpose and an efficient method for selecting mutants from it. The cell line is X63.653 transfected with the mu gene, whose tailpiece sequence was replaced with the transmembrane sequence of human EGF receptor to hold mu on the cell surface and whose CH1 sequence was removed to prevent mu from being retained in the endoplasmic reticulum. It efficiently and stably expressed mu chains of IgM on the cell surface (mu m+) without light chains. To obtain mutants lacking mu m (mu m-) from the mu m+ cell line by selectively killing mu m+ cells, a method with ricin A-conjugated anti-mu antibody was more reliable than complement lysis mediated by anti-mu antibody. Applying the system, we obtained a variety of mu m- mutants.

Molecular analysis of membrane immunoglobulin-negative variants

The mouse B-cell lymphoma WEHI 279.1 is a tumor which synthesizes both membrane and secreted immunoglobulin M (IgM). We have immunoselected variants which fail to express the membrane form (mIgM-); the most frequently isolated phenotype is a complete loss of both membrane expression and synthesis of the mu heavy chain within the cells. We have chosen four of these mIgM- mutants for detailed molecular investigation. One of these has suffered a large deletion which covers the region of chromosome 12 containing the expressed mu gene, but three have no detectable changes in the DNA arrangement of the mu gene. All of the mutants, including the deletion mutant, synthesize 10-30% of the wild-type level of cytoplasmic mu RNA; however, none is the appropriate size for membrane mu (mu m) or secreted mu (mu s) message. Based on our studies of the deletion mutant, which retains its nonproductively arranged allele, at least some of these RNAs may be 'sterile' transcripts from the nonproductively arranged allele. However, if all of these mRNAs derive from the other allele, they represent a substantial elevation of these sterile messages relative to the wild-type level. Furthermore, the three nondeletion mutants transcribe mu RNA at a level indistinguishable from the wild type. It is likely that their defects lie in the stability, processing, or transport of the mu RNA within the nucleus. Somatic cell hybrids between P3X and the IgM- variants produced mostly mIgM- hybrids. However, a few mIgM+ hybrids were produced, suggesting that the mu- defects may be partly complemented by the P3X fusion partner.

Variants of lymphoid lines produced with ricin A-chain monoclonal antibody conjugates

Conjugates of ricin A-chain with monoclonal anti-light chain antibodies specifically killed cells hearing kappa or lambda immunoglobulin (Ig) light chains. Exposure of cells from B-lymphoblastoid cell lines (B-LCL) to conjugate for less than 30 h had only a slight effect on cell growth, but on 48 h exposure a marked killing effect was achieved. After recovery of growth, cells were re-exposed to conjugate for 9-14 days. Treatment of cells from the EB4 line (sIgG lambda) in this way yielded 4 variants which showed a marked reduction in levels of surface Ig lambda and secreted Ig lambda with slight, or no, reduction in MHC class II expression and similar growth rates to the parent line. Variant lines retained their phenotype over long periods of culture.

LPS-nonresponsive variants of mouse B cell lymphoma, 70Z/3: isolation and characterization

We have used a genetic approach to study the differentiation of B lymphocytes. The cultured murine cell line 70Z/3 resembles pre-B cells in containing the heavy chain of the immunoglobulin IgM, mu, as an internal protein in the absence of light chain, L. However, overnight incubation with the B cell mitogen lipopolysaccharide (LPS) induces the cells to mature to a B lymphocyte-like state by the induction of L chain synthesis and the appearance of IgM on the cell surface. We have used immunoselection against surface-bound IgM to isolate LPS uninducible variants of 70Z/3. These fall into two complementation groups, LPS A and LPS B. LPS A variants predominated and were found at a frequency of 1/1200. These cells were completely unresponsive to LPS. LPS B was represented by a single variant in which a subset of cells was induced to display wild-type levels of membrane-bound IgM, and the proportion of induced cells increased with prolonged incubation with LPS. We detected no structural defects in either variant group, but LPS B may represent a defect in the decision to differentiate in response to LPS.

Biochemical characterization of mIgM- variants of the murine B-cell lymphoma, WEHI 279.1

The membrane immunoglobulin M (mIgM) of a B lymphocyte serves as a receptor for its cognate antigen. Our aim is to elucidate the structure and function of this membrane-bound receptor. The first step is to determine the requirements for proper membrane placement of IgM. We have used mIgM-positive B lymphocyte tumors from which we isolated mIgM negative variants by immunoselection. We report here the initial characterization of mIgM- variants isolated by repeated cycles of selection of the murine B lymphoma, WEHI 279.1, with goat anti-mouse immunoglobulin (G alpha MIg) and complement. These particular variants were chosen from a pool of more than 150 variants originally isolated because they resulted from several selection schemes and clearly had different origins. By analysis of their proteins, we have found three major phenotypes that do not produce mIgM: reduced microns, microseconds and L chain levels within cells, loss of microns and microseconds but retention of L chain synthesis, and loss of microns but retention of reduced amounts of microseconds and L chain. The defects underlying these phenotypes produce complex changes in the synthesis, turnover, and secretion of the mu or L chains involved. We performed experiments comparing the effects of the glycosylation inhibitor tunicamycin on variants with reduced mu and L levels with its effects on variants with L but no mu chains. These experiments suggested that mu and L chain synthesis are controlled coordinately at the level of protein synthesis. We have not yet isolated any variants lacking L chain synthesis or any appearing to have gross structural defects in the micron protein. This analysis is the first phase of the detailed characterization of the requirements for proper synthesis, processing, tetramer formation, and membrane display of mIgM on B lymphoma tumors in mice.