A universally applicable method of isolating somatic cell hybrids by two-colour flow sorting

Concomitant downregulation of IgH 3' enhancer activity and c-myc expression in a plasmacytoma x fibroblast environment: implications for dysregulation of translocated c-myc

Regulation of immunoglobulin heavy chain (IgH) gene expression is controlled by a B cell-specific promoter, intronic enhancer and additional B cell-specific enhancer elements identified recently in the 3' end of the IgH locus. One of the latter elements, the IgH 3' enhancer, is of particular interest: (1) it is B cell-specific and active only in late B cell development; (2) in rodent plasmacytomas and in some human Burkitt's lymphomas it is part of a locus control region (LCR) that is involved in deregulation of the c-myc oncogene as a result of translocation into the IgH locus; and (3) it has been implicated in the mechanisms that control Ig gene class switch recombination. We have used a somatic cell hybridization approach to genetically analyse regulation of the activity of the IgH 3' enhancer. When mouse MPC11 plasmacytoma cells, in which the IgH 3' enhancer is active, are fused with fibroblasts, Ig expression is extinguished at the level of transcription. Here we show that in a MPC11 plasmacytoma x fibroblast environment, the IgH 3' enhancer is transcriptionally inactive. Furthermore, we demonstrate that binding of several B cell-specific transcription factors, essential for IgH 3' enhancer activity, is lacking, which may explain 3' enhancer inactivity, although the binding of repressors cannot be excluded. Moreover, the high expression level of c-myc, characteristic of the parental MPC11 cells carrying the t(12;15) translocation, is down-regulated in the hybrids to that in unfused fibroblasts. Therefore, inactivation of the IgH 3' enhancer is a multifactorial process affecting several transcription factors that control the cell-specific and developmental activity of the enhancer.

Selection of hybrid hybridomas by flow cytometry using a new combination of fluorescent vital stains

A new combination of fluorescent dyes (rhodamine 123 and hydroethidine) was used to internally label hybridoma fusion partners. Murine hybridoma 520C9 (recognizing human c-erbB-2) was labeled with hydroethidine. Murine hybridoma 3G8 (recognizing human Fc gamma receptor III) was labeled with rhodamine 123, and verapamil was used to block rhodamine efflux via P-glycoprotein. Viability assays showed little cytotoxicity from these dyes at the concentrations used. The labeled cells were fused with polyethylene glycol, sorted for dual fluorescence on an Epics V cell sorter, and cloned. Hybrid hybridomas producing bispecific antibodies were selected for ability to promote lysis of SK-Br-3 breast cancer cells by human mononuclear cells. Several positive clones were obtained and shown to have a double content of DNA. Bispecific antibody produced by subclone 2B1 was purified by anion exchange chromatography and shown to bind both tumor cells and Fc gamma R III bearing cells. Using two parameter flow cytometric analysis, we were able to measure a 'bridging' effect of this bispecific antibody, which caused formation of complexes between PMNs and SK-Br-3 cells. Either parental antibody could compete with bispecific antibody to block such complexing. This fusion method provides several advantages over other techniques presently used (speed, convenience, low toxicity and automatic exclusion of dead cells) and can be applied to produce other hybrid hybridomas.

Extinction of an immunoglobulin kappa promoter in cell hybrids is mediated by the octamer motif and correlates with suppression of Oct-2 expression

When immunoglobulin-expressing B cells are fused with fibroblasts, immunoglobulin expression is rapidly and selectively suppressed. here we demonstrate that the conserved octamer motif of a kappa light chain gene promoter plays a crucial role in mediating this "extinction" phenomenon. Replacement of this octamer site by an Sp1 or NF1 binding site is sufficient to bypass extinction. Furthermore, in early cell hybrids, immunoglobulin suppression is correlated with absence of the cell-specific transcription factor Oct-2 and its transcripts. Such hybrids cannot support transcription of a transiently introduced reporter plasmid, driven by an octamer-containing promoter, unless an expression vector encoding Oct-2 is cotransfected. Transfection of the same Oct-2 expression vector into hybrid cells is also sufficient to "reactivate" an integrated kappa promoter construct. Thus, our data further establish the role of Oct-2 for immunoglobulin transcription and show that in B cell x fibroblast hybrids, the lack of a necessary cell-specific transcription factor is involved in the extinction of immunoglobulin expression.

Interspecific genetic complementation analysis of human and sheep fibroblasts with beta-galactosidase deficiency

Interspecific somatic cell hybrids were analyzed by genetic complementation to determine if a lysosomal storage disease in sheep associated with deficiencies of beta-galactosidase and alpha-neuraminidase was homologous with any of four beta-galactosidase-deficient human diseases. Fibroblasts from beta-galactosidase-deficient sheep, cats, and human patients were fused and assayed histochemically for beta-galactosidase, with 5-bromo-4-chloro-3-indolyl beta-D-galactoside. We observed complementation in heterokaryons consisting of fibroblasts from beta-galactosidase-deficient sheep and fibroblasts from patients with galactosialidosis or mucolipidosis type II, but no complementation in heterokaryons consisting of fibroblasts from beta-galactosidase-deficient sheep and fibroblasts from human or feline GM1 gangliosidosis (type I) or from human mucopolysaccharidosis type IVB fibroblasts. We conclude that the ovine disease is due to a mutation at the genetic locus homologous with that of GM1 gangliosidosis and mucopolysaccharidosis type IVB, suggesting that the primary defect in the ovine disease is a mutation of the beta-galactosidase structural gene.

Myeloma kappa gene transcription is blocked upon fusion with fibroblasts

Numerous studies on somatic cell hybrids have shown that expression of tissue-specific functions can be suppressed as a consequence of fusion with cells that do not express the given functions. We have further investigated this phenomenon, using as a model system the regulation of expression of kappa light chain genes in intraspecific hybrids between mouse myeloma cells and mouse fibroblasts. Hybrids containing only one genome equivalent from each parent cell (1s:1s) were isolated by fluorescence-activated cell sorting from within 10 h after fusion, and they were grown for no more than 16 days thereafter in order to ensure maximum integrity of the genomic constitution. Here we report that in hybrid cells, kappa gene transcription was specifically turned off as demonstrated by nuclear run-on assays performed on 16-day-old proliferating hybrids. Furthermore, a mechanism affecting mRNA stability may also contribute, at least initially, to the rapid depletion of cytoplasmic kappa transcripts, observed during the first few hours after fusion. Suppression was dominant and could not be overridden by increasing the relative myeloma ploidy at either the heterokaryon or the synkaryon stage. Nor could suppression be relieved by treating hybrids with cycloheximide.

Fluorescent vital stains for complementary labelling of protoplasts from Trichoderma spp

In this study several fluorescent vital stains were evaluated for their ability to provide complementary vital staining of protoplasts of Trichoderma spp. for selection of heterokaryons following protoplast fusion. Tetramethyl rhodamine isothiocyanate and fluorescein isothiocyanate were rejected because they stained only a small proportion of protoplasts. Fluorescein diacetate stained all protoplasts, but the chromophore leaked rapidly from stained cells. A mixture of FluoroBora T and acriflavine stained all cells, but intensity was low and fading upon illumination was rapid. Nile red stained lipid bodies in all cells, but the stain was lost upon protoplast fusion in polyethylene glycol. Rhodamine 6G, on the other hand, stained all cells, fluoresced green, and was stable through fusion and upon illumination. Hydroethidine also stained all protoplasts, and staining was relatively stable through fusion and upon illumination. Hydroethidine fluoresced red and stained nuclei more prominently than the cytoplasm. Rhodamine 6G and hydroethidine were tested on a number of strains to determine whether they were toxic to protoplasts. No toxicity to any strain was noted with rhodamine 6G. Hydroethidine, however, was toxic at the higher concentrations tested, especially when stained protoplasts were exposed to light. When protoplasts were stained with the minimum concentration giving ready visualization and were incubated in darkness, hydroethidine also was nontoxic. Hydroethidine and rhodamine 6G are useful complementary vital stains of Trichoderma protoplasts for visualization of frequency and type (dicell, multicell) of fusion.

In vitro differentiation of mouse embryonic yolk sac cells

The embryonic yolk sac is the first site in the mammalian embryo in which cells are found that can carry out cell-mediated immune functions, yet the relation of cells of this primitive hematopoietic organ to the development of the mature immune system has not been established. We have initiated a series of experiments to determine the potential of cells of the mouse yolk sac to differentiate in vitro, in order to get an insight into the development of immunocompetence in this primary population of hematopoietic stem cells. The present paper describes the conditions promoting stem-cell differentiation and provides an initial characterization of cell surface phenotypes of the cell lineages established in vitro. Yolk sac cells obtained from 10- to 13-day mouse embryos were maintained in culture for more than 18 months, giving rise to a variety of cell types belonging to the hematopoietic lineages and culminating in the establishment of long-term cell lines. Supernatants of secondary mixed leukocyte cultures were found to be an effective source of growth factors promoting the initial differentiation as well as the maintenance of these cells. Flow-cytometric analysis showed that, in contrast to freshly obtained yolk sac cells, which had no detectable Thy 1 antigen, cells expressing significant levels of Thy 1 were obtained after 1 week or more of culture. Ly1 and Lyt 2 antigens were detected only rarely and the L3T4 (GK 1.5) antigen was never expressed.(ABSTRACT TRUNCATED AT 250 WORDS)

Interspecific genetic complementation analysis with fibroblasts from humans and four species of animals with Chediak-Higashi syndrome

Although the autosomal recessive disease Chediak-Higashi syndrome (CHS) has been described in humans, cats, mink, cattle, mice, killer whales, blue foxes, and silver foxes, and these conditions appear quite similar, no direct evidence of the homology of this disease in the various species has been presented. To determine if CHS in humans, cats, mink, cattle, and mice is due to a mutant gene at the homologous genetic locus in each species, or alternatively, if these are merely similar syndromes, genetic complementation analysis after interspecific somatic cell (fibroblast) hybridization was performed. "Paracrystal" formation was the criterion used for the determination of complementation. The initial studies in this report were designed to characterize paracrystal formation in control and CHS fibroblasts of these five species. Most of the control fibroblasts from each species (91-96.6%) formed paracrystals upon incubation with 25 micrograms/ml of the microtubule depolymerizing agent vinblastine sulfate. A significantly (P less than 0.05) smaller percentage of the CHS fibroblasts formed paracrystals after the same incubation (except CHS mice, with 90.2% paracrystals). It was found that 52% of the human CHS fibroblasts, 60% of cat CHS fibroblasts, 47% of mink CHS fibroblasts, and 53.8% of cow CHS fibroblasts formed paracrystals. For genetic complementation analysis, human CHS fibroblasts were fused to cat, mink, cow, or mouse CHS fibroblasts with polyethylene glycol. Control fusions were human CHS fibroblasts fused with human, cat, mink, cow, and mouse normal fibroblasts. The results of complementation analysis after the fusion of human CHS with cow CHS and human CHS with mouse CHS fibroblasts were inconclusive. A lack of complementation of human CHS with cat CHS and human CHS with mink CHS fibroblasts indicates that the disease is homologous in these species.

Estimation of heterokaryon formation and hybridoma growth in murine and human cell fusions

Four mouse myelomas commonly used for cell fusions (X63.Ag8.653, SP2/0, NS1, P3U1), 3 human myeloma-like cell lines (ARH77, U-266, GM1500) and 3 human x mouse hybridomas (SPAZ4, SA2, SA3) were compared for their heterokaryon formation and successful hybridoma growth after cell fusion with polyethylene glycol. The cells were stained with different fluorescent dyes which do not alter hybridoma growth or antibody secretion. After fusion myeloma cells containing at least 1 nucleus from a lymphocyte (heterokaryons) were counted from fluorescence photomicrographs and the heterokaryon frequency was calculated. Mouse myelomas fused at a frequency of 1-7%, whereas human myeloma lines showed a higher heterokaryon frequency ranging from 3-25%. In mouse fusions almost every well contained growing hybridomas showing a minimum hybridoma frequency of 2/10(6) lymphocytes. In human fusions the SPAZ4 and SA2 lines showed a heterokaryon frequency nearly as good as mouse myelomas, whereas U-266 yielded no growing hybridomas despite 20% heterokaryon frequency. Furthermore, human cell lines showed a high tendency of multikaryon formation whereas this phenomenon was rarely observed with murine and murine x human heterohybrids. In individual fusion experiments no correlation was found between heterokaryon formation and the number of growing hybridomas. Thus, our study shows that defects in hybridoma growth may not always result from lack of a successful fusion and human hybridomas might be more sensitive to post-fusion conditions.

Bispecific antibody-producing hybrid hybridomas selected by a fluorescence activated cell sorter

Hybrid hybridomas (tetradomas) producing bispecific monoclonal antibodies reacting with both horseradish peroxidase (HRP) and human alpha-fetoprotein (AFP) were obtained by fusing two hybridoma lines and selecting the fused cells by a fluorescence activated cell sorter (FACS III). The hybridoma cells were labelled before fusion with fluorescein isothiocyanate (FITC) and tetramethyl rhodamine isothiocyanate (TRITC) respectively and heterofluorescent cells were sorted out after fusion. Several clones were found to produce bispecific antibodies, and one clone, designated T1, was subjected to growth in ascitic fluid in mice in order to obtain large quantities of hybrid antibodies. Bispecific antibodies could be separated from the monospecific antibody populations by one-step hydroxylapatite chromatography. SDS-polyacrylamide gel electrophoresis demonstrated that the hybrid antibody molecules contained the heavy chains of both anti-HRP and anti-AFP origin. The bispecific antibodies were used to build up a sensitive two-site binding enzyme immunoassay.

Time course of arrest of immunoglobulin expression in heterokaryons and early hybrids of human lymphoma cells and mouse fibroblasts. A study of transcriptional and translational events

Early events in arrest of immunoglobulin expression were investigated at the levels of both translation and transcription in heterokaryons and early hybrids between human Daudi lymphoma cells and mouse cl. 1D cells. Large populations of 1s: 1s hybrids, isolated by fluorescence-activated cell sorting (FACS) a few hours after fusion, were grown for up to 5 days. A survey at the light-microscopical level of peroxidase-antiperoxidase-immunostained cell populations showed that arrest of expression of IgM heavy chain (mu) occurred in up to 98% of the cells. Furthermore, quantitation of mu chain contents, by using an ELISA technique, suggested that synthesis of IgM was blocked shortly after fusion. The levels of cytoplasmic mRNA specific for mu and kappa chains, respectively, decreased at rates similar to those induced in unfused Daudi cells by treatment with actinomycin D. It is concluded that arrest of immunoglobulin expression in these hybrids occurs immediately or very shortly after fusion by mechanisms that affect the levels of their cytoplasmic mRNAs.

Homokaryon production by electrofusion: a convenient way to produce a large number of viable mammalian fused cells

A convenient technique to obtain homokaryons is described that provides large amounts of fused mammalian cells. Chinese hamster ovary cells grown in monolayers on a Petri dish are submitted to square wave electric pulses. Viability of cells is observed not to be affected by this electric treatment. The yield of fusion is strongly dependent on the strength of the field (KV/cm range) and on the duration of the pulse (microsecond range). The yield is not improved by accumulation of pulses. Yields up to 80% are obtained and under our experimental conditions 200 000 cells are fused per assay.