Optimization of electrofusion parameters for efficient production of murine hybridomas

Highly integrated microfluidic device for cell pairing, fusion and culture

In this study, we proposed a microfluidic device with compact structures integrating multiple modalities for cell capture, pairing, fusion, and culture. The microfluidic device is composed of upper and lower parts. The lower part configured with electrodes and capture wells is used for cell trapping/pairing/fusion, while the upper part configured with corresponding culture wells is used for cell culture. Dielectrophoresis is used to enable accurate cell trapping and pairing in capture wells. Moreover, the paired cells are fused flexibly by either electrical pulses or polyethylene glycol (PEG) buffer. The fused cells are then transferred to culture wells for on-chip culture simply by flipping the device. Using the device and HeLa cells, we demonstrated pairing efficiency of ∼78% and fusion efficiencies of ∼ 26% for electrical fusion or ∼ 21% for PEG fusion, and successful cell proliferation and migration after 72 h on-chip culture. We believe that this multifunction-integrated but structure-simplified microfluidic device would largely facilitate cell fusion oriented tasks.

Recent advances in detection of AGEs: Immunochemical, bioanalytical and biochemical approaches

Advanced glycation end products (AGEs) are a cohort of heterogeneous compounds that are formed after the nonenzymatic glycation of proteins, lipids and nucleic acids. Accumulation of AGEs in the body is implicated in various pathophysiological conditions like diabetes, cardiovascular diseases and atherosclerosis. Numerous studies have reported the connecting link between AGEs and the various complications associated with diseases. Hence, detection and measurement of AGEs becomes centrally important to understand and manage the menace created by AGEs inside the body. In recent years, an increasing number of immunotechniques as well as bioanalytical techniques have been developed to efficiently measure the levels of AGEs, but most of them are still far away from being clinically consistent, as relative disparity and ambiguity masks their standardization. This article is designed to critically review the recent advances and the emerging techniques for detection of AGEs. It is an attempt to summarize the major techniques that exist currently for the detection of AGEs both qualitatively and quantitatively. This review primarily focuses on the detection and quantification of AGEs which are formed in vivo. Immunochemical approach though costly but most effective and accurate method to measure the level of AGEs. Literature review suggests that detection of autoantibody targeting AGEs is a promising way that can be utilized for detection of AGEs. Future research efforts should be dedicated to develop this method in order to push forward the clinical applications of detection of AGEs.

Cell electrofusion using nanosecond electric pulses

Electrofusion is an efficient method for fusing cells using short-duration high-voltage electric pulses. However, electrofusion yields are very low when fusion partner cells differ considerably in their size, since the extent of electroporation (consequently membrane fusogenic state) with conventionally used microsecond pulses depends proportionally on the cell radius. We here propose a new and innovative approach to fuse cells with shorter, nanosecond (ns) pulses. Using numerical calculations we demonstrate that ns pulses can induce selective electroporation of the contact areas between cells (i.e. the target areas), regardless of the cell size. We then confirm experimentally on B16-F1 and CHO cell lines that electrofusion of cells with either equal or different size by using ns pulses is indeed feasible. Based on our results we expect that ns pulses can improve fusion yields in electrofusion of cells with different size, such as myeloma cells and B lymphocytes in hybridoma technology.

Increased concentration of two different advanced glycation end-products detected by enzyme immunoassays with new monoclonal antibodies in sera of patients with rheumatoid arthritis

Background

Levels of pentosidine (representative of advanced glycation end-products) in sera of patients with rheumatoid arthritis are increased when compared with sera of other diagnoses or healthy controls. These levels have been reported to correlate with clinical indices of rheumatoid arthritis activity and with laboratory markers of inflammation. The purpose of this study was to find out if these findings pertain to other advanced glycation end-products.

Methods

We have developed two immunoassays based on new monoclonal antibodies to advanced glycation end-products. Antibody 103-E3 reacts with an unidentified antigen, formed in the reaction of proteins with ribose, while antibody 8-C1 responds to N^ε-(carboxyethyl)lysine. We have used these monoclonal antibodies to measure levels of advanced glycation end-products in sera of patients with rheumatoid arthritis, systemic lupus erythematosus, osteoarthritis, and healthy controls. We calculated the correlations between advanced glycation end-product levels in rheumatoid arthritis sera and the Disease Activity Score 28 (DAS28), age, disease duration, CRP, anti-CCP, rheumatoid factor and treatment with corticosteroids, respectively.

Results

Levels of both glycation products were significantly higher in sera of patients with rheumatoid arthritis when compared with sera of patients with systemic lupus erythematosus, osteoarthritis, or the healthy controls. Neither the level of N^ε-(carboxyethyl)lysine nor the level of the 103-E3 antigen in rheumatoid arthritis sera correlated with the DAS28-scored rheumatoid arthritis activity. The levels of both antigens in rheumatoid arthritis sera did not correlate with age, gender, corticosteroid treatment, or levels of CRP, anti-CCP antibodies, and rheumatoid factor in sera.

Conclusions

We report highly specific increases in the levels of two advanced glycation end-products in sera of patients with rheumatoid arthritis. This increase could be explained neither by rheumatoid arthritis activity nor by inflammation. We propose a working hypothesis that presumes the existence of a link between advanced glycation end-product formation and induction of autoimmunity.

Development of a zona-free method of nuclear transfer in the mouse

In the present study, a zona-free nuclear transfer (NT) technique, which had been originally developed in cattle, was modified for the mouse. Steps involved in this approach include removing the zona pellucida and enucleating without a holding pipette; sticking donor cells to the cytoplast before electric pulses are applied to fuse them and culturing reconstructed embryos individually in single droplets, to prevent aggregation. Control zona-free and zona-intact embryos from mated donors showed no significant difference in development to blastocyst, but did show reduced development to term. Removal of the zona pellucida affected the response to activation by strontium in the absence of calcium as a significant proportion of zona-free control oocytes and embryos reconstructed by NT lysed during this treatment. A comparison between cumulus and ES cells as donor cells revealed significant differences in fusion efficiency (58.1 +/- 4.0%, n = 573 vs. 42.9 +/- 2.2%, n = 2064, respectively, p < 0.001), cleavage (77.2 +/- 3.4%, n = 334 vs. 40.8 +/- 2.7%, n = 903, respectively, p < 0.001) but not for development to morula/blastocyst (8.7 +/- 2.1%, n = 334 vs. 13.9 +/- 1.8%, n = 903, respectively, p < 0.1). The stage at which embryo development arrested was also affected by donor cell type. A majority of embryos reconstructed from cumulus cells arrested at two-cell stage, usually with two nuclei, whereas those reconstructed from ES cells arrested at one-cell stage, usually with two pseudo-pronuclei. After transfer of ES cell-derived NT embryos, a viable cloned mouse was produced (3.0% of transferred embryos developed to term). These observations establish that a zona-free cloning approach is possible in the mouse, although further research is required to increase the efficiency.

Improving electrotransfection efficiency by post-pulse centrifugation

We have demonstrated that the viability of electrotransfected adherent CHO and suspended NK-L, K-562, L1210 and MC2 cells is improved if pelleting by centrifugation is performed immediately after pulsing. The protection effect on cell viability is cell line- and pellet thickness-dependent. For forming CHO cell pellets, centrifugation force (300-13,000 g) and duration are not crucial; about five to 10 cell layers in the pellet provide the optimal protection effect. NK-L, K-562, L1210 and MC2 cell pellets are optimally formed by centrifugation at 13,000 g in an Eppendorf desktop centrifuge. Pelleting improves the cell viability over the whole range of the NK-L, K-562, L1210 and MC2 cell concentrations studied. When this pelleting method is applied to load CHO cells with FITC-dextran (41,000 MW), not only is the success rate close to 100%, but the growth rate is similar to the control, which is far better than the conventional electroporation method. Furthermore, the transfection efficiency of the five cell lines in pellet is significantly higher than that in suspension.

The role of helper lipids in cationic liposome-mediated gene transfer

In the procedure for cationic liposome-mediated transfection, the cationic lipid is usually mixed with a "helper lipid" to increase its transfection potency. The importance of helper lipids, including dioleoylphosphatidylcholine (DOPC) and phosphatidylethanolamine (dioleoyl PE), DO was examined. Freeze-fracture electron microscopy of DNA:cationic complexes containing the pSV-beta-GAL plasmid DNA, the cationic lipid dioleoyl trimethylammonium propane, and these helper lipids showed that the most efficient mixtures were aggregates of ensheathed DNA and fused liposomes. PE-containing complexes aggregated rapidly when added to culture media containing polyanions, whereas PC-containing complexes did not. However, more granules of PC-containing complexes were formed on cell surfaces after the complexes were added to Chinese hamster ovary (CHO) cells in transfection media. Pronase treatment inhibited transfection, whereas dilute poly-L-lysine enhanced transfection, indicating that the attachment of DNA:liposome complexes to cell surfaces was mediated by electrostatic interaction. Fluorescence spectroscopy studies confirmed that more PC-containing complexes than PE-containing complexes were associated with CHO cells, and that more PC-containing complexes were located in a low pH environment (likely to be within endosomes) with time. Cytochalasin-B had a stronger inhibitory effect on PC-containing liposome-mediated than on PE-containing liposome-mediated transfection. Confocal microscopic recording of the fluorescently label lipid and DNA uptake process indicated that many granules of DNA:cationic liposome complexes were internalized as a whole, whereas some DNA aggregates were left out on the cell surfaces after liposomes of the complexes fused with the plasma membranes. For CHO cells, endocytosis seems to be the main uptake pathway of DNA:cationic liposome complexes. More PC-containing granules than PE-containing granules were formed on cell surfaces by cytoskeleton-directed membrane motion, after their respective DNA:liposome complexes attached to cell surfaces by electrostatic means. Formation of granules on the cell surface facilitated and/or triggered endocytosis. Fusion between cationic liposomes and the cell membrane played a secondary role in determining transfection efficiency.

Electrofusion between heterogeneous-sized mammalian cells in a pellet: potential applications in drug delivery and hybridoma formation

High-efficiency electrofusion between cells of different sizes was achieved by application of fusing electric pulses to cells in centrifuged pellets. Larger target cells (Chinese hamster ovary or L1210 cells) were stacked among smaller human erythrocytes or erythrocyte ghosts by sequential centrifugation at 700 g to form five-tier pellets in a specially designed centrifugation-electrofusion chamber. The membranes of erythrocytes and ghost were labeled with fluorescent membrane dye (1,1' dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine (Dil)), and the contents of ghosts were loaded with water-soluble fluorescent dye (42-kDa fluorescein isothiocyanate dextran (FITC-dextran)), to monitor heterogeneous cell fusion. Fusion efficiency was assayed by the extent of either membrane dye mixing or contents (FITC-dextran) mixing with target cells. Four rectangular electric pulses at 300 V and 80 microseconds each were found to give the optimal fusion results of approximately 80% heterogeneous fusion by the content-mixing assay and approximately 95% by the membrane-dye-mixing assay. Cell viability remained greater than 80% after electrofusion. Because of the electric breakdown of cell membranes at the beginning of the pulse, the pellet resistance and hence the partial voltage across the pellet reduced rapidly during the remaining pulse time. This voltage redistribution favored the survival of fused cells. The limited colloidal-osmotic swelling of cells in pellets enhanced cell-cell contact and increased the pellet resistance after each pulse. As a result, the partial voltage across the pellet was restored when the next pulse was applied. This redistribution of pulse voltage in the pellet system permitted the breakdown of cell membranes at a lower applied voltage threshold than that required for electrofusion of cells in suspension or in dielectrophoretic cell chains. The cell viability and soluble dye retention within cells (FITC-dextran) remained at the same high levels for 3 h when the cells were incubated in respective culture media with serum at 37 degrees C. Viability and dye retention decreased significantly within 30 min when cells were incubated in phosphate-buffered saline without serum. The pellet technique was applied to form hybridomas by fusion of larger SP2/0 murine myelomas with smaller naive mouse lymphocytes. An optimum of 173 +/- 70 hypoxanthine aminopterin thymidine (HAT)-selected clones of the hybridomas was obtained from 40,000 SP2/0 cells and 1.5 x 10(6) lymphocytes used in each trial. This high-efficiency fusion technique may be adapted to mediate drug and gene transfer to target cells ex vivo as well as to form hybrid cells with limited cell sources.

A rapid non-selective method to generate quadromas by microelectrofusion

A simple non-selective methodology was developed and standardized to generate desired hybrid-hybridoma or quadroma secreting bifunctional antibodies. This novel protocol is based on microelectrofusion on a meander chamber using a few hundred cells of each of the two parental hybridomas with no laborious drug selection procedures. Seeding approximately 10 cells per well in a 96-well microtitre plate after fusion in 200 microliters standard medium containing 20% FBS and 10% Origen growth factor generated positive quadromas secreting bispecific antibodies with good stability after the second reclone. Compared to the conventional PEG fusion and other methods this simple protocol is both rapid and economical. Generally, conventional methods to make quadromas and triomas require the introduction of drug selection markers into one or both of the parental cells, a procedure that could take 3-6 months. Utilizing the non-selective microelectrofusion method described here, we have generated several quadromas in a very short time. Further, such a protocol could also be potentially adopted to generate human hybridomas with few B cells isolated from peripheral blood lymphocytes enriched by antigen specific panning or affinity microelectrofusions.

Characterization of PEG-mediated electrofusion of human erythrocytes

Polyethylene glycol (PEG) and electrofusion were applied together in a simple and highly efficient cell fusion method. PEG (8000 M(r)) was used to bring human erythrocytes into contact, and a single 4.4 kV/cm, 80 microseconds duration pulse was applied to cell suspensions. The fusion yield (FY) is PEG concentration-dependent. A maximum FY (50%) was found at about 10% PEG. Higher PEG concentrations (> 10%) suppressed FY caused by colloid osmotic shrinkage. Morphological changes, such as colloidal osmotic swelling and shrinking, and the expanding and contraction of fusion lumen, when suspension media were changed from PBS to isotonic 15% dextran solutions, was examined by microscopy. FY was found to depend on both simple osmotic and colloidal-osmotic swelling. From the swelling behavior, we propose two types of electropores: the pre-fusion sites between cell pairs, and electropores on each individual cell connecting intracellular and extracellular space. The latter type is responsible for the colloidal osmotic swelling and shrinking of cell which, together with simple osmotic swelling, is responsible for expanding the pre-fusion sites into fusion lumens. Resealing of electropores resulted in reducing FY, but the FY can be restored by simple osmotic shock. Apparently, PEG plays two opposite roles in this fusion method; one is to promote pre-pulse and post-pulse cell-cell contact, protecting pre-fusion sites, and the other suppresses FY by colloid osmotic shrinkage of cells after pulsing, especially when high PEG concentration is used. 10% PEG 8000 represents the optimal combination of these properties.

Dielectrophoresis of cell-size liposomes

The dielectrophoresis (DEP) behavior of cell size liposomes were studied in the frequency range from 20 kHz to 3 MHz. Liposomes in the size of about 10 microns in diameter were made from egg phosphatidylcholine (PC), egg phosphatidylethanolamine (PE), egg phosphatidylglycerol (PG) and brain phosphatidylserine (PS). These liposomes, having an internal conductivity of 58 microS/cm, were suspended in either PEG or Ficoll solutions of conductivities 9 and 20 microS/cm, respectively. The liposomes were induced to form pearl chains in an electric field gradient in specially designed chambers with either coaxial or flat electrodes. Liposome rotation and convection (mixed positive and negative dielectrophoresis) were observed just beyond the experimental frequency range, within which positive dielectrophoresis leading to pearl chain formation were found. The threshold voltages that caused pearl chain formation were recorded within the experimental frequency range. The threshold voltages remained more or less constant within the positive DEP frequency range, and increased at both ends of the range. Charged liposomes (PS, PG) were found to have lower threshold than uncharged ones (PC, PE). Theoretical DEP spectra were calculated using a model proposed by Kaler and Jones ((1990) Biophys. J. 57, 173-182), for uncharged liposomes. The surface current effect in charged liposomes was accounted for, using an approximation proposed by Schwartz ((1962) J. Phys. Chem. 66, 2636-42). The experimental data agreed with theoretical prediction in general. The higher cutoff frequencies observed experimentally were thought to represent a slight lowering of internal conductivity due to leakage. The higher than predicted ratio between threshold voltages of charged and uncharged liposomes was interpreted to be due to the slight difference in size of these two types of liposomes. The agreement between theory and experiment showed that the available theory was adequate, and that liposomes provide a good model to study the nature of dielectrophoretic forces.

Electrofusion of cell-size liposomes

Cell size liposomes of egg phosphatidylcholine (PC), trans-acylated egg phosphatidylethanolamine (PE), bovine brain phosphatidylserine (PS) and egg phosphatidylglycerol, suspended in 2.5% of polyethylene glycol (M(r) 8000), Ficoll (M(r) 400,000) or Dextran (M(r) 71,200) were aligned by dielectrophoresis and fused by applying a 1.7 kV/cm pulse of varying duration. Because the internal and external media of liposome suspension can be controlled, and the surface charge is known, the results can be described mathematically. The fusion yields (FY) at different pulse length were measured by microscopy. The FY curves were sigmoidal, with a common minimally required pulse length of 19 microseconds, and shifted to longer pulse length with increasing external media conductivity or vesicle surface charge. The types of polymer in solutions had little effect. The minimal required pulse length was interpreted to be the minimum rise time for the smallest vesicle capable of reaching the bilayer breakdown potential induced by the pulse, and the sigmoidal shape FY curves represented the cumulative vesicle size distribution curve. The shifting of FY curves upon changing media conductivity or vesicle surface charge were quantitatively accounted for by the balance of pulse-induced dipole-dipole attraction and electrostatic repulsion. Deviation from sigmoidal shape FY curves in the cases of charged liposomes was explained by increasing electrostatic repulsion due to vesicle deformation under pulse-induced dipole pressure. The data confirm the hypothesis that membrane potential breakdown is a pre-requisite or minimum requirement for electrofusion, and support our earlier proposition that pulse-induced dipole force plays an important role in the electrofusion process, and that electrostatic repulsion posts additional barrier to electrofusion.

Immortalization of antigen selected B cells

This paper reports the generation of monoclonal antibody producing hybridomas from a small number of antigen-specific B cells selected by panning on antigen-coated dishes and rosetting with antigen-coupled paramagnetic beads. Anti-HIV positive B cells from spleen could be recovered by panning with an efficiency of 5% and a purity of 24%. Immunobead selection of anti-HIV positive B cells from the same mice yielded a recovery of 17% and a purity of 7%. Various experimental conditions with respect to the selection of specific B cells were investigated, leading to an optimized protocol for the isolation of a limited subset of B cells. The selected cells retained their property to produce immunoglobulins and could be clonally expanded in the presence of human T cell supernatant and irradiated murine thymoma helper cells to generate sufficient cells for a mini-electrofusion with NS-1 myeloma cells. Up to 78 specific hybridomas could be generated from one anti-HIV positive B cell. An overall efficiency of specific B cell immortalization of up to 10% was obtained.

A human monoclonal antibody, produced following in vitro immunization, recognizing an epitope shared by HLA-A2 subtypes and HLA-A28

In vitro immunization and subsequent immortalization of peripheral blood cells of a multiparous woman has resulted in the production of a stable human mouse heterohybridoma, 5C2A2, secreting an HLA-A2/A28-specific human monoclonal antibody. Although possibly exposed to HLA-A2 by transfusions, the cell donor showed no HLA-A2-specific serum antibodies. The present protocol for in vitro immunization includes the elimination of suppressor cells from the responder cell population, the presence of irradiated allogeneic lymphocytes as a source of antigen, as well as stimuli--recombinant interleukin-2 and a B-cell specific nucleoside analogue--causing the proliferation of B lymphocytes, prior to immortalization. The ability of the antibody 5C2A2 to detect all known HLA-A2 subtypes, except A2.3, and A28, allows identification of the serological epitope on the HLA-A2 molecule. Application of this in vitro immunization method allows the production of a set of HLA monoclonal antibody-secreting human hybridomas, independent of the existence of serum HLA antibodies in the lymphocyte donors.

Selective production of hybridoma cells: antigenic-based pre-selection of B lymphocytes for electrofusion with myeloma cells

Methods for pre-selecting B lymphocytes were studied and investigated. First, biotinylated antigen was used for selecting B lymphocytes. These pre-selected B lymphocytes were then combined with biotinylated myeloma cells by adding streptavidin. The final formula of the selected B cell-myeloma cell was as follows: B cell-(antigen-biotin-strept-avidin-biotin)-myeloma cell. Then, this B cell-myeloma cell conjugate was fused by the pulsed electric field (PEF) method, which fused only those conjugated cells. The fusion efficiency obtained by this method was 3-15-times higher than that obtained by the non-specific poly(ethylene glycol) (PEG) fusion method. Second, avidin-antigen conjugate was used to select B lymphocytes. For this purpose, bifunctional cross-linkers such as N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP) and m-maleimidobenzoyl N-hydroxysuccinimide (MBS) were chosen. Each reagent contains two heterofunctional groups which can make covalent bond with both Lys and Cys residues. Typical avidin-antigen conjugate is expressed as avidin-SPDP (or MBS)-antigen. Thus, final B cell-myeloma cell conjugate was B cell-antigen-SPDP (or MBS)-avidin-biotin-myeloma cell. The yield of this procedure was of the order of 10(-2). Here, we suggest that the pre-selection of B lymphocytes by biotinylated antigen or avidin-antigen conjugate is a new method of obtaining selected hybridoma cells which produce specific monoclonal antibodies against the antigen used for selecting B lymphocytes.

Efficient hybridization of mouse-human cell lines by means of hypo-osmolar electrofusion

The fusion of a mouse-human heteromyeloma with a mouse hybridoma is used as a model to define parameters to generate human hybridomas. Electrofusion of these cells in 300 mosM and 75 mosM solutions showed that strong hypo-osmolar conditions resulted in a dramatic increase in the efficiency of hybridoma formation. In contrast to iso-osmolar electrofusion, a high hybrid yield could be obtained by injection of only a single field pulse. The field strength was adjusted in proportion to the increased size of the cells in hypo-osmolar solutions. Hypo-osmolar electrofusion allowed the generation of approximately 0.45% hybrids at a suspension density of 1.75 X 10(5) mouse-human cells/ml corresponding to an input number of 3.5 X 10(4) mouse-human cells. A further increase in the efficiency of hybridoma formation to about 0.6% was achieved by cell alignment in an alternating field of modulated field strength. Experiments in which the total cell number per fusion chamber was decreased at constant optimum suspension density showed that a further increase in the efficiency of hybridoma formation in hypo-osmolar solution was not possible because of the increasing influence of the heterogeneity of the cell lines with decreasing cell number. The results allow the conclusion that hypo-osmolar electrofusion is a potential tool to enhance successful immortalisation of human B lymphocytes.

Electroacoustic production of murine hybridomas

The optimal conditions for the production of murine hybridomas by electroacoustic fusion of cells in sugar solutions and of cells in ionic strength media (up to 115 mM NaCl) have been investigated. In the electroacoustic fusion technique cells were brought into contact in a 3 MHz ultrasonic standing wave field and were fused by application of an electric pulse. Hybridomas of murine splenocytes and X63-Ag8.653 mouse myelomas were successfully produced in low ionic strength mannitol solutions and in a range of salt concentrations up to 115 mM. The yield of hybridomas by electroacoustic fusion of cells in mannitol was at least as good as that obtained when cells were fused following conventional dielectrophoresis induced contact. The electroacoustic fusion yield was also comparable to conventional yields when cells were exposed to a pulse in higher ionic strength media where dielectrophoresis induces heating effects. Hybridomas were produced at similar electric field strengths when cells were suspended in high ionic strength media (up to 115 mM NaCl) or in mannitol. The effective electric field strength for hybridoma production was close to that at which trypan blue tests indicated membrane damage.

Enhanced hybridoma production by electrofusion in strongly hypo-osmolar solutions

Electrofusion of mammalian cells in strongly hypo-osmolar media containing sorbitol, small amounts of divalent cations and albumin resulted in high yields of hybrids. The number of viable hybrids was higher than any value for chemically- or electrically-mediated fusion reported in the literature. Optimum clone numbers were obtained for fusion of osmotically-stable subclones of murine myeloma cells with DNP-Hy-stimulated lymphocytes provided that the osmolarity of the fusion medium was as low as 75 mosmol/l. Similar results were obtained for fusion of osmotically stable subclones of myeloma cells with the murine hybridoma cell line G8. Due to the dramatic increase in volume the field strength of the breakdown pulse (leading to fusion of the dielectrophoretically aligned cells) has to be reduced, as predicted by theory. The efficacy of hypo-osmolar electrofusion allowed the use of very few cells (about 10(5) lymphocytes or G8 cells per fusion chamber). This figure is considerably smaller than that reported in the literature for iso-osmolar electrofusion. It is significant that, in contrast to iso-osmolar conditions, the fusion yield in hypo-osmolar electrofusion was reproducible over long periods of time and less dependent of variations between cultures. At suspension densities of about 10(6) cells per fusion chamber (normally used in iso-osmolar electrofusion) hypo-osmolar electrofusion of homogeneous cell suspensions resulted in the formation of many giant cells when the appropriate field conditions were applied. Similar high or, at some field strengths, even higher numbers of clones at low cell suspension density were obtained when G8 and myeloma cells were first exposed during the washing procedure to strongly hypo-osmolar media, but then transferred to iso-osmolar solutions for electrofusion. Similar experiments with lymphocytes and myeloma cells failed because of destruction of many lymphocytes by the two osmotic shock steps in rapid succession. Volume distribution measurements of G8 and myeloma cells showed that after re-incubation of the osmotically pre-stressed cells the original volume distribution is largely, but not completely re-established. This and other results indicate that osmotic pressure gradients and associated tensions in the membrane do not play a primary role in the initiation of the electrofusion process. The experiments suggest that due to the osmotic (pre-) stress the membrane permeability is slightly and uniformly increased presumably due to the dissolution of membrane- and cell-skeleton proteins. Obviously, this facilitates electrofusion in hypo-osmolar or subsequently in iso-osmolar solutions.