Guide for generating hybridomas by electrofusion
Natascha Weiss, Garnet Will, Claudia Hofmann, Eppendorf AG, Hamburg, Germany;
Susan Perkins, Department of Pathology, Stanford University School of Medicine, CA, USA
Introduction
Hybridoma technology
In 1975, Köhler and Milstein developed a
procedure to fuse myeloma cells with
B lymphocyte cells from the spleen of an
immunized animal. The aim was to generate
a cell that is characterized by both
the lymphocyte’s property of specific antibody
production and the immortal character
of the myeloma cells. The screening
of the resulting hybrid cells together
with limiting dilutions, enabled the generation
of clones from a single cell fusion
[1]. Hybridoma clones can be maintained
in culture, and will continue to secrete
antibodies.
Today, monoclonal antibodies (Mab) set
the stage for many different applications,
e.g. tests for the presence of specific
antigens, studies of cross-reactivity
among antigens, and antigen purification.
The development of techniques to produce
murine monoclonal antibodies in combination
with a variety of applications has
led to advances in many fields of biomedical
research and diagnostic procedures.
The hybridoma technique is now a routine
application performed world wide and
modified in many details since its first
publication. Up to now PEG (polyethylene
glycol) is the most frequently used agent
for inducing cell fusion. Nevertheless,
PEG-induced fusion is only partly controllable
since the success of a fusion
depends on minute details such as the
size and shape of cells as well as the
intensity of shaking, which are difficult to
standardize [2].
An efficient alternative is electric field-mediated cell fusion,
which offers several advantages over conventional biological
(virus-induced), and chemical (PEG-induced) fusion
methods:
● Considerably higher efficiencies for many cell types
● Better reproducibility
● Significantly lower amount of B cells required
● Fast and easy-to-use protocol
● Better growth properties in the early stage following fusion
● Possibility of tailor-made fusion protocols for specific cells
● Direct control of fusion-results
Basics of electrofusion
Applying an electrical field to cells in the form of short,
intense pulses increases the permeability of the membrane
as well as the membrane flow. The resulting local perforation
of the cell membrane allows cell fusion to form a
new cell with the properties of the fusion partners. The
prerequisite for successful electrofusion is the direct
membrane contact between the individual cells before
the electrical pulse is applied.
The process of electrofusion comprises three steps:
? Convergence and contact of the cells (pre-alignment)
The close membrane contact required for cell fusion
is induced by positive dielectrophoresis. An inhomogenous
electrical field causes a dipole to build up
within the cells, in turn causing them to move
toward the point of maximum strength (i.e. the electrodes)
and attract each other, forming long strings
(“pearl chains”).
? Membrane fusion
Very short field pulses with high intensity lead to a
reversible electrical breakthrough of the cell membranes.
This results in the formation of temporary pores
in the contact zone of two cells, which may then
become fused.
? Rounding off of the fusion product (post-alignment)
Rounding off the fusion products is carried out by
expanding the primary fused membrane sections,
resulting in a so-called plasmogamy.
Materials for electrofusion
Cell culture devices
● Sterile bench
● Incubator at 37 °C with CO2 atmosphere
● Microscope with a slider holder
● Centrifuge for cell centrifugation (e.g. Eppendorf Centrifuges 5702 R, 5804 R, 5810 R)
● Neubauer chamber or electronic cell analyzer
● Automated pipetting device (e.g. Eppendorf Easypet)
Cell culture disposal
● Cell culture flasks
● Conical centrifuge tubes
● Serologic pipettes (since many cells will adhere to glass, plastic pipettes or tips should be preferred)
● 24-well plates or 96-well microplates for cultivating the fusion products
Cell culture media
? Complete cell culture growth medium (CGM): e.g.
RPMI 1640, DMEM or IMDM as a basic medium
supplemented with FCS
Adding serum to media has a crucial effect on the
growth of hybrids. When fetal calf serum (FCS) is
used in the cell culture medium, the endotoxin
content must be kept to a minimum. Different manufacturers
offer FCS which has been specially tested
for hybrid growth after fusion.
? Post-fusion medium: e.g. complete cell culture growth
medium without phenol red
In a cell culture medium, phenol red acts as a pHindicator.
However, it has a toxic effect within the
cell. Treating the cells only with phenol red-free
medium following fusion, i.e. for as long as the cell
membranes are partially permeable, is therefore
recommended. Phenol red may be added to the
medium 24 hours after fusion.
● Selection medium: e.g. HAT (hypoxanthine-aminopterin-
thymidine) medium
100 x HT
Dissolve 136 mg hypoxanthine and 38.75 mg thymidine
in 100 ml of 0.02 M NaOH pre-warmed to
60 °C. Cool, filter sterilize, and store in aliquots at
–20 °C.
100 x A
Dissolve 1.9 mg aminopterin in 100 ml of 0.01 M
NaOH, filter sterilize and store in aliquots at – 20 °C.
HAT medium
Add 1 % (v/v) 100 x HT and 100 x A to the cell
culture medium.[3]
? Trypan blue (0.5 %) or propidium iodide (2 µg/ml) for
viability tests.
Reagents for cell isolation
● Ficoll™ separating solution (density: 1.077 g/ml)
● PBS (without Ca2+ and Mg2+)
● Collagenase II/DNase solution, final concentration 10 U/ml DNase, 1.5 mg/ml collagenase II
(e.g. dissolve 2,000 units of DNase in 10 ml cell culture
growth medium (CGM). Add 30 mg collagenase II to
1 ml of this solution and fill with CGM until the total
volume is 20 ml. Carry out sterile filtration.)
Reagents for B cell stimulation
Since small, unstimulated lymphocytes are usually very
difficult to fuse, fusion is always performed with activated
B cells [4]. The stimulation can be performed in vivo or in vitro.
One of the following reagents/methods can be used for
the in vitro stimulation, depending on the antigen system
and the source of B cells.
● LPS (lipopolysaccharide) activation [5,6]
● PHA (phytohemagglutinin) activation [7,8,9]
● EBV (Epstein-Barr-Virus) activation [10]
● PWM (pokeweed mitogen) activation [11]
● PWM plus IL-2 [7,12]
● IL-6 plus IL-2 plus muramyl dipeptide [13]
● Anti-CD40 antibodies in combination with IL-4 [7,14,15]
Electrofusion system
Multiporator with fusion module
The Eppendorf Multiporator is an innovative, compact
and easy-to-use tool for the electrofusion of cells. In
combination with a special hypoosmolar, low conductive
buffer system, factors which damage the cells (e.g. long
pulse times and excess voltage) can be reduced to levels
that have no adverse effect on the cell physiology [16].
The Multiporator regulates the voltage impulse used for
electrofusion as well as the alternating voltage required
for cell alignment.
Micro fusion chamber
Microscopic monitoring of the cells on the Micro fusion
chamber allows the optimization of the parameters for
alignment and for fusion. The ideal values can be transferred
into the Helix fusion chamber for further fusion
purposes. The Micro fusion chamber consists of a housing
containing a transparent reservoir with two electrodes
located 200 µm apart. It is connected to the
Multiporator by a coaxial cable.
Helix fusion chamber with
insert
The Helix fusion chamber is
specially designed for obtaining
large amounts of fusion
products (hybrids). It consists
of a conically tapered core
which bears the electrodes
(space between electrodes:
200 ìm) and a beaker in which
the cells are placed (250 ìl).
Both parts are screwed together
and linked to the Multiporator
via the insert.
Fusion medium (Eppendorf Hypoosmolar and
Isoosmolar Electrofusion Buffer)
The electrofusion media from Eppendorf are tailor-made
for the Multiporator system. An optimal result can be
expected under hypoosmolar conditions, i.e. the cells should swell slightly. The membrane and
cytoskeletons are temporarily loosened and fusion in the
electric field takes place much more efficiently.
Preparation of cells
General information concerning cell culture
? It is important to develop a consistent system of cell
selection, care, and feeding.
Fusion parameters can vary with different states of
cell activation, different feeding schedules, different
cell culture media and the individual cell populations.
Even a cloned cell population is not 100 %
homogeneous. Feeding the cells the day before
fusion ensures that they are all in the same cell
cycle [17]. The cell population should be in the
exponential growth phase.
● Cell cultures must be tested for mycoplasm.
If the cell membranes are attacked by mycoplasm,
the necessary cell contact during the alignment is
not possible, and fusion cannot take place.
● The centrifugal force necessary to pellet your cells
optimally with your equipment must be known in order
to reduce cell loss in the wash steps [10].
Isolation and cultivation of fusion partners
If not otherwise stated, all centrifugation steps are performed
at room temperature.
Cultivation of myeloma cell lines
An appropriate myeloma cell line with a selectable characteristic
(e.g. sensitivity to HAT medium) has to be chosen
as a fusion partner. The cells should be cultivated at least
six days before the fusion experiment. If they do not grow
rapidly and healthy a longer time may be necessary [18].
Split the cells one day before fusion. The cultivation and
harvesting procedure depends on the cell line.
Isolation of lymphocytes
Isolating lymphocytes from peripheral blood
● Add 20 ml Ficoll™ separating solution to a 50 ml centrifuge tube.
● Mix 10 ml blood sample with 10 ml PBS.
● Coat the Ficoll™ separating solution carefully with the same volume of diluted blood sample.
● Centrifuge for 20 min at 1,200 x g and room temperature (start with the slowest acceleration ramp and switch off the brake!).
● Transfer the lymphocyte band with a pipette to a new centrifuge tube.
● Dilute the lymphocytes in 10 ml PBS and centrifuge for 10 min at 300 x g and 8 °C.
● Dilute the lymphocyte pellet in complete cell culture growth medium and wash once again for 8 min at 200 x g and room temperature.
● Dilute the lymphocytes in complete cell culture growth medium. Set the cell density to 1-2 x 106 cells/ml.
Isolating lymphocytes from tumor material or from lymph
nodes
? Cut the tumor/lymph nodes using a pair of scissors.
? Incubate for 60 min in collagenase/DNase at 37 °C in
a CO2 atmosphere. Shake occasionally.
? Strain the tumor material/lymph nodes through a stainless
steel sieve (Sigma).
? Dilute the cell material in 10 ml complete cell culture
growth medium and centrifuge for 10 min at 200 x g.
? Resuspend the pellet in complete cell culture growth
medium and set to a cell density of 1-2 x 106 cells/ml.
Activation/stimulation of lymphocytes
In vivo activated B cells
In traditional murine MAb technology, subjecting the
mouse to extensive hyper-immunization programs with
highly immunogenic compounds usually causes B cell
activation. Human subjects may only be immunized with
a very limited number of immunogens (mostly after vaccinations
or naturally occurring infections) [4]. In this
case an in vitro activation is generally not necessary.
In vitro activated B cells
In vitro activated B cells, which are highly suitable for
electric field-mediated hybridization, can be obtained by
stimulation with polyclonal activators (see sec. II.) [4].
Note: If feeder cells are required (see sec. IV., 8.), prepare
these 1 to 3 days before electrofusion.
Electrofusion
For defining the electrofusion protocol the following
steps have to be performed:
● Determination of parameters for each cell population
separately in the Micro fusion chamber.
Before carrying out electrofusion in the Helix fusion
chamber, it is important to gain experience with the
myeloma cell line and a variety of stimulated B cells
in the Micro fusion chamber [10].
Determine the range and limits for the myeloma cell
line in advance. This cell line represents the only
constant of the fusion system [17].
● Optimization of parameters with the two cells together
in the Micro fusion chamber.
Use a range between the parameters determined in
previous experiments to begin the optimization [10].
● Electrofusion experiment in the Helix fusion chamber.
Parameters determined in the Micro fusion chamber
can be transferred directly to the Helix fusion chamber.
Points to consider before starting
? Make a table for the different conditions you will test in
one round ahead of time so that everything important
can be recorded quickly (see supplement for an example)
[17].
? Have all equipment and reagents ready and available
for use.
It is important to minimize the length of time the
cells spend in sub-optimal growing conditions
(e.g. incubation at room temperature, without CO2,
in solutions without nutrients) [10].
? The fusion temperature should be between 20 °C and
30 °C.
Outside of this temperature range, the fusion rate,
and thus the yield of hybrid cells, decreases drastically.
Preparing the electrofusion experiment
The following steps have to be carried out for each fusion
partner separately.
Testing of the fusion buffer
Prior to the fusion experiment an empirical test has to be
performed to determine the optimal conditions. A minimum
of 90 % should survive in the Eppendorf Hypoosmolar
Electrofusion Buffer after an incubation period of
30 minutes at room temperature.
For sensitive cells which undergo lysis, the osmolarity
should be increased by gradually mixing the Eppendorf
Hypoosmolar Buffer with the Eppendorf Isoosmolar
Buffer until the survival rate of the cells exceeds 90 %. In
some cases it might be necessary to use pure Eppendorf
Isoosmolar Buffer.
Determine the diameter of the cells after an incubation
period of 15 minutes in the optimized buffer mixture
under a microscope. The diameter will be the starting
point for the calculation of the pulse voltage (see equation
in supplement).
Washing of cells
Cells must be washed free of serum protein prior to fusion.
They should therefore be collected and washed twice
with the previously tested electrofusion buffer. Care must
be taken not to lose cells by centrifuging too gently, using
too great a volume of washing medium or centrifuging too
harshly.
Some populations, such as EBV activated B cells, have
increased fusibility in hypoosmolar media for a very short
period of time only. In this case washing is performed in
isoosmolar medium and cells are then fused after 10
minutes in the hypoosmolar media.
After washing, the cell number is set as required [10].
Note: The cells should not remain in the Eppendorf
Electrofusion Buffer longer than 30 minutes!
Adjusting the cell density
Count the cells before you start. A cell density of 1-3 x
106 cells/ml will be roughly equivalent to a fusion of 3 x
105 – 8 x 105 cells in the Helix fusion chamber. A higher
cell density, especially with large cells, will make it very
difficult to observe cell fusion in the Micro fusion chamber
[17].
Optimizing parameters for one cell line
? Make Micro fusion chambers available to optimize
fusion parameters and as many cell preparations as
possible beforehand [17].
Since timing is critical it is important that more than
one condition can be tested after washing [17].
? Pipette an aliquot of the cell suspension (20 to 50 µl)
onto both electrodes of the Micro fusion chamber so
as to completely cover them.
● Place the chamber under a microscope.
Either a normal or an inverted microscope is suitable.
A slide holder is advantageous to prevent the
chamber from moving. Connect the Micro fusion
chamber to the insert of the Multiporator. Focus the
microscope on the electrodes under high magnification
[10].
? In general standard values for electrofusion protocols
will be found in the following range:
Alignment: 4-6 V, 20-30 sec
Pulse: 20-60 V, 15-20 μsec, no. of pulses 1-3
Post-alignment: 4-6 V, 20-30 sec
These fusion parameters can be used as basic values for optimization.
● The first step is to determine the alignment parameters.
Before a square wave pulse can fuse the cells, they
must be aligned in chains between the electrodes.
Fusion will occur at the flattened areas of the cell
membranes, where the cells are in close contact
with one another. Using a higher field strength than
necessary during alignment will lead to fewer cells
remaining viable through the fusion process. An
insufficient field strength will cause the cells to rotate
and fail to produce the required membrane contact
between cells [10].
Observe the voltage at which the cells start to align
themselves. The length of time it takes cells to align
depends on several parameters, e.g. how well the
cells are washed, their number, viability and size.
For fusion with about 3 x 105 – 8 x 105 cells, 30 sec
are adequate for alignment. For greater numbers of
cells, the alignment time shows greater dependence
on the specific cells being fused [10,17].
Cells not washed sufficiently or with very poor viability
will not align well [10].
? The second step is to determine is the square wave
pulse needed for fusion.
The pulse voltage required for fusion depends on the
cell (e.g. size, shape and state of activation) and the
fusion medium. Higher voltages are required with
smaller and less activated cells, and lower voltages
with larger and more activated cells [10].
The field strength necessary for permeating the
membrane of a spherical cell (e.g. lymphocytes, myeloma/
hybridoma cells) can be calculated using an
equation (see sec. IV., 2. and VI., 1.).
To analyze the fusion process, observe the behavior
of the cells under the microscope. Fusion does not
take place immediately. Observe intermittently for 10-
30 min without removing the chamber from the
microscope during this period. If a removal of the
chamber is necessary be careful not to disturb the
cells [10].
Ideally, in a very homogeneous population chains of
fused cells form within 10-15 minutes in Hypoosmolar
Electrofusion Buffer and within 20-30 minutes in
Isoosmolar Electrofusion Buffer. If the voltage is too
high, many cells will lyse or start to lose membrane
integrity immediately. In this case try a lower voltage.
If no cells fuse, try a higher voltage [10]. If too many
giant cells are seen after the pulse a lower cell density
should be used [17].
● The last step is the optimization of the post-alignment
parameters.
Often, the parameters established for the first alignment
step can also be used for the post-alignment.
Sometimes, however a new optimization may be
required, especially for sensitive cells. In this case try
a lower voltage and a shorter time [10].
Optimizing parameters for two different cell lines
After determining the optimum fusion parameters for
each cell population, the two cell populations can be
fused at once. Following is an outline of the process [10].
? Wash each cell population separately as described
above [10].
? Pool cells in different ratios to work with in the Micro
fusion chamber. Start with a 1:1-2 ratio to determine
the voltage and time range. The optimal pulse voltage
depends on the particular cell ratio [10].
The goal of the process is to establish optimized
parameters for generating hybridomas consisting of
one B cell fused to one heteromyeloma. Because the
cell populations differ, they will tend to migrate to the
electrodes at different rates rather than randomly
distributed. This often leads to two cells of one
population fusing with a third cell of the other population.
With uneven cell ratios this tendency is increased,
leading to the formation of unstable hybrids [10].
? Start with the voltage and time range determined in the
previous experiments (see sec. IV. 3.).
Observe which population is lost when the voltage
becomes too high. In general, a relatively high voltage
is used to insure fusion of the activated B cells.
However, this intensity will tend to lyse significant
numbers of heteromyelomas. The loss can be tolerated
because the objective is the fusion of activated
B cells and their number is limited [10]. Therefore parameters
for the valuable B cells should be favored.
Cleaning the Micro fusion chamber
The content of the Micro fusion chamber is rinsed out
using bi-distilled water from a spray bottle. Particularly
stubborn cell residue can be removed by carefully cleaning
the electrodes with a soft toothbrush using vertical
strokes (the space between the electrodes must not be
changed during this procedure!). Rinsing the chamber
with 70 % non-denaturated ethanol accelerates the drying
process.
Transfer parameters to Helix fusion chamber
● Based on the parameters optimized with the Micro
fusion chamber determine the cell ratio and the cell
density necessary for fusion [10].
? Pool an appropriate cell number from both populations
of cells in one 15 ml conical tube and centrifuge as
above. Aspirate the medium [10].
? Wash the cells twice with 5-10 ml of fusion buffer (by
using lower cell numbers e.g. 3 x 105 cells, the number
of washing steps and volume must be reduced) [10].
Note: In order to guarantee a successful electrofusion
the overall incubation time in the Eppendorf
Electrofusion Buffer must not exceed 30 minutes.
● After the second wash aspirate to a dry pellet [10].
? Resuspend the cell pellet in Eppendorf Electrofusion
Buffer [10].
The exact volume to add depends on the cell number.
Hypoosmolar Electrofusion buffer fusions in the
Helix fusion chamber are performed with more than
2 x 105 and less than 106 cells total. In Isoosmolar
Electrofusion Buffer more than 106 cells are required
[10,17].
? Carefully pipette approx. 250 µl cell suspension onto
the bottom of the beaker of the Helix fusion chamber.
Avoid wetting the edges or the inner wall, since this
impairs the filling of the Helix fusion chamber.
Air bubbles may form, which reduce the effectiveness
of the experiment. Insert the electrodes carefully
into the beaker and screw both parts of the
chamber together slowly but continuously. This
action causes the gap to decrease in width, which
forces the cell suspension evenly upwards. Leave
the closed Helix fusion chamber(s) upside-down on
the coaxial connection until starting the fusion process.
● After attaching the Helix fusion chamber to the coaxial
connection of the insert by screwing it a quarter-turn,
place the insert into the Multiporator.
● Program the device and start the electrofusion.
Usually, in Hypoosmolar Electrofusion Buffer the
optimized parameters from the Micro fusion chamber
can be transferred directly to the Helix fusion
chamber. Using Isoosmolar Electrofusion Buffer
parameters can be approximated only because cell
density in the Helix fusion chamber is generally
much greater than what can be observed in the
Micro fusion chamber [17].
● Following fusion, the cells remain in the Helix fusion
chamber for 10 minutes at room temperature.
Using Eppendorf fusion buffer with higher osmolarity
than the Eppendorf Hypoosmolar Buffer may require
a longer time [17].
? After the chamber is opened, the core is rinsed into
the beaker with 1 ml post-fusion medium. Transfer the
contents of the beaker to a conical tube. The desired
cell concentration can be set by adding post-fusion
medium as required.
? The cells are plated out on special cloning plates or
microplates and incubated for 24 hours at 37 °C in a
CO2 incubator.
If the cell density of the fused cells is low, feeder
cells can be plated as well in order to support the
initial growth of the cloned cells.
Cleaning the Helix fusion chamber
The beaker and core of the Helix fusion chamber should
be rinsed with distilled water directly after the experiment
in order to prevent drying of the cell and buffer residue.
If heavily contaminated, the Helix fusion chamber should
be cleaned for 3 minutes in an ultrasonic bath (possibly
with a supplementary cleaning agent, such as Edinosite
Super) or with a very soft (tooth) brush. When cleaning
using brushes, make sure that to brush in the same
direction as the windings, since the electrodes may
otherwise move out of their correct position and thus
render the Helix fusion chamber unusable. Disinfect the
parts with ethanol (70 %, non-denatured) by filling the
beaker with 250 µl of ethanol and screwing beaker into
the core. Unscrew the core after 10 seconds and remove
the alcohol. Then set the beaker and the core in the
stand to dry under sterile conditions. After drying, the
Helix fusion chamber can be used again.
Post-fusion treatment
Feeder layers
Whether a feeder layer is required depends on the cell
line, the fusion parameters and the plating density. These
are found to be most useful under harsh conditions, such
as high voltage, and low plating density [17].
Feeder layers are commonly prepared from embryonic
fibroblasts, peripheral blood lymphocytes or peritoneal
exudate cells. Peritoneal or peripheral blood cells can be
used directly since the cells do not divide in culture.
Fibroblast feeder cells must be pre-treated by exposing
the cells to gamma radiation or incubating them with
mitomycin C [19].
Selection
A selective medium (HAT medium) which contains aminopterin
(A) to inhibit the growth of the myeloma fusion
partner is used following the fusion. Since lymphocytes
will die after a few days in culture, only the hybrid cells
which result from fusion of a myeloma cell and a lymphocyte
will survive [13, 20].
To eliminate all undesired cells, the selective pressure
must be maintained for at least 15 days. Following this,
the cells are transferred to medium containing a supplement
of HT (hypoxanthine, thymidine) for the same
length of time, allowing the adaption to the main
pathway. In the last step H and T can be removed and
the cultures can be kept in normal cell culture growth
medium [20].
Hybrids are fed 2-3 times/week after their removal from
the selection medium. When the medium becomes yellow
as a result of cell growth the supernatant is ready to
test for antibody secretion. Mouse/human heteromyeloma
fusion partner fusions are ready to assay for activity
approximately 2-6 weeks after fusion [17].
Screening
Many different procedures can be used to screen hybridoma
culture supernatants. Bound monoclonal antibodies
can be detected by using tagged secondary antibodies
(e.g. fluorescent, radio-labeled, enzyme-linked) or
functional assays, e.g. inhibiting the binding between the
ligand and its receptor. Here it is important that the
assay is fast, reliable, and specific. Most hybridomas
grow rapidly with a generation time of 10-12 hours (rat)
to 15-24 hours (mouse), precluding long-term assays [3].
Short protocol
Electrofusion in the Helix fusion chamber
? Harvest both cell lines by centrifugation.
? Count the number of cells and set the cell density.
? Pool an appropriate cell number of both populations of
cells, centrifuge and aspirate the medium.
? Wash cells twice with Eppendorf Electrofusion Buffer.
Note: In order to guarantee a successful electrofusion
the overall incubation time in the Eppendorf
Electrofusion Buffer must not exceed 30 minutes.
? After the second wash aspirate to a dry pellet.
? Resuspend the cell pellet in Eppendorf Electrofusion
Buffer.
? Carefully pipette approx. 250 µl cell suspension onto
the bottom of the beaker of the Helix fusion chamber
and fuse as soon as possible (during the waiting period
leading up to fusion, turn the chambers upsidedown).
? Attach the Helix fusion chamber to the insert and
place the insert into the Multiporator.
? Program the device and start the electrofusion.
? Following fusion, the cells remain in the Helix fusion
chamber for 10 minutes at room temperature.
? Unscrew the chamber(s) and rinse the electrode core
with 1 ml post-fusion medium in the beaker.
? Transfer the contents of the beaker to a conical tube.
? Plate the cells on special cloning plates or microplates
and incubate at 37 °C.
Optimization guide
During the optimization process using the Micro fusion
chamber the following phenomena can occur. Below are
some suggestions to help you obtain better results.
These parameters must not only be modified separately,
but also in combination with each other to achieve efficient
fusion [17].
1. What to do if not enough cells fuse?
● Higher pulse voltage
● More pulses
● Longer pulses
● Buffer with lower osmolarity
● Longer alignment times
● Wash the cells more thoroughly
2. What to do if too many cells die?
● Lower pulse voltage
● Fewer pulses
● Shorter pulses
● Buffer with higher osmolarity (more isoosmolar)
3. What to do if too many giant cells form?
Optimization of two cell fusions rather than multiple cells
● Reduce the total cell number in the chamber
● Decrease the alignment time
● Change the fusion ratio
● Try less hypoosmolar buffer
Appendix
Relation between cell radius and fusion voltage
Calculation of the critical field strength
The cell diameter is determined by incubating the cells in
(previously tested) Eppendorf Electrofusion Buffer
(15 minutes at room temperature) either microscopically
or by using a cell counter. With the aid of the diameter,
the critical field strength can be calculated:
Ec=Vc/0.75 x d
Vc: Permeation voltage
(for eukaryotic cells: 1 V at 22 °C) (V)
Ec: Critical field strength at which the membrane is permeated at the points on the cell surface oriented vertically to the applied field (V/cm)
d: Cell diameter (cm)
2 x Ec = Critical field strength for two-cell and three-cell fusions.
Calculation of the fusion voltage
U= 2 x Ec x d
U: Voltage to be set on the device (V)
2 x Ec: Critical field strength for two-cell and threecell fusions (V/cm)
d: Distance between electrodes (2 x 10-2 cm for Micro fusion chamber and Helix fusion chamber) (cm)
Example of an optimization table sheet [17]
Cell line
*Observations: Examine the cells fusing under the microscope with 10 x phase
● Are the cells still aligned?
● How quickly does fusion take place?
Table of pulse voltages for different cell sizes
References
[1] Köhler, G. et Milstein, C., 1975, Continuous cultures of fused cells secreting antibody of predefined specifity.
Nature 256, 495-497
[2] Karsten, U., Stolley, P., Walther, I., Papsdorf, G., Weber, S., Conrad, K., Pasternak, L., Kopp, J.,1988, Direct
comparison of electric field-mediated and PEG-mediated cell fusion for the generation of antibody producing
hybridomas. Hybridoma 7, 627-633
[3] Shepherd, P. et Dean, C., Preparation of rodent monoclonal antibodies by in vitro somatic hybridization. In
Shepherd, P. et Dean, C. (eds.), 2000, Monoclonal antibodies: a practical approach, Oxford University Press
[4] Zimmermann, U., Electrofusion of cells: State of the art and future directions. In Zimmermann, U. et Neil, G.A.,
1996, Electromanipulation of Cells, CRC Press, Inc.
[5] Yoshinari, K., Arai, K., Kimura, H., Matsumoto, K., Yamaguchi, Y., 1996, Efficient production of IgG human monoclonal
antibodies by lymphocytes stimulated by lipopolysaccharide, pokeweed mitogen, and interleukin 4. In Vitro
Cell Dev Biol Anim, June 1, 32 (6), 372-377
[6] Zimmermann, U., Love-Homann, L., Gessner, P., Clark, D., Klöck, G., Johlin, F.C., Neil, G.A., 1995, Generation of a
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General literature (available on the Eppendorf homepage at www.eppendorf.com)
● Multiporator Basic Applications Manual Electrofusion (B 4308 900.105)
● Multiporator Operating Manual (B 4308 900.016)
● Cell specific electrofusion protocols
Ordering information
Trademarks
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Acknowledgements
We are greatly indebted to Professor Steven Foung, Department of Pathology, Stanford University School of
Medicine, CA, USA, for his valuable assistance in the preparation of this application.
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