Preparation of Monoclonal Antibodies
In 1975 Kohler and Milstein reported the outcome of fusing antibody-producing mouse spleen cells with a mouse myeloma cell line. The monoclonal antibodies produced were of the specificity dictated by the spleen cells but in the quantity characterized by the myeloma cell line. From this beginning the preparation of monoclonal antibodies has become an important tool in almost every scientific discipline. No longer restricted to the immunology laboratory, monoclonal antibodies are used in a variety of experimental techniques and diagnostic assays and now, with improvements in molecular engineering techniques, are realizing their potential in therapeutic applications (Grillo-Lopez, 2000).
When the need arises to produce monoclonal antibodies to a particular antigen, the decision to establish the technology, use commercial enterprises, or approach private groups to produce them has to be made.
For laboratories that decide to establish the technology, this article describes a method that has evolved in our laboratory since the mid-1980s. The method is robust, in routine use, and has produced hundreds of monoclonal antibodies to a wide variety of antigens. The method uses commercial high-quality reagents, including cloning supplements that replace the messy and wasteful use of in vivo-derived feeder cells. The reagents detailed are the ones used in our laboratory but other manufacturers make equivalent reagents.
The intention of this article is to describe the reagents, equipment, and techniques used to prepare monoclonal antibodies. Full reviews of monoclonal antibody production, including methods for characterization, purification, use of monoclonal antibodies, and antibody engineering applications, are available in a number of monographs, including Campbell (1991), Donohoe et al. (1994) and Goding (1986).
Sterile culture technique and accurate record keeping are essential for success in monoclonal antibody preparation. The sterile culture technique is not difficult to learn, with attention to detail paramount for success. There are many excellent texts on basic and advanced tissue culture techniques, including Freshney (2002) and Harrison and Rae (2003).
II. MATERIALS AND INSTRUMENTATION
A. Essential Equipment
Sterile laminar flow hood, benchtop centrifuge, liquid nitrogen storage or equivalent, 37°C water bath, and a 37°C, 5% CO2 gassed tissue culture incubator. An inverted microscope to view cell growth is also useful.
B. Cell Culture Reagents
RPMI 1640 (10 × 500ml) (Cat. No. 21870-092), penicillin (10,000 U) / streptomycin (10,000 µg), L-glutamine (29.2 mg) (PSG×l00, Cat. No. 10378-016), 10 mM sodium hypoxanthine, 40µM aminopterin, 1.6mM thymidine (HAT×l00, Cat. No. 31062-011), 10mM sodium hypoxanthine, and 1.6mM thymidine (HT×l00, Cat. No. 11067-030) are from GIBCO Invitrogen Corporation. Fetal bovine serum (FBS, Cat. No. 12003-500M) is from JRH Biosciences; select batches of FBS that have been screened for hybridoma production. Complete Freund's adjuvant (CFA, Cat. No. 77140) and incomplete Freund's adjuvant (IFA, Cat. No. 77145) are from Pierce Biotechnology Inc. Hybridoma fusion cloning supplement (HFCS, Cat. No. 592247800) and polyethylene glycol 1500 in 75 mM HEPES, sterile and fusion tested (PEG, Cat. No. 783641), are from Roche Diagnostics. Red blood cell lysing buffer (8.3g/liter NH4Cl in 0.01M Tris-HCl) (Cat. No. R-7757) and trypan blue solution (Cat. No. T- 8154) are from Sigma. Dimethyl sulfoxide (DMSO, Cat. No. 102950) is from Merck. ELISA-based isotyping kits are available from a number of manufacturers, including Roche Diagnostics (Cat. No. 1183117) and Pierce Biotechnology Inc (Cat. No. 37502). Store all stock reagents as recommended by the manufacturer.
C. Cell Culture Materials
Tissue culture plates, 24-well (Cat. No. 143982) and 96-well (Cat. No. 167008) flat-bottom plates with lids, 25-cm2 (Cat. No. 136196) and 80-cm2 (Cat. No. 178891) tissue culture flasks, cryotubes (Cat. No. 375418), 10-ml disposable pipettes (Cat. No. 159633) and petri dishes (Cat. No. 150350) are from Nalge Nunc Int. V-bottom 30-ml tubes (Cat. No. 128A) and wide-bore graduated transfer pipettes (Cat. No. PP88SA) are from Bibby-Sterilin Ltd.
A container (Cat. No. 5100 Nalgene Nunc) or a foam box with walls of about 1 cm thick is required for the control rate freezing of cells.
In addition, we use Gilson Pipetman 5- to 20- and 20- to 200-µl pipettes and a Gilson Distriman repeating pipette for cloning, all with sterile tips. Hamilton 500-µl glass luer lock syringes are used for immunization (Cat. No. 1750 Hamilton, Reno. Nev). Sterile forceps and scissors are also required.
D. Myeloma Cell Line
Several myeloma cell lines are commonly used for fusion with murine spleen cells; most have been derived from the P3X63Ag8.653 murine myeloma line (Kearney et al., 1979). The myeloma clones SP2/0 (Shulman et al., 1978) and FO (de St Groth and Scheidegger, 1980), derived from P3X63Ag8.65, are commonly used. These cell lines are available from the American Type Culture Collection (ATCC) (http://www.atcc.org) as CRL1580, CRL1581, and CRL1646, respectively. We use P3X63Ag8.653 cells that have been maintained in our laboratory for many years, which are screened regularly for mycoplasma, and we select batches that have been previously successful in hybridoma production.
The preparation of monoclonal antibodies depends on a series of steps that require attention to detail and careful laboratory management. The major requirement is time; Table I gives an approximate time frame for monoclonal antibody preparation. Phase I is largely taken up by the immunization protocol, ensuring that the screening method is working, growing the myeloma cells, and finally performing the fusion. Phase 2 encompasses testing, cloning, and freezing and is more labor-intensive. However, the workload is variable and unless the screening method is cumbersome or many fusions are carried out, it is not a fulltime occupation.
In our experience the secret to success in monoclonal antibody preparation is the screening method. Ideally the method should be in place before the animals are immunized. The method needs to be robust and reliable and, as there may be 30 to 50 samples to screen a day, reagents and equipment need to be readily available. The method should be appropriate to the intended use of the monoclonal antibodies. Although many antibodies do perform in alternative protocols, there is no guarantee unless they have been screened appropriately. Some suggestions for screening methods are included in Table II, but detailed descriptions are outside the scope of this article.
B. Antigen Preparation and Immunization
A number of protocols that we have used to immunize mice to a variety of antigens are shown in Table II. Animals can make antibodies to a wide range of molecular structures with two important general exceptions: animals will not usually make antibodies to self-antigen (see Section V) and will not usually recognize small molecules. Small molecules need to be conjugated to a carrier protein. Selection of the carrier protein appears to be largely personal, but the ease of and position of conjugation are important criteria. When linking peptides to carrier proteins, Landsteiner's principle (Landsteiner, 1945) should be taken into account, that antibody specificity tends to be directed to epitopes of the hapten furthest removed from the functional group linked to the carrier protein.
Commonly used carrier proteins include keyhole limpet hemocyanin (KLH) and bovine serum albumin (BSA). If you are going to screen for monoclonal antibodies by ELISA, make both peptide-KLH and peptide-BSA combinations; immunize with one and screen against the other. There are a number of methods for the conjugation of peptides to carrier proteins and the protocol selected will depend on the amino acid sequence of the peptide and the epitope of interest. However, if you are having a peptide synthesized, consider adding a biotin tag; a streptavidinconjugated carrier protein can then make a convenient carrier.
If high-affinity IgG monoclonal antibodies are required, the immunization protocol needs to invoke T-cell help. Hence, adjuvant and several booster injections may be needed and test bleeds should be examined for the presence of antigen-specific IgG. If IgG isotype monoclonal antibodies are essential, they should be screened for specifically.
The protein antigen immunization schedule used in our laboratory is shown in Table III. Most protocols require antigen in adjuvant for immunization. Complete Freund's adjuvant (CFA) is highly effective but may cause toxic side effects. A number of new adjuvants are becoming available, including Hunter's TiterMax (http://www. titermax, com), Pierce' s AdjuPrime Immune Modulator (http://www. piercenet.com), and RIBI adjuvant systems (http://www.corixa.com) among others. Advice from the Institutional Animal Ethics Committee should be taken on which adjuvants are recommended.
If CFA is used, it should only ever be used subcutaneously for the primary immunization and Freund's incomplete used for subsequent boosts.
Complete Freund's adjuvant causes inflammation and granuloma formation in mice and will cause severe injury in humans. Take suitable precautions to prevent self-injection and employ eye protection.
Six-week-old female BALB/c mice are preferred for immunization as the myeloma lines used for fusion were derived from that strain. Routinely, we immunize six animals, in batches of two, with a 2-week interval between immunizing the next two animals.
Wear eye protection.
C. Test Bleeds
Analysis of a test bleed can allow the fusion to be postponed and the animals reimmunized if the antibody titer is low or nonexistent. Test bleeds should only be carried out by experienced personnel. Test bleeds should preferably be from the lateral tail vein using a 26-gauge needle and syringe. A maximum of 1% of the body weight of the animal should be collected, about 100-200 µl. Larger volumes of blood can be collected at the time of euthanasia and can provide a valuable positive control sample for the screening protocol.
D. Myeloma Cell Preparation
About 1 week before the fusion, thaw a vial of the myeloma cell line from the liquid nitrogen stock (see Section IV, B).
The myeloma cells should be of known pedigree; it is vital to maintain the myeloma line carefully. Mycoplasma-contaminated stock or stock that has been overgrown will result in poor production of hybridomas.
IV. CELL FUSION PROCEDURE
A. Background Notes
HAT medium is used to selectively grow hybrids following fusion. Aminopterin selects against unfused myeloma cells and myeloma:myeloma-fused cells by blocking the main synthetic pathway for DNA. Unfused spleen cells do not have the capacity to survive for more than a few days. In hybridomas, hypoxanthine and thymidine supply purines and pyrimidines for DNA synthesis via HGPRTase salvage pathways derived from the spleen cells. The HT medium acts as a rescue medium while the aminopterin is being diluted out. Although the HT medium can be withdrawn once the nucleoside biosynthesis pathways are re-established, in our experience it is rarely worth the effort.
Cells under pressure often require "supplements" to maintain growth. In many monoclonal antibody preparation methods, these are supplied by feeder cells, usually mouse thymocytes or peritoneal washout cells. In the method described, feeder cells are replaced with hybridoma fusion cloning supplement HFCS, which in addition to being preferred for reasons of animal ethics, is more convenient and, in our experience, more effective.
B. Fusion Protocol
With the exception of the 1-h incubation period, the entire fusion protocol should take no more then 2 to 3h.
C. Maintaining Hybridomas
Examine the wells using the inverted microscope on days following the fusion. Do not keep the plates out of the incubator for more than about 10min, as media in the wells will cool and the pH may also change. Large numbers of dead cells may be seen, this is normal. You may also see some small moving particles; this is due to Brownian movement of debris and does not indicate bacterial infection.
After 7 days, media needs to be replenished, this is also the beginning of diluting out the aminopterin from the HAT selection medium. To each well add 1 ml of HT medium +2% HFCS. Add the medium slowly so as not to break up any colonies; this is not critical, but it is easier to judge when to test a well if you can see the size of intact colonies.
From now on it is a matter of observing colony growth and changes in the pH of the medium. Screen for antibody from wells in which the medium is yellow (acidic) and colonies are about 25% confluent. Keep feeding wells on a 7-day cycle for slow growers and as needed for fast growers. For the second and subsequent feeds we use HT medium +1% HFCS.
D. Testing and Cloning
When testing for antibody, ensure that appropriate negative controls are used. A positive control can be a dilution of the test bleed or of serum collected when the spleen was removed. If screening by ELISA, falsepositive "antiplastic responses" can be a nuisance. The use of antigen-negative wells as a control can help screen these out. It is a waste of time cloning cells from wells that display an antiplastic response.
Cells from wells that are positive in the screening assay need to be cloned and stored in liquid nitrogen as soon as possible. Cloning is performed not only to produce a monoclonal population, but also to stabilize cell growth and eliminate antibody nonproducers. Nonproducers often grow faster than cells producing antibody and can quickly outgrow the antibodyproducing population.
We routinely clone at 3 cells per well and observe about two-thirds of the wells with cell growth. This approximates to Poisson's distribution of a probable cell cloning number of 1 cell per well. If you find more wells with cell growth, clone at a lower cell number per well, 1 or even 0.5 cells per well.
When storing positive wells in liquid nitrogen, we routinely store two vials from the initial positive 24- well plate. From subsequent 96-well cloning plates we keep no more than three or four positive wells and store about two cryotubes of each and the same at each subsequent cloning step.
F. lsotyping and Scaling up of Hybridomas
Isotyping of the monoclonal antibody can be performed with a commercial ELISA kit, this can help confirm monoclonality and determine the strategy for antibody purification from the culture supernatant.
Care should be taken in scaling up the cultures from wells to flasks, ensure that cell growth is stable, and split cells into a number of wells of the 24-well plate; we establish confluent growth in at least four wells before transferring the cells to a 25-cm2 flask.
V. LIQUID NITROGEN STORAGE PROCEDURE
The only means by which long-term survival of hybridomas can be achieved is by storing cells in liquid nitrogen. It is essential that an up-to-date catalogue of stocks is maintained. Valuable hybridoma stocks should be kept in more than one freezer, preferably at separate sites.
A. Cell Freezing Protocol
B. Cell Thawing Protocol
We strongly recommend that full-face protection be worn when retrieving cells from liquid nitrogen.
We have overcome tolerance to self-antigen by linking the conserved peptide to a fusion protein and using the construct as the immunogen. Using this strategy, we have been successful in making antibodies to a human peptide with 95% homology to the murine protein (Cavill et al., 1999).
If purchasing a tissue culture incubator, ensure that it can be cleaned easily, has readily accessible filters that are changed easily, and does not have "hidden" tubing that cannot be cleaned and acts as a harbor for infections.
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