Development of optimized transfectoma cell lines for production of chimeric antibodies in hollow fiber cell culture systems

Decreased chimeric antibody productivity of KR12H-1 transfectoma during long-term culture results from decreased antibody gene copy number

The stability of KR12H-1 transfectoma in regard to chimeric antibody production was examined during long-term, repeated batch culture without selection pressure using antibiotics. Both serum-supplemented and serum-free media were used. Regardless of the medium used, the specific antibody productivity (q(Ab)) of transfectoma decreased by 60% to 88% during 70-day culture. This loss of antibody productivity was not due mainly to the appearance of a nonproducing population (NP) of transfectoma. The percentage of a producing population (P), which was monitored by the limiting dilution method, remained over 90% until the end of culture, indicating that the q(Ab) of P decreased during the culture. Flow cytometric data also showed the increase of cell population with low fluorescence intensity during culture, indicating that the intracellular antibody content of P decreased. The subclones of P obtained at the end of long-term culture were further characterized. Compared with the q(Ab) of P at the beginning of long-term culture, the q(Ab) of most P subclones was significantly low, confirming that the loss of antibody productivity was due mainly to the decreased q(Ab) of P during long-term culture. The decreased antibody gene copy number of P subclones was found to be partly responsible for the decreased q(Ab) of P during long-term culture. (c) 1996 John Wiley & Sons, Inc.

Comparison of cell growth in T-flasks, in micro hollow fiber bioreactors, and in an industrial scale hollow fiber bioreactor system

In this article, cell growth in a novel micro hollow fiberbioreactor was compared to that in a T-flask and theAcuSyst-Maximizer(R), a large scale industrial hollowfiber bioreactor system. In T-flasks, there was relativelylittle difference in the growth rates of one murine hybridomacultured in three different media and for three other murinehybridomas cultured in one medium. However, substantialdifferences were seen in the growth rates of cells in themicro bioreactor under these same conditions. These differencecorrelated well with the corresponding rates of initial cellexpansion in the Maximizer. Quantitative prediction of thesteady-state antibody production rate in the Maximizer was moreproblematic. However, conditions which lead to faster initialcell growth and higher viable cell densities in the microbioreactor correlated with better performance of a cell line inthe Maximizer. These results demonstrate that the microbioreactor is more useful than a T-flask for determining optimalconditions for cell growth in a large scale hollow fiberbioreactor system.

Properties of ras-amplified recombinant BHK-21 cells in protein-free culture

We compared serum and protein-free cultures ofa ras-amplified recombinant BHK-21 cell line(ras-rBHK-IgG), which hyperproduces a lungcancer specific recombinant human monoclonal antibody. Ras-rBHK-IgG cells were shown to grow well, evenin protein-free medium and to be morphologicallysimilar to cells cultured in serum containing medium. However, the growth rate of ras-rBHK-IgG cellswas considerably slower in protein-free medium, whichresults in a longer maintenance period compared with cells cultured in serum containing medium. In addition, it was found that antibody production in protein-free culture had a ten times higher maximum than cells cultured in serum containing medium. On theother hand, in high density culture, using the hollowfiber bioreactor system, ras-rBHK-IgG cellscould be maintained for a month in protein-freeculture in contrast with serum culture, which onlylasted for half a month. However, the markedincrease of antibody production was not observed. A total amount of about 15 mg of the recombinantantibody, obtained in protein-free culture, was abouttwo times of that obtained in serum culture, and wasshown to be reactive to lung cancer cells in tissue. From these properties in protein-free medium, it isconcluded that protein-free culture of ras-rBHK-IgG cells is suitable for middle scaleproduction of recombinant human monoclonal antibody.

Selection and isolation of cells for optimal growth in hollow fiber bioreactors

Growth of a murine hybridoma in a hollow fiber microbioreactor was poor. This corresponded to slow initial growth in the Maximizer, a pilot scale hollow fiber bioreactor system. Medium screening experiments with the microbioreactor demonstrated that the slow growth was due to dialysis of low molecular weight serum components (under about 10 kDa) from the cell side of the fibers to the basal medium on the noncell side of the fibers. Better growth can be achieved by adding serum to both sides of the fibers, but this is an expensive option. As an alternative, the microbioreactor was used to select for a population of cells that did not require serum on both sides of the fiber for optimal growth. From this population, a stable subclone was isolated using limiting dilution followed by growth assessment in microbioreactors. The subclone was cultured in the Maximizer under conditions identical to the parental cell line. The subclone reached confluency in about 9 days compared with about 16 days for the parental cell line. At confluency, the subclone produced antibody at twice the rate of the parental cell line. These results demonstrate that the microbioreactor is a useful tool for quickly isolating subclones that are better suited for growth in a hollow fiber bioreactor.

Apoptosis-resistant E1B-19K-expressing NS/0 myeloma cells exhibit increased viability and chimeric antibody productivity under perfusion culture conditions

We have shown previously that recombinant NS/0 myelomas expressing sufficient amounts of E1B-19K were resistant to apoptosis occurring in the late phase of batch culture and under stressful conditions such as cultivation in glutamine-free medium or following heat shock. However, no significant increase in monoclonal antibodies (MAb) was observed during the prolonged stationary phase of these batch cultures. Here, we show that E1B-19K can enhance cell survival and improve MAb productivity in high cell density perfusion culture. Typically, lymphoid cells grown under steady state in perfusion exhibit decreasing viabilities with concomitant accumulation of apoptotic cells. By modulating the ability of these cells to resist to induction of apoptosis in low nutrient environment, a 3-fold decrease in specific death rate from 0.22 day-1 for NS/0 control to 0.07 day-1 for E1B-19K cells was achieved, resulting in a significant improvement in cell viability throughout perfusion. E1B-19K cells at the perfusion plateau phase also exhibited a 3-fold reduction in specific growth rate concomitant with a lower percentage of S and higher percentage of G1 phase cells. This was associated with a 40% decrease in specific oxygen consumption rate, likely related to a reduction in the specific consumption rates of limiting nutrient(s). Expression of E1B-19K consequently had a significant impact on the steady-state viable cell density, allowing maintenance of 11.5 x 10(6) E1B-19K cells/mL versus 5.9 x 10(6) control NS/0 cells/mL for the same amount of fresh medium brought into the system (half a volume per day). Whereas MAb concentrations found in perfusion culture of control NS/0 myelomas were almost 3-fold higher than those found in batch culture; in the case of E1B-19K-expressing myelomas, the MAb concentration in perfusion was more than 7-fold higher than in batch. This was attributable to the 2-fold increase in viable cell plateau and to a 40% increase in the perfusion to batch ratio of specific MAb productivity (2.2-fold for E1B-19K myelomas versus 1.6-fold for NS/0 control).

Generation of chimeric monoclonal antibodies from mice that carry human immunoglobulin Cgamma1 heavy of Ckappa light chain gene segments

Gene targeting in mouse embryonic stem (ES) cells was used to replace (i) the mouse immunoglobulin heavy chain (IgH) Cgamma2a gene segment (mCgamma2a) with the human Cgamma1 gene segment (hCgamma1), and (ii) the mouse immunoglobulin light chain (IgL) Ckappa gene segment (mC kappa) with its human counterpart (hC kappa). ES cells carrying these gene conversions were used to generate chimeric mice that transmitted the human alleles through the germ line. Mice homozygous for both gene alterations were generated by breeding. Serum from homozygous mutant mice contained comparable amounts of antibodies with chimeric kappa or mouse lambda light chains but only small fractions of basal serum IgG or antibodies elicited against immunizing agents contained chimeric heavy chains. A relative increase in immunogen-specific hCgamma1 antibodies was seen following immunization in combination with the saponin adjuvant QS-21. The effect of this was to shift the IgG1-dominated response to an IgG subclass profile that included significant amounts of IgG2a, IgG2b and IgG3 and chimeric IgG. The amounts of antibody secreted by hybridomas derived from mutant and wild-type mice were similar. Sequencing confirmed correct splicing of hCgamma1 and hCkappa gene segments to mouse J gene segments in hybridoma Ig gene transcripts. In conclusion, IgHhCgamma1/IgLhCkappa double mutant mice provide a useful animal model for deriving humanized antibodies with potential applications in immunotherapy and diagnostics in vivo as well as for investigating hCgamma1 associated functions.

Correlation between secreted and membrane-bound IgG in mouse myeloma cells transfected with chimeric immunoglobulin heavy and light chain genes

Mouse myeloma cells were transfected with pSV2-gpt and pSV2-neo based immunoglobulin expression vectors. Double transfectants were selected using the xanthine-guanine phosphoribosyl transferase (gpt) and the neomycin (neo) selection marker genes. A broad distribution in the level of mouse-human chimeric IgG expression was observed with series of independently isolated transfectoma clones. The relative amounts of secreted to membrane-bound antibodies correlated closely, which suggested, that fluorescence-activated cell sorting could be a valuable tool for the selection of high-yielding production cell lines. However, a single cycle of cell sorting did not steer the cloning process significantly toward cells that produce enhanced amounts of recombinant IgG. Only in cases in which the polyclonal transfectoma population contained a large percentage of nonproducing cells, these were successfully separated from the IgG-producing cell population. (c) 1996 John Wiley & Sons, Inc.