Several methods have been described to enhance antibody-dependent cellular cytotoxicity (ADCC) using different host cells that produce antibody with reduced levels of fucose on their carbohydrates. We compared the suitability of these methods for the serum-free fed-batch production of antibody for clinical trials and commercial uses. Recombinant anti-human CD20 chimeric IgG1-producing clones were established from host-cells that have been shown to produce more than 90% fucose-negative antibody. The cell lines were a FUT8 (alpha-1,6-fucosyltransferase) knockout Chinese hamster ovary (CHO) cell line, Ms704, and two Lens culinaris agglutinin (LCA)-resistant cell lines, one derived from a variant CHO line, Lec13 and the other from a rat hybridoma cell line, YB2/0. The amount of fucose-negative antibody produced by Lec13 and YB2/0 significantly decreased with the culture. The increase in fucosylation was due to remaining synthesis of GDP-fucose via de novo pathway for the CHO line and the elevation of FUT8 expression by the YB2/0 cells. In contrast, Ms704 cells stably produced fucose-negative antibody with a consistent carbohydrate structure until the end of the culture. The productivity of the Ms704 cells reached 1.76 g/L with a specific production rate (SPR) of 29 pg/cell/day for 17 days in serum-free fed-batch culture using a 1 L spinner bioreactor. Our results demonstrate that FUT8 knockout has the essential characteristics of host cells for robust manufacture of fucose-negative therapeutic antibodies with enhanced ADCC.

Antibody-dependent cellular cytotoxicity (ADCC) is dependent on the fucose content of oligosaccharides bound to monoclonal antibodies (MAbs). As MAbs with a low fucose content exhibit high ADCC activity, it is important to control the defucosylation levels (deFuc%) of MAbs and to analyze the factors that affect deFuc%. In this study, we observed that the deFuc% was inversely related to culture medium osmolality for MAbs produced in the rat hybridoma cell line YB2/0, with r 2 values as high as 0.92. Moreover, deFuc% exhibited the same correlation irrespective of the type of compound used for regulating osmolality (NaCl, KCl, fucose, fructose, creatine, or mannitol) at a culture scale ranging from 1 to 400 L. We succeeded in controlling MAb deFuc% by maintaining a constant medium osmolality in both perfusion and fed-batch cultures. In agreement with these observations, reverse transcription PCR analyses revealed decreased transcription of genes involved in glycolysis, GDP-fucose supply, and fucose transfer under hypoosmotic conditions.

The protease activity in serum-free conditioned medium of chinese hamster ovary (CHO) cells was measured using peptidyl (or aminoacyl)-4-methylcoumaryl-7-amides (MCAs) as the substrates. Aminopeptidase increased in level as amounts of nonviable cells increased during cultivation in serum-free medium, indicating that the activity seems to be originated from intracellular proteases. The activity toward Boc-Leu-Arg-Arg-MCA, which was strongly inhibited by p-chloromercuribenzonate and N-ethylmaleimide, was the strongest among those toward peptidyl-MCAs in the conditioned medium within 48 h-cultivation in serum-free medium. In contrast to the case of aminopeptidase activity, the endopeptidase activity decreased in level after 48 h-cultivation although amounts of nonviable cells increases. Thus, CHO cells continuously secrete the cysteine proteases.

A Namalwa cell line, KJM-1, which was adapted to serum-free medium is thought to be a very useful host cell line for recombinant DNA technology. Thus, the utility of Namalwa KJM-1 for expression of foreign genes was examined. As a model system human beta-interferon (beta-IFN) gene was engineered for expression in this cell line. For construction of the beta-IFN expression vector pSE1 beta 1-4, the expression vector pAGE107 was constructed and used. It contains simian virus 40 (SV40) early promoter, the rabbit beta-globin RNA processing signals for splicing and polyadenylation, and SV40 early RNA processing signal for polyadenylation. In addition to the above transcription unit, pAGE107 contains the ampicillin-resistance gene and G418-resistance gene. They can confer ampicillin resistance to Escherichia coli (E. coli) and G418 resistance to animal cells. To introduce plasmid DNA into cells, electroporation is a useful method (Wong, 1982; Potter, 1984). We have established conditions for DNA-mediated transfection of Namalwa KJM-1 cell line by electroporation. Among pSE1 beta 1-4-introduced cells, clone 1-3 was further examined for the expression of beta-IFN in serum-free medium. The production level of beta-IFN was elevated with the increase of the cell density. The results indicated that the Namalwa KJM-1 cell line is useful for production of foreign gene products.

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