Most biotherapeutic drugs are recombinant monoclonal antibodies which are mostly produced in monoclonal cell lines derived from Chinese hamster ovary (CHO) cells. Various clones expressing a monoclonal recombinant antibody were analyzed and a correlation of the antibody concentration and the relative mRNA level of calreticulin (CALR), glucose-regulated protein 78 and 94 kDa (GRP78, GRP94) and spliced X-box binding protein 1 (XPB1) was observed. By means of these results we were motivated to establish a novel selection system based on endoplasmic reticulum (ER) stress, which allows the rapid identification and isolation of high-expressing clones out of a pool mainly consisting of low- and medium-producing cells. Several ER stress responsive elements were tested with the aid of a recombinase mediated cassette exchange (RMCE) procedure. Very surprisingly, only GRP78 reporter constructs were strongly stimulated upon antibody expression. Furthermore we found that GRP78 reporter constructs are very suitable to reflect the level of antibody expression (IgG) in recombinant CHO cells. Based on these results, it is concluded, that the novel ER stress based selection system developed during this study is suitable to identify and isolate clones with a high level of antibody expression.
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