Evaluation of a combinatorial cell engineering approach to overcome apoptotic effects in XBP-1(s) expressing cells

Genetic engineering of producer cell lines for production of therapeutic antibodies in order to increase the yield of production processes remains a continuing challenge. Recently it was shown that heterologous expression of the active, spliced form of human X-box binding protein 1 (XBP-1(s)) can increase the amount of secreted protein products in mammalian cell culture processes. However, a prerequisite for the industrial application of any cell engineering approach is the ability to generate monoclonal cell lines that stably express the engineering gene to maintain the desired phenotype. Here, we show a decrease in heterologous human XBP-1(s) expression in CHO production cells producing a therapeutic antibody product monitored over a prolonged period in serial culture. Colony formation assays (CFA) in CHO-K1 cells reveal a general survival disadvantage conferred by XBP-1(s) in this cell type. We aimed to rescue this phenotype by expressing the caspase-inhibitor XIAP (x-linked inhibitor of apoptosis). Using a set of bicistronic expression vectors we engineered an antibody producing CHO cell line with XBP-1(s) and XIAP alone and in combination. Interestingly, co-expression of both genes resulted in the highest specific productivities (Qp) and final titers in a serum-free fed-batch process in chemically defined media. Thus, the combination of secretion and anti-apoptotic engineering provides an interesting approach for future applications in industrial mammalian cell culture.