Regulated readthrough: A new method for the alternative tagging and targeting of recombinant proteins

Fluorescent-protein co-expression to select CHO cells expressing high quantities of vaccine antigens

Cell line development for production of vaccine antigens or therapeutic proteins typically involves transfection, selection, and enrichment for high-expressing cells. Enrichment methods include minipool enrichment, antibody-based enrichment, and enrichment based on co-expressed fluorescent biosensor proteins. However, these methods have limitations regarding labor and cost intensity, the generation of antibodies and assurance of their viral safety, and potential expression-interference or signal-saturation of the co-expressed fluorescent protein. To improve the method of fluorescent-protein co-expression, expression constructs were created that constitutively express a model vaccine antigen together with one of three fluorescent proteins having translation initiation controlled by a wildtype or mutant internal ribosome entry site (IRES), for a total of six constructs. The constructs were transfected into Chinese hamster ovary cells (CHO) cells, enriched for high fluorescence, cultured, and tested in a mini bioreactor to identify the most promising construct. The fluorescent protein, Fluorescent Ubiquitination-based Cell Cycle Indicator (FUCCI) with a mutant IRES performed best and was further tested with three additional vaccine antigens. Across the four vaccine antigens, the FUCCI fluorescent protein yielded productivity enhancements, without the need for generating an antibody and assuring its viral safety. Furthermore, FUCCI protein was present in negligible quantities in the cell supernatant, indicating a low risk for contaminating drug substances or vaccine antigen.

Antibody display technologies: selecting the cream of the crop

Antibody display technologies enable the successful isolation of antigen-specific antibodies with therapeutic potential. The key feature that facilitates the selection of an antibody with prescribed properties is the coupling of the protein variant to its genetic information and is referred to as genotype phenotype coupling. There are several different platform technologies based on prokaryotic organisms as well as strategies employing higher eukaryotes. Among those, phage display is the most established system with more than a dozen of therapeutic antibodies approved for therapy that have been discovered or engineered using this approach. In recent years several other technologies gained a certain level of maturity, most strikingly mammalian display. In this review, we delineate the most important selection systems with respect to antibody generation with an emphasis on recent developments.

SPLICELECT™: an adaptable cell surface display technology based on alternative splicing allowing the qualitative and quantitative prediction of secreted product at a single-cell level

We describe a mammalian expression construct (SPLICELECT™) that allows the redirection of a proportion of a secreted protein onto the cell surface using alternative splicing: whereas the majority of the RNA is spliced into a transcript encoding a secreted protein, a weak splice donor site yields a secondary transcript encoding, in addition, a C-terminal transmembrane domain. The different sequence elements can be modified in order to modulate the level of cell surface display and of secretion in an independent manner. In this work, we demonstrated that the cell surface display of stable cell lines is correlated with the level of the secreted protein of interest, but also with the level of heterodimerization in the case of a bispecific antibody. It was also shown that this construct may be useful for rapid screening of multiple antibody candidates in binding assays following transient transfection. Thus, the correlation of product quantity and quality of the secreted and of membrane-displayed product in combination with the flexibility of the construct with regards to cell surface display/secretion levels make SPLICELECT™ a valuable tool with many potential applications, not limited to industrial cell line development or antibody engineering.

Amber suppression coupled with inducible surface display identifies cells with high recombinant protein productivity

Cell line development (CLD) for biotherapeutics is a time- and resource-intensive process requiring the isolation and screening of large numbers of clones to identify high producers. Novel methods aimed at enhancing cell line screening efficiency using markers predictive of productivity early in the CLD process are needed to reliably generate high-yielding cell lines. To enable efficient and selective isolation of antibody expressing Chinese hamster ovary cells by fluorescence-activated cell sorting, we developed a strategy for the expression of antibodies containing a switchable membrane-associated domain to anchor an antibody to the membrane of the expressing cell. The switchable nature of the membrane domain is governed by the function of an orthogonal aminoacyl transfer RNA synthetase/tRNApyl pair, which directs a nonnatural amino acid (nnAA) to an amber codon encoded between the antibody and the membrane anchor. The process is "switchable" in response to nnAA in the medium, enabling a rapid transition between the surface display and secretion. We demonstrate that the level of cell surface display correlates with productivity and provides a method for enriching phenotypically stable high-producer cells. The strategy provides a means for selecting high-producing cells with potential applications to multiple biotherapeutic protein formats.