Chinese Hamster Ovary (CHO) Cells

Over the years Chinese hamster ovary (CHO) cells became more and more important for life science research and commercial production of therapeutic enzymes and antibodies. This development has continued despite significant disadvantages such as limited growth, low productivity and stress resistance as well as higher expenses compared to bacterial or yeast based expression systems.¹ Nevertheless CHO cells have become the go-to cell line and represent the most frequently applied host cell system for industrial manufacturing of recombinant protein therapeutics.²

Mammalian post-translational modification

One of the major advantages is the high agreement in post-translational modification (PTM) pattern with human cells in contrast to the PTM patterns of bacteria (E. coli) or yeast. There is however no total consensus in pattern; CHO glycan can be capped with both N-glycolylneuraminic acid (Neu5Gc) and N-acetylneuraminic acid (Neu5Ac), whereas human cells only produce glycoproteins containing N-acetylneuraminic acid (fig.1).³

Glycan PTM pattern of CHO Cells

Galactose-alpha-1,3-galactose, or alpha gal, is a carbohydrate found in most mammalian cell membranes. However it is not found in primates, including humans, whose immune systems recognize it as a foreign body and produce immunoglobulin M antibodies which can induce severe anaphylactic shocks. Scientist try to optimize the CHO cell lines by eliminating the pathways that produce these glycan structures through genetic knockouts or overexpression.³

Origin of CHO cell line

Chinese hamsters had been used in research since 1919, originally for typing pneumococci. In 1957 it was used to derive the original Chinese hamster ovary cell line, named CHO-K1. Under laboratory conditions, scientists constantly try to improve CHO-based recombinant protein titer and quality achieving the biggest success by random cell-line mutagenesis and media optimization.⁴

A cell line with deficient DHFR alleles was one of the first mutagenized lines. Aim of this mutation is a better screening capability after genetic manipulation. Cells transfected with a gene of interest along with a functional copy of the DHFR gene can easily be screened for in thymidine-lacking media. The DHFR-deficient strain was called CHO-DG44.

One name – different cell types

CHO cells have been used in culture for more than 50 years and many recombinant protein–producing CHO cell lines derive from the CHO-K1, CHO-S and DG44 lineages. These CHO  cell lines are, however, immortalized cells, each representing different cell types, based on their inherent genetic diversity and their dynamic rate of genetic change. As is known for any biological system, this diversity is enhanced by selective forces when laboratories (no sharing of gene pools) grow cells under (diverse) conditions that are practical and useful.⁵

The continuing remodeling of genomic structure in clonal or non-clonal cell populations renders CHO cells a typical case for “quasispecies”.⁶

Quasispecies

The quasispecies model is a description of the process of the Darwinian evolution of certain families of related (genomic) sequence. A quasispecies is a large group or “cloud” of related genotypes that exist in an environment of high mutation rate, where a large fraction of offspring are expected to contain one or more mutations relative to the parent.

If the concept is transferred to CHO cells, interesting implications arise. CHO DNA and RNA analysis may provide only limited insights when done on relative old and poorly characterized CHO strains.⁷

In contrast, screening of clonal cell lines, derived from a well-defined starting material, may reveal a more narrow diversity of phenotypes with respect to physiological/metabolic activities and, thus, allow more precise and reliable predictions of the potential of a clone for high-yielding manufacturing processes.⁸