Plant cell culture has not received the attention it deserves from biopharmaceutical companies, but that may change as research-level developments reach the corporate level. One reason, said Karen McDonald, Ph.D., professor of chemical engineering and material science at the University of California, Davis, is that glycosylation is still an important issue.
“Although plant cells glycosylate, we aren’t quite at the point where we can introduce the biosynthetic machinery to have them perform mammalian-like glycosylation.”
Since glycosylation controls such important functions as immunogenicity and pharmacokinetics, transgenic plant cell cultures still lag significantly behind mammalian cells for important classes of therapeutic proteins, for example monoclonal antibodies (mAbs). As a consequence of plant cells lacking a broad manufacturing base, cell-line development, protein-expression levels, and research into media and feed strategies are also behind, so titers for plant-based cultures tend to be quite low by mammalian cell standards.
Notable exceptions include the lemna (duckweed) system developed by Biolex, which has produced N-glycosylation of mAbs that the company claims is superior to that formed in CHO cells. Dow AgroSciences has an approved vaccine, manufactured in transgenic tobacco, against Newcastle disease, and Protalix Biotherapeutics uses transgenic carrot cells to produce a recombinant protein, recombinant glucocerebrosidase, for treating Gaucher’s disease.
But to succeed and enter prime time, these methods must be made more generally applicable to the manufacture of biotherapeutics across molecular classes, including those for which glycosylation is not a concern. Efforts include development of efficient expression systems to raise titers, and scale-up.
Dr. McDonald’s preferred cultured cells are rice and tobacco, which are robust and easy to grow. “Generally speaking, plant cell cultures are not as particular as mammalian cells about living within a narrow range of pH, agitation, and dissolved oxygen.”
Another system she has experience with is Trichosanthes kirilowii, a flowering plant popular in Chinese traditional medicine. Dr. McDonald and colleague Alan Jackson have produced multiliter suspension cultures of T. kirilowii, which is a source of compounds useful in treating viruses and tumors.
Plant cell culture is attractive because plants do not harbor viruses that are pathogenic to humans and, therefore, do not require a downstream viral-clearance step. But, because plant cells grow slowly, in aggregates rather than free-floating cells, and produce at a lower level than mammalian cells, culturing for two weeks and going through conventional harvest and capture steps is not economical at industrial scale.
Dr. McDonald is also investigating ways to induce the cells to excrete proteins into the broth. Harvest and capture operations might even be combined by stopping agitation to allow cell aggregates to settle, and filtering off the supernatant. The longest such continuous culture Dr. McDonald has run lasted about 45 days, “but I am not sure that that is an inherent limitation.” One key to inducing protein excretion is development of secretion signal peptides.