Monoclonal antibodies produced in the diatom Phaeodactylum tricornutum are susceptible to serine proteases secreted in the culture medium

Microalgae have recently emerged as a green alternative cell factory for the production of therapeutic proteins, named Biologics, which include antibodies. For instance, efficient production of monoclonal antibodies (mAbs) has been performed in the diatom Phaeodactylum tricornutum through nuclear transformation. This allows the mAbs to be addressed to the endoplasmic reticulum and then to travel through the Golgi apparatus where its N-glycosylation occurs, before being secreted in the culture media. However, to date, the production yield of mAbs remains limited which suggests either a low secretion rate of mAbs in the culture media of P. tricornutum cells or their proteolytic degradation in the culture media where the mAbs accumulate. To discriminate between these two scenarios, we have investigated the protease activity and the proteolytic degradation of mAbs in the culture medium of the Pt1 (CCMP 2561) strain of P. tricornutum. We demonstrated that this diatom secretes proteases able to rapidly degrade mAbs. Such protease activities were also detected in 10 different P. tricornutum strains, although a large variability in proteolytic activity was observed between strains and over time. We also demonstrated that the stability of mAbs in P. tricornutum cell cultures can be strongly improve by addition of suitable protease inhibitors.