Biochemical Limitations to High-level Expression of Humanized Monoclonal Antibodies in Transgenic Maize Seed Endosperm

Transgenic plants are potentially valuable systems for the large scale manufacture of therapeutic proteins. To improve this technology, determining the importance of transgene transcript levels on protein accumulation in sink tissues during their development is crucial. In transgenic maize (Zea mays L.) plants expressing humanized monoclonal antibodies (mAbs) in their seed endosperm, steady-state kappa light chain (LC) and gamma heavy chain (HC) mRNA levels were quantified during development and compared to the levels of fully-assembled mAb protein present at seed maturity. RNA blots and non-reducing SDS-PAGE western immunoblots revealed that steady-state LC and HC mRNA and protein levels were undetectable at 10 days after pollination (DAP) but increased quickly thereafter in three transgenic events expressing different mAb molecules. Similar to gamma-zein mRNA, LC and HC messages were highly abundant between 15 and 25 DAP. Quantitative RNA blots and western immunoblots showed that steady-state LC transcript levels during development correlated extremely closely with protein levels in mature seed (r(2)=0.99). For HC, this correlation was not as strong (r(2)=0.85). Consistent with this finding, concomitantly increasing the zygosity levels of the LC and HC transgenes enhanced mAb concentration in mature seed, in contrast to increasing the copy number of the transgene insert, which did not correlate with high seed mAb levels. The results indicate that high-level expression of fully-assembled mAb protein in maize endosperm was favored by high LC and HC mRNA levels and was largely limited by HC protein concentration.