Knobs-into-Holes Engineering Enables Assembly of Bispecific Anti-HER2/Anti-HER3 Antibodies in Transient Plant Expression System

Cancer treatment often relies on antibodies, which are Y-shaped proteins that act like highly specific keys. These keys are designed to fit into locks on the surface of cancer cells, blocking the signals that tell the tumor to grow or survive. For years, scientists have focused on creating antibodies that target just one type of lock. However, cancer cells are clever; they can often find ways to bypass a single blockade, leading to treatment failure. To solve this, researchers are now trying to build "bispecific" antibodies. These are engineered molecules that can grab onto two different locks at the same time, effectively jamming the cancer cell's communication lines in two places at once. While this approach holds great promise, making these complex molecules is difficult. In traditional manufacturing, the parts of the antibody often get mixed up, snapping together in the wrong combinations and creating useless products.

A team of researchers at Chung-Ang University and their collaborators has found a new way to build these dual-targeting keys using plants. They focused on two specific locks found on certain breast cancers, known as HER2 and HER3. When these two locks join forces, they send a powerful signal that helps the cancer grow and resist standard treatments. The scientists wanted to create a single antibody that could bind to both HER2 and HER3 simultaneously. To do this, they turned to a method called "Knobs-into-Holes" engineering. Imagine trying to fit two puzzle pieces together; if both pieces have the same shape, they might snap together with a copy of themselves instead of the intended partner. The researchers modified the antibody parts so that one side had a protruding "knob" and the other had a matching "hole." This design forces the two different antibody chains to pair up correctly, preventing them from clumping with their own kind.

The team introduced the genetic instructions for these modified antibody parts into a common soil bacterium called Agrobacterium. They then used this bacterium to deliver the instructions directly into the leaves of a plant called Nicotiana benthamiana, a relative of the tobacco plant often used in research. This process, known as transient expression, allows the plant cells to temporarily act as tiny factories, reading the DNA and producing the protein. The researchers tested different mixtures of the two antibody instructions to see which ratio produced the best results. They found that when they mixed the instructions in a specific proportion, the plant cells successfully assembled the complex, dual-targeting antibody. By the seventh day after the injection, the production of the protein was at its peak.

To confirm that the plant had indeed made the correct antibody, the scientists extracted proteins from the leaves and analyzed them. They used a technique that separates proteins by size, which showed that the plant had produced a molecule with the exact weight expected for a correctly paired antibody. This molecule was heavier than a single antibody part but lighter than two separate antibodies stuck together, proving that the two different chains had joined to form a single, unified structure. The team then purified the antibody from the plant sap using a column that acts like a magnet for this specific type of protein. The final product was clean and ready for testing.

The researchers then checked if their plant-made antibody could actually do its job. They placed the antibody in contact with samples containing the HER2 protein and the HER3 protein. The tests showed that the antibody successfully grabbed onto both targets. It bound strongly to HER3 and held on to HER2, confirming that the molecule retained its ability to recognize both cancer locks despite the complex engineering required to build it. To understand why the antibody stayed together so well, the team used computer simulations to model the structure of the molecule. These simulations showed that the "knob" and "hole" design created a stable connection that was much stronger than the connections formed by unmodified parts. The computer models suggested that this specific pairing was energetically favorable, meaning the two parts naturally wanted to stay together in the way the scientists intended.

This study demonstrates that plants can be used as efficient factories to produce these complex, two-in-one antibodies. By combining the "Knobs-into-Holes" strategy with plant-based production, the researchers were able to bypass the common problem of parts mixing up incorrectly. The results suggest that this method could be a viable path for creating new cancer treatments that are both effective and potentially cheaper to produce than those made in animal cells. The work confirms that the plant system can handle the intricate task of assembling a bispecific antibody, opening the door for future developments in plant-made medicines.