A branch of our innate immune system recognizes “foreign” cells, such as disease-causing organisms and our own dying cells. These cells are thereby “tagged” with a protein called iC3b, which is found in our blood and in the fluid surrounding our own cells. This allows so-called phagocytes, specialized immune cells, to physically engulf the “tagged” cells, kill them and break them down. The entire process is called phagocytosis and is a central tool in our immune system. Phagocytosis driven by the iC3b protein is strongly stimulated if the “foreign” cell is also recognized by other immune molecules called antibodies.

In 2016, PhD student Rasmus K. Jensen began developing a method to produce the CR3 receptor and the iC3b protein under the supervision of Professor Gregers R. Andersen. Two years would pass before it became possible to study the receptor and the protein at the molecular level at Aarhus University using electron microscopy. However, it was not Rasmus who ultimately determined the structure of iC3b bound to the receptor. New forces were needed. Josefine Lorentzen, who had just started as a PhD student at the time, took on the task. Rather than focusing on a smaller version of the iC3b protein called C3d, which can also bind to the receptor, she developed a clever way to stabilize the C3d–CR3 complex. A breakthrough came during a stay in Hamburg, where she, for the first time, caught a glimpse of the structure that she and Rasmus had been chasing for so long. In spring 2024, Josefine achieved the final breakthrough. Suddenly, she and the rest of the research group understood in detail what happens when C3d binds to CR3.

Josefine describes the experience in her own words: “It has been a project filled with both frustrations and small victories along the way. Many times, the goal felt very far away, but that is exactly why it was so fantastic when we could finally see the structure clearly. It felt like putting the final pieces into a puzzle we had been working on for years.”

At the same time, she received help from postdoc Marlene U. Fruergaard to complete the work. In addition to studying the normal receptor, they investigated a variant that predisposes people to a rare hereditary autoimmune disease with the rather unwieldy name “systemic lupus erythematosus”.

Marlene explains: “The structure of the disease-associated variant turned out to be almost identical to that of the healthy receptor, which tells us that it is not the binding to C3d itself that is affected by the mutation. The reason for the increased predisposition to the disease must therefore be linked to the receptor having a reduced ability to stimulate phagocytosis of the ‘foreign’ cells. We hope to learn more about this in the coming years.”

Gregers R. Andersen is very pleased: “The course of this project illustrates what good basic research is built on: lots of talent, perseverance, technical advances and the ability to come up with fun ideas. But believe me, along the way we also went down many wrong paths. I am full of admiration for the efforts Rasmus, Josefine and Marlene have made.”

Many modern drugs for treating, for example, cancer are based on antibodies and exploit the interaction between antibodies and iC3b to achieve effective phagocytosis of cancer cells. At the same time, experiments in mice suggest that iC3b-driven phagocytosis is overactive in connection with diseases such as Alzheimer’s and Parkinson’s. The new research therefore has great potential to contribute to the development of even better drugs for treating cancer and alleviating these two feared neurological diseases.

The project’s results have opened the door to many new initiatives and collaborations. In addition to Marlene, two PhD students are now following up on the new results with similar receptors and other techniques. Hopefully, they will not have to wait 10 years to see the fruits of their work.

Along the way, the project was supported by the Lundbeck Foundation, Alexion, the LEO Foundation, the Novo Nordisk Foundation and the Independent Research Fund Denmark.

Contact:

Gregers R. Andersen
Email: gra@mbg.au.dk