Harvesting molecules from mice to fight cancer in humans
The cells of our immune system have the amazing ability to scan billions of molecules and precisely distinguish between those made by our own body and those of other people or other species. This helps protect us from pathogens such as viruses and bacteria; it's also the reason that the body rejects transplanted organs and tissues. The system often fails to protect us against cancer, but researchers believe that it might be possible to train immune cells to recognize and combat tumors. Thomas Blankenstein's lab at the MDC and Charité has now achieved an important milestone toward this goal. His team has transplanted key components of the human immune system into mice, hoping to use the animals to harvest cancer-detecting molecules that can be used in new therapies. The work appeared in the September 2010 issue of Nature Medicine.
The mice from Blankenstein's lab produce T cells with human receptors that recognize molecules associated with tumors. T cells extracted from the mice were sorted using a method called _flow cytometry_. In this case, three percent of the CD8+ T cells were found to respond to a molecule associated with human melanomas. This number is large enough to permit harvesting the cells and extracting the genetic code for their receptor.
Thomas' lab focuses on a type of white blood cell called a T cell which plays a major role in the detection of foreign molecules. As it develops, each cell is outfitted with a unique form of a protein called a T cell receptor (TCR). Immune responses usually begin when a TCR binds to a foreign molecule. This causes the cell to reproduce, yielding millions of T cells with identical TCRs that can recognize the pathogen and trigger a wider immune response.
TCRs can distinguish between a vast number of molecules because of the way they are made. As the T cell develops, it builds the receptor by selecting subunits from a catalogue of about 170 genes. It deletes the rest from its genome – the way you might assemble a note by cutting words out of a newspaper and pasting them together, then throwing the rest away. The process is completely random, and many of the TCRs that are made recognize the body's own proteins. If T cells with such molecules were allowed to roam the body, they might attack healthy tissues and launch an autoimmune disease. That doesn't usually happen because the body destroys them. As they develop in the thymus, T cells are exposed to nearly all the proteins that the body makes. If a TCR binds to one, its T cell is destroyed.
This process protects us from autoimmune diseases, but it also kills cells that might help fight off tumors. Most cases of cancer probably arise through mutations that damage or change the structures of one or more proteins. This creates a molecule that the body has never seen before, and it might appear on the surface of tumor cells, where it could potentially be spotted by the immune system. Many tumors are thought to bear such unique molecules, called tumor-specific antigens (TSAs). However, TSAs that potentially serve as target structures for T cells are rare and often unique to an individual tumor. Therefore, researchers aim to use tumor-associated antigens (TAAs) as targets for therapy. In principle, TAAs are more convenient as targets because they are shared between cancers of different individuals. But because TAAs are the body's own proteins and have not undergone mutations, they often fail to evoke a productive response. T cells that might recognize them have been destroyed during the training process.
"If those cells existed, we could extract them, multiply them in the lab, and return them to the patient," Thomas says. "They might go on to stimulate a full immune response that would attack the tumor. But since the body eliminates them, we can't harvest them from humans."
It might be possible, however, to grow them in another species. The mouse immune system closely parallels our own – it also creates T cells with unique TCRs and trains them in the thymus in a similar way. But in the process, it destroys T cells that recognize mouse proteins, and not those of humans. When a TAA or another human molecule is transplanted to a mouse, T cells recognize it, multiply, and mount an immune response.
Theoretically, Thomas says, this means you could harvest such cells from the mouse. You could extract the genetic code for its T cell receptor and insert it into human T cells. Reintroduced into the patient, they might attack cancer cells.
"But we can't use mouse TCRs because they are different from those of humans," Thomas says. "The patient's body would recognize a mouse TCR as foreign and reject it."
The solution found by Liang-Ping Li and other members of the lab was to transplant the entire set of human genes needed to build TCRs into the mouse, which would then build human versions of the receptors. This required a massive effort, because the instruction book for TCRs is huge, spread out over large regions of two chromosomes. Inserting so much information into the mouse genome posed technical challenges and took several years to achieve. Now the lab has succeeded and the result, Thomas believes, may be the largest set of human genes ever introduced into the mouse.
The lab has now begun testing the therapeutic potential of the new animals. The first task will be to find molecules associated with cancer in a patient and introduce it into the mice. Once it is recognized by a TCR, its T cell will reproduce itself and can then be harvested. Thomas and his colleagues will extract the gene for its TCR – which originally came from a human – and insert it into T cells taken from the patient. After the cells are grown in the lab and reimplanted, they will hopefully stimulate a full immune response that destroys the tumor.
The system should also permit Thomas and his colleagues to answer some questions about other types of diseases. Autoimmune conditions likely arise because of defects in the training of T cells. It may be a few T cells capable of recognizing human proteins slip past the controls and leave the thymus, only becoming a problem when something else goes wrong. Which TCRs are used by such self-destructive T cells remains an enigma, Thomas says. The mouse might help answer this question, because it allows the identification of all those TCRs against human self-proteins that are usually deleted, but spared in some individuals with autoimmune disease. Once these TCRs have been identified, it is easy to find T cells with specific TCRs that have accidentally survived in the individuals with autoimmune disease. This would permit the development of more selective therapies aiming to selectively eliminate the disease-causing T cells. So the strategy that Thomas' lab is using to fight cancer might work with other complex diseases as well.
- Russ Hodge
Highlight Reference:
Li LP, Lampert JC, Chen X, Leitao C, Popović J, Müller W, Blankenstein T. Transgenic mice with a diverse human T cell antigen receptor repertoire. Nat Med. 2010 Sep;16(9):1029-34
The full text of the paper
Homepage of "Sonderforschungsbereich TR36": Principles and Applications of Adoptive T Cell Therapy, coordinated by Thomas Blankenstein at the Charité
Wikipedia entry on T cell receptors
