A new study from Switzerland suggests why stem cell transplant works. The treatment is particularly suitable for younger people with aggressive forms of the disease.

During the treatment, several chemotherapies completely destroy the patients’ immune system – including the subset of T cells which mistakenly attack their own nervous system. The patients then receive a transplant of their own blood stem cells, which were harvested before the chemotherapy. The body then uses these cells to build a completely new immune system without any autoreactive cells.

Some unclear aspects were what exactly happens after the immune cells are eliminated, whether any of them survive the chemotherapy, and whether the autoreactive cells really do not return. In the recently published study, Department of Neuroimmunology and MS Research at the University of Zurich and the Department of Medical Oncology and Haematology Clinic at the University Hospital Zurich researchers systematically investigated these questions for the first time by analyzing the immune cells of 27 MS patients who received stem cell therapy in Zurich. The analysis was done before, during, and up to two years after treatment. This allowed the researchers to track how quickly the different types of immune cells regenerated.

Surprisingly, the cells known as memory T cells, which are responsible for ensuring the body remembers pathogens and can react quickly in case of a new infection, reappeared immediately after the transplant. Further analysis showed these cells had not re-formed, but had survived the chemotherapy. These remnants of the original immune system nevertheless pose no risk for a return of MS.

In the months and years following the transplant, the body gradually recreates the different types of immune cells. The thymus gland plays an important role in this process. This is where the T cells learn to distinguish foreign structures, such as viruses, from the body’s own. 

These findings have enabled the researchers to understand why stem cell transplants are usually so successful. But the treatment is not approved in many countries, as phase III studies are lacking.