Researchers at San Raffaele, coordinated by Professor Lorenzo Piemonti, director of the Diabetes Research Institute (DRI) at San Raffaele, demonstrated how it is possible to make induced pluripotent stem cells (iPSCs) and the insulin-secreting cells (β-cells) derived from them invisible to the immune system through genetic engineering techniques.

This is an important step forward in the field of regenerative medicine and the use of iPSCs for the treatment of type 1 diabetes, hampered by the auto-immune response typical of the disease.

The study, conducted in vitro and in vivo in experimental models of the disease, was published in the prestigious journal Cell Reports and conducted in collaboration with the San Raffaele Viral Transmission and Evolution Unit and the San Raffaele Telethon Institute for Gene Therapy (SR-Tiget).

Regenerative medicine and the immune system

One of the unresolved problems in the field of regenerative medicine is the impossibility of transplanting organs, tissues or cells in the absence of immunosuppressive therapy.

Every cell has a series of proteins on its surface, called the 'Major Histocompatibily Complex' (MHC), which represent a genuine barcode unique to each individual. Everyone's immune system recognizes its own barcode, but not those of others, and when it encounters a barcode other than its own it activates and rejects the unrecognized cells. If this mechanism is difficult to avoid in the field of organ transplantation, it is potentially avoidable if the source of new tissue is pluripotent stem cells.

In particular, iPSCs, by their nature, can in principle be transformed into all cells and tissues in our body that, having the barcode of the individual, escape aggression by the immune system.

However, this approach has limitations:

• it is excessively expensive and very labor intensive, having to produce IPSC lines from each different patient;
• it cannot be applied in autoimmune diseases, as in the case of type 1 diabetes, because the immune system has a defect in recognizing its own barcode and is activated anyway.

The problem of auto-immunity

Over the past few years, a number of solutions have been proposed to prevent the immune system of patients with type 1 diabetes from being unleashed against the patient's own β-cells, even those differentiated from iPSCs.

Among these strategies, the most promising lies in the ability to erase the barcode (MHC) on the surface of the cells. This prevents the immune system from distinguishing between foreign components of our body and parts of the body itself and thus from destroying tissues and cells created from scratch.

NK cells

The absence of the barcode is itself an alarm signal for the immune system, which can in turn trigger the response of another group of white blood cells, the Natural Killer or NK cells, which, as described by their name, once activated are able to eliminate anything that does not express the barcode.

"NK cells constitute a very ancient and powerful defense system that human beings developed during evolution and have some important functions, such as eliminating cancer cells or some pathogens. Our goal was to be able to hide stem cells not only from T lymphocytes, but also from NK cells, but without inhibiting their normal physiological function," Piemonti says.

The approach proposed by San Raffaele

To make stem cells doubly invisible to the immune system, researchers have on the one hand inactivated MHC; and on the other hand studied the mechanisms of interaction with NK cells.

Specifically, the researchers identified, on the surface of iPSCs, twp key molecules in the activation of NK cells and, subsequently, inhibited their expression through biological drugs and CRISPR/Cas9, the genetic engineering technique worth the Nobel Prize in Medicine in 2020.

Insulin-producing cells derived from the modified iPSCs thus showed the same ability to evade recognition by both T lymphocytes and NK cells. The study, conducted in vitro and in vivo in experimental models of the disease, led to very promising results that could have spin-offs in other fields as well.

Future applications

The approach proposed by San Raffaele researchers could provide the framework for the creation of 'universal' stem lines, that is, stem lines that can be used without rejection problems in all patients and for any tissue.

Other possible applications include the field of oncology and organ transplantation. "Tumors to escape the immune system operate with mechanisms similar to those we have studied to make iPSCs invisible," says Piemonti, "and understanding them could offer new perspectives to activate the immune system against cancer cells.

In the field of transplantation, on the other hand, we are trying to develop a new way of assessing donor-recipient compatibility that takes into account not only T lymphocytes, as has been the case so far, but also NK cells, which play an important role in aggression toward anything that is not recognized as their own, in this case the transplanted organ."