This translational research program is based on knowledge of the therapeutic effectiveness of adult stem cells on the experimental models tested, their modes of action, and their harmlessness once injected. In five years’ time, this program will allow us to validate the clinical transfer of this innovative therapeutic strategy for the treatment of severe radio-induced lesions.
Control of the production of adult stem cells of preclinical and clinical grade
The production of the adult stem cells required for this program will be essentially ensured by the IRSN, the Armed Forces Transfusion Center (CTSA) and the Saint-Antoine teaching hospital. The purity, sterility, phenotype characterization, genetic drift, and the expansion capacity of each type of stem cell studied will be systematically evaluated.
Defining the therapeutic effectiveness of the autologous grafting of adult stem cells on two preclinical models of localized irradiation in rodents and mini-pigs.
The experimental models are developed to confirm the therapeutic effectiveness of cell therapy in the treatment of lesions on two particularly radiation-sensitive target organs: skin tissue and the gastro-intestinal system. To do this, radio-induced tissue lesion models were developed and validated in rodents and mini-pigs. The ‘large animal’ model is essential in order to validate the clinical transfer of cell therapy.
These experimental models make it possible to establish the clinical criteria for the use of adult stem cells: type of injection, optimum number of cells to be injected, injection frequency, curative vs. preventive treatment, injection of fresh vs. frozen cells, therapeutic effectiveness of the main adult stem cells. The injection of adult stem cells can be preceded by a surgical procedure before the injection in certain cases that require it, and when possible. It can also be applied in injection-only mode
Characterization of the modes of action of adult stem cells (or products derived from cell therapy) on radio-induced tissue lesions
Sustaining this chronic inflammation blocks all healing processes. This type of radiation-induced lesion is also described as progressive ischemic lesions, which originate in an angiogenic deficit associated with a vascular dysfunction. This process participates in the ulceration of the irradiated tissue and the destruction of muscular cells. Thus, the development of therapeutic immune modulation strategies [Nasef et al. 2007 a,b, Nasef et al. 2008] and/or therapeutic angiogenic strategies [Ebrahimian et al. 2009] could therefore constitute a promising alternative in the treatment of radiological burns. The LRTE has demonstrated that injected human mesenchymal stem cells colonize organs that have been greatly altered by the ionizing radiation [Chapel et al. 2003 ; François et al 2006], such as the skin and the intestine. In the field of musculocutaneous effects, the injection of hMSCs induces faster healing and an acceleration of the functional recovery of the irradiated member. [François et al. 2007]. Moreover, in the case of radio-induced gastro-intestinal effects, our results show that the injection of hMSCs increases and accelerates the auto-renewal of the intestinal epithelium (stimulation of endogenous stem cells, i.e. increasing their proliferation and decreasing their apoptosis) as well as its functionality (secretion and absorption of nutrients) [Sémont et al. 2006; Sémont et al. 2010]. This last experimental result provides a hope of being able to treat the unwanted side-effects of abdominal or pelvic radiotherapy treatments, in particular the gastro-intestinal complications associated with a malfunction in the auto-renewal of the epithelium (partial or total radio-induced sterilization of the intestinal stem cells).
This stage should allow us:
to study the modes of action of adult stem cells,
to characterize the selected target biological functions (inflammation, immunity, angiogenesis, myogenesis, and neuromuscular control) and study their kinetics on experimental models,
to evaluate the effect of the administration of cells on these target functions and increase the effectiveness of the pre-clinical protocols for the administration of adult stem cells on experimental models,
to optimize the effectiveness of adult stem cells and identify the best therapy product (supernatant of conditioned adult stem cells, adult stem cells treated by extracorporeal photochemotherapy, adult stem cells of different tissue origins).
Evaluation of the harmlessness of adult stem cells
The generalized use of cell therapy will first require an analysis of the side-effects. The culture of MSCs taken from humans (in vitro amplification), a necessary step in order to inject a sufficient number of cells, could cause chromosomal instabilities in the cells. The injection of these cells could therefore lead to the development of carcinomas. The French Society for Bone Marrow Graft and Cell Therapy, in collaboration with the IRSN, evaluated this risk in 2009 [Tarte et al. 2010]. Cells from different donors were placed in a culture under controlled conditions and according to two different methods. This work shows that MSCs in a culture, with or without chromosomal alterations, gradually stop growing, and age without any sign of evolution toward tumor cells, whether in vitro or in vivo. This means that any genetic drift of the MSCs in expansion can probably be circumvented by constituting cryogenically preserved cell banks, with cells prepared from young donors, or the identification and selection of adult stem cells that are less sensitive to this possible genetic drift. A possible tumorigenic effect or the fibrogenic effect associated with MSC injection could probably be limited by using, if they have an effect, MSC culture supernatants, or by injecting MSCs genetically programmed by the extracorporeal photochemotherapy technique to disappear once implanted.
Conclusions and outlook
The introduction of cell therapy has opened new therapeutic hopes in the field of treating radiation-induced tissue lesions. The treatment of certain tissue lesions after irradiation, which until now had been strictly surgical, is therefore possible with medical protocols involving cell therapy. The situation of patients with cancerous pathologies is, however, more complex. Before considering a treatment using mesenchymal stem cells for patients receiving radiotherapy for solid cancers, it is essential to check that the MSCs do not favor (1) the proliferation of cancer cells which could remain in the organism after radiotherapy, and/or (2) the development of tumors from precancerous lesions that coexist with tumors in these patients. In collaboration with Prof. Larsen’s team, we are currently acquiring new knowledge concerning the harmlessness of MSCs on the evolution of solid rectal cancers after radiotherapy. Our aim is to enhance our knowledge of the effects of MSCs in order to reduce, as far as possible, the associated side-effects and thus make clinical transfer safer for the treatment of side-effects of radiotherapies.
The IRSN’s recent experimental work suggests that the scope of application of cell therapy in radiopathology could cover more than just radiological burns. If the effectiveness of mesenchymal stem cells in treating severe radiological lesions in the main physiological systems (digestive systems, central nervous system, pulmonary system, etc.) were to be confirmed, the treatment of radiotherapy side-effects, which can sometimes be very disabling, would be tremendously improved.
Some uncertainties nonetheless remain, and the IRSN’s experimental work is continuing, in particular to investigate the types of adult stem cell best suited, according to the damaged organ, to treat and determine the potential long-term side-effects of the use of these adult stem cells. Large-scale human clinical studies could then be envisaged on the basis of these results.