Hematopoietic stem cell transplantation in multiple sclerosis: experimental evidence to rethink the procedures

Acute immunosuppression with lymphocytic agents given at maximally tolerated doses, followed by hematopoietic stem cell rescue achieved by autologous bone marrow or peripheral blood stem cell transplantation (BMT), has proved effective in various experimental models of autoimmunity. The rationale for such an approach in autoimmune diseases is based on the concept of lymphoablation of self-reactive lymphocytes followed by de novo immune system reconstitution, which, in the presence of the autoantigens in the thymus, may reinduce self-tolerance. Our previous work shows that in experimental autoimmune encephalomyelitis (EAE), autologous/syngeneic BMT not only prevents the appearance of paralytic signs, but can also partially reverse chronic disease and induce long-term, antigen-specific tolerance. However, there are serious reservations to be considered when interpreting these data and before applying similar protocols in patients with multiple sclerosis. (1) The model of EAE is not a completely reliable model of multiple sclerosis. (2) In animals with chronic EAE, although further relapses were prevented, the established paralysis was usually not reversible. According to recent data, in chronic multiple sclerosis (MS) lesions, damage caused by axonal loss/transection and cortical/spinal cord atrophy is irreversible and probably amenable to immunotherapy. (3) Long-term, antigen-specific tolerance may be induced with BMT, but not in all cases; in passively induced CR-EAE, many of the mice relapsed upon challenge with myelin antigens, which may indicate that the presence of the immunizing, myelin antigens (on the site of immunization) during the process of immune reconstitution is critical for induction of tolerance. Finally, one should weigh the procedure-related risks (including mortality of up to 5%) of bone marrow or peripheral stem cell transplantation (SCT). A more radical solution for autoimmunity may involve the use of non-myeloablative allogeneic transplantation.