Multipotent mesenchymal stromal cell transplantation : Possible viral complications and alloimmunity

In the last few years, human multipotent mesenchymal stromal cells (MSC) have been increasingly used in novel therapeutic strategies due to their intrinsic immunosuppressive, anti-inflammatory and regenerative properties. In hematopoietic stem cell transplantation (HSCT), a curable treatment of hematological malignancies and several non-malignant conditions, MSC have been used as a therapy for graft-versus host-disease (GvHD) and other complications. The aim of MSC infusions in HSCT is to use the cells immunomodulatory effects to reduce the immunological reactions giving rise to GvHD and to achieve tissue regeneration. This thesis evaluates the clinical safety from a virological point of view and the immunogenicity of MSC in HSCT recipients. MSC were PCR screened for human herpesviruses and parvovirus B19 (B19), pathogens associated with severe infections in HSCT. The cells did not harbor herpesviruses, but presence of B19 DNA was detected in one MSC out of 20 screened. The presence of B19 is surprising since B19 is known for its extreme tropism for erythroid bone marrow cells. Upon exposure to the viruses, MSC supported infection of cytomegalovirus, herpes simplex virus and B19, but not Epstein-Barr virus as visualized by immunofluorescence. These infections could be passed to other uninfected cells implying that the infections of MSC were productive. Even though MSC are typically regarded as lowly immunogenic, data on rejection exist. MSC immunogenicity was evaluated at both the humoral and cellular levels. No alloantibodies could be detected by flow cytometric cross matches. However, MSC bound antibodies directed to FCS a component of the MSC culture medium. These antibodies are of uncertain clinical significance as they are constitutively expressed in humans. When evaluating MSC recipient lymphocytes in lymphocyte proliferation assays, there was no sign of allosensitization against the MSC donor, i.e. no immunological memory, 1 week to 6 months post-MSC infusion. In all instances, donor and third-party MSC failed to mount proliferative responses. In vitro studies revealed that MSC failed to prime responder cells to rechallenge with lymphocytes from the MSC donor and MSC rechallenge after PBL priming only gave weak responses. MSC failed to induce activated and effector CD4+ and CD8+ T lymphocyte subsets regardless of priming. To conclude, MSC occasionally carry viruses and may constitute a viral reservoir of persistent viruses associated with considerably disease in HSCT recipients. The MSC do not seem to induce humoral or cellular immune responses after infusions of HLA disparate MSC as therapy of complications to HSCT