Mesenchymal Stems Cells and Regulatory T Cells Inhibit Alloantigen Mediated Graft-Versus-Host Reaction but Also Impair Graft-Versus-Tumor Reactions Targeting Tumor Associated Antigens
Authors: Stelljes Matthias , Harmata Sarah , Schlösser Verena , Opitz Corinna , Albring Jörn , Rosenow Felix , Sargin Bülent , Berdel Wolfgang E. , Kienast Joachim 
Published: April 2020

Abstract
Graft-versus-host disease (GvHD) is a common and potentially life-threatening complication after allogeneic hematopoietic stem cell transplantation (HSCT). It is mediated by donor immune cells reacting against alloantigens presented by host or donor derived professional antigen presenting cells or expressed on host tissue. In addition, donor derived T-cells contribute to the beneficial graft-versus-tumor (GvT) reaction targeting minor histocompatibility antigens and, as previously demonstrated (Stelljes et al., Blood2004;104:1210-6), tumor associated antigens (TAAs). Several publications have shown that regulatory T-cells (Tregs) and mesenchymal stem cells (MSCs) can be used to prevent or treat GvHD. Since both cell populations react in a largely unspecific manner on alloantigen mediated immune responses, it remains a matter of debate whether intervention with these cell populations might interfere with GvT reactions directed against TAAs. In order to investigate the effects of MSCs and Tregs on GvHD and tumor specific GvT reactions, we used a murine parent-into-F1 transplant model. After total body irradiation with 9 Gy F1[BALB/c × C57BL/6] (CB6F1) mice were transplanted with parental bone marrow and splenocytes from C57BL/6 (B6) or BALB/c donors, respectively. Following transplantation, CB6F1 recipients received either three doses of 1×106 MSCs of B6 or BALB/c origin s.c. within the first two weeks or 1×106 Tregs together with the transplant. To evaluate TAA specific GvT effects, transplanted recipients were inoculated with tumor cells sharing the MHC background of the donor strain. GvHD score and tumor size were monitored every other day. Treatment with unstimulated MSCs of B6 origin had only marginal effects on GvHD intensity. In contrast, mice treated with unstimulated MSCs of BALB/c origin or BL/6 MSCs stimulated in vitro with supernatant of allogeneic mixed lymphocyte cultures, showed no signs of GvHD like weight loss or skin damage. TAA specific GvT reactions were significantly impaired by MSCs of either origin resulting in markedly increased tumor growth. Analysis of in vivo primed T cells 25 days after transplantation by IFN-γ ELIspot assay revealed significantly reduced TAA specific T-cells in recipients treated with MSCs as compared to transplanted tumor bearing control animals. In our experimental setting, Tregs also reduced GvHD compared with transplanted controls. Again, specific immune responses targeting TAAs were also abolished. In conclusion, cellular immune modulation using MSCs or Tregs might be effective to modulate GvHD. However, both strategies clearly impair tumor specific GvT reactions in our experimental setting suggesting that these interventions might bear the risk of higher relapse rates after allogeneic HSCT. The different effects on GvHD by stimulated and unstimulated MSCs of B6 or BALB/c origin are being explored.