Myeloid-Derived Suppressor Cells (MDSCs) Induced In Vivo Following Activation of Invariant Natural Killer T (iNKT) Cells Prolong the Survival of Skin Allografts

Abstract

Introduction: Transplantation is the most effective treatment for end-stage organ failure but long-term outcomes are hindered by chronic rejection and the side effects associated with prolonged immunosuppression. The induction of tolerance is an ideal solution to these problems and the contribution of immune cells other than T cells to this process is being increasingly recognized. In this study we investigated the potential impact of iNKT cells on the differentiation of myeloid-derived suppressor cells (MDSCs). We also examined the possibility of these in vivo expanded MDSCs to extend survival of an allogeneic skin transplant following adoptive transfer into a CD4+ TCR-transgenic (TCR-Tg) mouse model. Methods: B6 mice were injected i.v. with 1×106 TEa TCR-Tg CD4+ T cells (Va2+CD44-) on Day -1. To activate the TEa T cells these mice received 10x106 allogeneic B6xBALB/cF1 splenocytes i.v. on Day 0. Mice were also inoculated i.p. with either a-galactosylceramide (a-Gal) on Day 0 and 3 (to concomitantly activate endogenous iNKT cells) or PBS (as a control). On Day 5 Gr-1+CD11b+ MDSCs were flow sorted from the spleens of treated mice. Subsequently, B6 Rag-/- mice received 5x104 TEa T cells plus 5×104 sorted MDSCs from a-Gal or PBS treated mice. One day later all mice received an allogeneic B6xBALB/cF1 skin transplant. Results: As expected, transplanted mice that received TEa T cells alone rejected their grafts and those that received no cells accepted skin allografts long-term (median survival times (MST) 19.7 (n= 10) and >100 (n=10) days respectively). Mice that received a co-transfer of TEa T cells and MDSC from PBS treated mice rejected their grafts with a MST of 26.4 days (n=7). However, the addition of MDSC from a-Gal treated mice significantly extended graft survival compared to mice that received MDSC from PBS treated mice (MST=55 days (n=7), p=0.0002. Furthermore, flow sorted MDSC from a-Gal treated mice produced significantly more IFN-g and IL-10 compared to MDSC from PBS treated mice when stimulated in vitro (p=0.006 and p=0.0012 respectively). Conclusion: In summary, we have demonstrated that adoptive transfer of MDSCs, expanded in the presence of activated iNKT cells in vivo, significantly prolongs allograft survival in a mouse model of skin transplantation. We hypothesise that this extension in survival is IFN-g dependent as MDSC suppressive activity is lost in vivo and in vitro when IFN-g production is blocked.