The mobilisation of stem cells by cytokines such as G-CSF has become standard therapy in haematology practice for the purpose of transplantation and has largely replaced bone marrow as a stem cell source. Allogeneic Stem Cell Transplantation (SCT) has improved patient outcome relative to traditional bone marrow transplantation. The major complication of allogeneic SCT remains graft-versus-host disease (GVHD) in which the skin, GI tract, and liver are preferentially damaged by the transplanted donor immune system. The therapeutic potential of SCT relates to the graft-versus-leukaemia (GVL) effect, which eradicates host malignancy after SCT and is mediated by donor T and NK cells. Although GVL effects are associated with GVHD, evidence to date suggests that they are separable phenomena.
GVHD occurs in the majority (50-70 %) of SCT recipients and is responsible for the transplant-related mortality of up to 50 %. Approximately 20% of SCT recipients will develop severe GVHD that is refractory to steroid and cyclosporine-A based therapeutic protocols. These patients have a dismal prognosis and are traditionally treated with preparations of anti-thymocyte globulin or inhibitory antibodies to the IL-2 receptor. Despite initial response rates of up to 50 %, the vast majority of patients (80-95 %) eventually die from refractory or relapsed GVHD and the infective complications relating to severe and prolonged immunosuppression. Disappointingly, the majority of these patients are cured of their underlying malignancy. Clearly there is a pressing need for new treatment approaches to both prevent and especially, to treat GVHD.
The principal aim of our studies is to determine the potential efficacy of novel new strategies for the induction of antigen-specific tolerance to prevention and treatment of GVHD in well-characterised pre-clinical models. The mechanisms of action of the therapeutic manoeuvres are elucidated, which provide important insights into the pathophysiology of the disease as well as the induction of tolerance in established inflammation.
The pathophysiology of acute GVHD can be viewed as a three-step process and is absolutely dependent on the presence and function of donor T cells in the donor inoculum. In step 1, the conditioning regimen (irradiation and/or chemotherapy) leads to the damage and activation of host tissue by the release of the inflammatory cytokines TNF-a and IL-1. These cytokines can increase the expression of MHC antigens and adhesion molecules on host antigen presenting cells (dendritic cells), enhancing the recognition of host MHC and/or minor histocompatibility antigens by mature donor T cells. Donor T cell activation in step 2 is characterised by proliferation of Th1 T cells and secretion of IL-2 and IFN-y, promoting T cell expansion, CTL and NK cell responses. The effectors of tissue damage in step 3 include the products of mononuclear phagocytes (IL-1 and TNF-a) which are triggered via signals provided by LPS. Damage to the intestinal mucosa in step 1 and by cytolytic effectors activated in step 2 allows translocation of LPS from the intestinal lumen into the circulation. This mechanism may amplify local tissue injury and further promote an inflammatory response which, together with the CTL and NK component, leads to target tissue destruction in the BMT host. The importance of T cells for GVHD and GVL effects is demonstrated by depletion of T cells from the donor graft. This prevents GVHD, but is also associated with increased rates of relapse and infective complications which negate any beneficial effect. However, if inflammatory cytokine dysregulation during GVHD is prevented, while maintaining donor T cell cytotoxicity to host and haematopoetic antigens, leukaemia eradication after BMT can be demonstrated in the absence of GVHD. The “holy grail” of BMT remains complete separation of GVHD and GVL while preserving cognate T cell responses to non-host (e.g. infectious) antigens. The key to this achievement lies in administration of antigen-specific immunotherapy.
The effect of cytokine therapy on the outcome of bone marrow transplantation
We are detailing the effects of growth factors on transplant outcome, either when used to mobilise stem cells, to accelerate granulocyte recovery after transplantation, or as cytoprotective agents. These studies have lead to a number of novel discoveries and most recently a Nature Medicine publication on the subject.
The pathways of antigen presentation in graft-versus-host disease and transplant outcome
In these studies we are dissecting the role of various antigen presenting cell (APC) subsets on the initiation and maintenance of graft versus host disease (GVHD). These studies use novel reagents that allow the specific and quantitative determination of antigen presentation in both host and donor subsets, as well as the relative contribution of classical and non-classical APC subsets. It is hoped that this will allow modulation of alloreactive responses at the level of antigen presentation rather than current T cell directed approaches.
The role of TH17 differentiation in graft-versus-host disease and graft-versus-leukemia (GVL) effects
This new pathway of T cell differentiation may explain some of the immunological complexities in transplantation that fail to fit into previously described TH1 and TH2 paradigms. We are using a number of novel reagents to fully dissect this pathway and its’ control of specific GVHD and GVL responses.
Modulation of GVL via NKT activation
It is now clear that NKT cells can profoundly modulate immune responses, in both directions. In these studies we are focusing on NKT activation with glycolipid variants to improve GVL responses after BMT.
All these studies are based on solid preclinical data and have moved into translation phases at the Bone Marrow Transplant Unit in the Royal Brisbane and Women’s Hospital.