Manipulating adrenergic stress receptor signalling to enhance immunosuppression and prolong survival of vascularized composite tissue transplants

Background: Vascularized composite tissue allotransplantation (VCA) to replace limbs or faces damaged beyond repair is now possible. The resulting clear benefit to quality of life is a compelling reason to attempt this complex procedure. Unfortunately, the high doses of immunosuppressive drugs required to protect this type of allograft result in significant morbidity and mortality giving rise to ethical concerns about performing this surgery in patients with non-life-threatening conditions. Here we tested whether we could suppress anti-graft immune activity by using a safe beta(2)-adrenergic receptor (AR) agonist, terbutaline, to mimic the natural immune suppression generated by nervous system-induced signalling through AR. Methods: A heterotopic hind limb transplantation model was used with C5713L/6 (H-2b) as recipients and BALB/c (H -2d) mice as donors. To test the modulation of the immune response, graft survival was investigated after daily intraperitoneal injection of beta(2)-AR agonist with and without tacrolimus. Analyses of immune compositions and quantification of pro-inflammatory cytokines were performed to gauge functional immunomodulation. The contributions to aliograft survival of beta(2)-AR signalling in donor and recipient tissue were investigated with beta(2)-AR(-/-) strains. Results: Treatment with the beta(2)-AR agonist delayed VCA rejection, even with a subtherapeutic dose of tacrolimus. beta(2)-AR agonist decreased T-cell infiltration into the transplanted grafts and decreased memory T-cell populations in recipient's circulation. In addition, decreased levels of inflammatory cytokines (IFN-gamma, IL-6, TNF-alpha, CXCL-1/10 and CCL3/4/5/7) were detected following beta(2)-AR agonist treatment, and there was a decreased expression of ICAM-1 and vascular cell adhesion molecule-1 in donor stromal cells. Conclusions: beta(2)-AR agonist can be used safely to mimic the natural suppression of immune responses, which occurs during adrenergic stress-signalling and thereby can be used in combination regimens to reduce the dose needed of toxic immunosuppressive drugs such as tacrolimus. This strategy can be further evaluated for feasibility in the clinic.