Efficacy of single-agent immunosuppressive regimens in a murine model of vascularized composite allotransplantation

Methods of ex vivo analysis of tissue status in vascularized composite allografts

Vascularized composite allotransplantation can improve quality of life and restore functionality. However, the complex tissue composition of vascularized composite allografts (VCAs) presents unique clinical challenges that increase the likelihood of transplant rejection. Under prolonged static cold storage, highly damage-susceptible tissues such as muscle and nerve undergo irreversible degradation that may render allografts non-functional. Skin-containing VCA elicits an immunogenic response that increases the risk of recipient allograft rejection. The development of quantitative metrics to evaluate VCAs prior to and following transplantation are key to mitigating allograft rejection. Correspondingly, a broad range of bioanalytical methods have emerged to assess the progression of VCA rejection and characterize transplantation outcomes. To consolidate the current range of relevant technologies and expand on potential for development, methods to evaluate ex vivo VCA status are herein reviewed and comparatively assessed. The use of implantable physiological status monitoring biochips, non-invasive bioimpedance monitoring to assess edema, and deep learning algorithms to fuse disparate inputs to stratify VCAs are identified.

Small extracellular vesicles derived from PD-L1-modified mesenchymal stem cell promote Tregs differentiation and prolong allograft survival

We aimed to explore whether programmed cell death protein-1 ligand (PD-L1) modification on small extracellular vesicles (sEVs) could promote T regulatory cells (Tregs) differentiation. In this study, it was confirmed that under physiological conditions, PD-L1 expression was minimal in the MSCs and absent in the MSC-sEVs. A vector harboring the PD-L1 gene was constructed and transfected into bone marrow mesenchymal stem cells (BM-MSCs). By extracting the sEVs of these modified BM-MSCs and monitoring the expression of the PD-L1 protein, however, PD-L1 expression was substantially increased in the MSCs and concentrated in the sEVs. Then, the rat naïve CD4 + T cells were cocultured with the sEVs derived from the PD-L1-modified MSCs (sEVs^PD-L1). By flow cytometry, a higher percentage of Tregs and anti-inflammatory downstream cytokines (including IL-2, IFN-γ, TGF-β, IL-10) was detected in the sEVs^PD-L1 group than that in the control group treated by either sEVs in wild type, modified by empty vector, or blank control. Suppressive effect on CD4 + T cell proliferation serves as additional evidence to support the immunoregulation capacity of sEVs^PD-L1. The animal model of vascularized composite allograft further confirmed that PD-L1-modified sEVs induce an immune tolerance, by clinically observation, histopathology, T cell fate and cell product. In conclusion, sEVs^PD-L1 efficiently promotes Treg cell differentiation in vitro and in vivo, which suggests their therapeutic potential in the treatment of allograft rejection.

Pharmacokinetics and Biodistribution of Tacrolimus after Topical Administration: Implications for Vascularized Composite Allotransplantation

AIM

The high doses of oral tacrolimus (TAC) (1,2) necessary to prevent acute rejection (AR) after vascularized composite allotransplantation (VCA) are associated with systemic adverse effects. The skin is the most antigenic tissue in VCA and the primary target of AR. However, the short-term use of topical TAC (Protopic®), as an off-label adjunct to oral TAC, to treat AR episodes pro re nata (PRN), has yielded inconsistent results. There is lack of data on the pharmacokinetics and tissue distribution of topical TAC in VCA, that hampers our understanding of the reasons for unreliable efficacy. Toward this goal, we evaluated the ability of topical TAC to achieve high local tissue concentrations at the site of application with low systemic concentrations.

MATERIALS AND METHODS

We assessed the pharmacokinetics and tissue distribution of topical TAC (Protopic®, 0.03%) after single or repeated topical application in comparison to those after systemic delivery in rats. Animals received a single topical application of TAC ointment (Group 1) or an intravenous (IV) injection of TAC (Group 2) at a dose of 0.5 mg/kg. In another experiment, animals received daily topical application of TAC ointment (Group 3), or daily intraperitoneal (IP) injection of TAC (Group 4) at a dose of 0.5 mg/kg for 7 days. TAC concentrations in blood and tissues were analyzed by Liquid Chromatography-Mass Spectrometry (LC/MS-MS).

RESULTS

Following single topical administration, TAC was absorbed slowly with a Tmax of 4 h and an absolute bioavailability of 11%. The concentrations of TAC in skin and muscle were several folds higher than whole blood concentrations. Systemic levels remained subtherapeutic (< 3 ng/ml) with repeated once daily applications.

CONCLUSION

Topical application of TAC ointment (Protopic®, 0.03%) at a dose of 0.5 mg/kg/day provided high concentrations in the local tissues with low systemic exposure. Repeated topical administration of TAC is well tolerated with no local or systemic adverse effects. This study confirms the feasibility of topical application of TAC for site specific graft immunosuppression and enables future applications in VCA.

Limited efficacy of rapamycin monotherapy in vascularized composite allotransplantation

BACKGROUND

Vascularized composite allotransplantation (VCA) is a novel and life-enhancing procedure to restore a patient's function and/or appearance. Current immunosuppression in VCA recipients is based on calcineurin inhibitor (CNI) therapy that can lead to severe complications, such that inducing immune tolerance is a major goal of VCA research. In contrast to CNI, rapamycin (RPM) is thought to be beneficial to the development of immune tolerance by suppressing T-effector cells (Teffs) and expanding T-regulatory (Treg) cells. However, we found high dose RPM monotherapy prolonged VCA survival by only a few days, leading us to explore the mechanisms responsible.

METHODS

A mouse orthotopic forelimb transplantation model (BALB/c- > C57BL/6) was established using WT mice, as well as C57BL/6 recipients with conditional deletion of T-bet within their Treg cells. Events in untreated VCA recipients or those receiving RPM or FK506 therapy were analyzed by flow-cytometry, histopathology and real-time qPCR.

RESULTS

Therapy with RPM (2 mg/kg/d, p < .005) or FK506 (2 mg/kg/d, p < .005) each prolonged VCA survival. In contrast to FK506, RPM increased the ratio of splenic Treg to Teff cells (p < .05) by suppressing Teff and expanding Treg cells. While the proportion of activated splenic CD4 + Foxp3- T cells expressing IFN-γ were similar in control and RPM-treated groups, RPM decreased the proportions ICOS+ and CD8+ IFN-γ + splenic T cells. However, RPM also downregulated CXCR3+ expression by Tregs, and forelimb allografts had reduced infiltration by CXCR3+ Treg cells. In addition, allograft recipients whose Tregs lacked T-bet underwent accelerated rejection compared to WT mice despite RPM therapy.

CONCLUSIONS

We demonstrate that while RPM increased the ratio of Treg to Teff cells and suppressed CD8+ T cell allo-activation, it failed to prevent CD4 Teff cell activation and impaired CXCR3-dependent Treg graft homing, thereby limiting the efficacy of RPM in VCA recipients.