T-regulatory cell treatment prevents chronic rejection of heart allografts in a murine mixed chimerism model

Abstract

BackgroundThe mixed chimerism approach induces donor-specific tolerance in both pre-clinical models and clinical pilot trials. However, chronic rejection of heart allografts and acute rejection of skin allografts were observed in some chimeric animals despite persistent hematopoietic chimerism and tolerance toward donor antigens in vitro. We tested whether additional cell therapy with regulatory T cells (Tregs) is able to induce full immunologic tolerance and prevent chronic rejection.MethodsWe recently developed a murine “Treg bone marrow (BM) transplantation (BMT) protocol” that is devoid of cytoreductive recipient pre-treatment. The protocol consists of a moderate dose of fully mismatched allogeneic donor BM under costimulation blockade, together with polyclonal recipient Tregs and rapamycin. Control groups received BMT under non-myeloablative irradiation and costimulation blockade without Treg therapy. Multilineage chimerism was followed by flow cytometry, and tolerance was assessed by donor-specific skin and heart allografts.ResultsDurable multilineage chimerism and long-term donor skin and heart allograft survival were successfully achieved with both protocols. Notably, histologic examination of heart allografts at the end of follow-up revealed that chronic rejection is prevented only in chimeras induced with the Treg protocol.ConclusionsIn a mouse model of mixed chimerism, additional Treg treatment at the time of BMT prevents chronic rejection of heart allografts. As the Treg-chimerism protocol also obviates the need for cytoreductive recipient treatment it improves both efficacy and safety over previous non-myeloablative mixed chimerism regimens. These results may significantly impact the development of protocols for tolerance induction in cardiac transplantation.

Deletional and regulatory mechanisms coalesce to drive transplantation tolerance through mixed chimerism

“…Relatively low chimerism levels were observed with this protocol, but transplantation of BM retrieved from chimeras into secondary BMT recipients demonstrated that HSC engraftment had been achieved in the primary BMT recipients [40]. Donor skin and heart transplants were shown to be permanently accepted (while third party grafts are rejected) and in vitro assays demonstrated donor-specific T-and B-cell tolerance in the recipients [40,41]. Several types of tested recipient Treg cells (i.e., FoxP3-transduced, activated Treg cells and TGF-β-induced Treg cells) were shown to be effective in this setting [40] with recipient Treg cells being more potent in inducing chimerism than donor or third party Treg cells (unpublished data).…”

“…Along these lines, regulation is critical in the NHP setting and in clinical trials (see below) in which permanent mixed chimerism (which is a prerequisite for maintaining continuous central deletion) is not achieved [69]. Combining Treg-cell therapy with noncytoreductive BMT actively has been shown to potentiate regulatory mechanisms [40,41]. Clonal deletion has been shown with such a Treg-cell chimerism regimen, but is less pronounced, while regulation is predominant throughout follow up.…”

“…Increased regulatory activity might thus also provide a potential solution for the phenomenon of "split tolerance" seen in some chimeras, in which some types of grafts (e.g., BM) are accepted while other types (e.g., skin or heart) are rejected [20,72]. Tissuespecific minor antigens that have been identified to cause split tolerance in several models [73,74], are now known to be amenable to active regulation [20,41]. Thus, deliberately shifting the contributing mechanisms toward regulation indeed appears beneficial for the robustness of the state of tolerance achieved through mixed chimerism and increases the efficacy of such protocols.…”

“…Combining Treg-cell therapy with noncytoreductive BMT actively has been shown to potentiate regulatory mechanisms [40,41]. Clonal deletion has been shown with such a Treg-cell chimerism regimen, but is less pronounced, while regulation is predominant throughout follow up.…”

“…Clonal deletion has been shown with such a Treg-cell chimerism regimen, but is less pronounced, while regulation is predominant throughout follow up. Intriguingly, the degree of tolerance achieved with Treg-cell therapy is more complete -despite incomplete clonal deletion -and more fully encompasses tolerance toward minor donor antigens, than without Treg-cell therapy [41]. Increased regulatory activity might thus also provide a potential solution for the phenomenon of "split tolerance" seen in some chimeras, in which some types of grafts (e.g., BM) are accepted while other types (e.g., skin or heart) are rejected [20,72].…”