Contrasting Effects of Cyclosporine and Rapamycin in De Novo Generation of Alloantigen-Specific Regulatory T Cells

Abstract

The outcome of T-cell-mediated responses, immunity or tolerance, critically depends on the balance of cytopathic versus regulatory T (T reg ) cells. In the creation of stable tolerance to MHC incompatible allografts, reducing the unusually large mass of donorreactive cytopathic T effector (T eff ) cells via apoptosis is often required. Cyclosporine (CsA) blocks activationinduced cell death (AICD) of T eff cells, and is detrimental to tolerance induction by costimulation blockade, whereas Rapamycin (RPM) preserves AICD, and augments the potential of costimulation blockade to create tolerance. While differences between CsA and RPM in influencing apoptosis of activated graft-destructive T eff cells are apparent, their effects on graft-protective T reg cells remain enigmatic. Moreover, it is unclear whether tolerizing regimens foster conversion of naïve peripheral T cells into alloantigen-specific T reg cells for graft protection. Here we show, using reporter mice for T reg marker Foxp3, that RPM promotes de novo conversion of alloantigen-specific T reg cells, whereas CsA completely inhibits this process. Upon transfer, in vivo converted T reg cells potently suppress the rejection of donor but not third party skin grafts. Thus, the differential effects of RPM and CsA on T eff and T reg cells favor the use of RPM in shifting the balance of aggressive to protective type alloimmunity.

Rapamycin inhibits differentiation of Th17 cells and promotes generation of FoxP3+ T regulatory cells

“…Furthermore, we found that rapamycin, but not CsA, promoted de novo generation of Tregs while potently inhibiting the differentiation of Th17 cells. During the preparation of this manuscript, Gao and colleagues reported that rapamycin, but not CsA, had the capacity to promote de novo generation of alloantigen-specific Tregs as demonstrated by a series of elegant in vitro as well as in vivo studies (16). Our data thus support and extend this report.…”

“…At a similar concentration usually used by other in vitro studies (12,13,15,16), rapamycin at 10 ng/ml resulted in the reduction of TGFβ/ IL-6-stimulated CD4+ IL-17-producing cells from 5.8% to 2.8%. Similarly, CsA (10 nM) also markedly inhibited the generation of IL-17-producing cells to 1.2%.…”

“…Our data also suggest that rapamycin does not block IL-2 signaling leading to the de novo generation of Tregs from naïve CD4 cells because TGFβ-induced expression of FoxP3 was not inhibited by this compound. It must be pointed out that even though rapamycin has the capacity to permit TGFβ-mediated generation of Tregs as shown in this report and others (16), the absolute number of Tregs produced may be compromised by rapamycin (1~100 ng/ml). This is supported by FACS analysis showing scatter properties of the cells treated with rapamycin suggestive of their lesser activation in response to plate-bound CD3 and soluble CD28 Abs (data not shown).…”

“…In addition, CD4 expression by some cells was reduced by rapamycin treatment. It has been reported that rapamycin has the capacity to preserve activation-induced cell death (AICS) (16). Those cells may represent dead or dying cells (and thus were gated out in the analysis) resulting from anti CD3 and anti CD28 Abs stimulation.…”

T cell receptor signaling controls Foxp3 expression via PI3K, Akt, and mTOR

“…Adaptive Treg cells can arise from naive peripheral CD4 T cells, for example by immunisation with low dose antigen and limited costimulation (13). TGF␤ is a potent inducer of Foxp3 expression in vitro (14) and in vivo (15)(16)(17) and immunosuppressive drugs, such as rapamycin (18)(19)(20), act by as yet undefined mechanisms to induce Foxp3 expression (18) or to expand preexisting Treg cells (19,20). To clarify the determinants of the Treg cell fate choice, we set out to identify signaling events that control Foxp3 expression.…”