Sirolimus Inhibits Proliferation but Enhances Suppressive Activity of Rhesus Macaque Natural CD4 Regulatory T Cells
Authors: Singh Karnail , Kozyr Natalia , Stempora Linda , Kirk Allan D , Larsen Christian P , Blazar Bruce R. , Kean Leslie S. 
Published: October 2019

Abstract
Abstract 1008
Regulatory T cells (Tregs) have been shown to be potent inhibitors of autoimmunity, and to be capable of suppressing alloimmune responses that occur during both allograft rejection and graft-versus host disease. However, they have yet to gain widespread use clinically, due in part to the fact that it remains extremely costly and difficult to produce them in sufficient numbers and with sufficient suppressive capacity to significantly impact the alloimmune response. Here we have used our established non-human primate model to demonstrate that significant Treg expansion (up to 600-fold in 21 days) can be maintained, and suppressive capacity enhanced by exposing Treg cultures to a short burst of sirolimus at the end of the culture period.
Using a highly sensitive and specific in vitro CFSE-MLR assay we show that Tregs significantly inhibit allo-proliferation of multiple T cell subpopulations including both CD4+ and CD8+ T cells (3.2 and 2.7-fold inhibition of proliferation, respectively), as well as their CD28+CD95+ and CD28-CD95+ subpopulations (2.2 and 2.1 and 1.9 and 2.7-fold inhibition of CD4+ and CD8+ subpopulation proliferation, respectively). Tregs were able to combine in vitro with the newly FDA-approved CTLA4-Ig analog belatacept to enhance the inhibition of alloproliferation that occurred with either agent alone (4.8-fold inhibition of CD8 T cell proliferation with Tregs + belatacept, compared to 3.0-fold or 1.9-fold inhibition of CD8 T cell proliferation with Tregs or belatacept alone, respectively).
Importantly, we have found that the suppressive activity of ex-vivo expanded Tregs could be further enhanced by pulsing with sirolimus. Thus, while long-term culture of Tregs in the presence of sirolimus (1–1000 nM) profoundly inhibited Treg expansion (50–800 fold inhibition of expansion when cultured in the presence of 1–1000 nM sirolimus), a 48 hour pulse of sirolimus (100 nM) on days 20–21 of culture completely preserved Treg yields while doubling their suppressive function against CD8 proliferation when compared to unpulsed Tregs, p<0.01) A mechanistic evaluation of the increase potency observed with sirolimus pulsed Tregs (SPTs) has revealed several key differences that distinguish these cells from the less-potent unpulsed Tregs: SPTs were found to undergo fewer rounds of proliferation in an MLR when compared with unpulsed Tregs (14% proliferation in SPTs versus 37% proliferation in un-pulsed Tregs, p= 0.015), suggesting that the suppressive capability of Tregs may be inversely related to their proliferative capacity. SPTs were also shown to have significantly increased expression of CD25 (p=0.04) and total CTLA4 (p= 0.009) compared to unpulsed Tregs, implicating signaling through both of these molecules in their enhanced function.
Our results suggest that the creation of SPTs may provide a novel avenue by which to achieve enhanced Treg-based suppression of alloimmunity, in a manner that is amenable to large-scale ex-vivo expansion and to combinatorial therapy with novel, costimulation-blockade-based immunosuppression strategies.

Disclosures:
No relevant conflicts of interest to declare.