Equal Expansion of Endogenous Transplant-Specific Regulatory T Cell and Recruitment Into the Allograft During Rejection and Tolerance.

James S Young,Dengping Yin,Augustin Georges Louis Vannier,Anita S Chong,Maria-Luisa Alegre

doi:10.3389/fimmu.2018.01385

Abstract

Despite numerous advances in the definition of a role for regulatory T cells (Tregs) in facilitating experimental transplantation tolerance, and ongoing clinical trials for Treg-based therapies, critical issues related to the optimum dosage, antigen-specificity, and Treg-friendly adjunct immunosuppressants remain incompletely resolved. In this study, we used a tractable approach of MHC tetramers and flow cytometry to define the fate of conventional (Tconvs) and Tregs CD4+ T cells that recognize donor 2W antigens presented by I-Ab on donor and recipient antigen-presenting cells (APCs) in a mouse cardiac allograft transplant model. Our study shows that these endogenous, donor-reactive Tregs comparably accumulate in the spleens of recipients undergoing acute rejection or exhibiting costimulation blockade-induced tolerance. Importantly, this expansion was not detected when analyzing bulk splenic Tregs. Systemically, the distinguishing feature between tolerance and rejection was the inhibition of donor-reactive conventional T cell (Tconv) expansion in tolerance, translating into increased percentages of splenic FoxP3+ Tregs within the 2W:I-Ab CD4+ T cell subset compared to rejection (~35 vs. <5% in tolerance vs. rejection). We further observed that continuous administration of rapamycin, cyclosporine A, or CTLA4-Ig did not facilitate donor-specific Treg expansion, while all three drugs inhibited Tconv expansion. Finally, donor-specific Tregs accumulated comparably in rejecting tolerant allografts, whereas tolerant grafts harbored <10% of the donor-specific Tconv numbers observed in rejecting allografts. Thus, ~80% of 2W:I-Ab CD4+ T cells in tolerant allografts expressed FoxP3+ compared to ≤10% in rejecting allografts. A similar, albeit lesser, enrichment was observed with bulk graft-infiltrating CD4+ cells, where ~30% were FoxP3+ in tolerant allografts, compared to ≤10% in rejecting allografts. Finally, we assessed that the phenotype of 2W:I-Ab Tregs and observed that the percentages of cells expressing neuropilin-1 and CD73 were significantly higher in tolerance compared to rejection, suggesting that these Tregs may be functionally distinct. Collectively, the analysis of donor-reactive, but not of bulk, Tconvs and Tregs reveal a systemic signature of tolerance that is stable and congruent with the signature within tolerant allografts. Our data also underscore the importance of limiting Tconv expansion for high donor-specific Tregs:Tconv ratios to be successfully attained in transplantation tolerance.

Highlights

Life-long pharmacological immunosuppression is necessary to prevent the rejection of allografts; side-effects, on-target toxicities, and high costs of drugs, together with emergent chronic allograft rejection, have prompted research toward inducing longterm graft acceptance following transient immunosuppressive therapy [1]
Regulatory T cells have been implicated in the induction and maintenance of donor-specific transplantation tolerance
Using a powerful new approach to track the fate of donor-specific Tconvs and Tregs in both the periphery and the allografts, we show that endogenous polyclonal CD4+ T cells recognizing a single donor-derived antigen can exist as Tregs or Tconvs prior to transplantation, and undergo different expansion profiles during acute rejection and tolerance

Summary

Introduction

Life-long pharmacological immunosuppression is necessary to prevent the rejection of allografts; side-effects, on-target toxicities, and high costs of drugs, together with emergent chronic allograft rejection, have prompted research toward inducing longterm graft acceptance following transient immunosuppressive therapy [1]. Regulatory T cells (Tregs) that express the transcription factor FoxP3 (FoxP3+ Tregs) have been shown to be critical for the successful induction and maintenance of peripherally induced transplantation tolerance in many experimental models. Their importance has been inferred from observations of Treg accumulation in tolerant allografts [2,3,4,5,6,7], peripheral conversion of FoxP3− CD4+ T [conventional T cell (Tconv)] cells into FoxP3+ Tregs under tolerance-inducing therapy [8, 9], and the inability to develop, as well as the reversal of, transplantation tolerance in recipients depleted of Tregs [6, 7, 10, 11]. While some caveats can be raised that experimental mouse models are highly reductionist and/or attenuated, observations made with these models have provided the rationale for adoptive Treg therapy in transplantation [1]

Methods

Results

Conclusion