Interleukin-21 promotes Type-1 activation and cytotoxicity of CD56dimCD16bright natural killer cells during kidney allograft antibody-mediated rejection showing a new link between adaptive and innate humoral allo-immunity.

The role of Natural killer (NK) cells during kidney allograft antibody-mediated rejection (ABMR) is increasingly recognized, but an in-depth characterization of mechanisms that contribute to such immune response is still under investigation. Here, we characterized phenotypic, functional, and transcriptomic profiles of peripheral blood circulating and allograft infiltrating CD56dimCD16bright NK cells during anti-HLA donor-specific antibody (DSA)+ ABMR. Cross-sectional analyses performed in 71 kidney transplant recipients identified a unique phenotypic circulating CD56dimCD16bright NK cell cluster expanded in DSA+ ABMR. This cluster co-expressed high levels of the interleukin-21 Receptor (IL-21R); Type-1 transcription factors T-bet and EOMES, CD160 and natural killer group 2D cytotoxic and activating co-stimulatory receptors. CD160+ IL-21R+ NK cells correlated with elevated plasma IL-21, Ki-67+ ICOS+ (CD278) IL-21-producing circulating T follicular helper cells, enhanced Type-1 pro-inflammatory cytokines, NK cell cytotoxicity, worse microvascular inflammation and graft loss. Single-cell transcriptomic analysis of circulating NK cells delineated an expanded cluster in DSA+ ABMR characterized by elevated pro-inflammatory/cytotoxic pathways, IL-21/STAT3 signaling, and leukocyte trans-endothelial migration pathways. Infiltration of CD160+ IL-21R+ NK cells with similar transcriptomic profile was detected in DSA+ ABMR allograft biopsies, potentially contributing to allograft injury. Thus, the IL-21/IL-21R axis, linking adaptive and innate humoral allo-immunity, or NK cells may represent appealing immunotherapy targets in DSA+ ABMR.

Breakdown of Immune Tolerance in AIRE-Deficient Rats Induces a Severe Autoimmune Polyendocrinopathy-Candidiasis-Ectodermal Dystrophy-like Autoimmune Disease.

Autoimmune regulator (AIRE) deficiency in humans induces a life-threatening generalized autoimmune disease called autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED), and no curative treatments are available. Several models of AIRE-deficient mice have been generated, and although they have been useful in understanding the role of AIRE in central tolerance, they do not reproduce accurately the APECED symptoms, and thus there is still a need for an animal model displaying APECED-like disease. We assessed, in this study, the potential of the rat as an accurate model for APECED. In this study, we demonstrate that in rat, AIRE is expressed by MHC class II (MCH-II)+ and MHC-II- medullary thymic epithelial cells in thymus and by CD4int conventional dendritic cells in periphery. To our knowledge, we generated the first AIRE-deficient rat model using zinc-finger nucleases and demonstrated that they display several of the key symptoms of APECED disease, including alopecia, skin depigmentation, and nail dystrophy, independently of the genetic background. We observed severe autoimmune lesions in a large spectrum of organs, in particular in the pancreas, and identified several autoantibodies in organs and cytokines such as type I IFNs and IL-17 at levels similar to APECED. Finally, we demonstrated a biased Ab response to IgG1, IgM, and IgA isotypes. Altogether, our data demonstrate that AIRE-deficient rat is a relevant APECED animal model, opening new opportunity to test curative therapeutic treatments.

Compensatory Regulatory Networks between CD8 T, B, and Myeloid Cells in Organ Transplantation Tolerance.

In transplantation tolerance, numerous regulatory populations have the capacity to inhibit allograft rejection; however, their compensatory capacities have never been clearly evidenced. We have previously demonstrated that the tolerogenic effect mediated by CD8(+)CD45RC(low) regulatory T cells (Tregs) in a model of organ transplantation with CD40Ig could be abrogated by permanent depletion of CD8(+) cells that resulted in allograft rejection in half of the recipients. This result demonstrated that CD8(+) Tregs were essential, but also that half of the recipients still survived indefinitely. We also demonstrated that no other regulatory populations, besides CD8(+) Tregs, could induce and maintain allograft tolerance in CD40Ig-treated tolerant animals. In the current study, we analyzed the mechanisms that arose following CD8(+) Treg depletion and allowed establishment of networks of new regulatory cells to maintain allograft survival. We identified regulatory B cells (Bregs) and regulatory myeloid cells (RegMCs) as being responsible of the maintenance of the long-term allograft survival. We demonstrated that both regulatory cell subsets efficiently inhibited antidonor immune responses in adoptively transferred recipients. Although Bregs were induced, they were not essential for the maintenance of the graft as demonstrated in IgM-deficient recipients. In addition, we showed that RegMCs were the most suppressive and acted alone, whereas Bregs activity was associated with increased suppressive activity of other subsets in adoptively transferred recipients. Altogether, to our knowledge, we demonstrated in this study for the first time the emergence of both Bregs and RegMCs following Tregs depletion and highlighted the importance of regulatory cell networks and their synergistic potential in transplantation.

Anti-CD45RC antibody immunotherapy prevents and treats experimental autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy syndrome.

Targeted monoclonal antibody (mAb) therapies show great promise for the treatment of transplant rejection and autoimmune diseases by inducing more specific immunomodulatory effects than broadly immunosuppressive drugs routinely used. We recently described the therapeutic advantage of targeting CD45RC, expressed at high levels by conventional T (Tconv) cells (CD45RChi), their precursors, and terminally differentiated T (TEMRA) cells, but not by regulatory T cells (Tregs; CD45RClo/-). We demonstrated efficacy of anti-CD45RC mAb treatment in transplantation, but its potential has not been examined in autoimmune diseases. Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is a rare genetic syndrome caused by loss-of-function mutations of autoimmune regulator (AIRE), a key central tolerance mediator, leading to abnormal autoreactive T cell responses and autoantibody production. Herein, we show that, in a rat model of APECED syndrome, anti-CD45RC mAb was effective for both prevention and treatment of autoimmune manifestations and inhibited autoantibody development. Anti-CD45RC mAb intervention depleted CD45RChi T cells, inhibited CD45RChi B cells, and restored the Treg/Tconv cell ratio and the altered Treg transcriptomic profile. In APECED patients, CD45RC was significantly increased in peripheral blood T cells, and lesioned organs from APECED patients were infiltrated by CD45RChi cells. Our observations highlight the potential role for CD45RChi cells in the pathogenesis of experimental and human APECED syndrome and the potential of anti-CD45RC antibody treatment.

Cross-Reactive Donor-Specific CD8+ Tregs Efficiently Prevent Transplant Rejection.

To reduce the use of non-specific immunosuppressive drugs detrimental to transplant patient health, therapies in development aim to achieve antigen-specific tolerance by promoting antigen-specific regulatory T cells (Tregs). However, identification of the natural antigens recognized by Tregs and the contribution of their dominance in transplantation has been challenging. We identify epitopes derived from distinct major histocompatibility complex (MHC) class II molecules, sharing a 7-amino acid consensus sequence positioned in a central mobile section in complex with MHC class I, recognized by cross-reactive CD8+ Tregs, enriched in the graft. Antigen-specific CD8+ Tregs can be induced in vivo with a 16-amino acid-long peptide to trigger transplant tolerance. Peptides derived from human HLA class II molecules, harboring the rat consensus sequence, also activate and expand human CD8+ Tregs, suggesting its potential in human transplantation. Altogether, this work should facilitate the development of therapies with peptide epitopes for transplantation and improve our understanding of CD8+ Treg recognition.

IL-34 is a Treg-specific cytokine and mediates transplant tolerance.

Cytokines and metabolic pathway-controlling enzymes regulate immune responses and have potential as powerful tools to mediate immune tolerance. Blockade of the interaction between CD40 and CD40L induces long-term cardiac allograft survival in rats through a CD8+CD45RClo Treg potentiation. Here, we have shown that the cytokine IL-34, the immunoregulatory properties of which have not been previously studied in transplantation or T cell biology, is expressed by rodent CD8+CD45RClo Tregs and human FOXP3+CD45RCloCD8+ and CD4+ Tregs. IL-34 was involved in the suppressive function of both CD8+ and CD4+ Tregs and markedly inhibited alloreactive immune responses. Additionally, in a rat cardiac allograft model, IL-34 potently induced transplant tolerance that was associated with a total inhibition of alloantibody production. Treatment of rats with IL-34 promoted allograft tolerance that was mediated by induction of CD8+ and CD4+ Tregs. Moreover, these Tregs were capable of serial tolerance induction through modulation of macrophages that migrate early to the graft. Finally, we demonstrated that human macrophages cultured in the presence of IL-34 greatly expanded CD8+ and CD4+ FOXP3+ Tregs, with a superior suppressive potential of antidonor immune responses compared with non-IL-34-expanded Tregs. In conclusion, we reveal that IL-34 serves as a suppressive Treg-specific cytokine and as a tolerogenic cytokine that efficiently inhibits alloreactive immune responses and mediates transplant tolerance.