Dendritic cells of myeloid lineage: the masterminds behind acute allograft rejection

Biomarkers from subcutaneous engineered tissues predict acute rejection of organ allografts

Invasive graft biopsies assess the efficacy of immunosuppression through lagging indicators of transplant rejection. We report on a microporous scaffold implant as a minimally invasive immunological niche to assay rejection before graft injury. Adoptive transfer of T cells into Rag2-/- mice with mismatched allografts induced acute cellular allograft rejection (ACAR), with subsequent validation in wild-type animals. Following murine heart or skin transplantation, scaffold implants accumulate predominantly innate immune cells. The scaffold enables frequent biopsy, and gene expression analyses identified biomarkers of ACAR before clinical signs of graft injury. This gene signature distinguishes ACAR and immunodeficient respiratory infection before injury onset, indicating the specificity of the biomarkers to differentiate ACAR from other inflammatory insult. Overall, this implantable scaffold enables remote evaluation of the early risk of rejection, which could potentially be used to reduce the frequency of routine graft biopsy, reduce toxicities by personalizing immunosuppression, and prolong transplant life.

Tregs integrate native and CAR-mediated costimulatory signals for control of allograft rejection

Tregs expressing chimeric antigen receptors (CAR-Tregs) are a promising tool to promote transplant tolerance. The relationship between CAR structure and Treg function was studied in xenogeneic, immunodeficient mice, revealing advantages of CD28-encoding CARs. However, these models could underrepresent interactions between CAR-Tregs, antigen-presenting cells (APCs), and donor-specific Abs. We generated Tregs expressing HLA-A2-specific CARs with different costimulatory domains and compared their function in vitro and in vivo using an immunocompetent model of transplantation. In vitro, the CD28-encoding CAR had superior antigen-specific suppression, proliferation, and cytokine production. In contrast, in vivo, Tregs expressing CARs encoding CD28, ICOS, programmed cell death 1, and GITR, but not 4-1BB or OX40, all extended skin allograft survival. To reconcile in vitro and in vivo data, we analyzed effects of a CAR encoding CD3ζ but no costimulatory domain. These data revealed that exogenous costimulation from APCs can compensate for the lack of a CAR-encoded CD28 domain. Thus, Tregs expressing a CAR with or without CD28 are functionally equivalent in vivo, mediating similar extension of skin allograft survival and controlling the generation of anti-HLA-A2 alloantibodies. This study reveals a dimension of CAR-Treg biology and has important implications for the design of CARs for clinical use in Tregs.

CAR-T Regulatory (CAR-Treg) Cells: Engineering and Applications

Regulatory T cells are critical for maintaining immune tolerance. Recent studies have confirmed their therapeutic suppressive potential to modulate immune responses in organ transplant and autoimmune diseases. However, the unknown and nonspecific antigen recognition of polyclonal Tregs has impaired their therapeutic potency in initial clinical findings. To address this limitation, antigen specificity can be conferred to Tregs by engineering the expression of transgenic T-cell receptor (TCR) or chimeric antigen receptor (CAR). In contrast to TCR Tregs, CAR Tregs are major histocompatibility complex (MHC) independent and less dependent on interleukin-2 (IL-2). Furthermore, CAR Tregs maintain Treg phenotype and function, home to the target tissue and show enhanced suppressive efficacy compared to polyclonal Tregs. Additional development of engineered CAR Tregs is needed to increase Tregs' suppressive function and stability, prevent CAR Treg exhaustion, and assess their safety profile. Further understanding of Tregs therapeutic potential will be necessary before moving to broader clinical applications. Here, we summarize recent studies utilizing CAR Tregs in modulating immune responses in autoimmune diseases, transplantation, and gene therapy and future clinical applications.

Dendritic Cells: Versatile Players in Renal Transplantation

Dendritic cells (DCs) induce and regulate adaptive immunity through migrating and maturing in the kidney. In this procedure, they can adopt different phenotypes—rejection-associated DCs promote acute or chronic injury renal grafts while tolerogenic DCs suppress the overwhelmed inflammation preventing damage to renal functionality. All the subsets interact with effector T cells and regulatory T cells (Tregs) stimulated by the ischemia–reperfusion procedure, although the classification corresponding to different effects remains controversial. Thus, in this review, we discuss the origin, maturation, and pathological effects of DCs in the kidney. Then we summarize the roles of divergent DCs in renal transplantation: taking both positive and negative stages in ischemia–reperfusion injury (IRI), switching phenotypes to induce acute or chronic rejection, and orchestrating surface markers for allograft tolerance via alterations in metabolism. In conclusion, we prospect that multidimensional transcriptomic analysis will revolute researches on renal transplantation by addressing the elusive mononuclear phagocyte classification and providing a holistic view of DC ontogeny and subpopulations.

Recent advances in our understanding of the allograft response

Organ transplantation is a life-saving treatment for end-stage organ failure. However, despite advances in immunosuppression, donor matching, tissue typing, and organ preservation, many organs are still lost each year to rejection. Ultimately, tolerance in the absence of immunosuppression is the goal, and although this seldom occurs spontaneously, a deeper understanding of alloimmunity may provide avenues for future therapies which aid in its establishment. Here, we highlight the recent key advances in our understanding of the allograft response. On the innate side, recent work has highlighted the previously unrecognised role of innate lymphoid cells as well as natural killer cells in promoting the alloresponse. The two major routes of allorecognition have recently been joined by a third newly identified pathway, semi-direct allorecognition, which is proving to be a key active pathway in transplantation. Through this review, we detail these newly defined areas in the allograft response and highlight areas for potential future therapeutic intervention.

A post-preservation vascular flush removes significant populations of donor leukocytes prior to lung transplantation

BACKGROUND

Donor leukocytes are intrinsically involved in acute lung allograft rejection, via self-presentation of donor antigens to recipient leukocytes. Therapeutic modalities to remove donor leukocytes are currently unavailable. We evaluated if a vascular flush immediately following preservation can be used for this purpose.

METHODS

A post-preservation flush was performed with STEEN solution in n = 6 porcine lungs following static cold storage. The first 500 ml effluent from the left atrium was collected and an inflammatory profile performed.

RESULTS

A total of 1.17 billion (±2.8 × 10⁸) viable leukocytes were identified within the effluent. T cells were the dominant cell population, representing 82% of the total mobilised leukocytes, of which <0.01% were regulatory T cells. IL-18 was the most abundant cytokine, with a mean concentration of 84,216 pg (±153,552 pg). In addition, there was a mean concentration of 8819 ng (±4415) cell-free mitochondrial DNA.

CONCLUSION

There is an immediate transfer of donor leukocytes, cytokines and damage-associated molecular patterns following reperfusion. Such a pro-inflammatory donor load may enhance alloantigen presentation and drive recipient alloimmune responses. A post-preservation flush may therefore be an effective method for reducing the immune burden of the donor lung prior to transplantation.

Galectin-1-induced tolerogenic dendritic cells combined with apoptotic lymphocytes prolong liver allograft survival

Donor-derived tolerogenic dendritic cells (DCs) and apoptotic lymphocytes (ALs) are practical tools for controlling rejection after transplantation by targeting direct and indirect allorecognition pathways, respectively. To date, few studies have investigated the combination of donor-derived tolerogenic DCs and ALs infusion in organ transplantation protection. In the present study, we generated galectin-1-induced tolerogenic DCs (DC_gal-1s) and ultraviolet irradiation-induced ALs with stable immune characteristics in vitro and potential immune regulatory activity in vivo. A rat model of acute liver transplant rejection was established, and the intrinsic tolerogenic profiles associated with the short-term alleviation of rejection and the long-term maintenance of tolerance in the absence of immunosuppressive drugs were evaluated. The DC_gal-1-AL treatment prolonged allograft survival more significantly than a transfusion of DC_gal-1s or ALs alone. This benefit was associated with CD4⁺ Treg cell expansion and decreased interferon (IFN)-γ⁺ T cell levels. Moreover, DC_gal-1-AL treatment led to different cytokine/chemokine changes in the allograft and peripheral blood, that indicated an alleviation of local and systemic inflammation on day 7 post-transplantation. TGF-β1 and TGF-β2 were significantly increased in the long-term surviving allografts after DC_gal-1-AL treatment. Our results indicate that the combination of DC_gal-1s with ALs effectively prolongs liver allograft survival and represents a novel therapeutic strategy for liver transplant rejection.

GDF15 Regulates Malat-1 Circular RNA and Inactivates NFκB Signaling Leading to Immune Tolerogenic DCs for Preventing Alloimmune Rejection in Heart Transplantation

Recombinant human growth differentiation factor 15 (rhGDF15) affects dendritic cell (DC) maturation. However, whether GDF15 is expressed in DCs and its roles and signaling in DCs remain largely unknown. It is unclear whether GDF15-DCs can induce immune tolerance in heart transplantation (HT). This study aims to understand the impact of endogenous GDF15 on DC's development, function, underlying molecular mechanism including circular RNA (circRNA). This study will also explore GDF15-DC-mediated immune modulation in HT. Bone marrow (BM) derived DCs were cultured and treated to up- or down regulate GDF15 expression. Phenotype and function of DCs were detected. Expression of genes and circRNAs was determined by qRT-PCR. The signaling pathways activated by GDF15 were examined. The impact of GDF15 treated DCs on preventing allograft immune rejection was assessed in a MHC full mismatch mouse HT model. Our results showed that GDF15 was expressed in DCs. Knockout of GDF15 promoted DC maturation, enhanced immune responsive functions, up-regulated malat-1 circular RNA (circ_Malat 1), and activated the nuclear factor kappa B (NFκB) pathway. Overexpression of GDF15 in DCs increased immunosuppressive/inhibitory molecules, enhanced DCs to induce T cell exhaustion, and promoted Treg generation through IDO signaling. GDF15 utilized transforming growth factor (TGF) β receptors I and II, not GFAL. Administration of GDF15 treated DCs prevented allograft rejection and induced immune tolerance in transplantation. In conclusion, GDF15 induces tolerogenic DCs (Tol-DCs) through inhibition of circ_Malat-1 and the NFκB signaling pathway and up-regulation of IDO. GDF15-DCs can prevent alloimmune rejection in HT.

Release of pig leukocytes and reduced human NK cell recruitment during ex vivo perfusion of HLA-E/human CD46 double-transgenic pig limbs with human blood

BACKGROUND

In pig-to-human xenotransplantation, interactions between human natural killer (NK) cells and porcine endothelial cells (pEC) are characterized by recruitment and cytotoxicity. Protection from xenogeneic NK cytotoxicity can be achieved in vitro by the expression of the non-classical human leukocyte antigen-E (HLA-E) on pEC. Thus, the aim of this study was to analyze NK cell responses to vascularized xenografts using an ex vivo perfusion system of pig limbs with human blood.

METHODS

Six pig forelimbs per group, respectively, stemming from either wild-type (wt) or HLA-E/hCD46 double-transgenic (tg) animals, were perfused ex vivo with heparinized human blood for 12 hours. Blood samples were collected at defined time intervals, cell numbers counted, and peripheral blood mononuclear cells analyzed for phenotype by flow cytometry. Muscle biopsies were analyzed for NK cell infiltration. In vitro NK cytotoxicity assays were performed using pEC derived from wt and tg animals as target cells.

RESULTS

Ex vivo, a strong reduction in circulating human CD45 leukocytes was observed after 60 minutes of xenoperfusion in both wt and tg limb groups. NK cell numbers dropped significantly. Within the first 10 minutes, the decrease in NK cells was more significant in the wt limb perfusions as compared to tg limbs. Immunohistology of biopsies taken after 12 hours showed less NK cell tissue infiltration in the tg limbs. In vitro, NK cytotoxicity against hCD46 single tg pEC and wt pEC was similar, while lysis of double tg HLA-E/hCD46 pEC was significantly reduced. Finally, circulating cells of pig origin were observed during the ex vivo xenoperfusions. These cells expressed phenotypes mainly of monocytes, B and T lymphocytes, NK cells, as well as some activated endothelial cells.

CONCLUSIONS

Ex vivo perfusion of pig forelimbs using whole human blood represents a powerful tool to study humoral and early cell-mediated rejection mechanisms of vascularized pig-to-human xenotransplantation, although there are several limitations of the model. Here, we show that (i) transgenic expression of HLA-E/hCD46 in pig limbs provides partial protection from human NK cell-mediated xeno responses and (ii) the emergence of a pig cell population during xenoperfusions with implications for the immunogenicity of xenografts.

Donor-derived exosomes: the trick behind the semidirect pathway of allorecognition

PURPOSE OF REVIEW

The passenger leukocyte hypothesis predicts that after transplantation, donor antigen-presenting cells (APCs) from the graft present donor MHC molecules to directly alloreactive T cells in lymphoid organs. However, in certain transplantation models, recent evidence contradicts this long-standing concept. New findings demonstrate that host, instead of donor, APCs play a prominent role in allosensitization against donor MHC molecules via the semidirect pathway. A similar mechanism operates in development of T-cell split tolerance to noninherited maternal antigens.

RECENT FINDINGS

Following fully mismatch skin or heart transplantation in mice, no or extremely few donor migrating APCs (i.e. conventional dendritic cells) are detected in the draining lymphoid organs. Instead, recipient dendritic cells that have captured donor extracellular vesicles (i.e. exosomes) carrying donor MHC molecules and APC costimulatory signals present donor MHC molecules to directly alloreactive T cells. This semidirect pathway can also give rise to a form of 'split' tolerance during chronic alloantigen exposure, as indirectly alloreactive T helper cells and directly alloreactive T-cell effectors are differentially impacted by host dendritic cells 'cross-dressed' with extracellular vesicles/exosomes derived from maternal microchimerism.

SUMMARY

Acquisition by recipient APCs of donor exosomes (and likely other extracellular vesicles) released by passenger leukocytes or the graft explains the potent T-cell allosensitization against donor MHC molecules, in the absence or presence of few passenger leukocytes in lymphoid organs. It also provides the basic mechanism and in-vivo relevance of the elusive semidirect pathway. Its degree of coordination with the allopeptide - specific, indirect pathway of T-cell help may determine whether semidirect allopresentation results in a sustained, effective, acute rejection response, or rather, in abortive acute rejection and 'split' tolerance.

Antigen Presentation in Transplantation

Transplantation of solid organs between genetically distinct individuals leads, in the absence of immunosuppression, to T cell-dependent transplant rejection. Activation of graft-reactive T cells relies on the presentation of transplant-derived antigens (intact donor MHC molecules or processed peptides on host MHC molecules) by mature dendritic cells (DCs). This review focuses on novel insights regarding the steps for maturation and differentiation of DCs that are necessary for productive presentation of transplant antigens to host T cells. These steps include the licensing of DCs by the microbiota, their activation and maturation following recognition of allogeneic non-self, and their capture of donor cell exosomes to amplify the presentation of transplant antigens.

Complement component 3 deficiency prolongs MHC-II disparate skin allograft survival by increasing the CD4(+) CD25(+) regulatory T cells population

Recent reports suggest that complement system contributes to allograft rejection. However, its underlying mechanism is poorly understood. Herein, we investigate the role of complement component 3 (C3) in a single MHC-II molecule mismatched murine model of allograft rejection using C3 deficient mice (C3^−/−) as skin graft donors or recipients. Compared with C3^+/+ B6 allografts, C3^−/− B6 grafts dramatically prolonged survival in MHC-II molecule mismatched H-2^bm12 B6 recipients, indicating that C3 plays a critical role in allograft rejection. Compared with C3^+/+ allografts, both Th17 cell infiltration and Th1/Th17 associated cytokine mRNA levels were clearly reduced in C3^−/− allografts. Moreover, C3^−/− allografts caused attenuated Th1/Th17 responses, but increased CD4⁺CD25⁺Foxp3⁺ regulatory T (Treg) cell expression markedly in local intragraft and H-2^bm12 recipients. Depletion of Treg cells by anti-CD25 monoclonal antibody (mAb) negated the survival advantages conferred by C3 deficiency. Our results indicate for the first time that C3 deficiency can prolong MHC-II molecule mismatched skin allograft survival, which is further confirmed to be associated with increased CD4⁺ CD25⁺ Treg cell population expansion and attenuated Th1/Th17 response.

Graft-infiltrating host dendritic cells play a key role in organ transplant rejection

Successful engraftment of organ transplants has traditionally relied on preventing the activation of recipient (host) T cells. Once T-cell activation has occurred, however, stalling the rejection process becomes increasingly difficult, leading to graft failure. Here we demonstrate that graft-infiltrating, recipient (host) dendritic cells (DCs) play a key role in driving the rejection of transplanted organs by activated (effector) T cells. We show that donor DCs that accompany heart or kidney grafts are rapidly replaced by recipient DCs. The DCs originate from non-classical monocytes and form stable, cognate interactions with effector T cells in the graft. Eliminating recipient DCs reduces the proliferation and survival of graft-infiltrating T cells and abrogates ongoing rejection or rejection mediated by transferred effector T cells. Therefore, host DCs that infiltrate transplanted organs sustain the alloimmune response after T-cell activation has already occurred. Targeting these cells provides a means for preventing or treating rejection.

A novel cyclic helix B peptide inhibits dendritic cell maturation during amelioration of acute kidney graft rejection through Jak-2/STAT3/SOCS1

We recently synthesized a novel proteolysis-resistant cyclic helix B peptide (CHBP) that exhibits promising renoprotective effects. Dendritic cells (DCs) play an activation role in acute rejection (AR). Thus, the present study was designed to investigate the effects of CHBP on DCs in a rat renal transplantation model. The left kidney was harvested from male Lewis rats and then transplanted into male Wistar rats with or without CHBP treatment. Five successive treatment doses of CHBP after transplantation significantly ameliorated AR with lower histological injury, apoptosis and CD4⁺ and CD8⁺ T-cell infiltration in renal allografts. CHBP reduced IFN-γ and IL-1β levels but increased IL-4 and IL-10 levels in the serum. The number of mature DCs was significantly decreased in renal allografts treated with CHBP. In addition, incubating DCs with CHBP in vitro led to reduction in TNF-α, IFN-γ, IL-1β and IL-12 levels and increase of IL-10 expression at the protein level in the supernatant. Mechanistically, CHBP inhibited TLR activation-induced DC maturation by increasing SOCS1 expression through Jak-2/STAT3 signaling. In conclusion, CHBP suppresses renal allograft AR by inhibiting the maturation of DCs via Jak-2/STAT3/SOCS1 signaling, suggesting that CHBP may be an potential therapeutic drug for treating renal AR.

Introduction of a Framework for Dynamic Knowledge Representation of the Control Structure of Transplant Immunology: Employing the Power of Abstraction with a Solid Organ Transplant Agent-Based Model

Agent-based modeling has been used to characterize the nested control loops and non-linear dynamics associated with inflammatory and immune responses, particularly as a means of visualizing putative mechanistic hypotheses. This process is termed dynamic knowledge representation and serves a critical role in facilitating the ability to test and potentially falsify hypotheses in the current data- and hypothesis-rich biomedical research environment. Importantly, dynamic computational modeling aids in identifying useful abstractions, a fundamental scientific principle that pervades the physical sciences. Recognizing the critical scientific role of abstraction provides an intellectual and methodological counterweight to the tendency in biology to emphasize comprehensive description as the primary manifestation of biological knowledge. Transplant immunology represents yet another example of the challenge of identifying sufficient understanding of the inflammatory/immune response in order to develop and refine clinically effective interventions. Advances in immunosuppressive therapies have greatly improved solid organ transplant (SOT) outcomes, most notably by reducing and treating acute rejection. The end goal of these transplant immune strategies is to facilitate effective control of the balance between regulatory T cells and the effector/cytotoxic T-cell populations in order to generate, and ideally maintain, a tolerant phenotype. Characterizing the dynamics of immune cell populations and the interactive feedback loops that lead to graft rejection or tolerance is extremely challenging, but is necessary if rational modulation to induce transplant tolerance is to be accomplished. Herein is presented the solid organ agent-based model (SOTABM) as an initial example of an agent-based model (ABM) that abstractly reproduces the cellular and molecular components of the immune response to SOT. Despite its abstract nature, the SOTABM is able to qualitatively reproduce acute rejection and the suppression of acute rejection by immunosuppression to generate transplant tolerance. The SOTABM is intended as an initial example of how ABMs can be used to dynamically represent mechanistic knowledge concerning transplant immunology in a scalable and expandable form and can thus potentially serve as useful adjuncts to the investigation and development of control strategies to induce transplant tolerance.

Current understanding of alloimmunity of the intestinal graft

PURPOSE OF REVIEW

This review focuses on the known mechanisms of alloimmunity that occur after transplantation and what is being done in order to improve graft and patient survival, particularly in the long term.

RECENT FINDINGS

The presence of mismatched antigens and epitopes might relate directly to the development of de-novo donor-specific antibodies (DSA), and thus, rejection. In an abdominal wall transplant, the skin graft could be the first to show signs of rejection. The epithelial or endothelial cells are the main targets in acute and chronic rejection, respectively. Possible therapeutical targets are gut homing T cells and cells of the innate immune system. Chimerism development might mostly occur in isolated lymph nodes, but also in the epithelium, particularly after transplantation of bone marrow mesenchymal stromal cells.

SUMMARY

Ischemia-reperfusion, surgical injury, and bacterial translocation trigger the innate immune system, starting acute rejection. Interaction between donor and recipient immune cells generate injury and tolerance, which occur mostly in secondary lymphoid organs, lamina propria, and epithelium. Chronic rejection mostly affects the endothelial cells, generating graft dysfunction. DSA increase the risk of graft rejection both acutely and chronically, and the liver protects against their effects. Induction therapies deplete lymphocytes prior to implantation, and maintenance therapies inhibit T-cell expansion. Rejection rates are the lowest when depleting drugs and a combination of interleukin 2 receptor blockade, inhibition of T-cell expansion, and steroids are used as maintenance therapy. Chimerism and tolerogenic regiments that induce Tregs and prevent the development of DSA are important treatment goals for the future.

Clinical implications of basic science discoveries: nociceptive neurons as targets to control immunity--potential relevance for transplantation

Increasing evidence indicates the existence of a complex cross-regulation between the most important biosensors of the human body: The immune and nervous systems. Cytokines control body temperature and trigger autoimmune disorders in the central nervous system, whereas neuropeptides released in peripheral tissues and lymphoid organs modulate inflammatory (innate) and adaptive immune responses. Surprisingly, the effects of nerve fibers and the antidromic release of its pro-inflammatory neuropeptides on the leukocytes of the immune system that mediate graft rejection are practically unknown. In the transplantation field, such area of research remains practically unexplored. A recent study by Riol-Blanco et al has revealed new details on how nociceptive nerves regulate the pro-inflammatory function of leukocytes in peripheral tissues. Although the mechanism(s) by which neuroinflammation affects the immune response against the allograft remains unknown, recent data suggest that this new area of research is worth exploring for potential development of novel complementary therapies for prevention/treatment of graft rejection.

Dendritic cells and innate immunity in kidney transplantation

This review summarizes emerging concepts related to the roles of dendritic cells and innate immunity in organ transplant rejection. First, it highlights the primary role that recipient, rather than donor, dendritic cells have in rejection and reviews their origin and function in the transplanted kidney. Second, it introduces the novel concept that recognition of allogeneic non-self by host monocytes (referred to here as innate allorecognition) is necessary for initiating rejection by inducing monocyte differentiation into mature, antigen-presenting dendritic cells. Both concepts provide opportunities for preventing rejection by targeting monocytes or dendritic cells.