Belatacept Does Not Inhibit Follicular T Cell-Dependent B-Cell Differentiation in Kidney Transplantation

Cellular Immunobiology and Molecular Mechanisms in Alloimmunity-Pathways of Immunosuppression

Current maintenance immunosuppression commonly comprises a synergistic combination of tacrolimus as calcineurin inhibitor (CNI), mycophenolic acid, and glucocorticoids. Therapy is often individualized by steroid withdrawal or addition of belatacept or inhibitors of the mechanistic target of rapamycin. This review provides a comprehensive overview of their mode of action, focusing on the cellular immune system. The main pharmacological action of CNIs is suppression of the interleukin-2 pathway that leads to inhibition of T cell activation. Mycophenolic acid inhibits the purine pathway and subsequently diminishes T and B cell proliferation but also exerts a variety of effects on almost all immune cells, including inhibition of plasma cell activity. Glucocorticoids exert complex regulation via genomic and nongenomic mechanisms, acting mainly by downregulating proinflammatory cytokine signatures and cell signaling. Belatacept is potent in inhibiting B/T cell interaction, preventing formation of antibodies; however, it lacks the potency of CNIs in preventing T cell-mediated rejections. Mechanistic target of rapamycin inhibitors have strong antiproliferative activity on all cell types interfering with multiple metabolic pathways, partly explaining poor tolerability, whereas their superior effector T cell function might explain their benefits in the case of viral infections. Over the past decades, clinical and experimental studies provided a good overview on the underlying mechanisms of immunosuppressants. However, more data are needed to delineate the interaction between innate and adaptive immunity to better achieve tolerance and control of rejection. A better and more comprehensive understanding of the mechanistic reasons for failure of immunosuppressants, including individual risk/benefit assessments, may permit improved patient stratification.

Helper T Cell (CD4+) Targeted Tacrolimus Delivery Mediates Precise Suppression of Allogeneic Humoral Immunity

Antibody-mediated rejection (ABMR) is a major cause of dysfunction and loss of transplanted kidney. The current treatments for ABMR involve nonspecific inhibition and clearance of T/B cells or plasma cells. However, the prognosis of patients following current treatment is poor. T follicular helper cells (Tfh) play an important role in allograft-specific antibodies secreting plasma cell (PC) development. Tfh cells are therefore considered to be important therapeutic targets for the treatment of antibody hypersecretion disorders, such as transplant rejection and autoimmune diseases. Tacrolimus (Tac), the primary immunosuppressant, prevents rejection by reducing T cell activation. However, its administration should be closely monitored to avoid serious side effects. In this study, we investigated whether Tac delivery to helper T (CD4+) cells using functionalized mesoporous nanoparticles can block Tfh cell differentiation after alloantigen exposure. Results showed that Tac delivery ameliorated humoral rejection injury in rodent kidney graft by suppressing Tfh cell development, PC, and donor-specific antibody (DSA) generation without causing severe side effects compared with delivery through the drug administration pathway. This study provides a promising therapeutic strategy for preventing humoral rejection in solid organ transplantation. The specific and controllable drug delivery avoids multiple disorder risks and side effects observed in currently used clinical approaches.

Belatacept Treatment of Recurrent Late-onset T Cell-mediated Rejection/Antibody-mediated Rejection With De Novo Donor-specific Antibodies in a Liver Transplant Patient

T cell-mediated rejection that appears and persists late after transplantation is often associated with development of de novo donor-specific antibodies. Treatment of this condition often presents a conundrum because of the uncertainty regarding the trade-off between immunosuppression-related toxicities/complications and restoration of allograft function and structure.

Methods

Herein, we report an illustrative case of a young 20-y-old otherwise healthy woman who underwent liver replacement for Alagille's syndrome from an ABO-compatible, 6 antigen-mismatched crossmatch-negative 24-y-old man. Although triple baseline immunosuppression was used (tacrolimus, mycophenolate mofetil, and prednisone), she developed rejection 3 d after liver replacement. Despite verified continual immunosuppression compliance, 1.5 y after liver replacement she experienced 6 more rejection episodes over the following 18 mo and development of de novo donor-specific antibody.

Results

Treatment with belatacept began 3.5 y after transplantation, normalizing her liver tests with no further rejections. A biopsy obtained 6 y after transplantation (postoperative day 2221) was normal, appearing without inflammation or residual fibrosis.

Conclusions

Belatacept may be a useful treatment approach in this setting.

TIGIT regulates apoptosis of risky memory T cell subsets implicated in belatacept-resistant rejection

Belatacept confers increased patient and graft survival in renal transplant recipients relative to calcineurin inhibitors, but is associated with an increased rate of acute rejection. Recent immunophenotypic studies comparing pretransplant T cell phenotypes of patients who reject versus those who remain stable on belatacept identified three potential "risky" memory T cell subsets that potentially underlie belatacept-resistant rejection: CD4⁺ CD28⁺ T_EM , CD8⁺ CD28^null , and CD4⁺ CD57⁺ PD1^- subsets. Here, we compared key phenotypic and functional aspects of these human memory T cell subsets, with the goal of identifying additional potential targets to modulate them. Results demonstrate that TIGIT, an increasingly well-appreciated immune checkpoint receptor, was expressed on all three risky memory T cell subsets in vitro and in vivo in the presence of belatacept. Coculture of human memory CD4⁺ and CD8⁺ T cells with an agonistic anti-TIGIT mAb significantly increased apoptotic cell death of all three risky memory T cell subsets. Mechanistically, TIGIT-mediated apoptosis of risky memory T cells was dependent on FOXP3⁺ Treg, suggesting that agonism of the TIGIT pathway increases FOXP3⁺ Treg suppression of human memory T cell populations. Overall, these data suggest that TIGIT agonism could represent a new therapeutic target to inhibit belatacept-resistant rejection during transplantation.

The role of circulating T follicular helper cells in kidney transplantation

Humoral rejection plays a crucial role in the chronic deterioration of kidney allografts, but there is no effective therapeutic strategy to prevent or treat it. T follicular helper (Tfh) cells provide help to B cells, subsequently contributing to humoral rejection. Investigation of Tfh cells may be a useful strategy for assessing the risk and level of humoral rejection. However, it is difficult to investigate Tfh cells from patient-derived lymphoid tissue. Recent studies have shown that circulating Tfh (cTfh) cells, working in parallel to Tfh cells, have the capacity to promote antibody-secreting B cell differentiation and antibody secretion. Here, we review recent studies of cTfh cells in kidney transplantation and discuss the characteristics and functions of cTfh cells in kidney transplant recipients.

Therapies for Chronic Allograft Rejection

Remarkable advances have been made in the pathophysiology, diagnosis, and treatment of antibody-mediated rejection (ABMR) over the past decades, leading to improved graft outcomes. However, long-term failure is still high and effective treatment for chronic ABMR, an important cause of graft failure, has not yet been identified. Chronic ABMR has a relatively different phenotype from active ABMR and is a slowly progressive disease in which graft injury is mainly caused by de novo donor specific antibodies (DSA). Since most trials of current immunosuppressive therapies for rejection have focused on active ABMR, treatment strategies based on those data might be less effective in chronic ABMR. A better understanding of chronic ABMR may serve as a bridge in establishing treatment strategies to improve graft outcomes. In this in-depth review, we focus on the pathophysiology and characteristics of chronic ABMR along with the newly revised Banff criteria in 2017. In addition, in terms of chronic ABMR, we identify the reasons for the resistance of current immunosuppressive therapies and look at ongoing research that could play a role in setting better treatment strategies in the future. Finally, we review non-invasive biomarkers as tools to monitor for rejection.

The Implications of B-lineage Cells in Kidney Allografts

The majority of cells comprising the inflammatory infiltrates in kidney allografts undergoing acute and/or chronic rejection are typically T cells and monocyte/macrophages with B cells, plasma cells, and eosinophils accounting for <5%. In a significant minority of biopsies, B lineage cells (B cells and/or plasma cells) may be found more abundantly. Although plasma cell infiltrates tend to be more diffuse, B cells tend to aggregate into nodules that may mature into tertiary lymphoid organs. Given the ability to target B cells with anti-CD20 monoclonal antibodies and plasma cells with proteasome inhibitors and anti-CD38 monoclonal antibodies, it is increasingly important to determine the significance of such infiltrates. Both cell types are potential effectors of rejection, but both also have a tolerizing potential. B cell infiltrates have been associated with steroid resistance and reduced graft survival in some studies but not in others, and their presence should not prompt automatic depletional therapy. Plasma cell-rich infiltrates tend to occur later, may be associated with cell-mediated and/or antibody-mediated rejection, and portend an adverse outcome. Viral infection and malignancy must be ruled out. Randomized controlled trials are needed to determine the appropriateness of specific therapy when B cells and/or plasma cells are found. No strong therapeutic recommendations can be made at this time.

Memory T Cells in Transplantation: Old Challenges Define New Directions

Immunologic memory is the ability of adaptive immune system to quickly and specifically recognize previously encountered antigens and initiate an effector response. Alloreactive memory cells can mount rapid and robust responses to the transplanted organ resulting in allograft injury. Thus preexisting humoral or cellular memory alloresponses are typically associated with poor graft outcomes in experimental and clinical transplantation. While both B and T lymphocytes exhibit memory responses, this review discusses recent updates on the biology of memory T cells and their relevance to the field of transplantation. Three major areas of focus are the emergence and characterization of tissue resident memory T cells, manipulation of T cell metabolic pathways, and the latest promising approaches to targeting detrimental T cell memory in the settings of organ transplantation.

Potential Application of T-Follicular Regulatory Cell Therapy in Transplantation

Regulatory T cells (Tregs) constitute a small proportion of circulating CD4⁺ T cells that function to maintain homeostasis and prevent autoimmunity. In light of their powerful immunosuppressive and tolerance-promoting properties, Tregs have become an interesting potential candidate for therapeutic use in conditions such as solid organ transplant or to treat autoimmune and inflammatory conditions. Clinical studies have demonstrated the safety of polyclonally expanded Tregs in graft-versus-host disease, type 1 diabetes, and more recently in renal and liver transplantation. However, Tregs are heterogenous. Recent insights indicate that only a small proportion of Tregs, called T follicular regulatory cells (Tfr) regulate interactions between B cells and T follicular helper (Tfh) cells within the germinal center. Tfr have been mainly described in mouse models due to the challenges of sampling secondary lymphoid organs in humans. However, emerging human studies, characterize Tfr as being CD4⁺CD25⁺FOXP3⁺CXCR5⁺ cells with different levels of PD-1 and ICOS expression depending on their localization, in the blood or the germinal center. The exact role they play in transplantation remains to be elucidated. However, given the potential ability of these cells to modulate antibody responses to allo-antigens, there is great interest in exploring translational applications in situations where B cell responses need to be regulated. Here, we review the current knowledge of Tfr and the role they play focusing on human diseases and transplantation. We also discuss the potential future applications of Tfr therapy in transplantation and examine the evidence for a role of Tfr in antibody production, acute and chronic rejection and tertiary lymphoid organs. Furthermore, the potential impact of immunosuppression on Tfr will be explored. Based on preclinical research, we will analyse the rationale of Tfr therapy in solid organ transplantation and summarize the different challenges to be overcome before Tfr therapy can be implemented into clinical practice.

The direct and indirect regulation of follicular T helper cell differentiation in inflammation and cancer

Follicular T helper (Tfh) cells play important roles in facilitating B-cell differentiation and inducing the antibody response in humoral immunity and immune-associated inflammatory diseases, including infections, autoimmune diseases, and cancers. However, Tfh cell differentiation is mainly achieved through self-directed differentiation regulation and the indirect regulation mechanism of antigen-presenting cells (APCs). During the direct intrinsic differentiation of naïve CD4⁺ T cells into Tfh cells, Bcl-6, as the characteristic transcription factor, plays the core role of transcriptional regulation. APCs indirectly drive Tfh cell differentiation mainly by changing cytokine secretion mechanisms. Altered metabolic signaling is also critically involved in Tfh cell differentiation. This review summarizes the recent progress in understanding the direct and indirect regulatory signals and metabolic mechanisms of Tfh cell differentiation and function in immune-associated diseases.

Avoidance of CNI and steroids using belatacept-Results of the Clinical Trials in Organ Transplantation 16 trial

Immunosuppression devoid of corticosteroids has been investigated to avoid long-term comorbidities. Likewise, alternatives to calcineurin inhibitors have been investigated as a strategy to improve long-term kidney function following transplanion. Costimulatory blockade strategies that include corticosteroids have recently shown promise, despite their higher rates of early acute rejection. We designed a randomized clinical trial utilizing depletional induction therapy to mitigate early rejection risk while limiting calcineurin inhibitors and corticosteroids. This trial, Clinical Trials in Organ Transplantation 16 (CTOT-16), sought to evaluate novel belatacept-based strategies employing tacrolimus and corticosteroid avoidance. Sixty-nine kidney transplant recipients were randomized from 4 US transplant centers comparing a control group of with rabbit antithymocyte globulin (rATG) induction, rapid steroid taper, and maintenance mycophenolate and tacrolimus, to 2 arms using maintenance belatacept. There were no graft losses but there were 2 deaths in the control group. However, the trial was halted early because of rejection in the belatacept treatment groups. Serious adverse events were similar across groups. Although rejection was not uniform in the belatacept maintenance therapy groups, the frequency of rejection limits the practical implementation of this strategy to avoid both calcineurin inhibitors and corticosteroids at this time.

Conversion From Calcineurin Inhibitors to Belatacept in HLA-sensitized Kidney Transplant Recipients With Low-level Donor-specific Antibodies

BACKGROUND

Belatacept could be the treatment of choice in renal-transplant recipients with renal dysfunction attributed to calcineurin inhibitor (CNI) nephrotoxicity. Few studies have described its use in patients with donor-specific antibody (DSA).

METHODS

We retrospectively evaluated conversion from CNIs to belatacept in 29 human leukocyte antigen-immunized renal-transplant recipients. Data about acute rejection, DSA, and renal function were collected. These patients were compared with 42 nonimmunized patients treated with belatacept.

RESULTS

Patients were converted from CNIs to belatacept a median of 444 days (interquartile range, 85-1200) after transplantation and were followed up after belatacept conversion, for a median of 308 days (interquartile range, 125-511). At conversion, 16 patients had DSA. Nineteen DSA were observed in these 16 patients, of which 11/19 were <1000 mean fluorescence intensity (MFI), 7/19 were between 1000 and 3000 MFI, and one was >3000 MFI. At last follow-up, preexisting DSA had decreased or stabilized. Seven patients still had DSA with a mean MFI of 1298 ± 930 at the last follow-up. No patient developed a de novo DSA in the DSA-positive group. In the nonimmunized group, one patient developed de novo DSA (A24-MFI 970; biopsy for cause did not show biopsy-proven acute rejection or microinflammation score). After belatacept conversion, one antibody-mediated rejection was diagnosed. The mean estimated glomerular filtration rate improved from 31.7 ± 14.2 mL/min/1.73 m to 40.7 ± 12.3 mL/min/1.73 m (P < 0.0001) at 12 months after conversion. We did not find any significant difference between groups in terms of renal function, proteinuria, or biopsy-proven acute rejection.

CONCLUSIONS

We report on a safe conversion to belatacept in human leukocyte antigen-immunized patients with low DSA levels.

An overview of T follicular cells in transplantation: spotlight on their clinical significance

Introduction: For late stage organ failure patients, transplantation is the best option to increase life expectancy with a superior quality of life. Unfortunately, after transplantation many patients are at risk of cellular and antibody-mediated rejection (ABMR). The latter is initiated by donor specific antibodies (DSA) which depend on the actions of B cells, T follicular helper (Tfh) cells and T follicular regulatory (Tfr) cells that are present in the germinal center of lymphoid organs.Areas covered: In this overview paper, we discuss the biology and function of Tfh and Tfr cells in lymphoid tissues, transplanted organs and their circulating counterparts. We report on their relevance to alloimmunity and on the effects of immunosuppressive drugs on these immunocompetent cell populations.Expert opinion: Growing knowledge about the actions of Tfh and Tfr allows for a better understanding of the immunological mechanisms of ABMR after organ transplantation. This understanding feeds the hypothesis that immunosuppressive drugs targeting the actions of Tfh cells have huge therapeutic potential. This new concept in the treatment of the humoral rejection response will improve graft and patient survival after organ transplantation.

The role of follicular T helper cells in the humoral alloimmune response after clinical organ transplantation

Over the past decade, antibody‐mediated or humoral rejection in combination with development of de novo donor‐specific antibodies (DSA) has been recognized as a distinct and common cause of transplant dysfunction and is responsible for one‐third of the failed allografts. Detailed knowledge of the mechanisms that initiate and maintain B‐cell driven antidonor reactivity is required to prevent and better treat this antidonor response in organ transplant patients. Over the past few years, it became evident that this response largely depends on the actions of both T follicular helper (Tfh) cells and the controlling counterparts, the T follicular regulatory (Tfr) cells. In this overview paper, we review the latest insights on the functions of circulating (c)Tfh cells, their subsets Tfh1, Tfh2 and Tfh17 cells, IL‐21 and Tfr cells in antibody mediated rejection (ABMR). This may offer new insights in the process to reduce de novo DSA secretion resulting in a decline in the incidence of ABMR. In addition, monitoring these cell populations could be helpful for the development of biomarkers identifying patients at risk for ABMR and provide novel therapeutic drug targets to treat ABMR.

Follicular helper T cells and humoral response in organ transplantation

Antibody mediated rejection has been recognized as an important contributor to long-term graft loss in most solid organ transplants. Current immunosuppressive regimes are not capable of preventing anti-HLA antibody formation and eventual damage to the graft, and there is a need to develop drugs directed against novel targets to avoid graft allorecognition. In this review we introduce follicular helper T cells (Tfh), a subtype of lymphocyte specialized in helping B cells to differentiate into plasmablasts and produce class-switched antibodies. We focus on the role of Tfh in solid organ transplantation, what is known about Tfh and the production of alloantibodies, how current immunosuppressive therapies affect Tfh and what new molecules could be used to target Tfh in transplantation, with the goal of improving graft survival.

T Follicular Regulatory Cells and Antibody Responses in Transplantation

De novo donor-specific antibody (DSA) formation is a major problem in transplantation, and associated with long-term graft decline and loss as well as sensitization, limiting future transplant options. Forming high-affinity, long-lived antibody responses involves a process called the germinal center (GC) reaction, and requires interaction between several cell types, including GC B cells, T follicular helper (Tfh) and T follicular regulatory (Tfr) cells. T follicular regulatory cells are an essential component of the GC reaction, limiting its size and reducing nonspecific or self-reactive responses.An imbalance between helper function and regulatory function can lead to excessive antibody production. High proportions of Tfh cells have been associated with DSA formation in transplantation; therefore, Tfr cells are likely to play an important role in limiting DSA production. Understanding the signals that govern Tfr cell development and the balance between helper and regulatory function within the GC is key to understanding how these cells might be manipulated to reduce the risk of DSA development.This review discusses the development and function of Tfr cells and their relevance to transplantation. In particular how current and future immunosuppressive strategies might allow us to skew the ratio between Tfr and Tfh cells to increase or decrease the risk of de novo DSA formation.

The Calcineurin Inhibitor Tacrolimus Specifically Suppresses Human T Follicular Helper Cells

Background

T follicular helper (Tfh) cells are key players in the production of antibody-producing B cells via the germinal center reaction. Therapeutic strategies targeting Tfh cells are important where antibody formation is implicated in disease, such as transplant rejection and autoimmune diseases. We investigated the impact of the immunosuppressive agent tacrolimus on human Tfh cell differentiation and function in transplant recipients.

Methods

Paired blood and lymph node (LN) samples were obtained from 61 transplant recipients immediately prior to organ implantation. Living-donor recipients received a week of tacrolimus prior to kidney transplantation. Deceased-donor recipients served as controls, as tacrolimus was not administered until after the transplant operation. Flow cytometry was used to compare LN and circulating cell subsets.

Results

The calcineurin inhibitor (CNIs) tacrolimus specifically suppresses both LN Tfh cells and circulating Tfh cells, but not their regulatory counterparts or other CD4 T cell subsets.

Conclusion

Our findings suggest that CNIs may have a more important role in the prevention of antibody formation than previously understood and, therefore, have potential for antibody-associated conditions in which aberrant Tfh function has been implicated in disease.

T Follicular Helper Cells As a New Target for Immunosuppressive Therapies

Over the past decade, antibody-mediated (humoral) rejection has been recognized as a common cause of graft dysfunction after organ transplantation and an important determinant for graft loss. In humoral alloimmunity, T follicular helper (Tfh) cells play a crucial role, because they help naïve B cells to differentiate into memory B cells and alloantibody-producing plasma cells within germinal centers. In this way, they contribute to the induction of donor-specific antibodies, which are responsible for the humoral immune response to the allograft. In this article, we provide an overview of the current knowledge on the effects of immunosuppressive therapies on Tfh cell development and function, and discuss possible new approaches to influence the activity of Tfh cells. In addition, we discuss the potential use of Tfh cells as a pharmacodynamic biomarker to improve alloimmune-risk stratification and tailoring of immunosuppression to individualize therapy after transplantation.