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BACKGROUND AND AIMS Although considerable advances have been made in understanding the cellular effector mechanisms responsible for donor-specific antibody (DSA) generation leading to antibody-mediated rejection (ABMR), the identification of cellular regulators of such immune responses is lacking. METHOD In a cohort of 96 kidney transplant recipients, we used high-dimensional flow cytometry to concomitantly profile and track the two major subsets of regulatory lymphocytes in the blood: T regulatory (TREG) and transitional B (TrB) cells. Additionally, we established co-culture assays to address their respective capacities to suppress antibody responses in vitro. RESULTS TREG and TrB were potent suppressors of T follicular helper-mediated B cell differentiation into plasmablast and antibody generation. While TREG and TrB were both durably expanded in patients who did not develop DSA post-transplant, patients who manifested DSA and progressed to ABMR displayed a marked and persistent numerical reduction in TREG and TrB cells. Strikingly, specific cell clusters expressing the transcription factor T-bet were selectively depleted in both TREG and TrB compartments in patients with ABMR. Importantly, the coordinated loss of these T-bet + CXCR5+ TREG and T-bet+CD21–TrB cell clusters was correlated with increased and inflammatory DSA responses, more extensive microvascular inflammation and a higher rate of kidney allograft loss. CONCLUSION Our study identified coordinated and persistent defects in the regulatory T and B cell responses in patients undergoing ABMR, which may contribute to their loss of humoral immune regulation and thus warranting timely therapeutic interventions to replenish and sustain TREG and TrB cells in these patients.

Humoral alloimmunity of organ transplant recipient to donor can lead to antibody-mediated rejection (ABMR), causing thousands of organ transplants to fail each year worldwide. However, the mechanisms of adaptive immune cell responses at the basis of humoral alloimmunity have not been entirely understood. In this review, we discuss how recent investigations have allowed to uncover the key contributions of T follicular helper and B cells, and their coordinated actions in driving donor-specific antibody generation and the immune progression towards ABMR. We show how recognition of the role of T follicular helper-B cell interactions may allow to elaborate improved clinical strategies for immune monitoring and to identify novel therapeutic targets to tackle ABMR that will ultimately allow to improve organ transplant survival.

Humoral allogeneic immunity driven by anti-HLA donor-specific antibodies (DSAs) and antibody-mediated mediated rejection (ABMR) significantly impede prolonged survival of organ allografts after transplantation. Although the importance of T follicular helper (T(FH)) cells in controlling antibody responses has been long established, their role in directing DSA generation leading to ABMR was only recently appreciated in the clinical setting of organ transplantation. In this review, we provide a comprehensive summary of the current knowledge on the biology of human T(FH) cells as well as their circulating (cT(FH)) counterparts and describe their pivotal role in driving humoral alloimmunity. In addition, we discuss the intrinsic effects of current induction therapies and maintenance immunosuppressive drugs as well as of biotherapies on T(FH) cells; and provide future directions and novel opportunities of biotherapeutic targeting of T(FH) cells that have the potential of bringing the prophylactic and curative treatments of ABMR towards personalized and precision medicine.

Although considerable advances have been made in understanding the cellular effector mechanisms responsible for donor-specific antibody generation leading to antibody-mediated rejection (ABMR), the identification of cellular regulators of such immune responses is lacking. To clarify this, we used high dimensional flow cytometry to concomitantly profile and track the two major subsets of regulatory lymphocytes in blood: T regulatory (T

Current immunosuppressive (IS) regimens used to prevent organ allograft rejection have well-recognized side effects, that include enhanced risk of infection and certain types of cancer, metabolic disorders, cardiovascular disease, renal complications and failure to control chronic allograft rejection. The life-long dependency of patients on these IS agents reflects their inability to induce donor-specific tolerance. Extensive studies in rodent and non-human primate models have demonstrated the ability of adoptively-transferred regulatory immune cells (either regulatory myeloid cells or regulatory T cells) to promote transplant tolerance. Consequently, there is considerable interest in the potential of regulatory immune cell therapy to allow safe minimization/complete withdrawal of immunosuppression and the promotion of organ transplant tolerance in the clinic. Here, we review the properties of regulatory dendritic cells (DCreg) with a focus on the approaches being taken to generate human DCreg for clinical testing. We also document the early phase clinical trials that are underway to assess DCreg therapy in clinical organ transplantation as well as in autoimmune disorders.

Alloimmune responses driven by donor-specific antibodies (DSAs) can lead to antibody-mediated rejection (ABMR) in organ transplantation. Yet, the cellular states underlying alloreactive B cell responses and the molecular components controlling them remain unclear. Using high-dimensional profiling of B cells in a cohort of 96 kidney transplant recipients, we identified expanded numbers of CD27+CD21– activated memory (AM) B cells that expressed the transcription factor T-bet in patients who developed DSAs and progressed to ABMR. Notably, AM cells were less frequent in DSA+ABMR– patients and at baseline levels in DSA– patients. RNA-Seq analysis of AM cells in patients undergoing ABMR revealed these cells to be poised for plasma cell differentiation and to express restricted IGHV sequences reflective of clonal expansion. In addition to T-bet, AM cells manifested elevated expression of interferon regulatory factor 4 and Blimp1, and upon coculture with autologous T follicular helper cells, differentiated into DSA-producing plasma cells in an IL-21–dependent manner. The frequency of AM cells was correlated with the timing and severity of ABMR manifestations. Importantly, T-bet+ AM cells were detected within kidney allografts along with their restricted IGHV sequences. This study delineates a pivotal role for AM cells in promoting humoral responses and ABMR in organ transplantation and highlights them as important therapeutic targets.

EBV is a γ herpesvirus that infects more than 95% of the world’s adult population. Primary EBV infection occurs during childhood when it usually is asymptomatic or during adolescence when it may manifest as infectious mononucleosis (IM). Regardless of the clinical manifestation of primary infection, EBV persists in the B cell compartment of the infected host in a latent state of infection that may be occasionally interrupted by episodic lytic viral reactivations. Although EBV elicits and maintains potent immune responses that successfully keep both its latency and reactivation phases under tight control in healthy humans, the virus may associate with several malignancies in individuals with immunodeficiencies or individuals whose cellular immunity is impaired, because of the growth-transforming function of some of the EBV latent genes, which are true oncogenes. This chapter will discuss the features of innate and adaptive immunity to EBV in healthy and in immunosuppressed organ transplant patients.

Regulatory dendritic cells (DCreg) promote transplant tolerance following their adoptive transfer in experimental animals. We investigated the feasibility, safety, fate, and impact on host T cells of donor monocyte-derived DCreg infused into prospective, living donor liver transplant patients, 7 days before transplantation. The DCreg expressed a tolerogenic gene transcriptional profile, high cell surface programed death ligand-1 (PD-L1):CD86 ratios, high IL-10/no IL-12 productivity and poor ability to stimulate allogeneic T cell proliferation. Target DCreg doses (range 2.5-10 × 106 cells/kg) were achieved in all but 1 of 15 recipients, with no infusion reactions. Following DCreg infusion, transiently elevated levels of donor HLA and immunoregulatory PD-L1, CD39, and CD73 were detected in circulating small extracellular vesicles. At the same time, flow and advanced image stream analysis revealed intact DCreg and "cross-dressing" of host DCs in blood and lymph nodes. PD-L1 co-localization with donor HLA was observed at higher levels than with recipient HLA. Between DCreg infusion and transplantation, T-bethi Eomeshi memory CD8+ T cells decreased, whereas regulatory (CD25hi CD127- Foxp3+ ): T-bethi Eomeshi CD8+ T cell ratios increased. Thus, donor-derived DCreg infusion may induce systemic changes in host antigen-presenting cells and T cells potentially conducive to modulated anti-donor immune reactivity at the time of transplant.

BACKGROUND: Although antibody-mediated rejection (ABMR) has been long recognized as a leading cause of allograft failure after kidney transplantation, the cellular and molecular processes underlying the induction of deleterious donor-specific antibody (DSA) responses remain poorly understood. METHODS: Using high-dimensional flow cytometry, in vitro assays, and RNA sequencing, we concomitantly investigated the role of T follicular helper (T(FH)) cells and B cells during ABMR in 105 kidney transplant recipients. RESULTS: There were 54 patients without DSAs; of those with DSAs, ABMR emerged in 20 patients, but not in 31 patients. We identified proliferating populations of circulating T(FH) cells and activated B cells emerging in blood of patients undergoing ABMR. Although these circulating T(FH) cells comprised heterogeneous phenotypes, they were dominated by activated (ICOS(+)PD-1(+)) and early memory precursor (CCR7(+)CD127(+)) subsets, and were enriched for the transcription factors IRF4 and c-Maf. These circulating T(FH) cells produced large amounts of IL-21 upon stimulation with donor antigen and induced B cells to differentiate into antibody-secreting cells that produced DSAs. Combined analysis of the matched circulating T(FH) cell and activated B cell RNA-sequencing profiles identified highly coordinated transcriptional programs in circulating T(FH) cells and B cells among patients with ABMR, which markedly differed from those of patients who did not develop DSAs or ABMR. The timing of expansion of the distinctive circulating T(FH) cells and activated B cells paralleled emergence of DSAs in blood, and their magnitude was predictive of IgG3 DSA generation, more severe allograft injury, and higher rate of allograft loss. CONCLUSIONS: Patients undergoing ABMR may benefit from monitoring and therapeutic targeting of T(FH) cell–B cell interactions.

Introduction The cellular events that contribute to generation of donor-specific anti-HLA antibodies (DSA) post-kidney transplantation (KTx) are not well understood. Characterization of such mechanisms could allow tailoring of immunosuppression to benefit sensitized patients. Methods We prospectively monitored circulating T follicular helper (cTFH) cells in KTx recipients who received T-cell depleting (thymoglobulin, n = 54) or T-cell nondepleting (basiliximab, n = 20) induction therapy from pre-KTx to 1 year post-KTx and assessed their phenotypic changes due to induction and DSA occurrence, in addition to healthy controls (n = 13), for a total of 307 blood samples. Results Before KTx, patients displayed comparable levels of resting, central memory cTFH cells with similar polarization to those of healthy controls. Unlike basiliximab induction, thymoglobulin induction significantly depleted cTFH cells, triggered lymphopenia-induced proliferation that skewed cTFH cells toward increased Th1 polarization, effector memory, and elevated programmed cell death protein 1 (PD-1)int/hi expression, resembling activated phenotypes. Regardless of induction, patients who developed DSA post-KTx, harbored pre-KTx donor-reactive memory interleukin (IL)-21+ cTFH cells and showed higher % cTFH and lower % of T regulatory (TREG) cells post-KTx resulting in elevated cTFH:TREG ratio at DSA occurrence. Conclusion Induction therapy distinctly shapes cTFH cell phenotype post-KTx. Monitoring cTFH cells before and after KTx may help detect those patients prone to DSA generation post-KTx., Graphical abstract

Memory T cell (Tmem) responses play a critical role in the outcome of allo-transplantation. While the role of the T-box transcription factor Eomesodermin (Eomes) in the maintenance of antigen-specific Tmem is well studied, little is known about Eomes+CD8+T cell responses after transplantation. We evaluated the phenotype and function of allo-reactive Eomes+CD8+T cells in healthy volunteers and kidney transplant patients and their relation to transplant outcome. High Eomes expression by steady-state CD8+T cells correlated with effector and memory phenotype. Following allo-stimulation, the expression of both the T-box proteins Eomes and T-bet by proliferating cells increased significantly, where high expression of Eomes and T-bet correlated with higher incidence of allo-stimulated IFNγ+TNFα+ CD8+T cells. In patients with no subsequent rejection, Eomes but not T-bet expression by donor-stimulated CD8+T cells, increased significantly after transplantation. This was characterized by increased EomeshiT-bet-/lo and decreased Eomes-/loT-bethi CD8+T cell subsets, with no significant changes in the EomeshiT-bethi CD8+T cell subset. No upregulation of exhaustion markers programmed-death-1 (PD-1) and cytotoxic-T-lymphocyte-associated-antigen-4 (CTLA4) by donor-stimulated Eomes+CD8+T cells was observed. Before transplantation, in patients without rejection, there were higher incidences of EomeshiT-bet-/lo, and lower incidences of EomeshiT-bethi and Eomes-/loT-bethi donor-stimulated CD8+T cell subsets, compared to those with subsequent rejection. Overall, our findings indicate that high Eomes expression by allo-stimulated T-bet+CD8+T cells is associated with enhanced effector function, and that an elevated incidence of donor-stimulated CD8+T cells co-expressing high levels of Eomes and T-bet before transplantation, may correlate with an increased incidence of acute cellular rejection.

Dendritic cells (DC) are rare, bone marrow (BM)-derived innate immune cells that critically maintain self-tolerance in the healthy steady-state. Regulatory DC (DCreg) with capacity to suppress allograft rejection and promote transplant tolerance in pre-clinical models can readily be generated from BM precursors or circulating blood monocytes. These DCreg enhance allograft survival via various mechanisms, including promotion of regulatory T cells. In non-human primates receiving minimal immunosuppressive drug therapy (IS), infusion of DCreg of donor origin, one week before transplant, safely prolongs renal allograft survival and selectively attenuates anti-donor CD8+ memory T cell responses in the early post-transplant period. Based on these observations, and in view of the critical need to reduce patient dependence on non-specific IS agents that predispose to cardiometabolic side effects and renal insufficiency, we will conduct a first-in-human safety and preliminary efficacy study of donor-derived DCreg infusion to achieve early (18 months post-transplant) complete IS withdrawal in low-risk, living donor liver transplant recipients receiving standard-of-care IS (mycophenolate mofetil, tacrolimus and steroids). We will test the hypothesis that, although donor-derived DCreg are short-lived, they will induce robust donor-specific T cell hyporesponsiveness. We will examine immunological mechanisms by sequential analysis of blood and tissue samples, incorporating cutting-edge technologies.

Human regulatory dendritic cells (DCreg) were generated from CD14 immunobead-purified or elutriated monocytes in the presence of vitamin D3 and IL-10. They exhibited similar, low levels of costimulatory CD80 and CD86, but comparatively high levels of co-inhibitory programed death ligand-1 (PD-L1) and IL-10 production compared to control immature DC (iDC). Following Toll-like receptor 4 ligation, unlike control iDC, DCreg resisted phenotypic and functional maturation and further upregulated PD-L1:CD86 expression. Whereas LPS-stimulated control iDC (mature DC; matDC) secreted pro-inflammatory tumor necrosis factor but no IL-10, the converse was observed for LPS-stimulated DCreg. DCreg weakly stimulated naive and memory allogeneic CD4+ and CD8+ T cell proliferation and IFNγ, IL-17A and perforin/granzyme B production in MLR. Their stimulatory function was enhanced however, by blocking PD-1 ligation. High-throughput T cell receptor (TCR) sequencing revealed that, among circulating T cell subsets, memory CD8+ T cells contained the most alloreactive TCR clonotypes and that, while matDC expanded these alloreactive memory CD8 TCR clonotypes, DCreg induced more attenuated responses. These findings demonstrate the feasibility of generating highly-purified GMP-grade DCreg for systemic infusion, their influence on the alloreactive T cell response, and a key mechanistic role of the PD1 pathway.

International audience; Background. The human regulatory macrophage (Mreg) has emerged as a promising cell type for use as a cell-based adjunct immunosuppressive therapy in solid organ transplant recipients. In this brief report, dehydrogenase/reductase 9 (DHRS9) is identified as a robust marker of human Mregs.Methods. The cognate antigen of a mouse monoclonal antibody raised against human Mregs was identified as DHRS9 by immunoprecipitation and MALDI-MS sequencing. Expression of DHRS9 within a panel of monocyte-derived macrophages was investigated by quantitative PCR, immunoblotting and flow cytometry.Results. DHRS9 expression discriminated human Mregs from a panel of in vitro derived macrophages in other polarisation states. Likewise, DHRS9 expression distinguished Mregs from a variety of human monocyte-derived tolerogenic antigen-presenting cells in current development as cell-based immunotherapies, including Tol-DC, Rapa-DC, DC-10, and PGE 2-induced myeloid-derived suppressor cells. A subpopulation of DHRS9-expressing human splenic macrophages was identified by immunohistochemistry. Expression of DHRS9 was acquired gradually during in vitro development of human Mregs from CD14 + monocytes and was further enhanced by IFN-γ treatment on day 6 of culture. Stimulating Mregs with 100 ng/mL lipopolysaccharide for 24 hours did not extinguish DHRS9 expression. Dhrs9 was not an informative marker of mouse Mregs. Conclusion. DHRS9 is a specific and stable marker of human Mregs.

Th17 and TfH cells are thought to promote tissue inflammation and autoantibody production, respectively, in autoimmune diseases including rheumatoid arthritis (RA). TfH cells that co-express Th17 markers (CXCR5+Th17) encompass both of these pathogenic functions, and are increased in some human autoimmune settings including juvenile dermatomyositis. We investigated CXCR5+Th17 cells in RA subjects with stable or active disease and before and after TNF inhibitor therapy. CXCR5+Th17 cell frequency was increased in RA compared to healthy controls, but other helper T cell subsets were not different. CXCR5+Th17 cells correlated with disease activity in subjects with active RA prior to initiation of TNF inhibitor therapy. Baseline CXCR5+Th17 cells also correlated with numbers of swollen joints as late as one year post-therapy. CXCR5+Th17 cell frequencies were unaltered by TNF blockade and in fact remained remarkably stable within individuals. We conclude that CXCR5+Th17 cells are not a direct target of TNF blockade and therefore cannot serve as a biomarker of current disease activity. However, basal CXCR5+Th17 cell frequency may indicate underlying differences in disease phenotype between patients and predict ultimate success of TNF inhibitor therapy.

The recently described T follicular helper (Tfh) cell is required for the production of high affinity antibody. After contact with follicular dendritic cells, Tfh cells move into the germinal centre and provide help to B cells both by direct B cell-T cell interaction and production of IL-21. This drives proliferation, differentiation, and affinity maturation of the B cells to produce plasma cells capable of secreting high-affinity antibody. Circulating Tfh cells are produced by movement of Tfh cells from lymph nodes after dendritic cell contact. A reduction of Tfh cell-associated molecules is linked with increased expression of other chemokine receptors to form Th1-, Th2-, and Th17-like Tfh cells. These circulating Tfh cells are able to help B cells in vitro and to move into target tissues to support antibody production. Alloantibody production is dependent on T-cell help via the indirect pathway. Antibody-mediated rejection is therefore dependent on Tfh cells. Animal data suggest that Tfh cells and B cells migrate to the allograft and are involved in alloantibody production within a tertiary lymphoid organ. There is some data supporting the same process within human allografts. The requirement for T-cell help provides a potential therapeutic target in the treatment of antibody-mediated rejection.

Dendritic cells (DC) are rare, professional antigen-presenting cells with ability to induce or regulate alloimmune responses. Regulatory DC (DCreg) with potential to down-modulate acute and chronic inflammatory conditions that occur in organ transplantation can be generated in vitro under a variety of conditions. Here, we provide a rationale for evaluation of DCreg therapy in clinical organ transplantation with the goal of promoting sustained, donor-specific hyporesponsiveness, while lowering the incidence and severity of rejection and reducing patients’ dependence on anti-rejection drugs. Generation of donor- or recipient-derived DCreg that suppress T cell responses and prolong transplant survival in rodents or non-human primates has been well-described. Recently, good manufacturing practice (GMP)-grade DCreg have been produced at our Institution for prospective use in human organ transplantation. We briefly review experience of regulatory immune therapy in organ transplantation and describe our experience generating and characterizing human monocyte-derived DCreg. We propose a phase I/II safety study in which the influence of donor-derived DCreg combined with conventional immunosuppression on subclinical and clinical rejection and host alloimmune responses will be examined in detail.

Mixed leukocyte culture (MLC) is perhaps the most widely used of cellular assays. MLC has been used clinically for donor selection, predominantly for bone marrow transplantation. In contrast to MLC, which measures a bulk response, limiting dilution assay (LDA) is a quantitative tool. Mycoplasma contains the thymine kinase enzyme, which interferes with nucleic and amino acid metabolism and subsequently with DNA replication in the contaminated cell culture. In solid-organ transplantation, the histological analysis of allograft biopsies remains the "gold standard" for diagnosing rejection. Since the ultimate goal is the early detection of activated cells infiltrating the allograft and not in vitro priming, it also deals with methods that minimize the possibility of priming against the donor antigens in vitro. Several of the assays outlined in this chapter are being implemented in the clinical laboratory. Researchers compared helper and cytotoxic antirecipient T-cell frequencies in recipients who had received unrelated-donor bone marrow. They found that the helper T lymphocytes (HTLp) and cytotoxic T lymphocytes (CTLp) assays provided similar predictive information for outcome. The HTLp method is more rapid and less labor-intensive and, thus, may be more useful for donor selection in unrelated-donor bone marrow transplantation. A recent multicenter study evaluated the Cylex ImmuKnow assay for the measurement of immune response in immunosuppressed transplant recipients. The long-term goals of using the cellular assays described in this chapter are to assess accurately the changing immuno-logical statuses of transplant recipients, predict long-term graft outcome, and provide the immune response information needed to individualize immunosuppression therapy.

Human monocytes have been grouped into classical (CD14++CD16−), non-classical (CD14dimCD16++), and intermediate (CD14++CD16+) subsets. Documentation of normal function and variation in this complement of subtypes, particularly their differentiation potential to dendritic cells (DC) or macrophages, remains incomplete. We therefore phenotyped monocytes from peripheral blood of healthy subjects and performed functional studies on high-speed sorted subsets. Subset frequencies were found to be tightly controlled over time and across individuals. Subsets were distinct in their secretion of TNFα, IL-6, and IL-1β in response to TLR agonists, with classical monocytes being the most producers and non-classical monocytes the least. Monocytes, particularly those of the non-classical subtype, secreted interferon-α (IFN-α) in response to intracellular TLR3 stimulation. After incubation with IL-4 and GM-CSF, classical monocytes acquired monocyte-derived DC (mo-DC) markers and morphology and stimulated allogeneic T cell proliferation in MLR; intermediate and non-classical monocytes did not. After incubation with IL-3 and Flt3 ligand, no subset differentiated to plasmacytoid DC. After incubation with GM-CSF (M1 induction) or macrophage colony-stimulating factor (M-CSF) (M2 induction), all subsets acquired macrophage morphology, secreted macrophage-associated cytokines, and displayed enhanced phagocytosis. From these studies we conclude that classical monocytes are the principal source of mo-DCs, but all subsets can differentiate to macrophages. We also found that monocytes, in particular the non-classical subset, represent an alternate source of type I IFN secretion in response to virus-associated TLR agonists.