Critical, but conditional, role of OX40 in memory T cell-mediated rejection

Dysfunction of infiltrating cytotoxic CD8+ T cells within the graft promotes murine kidney allotransplant tolerance

Tolerance of mouse kidney allografts arises in grafts that develop regulatory tertiary lymphoid organs (rTLOs). Single-cell RNA-seq (scRNA-seq) data and adoptive transfer of alloreactive T cells after transplantation showed that cytotoxic CD8+ T cells are reprogrammed within the accepted graft to an exhausted/regulatory-like phenotype mediated by IFN-γ. Establishment of rTLOs was required because adoptive transfer of alloreactive T cells prior to transplantation results in kidney allograft rejection. Despite the presence of intragraft CD8+ cells with a regulatory phenotype, they were not essential for the induction and maintenance of kidney allograft tolerance since renal allotransplantation into CD8-KO recipients resulted in acceptance and not rejection. Analysis of scRNA-seq data from allograft kidneys and malignant tumors identified similar regulatory-like cell types within the T cell clusters and trajectory analysis showed that cytotoxic CD8+ T cells are reprogrammed into an exhausted/regulatory-like phenotype intratumorally. Induction of cytotoxic CD8+ T cell dysfunction of infiltrating cells appears to be a beneficial mechanistic pathway that protects the kidney allotransplant from rejection through a process we call "defensive tolerance." This pathway has implications for our understanding of allotransplant tolerance and tumor resistance to host immunity.

T cell specific deletion of IRF4 with Ox40-Cre impairs effector and memory T cell responses in heart transplantation

BACKGROUND

IRF4 is the pioneer factor for effector T cell maturation. Here we investigated the function of IRF4 in maintaining OX40-related T cell responses following alloantigen activation in a mouse heart transplantation model.

METHODS

Irf4^flox/flox mice were bred with Ox40^cre/+ mice to generate Irf4^flox/floxOx40^cre/+ mice. Wild type C57BL/6, Irf4^flox/floxOx40^cre/+ mice were transplanted with BALB/c heart allografts, with or without BALB/c skin-sensitization. CD4⁺ TEa T cells co-transfer experiments and flow cytometric analysis were conducted to investigate the amount of CD4⁺ T cells and the percentage of the T effector subset.

RESULTS

Irf4^flox/floxOx40^cre/+ and Irf4^flox/floxOx40^cre/+ TEa mice were constructed successfully. IRF4 ablation in activated OX40-mediated alloantigen specific CD4⁺ TEa T cells reduced effector T cell differentiation (CD44^hiCD62L^lo, Ki67, IFN-γ), which caused long-term allograft survival (> 100 d) in the chronic rejection model. In the donor skin-sensitized heart transplantation model, the formation and function of alloantigen-specific memory CD4⁺ TEa cells were also impaired in Irf4^flox/floxOx40^cre/+ mice. Additionally, deletion of IRF4 after T cell activation in Irf4^flox/floxOx40^cre/+ mice reduced T cell reactivation in vitro.

CONCLUSIONS

IRF4 ablation after OX40-related T cell activation could reduce effector and memory T cell formation and inhibit their function in response to alloantigen stimulation. These findings could have significant implications in targeting activated T cells to induce transplant tolerance.

Integrated analysis of an in vivo model of intra-nasal exposure to instilled air pollutants reveals cell-type specific responses in the placenta

The placenta is a heterogeneous organ whose development involves complex interactions of trophoblasts with decidual, vascular, and immune cells at the fetal-maternal interface. It maintains a critical balance between maternal and fetal homeostasis. Placental dysfunction can lead to adverse pregnancy outcomes including intra-uterine growth restriction, pre-eclampsia, or pre-term birth. Exposure to environmental pollutants contributes to the development of placental abnormalities, with poorly understood molecular underpinning. Here we used a mouse (C57BL/6) model of environmental pollutant exposure by administration of a particulate matter (SRM1649b at 300 μg/day/mouse) suspension intra-nasally beginning 2 months before conception and during gestation, in comparison to saline-exposed controls. Placental transcriptomes, at day 19 of gestation, were determined using bulk RNA-seq from whole placentas of exposed (n = 4) and control (n = 4) animals and scRNAseq of three distinct placental layers, followed by flow cytometry analysis of the placental immune cell landscape. Our results indicate a reduction in vascular placental cells, especially cells responsible for structural integrity, and increase in trophoblast proliferation in animals exposed to particulate matter. Pollution-induced inflammation was also evident, especially in the decidual layer. These data indicate that environmental exposure to air pollutants triggers changes in the placental cellular composition, mediating adverse pregnancy outcomes.

Lysine Acetylation/Deacetylation Modification of Immune-Related Molecules in Cancer Immunotherapy

As major post-translational modifications (PTMs), acetylation and deacetylation are significant factors in signal transmission and cellular metabolism, and are modulated by a dynamic process via two pivotal categories of enzymes, histone acetyltransferases (HATs) and histone deacetylases (HDACs). In previous studies, dysregulation of lysine acetylation and deacetylation has been reported to be associated with the genesis and development of malignancy. Scientists have recently explored acetylation/deacetylation patterns and prospective cancer therapy techniques, and the FDA has approved four HDAC inhibitors (HDACi) to be used in clinical treatment. In the present review, the most recent developments in the area of lysine acetylation/deacetylation alteration in cancer immunotherapy were investigated. Firstly, a brief explanation of the acetylation/deacetylation process and relevant indispensable enzymes that participate therein is provided. Subsequently, a multitude of specific immune-related molecules involved in the lysine acetylation/deacetylation process are listed in the context of cancer, in addition to several therapeutic strategies associated with lysine acetylation/deacetylation modification in cancer immunotherapy. Finally, a number of prospective research fields related to cancer immunotherapy concepts are offered with detailed analysis. Overall, the present review may provide a reference for researchers in the relevant field of study, with the aim of being instructive and meaningful to further research as well as the selection of potential targets and effective measures for future cancer immunotherapy strategies.

Clinical Significance of OX40 and OX40 Ligand in the Peripheral Blood of Patients with Myasthenia Gravis

Background

A previous study on thymomas in myasthenia gravis (MG) patients indicated that OX40 expression may be upregulated in thymic tissues adjacent to germinal centers (GCs) and thymomas, and OX40 may interact with OX40L in GCs to enhance anti-acetylcholine receptor antibody production. However, little is known about the clinical significance of the expression of OX40 and OX40L in the peripheral blood of patients with MG. We aimed to characterize the expression of membrane-bound and soluble OX40 and OX40L in the peripheral blood of patients with MG and to identify their clinical significance.

Methods

For membrane molecules, we collected peripheral blood (PB) from 39 MG patients at baseline, 22 patients in relapse, and 42 patients in remission, as well as from 36 healthy participants as controls. For soluble molecules, plasma from 37 MG patients at baseline, 34 patients in relapse, and 30 patients in remission, as well as plasma from 36 healthy controls (HC), was retrospectively collected from the sample bank of the First Hospital of Soochow University. The expression of membrane-bound OX40 and OX40L (mOX40 and mOX40L) by immune cells was measured using flow cytometry. Plasma levels of soluble OX40 and OX40L (sOX40 and sOX40L) were measured by ELISA.

Results

(1) The expression of OX40 on CD4+ T cells and that of OX40L on B cells and monocytes were significantly increased, and the levels of sOX40 were significantly decreased in MG patients at baseline compared with HC, while the expression of sOX40L was not significantly different between the two groups. (2) Dynamic observation of the molecules showed significantly higher expression of OX40 on CD4⁺ T cells and higher levels of sOX40 in MG patients in relapse than in MG patients at baseline and MG patients in remission. Furthermore, the expression levels of sOX40 were significantly elevated in MG patients in remission compared with MG patients at baseline, and the expression of sOX40L was significantly lower in MG patients in remission than in MG patients at baseline and MG patients in relapse. (3) Plasma levels of sOX40 and sOX40L were significantly decreased in 13 patients with relapsed MG after immunosuppressive treatment compared with those before treatment. (4) Correlation analysis showed that the expression of OX40 on CD4⁺ T cells in patients with relapsed MG was positively correlated with the concentration of acetylcholine receptor antibodies (AchR-Ab), whereas the expression of OX40L on CD19⁺ B cells and CD14⁺ monocytes was negatively correlated with disease duration. (5) Binary regression analysis showed that patients with high CD4⁺ OX40 expression and high sOX40L levels had an increased risk of relapse.

Conclusions

OX40 and OX40L are abnormally expressed in the peripheral blood of patients with MG and may be closely associated with disease status and treatment. The OX40/OX40L pathway may be involved in the immunopathological process of MG and may play a role mainly in the later stage of MG.

Genetically targeting the BATF family transcription factors BATF and BATF3 in the mouse abrogates effector T cell activities and enables long-term heart allograft survival

T cells must be activated and become effectors first before executing allograft rejection, a process that is regulated by diverse signals and transcription factors. In this study, we studied the basic leucine zipper ATF-like transcription factor (BATF) family members in regulating T cell activities in a heart transplant model and found that mice deficient for both BATF and BATF3 (Batf-/- Batf3-/- mice) spontaneously accept the heart allografts long-term without tolerizing therapies. Similarly, adoptive transfer of wild type T cells into Rag1-/- hosts induced prompt rejection of heart and skin allografts, whereas the Batf-/- Batf3-/- T cells failed to do so. Analyses of graft-infiltrating cells showed that Batf-/- Batf3-/- T cells infiltrate the graft but fail to acquire an effector phenotype (CD44high KLRG1+ ). Co-transfer experiments in a T cell receptor transgenic TEa model revealed that the Batf-/- Batf3-/- T cells fail to expand in vivo, retain a quiescent phenotype (CD62L+ CD127+ ), and unable to produce effector cytokines to alloantigen stimulation, which contrasted sharply to that of wild type T cells. Together, our data demonstrate that the BATF and BATF3 are critical regulators of T effector functions, thus making them attractive targets for therapeutic interventions in transplant settings.

Targeting T Follicular Helper Cells to Control Humoral Allogeneic Immunity

Humoral allogeneic immunity driven by anti-HLA donor-specific antibodies and antibody-mediated rejection (AMR) significantly impede prolonged survival of organ allografts after transplantation. Although the importance of T follicular helper (TFH) cells in controlling antibody responses has been long established, their role in directing donor-specific antibody generation leading to AMR 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 TFH cells as well as their circulating 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 TFH cells and provide future directions and novel opportunities of biotherapeutic targeting of TFH cells that have the potential of bringing the prophylactic and curative treatments of AMR toward personalized and precision medicine.

Generation and characterization of a high-affinity chimeric anti-OX40 antibody with potent antitumor activity

OX40 is a costimulatory molecule that belongs to the tumor necrosis factor receptor (TNFR) superfamily. OX40 agonist-based combinations are emerging as promising candidates for novel cancer immunotherapy. Clinical trials have shown that OX40 agonist antibodies could lead to better results in cancer patients. Using a hybridoma platform and three different types of immunization strategies, namely recombinant protein, DNA, and overexpressing cells, we identified a chimeric anti-OX40 antibody (mAb035-hIgG1 from DNA immunization) that shows excellent binding specificity, and slightly stronger activation of human memory CD4⁺ T cells and similar potent antitumor activity compared with BMS 986178, an anti-OX40 antibody currently being evaluated for the treatment of solid tumors. This paper further systematically investigates the antigen-specific immune response, the number of binders, epitope bins, and functional activities of antibodies among different immunization strategies. Interestingly, we found that different immunization strategies affect the biological activity of monoclonal antibodies.

The Redox-Metabolic Couple of T Lymphocytes: Potential Consequences for Hypertension

Significance: T lymphocytes, as part of the adaptive immune system, possess the ability to activate and function in extreme cellular microenvironments, which requires these cells to remain highly malleable. One mechanism in which T lymphocytes achieve this adaptability is by responding to cues from both reactive oxygen and nitrogen species, as well as metabolic flux, which together fine-tune the functional fate of these adaptive immune cells. Recent Advances: To date, examinations of the redox and metabolic effects on T lymphocytes have primarily investigated these biological processes as separate entities. Given that the redox and metabolic environments possess significant overlaps of pathways and molecular species, it is inevitable that perturbations in one environment affect the other. Recent consideration of this redox-metabolic couple has demonstrated the strong link and regulatory consequences of these two systems in T lymphocytes. Critical Issues: The redox and metabolic control of T lymphocytes is essential to prevent dysregulated inflammation, which has been observed in cardiovascular diseases such as hypertension. The role of the adaptive immune system in hypertension has been extensively investigated, but the understanding of how the redox and metabolic environments control T lymphocytes in this disease remains unclear. Future Directions: Herein, we provide a discussion of the redox and metabolic control of T lymphocytes as separate entities, as well as coupled to one another, to regulate adaptive immunity. While investigations examining this pair together in T lymphocytes are sparse, we speculate that T lymphocyte destiny is shaped by the redox-metabolic couple. In contrast, disrupting this duo may have inflammatory consequences such as hypertension.

Emerging immune checkpoint inhibitors for the treatment of head and neck cancers

Introduction: The benefits of immune checkpoint inhibitors (ICIs) in recurrent and/or metastatic (R/M) head and neck squamous cell carcinoma (HNSCC) have been demonstrated through multiple studies to improve overall survival (OS) with decreased side effects when compared to the standard of care (SOC) treatment regimens in place for decades, leading to the approval of two ICIs, nivolumab and pembrolizumab. There has been a subsequent influx in the development of novel immunotherapy agents for the treatment of HNSCC. Areas covered: Data for anti-programmed cell death protein 1 (PD-1)/programmed death ligand 1 (PD-L1) and anti-cytotoxic T-lymphocyte associated protein 4 (CTLA-4) antibodies in treatment of R/M HNSCC will be reviewed. Emerging immune checkpoint inhibitors as well as combined therapies in HNSCC will be discussed. The role of predictive biomarkers, HPV-status, PD-L1 expression, and challenges related to treating patients with ICIs will be summarized. Expert opinion: A shift toward ICIs as SOC for the treatment of R/M HNSCC will continue as emerging immune checkpoints and combination therapies are evaluated. Response rates are variable in this patient population underlying the importance of identifying predictive biomarkers to aid in patient selection for ICI treatment.

Combination blockade of OX40L and CD30L inhibits allergen-driven memory TH2 cell reactivity and lung inflammation

BACKGROUND

The selective reduction of memory T_H2 cell responses could be key to affording tolerance and protection from the recurrence of damaging allergic pathology.

OBJECTIVE

We asked whether TNF family costimulatory molecules cooperated to promote accumulation and reactivity of effector memory CD4 T cells to inhaled complex allergen, and whether their neutralization could promote airway tolerance to subsequent reexposure to allergen.

METHODS

Mice were sensitized intraperitoneally or intranasally with house dust mite and challenged with intranasal allergen after memory had developed. We assessed whether single or combined blockade of OX40L/CD252 and CD30L/CD153 inhibited memory T cells from driving acute asthmatic lung inflammation and protected mice following exposure to allergen at a later time.

RESULTS

OX40- or CD30-deficient animals showed strong or partial protection against allergic airway inflammation; however, neutralizing either molecule alone during the secondary response to allergen had little effect on the frequency of effector memory CD4 T cells formed and acute lung inflammation. In contrast, a significant reduction in eosinophilic inflammation was observed when OX40L and CD30L were simultaneously neutralized, with dual blockade inhibiting effector memory T_H2 cell expansion in the lungs, whereas formation of peripherally induced regulatory T cells remained intact. Moreover, dual blockade during the secondary response resulted in a tolerogenic state such that mice did not develop a normal tertiary memory T_H2 cell and lung inflammatory response when challenged weeks later with allergen.

CONCLUSION

Memory T-cell responses to complex allergens are controlled by several TNF costimulatory interactions, and their combination targeting might represent a strategy to reduce the severity of inflammatory reactions following reexposure to allergen.

Allogeneic ADSCs Induce the Production of Alloreactive Memory-CD8 T Cells through HLA-ABC Antigens

We investigated the immunogenicity of allogeneic human adipose-derived mesenchymal stem cells (ADSCs) through the production of alloreactive-CD8 T and -memory CD8 T cells, based on their human leukocyte antigen (HLA) expression. In surface antigen analysis, ADSCs do not express co-stimulatory molecules, but expresses HLA-ABC, which is further increased by exposure to the pro-inflammatory cytokines as well as IFN-γ alone. For immunogenicity analysis, allogeneic ADSCs cultured in xenofree medium (XF-ADSCs) were incubated with the recipient immune cells for allogeneic-antigen stimulation. As a result, XF-ADSCs induced IFN-γ and IL-17A release by alloreactive-CD8 T cells and the production of alloreactive-CD8 T cell through a direct pathway, although they have immunomodulatory activity. In the analysis of alloreactive memory CD8 T cells, XF-ADSCs also significantly induced the production of CFSE-low-CD8 TEM and -CD8 TCM cells. However, HLA-blocking antibodies significantly inhibited the production of CFSE-low memory-CD8 T cells, indicating that HLAs are the main antigens responsible for the development of allogeneic ADSCs' immunogenicity. These results suggested that HLA surface antigens expressed in allogeneic MSCs should be solved in order to address concerns related to the immunogenicity problem.

Blockade of OX40/OX40L pathway combined with ethylene-carbodiimide-fixed donor splenocytes induces donor-specific allograft tolerance in presensitized recipients

Background

Memory T cells (Tms) are the major barrier preventing long-term allograft survival in presensitized transplant recipients. The OX40/OX40L pathway is important in the induction and maintenance of Tms.

Methods

In this study, we added anti-OX40L mAb to ethylene-carbodiimide-fixed donor splenocytes (ECDI-SPs)-a method which is effective in inducing allograft tolerance in non-presensitized mouse heart transplant model. Recipient mice received heart transplantation after 6 weeks of donor skin presensitization and were treated with anti-OX40L mAb, ECDI-SPs or anti-OX40L mAb + ECDI-SPs, respectively.

Results

Our data showed that the combination of ECDI-SPs and anti-OX40L mAb induced donor-specific tolerance in skin-presensitized heart transplant recipients, with the mechanism for this being associated with suppression of Tms and upregulation of CD4⁺CD25⁺Foxp3⁺ T regulatory cells (Tregs). Importantly, CD25⁺ T-cell depletion in the combined therapy-treated recipients broke the establishment of allograft tolerance, whereas adoptive transfer of presensitization-derived T cells into tolerant recipients suppressed Tregs expansion and abolished established tolerance.

Conclusions

Blockade of OX40/OX40L pathway in combination with ECDI-SPs appears to modulate the Tms/Tregs imbalance so as to create a protective milieu and induce graft tolerance in presensitized recipients.

Costimulation Blockade in Transplantation

T cells play a pivotal role in orchestrating immune responses directed against a foreign (allogeneic) graft. For T cells to become fully activated, the T-cell receptor (TCR) must interact with the major histocompatibility complex (MHC) plus peptide complex on antigen-presenting cells (APCs), followed by a second "positive" costimulatory signal. In the absence of this second signal, T cells become anergic or undergo deletion. By blocking positive costimulatory signaling, T-cell allo-responses can be aborted, thus preventing graft rejection and promoting long-term allograft survival and possibly tolerance (Alegre ML, Najafian N, Curr Mol Med 6:843-857, 2006; Li XC, Rothstein DM, Sayegh MH, Immunol Rev 229:271-293, 2009). In addition, costimulatory molecules can provide negative "coinhibitory" signals that inhibit T-cell activation and terminate immune responses; strategies to promote these pathways can also lead to graft tolerance (Boenisch O, Sayegh MH, Najafian N, Curr Opin Organ Transplant 13:373-378, 2008). However, T-cell costimulation involves an incredibly complex array of interactions that may act simultaneously or at different times in the immune response and whose relative importance varies depending on the different T-cell subsets and activation status. In transplantation, the presence of foreign alloantigen incites not only destructive T effector cells but also protective regulatory T cells, the balance of which ultimately determines the fate of the allograft (Lechler RI, Garden OA, Turka LA, Nat Rev Immunol 3:147-158, 2003). Since the processes of alloantigen-specific rejection and regulation both require activation of T cells, costimulatory interactions may have opposing or synergistic roles depending on the cell being targeted. Such complexities present both challenges and opportunities in targeting T-cell costimulatory pathways for therapeutic purposes. In this chapter, we summarize our current knowledge of the various costimulatory pathways in transplantation and review the current state and challenges of harnessing these pathways to promote graft tolerance (summarized in Table 10.1).

OX40 (CD134) and OX40 ligand, important immune checkpoints in cancer

Immunotherapy has shown promising results in cancer treatment. Research shows that most patients might be resistant to these therapies. So, new immune therapies are needed. OX40 (CD134) and OX40 ligand (OX40L), costimulatory molecules, express on different types of immune cells. The interaction between OX40 and OX40L (OX40/OX40L) induces the expansion and proliferation of T cells and decreases the immunosuppression of regulatory T (Treg) cells to enhance the immune response to the specific antigen. For the important role OX40 takes in the process of immunity, many clinical trials are focusing on OX40 to find out whether it may have active effects in clinical cancer treatment. The results of clinical trials are still not enough. So, we reviewed the OX40 and its ligand (OX40L) function in cancer, clinical trials with OX40/OX40L and the correlation between OX40/OX40L and other immune checkpoints to add more ideas to tumor feasible treatment.

Antitumor T-cell Homeostatic Activation Is Uncoupled from Homeostatic Inhibition by Checkpoint Blockade

T-cell transfer into lymphodepleted recipients induces homeostatic activation and potentiates antitumor efficacy. In contrast to canonical T-cell receptor-induced activation, homeostatic activation yields a distinct phenotype and memory state whose regulatory mechanisms are poorly understood. Here, we show in patients and murine models that, following transfer into lymphodepleted bone marrow transplant (BMT) recipients, CD8⁺ T cells undergo activation but also simultaneous homeostatic inhibition manifested by upregulation of immune-checkpoint molecules and functional suppression. T cells transferred into BMT recipients were protected from homeostatic inhibition by PD-1/CTLA4 dual checkpoint blockade (dCB). This combination of dCB and BMT-"immunotransplant"-increased T-cell homeostatic activation and antitumor T-cell responses by an order of magnitude. Like homeostatic activation, homeostatic inhibition is IL7/IL15-dependent, revealing mechanistic coupling of these two processes. Marked similarity in ex vivo modulation of post-BMT T cells in mice and patients is promising for the clinical translation of immunotransplant (NCT03305445) and for addressing homeostatic inhibition in T-cell therapies. SIGNIFICANCE: For optimal anticancer effect, T-cell therapies including chimeric antigen receptor T-cell, tumor-infiltrating lymphocyte, and transgenic T-cell therapies require transfer into lymphodepleted recipients and homeostatic activation; however, concomitant homeostatic inhibition mitigates T-cell therapies' efficacy. Checkpoint blockade uncouples homeostatic inhibition from activation, amplifying T-cell responses. Conversely, tumors nonresponsive to checkpoint blockade or BMT are treatable with immunotransplant.See related commentary by Ansell, p. 1487.This article is highlighted in the In This Issue feature, p. 1469.

Crucial role of OX40/OX40L signaling in a murine model of asthma

The aim of the present study was to explore the roles of OX40/OX40 ligand (OX40L) signaling and OX40⁺ T cells in ovalbumin (OVA)-induced mouse asthma model. Asthma was induced by OVA exposure and subsequent co-treatment with OX40L protein, neutralizing anti-OX40L blocking antibody, OX40⁺ T cells or PBS. The protein expression levels of interleukin (IL)-4, IL-6, IL-13, IL-17, tumor necrosis factor (TNF)-α and interferon (IFN)-γ in bronchoalveolar lavage fluid (BALF) were examined using murine cytokine-specific ELISA. Eosinophil accumulation as well as proliferation and apoptosis of T cells in BALF were detected by Cell Counting kit-8 and flow cytometric assays. Expression of the apoptosis-related protein cleaved caspase-3 was examined in OX40⁺ T cells using western blot assay. Flow cytometric analysis revealed that OVA-treated mice that were co-treated with OX40L or OX40⁺ T cells exhibited higher eosinophil infiltration compared with control mice treated only with OVA, whereas neutralizing anti-OX40L blocking antibody inhibited eosinophil infiltration. ELISA assays demonstrated that the expression of IL-4, IL-6, IL-13, IL-17, TNF-α and IFN-γ in BALF in OX40L-treated and OX40⁺ T cell-treated mice was increased compared with expression levels in control mice. Treatment with OX40L protein effectively reduced apoptosis of T cells and the expression of cleaved caspase-3 in T cells. OX40L-treated and OX40⁺ T cell-treated mice exhibited increased asthma through OX40/OX40L signaling, which probably promoted inflammatory factor expression, eosinophil infiltration and T cell proliferation.

Harnessing Apoptotic Cells for Transplantation Tolerance: Current Status and Future Perspectives

PURPOSE OF REVIEW

The use of donor apoptotic cells is an emerging therapy for inducing transplantation tolerance. In this review, we will discuss current understanding of mechanisms of this approach, as well as crucial aspects necessary for successful translation of this approach to clinical transplantation.

RECENT FINDINGS

Transplantation tolerance by donor apoptotic cells is mediated by their homeostatic interaction with recipient phagocytes, and subsequent expansion of suppressor cell populations as well as inhibition of effector T cells via deletion and anergy. To ensure their tolerogenicity, it is critical to procure non-stressed donor cells, and to induce and arrest their apoptosis at the appropriate stage prior to their administration. Equally important is the monitoring of dynamics of recipient immunological status, and its influences on tolerance efficacy and longevity. Emerging concepts and technologies may significantly streamline tolerogen manufacture and delivery of this approach, and smooth its transition to clinical application.

SUMMARY

Hijacking homeostatic clearance of donor apoptotic cells is a promising strategy for transplantation tolerance. Timing is now mature for concerted efforts for transitioning this strategy to clinical transplantation.

Interruption of OX40L signaling prevents costimulation blockade-resistant allograft rejection

The potential of costimulation blockade to serve as a novel transplant immunosuppression strategy has been explored for over 20 years, culminating in the recent clinical approval of belatacept for renal transplant patients. Despite improving long-term graft function and survival compared with calcineurin inhibitors, clinical acceptance of belatacept has been hindered by elevated rates of acute rejection. We examined the signaling pathways required to activate costimulation blockade-resistant alloreactive T cells and identified the OX40/OX40L secondary costimulatory pathway as a promising target. We next sought to improve the clinical efficacy of traditional costimulation blockade using belatacept by coupling it with anti-OX40L. Using a murine transplant model, we demonstrate that combined blockade enhances the suppression of alloreactive T cell proliferation and effector functions including both cytokine release and cytotoxic degranulation. We also show that anti-OX40L may be particularly useful in targeting alloreactive memory T cell responses that are relatively unaffected by traditional costimulation blockade regimens. Finally, we translated this therapy to a clinically relevant nonhuman primate renal transplant model, validating the efficacy of this regimen in a potentially novel steroid- and calcineurin inhibitor-free immunosuppression regimen.

Advances in targeting co-inhibitory and co-stimulatory pathways in transplantation settings: the Yin to the Yang of cancer immunotherapy

In the past decade, the power of harnessing T-cell co-signaling pathways has become increasingly understood to have significant clinical importance. In cancer immunotherapy, the field has concentrated on two related modalities: First, targeting cancer antigens through highly activated chimeric antigen T cells (CAR-Ts) and second, re-animating endogenous quiescent T cells through checkpoint blockade. In each of these strategies, the therapeutic goal is to re-ignite T-cell immunity, in order to eradicate tumors. In transplantation, there is also great interest in targeting T-cell co-signaling, but with the opposite goal: in this field, we seek the Yin to cancer immunotherapy's Yang, and focus on manipulating T-cell co-signaling to induce tolerance rather than activation. In this review, we discuss the major T-cell signaling pathways that are being investigated for tolerance induction, detailing preclinical studies and the path to the clinic for many of these molecules. These include blockade of co-stimulation pathways and agonism of coinhibitory pathways, in order to achieve the delicate state of balance that is transplant tolerance: a state which guarantees lifelong transplant acceptance without ongoing immunosuppression, and with preservation of protective immune responses. In the context of the clinical translation of immune tolerance strategies, we discuss the significant challenge that is embodied by the fact that targeted pathway modulators may have opposing effects on tolerance based on their impact on effector vs regulatory T-cell biology. Achieving this delicate balance holds the key to the major challenge of transplantation: lifelong control of alloreactivity while maintaining an otherwise intact immune system.

OX40, OX40L and Autoimmunity: a Comprehensive Review

The tumour necrosis factor receptor OX40 (CD134) is activated by its cognate ligand OX40L (CD134L, CD252) and functions as a T cell co-stimulatory molecule. OX40-OX40L interactions have been proposed as a potential therapeutic target for treating autoimmunity. OX40 is expressed on activated T cells, and in the mouse at rest on regulatory T cells (Treg). OX40L is found on antigen-presenting cells, activated T cells and others including lymphoid tissue inducer cells, some endothelia and mast cells. Expression of both molecules is increased after antigen presentation occurs and also in response to multiple other pro-inflammatory factors including CD28 ligation, CD40L ligation and interferon-gamma signaling. Their interactions promote T cell survival, promote an effector T cell phenotype, promote T cell memory, tend to reduce regulatory function, increase effector cytokine production and enhance cell mobility. In some circumstances, OX40 agonism may be associated with increased tolerance, although timing with respect to antigenic stimulus is important. Further, recent work has suggested that OX40L blockade may be more effective than OX40 blockade in reducing autoimmunity. This article reviews the expression of OX40 and OX40L in health, the effects of their interactions and insights from their under- or over-expression. We then review OX40 and OX40L expression in human autoimmune disease, identified associations of variations in their genes (TNFRSF4 and TNFSF4, respectively) with autoimmunity, and data from animal models of human diseases. A rationale for blocking OX40-OX40L interaction in human autoimmunity is then presented along with commentary on the one trial of OX40L blockade in human disease conducted to date. Finally, we discuss potential problems with clinical use of OX40-OX40L directed pharmacotherapy.

Patterns of Early Rejection in Renal Retransplantation: A Single-Center Experience

It has been reported that kidney retransplant patients had high rates of early acute rejection due to previous sensitization. In addition to the acute antibody-mediated rejection (ABMR) that has received widespread attention, the early acute T-cell-mediated rejection (TCMR) may be another important issue in renal retransplantation. In the current single-center retrospective study, we included 33 retransplant patients and 90 first transplant patients with similar protocols of induction and maintenance therapy. Analysis focused particularly on the incidence and patterns of early acute rejection episodes, as well as one-year graft and patient survival. Excellent short-term clinical outcomes were obtained in both groups, with one-year graft and patient survival rates of 93.9%/100% in the retransplant group and 92.2%/95.6% in the first transplant group. Impressively, with our strict immunological selection and desensitization criteria, the retransplant patients had a very low incidence of early acute ABMR (6.1%), which was similar to that in the first transplant patients (4.4%). However, a much higher rate of early acute TCMR was observed in the retransplant group than in the first transplant group (30.3% versus 5.6%, P < 0.001). Acute TCMR that develops early after retransplantation should be monitored in order to obtain better transplant outcomes.

The CD8 T-cell response during tolerance induction in liver transplantation

Both experimental and clinical studies have shown that the liver possesses unique tolerogenic properties. Liver allografts can be spontaneously accepted across complete major histocompatibility mismatch in some animal models. In addition, some liver transplant patients can be successfully withdrawn from immunosuppressive medications, developing ‘operational tolerance'. Multiple mechanisms have been shown to be involved in inducing and maintaining alloimmune tolerance associated with liver transplantation. Here, we focus on CD8 T-cell tolerance in this setting. We first discuss how alloreactive cytotoxic T-cell responses are generated against allografts, before reviewing how the liver parenchyma, donor passenger leucocytes and the host immune system function together to attenuate alloreactive CD8 T-cell responses to promote the long-term survival of liver transplants.

Role of Wnt3a expressed by dendritic cells in the activation of canonical Wnt signaling and generation of memory T cells during primary immune responses

The presence of memory T cells (T_Ms) hinders transplant survival. Dendritic cells (DCs) induce the generation of T_Ms during primary immune responses. However, the specific mechanisms are unclear. In this study, we constructed a Wnt3a-expressing adenovirus and used small interfering RNA (siRNA) targeting Wnt3a to investigate the influence of Wnt3a expression in DCs on the generation of T_Ms during primary immune responses. Our results demonstrated that the Wnt3a expression levels in DCs influenced the generation of T_Ms after 5days in co-culture with naïve T cells through activation of the Wnt canonical pathway. Interleukin-7 secretion levels in supernatants of DC/T_Ns co-cultures showed a similar pattern of Wnt3a expression levels in DCs. These findings provide a better understanding of T_Ms generation mechanisms that might be useful to improve transplant outcomes.

T Cell Cosignaling Molecules in Transplantation

The ultimate outcome of alloreactivity versus tolerance following transplantation is potently influenced by the constellation of cosignaling molecules expressed by immune cells during priming with alloantigen, and the net sum of costimulatory and coinhibitory signals transmitted via ligation of these molecules. Intense investigation over the last two decades has yielded a detailed understanding of the kinetics, cellular distribution, and intracellular signaling networks of cosignaling molecules such as the CD28, TNF, and TIM families of receptors in alloimmunity. More recent work has better defined the cellular and molecular mechanisms by which engagement of cosignaling networks serve to either dampen or augment alloimmunity. These findings will likely aid in the rational development of novel immunomodulatory strategies to prolong graft survival and improve outcomes following transplantation.

Down-Regulation of Surface CD28 under Belatacept Treatment: An Escape Mechanism for Antigen-Reactive T-Cells

Background

The co-stimulatory inhibitor of the CD28-CD80/86-pathway, belatacept, allows calcineurin-inhibitor-free immunosuppression in kidney transplantation. However, aggressive T-cell mediated allogeneic responses have been observed in belatacept-treated patients, which could be explained by effector-memory T-cells that lack membrane expression of CD28, i.e. CD28-negative (CD28^NULL) T-cells. CD28-positive (CD28^POS) T-cells that down regulate their surface CD28 after allogeneic stimulation could also pose a threat against the renal graft. The aim of this study was to investigate this potential escape mechanism for CD28^POS T-cells under belatacept treatment.

Materials & Methods

PBMCs, isolated T-cell memory subsets and isolated CD28^POS T-cells were obtained from end-stage renal disease (ESRD) patients and co-cultured with allo-antigen in the presence of belatacept to mimic allogeneic reactions in kidney-transplant patients under belatacept treatment. As a control, IgG was used in the absence of belatacept.

Results

Despite high in vitro belatacept concentrations, a residual T-cell growth of ±30% was observed compared to the IgG control after allogeneic stimulation. Of the alloreactive T-cells, the majority expressed an effector-memory phenotype. This predominance for effector-memory T-cells within the proliferated cells was even larger when a higher dose of belatacept was added. Contrary to isolated naïve and central-memory T cells, isolated effector-memory T cells could not be inhibited by belatacept in differentiation or allogeneic IFNγ production. The proportion of CD28-positive T cells was lower within the proliferated T cell population, but was still substantial. A fair number of the isolated initially CD28^POS T-cells differentiated into CD28^NULL T-cells, which made them not targetable by belatacept. These induced CD28^NULL T-cells were not anergic as they produced high amounts of IFNγ upon allogeneic stimulation. The majority of the proliferated isolated originally CD28^POS T-cells, however, still expressed CD28 and also expressed IFNγ.

Conclusion

This study provides evidence that, apart from CD28^NULL T-cells, also CD28^POS, mostly effector-memory T-cells can mediate allogeneic responses despite belatacept treatment.

The Evolving Roles of Memory Immune Cells in Transplantation

Memory cells are the products of immune responses but also exert significant impact on subsequent immunity and immune tolerance, thus placing them in a unique position in transplant research. Memory cells are heterogeneous, including not only memory T cells but also memory B cells and innate memory cells. Memory cells are a critical component of protective immunity against invading pathogens, especially in immunosuppressed patients, but they also mediate graft loss and tolerance resistance. Recent studies suggest that some memory cells unexpectedly act as regulatory cells, promoting rather than hindering transplant survival. This functional diversity makes therapeutic targeting of memory cells a challenging task in transplantation. In this article, we highlight recent advances in our understanding of memory cells, focusing on diversity of memory cells and mechanisms involved in their induction and functions. We also provide a broad overview on the challenges and opportunities in targeting memory cells in the induction of transplant tolerance.

Role of Memory T Cells and Perspectives for Intervention in Organ Transplantation

Memory T cells are necessary for protective immunity against invading pathogens, especially under conditions of immunosuppression. However, their presence also threatens transplant survival, making transplantation a great challenge. Significant progress has been achieved in recent years in advancing our understanding of the role that memory T cells play in transplantation. This review focuses on the latest advances in our understanding of the involvement of memory T cells in graft rejection and transplant tolerance and discusses potential strategies for targeting memory T cells in order to minimize allograft rejection and optimize clinical outcomes.

Update on CD40 and CD154 blockade in transplant models

Generation of an effective immune response against foreign antigens requires two distinct molecular signals: a primary signal provided by the binding of antigen-specific T-cell receptor to peptide-MHC on antigen-presenting cells and a secondary signal delivered via the engagement of costimulatory molecules. Among various costimulatory signaling pathways, the interactions between CD40 and its ligand CD154 have been extensively investigated given their essential roles in the modulation of adaptive immunity. Here, we review current understanding of the role CD40/CD154 costimulation pathway has in alloimmunity, and summarize recent mechanistic and preclinical advances in the evaluation of candidate therapeutic approaches to target this receptor-ligand pair in transplantation.

Arsenic trioxide inhibits accelerated allograft rejection mediated by alloreactive CD8(+) memory T cells and prolongs allograft survival time

CD8(+) memory T (Tm) cells are a significant barrier to transplant tolerance induction in alloantigen-primed recipients, and are insensitive to existing clinical immunosuppressants. Here, we studied the inhibition of CD8(+) Tm cells by arsenic trioxide (As2O3) for the first time. Alloantigen-primed CD8(+) Tm cells were transferred to T cell immunodeficient nude mice. The mice were subjected to heart allotransplantation, and treated with As2O3. The transplant survival time was determined, and the inhibitory effects of As2O3 on CD8(+) Tm cell-mediated immune rejection were assessed through serological studies and inspection of the transplanted heart and lymphoid organs. We found that As2O3 treatment prolonged the mean survival time of the graft and reduced the number of CD8(+) Tm cells in the spleen and lymph nodes. The expression of the genes encoding interleukin (IL)-2, and IFN-γ was reduced, while expression of IL-10 and transforming growth factor-β was increased in the transplant. Our findings show that As2O3 treatment inhibits allograft rejection mediated by alloreactive CD8(+) Tm cells in the mouse heart transplantation model.

Taming inflammation by targeting cytokine signaling: new perspectives in the induction of transplantation tolerance

Transplantation tolerance remains an elusive goal, partly due to limitations in our understanding of the interplay between inflammatory mediators and their role in the activation and regulation of T lymphocytes. Although multiple mechanisms acting both centrally and peripherally are responsible for tolerance induction, the signaling pathways leading to activation or regulation of adaptive immunity are often complex, branched, redundant and modulated by the microenvironment's inflammatory milieu. Accumulating evidence clearly indicates that inflammatory cytokines limit the tolerogenic potential of immunomodulatory protocols by supporting priming of the immune system and counteracting regulatory mechanisms, ultimately promoting rejection. In this review, we summarize recent progress in the development of novel therapeutics to manipulate this inflammatory environment and achievements in targeted inhibition of inflammatory cytokine signaling. Ultimately, robust transplant tolerance induction will probably require a multifaceted, holistic approach that integrates the various mechanisms of tolerance induction, incorporates the dynamic alterations in costimulatory requirements of alloreactive T cells, while maintaining endogenous mechanisms of immune regulation.

Anti-OX40L monoclonal antibody prolongs secondary heart allograft survival based on CD40/CD40L and LFA-1/ICAM-1 blockade

BACKGROUND

Memory T cells (Tms) form a barrier against long-term allograft survival; however, CD4(+)Foxp3(+) regulatory T cells (Tregs) can suppress allograft rejection. The OX40/OX40L pathway is critical to the generation of Tms and turns off Treg suppressor function.

METHODS

B6 mice that rejected BALB/c skin grafts after 4 weeks were used as the secondary heart transplant recipients. The skin recipient mice, termed S0, S2 and S3, were treated with the isotype antibodies, anti-CD40L/LFA-1 or anti-OX40L combined with anti-CD40L/LFA-1 mAbs, respectively. The secondary heart recipients, termed H0 and H2, received anti-CD40L/LFA-1 mAbs or not, respectively (Fig. 1).

RESULTS

Four weeks after primary skin transplantation, the Tms in the S3 group that received anti-OX40L with anti-CD40L/LFA-1 mAbs were reduced compared to those in the S2 group (CD4(+) Tm: 32.61 ± 2.20% in S2 vs. 25.36 ± 1.16% in S3; CD8(+) Tm: 27.76 ± 1.96% in S2 vs. 20.95 ± 1.30% in S3; P < 0.01). Meanwhile, the proportions of Tregs in S3 increased compared to those in S2 (P < 0.05). The anti-OX40L with anti-CD40L/LFA-1 mAbs group (S3H2) prolonged the mean survival time (MST) following secondary heart transplantation from 9.5 days to 21 days (P < 0.001). Furthermore, allogeneic proliferation of recipient splenic T cells and graft-infiltrating lymphocytes were significantly inhibited in the S3H2 group. Additionally, a higher level of IL-10 was detected in sera and allografts.

CONCLUSIONS

Anti-OX40L mAb could prolong secondary heart allograft survival based on CD40/CD40L and LFA-1/ICAM-1 blockade. The mechanism of protecting allografts using anti-OX40L mAb involved impairing the generation of Tm and up-regulating IL-10 producing Tregs, inhibiting the function of T cells.

Memory T Cells in Transplantation

Following infections and environmental exposures, memory T cells are generated that provide long-term protective immunity. Compared to their naïve T cell counterparts, memory T cells possess unique characteristics that endow them with the ability to quickly and robustly respond to foreign antigens. While such memory T cells are beneficial in protecting their hosts from recurrent infection, memory cells reactive to donor antigens pose a major barrier to successful transplantation and tolerance induction. Significant progress has been made over the past several decades contributing to our understanding of memory T cell generation, their distinct biology, and their detrimental impact in clinical and animal models of transplantation. This review focuses on the unique features which make memory T cells relevant to the transplant community and discusses potential therapies targeting memory T cells which may ameliorate allograft rejection.

Memory T Cells Mediate Cardiac Allograft Vasculopathy and are Inactivated by Anti-OX40L Monoclonal Antibody

PURPOSE

Cardiac allograft vasculopathy (CAV) is a major complication limiting the long-term survival of cardiac transplants. The role of memory T cells (Tmem) in the pathogenesis of CAV remains elusive. This study investigated the role of Tmem cells in the development of CAV and the therapeutic potential of targeting the OX40/OX40L pathway for heart transplant survival.

METHODS

Tmem cells were generated in Rag-1(-/-) C57BL/6 (B6) mice by homeostatic proliferation (HP) of CD40L null CD3(+) T cells from B6 mice. Rag-1(-/-) B6 mice (H-2(b)) harboring Tmem cells received cardiac allografts from BALB/c mice (H-2(d)), and were either untreated or treated with anti-OX40L monoclonal antibody (mAb) (0.5 mg/mouse/day) for 10 days.

RESULTS

Six weeks after HP, the majority of transferred CD40L(-/-) T cells in Rag-1(-/-) B6 mice were differentiated to CD44(high) and CD62L(low) Tmem cells. BALB/c heart allografts in Rag-1(-/-) B6 recipient mice in the presence of these Tmem cells developed a typical pathological feature of CAV; intimal thickening, 100 days after transplantation. However, functionally blocking the OX40/OX40L pathway with anti-OX40L mAb significantly prevented CAV development and reduced the Tmem cell population in recipient mice. Anti-OX40L mAb therapy also significantly decreased cellular infiltration and cytokine (IFN-γ, TNF-α and TGF-β) expression in heart allografts.

CONCLUSIONS

Tmem cells mediate CAV in heart transplants. Functionally blocking the OX40/OX40L pathway using anti-OX40L mAb therapy prevents Tmem cell-mediated CAV, suggesting therapeutic potential for disrupting OX40-OX40L signaling in order to prevent CAV in heart transplant patients.

Targeting co-stimulatory pathways: transplantation and autoimmunity

The myriad of co-stimulatory signals expressed, or induced, upon T-cell activation suggests that these signalling pathways shape the character and magnitude of the resulting autoreactive or alloreactive T-cell responses during autoimmunity or transplantation, respectively. Reducing pathological T-cell responses by targeting T-cell co-stimulatory pathways has met with therapeutic success in many instances, but challenges remain. In this Review, we discuss the T-cell co-stimulatory molecules that are known to have critical roles during T-cell activation, expansion, and differentiation. We also outline the functional importance of T-cell co-stimulatory molecules in transplantation, tolerance and autoimmunity, and we describe how therapeutic blockade of these pathways might be harnessed to manipulate the immune response to prevent or attenuate pathological immune responses. Ultimately, understanding the interplay between individual co-stimulatory and co-inhibitory pathways engaged during T-cell activation and differentiation will lead to rational and targeted therapeutic interventions to manipulate T-cell responses and improve clinical outcomes.

Lessons and limits of mouse models

Seminal studies in rabbits and rodent transplantation models by Peter Medawar revealed that cellular processes, rather than humoral antibodies, are central to the acute rejection of transplanted organs, and much of basic transplantation research continues to be focused on the biology and control of these cells, which were subsequently shown to be T cells. However, the success of current immunosuppression at controlling T-cell-mediated rejection has resulted in an increasing awareness of antibody-mediated rejection in the clinic. This, in turn, has fueled an emerging interest in the biology of allospecific antibodies, the B cells that produce these antibodies, and the development of mouse models that allow their investigation. Here we summarize some of the more widely used mouse models that have been developed to study the immunobiology of alloreactivity, transplantation rejection and tolerance, and used to identify therapeutic strategies that modulate these events.

T-cell co-stimulatory blockade in transplantation: two steps forward one step back!

INTRODUCTION

The concern about nephrotoxicity with calcineurin inhibitors led to the search of novel agents for immunosuppression. Based on the requirement of T-cell co-stimulatory signals to fully activated naïve T cells, it became clear that blocking these pathways could be an appealing therapeutic target. However, some unexpected findings were noticed in the recent clinical trials of belatacept, including a higher rate of rejection, which warranted further investigation with some interesting concepts emerging from the bench.

AREAS COVERED

This article aims to review the literature of the B7:CD28 co-stimulatory blockade in transplantation, including the basic immunology behind its development, clinical application and potential limitations.

EXPERT OPINION

Targeting co-stimulatory pathways were found to be much more complex than initially anticipated due to the interplay between not only various co-stimulatory pathways but also various co-inhibitory ones. In addition, co-stimulatory signals have different roles in diverse immune cell types. Therefore, targeting CD28 ligands with cytotoxic T lymphocyte antigen-4 (CTLA4)-Ig may have some deleterious effects, including the inhibition of regulatory T cells, blockade of co-inhibitory signals (CTLA4) and promotion of Th17 cells. Co-stimulatory independence of memory T cells was another unforeseen limitation. Learning how to better integrate co-stimulatory targeting with other immunosuppressive agents will be critical for the improvement of long-term graft survival.

Memory T cells in transplantation - progress and challenges

PURPOSE OF REVIEW

Memory T cells present a different set of challenges to transplant patients; they are needed for protection against invading pathogens, especially under conditions of immunosuppression. But their presence also threatens transplant survival, as some of them are alloreactive. Efforts to resolve this paradox will be critical in the induction of transplant tolerance.

RECENT FINDINGS

There has been significant progress made in the past few years in the areas of population diversity of memory T cells, metabolic control of their induction, and mechanisms and pathways involved in memory cell exhaustion. Multiple targets on memory T cells have been identified, some of which are under vigorous testing in various transplant models.

SUMMARY

Memory T cells are both friends and foes to transplant patients, and tolerance strategies should selectively target alloreactive memory T cells and leave other memory cells unaltered. This situation remains a major challenge in the clinic.

A diametric role for OX40 in the response of effector/memory CD4+ T cells and regulatory T cells to alloantigen

OX40 is a member of the TNFR superfamily that has potent costimulatory properties. Although the impact of blockade of the OX40-OX40 ligand (OX40L) pathway has been well documented in models of autoimmune disease, its effect on the rejection of allografts is less well defined. In this article, we show that the alloantigen-mediated activation of naive and memory CD4(+) T cells results in the induction of OX40 expression and that blockade of OX40-OX40L interactions prevents skin allograft rejection mediated by either subset of T cells. Moreover, a blocking anti-OX40 had no effect on the activation and proliferation of T cells; rather, effector T cells failed to accumulate in peripheral lymph nodes and subsequently migrate to skin allografts. This was found to be the result of an enhanced degree of cell death among proliferating effector cells. In clear contrast, blockade of OX40-OX40L interactions at the time of exposure to alloantigen enhanced the ability of regulatory T cells to suppress T cell responses to alloantigen by supporting, rather than diminishing, regulatory T cell survival. These data show that OX40-OX40L signaling contributes to the evolution of the adaptive immune response to an allograft via the differential control of alloreactive effector and regulatory T cell survival. Moreover, these data serve to further highlight OX40 and OX40L as therapeutic targets to assist the induction of tolerance to allografts and self-Ags.

Islet allograft rejection in sensitized mice is refractory to control by combination therapy of immune-modulating agents

Retransplantation is common in allogeneic islet transplantation, and therefore, memory responses in previously sensitized recipients present a distinct obstacle for successful islet transplantation. Given the difficulties in controlling memory responses contributing to allograft rejection, it is worth investigating the effects of new immune-modulating agents against islet allograft rejection in the sensitized recipients. In this study, we investigated immune-modulating agents including 5-azacytidine and IL-2/anti-IL-2 complex to ascertain their suppressive effects on memory responses. In suppression assays, rapamycin effectively suppressed the proliferation of memory T cells, whereas 5-azacytidine, a methylation inhibitor suppressed the survival and proliferation of memory T cells. Combination therapy of anti-CD40L, anti-OX40L, and rapamycin slightly prolonged BALB/c islet allograft survival in sensitized C57BL6 mice, and reduced intragraft infiltration of macrophages, T cells, and B cells. However, the addition of IL-2/anti-IL-2 complex, an inducer of regulatory T cells, did not exhibit additional suppression against rejection in sensitized mice. Although a combination of 5-azacytidine and rapamycin markedly suppressed islet allograft rejection in naïve mice, it failed to achieve long-term graft survival even when combined with anti-CD40L and anti-OX40 in sensitized mice. In short, 5-azacytidine-based or IL-2/anti-IL-2 complex-based regimens can suppress islet allograft rejection in naïve recipients, but fail to control islet allograft rejection in sensitized recipients.

CD18 is required for optimal lymphopenia-induced proliferation of mouse T cells

Lymphocyte numbers are tightly regulated; with acute lymphopenia, T cell numbers are reestablished through lymphopenia-induced proliferation. In contrast to the costimulation requirements of antigen-driven proliferation, a number of costimulatory molecules are not required for lymphopenia-induced proliferation. However, the requirement for major histocompatibility complex (MHC)-T cell receptor (TCR) interactions and the enhanced lymphopenia-induced proliferation in T cells with higher TCR affinity argue for a role for surface molecules that contribute to efficient MHC-TCR interactions, in particular adhesion molecules. CD18 is an integrin that contributes to the activation of peripheral and intestinal T cells through adhesive and costimulatory mechanisms. We found that CD18 is required for optimal polyclonal and monoclonal CD4+ T cell lymphopenia-induced proliferation in recombination-activating gene 1-deficient (RAG-1-/-) mice; this requirement persisted over time. Uniquely, the dependency on CD18 in CD4+ T cells is in the rapid proliferation in RAG-1-/- recipients and in the slow homeostatic proliferation in irradiated Balb/c recipients. Consistent with the proposed role for intestinal microbiota in lymphopenia-induced rapid proliferation in RAG-/- mice, we observed a significant reduction in rapid proliferation upon treatment of mice with antibiotics; however, the dependency on CD18 for optimal lymphopenia-induced proliferation persisted. Moreover, the dependency for CD18 is maintained over a wide range of numbers of initially transferred T cells, including a low number of initially transferred T cells, when the drive for proliferation is very strong and proliferation is more rapid. Overall, these data argue for an essential and broad role for CD18 in lymphopenia-induced proliferation.

OX40 signaling favors the induction of T(H)9 cells and airway inflammation

The mechanisms that regulate the T(H)9 subset of helper T cells and diseases mediated by T(H)9 cells remain poorly defined. Here we found that the costimulatory receptor OX40 was a powerful inducer of T(H)9 cells in vitro and T(H)9 cell-dependent airway inflammation in vivo. In polarizing conditions based on transforming growth factor-β (TGF-β), ligation of OX40 inhibited the production of induced regulatory T cells and the T(H)17 subset of helper T cells and diverted CD4(+)Foxp3(-) T cells to a T(H)9 phenotype. Mechanistically, OX40 activated the ubiquitin ligase TRAF6, which triggered induction of the kinase NIK in CD4(+) T cells and the noncanonical transcription factor NF-κB pathway; this subsequently led to the generation of T(H)9 cells. Thus, our study identifies a previously unknown mechanism for the induction of T(H)9 cells and may have important clinical implications in allergic inflammation.

Targeting T-cell memory: where do we stand?

PURPOSE OF REVIEW

The need to control donor-reactive T cell memory for successful organ transplantation is widely acknowledged. Alloreactive memory T cells are present in many human transplant recipients prior to transplantation and are less susceptible to the effects of currently used immunosuppression than naïve T cells. This review brings together recent advances in various fields of immunology that are potentially applicable for targeting memory T cells in sensitized transplant patients.

RECENT FINDINGS

The topics of the discussion are evoked by the latest findings on immunobiology of memory T cells and include functional diversity of T-cell memory, characteristic features of memory T-cell homeostasis and signaling, costimulatory requirements of memory T cells and their susceptibility to regulation.

SUMMARY

Despite apparent resistance of memory T cells to currently used therapies, recent findings suggest that recall responses by memory T cells can be controlled at many different levels. Use of this information may facilitate development of future tools managing T-cell memory in transplant settings.

Heterogeneity within T Cell Memory: Implications for Transplant Tolerance

Adaptive immunity in both mouse and man results in the generation of immunological memory. Memory T cells are both friend and foe to transplant recipients, as they are intimately involved and in many cases absolutely required for the maintenance of protective immunity in the face immunosuppression, yet from the evidence presented herein they clearly constitute a formidable barrier for the successful implementation of tolerance induction strategies in transplantation. This review describes the experimental evidence demonstrating the increased resistance of memory T cells to many distinct tolerance induction strategies, and outlines recent advances in our knowledge of the ways in which alloreactive memory T cells arise in previously untransplanted individuals. Understanding the impact of alloreactive memory T cell specificity, frequency, and quality might allow for better donor selection in order to minimize the donor-reactive memory T cell barrier in an individual transplant recipient, thus allowing stratification of relative risk of alloreactive memory T cell mediated rejection, and conversely increase the likelihood of successful establishment of tolerance. However, further research into the molecular and cellular pathways involved in alloreactive memory T cell-mediated rejection is required in order to design new strategies to overcome the memory T cell barrier, without critically impairing protective immunity.