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Key Points The atypical serine/threonine protein kinase mammalian target of rapamycin (mTOR) has an important role in the modulation of both innate and adaptive immune responses. A complex formed between the immunosuppressive drug rapamycin and the immunophilin FK506-binding protein 1A, 12 kDA (FKBP12) inhibits mTOR kinase activity. mTOR functions in at least two multi-protein complexes: mTOR complex 1 (mTORC1) and mTORC2. mTOR in mTORC1 is highly sensitive to inhibition by rapamycin, whereas mTOR in mTORC2 is resistant to rapamycin. mTORC1 regulates cell growth downstream of phosphoinositide 3-kinase–AKT signalling, in which active mTORC1 phosphorylates S6 kinase (S6K1) and the eukaryotic translation initiation factor-binding protein 1 (EIF4EBP1). Both of these activities promote mRNA translation and cell growth. Rapamycin exerts many effects on the differentiation and function of professional antigen-presenting cells (APCs). mTOR inhibition by rapamycin impedes antigen uptake and can modulate antigen presentation by dendritic cells (DCs); its differential effects on cytokine production and chemokine receptor expression by DCs regulate interactions between innate and adaptive immune cells. Recent findings have shed light on previously unappreciated effects of mTOR inhibition on T cells. Rapamycin induces thymic involution, whereas the ontogeny of naturally occurring regulatory T (T Reg ) cells seems to be less affected. During conventional T cell activation, rapamycin-mediated mTOR inhibition blocks cell cycle progression and can sequester activated T cells in secondary lymphoid tissues. By contrast, rapamycin causes an increase in the frequency of FOXP3 (forkhead box P3) + T cells, reflecting both the ability of T Reg cells to proliferate in the presence of rapamycin and the promotion of FOXP3 expression in peripheral T cells that are then converted into modulators of immune reactivity. mTOR inhibition is a promising therapeutic strategy to prevent rejection in transplantation and for autoimmune disease. Differential effects of rapamycin on T cells and T Reg cells (both naturally occurring and inducible) favour its ability to promote tolerance in tolerance-enhancing protocols. In addition, adoptively transferred rapamycin-conditioned APCs inhibit organ allograft rejection and graft-versus-host disease following haematopoietic cell transplantation. Ongoing and future areas of enquiry, which could prove fruitful, include distinguishing the role of mTORC1 and mTORC2 in the regulation of immune responses and tolerance, investigating the role of the mTOR–survivin–aurora B complex in T cell activation and ascertaining the mechanisms that determine T Reg cell resistance to rapamycin and mTOR-mediated regulation of FOXP3 expression, as well as their relevance to therapy. Angus Thomson and colleagues describe the consequences of mammalian target of rapamycin (mTOR) inhibition by rapamycin on dendritic cells, effector T cells and regulatory T cells. These effects make mTOR inhibition a promising immunosuppressive, but tolerance-promoting, therapeutic strategy. The potent immunosuppressive action of rapamycin is commonly ascribed to inhibition of growth factor-induced T cell proliferation. However, it is now evident that the serine/threonine protein kinase mammalian target of rapamycin (mTOR) has an important role in the modulation of both innate and adaptive immune responses. mTOR regulates diverse functions of professional antigen-presenting cells, such as dendritic cells (DCs), and has important roles in the activation of effector T cells and the function and proliferation of regulatory T cells. In this Review, we discuss our current understanding of the mTOR pathway and the consequences of mTOR inhibition, both in DCs and T cells, including new data on the regulation of forkhead box P3 expression.

Alarmins, sequestered self-molecules containing damage-associated molecular patterns, are released during tissue injury to drive innate immune cell proinflammatory responses. Whether endogenous negative regulators controlling early immune responses are also released at the site of injury is poorly understood. Herein, we establish that the stromal cell-derived alarmin interleukin 33 (IL-33) is a local factor that directly restricts the proinflammatory capacity of graft-infiltrating macrophages early after transplantation. By assessing heart transplant recipient samples and using a mouse heart transplant model, we establish that IL-33 is upregulated in allografts to limit chronic rejection. Mouse cardiac transplants lacking IL-33 displayed dramatically accelerated vascular occlusion and subsequent fibrosis, which was not due to altered systemic immune responses. Instead, a lack of graft IL-33 caused local augmentation of proinflammatory iNOS+ macrophages that accelerated graft loss. IL-33 facilitated a metabolic program in macrophages associated with reparative and regulatory functions, and local delivery of IL-33 prevented the chronic rejection of IL-33-deficient cardiac transplants. Therefore, IL-33 represents what we believe is a novel regulatory alarmin in transplantation that limits chronic rejection by restraining the local activation of proinflammatory macrophages. The local delivery of IL-33 in extracellular matrix-based materials may be a promising biologic for chronic rejection prophylaxis.

Antigen-presenting cells (APCs) integrate signals emanating from local pathology and program appropriate T cell responses. In allogeneic hematopoietic stem cell transplantation (alloHCT), recipient conditioning releases damage-associated molecular patterns (DAMPs) that generate proinflammatory APCs that secrete IL-12, which is a driver of donor Th1 responses, causing graft-versus-host disease (GVHD). Nevertheless, other mechanisms exist to initiate alloreactive T cell responses, as recipients with disrupted DAMP signaling or lacking IL-12 develop GVHD. We established that tissue damage signals are perceived directly by donor CD4+ T cells and promoted T cell expansion and differentiation. Specifically, the fibroblastic reticular cell-derived DAMP IL-33 is increased by recipient conditioning and is critical for the initial activation, proliferation, and differentiation of alloreactive Th1 cells. IL-33 stimulation of CD4+ T cells was not required for lymphopenia-induced expansion, however. IL-33 promoted IL-12-independent expression of Tbet and generation of Th1 cells that infiltrated GVHD target tissues. Mechanistically, IL-33 augmented CD4+ T cell TCR-associated signaling pathways in response to alloantigen. This enhanced T cell expansion and Th1 polarization, but inhibited the expression of regulatory molecules such as IL-10 and Foxp3. These data establish an unappreciated role for IL-33 as a costimulatory signal for donor Th1 generation after alloHCT.

Chronic rejection (CR) after organ transplantation is alloimmune injury manifested by graft vascular remodeling and fibrosis that is resistant to immunosuppression. Single-cell RNA-Seq analysis of MHC class II-mismatched (MHCII-mismatched) heart transplants developing chronic rejection identified graft IL-33 as a stimulator of tissue repair pathways in infiltrating macrophages and Tregs. Using IL-33-deficient donor mice, we show that graft fibroblast-derived IL-33 potently induced amphiregulin (Areg) expression by recipient Tregs. The assessment of clinical samples also confirmed increased expression of Areg by intragraft Tregs also during rejection. Areg is an EGF secreted by multiple immune cells to shape immunomodulation and tissue repair. In particular, Areg is proposed to play a major role in Treg-mediated muscle, epithelium, and nerve repair. Assessment of recipient mice with Treg-specific deletion of Areg surprisingly uncovered that Treg secretion of Areg contributed to CR. Specifically, heart transplants from recipients with Areg-deficient Tregs showed less fibrosis, vasculopathy, and vessel-associated fibrotic niches populated by recipient T cells. Mechanistically, we show that Treg-secreted Areg functioned to increase fibroblast proliferation. In total, these studies identify how a dysregulated repair response involving interactions between IL-33+ fibroblasts in the allograft and recipient Tregs contributed to the progression of CR.

Detrimental inflammatory responses after solid organ transplantation are initiated when immune cells sense pathogen-associated molecular patterns (PAMPs) and certain damage-associated molecular patterns (DAMPs) released or exposed during transplant-associated processes, such as ischemia/reperfusion injury (IRI), surgical trauma, and recipient conditioning. These inflammatory responses initiate and propagate anti-alloantigen (AlloAg) responses and targeting DAMPs and PAMPs, or the signaling cascades they activate, reduce alloimmunity, and contribute to improved outcomes after allogeneic solid organ transplantation in experimental studies. However, DAMPs have also been implicated in initiating essential anti-inflammatory and reparative functions of specific immune cells, particularly Treg and macrophages. Interestingly, DAMP signaling is also involved in local and systemic homeostasis. Herein, we describe the emerging literature defining how poor outcomes after transplantation may result, not from just an over-abundance of DAMP-driven inflammation, but instead an inadequate presence of a subset of DAMPs or related molecules needed to repair tissue successfully or re-establish tissue homeostasis. Adverse outcomes may also arise when these homeostatic or reparative signals become dysregulated or hijacked by alloreactive immune cells in transplant niches. A complete understanding of the critical pathways controlling tissue repair and homeostasis, and how alloimmune responses or transplant-related processes disrupt these will lead to new immunotherapeutics that can prevent or reverse the tissue pathology leading to lost grafts due to chronic rejection.

Mechanisms that mediate allograft tolerance differ between organs. We have previously shown that Foxp3+ T cell-enriched bronchus-associated lymphoid tissue (BALT) is induced in tolerant murine lung allografts and that these Foxp3+ cells suppress alloimmune responses locally and systemically. Here, we demonstrated that Foxp3+ cells that reside in tolerant lung allografts differed phenotypically and transcriptionally from those in the periphery and were clonally expanded. Using a mouse lung retransplant model, we showed that recipient Foxp3+ cells were continuously recruited to the BALT within tolerant allografts. We identified distinguishing features of graft-resident and newly recruited Foxp3+ cells and showed that graft-infiltrating Foxp3+ cells acquired transcriptional profiles resembling those of graft-resident Foxp3+ cells over time. Allografts underwent combined antibody-mediated rejection and acute cellular rejection when recruitment of recipient Foxp3+ cells was prevented. Finally, we showed that local administration of IL-33 could expand and activate allograft-resident Foxp3+ cells, providing a platform for the design of tolerogenic therapies for lung transplant recipients. Our findings establish graft-resident Foxp3+ cells as critical orchestrators of lung transplant tolerance and highlight the need to develop lung-specific immunosuppression.

[...]the combination of donor AlloHCT and SOTx has provided evidence that tolerance to donor antigens can be induced, yet the risk of graft-versus-host disease (GVHD) and unidentified barriers to routine tolerance induction with these protocols remain (6). Articles in this Research Topic encompass Original Research and Reviews from transplant researchers seeking to understand the mechanisms controlling transplant outcomes beyond T cell recognition of donor MHC. A current picture of alloreactive T cell metabolism during AlloHSCT is provided, with roles for glycolysis, fat oxidation, and glutamine metabolism as well as a potential explanation for how presumably contradictory metabolic findings might be reconciled. [...]the caveats and challenges of assigning causality using the current metabolic toolbox, as well as future directions in the field, are summarized.

Aims/hypothesis Weak stimulation of CD4 + T cells induces expansion of CD4 + forkhead box P3 + regulatory T cells (Tregs) and can also promote T helper (Th) 2 responses, which have demonstrable beneficial effects on autoimmune diabetes. This study explored the feasibility of combined Treg/Th2 expansion for immunotherapy of type 1 diabetes in NOD mice. Methods We compared Treg and Th responses to dendritic cells (DC) presenting scaled antigen doses to islet-specific NOD CD4 + T cells. Flow cytometric and Luminex analyses were performed to determine the phenotype and cytokine profile of expanded T cells. The ability of expanded T cells to prevent type 1 diabetes was tested in an adoptive transfer model. Results In vitro studies revealed a hierarchical, selective expansion of Treg and T effector (Teff) populations at different antigen doses. Thus, a single low dose produced a mixture of Tregs Th2 and type 1 regulatory (Tr1) cells, which prevented diabetes in NOD-SCID mice and increased the ratio of Treg/Teff cells infiltrating the pancreatic islets. Subcutaneous injection of DC, previously shown to prevent diabetes in NOD mice, induced expansion of the same mixture of Tregs Tr1 and Th2 cells. Low-dose expansion of Treg required MHC–T cell receptor interaction and was partly dependent on T cell derived TGF-β and IL-2. Autocrine IFN-γ was required for the promotion of diabetogenic Th1 cells at high antigen doses. Conclusions/interpretation Weak stimulation of CD4 + T cells with DC and low-dose antigen expands a combination of antigen-specific Tregs Th2 and Tr1 cells that prevent autoimmunity, without the need to target or purify specific Treg populations.

Mechanistic target of rapamycin inhibitors (mTORi) have a complex immunoregulatory profile in both animal models and transplant patients. Studies suggest that mTORi act as tolerance-supporting and regulatory T cell (Treg)-promoting immunosuppressants. Yet proinflammatory influences on myeloid dendritic cells have been established. Insight is needed into the impact of mTORi on immune cells. Stallone et al. describe a clinical study identifying a potential immunoregulatory pathway involving plasmacytoid dendritic cells and Tregs in renal transplant patients on mTORi.