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•Review identified 23 possible primary determinants of suboptimal vaccination coverage.•The 5As dimensions are Access, Affordability, Awareness, Acceptance, Activation.•This intuitive, pragmatic taxonomy organised all determinants of uptake.•This taxonomy facilitates mutual understanding of root causes of poor uptake. Suboptimal vaccine uptake in both childhood and adult immunisation programs limits their full potential impact on global health. A recent progress review of the Global Vaccine Action Plan stated that “countries should urgently identify barriers and bottlenecks and implement targeted approaches to increase and sustain coverage”. However, vaccination coverage may be determined by a complex mix of demographic, structural, social and behavioral factors. To develop a practical taxonomy to organise the myriad possible root causes of a gap in vaccination coverage rates, we performed a narrative review of the literature and tested whether all non-socio-demographic determinants of coverage could be organised into 4 dimensions: Access, Affordability, Awareness and Acceptance. Forty-three studies were reviewed, from which we identified 23 primary determinants of vaccination uptake. We identified a fifth domain, Activation, which captured interventions such as SMS reminders which effectively nudge people towards getting vaccinated. The 5As taxonomy captured all identified determinants of vaccine uptake. This intuitive taxonomy has already facilitated mutual understanding of the primary determinants of suboptimal coverage within inter-sectorial working groups, a first step towards them developing targeted and effective solutions.

In the past 40 years, liver transplantation has evolved from a high-risk procedure to one that offers high success rates for reversal of liver dysfunction and excellent patient and graft survival. The liver is the most tolerogenic of transplanted organs; indeed, immunosuppressive therapy can be completely withdrawn without rejection of the graft in carefully selected, stable long-term liver recipients. However, in other recipients, chronic allograft injury, late graft failure and the adverse effects of anti-rejection therapy remain important obstacles to improved success. The liver has a unique composition of parenchymal and immune cells that regulate innate and adaptive immunity and that can promote antigen-specific tolerance. Although the mechanisms underlying liver transplant tolerance are not well understood, important insights have been gained into how the local microenvironment, hepatic immune cells and specific molecular pathways can promote donor-specific tolerance. These insights provide a basis for the identification of potential clinical biomarkers that might correlate with tolerance or rejection and for the development of novel therapeutic targets. Innovative approaches aimed at promoting immunosuppressive drug minimization or withdrawal include the adoptive transfer of donor-derived or recipient-derived regulatory immune cells to promote liver transplant tolerance. In this Review, we summarize and discuss these developments and their implications for liver transplantation.In this Review, Thomson et al. describe the immunobiology underlying liver graft tolerance and failure, and discuss therapeutic approaches for minimization or withdrawal of anti-rejection immunosuppressive drug therapy post transplantation.

Key Points The liver is a solid organ with unique immunoregulatory functions that are determined by the hepatic microenvironment, which is rich in regulatory soluble mediators, and by local antigen-presenting cells (APCs) with tolerogenic capabilities located within a unique anatomical microarchitecture. Local hepatic APCs with tolerogenic function are myeloid and plasmacytoid dendritic cells, liver sinusoidal endothelial cells, Kupffer cells and hepatocytes. Under steady-state conditions, these cells induce T cell tolerance by numerous mechanisms, including clonal elimination, the induction of T cell anergy and the induction, recruitment or proliferation of regulatory T (T Reg ) cells. Tolerogenic hepatic APCs characteristically resist functional maturation in response to pathogen- or danger-associated molecular patterns (PAMPs; DAMPs), which are present physiologically in portal venous blood, through the development of hyporesponsiveness towards these stimuli or through non-responsiveness due to cell-intrinsic regulatory mechanisms. Microbial infection leading to the functional maturation of tolerogenic into immunogenic APCs, either by cell-autonomous mechanisms or cell–cell interactions, can result in the local induction of T cell immunity in the liver by mechanisms that still need to be defined. The abundance of tolerogenic APCs within the hepatic sinusoids facilitates interaction with circulating T cells and allows the liver to function as a large immunoregulatory platform aimed at skewing hepatic, as well as extrahepatic, immune responses. The principles governing hepatic tolerance or immunity may be exploited to develop therapeutic options to mitigate autoimmunity or allograft rejection, to prolong hepatic transgene expression or to overcome tolerogenic barriers in persistent infection and cancer. The mechanisms by which local antigen-presenting cells, such as myeloid and plasmacytoid dendritic cells, liver sinusoidal endothelial cells, Kupffer cells and hepatocytes, mediate tolerance to antigens metabolized in the liver are described here. These insights into hepatic tolerance may be harnessed in the clinic for the treatment of various diseases. The demands that are imposed on the liver as a result of its function as a metabolic organ that extracts nutrients and clears gut-derived microbial products from the blood are met by a unique microanatomical and immunological environment. The inherent tolerogenicity of the liver and its role in the regulation of innate and adaptive immunity are mediated by parenchymal and non-parenchymal antigen-presenting cells (APCs), cell-autonomous molecular pathways and locally produced factors. Here, we review the central role of liver APCs in the regulation of hepatic immune function and also consider how recent insights may be applied in strategies to target liver tolerance for disease therapy.

Unrestrained excessive inflammatory responses exacerbate ischemic brain injury and impede post-stroke brain recovery. CD4+CD25+Foxp3+ regulatory T (Treg) cells play important immunosuppressive roles to curtail inflammatory responses and regain immune homeostasis after stroke. Accumulating evidence confirms that Treg cells are neuroprotective at the acute stage after stroke and promote brain repair at the chronic phases. The beneficial effects of Treg cells are mediated by diverse mechanisms involving cell–cell interactions and soluble factor release. Multiple types of cells, including both immune cells and non-immune CNS cells, have been identified to be cellular targets of Treg cells. In this review, we summarize recent findings regarding the function of Treg cells in ischemic stroke and the underlying cellular and molecular mechanisms. The protective and reparative properties of Treg cells endorse them as good candidates for immune therapy. Strategies that boost the numbers and functions of Treg cells have been actively developing in the fields of transplantation and autoimmune diseases. We discuss the approaches for Treg cell expansion that have been tested in stroke models. The application of these approaches to stroke patients may bring new hope for stroke treatments.

Key Points The mTOR pathway has a central role in the regulation of cell metabolism, growth and proliferation Pharmacological inhibition of mTOR and selective gene targeting of mTORC1 or mTORC2 in podocytes and tubular epithelial cells has helped to elucidate their role in renal cell homeostasis, including in autophagy Mechanistic insights into the roles of mTOR complexes in regulating immune cell function are helping to improve understanding of the effects of mTOR inhibitors in renal disease and transplant rejection mTOR is increasingly recognized as having a fundamental role in the development of glomerular disease and in acute kidney injury; its role in fibrotic kidney disease is less certain New generation dual mTORC1 and mTORC2 inhibitors offer potential for the treatment of renal cell carcinoma mTOR inhibitors are associated with reduced rates of skin cancer and cytomegalovirus infection in renal transplant recipients The mTOR pathway has a role in the development of renal disease, kidney transplant rejection and malignancies. Here, the authors discuss the mechanisms by which mTOR complexes drive the pathogenesis of these diseases as well as the therapeutic potential of mTOR inhibitors. The mTOR pathway has a central role in the regulation of cell metabolism, growth and proliferation. Studies involving selective gene targeting of mTOR complexes (mTORC1 and mTORC2) in renal cell populations and/or pharmacologic mTOR inhibition have revealed important roles of mTOR in podocyte homeostasis and tubular transport. Important advances have also been made in understanding the role of mTOR in renal injury, polycystic kidney disease and glomerular diseases, including diabetic nephropathy. Novel insights into the roles of mTORC1 and mTORC2 in the regulation of immune cell homeostasis and function are helping to improve understanding of the complex effects of mTOR targeting on immune responses, including those that impact both de novo renal disease and renal allograft outcomes. Extensive experience in clinical renal transplantation has resulted in successful conversion of patients from calcineurin inhibitors to mTOR inhibitors at various times post-transplantation, with excellent long-term graft function. Widespread use of this practice has, however, been limited owing to mTOR-inhibitor- related toxicities. Unique attributes of mTOR inhibitors include reduced rates of squamous cell carcinoma and cytomegalovirus infection compared to other regimens. As understanding of the mechanisms by which mTORC1 and mTORC2 drive the pathogenesis of renal disease progresses, clinical studies of mTOR pathway targeting will enable testing of evolving hypotheses.

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.

Key Points Tolerogenic dendritic cells (DCs) of various subsets have been described in rodents and humans. They offer potential as therapeutic tools to ameliorate or prevent transplant rejection or graft-versus-host disease (GVHD), or to treat autoimmune disorders. Tolerogenic DCs include immature, maturation-resistant or alternatively activated DCs that express surface MHC class I and class II molecules, have a low co-stimulatory to inhibitory signal ratio and have an impaired ability to synthesize T helper 1 (T H 1)-cell-driving cytokines (such as interleukin-12p70). Various anti-inflammatory and immunosuppressive agents potentiate or confer tolerogenicity on DCs ( in vitro or in vivo ). Growth-factor-induced DC expansion (mobilization) in donor or host tissues has resulted in variable transplant outcomes leading to tolerance or exacerbation of rejection, depending on the model. Donor- or host-derived DCs, adoptively transferred to allograft recipients or targeted in situ (allopeptides, apoptotic cells or exosomes) can potentiate long-term transplant survival in normal hosts; this effect is potentiated by conventional and experimental immunosuppressive agents, including the co-stimulation blocking molecules. Mechanisms by which DCs mediate their tolerogenic properties include T-cell deletion or anergy, polarization of T H 2-cell responses and expansion or induction of regulatory T cells (with the ability to suppress T cells that recognize alloantigen through the direct or indirect pathways). DC function may be modified in situ by local microenvironmental factors (for example, in the liver) such that they acquire tolerogenic properties. There is a pressing need to ascertain whether the ability of rodent DCs to promote transplant tolerance can be extrapolated from rodents to non-human primates, which is likely to provide a better index of their potential for clinical application. Proof-of-principle studies show that autologous immature DCs can promote T-cell tolerance to model antigens in humans. Tolerogenic dendritic cells (DCs) have potential as therapeutic tools for preventing transplant rejection. This Review describes what constitutes a tolerogenic DC, how tolerogenic DCs can be induced, the mechanisms by which they mediate tolerance and the future challenges facing DC-based immunotherapy. In recent years, there has been a shift from the perception of dendritic cells (DCs) solely as inducers of immune reactivity to the view that these cells are crucial regulators of immunity, which includes their ability to induce and maintain tolerance. Advances in our understanding of the phenotypical and functional plasticity of DCs, and in our ability to manipulate their development and maturation in vitro and in vivo , has provided a basis for the therapeutic harnessing of their inherent tolerogenicity. In this Review, we integrate the available information on the role of DCs in the induction of tolerance, with a focus on transplantation.

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor that affects cellular phenotypes by modulating phagocytosis and metabolism, promoting cell survival, and counteracting inflammation. Its role in renal injury, in particular, unilateral ureteral obstruction (UUO) or ischemia-reperfusion injury (IRI)-induced renal injury remains unclear. In our study, WT and Trem2 −/− mice were employed to evaluate the role of TREM2 in renal macrophage infiltration and tissue injury after UUO. Bone marrow-derived macrophages (BMDM) from both mouse genotypes were cultured and polarized for in vitro experiments. Next, the effects of TREM2 on renal injury and macrophage polarization in IRI mice were also explored. We found that TREM2 expression was upregulated in the obstructed kidneys. TREM2 deficiency exacerbated renal inflammation and fibrosis 3 and 7 days after UUO, in association with reduced macrophage infiltration. Trem2 −/− BMDM exhibited increased apoptosis and poorer survival compared with WT BMDM. Meanwhile, TREM2 deficiency augmented M1 and M2 polarization after UUO. Consistent with the in vivo observations, TREM2 deficiency led to increased polarization of BMDM towards the M1 proinflammatory phenotype. Mechanistically, TREM2 deficiency promoted M1 and M2 polarization via the JAK-STAT pathway in the presence of TGF-β1, thereby affecting cell survival by regulating mTOR signaling. Furthermore, cyclocreatine supplementation alleviated cell death caused by TREM2 deficiency. Additionally, we found that TREM2 deficiency promoted renal injury, fibrosis, and macrophage polarization in IRI mice. The current data suggest that TREM2 deficiency aggravates renal injury by promoting macrophage apoptosis and polarization via the JAK-STAT pathway. These findings have implications for the role of TREM2 in the regulation of renal injury that justify further evaluation.

Immunomodulation holds therapeutic promise against brain injuries, but leveraging this approach requires a precise understanding of mechanisms. We report that [CD8.sup.+][CD122.sup.+][CD49d.sup.lo] T regulatory-like cells ([CD8.sup.+] TRLs) are among the earliest lymphocytes to infiltrate mouse brains after ischemic stroke and temper inflammation; they also confer neuroprotection. TRL depletion worsened stroke outcomes, an effect reversed by [CD8.sup.+] TRL reconstitution. The CXCR3/CXCL10 axis served as the brain-homing mechanism for [CD8.sup.+] TRLs. Upon brain entry, [CD8.sup.+] TRLs were reprogrammed to upregulate leukemia inhibitory factor (LIF) receptor, epidermal growth factor-like transforming growth factor (ETGF), and interleukin 10 (IL-10). LIF/LIF receptor interactions induced ETGF and IL-10 production in [CD8.sup.+] TRLs. While IL-10 induction was important for the Anti-inflammatory effects of [CD8.sup.+] TRLs, ETGF provided direct neuroprotection. Poststroke intravenous transfer of [CD8.sup.+] TRLs reduced infarction, promoting long-term neurological recovery in young males or aged mice of both sexes. Thus, these unique [CD8.sup.+] TRLs serve as early responders to rally defenses against stroke, offering fresh perspectives for clinical translation.

Dendritic cells (DCs) are highly specialized, bone marrow (BM)-derived antigen-processing and -presenting cells crucial to the induction, integration and regulation of innate, and adaptive immunity. They are stimulated by damage-associated molecular patterns (DAMPS) via pattern recognition receptors to promote inflammation and initiate immune responses. In addition to residing within the parenchyma of all organs as part of the heterogeneous mononuclear phagocyte system, DCs are an abundant component of the inflammatory cell infiltrate that appears in response to ischemia reperfusion injury (IRI). They can play disparate roles in the pathogenesis of IRI since their selective depletion has been found to be protective, deleterious, or of no benefit in mouse models of IRI. In addition, administration of DC generated and manipulated can protect organs from IRI by suppressing inflammatory cytokine production, limiting the capacity of DCs to activate NKT cells, or enhancing regulatory T cell function. Few studies however have investigated specific signal transduction mechanisms underlying DC function and how these affect IRI. Here, we address current knowledge of the role of DCs in regulation of IRI, current gaps in understanding and prospects for innovative therapeutic intervention at the biological and pharmacological levels.