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.

The macrolide rapamycin and its analogues (rapalogs) constitute the first generation of mammalian target of rapamycin (mTOR) inhibitors. Since the introduction of rapamycin as an immunosuppressant, there has been extensive progress in understanding its complex mechanisms of action. New insights into the function of mTOR in different immune cell types, vascular endothelial cells and neoplastic cells have opened new opportunities and challenges regarding mTOR as a pharmacological target. Currently, the two known mTOR complexes, mTOR complex (mTORC) 1 and mTORC2, are the subject of intense investigation, and the introduction of second‐generation dual mTORC kinase inhibitors (TORKinibs) and gene knockout mice is helping to uncover the distinct roles of these complexes in different cell types. While the pharmacological profiling of rapalogs is advanced, much less is known about the properties of TORKinibs. A potential benefit of mTOR inhibition in transplantation is improved protection against transplant‐associated viral infections compared with standard calcineurin inhibitor‐based immunosuppression. Preclinical and clinical data also underscore the potentially favourable antitumour effects of mTOR inhibitors in regard to transplant‐associated malignancies and as a novel treatment option for various other cancers. Many aspects of the mechanisms of action of mTOR inhibitors and their clinical implications remain unknown. In this brief review we discuss new findings and perspectives of mTOR inhibitors in transplantation.

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Background Little is known about how new generation adenosine triphosphate (ATP)-competitive mechanistic target of rapamycin (mTOR) kinase inhibitors (TORKinibs) affect immunity and allograft rejection. Methods mTOR complex (C) 1 and 2 signaling in dendritic cells (DC) and T cells was analyzed by Western blotting, while immune cell populations in normal and heart allograft recipient mice were analyzed by flow cytometry. Alloreactive T cell proliferation was quantified in MLR; intracellular cytokine production and serum antidonor IgG levels were determined by flow analysis and immunofluorescence staining used to detect IgG in allografts. Results The novel TORKinib AZD2014 impaired DC differentiation and T cell proliferation in vitro and depressed immune cells and allospecific T cell responses in vivo. A 9-day course of AZD2014 (10 mg/kg ip twice daily) or rapamycin (RAPA; 1mg/kg ip daily) prolonged median heart allograft survival time significantly (25 days for AZD2014; 100 days for RAPA; 9.5 days for control). Like RAPA, AZD2014 suppressed graft mononuclear cell infiltration, increased regulatory T cell (Treg) to effector memory T cell (Tem) ratios and reduced T follicular helper (Tfh) and B cells 7 days posttransplant. By 21 days (10 days after drug withdrawal), however, Tfh and B cells and donor-specific IgG1 and IgG2c antibody titers were significantly lower in RAPA- compared with AZD2014-treated mice. Elevated Treg to Tem ratios were maintained after RAPA, but not AZD2014 withdrawal. Conclusions Immunomodulatory effects of AZD2014, unlike those of RAPA, were not sustained after drug withdrawal, possibly reflecting distinct pharmacokinetics or/and inhibitory effects of AZD2014 on mTORC2.

Dendritic cells (DCs) are critical initiators of innate immunity in the kidney and orchestrate inflammation following ischemia-reperfusion injury. The role of the mammalian/mechanistic target of rapamycin (mTOR) in the pathophysiology of renal ischemia-reperfusion injury has been characterized. However, the influence of DC-based alterations in mTOR signaling is unknown. To address this, bone marrow-derived mTORC2-deficient (Rictor) DCs underwent hypoxia-reoxygenation and then analysis by flow cytometry. Adoptive transfer of wild-type or Rictor DC to C57BL/6 mice followed by unilateral or bilateral renal ischemia-reperfusion injury (20 min ischemia) was used to assess their in vivo migratory capacity and influence on tissue injury. Age-matched male DC-specific Rictor mice or littermate controls underwent bilateral renal ischemia-reperfusion, followed by assessment of renal function, histopathology, and biomolecular and cell infiltration analysis. Rictor DCs expressed more costimulatory CD80/CD86 but less coinhibitory programmed death ligand 1 (PDL1), a pattern that was enhanced by hypoxia-reoxygenation. They also demonstrated enhanced migration to the injured kidney and induced greater tissue damage. Following ischemia-reperfusion, Rictor DC mice developed higher serum creatinine levels, more severe histological damage, and greater proinflammatory cytokine production compared to littermate controls. Additionally, a greater influx of both neutrophils and T cells was seen in Rictor DC mice, along with CD11cMHCIICD11bF4/80 renal DC, that expressed more CD86 but less PDL1. Thus, DC-targeted elimination of Rictor enhances inflammation and migratory responses to the injured kidney, highlighting the regulatory roles of both DCs and Rictor in the pathophysiology of acute kidney injury.

Background and objectivesMany patients, providers, and potential living donors perceive the living kidney donor evaluation process to be lengthy and difficult to navigate.Design, setting, participants, & measurementsWe sought consensus on key terms and process and outcome indicators that can be used to measure how efficiently a transplant center evaluates persons interested in becoming a living kidney donor. Using a RAND-modified Delphi method, 77 participants (kidney transplant recipients or recipient candidates, living kidney donors or donor candidates, health care providers, and health care administrators) completed an online survey to define the terms and indicators. The definitions were then further refined during an in-person meeting with ten stakeholders.ResultsWe identified 16 process indicators (e.g., average time to evaluate a donor candidate), eight outcome indicators (e.g., annual number of preemptive living kidney donor transplants), and two measures that can be considered both process and outcome indicators (e.g., average number of times a candidate visited the transplant center for the evaluation). Transplant centers wishing to implement this set of indicators will require 22 unique data elements, all of which are either readily available or easily collected prospectively.ConclusionsWe identified a set of indicators through a consensus-based approach that may be used to monitor and improve the performance of a transplant center in how efficiently it evaluates persons interested in becoming a living kidney donor.

Background. Urine CXCL10 (C-X-C motif chemokine ligand 10, interferon gamma-induced protein 10 [IP10]) outperforms standard-of-care monitoring for detecting subclinical and early clinical T-cell–mediated rejection (TCMR) and may advance TCMR therapy development through biomarker-enriched trials. The goal was to perform an international multicenter validation of a CXCL10 bead-based immunoassay (Luminex) for transplant surveillance and compare with an electrochemiluminescence-based (Meso Scale Discovery [MSD]) assay used in transplant trials. Methods. Four laboratories participated in the Luminex assay development and evaluation. Urine CXCL10 was measured by Luminex and MSD in 2 independent adult kidney transplant trial cohorts (Basel and TMCT04). In an independent test and validation set, a linear mixed-effects model to predict (log10-transformed) MSD CXCL10 from Luminex CXCL10 was developed to determine the conversion between assays. Net reclassification was determined after mathematical conversion. Results. The Luminex assay was precise, with an intra- and interassay coefficient of variation 8.1% and 9.3%; showed modest agreement between 4 laboratories (R 0.96 to 0.99, P < 0.001); and correlated with known CXCL10 in a single- (n = 100 urines, R 0.94 to 0.98, P < 0.001) and multicenter cohort (n = 468 urines, R 0.92, P < 0.001) but the 2 assays were not equivalent by Passing–Bablok regression. Linear mixed-effects modeling demonstrated an intercept of −0.490 and coefficient of 1.028, showing Luminex CXCL10 are slightly higher than MSD CXCL10, but the agreement is close to 1.0. After conversion of the biopsy thresholds, the decision to biopsy would be changed for only 6% (5/85) patients showing acceptable reclassification. Conclusions. These data demonstrate this urine CXCL10 Luminex immunoassay is robust, reproducible, and accurate, indicating it can be readily translated into clinical HLA laboratories for serial posttransplant surveillance.

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