Establishment of CD8+ T Cell Thymic Central Tolerance to Tissue-Restricted Antigen Requires PD-1

Highly self-reactive T cells are censored from the repertoire by both central and peripheral tolerance mechanisms upon receipt of high-affinity TCR signals. Clonal deletion is considered a major driver of central tolerance; however, other mechanisms such as induction of regulatory T cells and functional impairment have been described. An understanding of the interplay between these different central tolerance mechanisms is still lacking. We previously showed that impaired clonal deletion to a model tissue-restricted Ag did not compromise tolerance. In this study, we determined that murine T cells that failed clonal deletion were rendered functionally impaired in the thymus. Programmed cell death protein 1 (PD-1) was induced in the thymus and was required to establish cell-intrinsic tolerance to tissue-restricted Ag in CD8+ thymocytes independently of clonal deletion. In bone marrow chimeras, tolerance was not observed in PD-L1-deficient recipients, but tolerance was largely maintained following adoptive transfer of tolerant thymocytes or T cells to PD-L1-deficient recipients. However, CRISPR-mediated ablation of PD-1 in tolerant T cells resulted in broken tolerance, suggesting different PD-1 signaling requirements for establishing versus maintaining tolerance. Finally, we showed that chronic exposure to high-affinity Ag supported the long-term maintenance of tolerance. Taken together, our study identifies a critical role for PD-1 in establishing central tolerance in autoreactive T cells that escape clonal deletion. It also sheds light on potential mechanisms of action of anti-PD-1 pathway immune checkpoint blockade and the development of immune-related adverse events.

On antigen-specific signals, immune class regulation and energetics: Report III from the workshops on foundational concepts of immune regulation

This is a report from a one-week workshop held in Athens, Greece in July of 2022. The workshop aimed to identify emerging concepts relevant to the fundamentals of immune regulation and areas for future research. Theories of immune regulation emphasize the role of T cell help or co-stimulation (signal 2). The workshop participants considered how new data on the characteristics of agonist antigens, the role of the antigen receptor signals (signal 1) in driving fate decisions, the effect of energetics on immunity and a better understanding of class-control in the immune response, may impact theories of immune regulation. These ideas were discussed in the context of tumour immunology, autoimmunity, pregnancy and transplantation. Here we present the discussions as a narrative of different viewpoints to allow the reader to join the conversation. These discussions highlight the evolving understanding of the nature of specific antigen recognition and how both antigen-specific and non-specific mechanisms impact immune responses.

Long-Term Survival and Induction of Operational Tolerance to Murine Islet Allografts by Co-Transplanting Cyclosporine A Microparticles and CTLA4-Ig

One strategy to prevent islet rejection is to create a favorable immune-protective local environment at the transplant site. Herein, we utilize localized cyclosporine A (CsA) delivery to islet grafts via poly(lactic-co-glycolic acid) (PLGA) microparticles to attenuate allograft rejection. CsA-eluting PLGA microparticles were prepared using a single emulsion (oil-in-water) solvent evaporation technique. CsA microparticles alone significantly delayed islet allograft rejection compared to islets alone (p < 0.05). Over 50% (6/11) of recipients receiving CsA microparticles and short-term cytotoxic T lymphocyte-associated antigen 4-Ig (CTLA4-Ig) therapy displayed prolonged allograft survival for 214 days, compared to 25% (2/8) receiving CTLA4-Ig alone. CsA microparticles alone and CsA microparticles + CTLA4-Ig islet allografts exhibited reduced T-cell (CD4+ and CD8+ cells, p < 0.001) and macrophage (CD68+ cells, p < 0.001) infiltration compared to islets alone. We observed the reduced mRNA expression of proinflammatory cytokines (IL-6, IL-10, INF-γ, and TNF-α; p < 0.05) and chemokines (CCL2, CCL5, CCL22, and CXCL10; p < 0.05) in CsA microparticles + CTLA4-Ig allografts compared to islets alone. Long-term islet allografts contained insulin+ and intra-graft FoxP3+ T regulatory cells. The rapid rejection of third-party skin grafts (C3H) in islet allograft recipients suggests that CsA microparticles + CTLA4-Ig therapy induced operational tolerance. This study demonstrates that localized CsA drug delivery plus short-course systemic immunosuppression promotes an immune protective transplant niche for allogeneic islets.

Pyruvate metabolism controls chromatin remodeling during CD4+ T cell activation

Upon antigen-specific T cell receptor (TCR) engagement, human CD4+ T cells proliferate and differentiate, a process associated with rapid transcriptional changes and metabolic reprogramming. Here, we show that the generation of extramitochondrial pyruvate is an important step for acetyl-CoA production and subsequent H3K27ac-mediated remodeling of histone acetylation. Histone modification, transcriptomic, and carbon tracing analyses of pyruvate dehydrogenase (PDH)-deficient T cells show PDH-dependent acetyl-CoA generation as a rate-limiting step during T activation. Furthermore, T cell activation results in the nuclear translocation of PDH and its association with both the p300 acetyltransferase and histone H3K27ac. These data support the tight integration of metabolic and histone-modifying enzymes, allowing metabolic reprogramming to fuel CD4+ T cell activation. Targeting this pathway may provide a therapeutic approach to specifically regulate antigen-driven T cell activation.

Endothelial phosphoinositide 3-kinase-β inactivation confers protection from immune-mediated vascular injury

Heart transplant and recipient survival are limited by immune cell-mediated injury of the graft vasculature. We examined the role of the phosphoinositide 3-kinase-β (PI3Kβ) isoform in endothelial cells (EC) during coronary vascular immune injury and repair in mice. In minor histocompatibility-antigen mismatched allogeneic heart grafts, a robust immune response was mounted to each wild-type, PI3Kβ inhibitor-treated, or endothelial-selective PI3Kβ knockout (ECβKO) graft transplanted to wild-type recipients. However, microvascular EC loss and progressive occlusive vasculopathy only developed in control, but not PI3Kβ-inactivated hearts. We observed a delay in inflammatory cell infiltration of the ECβKO grafts, particularly in the coronary arteries. Surprisingly, this was accompanied by an impaired display of proinflammatory chemokine and adhesion molecules by the ECβKO ECs. In vitro, tumor necrosis factor α-stimulated endothelial ICAM1 and VCAM1 expression was blocked by PI3Kβ inhibition or RNA interference. Selective PI3Kβ inhibition also blocked tumor necrosis factor α-stimulated degradation of inhibitor of nuclear factor kappa Bα and nuclear translocation of nuclear factor kappa B p65 in EC. These data identify PI3Kβ as a therapeutic target to reduce vascular inflammation and injury.

CD8 T Cells Target Antigen Cross-Presented by Bone Marrow Derived Cells to Induce Bystander Rejection of Grafts Lacking the Cognate Peptide-MHC

CD8 T cells play a key role in cancer immunotherapy and allograft rejection. However, it is not clear how they kill cells and tissues that do not have the agonist peptide-major histocompatibility complex (MHC) on their surface, as in the settings of MHC class I deficient tumors and indirect rejection of MHC-mismatched transplants. CD8 T cells might respond to agonist antigen cross-presented on hematopoietic cells, leading to a "bystander" rejection. Alternatively, they may recognize agonist antigen cross-presented on recipient endothelial cells and kill the tissue's vital blood supply. The latter mechanism predicts that all non-vascularized grafts, grafts dependent on in-growth of recipient blood vessels, will be susceptible to CD8 T cell mediated indirect rejection. In contrast, we show here that non-vascularized transplants, bearing the same agonist antigen, are not universally susceptible to this rejection pathway. Non-vascularized skin, but not islet or heart tissue transplants were indirectly rejected by CD8 T cells. Furthermore, CD8 T cells were able to indirectly reject skin grafts when recipient MHC class I expression was restricted to bone marrow derived cells but not when it was restricted to radioresistant cells (e.g. endothelial cells). These findings argue against a major role for endothelial cell cross-presentation in killing of tissue that does not present the agonist peptide-MHC class I. Instead, the data suggests that cross-presentation by recipient hematopoietic cells underlies the CD8 T cell mediated killing of tissue that is unable to directly present the target peptide-MHC class I.

Oxidative stress as a candidate mechanism for accelerated neuroectodermal differentiation due to trisomy 21

The ubiquity of cognitive deficits and early onset Alzheimer's disease in Down syndrome (DS) has focused much DS iPSC-based research on neuron degeneration and regeneration. Despite reports of elevated oxidative stress in DS brains, few studies assess the impact of this oxidative burden on iPSC differentiation. Here, we evaluate cellular specific redox differences in DS and euploid iPSCs and neural progenitor cells (NPCs) during critical intermediate stages of differentiation. Despite successful generation of NPCs, our results indicate accelerated neuroectodermal differentiation of DS iPSCs compared to isogenic, euploid controls. Specifically, DS embryoid bodies (EBs) and neural rosettes prematurely develop with distinct morphological differences from controls. Additionally, we observed developmental stage-specific alterations in mitochondrial superoxide production and SOD1/2 abundance, coupled with modulations in thioredoxin, thioredoxin reductase, and peroxiredoxin isoforms. Disruption of intracellular redox state and its associated signaling has the potential to disrupt cellular differentiation and development in DS lending to DS-specific phenotypes.

Regulation of T cell CD5 levels by BTLA

Many coinhibitory receptors are absent from naïve T cells and upregulated upon activation. In contrast, there are a small number of coinhibitory receptors expressed constitutively by naïve T cells, including CD5, BTLA, and VISTA. The relationship between these constitutively expressed coinhibitors is unknown. We examined the relationship between the constitutively expressed BTLA and CD5 in T cell ontogeny. We found an inverse relationship between CD5 and BTLA expression levels, with low BTLA expression in the thymus and higher BTLA expression in the periphery, corresponding with high and low CD5 expression, respectively. To determine if there is a causal relationship between BTLA expression and CD5 expression, we examined CD5 expression in wild type (WT) vs. btla−/− T cells. Interestingly, btla−/− T cells consistently expressed higher levels of CD5 both in thymic and splenic CD4 and CD8 T cells, indicating that BTLA expression directly or indirectly determines the level of CD5 expression broadly across T cells. In contrast, the loss of an inducible coinhibitor, PD-1, only affected CD5 levels on splenic CD8+ T cells. The loss of BTLA expression early in ontogeny might alter the TCR repertoire indirectly affecting CD5 levels, or instead its loss might affect CD5 expression rapidly within mature T cells. To examine the latter, we deleted btla by administering tamoxifen to adult btlafl/fl CreERT2+/− mice and WT CreERT2+/− controls. Loss of btla in adults led only to a transiently heightened CD5 expression. Together our data indicates that loss of BTLA early, but not later in ontogeny, leads to a long-term increase in CD5 expression by T cells. Such calibration of CD5 levels early in ontogeny might serve to reduce autoimmunity.

Funded by the CIHR, NSERC, and an AAI fellowship; btlafl/fl mice provided by the La Jolla Institute for Immunology.

Loss of thymic function promotes EAE relapse in anti-CD52-treated mice

Anti-CD52 treatment creates a long-lasting CD4 T cell lymphopenia and reduces multiple sclerosis (MS) relapses in humans. In contrast, anti-CD52 therapy at disease onset more fully suppresses experimental autoimmune encephalomyelitis (EAE) in mice, and T cell repopulation is rapid. To test whether prolonged T cell lymphopenia promotes relapses, we thymectomized mice prior to EAE induction and anti-CD52 treatment. Thymectomy greatly reduced the number of recent thymic emigrant T cells and was associated with a prolonged reduction in CD4 T cells in peripheral blood. Two-thirds of thymectomized C57BL/6 mice had an EAE relapse post anti-CD52 treatment, while no surgery and sham surgery euthymic controls remained relapse-free. These data demonstrate that thymus function can alter the effectiveness of anti-CD52 treatment.

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Thymectomy significantly reduces the proportion of newly generated T cells.

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Thymectomy predisposes anti-CD52-treated mice to EAE relapse.

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Thymectomy-promoted EAE relapse in anti-CD52 treated mice is associated with weight decline and prolonged T cell lymphopenia.

Metabolic Consequences of IgE- and Non-IgE-Mediated Mast Cell Degranulation

Mast cells are important effector cells in the immune system and undergo activation (i.e., degranulation) by two major mechanisms: IgE-mediated and non-IgE-mediated mechanisms. Although IgE-mediated degranulation is well researched, the cellular mechanisms of non-IgE-mediated mast cell activation are poorly understood despite the potential to induce similar pathophysiological effects. To better understand non-IgE mast cell degranulation, we characterized and compared cellular metabolic shifts across several mechanisms of degranulation (allergen-induced [IgE-mediated], 20 nm of silver nanoparticle-mediated [non-IgE], and compound 48/80-mediated [non-IgE]) in murine bone marrow-derived mast cells. All treatments differentially impacted mitochondrial activity and glucose uptake, suggesting diverging metabolic pathways between IgE- and non-IgE-mediated degranulation. Non-IgE treatments depleted mast cells' glycolytic reserve, and compound 48/80 further inhibited the ability to maximize mitochondrial respiration. This cellular reprogramming may be indicative of a stress response with non-IgE treatments. Neither of these outcomes occurred with IgE-mediated degranulation, hinting at a separate programmed response. Fuel flexibility between the three primary mitochondrial nutrient sources was also eliminated in activated cells and this was most significant in non-IgE-mediated degranulation. Lastly, metabolomics analysis of bone marrow-derived mast cells following degranulation was used to compare general metabolite profiles related to energetic pathways. IgE-mediated degranulation upregulated metabolite concentrations for the TCA cycle and glycolysis compared with other treatments. In conclusion, mast cell metabolism varies significantly between IgE- and non-IgE-mediated degranulation suggesting novel cell regulatory mechanisms are potentially driving unexplored pathways of mast cell degranulation.

Trisomy 21 results in modest impacts on mitochondrial function and central carbon metabolism

Down syndrome (DS) is the most common genetic cause of intellectual disability. Mechanistically, oxidative stress and mitochondrial dysfunction are reported to be etiological factors for many of the DS-related comorbidities and have previously been reported in a number of in vitro and in vivo models of DS. The purpose of this study was to test for the presence of mitochondrial dysfunction in fibroblast cells obtained via skin biopsy from individuals with DS, and to assess the impact of trisomy 21 on central carbon metabolism. Using extracellular flux assays in matched dermal fibroblasts from euploid and DS individuals, we found that basal mitochondrial dysfunction is quite mild. Stressing the cells with a cocktail of mitochondrial stressors revealed a significant mitochondrial deficit in DS cells compared to euploid controls. Evaluation of extracellular acidification rate did not reveal a baseline abnormality in glycolysis; however, metabolomic assessments utilizing isotopically labeled glucose and glutamine revealed altered central carbon metabolism in DS cells. Specifically, we observed greater glucose dependency, uptake and flux into the oxidative phase of the pentose phosphate pathway in DS fibroblasts. Furthermore, using induced pluripotent stem cells (iPSC) we found that mitochondrial function in DS iPSCs was similar to the previously published studies employing fetal cells. Together, these data indicate that aberrant central carbon metabolism is a candidate mechanism for stress-related mitochondrial dysfunction in DS.

BTLA signaling is needed in newly generated T cells to block autoimmunity

Radiotherapy and/or chemotherapy in cancer treatment causes lymphopenia, which favors autoimmunity. Also, coinhibitory receptor blockade triggers immune-related adverse events in more patients than it cures of cancer. Having shown that PD-1 coinhibitory receptor is required in newly generated T cells (NGTC) to prevent lymphopenia-induced autoimmunity, our objective was to determine if the coinhibitory receptor BTLA is also required to establish tolerance in NGTC during lymphopenia. NGTC from thymocytes or established T cells from splenocytes of wildtype (WT) or BTLA−/− B6 mice were adoptively transferred to lymphopenic Rag−/− B6 mice. Sorted WT or BTLA−/− CD8 or CD4 single-positive (SP) thymocytes were adoptively transferred to Rag−/− mice not expressing MHC II (CiiTA−/−) or lacking MHC I (KbDb−/−), respectively. Disease was assessed by macroscopic appearance and weight loss. About 65% (5/8) recipients of BTLA−/− NGTC developed overt autoimmunity, while 4/4 recipients of BTLA−/− splenocytes were disease-free. 10/10 recipients of BTLA−/− CD8 SP T cells showed progressive weight gain. In contrast, 11/11 BTLA−/− CD4 SP T cell recipients had diarrhea, piloerection, hunched appearance, and progressive weight loss from 3–8 weeks post thymocyte transfer (p&lt;0.0001). 3/11 BTLA−/− CD4 SP T cell recipients developed dermatitis post thymocyte transfer. Overt autoimmunity was evident in all WT Rag−/− (n=11) and Rag−/− KbDb−/− (n=4) recipients of thymocytes, but not in any of the CiiTA−/− recipients (n=4). There were no signs of disease in the recipients of WT NGTC. Our study suggests that BTLA is required to establish tolerance in NGTC. Disease in BTLA−/− thymocyte recipients was mediated by the CD4+ T cells and was dependent on MHC II.

CD5 levels reveal distinct basal T-cell receptor signals in T cells from non-obese diabetic mice

Type 1 diabetes in non-obese diabetic (NOD) mice occurs when autoreactive T cells eliminate insulin producing pancreatic β cells. While extensively studied in T-cell receptor (TCR) transgenic mice, the contribution of alterations in thymic selection to the polyclonal T-cell pool in NOD mice is not yet resolved. The magnitude of signals downstream of TCR engagement with self-peptide directs the development of a functional T-cell pool, in part by ensuring tolerance to self. TCR interactions with self-peptide are also necessary for T-cell homeostasis in the peripheral lymphoid organs. To identify differences in TCR signal strength that accompany thymic selection and peripheral T-cell maintenance, we compared CD5 levels, a marker of basal TCR signal strength, on immature and mature T cells from autoimmune diabetes-prone NOD and -resistant B6 mice. The data suggest that there is no preferential selection of NOD thymocytes that perceive stronger TCR signals from self-peptide engagement. Instead, NOD mice have an MHC-dependent increase in CD4⁺ thymocytes and mature T cells that express lower levels of CD5. In contrast, T cell-intrinsic mechanisms lead to higher levels of CD5 on peripheral CD8⁺ T cells from NOD relative to B6 mice, suggesting that peripheral CD8⁺ T cells with higher basal TCR signals may have survival advantages in NOD mice. These differences in the T-cell pool in NOD mice may contribute to the development or progression of autoimmune diabetes.

Trisomy 21 impairs PGE2 production in dermal fibroblasts

The triplication of human chromosome 21 results in Down syndrome (DS), the most common genetic form of intellectual disability. This aneuploid condition also results in an enhanced risk of a spectrum of comorbid conditions, such as leukemia, early onset Alzheimer's disease, and diabetes. Individuals with DS also display an increased incidence of wound healing complications and resistance to solid tumor development. Due to this unique phenotype and the involvement of eicosanoids in key comorbidities like poor healing and tumor development, we hypothesized that cells from DS individuals would display altered eicosanoid production. Using age- and sex-matched dermal fibroblasts we interrogated this hypothesis. Briefly, assessment of over 90 metabolites derived from cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome p450 systems revealed a possible deficiency in the COX system. Basal gene expression and Western blotting experiments showed significantly decreased gene expression of COX1 and 2, and COX2 protein abundance in DS fibroblasts compared to euploid controls. Further, using two different stressors, scratch wound or LPS, we found that DS fibroblasts could not upregulate COX2 abundance and prostaglandin E2 production. Together, these findings show that dermal fibroblasts from DS individuals have a deficient COX2 response, which may contribute to wound healing complications and tumor resistance in DS.

Maneb alters central carbon metabolism and thiol redox status in a toxicant model of Parkinson's disease

The dithiocarbamate fungicide maneb (MB) has attracted interest due to increasing concern of the negative health effects of pesticides, as well as its association with Parkinson's disease (PD). Our laboratory has previously reported distinct phenotypic changes of neuroblastoma cells exposed to acute, sub-toxic levels of MB, including decreased mitochondrial respiration, altered lactate dynamics, and metabolic stress. In this study, we aimed to further define the specific molecular mechanisms of MB toxicity through the comparison of several thiol-containing compounds and their effects on cellular energy metabolism and thiol redox nodes. Extracellular flux analyses and stable isotope labeled tracer metabolomics were employed to evaluate alterations in energy metabolism of SK-N-AS human neuroblastoma cells after acute exposure of an array of compounds, including dithiocarbamates (maneb, nabam, zineb) and other thiol-containing small molecules (glutathione, N-acetylcysteine). These studies revealed MB and its methylated form (MeDTC) as unique toxicants with significant alterations to mitochondrial respiration, proliferation, and glycolysis. We observed MB to significantly impact cellular thiol redox status by oxidizing cellular glutathione and altering the thiol redox status of peroxiredoxin 3 (Prx3, mitochondrial) after acute exposure. Redox Western blotting revealed a MB-specific modification of cellular Prx3, strengthening the argument that MB can preferentially target mitochondrial enzymes containing reactive cysteine thiols. Further, stable isotope tracer metabolomics confirmed our energetics assessments, and demonstrated that MB exposure results in acute derangement of central carbon metabolism. Specifically, we observed shunting of cellular glucose into the pentose-phosphate pathway and reduction of TCA intermediates derived from glucose and glutamine. Also, we report novel lactate utilization for TCA enrichment and glutathione synthesis after MB exposure. In summary, our results further confirm that MB exerts its toxic effects via thiol modification, and significantly transforms central carbon metabolism.

Desensitization using imlifidase and EndoS enables chimerism induction in allosensitized recipient mice

Mixed hematopoietic chimerism induction as a way to foster tolerance to donor organs in recipients who have been sensitized to donor antigens is challenging. Donor-specific antibodies (DSA) are a dominant barrier toward successful donor bone marrow engraftment. Although desensitization methods are routinely used in recipients with allosensitization for allogeneic bone marrow transplantation, engraftment is frequently unsuccessful. To overcome the barrier of prior sensitization we tested enzymatic desensitization of donor-specific IgG using imlifidase and endoglycosidase of Streptococcus pyogenes (EndoS), which both partially block the function of DSA in mice, as a novel approach to improve murine bone marrow engraftment in primed hosts. We found that EndoS was capable of inhibiting antibody-mediated killing of donor cells in vivo. Furthermore, the effect of EndoS depended on the titer of DSA and the genetic background of the recipients. In combination with imlifidase, EndoS improved the survival of donor bone marrow cells. Together with cyclophosphamide, bortezomib, T cell depletion, and nonlethal irradiation, imlifidase in combination with EndoS allowed allogeneic bone marrow engraftment in sensitized recipients. We conclude that enzymatic inactivation of DSA, using the combination of imlifidase and EndoS, can be used for inducing donor hematopoietic chimerism in allosensitized recipient mice in combination with other desensitization strategies.

Apelin directs endothelial cell differentiation and vascular repair following immune-mediated injury

Sustained, indolent immune injury of the vasculature of a heart transplant limits long-term graft and recipient survival. This injury is mitigated by a poorly characterized, maladaptive repair response. Vascular endothelial cells respond to proangiogenic cues in the embryo by differentiation to specialized phenotypes, associated with expression of apelin. In the adult, the role of developmental proangiogenic cues in repair of the established vasculature is largely unknown. We found that human and minor histocompatibility-mismatched donor mouse heart allografts with alloimmune-mediated vasculopathy upregulated expression of apelin in arteries and myocardial microvessels. In vivo, loss of donor heart expression of apelin facilitated graft immune cell infiltration, blunted vascular repair, and worsened occlusive vasculopathy in mice. In vitro, an apelin receptor agonist analog elicited endothelial nitric oxide synthase activation to promote endothelial monolayer wound repair and reduce immune cell adhesion. Thus, apelin acted as an autocrine growth cue to sustain vascular repair and mitigate the effects of immune injury. Treatment with an apelin receptor agonist after vasculopathy was established markedly reduced progression of arterial occlusion in mice. Together, these initial data identify proangiogenic apelin as a key mediator of coronary vascular repair and a pharmacotherapeutic target for immune-mediated injury of the coronary vasculature.

Anti-CD52 blocks EAE independent of PD-1 signals and promotes repopulation dominated by double-negative T cells and newly generated T and B cells

Lymphocyte depletion using anti-CD52 antibody effectively reduces relapses of multiple sclerosis (MS). To begin to understand what mechanisms might control this outcome, we examined the effect of a murine-CD52-specific mAb on the depletion and repopulation of immune cells in mice with experimental autoimmune encephalomyelitis (EAE), a model of MS. We tested whether the tolerance-promoting receptor programmed cell death protein-1 (PD-1) is required for disease remission post anti-CD52, and found that PD-1-deficient mice with a more severe EAE were nevertheless effectively treated with anti-CD52. Anti-CD52 increased the proportions of newly generated T cells and double-negative (DN) T cells while reducing newly generated B cells; the latter effect being associated with a higher expression of CD52 by these cells. In the longer term, anti-CD52 caused substantial increases in the proportion of newly generated lymphocytes and DN T cells in mice with EAE. Thus, the rapid repopulation of lymphocytes from central lymphoid organs post anti-CD52 may limit further disease. Furthermore, these data identify DN T cells, a subset with immunoregulatory potential, as a significant hyperrepopulating subset following CD52-mediated depletion.

Best practices for enhancing surgical research: a perspective from the Canadian Association of Chairs of Surgical Research

Summary

The Canadian Association of Chairs of Surgical Research was created in 2014, with representation from every departmental surgical research committee across Canada, to establish Canadian surgical research as a beacon for health care innovation and to propose solutions for the daily challenges facing surgeon-researchers. Our key mandate has been to identify challenges for surgeons and scientists performing research to prevent further erosion of this vital area of activity that benefits patients, health care service providers and Canadian society. This article outlines the findings of a nationwide survey sent to all members of departments of surgery across Canada, seeking input on current threats and potential solutions. The results suggest that surgical research in Canada is experiencing a decline in funding and an increase in challenges affecting research productivity of academic surgeons, such as pressures to be clinically active, unpredictable surgical schedules, growing administrative demands, and increasing complexity of patient populations. Although surgeons are productive in their research endeavours, institutional changes and sharing of best practices are needed to ensure sustainable growth of research programs.

Potential role of thioredoxin-interacting protein in non-IgE mast cell degranulation

Non-IgE mediated pathways of mast cell degranulation result in similar pathological outcomes as IgE mediated degranulation without prior IgE sensitization thereby making it difficult to identify causative agents and develop therapeutics for patients. We have demonstrated that 20 nm silver nanoparticles (AgNP) induce non-IgE mast cell degranulation through an unknown mechanism. However, we have established that this non-IgE response is strain dependent with C57BL/6 bone marrow derived mast cells (BMMCs) being high responders and LP/J BMMCs being low responders. Further, RNA sequencing of high-responder and low-responder BMMCs demonstrated a significant increase in thioredoxin-interacting protein (txnip) in the low-responders after AgNP mediated degranulation. We explored this protein to determine its regulatory potential in non-IgE mast cell degranulation. At 1hr following exposure to AgNP, txnip mRNA levels increased in LP/J but not C57BL/6 which confirmed RNAseq results. Txnip protein levels were similarly decreased in BMMCs from both strains at 1hr; however, there was an increase in the low-responder BMMCs at six hours post-exposure that did not occur in the high-responder BMMCs. siRNA knockdown of txnip resulted in increased BMMC degranulation by AgNP. Txnip is strongly involved in cellular metabolism. Seahorse XfP results demonstrated a lower energy and glycolytically focused phenotype in low-responders compared to high-responders. C57BL/6 have significantly lowered glycolytic capacity after AgNP-mediated degranulation not present in LP/J. Our data suggests that txnip plays a role in non-IgE mediated mast cell degranulation and may modulate cellular metabolism.

Time-dependent simvastatin administration enhances doxorubicin toxicity in neuroblastoma

Statins have a primary indication for the reduction and management of hypercholesterolemia; however, evidence shows that statins have the ability to increase the toxicity of chemotherapeutics within cancer cells by inducing anti-proliferative, anti-metastatic, and anti-angiogenic effects. More recently, lipophilic statins have shown complex interaction with energy metabolism, specifically acute mitochondrial dysfunction and delayed inhibition of glycolysis. With the goal to demonstrate that statin-mediated enhancement of chemotherapeutics is time-dependent, we hypothesized that the lipophilic statin simvastatin, in conjunction with variable co-exposure of doxorubicin or cisplatin, will enhance the toxicity of these drugs in neuroblastoma. Utilizing human SK-N-AS neuroblastoma cells, we assessed cell proliferation, necrosis, caspase activation, and overall apoptosis of these cells. After determining the toxicity of simvastatin at 48 h post-treatment, 10μM was chosen as the intervention concentration. We found that significant cell death resulted from 1.0μM dose of doxorubicin with 24 h pre-treatment of simvastatin. On the other hand, simvastatin enhancement of cisplatin toxicity was only observed in the co-exposure model. As doxorubicin has strict dosage limits due to its primary off-target toxicity in cardiac muscle, we further compared the effects of this drug combination on rat H9C2 cardiomyoblasts. We found that simvastatin did not enhance doxorubicin toxicity in this cell line. We conclude that simvastatin provides time-dependent sensitization of neuroblastoma cells to doxorubicin toxicity, and our results provide strong argument for the consideration of simvastatin as an adjuvant in doxorubicin-based chemotherapy programs.

Significant association between tumor mutational burden and immune-related adverse events during immune checkpoint inhibition therapies

More than 2000 immuno-oncology agents are being tested or are in use as a result of the cancer immunotherapy revolution. Manipulation of co-inhibitory receptors has achieved tumor eradication in a minority of patients, but widespread immune-related adverse events (irAEs) compromised tolerance to healthy self-tissues in the majority. We have proposed that a major mechanism of irAEs is similar to a graft-versus-malignancy effect of graft-versus-host disease. To verify our hypothesis, we retrieved post-marketing data of adverse events from the U.S. Food and Drug Administration Adverse Event Reporting System. A significant positive correlation was revealed in 7677 patients between the reporting odds ratio of irAEs during immune checkpoint inhibitor therapy and the corresponding tumor mutational burden across 19 cancer types. These results can be interpreted to mean that the ICI drugs unleashed T cells against “altered-self,” self, and tumors resulting in better overall survival.

Simvastatin Induces Delayed Apoptosis Through Disruption of Glycolysis and Mitochondrial Impairment in Neuroblastoma Cells

Simvastatin, a commonly used cholesterol-lowering drug, inhibits the mevalonate pathway involved in the synthesis of the mitochondrial electron carrier coenzyme Q10 (CoQ10), as well as other bioenergetics substrates. The purpose of this study was to investigate simvastatin exposure on mitochondrial respiration, metabolic fuel preferences, and glucose utilization. We hypothesized that simvastatin at a noncytotoxic dose will impair energy metabolism in human neuroblastoma cells. SK-N-AS cells were exposed at acute and chronic time points and evaluated in a Seahorse XF analyzer, revealing decreased mitochondrial and glycolytic parameters. Flow cytometry showed a significant induction of apoptosis in simvastatin-treated cells at 48 hours. Finally, multiple techniques were used to show that simvastatin-mediated impairment of bioenergetics is more complex than CoQ10 depletion or hampered glucose uptake. Therefore, the data reported here represent a biphasic hit to mitochondria followed by reduction in glucose and glutamine metabolism in neuroblastoma; adding mechanism to potential pleotropic effects of statins.

SNARE proteins rescue impaired autophagic flux in Down syndrome

Down syndrome (DS) is a chromosomal disorder caused by trisomy of chromosome 21 (Ts21). Unbalanced karyotypes can lead to dysfunction of the proteostasis network (PN) and disrupted proteostasis is mechanistically associated with multiple DS comorbidities. Autophagy is a critical component of the PN that has not previously been investigated in DS. Based on our previous observations of PN disruption in DS, we investigated possible dysfunction of the autophagic machinery in human DS fibroblasts and other DS cell models. Following induction of autophagy by serum starvation, DS fibroblasts displayed impaired autophagic flux indicated by autophagolysosome accumulation and elevated p62, NBR1, and LC3-II abundance, compared to age- and sex-matched, euploid (CTL) fibroblasts. While lysosomal physiology was unaffected in both groups after serum starvation, we observed decreased basal abundance of the Soluble N-ethylmaleimide-sensitive-factor Attachment protein Receptor (SNARE) family members syntaxin 17 (STX17) and Vesicle Associated Membrane Protein 8 (VAMP8) indicating that decreased autophagic flux in DS is due at least in part to a possible impairment of autophagosome-lysosome fusion. This conclusion was further supported by the observation that over-expression of either STX17 or VAMP8 in DS fibroblasts restored autophagic degradation and reversed p62 accumulation. Collectively, our results indicate that impaired autophagic clearance is a characteristic of DS cells that can be reversed by enhancement of SNARE protein expression and provides further evidence that PN disruption represents a candidate mechanism for multiple aspects of pathogenesis in DS and a possible future target for therapeutic intervention.

Exploiting autoimmunity unleashed by low-dose immune checkpoint blockade to treat advanced cancer

As a result of the cancer immunotherapy revolution, more than 2000 immuno-oncology agents are currently being tested or are in use to improve responses. Not unexpectedly, the 2018 Nobel Prize in Physiology or Medicine was awarded to James P. Allison and Tasuku Honjo for their development of cancer therapy by the blockade of co-inhibitory signals. Unfortunately, manipulation of the co-inhibitory receptors has also resulted in a safety issue: widespread iatrogenic immune-related adverse events (irAEs). Autoimmunity is emerging as the nemesis of immunotherapy. Originally, it was assumed that CTLA-4 blockade selectively targets T cells relevant to the antitumour immune response. However, an uncontrolled pan T cell activation was induced compromising tolerance to healthy self-tissues. The irAEs are very similar to that of a chronic graft-versus-host-disease (GVHD) reaction following allogeneic bone marrow transplantation (BMT). We hypothesized that ipilimumab induced a graft-versus-malignancy (GVM) effect, which eradicated metastatic melanoma in a minority of patients, but also involved an auto-GVHD reaction that resulted in widespread autoimmunity in the majority. Therefore, we argued for a profound theoretical point against the consensus of experts. The task is not to desperately put the genie back in the bottle by immune-suppressive treatments, but instead to harness the autoimmune forces. In this way, the same goal could be achieved by an antibody as by the adoptive transfer of alloreactive donor lymphocytes, but without severe GVHD. The proof-of-principle of a low-dose-combination immune checkpoint therapy, consisting only of approved drugs and treatments, was demonstrated in 111 stage IV cancer patients.

Acute Maneb Exposure Significantly Alters Both Glycolysis and Mitochondrial Function in Neuroblastoma Cells

The pesticides paraquat (PQ) and maneb (MB) have been described as environmental risk factors for Parkinson's disease (PD), with mechanisms associated with mitochondrial dysfunction and reactive oxygen species generation. A combined exposure of PQ and MB in murine models and neuroblastoma cells has been utilized to further advance understanding of the PD phenotype. MB acts as a redox modulator through alkylation of protein thiols and has been previously characterized to inhibit complex III of the electron transport chain and uncouple the mitochondrial proton gradient. The purpose of this study was to analyze ATP-linked respiration and glycolysis in human neuroblastoma cells utilizing the Seahorse extracellular flux platform. Employing an acute, subtoxic exposure of MB, this investigation revealed a MB-mediated decrease in mitochondrial oxygen consumption at baseline and maximal respiration, with inhibition of ATP synthesis and coupling efficiency. Additionally, MB-treated cells showed an increase in nonmitochondrial respiration and proton leak. Further investigation into mitochondrial fuel flex revealed an elimination of fuel flexibility across all 3 major substrates, with a decrease in pyruvate capacity as well as glutamine dependency. Analyses of glycolytic function showed a substantial decrease in glycolytic acidification caused by lactic acid export. This inhibition of glycolytic parameters was also observed after titrating the MB dose as low as 6 μM, and appears to be dependent on the dithiocarbamate functional group, with manganese possibly potentiating the effect. Further studies into cellular ATP and NAD levels revealed a drastic decrease in cells treated with MB. In summary, MB significantly impacted both aerobic and anaerobic energy production; therefore, further characterization of MB's effect on cellular energetics may provide insight into the specificity of PD to dopaminergic neurons.

The case for allele-specific recognition by the TCR

There is a sharp difference in how one views TCR structure-function-behaviour dependent on whether its recognition of major histocompatibility complex-encoded restriction elements (R) is germline selected or somatically generated. The generally accepted or Standard model is built on the assumption that recognition of R is by the V regions of the αβ TCR, which is not driven by allele specificity, whereas the competing model posits that recognition of R is allele-specific. The establishing of allele-specific recognition of R by the TCR would rule out the Standard model and clear the road to a consideration of a competing construct, the Tritope model. Here, the case for allele-specific recognition (germline selected) is detailed making it obvious that the Standard model is untenable.

Comparing mast cell immunometabolism shifts induced by IgE mediated and non-IgE mediated degranulation

Mast cells are immune effector cells with the ability to immediately release an array of pre-formed mediators in response to various stimuli, a process termed degranulation. Degranulation is generally divided into two categories: IgE mediated and non-IgE mediated. Non-IgE mast cell degranulation occurs in response to various environmental agents without prior IgE sensitization often making it difficult to identify the causative agent in patients. To better understand non-IgE mast cell degranulation, we characterized and compared metabolic shifts in response to both mechanisms of degranulation (IgE vs non-IgE) in bone marrow-derived mast cells (BMMCs) grown from C57BL/6 mice. Compound 48/80 and silver nanoparticles were used to trigger non-IgE degranulation. To explore metabolic changes in BMMCs, we used Seahorse XF technology to measure: 1) respiratory mitochondrial metabolism, 2) anaerobic glycolytic metabolism, and 3) performed a phenotype stress test to define metabolic pathways. The mito stress test revealed a decrease in respiration for both mechanisms of degranulation but was consistently lower for non-IgE triggers. Secondly, we observed an increase in glycolysis from both mechanisms which was more prominent in non-IgE degranulation. However, there was a complete depletion of glycolytic reserve with non-IgE degranulation that did not occur with IgE mediated degranulation. The cell phenotype test revealed that BMMCs shift towards glycolysis when activated via a non-IgE pathway more prominently than via an IgE-pathway. In conclusion, mast cell metabolism varies significantly between IgE and non-IgE degranulation with an apparent shift towards glycolytic dependence for non-IgE activated mast cells.

Toxicant-mediated redox control of proteostasis in neurodegeneration

Disruption in redox signaling and control of cellular processes has emerged as a key player in many pathologies including neurodegeneration. As protein aggregations are a common hallmark of several neuronal pathologies, a firm understanding of the interplay between redox signaling, oxidative and free radical stress, and proteinopathies is required to sort out the complex mechanisms in these diseases. Fortunately, models of toxicant-induced neurodegeneration can be utilized to evaluate and report mechanistic alterations in the proteostasis network (PN). The epidemiological links between environmental toxicants and neurological disease gives further credence into characterizing the toxicant-mediated PN disruptions observed in these conditions. Reviewed here are examples of mechanistic interaction between oxidative or free radical stress and PN alterations. Additionally, investigations into toxicant-mediated PN disruptions, specifically focusing on environmental metals and pesticides, are discussed. Finally, we emphasize the need to distinguish whether the presence of protein aggregations are contributory to phenotypes related to neurodegeneration, or if they are a byproduct of PN deficiencies.

Stability of Chimerism in Non-Obese Diabetic Mice Achieved By Rapid T Cell Depletion Is Associated With High Levels of Donor Cells Very Early After Transplant

Stable mixed hematopoietic chimerism is a robust method for inducing donor-specific tolerance with the potential to prevent rejection of donor islets in recipients with autoimmune type-1 diabetes. However, with reduced intensity conditioning, fully allogeneic chimerism in a tolerance resistant autoimmune-prone non-obese diabetic (NOD) recipient has rarely been successful. In this setting, successful multilineage chimerism has required either partial major histocompatability complex matching, mega doses of bone marrow, or conditioning approaches that are not currently clinically feasible. Irradiation free protocols with moderate bone marrow doses have not generated full tolerance; donor skin grafts were rejected. We tested whether more efficient recipient T cell depletion would generate a more robust tolerance. We show that a combination of donor-specific transfusion-cyclophosphamide and multiple T cell depleting antibodies could induce stable high levels of fully allogeneic chimerism in NOD recipients. Less effective T cell depletion was associated with instability of chimerism. Stable chimeras appeared fully donor-specific tolerant, with clonal deletion of allospecific T cells and acceptance of donor skin grafts, while recovering substantial immunocompetence. The loss of chimerism months after transplant was significantly associated with a lower level of chimerism and donor T cells within the first 2 weeks after transplant. Thus, rapid and robust recipient T cell depletion allows for stable high levels of fully allogeneic chimerism and robust donor-specific tolerance in the stringent NOD model while using a clinically feasible protocol. In addition, these findings open the possibility of identifying recipients whose chimerism will later fail, stratifying patients for early intervention.

Prior to Peripheral Tolerance, Newly Generated CD4 T Cells Maintain Dangerous Autoimmune Potential: Fas- and Perforin-Independent Autoimmunity Controlled by Programmed Death-1

Lymphopenia can result from various factors, including viral infections, clinical interventions, or as a normal property of the fetal/neonatal period. T cells in a lymphopenic environment undergo lymphopenia-induced proliferation (LIP) to fill the available “niche” as defined by peptide–MHC and homeostatic cytokine resources. We recently reported systemic autoimmunity following reconstitution of the lymphoid compartment of Rag1^−/− mice with PD-1^−/− hematopoietic stem cells or by transfer of thymocytes, but not splenocytes, suggesting that programmed death-1 (PD-1) plays a crucial role in controlling recent thymic emigrants (RTE) and preventing autoimmunity upon their LIP. However, it is unclear whether RTE residing within the periphery of a lymphoreplete host maintain enhanced autoimmune generating potential or if this property only manifests if RTE experience a lymphopenic periphery immediately after export from the thymus. Furthermore, it is unclear which of a variety of T cell effector mechanisms generate pathology when control of RTE by PD-1 is lacking. Herein, we determined that PD-1 is upregulated on CD4 T cells undergoing the natural LIP characteristic of the neonatal period. Newly generated T cells lacking PD-1 maintained an enhanced autoimmune potential even after residence in a lymphoreplete periphery, emphasizing the importance of PD-1 in the establishment of peripheral tolerance. Neither Fas nor perforin-dependent killing mechanisms were required for autoimmunity, while host MHC-II expression was critical, suggesting that LIP-driven autoimmunity in the absence of PD-1 may primarily result from a CD4 T cell-mediated systemic cytokinemia, a feature potentially shared by other autoimmune or inflammatory syndromes associated with immune reconstitution and LIP.

PD-1 Controls Tonic Signaling and Lymphopenia-Induced Proliferation of T Lymphocytes

Recovery of the T lymphocyte compartment within a lymphopenic host by lymphopenia-induced proliferation (LIP) is regulated by inter- and intraclonal competition for limited resources, including homeostatic cytokines and peptide:MHC (pMHC) complexes with which the TCR can interact at least weakly to yield a tonic signal. Importantly, the process of LIP can synergize with other factors that promote T cell activation to drive inflammatory disease. While reconstitution of the lymphoid compartment of immune deficient Rag^-/- mice by transfer of wild-type hematopoietic stem cells (HSC) does not generally result in an overt disease phenotype, transfer of HSC deficient in expression of the co-inhibitory molecule PD-1 results in severe systemic autoimmunity driven by newly generated T cells that emerge from the thymus into the periphery and undergo LIP. Importantly, autoimmunity does not appear to depend on a response to exogenous (i.e., gut flora-derived) antigens. PD-1 is well known to be upregulated during T cell activation in response to cognate antigens, but it is unclear whether PD-1 has a role in controlling LIP of T cells in the absence of cognate antigen, i.e., in response to tonic pMHC. We examined whether PD-1 controls LIP of newly generated T cells by controlling the response to tonic pMHC or the homeostatic cytokine IL-7. We found that PD-1-deficient T cells have a proliferative advantage over WT T cells during LIP and this effect is MHC-II dependent and independent of IL-7Rα signaling. Furthermore, our data suggest that signals through IL-7Rα can be dispensable for LIP and may instead be of increased importance for T cell survival in conditions of high competition for limited pMHC (e.g., post-LIP, in a lymphoreplete host). We hypothesize that autoimmunity post-PD-1^-/- HSC transplant is the result of an overzealous T cell response to normally tonic self-pMHC precipitated by the synergy of LIP and PD-1 deficiency. Furthermore, potentiation of TCR signals in response to normally tonic self-pMHC may contribute to the success of PD-1 blockade in cancer immunotherapy.

Divide and conquer: Blocking graft versus host but not graft versus leukemia T cells with agonist BTLA co-inhibitory signals

One of the main objectives in allogeneic hematopoietic stem cell transplantation (aHSCT) research is the prevention of graft versus host disease (GVHD) while maintaining the graft versus leukemia/lymphoma (GVL) effect. Whether these two responses generated by donor T cells can be sufficiently separated and controlled remains controversial. While various approaches have been tested to achieve this goal, success has been relatively limited. Lymphocyte responses are negatively regulated by a series of receptors that function along with antigen receptors to deliver co-inhibitory signals. B and T lymphocyte associated (BTLA) is a novel co-inhibitory molecule expressed by activated T cells, B cells and other immune cells. A study by Albring et al. has now shown in a murine model that a single injection of agonistic anti-BTLA monoclonal antibody can inhibit GVHD long-term while maintaining GVL responses and immunity to infection. These studies suggest that future development of biologics to harness the function of co-inhibitory signals will be an important approach in the prevention of autoimmunity and GVHD and in protocols to achieve transplantation tolerance.

Chemokines and Their Receptors in Islet Allograft Rejection and as Targets for Tolerance Induction

Graft rejection is a major barrier to successful outcome of transplantation surgery. Islet transplantation introduces insulin secreting tissue into type 1 diabetes mellitus recipients, relieving patients from exogenous insulin injection. However, insulitis of grafted tissue and allograft rejection prevent long-term insulin independence. Leukocyte trafficking is necessary for the launch of successful immune responses to pathogen or allograft. Chemokines, small chemotactic cytokines, direct the migration of leukocytes through their interaction with chemokine receptors found on cell surfaces of immune cells. Unique receptor expression of leukocytes, and the specificity of chemokine secretion during various states of immune response, suggest that the extracellular chemokine milieu specifically homes certain leukocyte subsets. Thus, only those leukocytes required for the current immune task are attracted to the inflammatory site. Chemokine blockade, using antagonists and monoclonal antibodies directed against chemokine receptors, is an emerging and specific immunosuppressive strategy. Importantly, chemokine blockade may potentiate tolerance induction regimens to be used following transplantation surgery, and prevent the need for life-long immunosuppression of islet transplant recipients. Here, the role for chemokine blockade in islet transplant rejection and tolerance is reviewed.

A Minimum Conditioning Protocol towards Transplantation Tolerance in NOD Mice by Mixed Hematopoietic Chimerism

Stable mixed hematopoietic chimerism is a robust method for inducing donor specific tolerance. However, its clinical application is dampened by the toxicity of current recipient conditioning regimens. We previously showed an irradiation-free mixed chimerism protocol in diabetes prone NOD mice is achievable with antibodies to T cells and CD40L together with busulfan and high dose rapamycin. We sought to generate a more clinically feasible chimerism protocol and tested the hypothesis that more efficient recipient T cell depletion would eliminate the need for anti-CD40L (known to cause thromboembolism in humans) and rapamycin.

We preconditioned NOD mice with donor specific transfusion from fully mismatched mice (d −10), cyclophosphamide (CYP) (d −8), antibodies against CD90 and/or CD4 + CD8 (d −6, −1, 4, 9, 14), busulfan (d −1) and donor bone marrow transplant (d 0). Flow cytometry was used to detect chimerism.

Mixed chimerism was induced in 33/45 NOD mice. Stable chimerism with multilineage donor cells was maintained in 21/33 recipients. Loss of chimerism could be predicted by a lower early level of chimerism at d 4, 9 or 14. Inclusion of αCD90 mAb, busulfan and CYP was critical for chimerism induction. With anti-CD90 17/23 mice became stable chimeras. 5/5 chimeric mice accepted skin from bone marrow donors but rejected skin from MHC-matched and minor antigen mismatched donors. The loss of anti-donor Vβ11+ T cells and anti-recipient Vβ17+ T cells in stable chimeric mice indicated the establishment of chimerism involves clonal deletion.

Conclusion

A protocol causing rapid and robust recipient T cell depletion generated chimerism without the need for anti-CD40L or rapamycin in tolerance induction resistant NOD mice.

Application of central immunologic concepts to cancer: helping T cells and B cells become intolerant of tumors

CD4-mediated T-cell help in the activation of CD8(+) T cells and B cells, through linked-recognition of antigenic determinants, is a long-standing concept foundational to our understanding of immunity (presence of help) versus tolerance (lack of help). Surprisingly, this function of CD4(+) T cells has not been extensively examined as a means to overcome immune tolerance of the self-antigens made by tumor cells. Hesitation to employ this powerful mechanism may be due to the potential to cause unwanted autoimmune pathology. In this issue of the European Journal of Immunology, Snook et al. [Eur. J. Immunol. 2014. 44: 1956-1966] identify a state of split tolerance, showing that CD4(+) T cells specific for a number of tumor-associated self-antigens are robustly tolerant, while their CD8(+) T-cell and B-cell counterparts are far less tolerant. Furthermore, the authors demonstrate that provision of linked foreign helper epitopes, such as influenza hemagglutinin, substantially enhances both CD8(+) T-cell and B-cell responses to tumor self-antigens without causing any overt autoimmune pathology. These findings provide a strong rationale to employ foreign helper epitopes in cancer vaccines and highlight the need to fully explore therapeutic strategies that are based on well-established immunologic concepts.

T cells generated in the absence of a thoracic thymus fail to establish homeostasis

Cervical thymus mimics the thoracic thymus in supporting T-cell development and exists in a subset of mice and humans. Importantly, it remains unknown whether the cervical thymus can generate T cells that are self-tolerant in the complete absence of signals from the thoracic thymus. Using a fetal liver reconstitution model in thoracic thymectomized RAG(-/-) mice, we found that T cells could be generated without contribution from the thoracic thymus. However, these mice had decreased T cells, increased proportions of effector memory T cells and Treg phenotype cells, increased serum IgG1/2b, and increased frequency of T cells expressing IFN-γ, IL-17 or IL-10. Half of the mice that received a thoracic thymectomy and fetal liver cells, unlike sham surgery controls, developed substantial morbidity with age. Disease was associated with lymphopenia-driven activation rather than inherent defects in the cervical thymus, as both thoracic and cervical thymocytes could generate disease in lymphopenic recipients. Administration of the homeostatic cytokine IL-7 caused a rapid, transient increase in T-cell numbers and reduced the time to disease onset. Together the data suggests that the cervical thymus can function in the complete absence of the thoracic thymus; however, the T cells generated do not establish homeostasis.

Control of in vivo collateral damage generated by T cell immunity

An ongoing dilemma faced during an immune response is generating an effective, often proinflammatory response to eliminate pathogens and/or infected cells while also minimizing collateral damage to adjacent noninfected tissues. The factors limiting bystander cell injury during an Ag-specific immune response in vivo are largely unknown. In this study, using an in vivo model of islet transplants in TCR transgenic mice, we show that both CD4 and CD8 T cells do have the capacity to inflict adjacent tissue damage and that this injury is greatly enhanced in sensitized hosts. CD4 T cell-mediated killing of specific and bystander cells occurred via different mechanisms. Unlike specific target cell killing, CD4-mediated bystander injury required tissue Fas expression and was inhibited with anti-IFN-γ Ab treatment in vivo. Moreover, bystander cell injury was not entirely nonspecific but rather required, in naive recipients, that the MHC allele expressed by the bystanders was self. Importantly, the coinhibitor programmed death-1 plays an important role in restraining bystander cell injury mediated either by defined TCR transgenic T cells or by polyclonal T cell populations. Thus, the differential requirements for specific versus bystander cell injury suggest that there are opportunities for inhibiting immune pathology without compromising Ag-specific immunity in vivo.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

Toward minimal conditioning protocols for allogeneic chimerism in tolerance resistant recipients

Mixed chimerism is a promising approach toward generating donor-specific immunological tolerance. However, chimerism induction can be toxic; therefore, there is an effort to develop non-myeloablative, minimal intensity protocols that can generate chimerism without the toxic side effects. Recently, with the goal of creating a minimalistic chimerism induction protocol in the tolerance resistant non-obese diabetic (NOD) mouse model, we identified pre-existing T cells as cells that resist fully allogeneic chimerism. With monoclonals targeting NOD T cells, we showed that long-term chimerism and tolerance toward donor islets could be established. However, this promising new protocol relied on the administration of a single dose of anti-CD40 ligand, which is not clinically applicable. In refining protocols to move even closer to clinical utility, we report here initial success at generating fully allogeneic mixed chimerism in NOD mice by adding cyclophosphamide to the conditioning regimen in place of anti-CD40 ligand antibodies.

Targeting cells causing split tolerance allows fully allogeneic islet survival with minimal conditioning in NOD mixed chimeras

Donor-specific tolerance induced by mixed chimerism is one approach that may eliminate the need for long-term immunosuppressive therapy, while preventing chronic rejection of an islet transplant. However, even in the presence of chimerism it is possible for certain donor tissues or cells to be rejected whereas others from the same donor are accepted (split tolerance). We previously developed a nonmyeloablative protocol that generated mixed chimerism across full major histocompatability complex plus minor mismatches in NOD (nonobese diabetic) mice, however, these chimeras demonstrated split tolerance. In this study, we used radiation chimeras and found that the radiosensitive component of NOD has a greater role in the split tolerance NOD mice develop. We then show that split tolerance is mediated primarily by preexisting NOD lymphocytes and have identified T cells, but not NK cells or B cells, as cells that both resist chimerism induction and mediate split tolerance. Finally, after recognizing the barrier that preexisting T cells impose on the generation of fully tolerant chimeras, the chimerism induction protocol was refined to include nonmyeloablative recipient NOD T cell depletion which generated long-term mixed chimerism across fully allogeneic barriers. Furthermore, these chimeric NOD mice are immunocompetent, diabetes free and accept donor islet allografts.

A nude mouse model of hypertrophic scar shows morphologic and histologic characteristics of human hypertrophic scar

Hypertrophic scar (HSc) is a fibroproliferative disorder that occurs following deep dermal injury. Lack of a relevant animal model is one barrier toward better understanding its pathophysiology. Our objective is to demonstrate that grafting split-thickness human skin onto nude mice results in survival of engrafted human skin and murine scars that are morphologically, histologically, and immunohistochemically consistent with human HSc. Twenty nude mice were xenografted with split-thickness human skin. Animals were euthanized at 30, 60, 120, and 180 days postoperatively. Eighteen controls were autografted with full-thickness nude mouse skin and euthanized at 30 and 60 days postoperatively. Scar biopsies were harvested at each time point. Blinded scar assessment was performed using a modified Manchester Scar Scale. Histologic analysis included hematoxylin and eosin, Masson's trichrome, toluidine blue, and picrosirius red staining. Immunohistochemistry included anti-human human leukocyte antigen-ABC, α-smooth muscle actin, decorin, and biglycan staining. Xenografted mice developed red, shiny, elevated scars similar to human HSc and supported by blinded scar assessment. Autograft controls appeared morphologically and histologically similar to normal skin. Xenografts survived up to 180 days and showed increased thickness, loss of hair follicles, adnexal structures and rete pegs, hypercellularity, whorled collagen fibers parallel to the surface, myofibroblasts, decreased decorin and increased biglycan expression, and increased mast cell density. Grafting split-thickness human skin onto nude mice results in persistent scars that show morphologic, histologic, and immunohistochemical consistency with human HSc. Therefore, this model provides a promising technique to study HSc formation and to test novel treatment options.

Comment on: Ten experiments that would make a difference in understanding immune mechanisms

Melvin Cohn provides a list of experiments to test predictions of his associative antigen recognition (AAR) and Tritope models in relation to standard models of immunity. The manuscript highlights important questions and some decisive experiments testing the competing models. A central aspect of good science is the ability to pinpoint the important questions, something Cohn has, and continues here, to do with great clarity. The problems posed are presented succinctly, although knowledge of Cohn's previous theoretical contributions are needed in some areas to fully understand the path he is taking here. The importance of theory, as championed by Cohn, is a message that needs repeating, as it seems increasingly that immunologists favor description over theory. I briefly comment on each of Cohn's ten experiments and then discuss in detail a critical experiment that is missing from his list-an experiment testing the timing of antigen encounter in ontogeny as a central principle of the self non-self discrimination.