Molecular imaging of innate immune cell function in transplant rejection

The role and research progress of macrophages after heart transplantation

Since the 60s of the 20th century, heart transplantation has been the best treatment for patients with end-stage heart failure. Due to the increasing number of patients, how to expand the number of donor organs and enhance immune compatibility has become an urgent problem to be solved at this stage. Although current immunosuppression is effective, its side effects are also quite obvious, such as opportunistic infections and malignant tumors. In this review, we focus on the important role in macrophages after heart transplantation and their potential targets for achieving allogeneic graft tolerance, in order to improve effective graft survival and reduce infection and the occurrence of malignant tumors.

Single-cell analysis of graft-infiltrating host cells identifies caspase-1 as a potential therapeutic target for heart transplant rejection

Aims

Understanding the cellular mechanisms underlying early allograft rejection is crucial for the development of effective immunosuppressant strategies. This study aims to investigate the cellular composition of graft-infiltrating cells during the early rejection stage at a single-cell level and identify potential therapeutic targets.

Methods

A heterotopic heart transplant model was established using enhanced green fluorescent protein (eGFP)-expressing mice as recipients of allogeneic or syngeneic grafts. At 3 days post-transplant, eGFP-positive cells infiltrating the grafts were sorted and subjected to single-cell RNA-seq analysis. Potential molecular targets were evaluated by assessing graft survival and functions following administration of various pharmacological inhibitors.

Results

A total of 27,053 cells recovered from syngrafts and allografts were classified into 20 clusters based on expression profiles and annotated with a reference dataset. Innate immune cells, including monocytes, macrophages, neutrophils, and dendritic cells, constituted the major infiltrating cell types (>90%) in the grafts. Lymphocytes, fibroblasts, and endothelial cells represented a smaller population. Allografts exhibited significantly increased proportions of monocyte-derived cells involved in antigen processing and presentation, as well as activated lymphocytes, as compared to syngrafts. Differential expression analysis revealed upregulation of interferon activation-related genes in the innate immune cells infiltrating allografts. Pro-inflammatory polarization gene signatures were also enriched in these infiltrating cells of allografts. Gene profiling and intercellular communication analysis identified natural killer cells as the primary source of interferon-γ signaling, activating inflammatory monocytes that displayed strong signals of major histocompatibility complexes and co-stimulatory molecules. The inflammatory response was also associated with promoted T cell proliferation and activation in allografts during the early transplant stages. Notably, caspase-1 exhibited specific upregulation in inflammatory monocytes in response to interferon signaling. The regulon analysis also revealed a significant enrichment of interferon-related motifs within the transcriptional regulatory network of downstream inflammatory genes including caspase-1. Remarkably, pharmacological inhibition of caspase-1 was shown to reduce immune infiltration, prevent acute graft rejection, and improve cardiac contractile function.

Conclusion

The single-cell transcriptional profile highlighted the crucial role of caspase-1 in interferon-mediated inflammatory monocytes infiltrating heart transplants, suggesting its potential as a therapeutic target for attenuating rejection.

Intragraft immune cells: accomplices or antagonists of recipient-derived macrophages in allograft fibrosis?

Organ fibrosis caused by chronic allograft rejection is a major concern in the field of transplantation. Macrophage-to-myofibroblast transition plays a critical role in chronic allograft fibrosis. Adaptive immune cells (such as B and CD4⁺ T cells) and innate immune cells (such as neutrophils and innate lymphoid cells) participate in the occurrence of recipient-derived macrophages transformed to myofibroblasts by secreting cytokines, which eventually leads to fibrosis of the transplanted organ. This review provides an update on the latest progress in understanding the plasticity of recipient-derived macrophages in chronic allograft rejection. We discuss here the immune mechanisms of allograft fibrosis and review the reaction of immune cells in allograft. The interactions between immune cells and the process of myofibroblast formulation are being considered for the potential therapeutic targets of chronic allograft fibrosis. Therefore, research on this topic seems to provide novel clues for developing strategies for preventing and treating allograft fibrosis.

HLA antibody affinity determination: From HLA-specific monoclonal antibodies to donor HLA specific antibodies (DSA) in patient serum

Organs transplanted across donor-specific HLA antibodies (DSA) are associated with a variety of clinical outcomes, including a high risk of acute kidney graft rejection. Unfortunately, the currently available assays to determine DSA characteristics are insufficient to clearly discriminate between potentially harmless and harmful DSA. To further explore the hazard potential of DSA, their concentration and binding strength to their natural target, using soluble HLA, may be informative. There are currently a number of biophysical technologies available that allow the assessment of antibody binding strength. However, these methods require prior knowledge of antibody concentrations. Our objective within this study was to develop a novel approach that combines the determination of DSA-affinity as well as DSA-concentration for patient sample evaluation within one assay. We initially tested the reproducibility of previously reported affinities of human HLA-specific monoclonal antibodies and assessed the technology-specific precision of the obtained results on multiple platforms, including surface plasmon resonance (SPR), bio-layer interferometry (BLI), Luminex (single antigen beads; SAB), and flow-induced dispersion analysis (FIDA). While the first three (solid-phase) technologies revealed comparable high binding-strengths, suggesting measurement of avidity, the latter (in-solution) approach revealed slightly lower binding-strengths, presumably indicating measurement of affinity. We believe that our newly developed in-solution FIDA-assay is particularly suitable to provide useful clinical information by not just measuring DSA-affinities in patient serum samples but simultaneously delivering a particular DSA-concentration. Here, we investigated DSA from 20 pre-transplant patients, all of whom showed negative CDC-crossmatch results with donor cells and SAB signals ranging between 571 and 14899 mean fluorescence intensity (MFI). DSA-concentrations were found in the range between 11.2 and 1223 nM (median 81.1 nM), and their measured affinities fall between 0.055 and 24.7 nM (median 5.34 nM; 449-fold difference). In 13 of 20 sera (65%), DSA accounted for more than 0.1% of total serum antibodies, and 4/20 sera (20%) revealed a proportion of DSA even higher than 1%. To conclude, this study strengthens the presumption that pre-transplant patient DSA consists of various concentrations and different net affinities. Validation of these results in a larger patient cohort with clinical outcomes will be essential in a further step to assess the clinical relevance of DSA-concentration and DSA-affinity.

N-methyl-d-aspartate receptors induce M1 polarization of macrophages: Feasibility of targeted imaging in inflammatory response in vivo

Background

N-methyl-d-aspartate receptors (NMDARs) are considered to be involved in several physiological and pathophysiological processes in addition to the progression of neurological disorders. However, how NMDARs are involved in the glycolytic phenotype of M1 macrophage polarization and the possibility of using them as a bio-imaging probe for macrophage-mediated inflammation remain unclear.

Methods

We analyzed cellular responses to NMDAR antagonism and small interfering RNAs using mouse bone marrow-derived macrophages (BMDMs) treated with lipopolysaccharide (LPS). An NMDAR targeting imaging probe, N-TIP, was produced via the introduction of NMDAR antibody and the infrared fluorescent dye FSD Fluor™ 647. N-TIP binding efficiency was tested in intact and LPS-stimulated BMDMs. N-TIP was intravenously administered to mice with carrageenan (CG)- and LPS-induced paw edema, and in vivo fluorescence imaging was conducted. The anti-inflammatory effects of dexamethasone were evaluated using the N-TIP-mediated macrophage imaging technique.

Results

NMDARs were overexpressed in LPS-treated macrophages, subsequently inducing M1 macrophage polarization. Mechanistically, NMDAR-mediated Ca2+ accumulation resulted in LPS-stimulated glycolysis via upregulation of PI3K/AKT/mTORC1 signaling. In vivo fluorescence imaging with N-TIP showed LPS- and CG-induced inflamed lesions at 5 h post-inflammation, and the inflamed lesions could be detected until 24 h. Furthermore, our N-TIP-mediated macrophage imaging technique helped successfully visualize the anti-inflammatory effects of dexamethasone in mice with inflammation.

Conclusion

This study demonstrates that NMDAR-mediated glycolysis plays a critical role in M1 macrophage-related inflammation. Moreover, our results suggest that NMDAR targeting imaging probe may be useful in research on inflammatory response in vivo.

Training vs. Tolerance: The Yin/Yang of the Innate Immune System

For almost nearly a century, memory functions have been attributed only to acquired immune cells. Lately, this paradigm has been challenged by an increasing number of studies revealing that innate immune cells are capable of exhibiting memory-like features resulting in increased responsiveness to subsequent challenges, a process known as trained immunity (known also as innate memory). In contrast, the refractory state of endotoxin tolerance has been defined as an immunosuppressive state of myeloid cells portrayed by a significant reduction in the inflammatory capacity. Both training as well tolerance as adaptive features are reported to be accompanied by epigenetic and metabolic alterations occurring in cells. While training conveys proper protection against secondary infections, the induction of endotoxin tolerance promotes repairing mechanisms in the cells. Consequently, the inappropriate induction of these adaptive cues may trigger maladaptive effects, promoting an increased susceptibility to secondary infections—tolerance, or contribute to the progression of the inflammatory disorder—trained immunity. This review aims at the discussion of these opposing manners of innate immune and non-immune cells, describing the molecular, metabolic and epigenetic mechanisms involved and interpreting the clinical implications in various inflammatory pathologies.

Macrophages in transplant rejection

Transplant rejection is one of the primary factors leading to loss of allograft function, and macrophages are involved in allograft rejection. Macrophages polarize into different phenotypes according to stimulation by different external factors. Different types of macrophages play distinct roles in inflammation, tumors, and autoimmune diseases and are involved in transplant rejection. In this review, we introduce the origin and migration of macrophages, outline the classification of macrophages and their polarization mechanisms, and review the currently understood mechanisms of their involvement in transplant rejection. Finally, we discuss the regulation of macrophage polarization and miRNA expression with respect to transplant rejection, which is important for the development of new anti-rejection therapies.

Reciprocal organ interactions during heart failure: a position paper from the ESC Working Group on Myocardial Function

Heart failure-either with reduced or preserved ejection fraction (HFrEF/HFpEF)-is a clinical syndrome of multifactorial and gender-dependent aetiology, indicating the insufficiency of the heart to pump blood adequately to maintain blood flow to meet the body's needs. Typical symptoms commonly include shortness of breath, excessive fatigue with impaired exercise capacity, and peripheral oedema, thereby alluding to the fact that heart failure is a syndrome that affects multiple organ systems. Patients suffering from progressed heart failure have a very limited life expectancy, lower than that of numerous cancer types. In this position paper, we provide an overview regarding interactions between the heart and other organ systems, the clinical evidence, underlying mechanisms, potential available or yet-to-establish animal models to study such interactions and finally discuss potential new drug interventions to be developed in the future. Our working group suggests that more experimental research is required to understand the individual molecular mechanisms underlying heart failure and reinforces the urgency for tailored therapeutic interventions that target not only the heart but also other related affected organ systems to effectively treat heart failure as a clinical syndrome that affects and involves multiple organs.

Biomimetic Glucan Particles with Aggregation-Induced Emission Characteristics for Noninvasive Monitoring of Transplant Immune Response

Real-time monitoring of post-transplant immune response is critical to prolong the survival of grafts. The current gold standard for assessing the immune response to graft is biopsy. However, such a method is invasive and prone to false negative results due to limited tissue size available and the heterogeneity of the rejection site. Herein, we report biomimetic glucan particles with aggregation-induced emission (AIE) characteristics (HBTTPEP/GPs) for real-time noninvasive monitoring of post-transplant immune response. We have found that the positively charged near-infrared AIEgens can effectively aggregate in the confined space of glucan particles (GPs), thereby turning on the fluorescence emission. HBTTPEP/GPs can track macrophages for 7 days without hampering the bioactivity. Oral administration of HBTTPEP/GPs can specially target macrophages by mimicking yeast, which then migrate to the transplant rejection site. The fluorescence emitted from HBTTPEP/GPs correlated well with the infiltration of macrophages and the degree of allograft rejection. Furthermore, a single oral HBTTPEP/GPs dose can dynamically evaluate the therapeutic response to immunosuppressive therapy. Consequently, the biomimetic AIE-active glucan particles can be developed as a promising probe for immune-monitoring in solid organ transplantation.

Molecular probes for selective detection of cysteine cathepsins

Cysteine cathepsins are proteases critical in physiopathological processes and show potential as targets or biomarkers for diseases and medical conditions. The 11 members of the cathepsin family are redundant in some cases but remarkably independent of others, demanding the development of both pan-cathepsin targeting tools as well as probes that are selective for specific cathepsins with little off-target activity. This review addresses the diverse design strategies that have been employed to accomplish this tailored selectivity among cysteine cathepsin targets and the imaging modalities incorporated. The power of these diverse tools is contextualized by briefly highlighting the nature of a few prominent cysteine cathepsins, their involvement in select diseases, and the application of cathepsin imaging probes in research spanning basic biochemical studies to clinical applications.

Imaging alloreactive T cells provides early warning of organ transplant rejection

Diagnosis of organ transplant rejection relies upon biopsy approaches to confirm alloreactive T cell infiltration in the graft. Immune molecular monitoring is under investigation to screen for rejection, though these techniques have suffered from low specificity and lack of spatial information. ImmunoPET utilizing antibodies conjugated to radioisotopes has the potential to improve early and accurate detection of graft rejection. ImmunoPET is capable of noninvasively visualizing the dynamic distribution of cells expressing specific immune markers in the entire body over time. In this work, we identify and characterize OX40 as a surrogate biomarker for alloreactive T cells in organ transplant rejection and monitor its expression by utilizing immunoPET. In a dual murine heart transplant model that has both syngeneic and allogeneic hearts engrafted in bilateral ear pinna on the recipients, OX40 immunoPET clearly depicted alloreactive T cells in the allograft and draining lymph node that were not observed in their respective isograft counterparts. OX40 immunoPET signals also reflected the subject’s immunosuppression level with tacrolimus in this study. OX40 immunoPET is a promising approach that may bridge molecular monitoring and morphological assessment for improved transplant rejection diagnosis.

Non-invasive imaging for the diagnosis of acute rejection in transplantation: The next frontier

Acute rejection is a leading cause of organ transplant failure and the most common indication for re-transplantation. Clinically, suspicion of acute rejection is often dependent upon serum laboratory values which may only manifest after organ injury. The gold standard for diagnosis requires an invasive biopsy which can carry serious clinical risks including bleeding and graft loss as well as the possibility of sampling error. The use of noninvasive imaging modalities to monitor transplanted organs is of great clinical value, particularly as a tool for early detection of graft dysfunction or acute rejection. Herein, we provide an overview of the existing literature evaluating noninvasive imaging modalities of solid organ and cellular allografts after transplantation, including both preclinical and clinical studies.

Magnetic resonance imaging of stem cell-macrophage interactions with ferumoxytol and ferumoxytol-derived nanoparticles

"Off the shelf" allogeneic stem cell transplants and stem cell nano-composites are being used for the treatment of degenerative bone diseases. However, major and minor histocompatibility antigens of therapeutic cell transplants can be recognized as foreign and lead to their rejection by the host immune system. If a host immune response is identified within the first week post-transplant, immune modulating therapies could be applied to prevent graft failure and support engraftment. Ferumoxytol (Feraheme™) is an FDA approved iron oxide nanoparticle preparation for the treatment of anemia in patients. Ferumoxytol can be used "off label" as an magnetic resonance (MR) contrast agent, as these nanoparticles provide measurable signal changes on magnetic resonance imaging (MRI). In this focused review article, we will discuss three methods to localize and identify innate immune responses to stem cell transplants using ferumoxytol-enhanced MRI, which are based on tracking stem cells, tracking macrophages or detecting mediators of cell death: (a) monitor MRI signal changes of ferumoxytol-labeled stem cells in the presence or absence of innate immune responses, (b) monitor influx of ferumoxytol-labeled macrophages into stem cell implants, and (c) monitor apoptosis of stem cell implants with caspase-3 activatable nanoparticles. These techniques can detect transplant failure at an early stage, when immune-modulating interventions can potentially preserve the viability of the cell transplants and thereby improve bone and cartilage repair outcomes. Approaches 1 and 2 are immediately translatable to clinical practice. This article is categorized under: Diagnostic Tools > in vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Cells at the Nanoscale Diagnostic Tools > Biosensing.

Molecular Imaging of the Heart

Multimodality cardiovascular imaging is routinely used to assess cardiac function, structure, and physiological parameters to facilitate the diagnosis, characterization, and phenotyping of numerous cardiovascular diseases (CVD), as well as allows for risk stratification and guidance in medical therapy decision-making. Although useful, these imaging strategies are unable to assess the underlying cellular and molecular processes that modulate pathophysiological changes. Over the last decade, there have been great advancements in imaging instrumentation and technology that have been paralleled by breakthroughs in probe development and image analysis. These advancements have been merged with discoveries in cellular/molecular cardiovascular biology to burgeon the field of cardiovascular molecular imaging. Cardiovascular molecular imaging aims to noninvasively detect and characterize underlying disease processes to facilitate early diagnosis, improve prognostication, and guide targeted therapy across the continuum of CVD. The most-widely used approaches for preclinical and clinical molecular imaging include radiotracers that allow for high-sensitivity in vivo detection and quantification of molecular processes with single photon emission computed tomography and positron emission tomography. This review will describe multimodality molecular imaging instrumentation along with established and novel molecular imaging targets and probes. We will highlight how molecular imaging has provided valuable insights in determining the underlying fundamental biology of a wide variety of CVDs, including: myocardial infarction, cardiac arrhythmias, and nonischemic and ischemic heart failure with reduced and preserved ejection fraction. In addition, the potential of molecular imaging to assist in the characterization and risk stratification of systemic diseases, such as amyloidosis and sarcoidosis will be discussed. © 2019 American Physiological Society. Compr Physiol 9:477-533, 2019.

Acute and chronic phagocyte determinants of cardiac allograft vasculopathy

Post-transplant immunosuppression has reduced the incidence of T cell-mediated acute rejection, yet long-term cardiac graft survival rates remain a challenge. An important determinant of chronic solid organ allograft complication is accelerated vascular disease of the transplanted graft. In the case of cardiac allograft vasculopathy (CAV), the precise cellular etiology remains inadequately understood; however, histologic evidence hints at the accumulation and activation of innate phagocytes as a causal contributing factor. This includes monocytes, macrophages, and immature dendritic cell subsets. In addition to crosstalk with adaptive T and B immune cells, myeloid phagocytes secrete paracrine signals that directly activate fibroblasts and vascular smooth muscle cells, both of which contribute to fibrous intimal thickening. Though maladaptive phagocyte functions may promote CAV, directed modulation of myeloid cell function, at the molecular level, holds promise for tolerance and prolonged cardiac graft function.

Application of animal and human PET in cardiac research

PURPOSE OF REVIEW

After a warm-up period of imaging research, several modalities of positron emission tomography (PET) are under development for evaluating ischemic heart disease.

RECENT FINDINGS

Several types of well-documented stem/progenitor PET imaging have been utilized for changes in myocardial blood flow and carbohydrate metabolism. Some recent experimental and human studies reported that these data may have beneficial effects on cardiac research.

SUMMARY

Although the role of PET in the pathology of ischemic heart disease has not been sufficiently elucidated, many studies attempting imaging research of myocardial metabolism and neural regulation have been reported. Further studies are needed to better evaluate the potential of PET in evaluating ischemic heart disease.

Efferocytosis and Outside-In Signaling by Cardiac Phagocytes. Links to Repair, Cellular Programming, and Intercellular Crosstalk in Heart

Phagocytic sensing and engulfment of dying cells and extracellular bodies initiate an intracellular signaling cascade within the phagocyte that can polarize cellular function and promote communication with neighboring non-phagocytes. Accumulating evidence links phagocytic signaling in the heart to cardiac development, adult myocardial homeostasis, and the resolution of cardiac inflammation of infectious, ischemic, and aging-associated etiology. Phagocytic clearance in the heart may be carried out by professional phagocytes, such as macrophages, and non-professional cells, including myofibrolasts and potentially epithelial cells. During cardiac development, phagocytosis initiates growth cues for early cardiac morphogenesis. In diseases of aging, including myocardial infarction, heightened levels of cell death require efficient phagocytic debridement to salvage further loss of terminally differentiated adult cardiomyocytes. Additional risk factors, including insulin resistance and other systemic risk factors, contribute to inefficient phagocytosis, altered phagocytic signaling, and delayed cardiac inflammation resolution. Under such conditions, inflammatory presentation of myocardial antigen may lead to autoimmunity and even possible rejection of transplanted heart allografts. Increased understanding of these basic mechanisms offers therapeutic opportunities.

Molecular Imaging of Acute Cardiac Transplant Rejection: Animal Experiments and Prospects

Acute rejection (AR) remains the biggest challenge during the first year after heart transplantation despite advances in immunosuppressive therapy. The early detection and curbing of AR are crucial to the survival of transplant recipients. However, as the criterion standard for AR, endomyocardial biopsy has several limitations because of its inherent invasiveness and morbidity. Traditional imaging techniques, such as echocardiography and cardiac magnetic resonance imaging, are of certain value for AR, but their diagnostic criteria and accuracy remain in question. Molecular imaging sheds new light on AR diagnosis because it can provide information about gene expression and the location of molecules and cells. This article reviews the latest research and applications of several typical modalities of molecular imaging used in AR and discusses their advantages and disadvantages.

Characterization of Cardiac Dynamics in an Acute Myocardial Infarction Model by Four-Dimensional Optoacoustic and Magnetic Resonance Imaging

Extraction of murine cardiac functional parameters on a beat-by-beat basis is limited with the existing imaging modalities due to insufficient three-dimensional temporal resolution. Faster volumetric imaging methods enabling in vivo characterization of functional parameters are poised to advance cardiovascular research and provide a better understanding of the mechanisms underlying cardiac diseases. We present a new approach based on analyzing contrast-enhanced optoacoustic (OA) images acquired at high volumetric frame rate without using cardiac gating or other approaches for motion correction. We apply an acute murine myocardial infarction model optimized for acquisition of artifact-free optoacoustic imaging data to study cardiovascular hemodynamics. Infarcted hearts (n = 21) could be clearly differentiated from healthy controls (n = 9) based on a significantly higher pulmonary transit time (PTT) (2.25 [2.00-2.41] s versus 1.34 [1.25-1.67] s, p = 0.0235), while no statistically significant difference was observed in the heart rate (318 [252-361] bpm versus 264 [252-320] bpm, p = 0.3129). Nevertheless, nonlinear heartbeat dynamics was stronger in the healthy hearts, as evidenced by the third harmonic component in the heartbeat spectra. MRI data acquired from the same mice further revealed that the PTT increases with the size of infarction and similarly increases with reduced ejection fraction. Moreover, an inverse relationship between infarct PTT and time post-surgery was found, which suggests the occurrence of cardiac healing. In combination with the proven ability of optoacoustics to track targeted probes within the injured myocardium, our method can depict cardiac anatomy, function, and molecular signatures, with both high spatial and temporal resolution. Volumetric four-dimensional optoacoustic characterization of cardiac dynamics with supreme temporal resolution can capture cardiovascular dynamics on a beat-by-beat basis in mouse models of myocardial ischemia.

Myeloid suppressor cells in cancer and autoimmunity

A bottleneck for immunotherapy of cancer is the immunosuppressive microenvironment in which the tumor cells proliferate. Cancers harness the immune regulatory mechanism that prevents autoimmunity from evading immunosurveillance and promoting immune destruction. Regulatory T cells, myeloid suppressor cells, inhibitory cytokines and immune checkpoint receptors are the major components of the immune system acting in concert with cancer cells and causing the subversion of anti-tumor immunity. This redundant immunosuppressive network poses an impediment to efficacious immunotherapy by facilitating tumor progression. Tumor-associated myeloid cells comprise heterogeneous populations acting systemically (myeloid-derived suppressor cells/MDSCs) and/or locally in the tumor microenvironment (MDSCs and tumor-associated macrophages/TAMs). Both populations promote cancer cell proliferation and survival, angiogenesis and lymphangiogenesis and elicit immunosuppression through different pathways, including the expression of immunosuppressive cytokines and checkpoint inhibitors. Several evidences have demonstrated that myeloid cells can express different functional programs in response to different microenvironmental signals, a property defined as functional plasticity. The opposed extremes of this functional flexibility are generally represented by the classical macrophage activation, which identifies inflammatory and cytotoxic M1 polarized macrophages, and the alternative state of macrophage activation, which identifies M2 polarized anti-inflammatory and immunosuppressive macrophages. Functional skewing of myeloid cells occurs in vivo under physiological and pathological conditions, including cancer and autoimmunity. Here we discuss how myeloid suppressor cells can on one hand support tumor growth and, on the other, limit autoimmune responses, indicating that their therapeutic reprogramming can generate opportunities in relieving immunosuppression in the tumor microenvironment or reinstating tolerance in autoimmune conditions.

Towards clinically translatable in vivo nanodiagnostics

Nanodiagnostics as a field makes use of fundamental advances in nanobiotechnology to diagnose, characterize and manage disease at the molecular scale. As these strategies move closer to routine clinical use, a proper understanding of different imaging modalities, relevant biological systems and physical properties governing nanoscale interactions is necessary to rationally engineer next-generation bionanomaterials. In this Review, we analyse the background physics of several clinically relevant imaging modalities and their associated sensitivity and specificity, provide an overview of the materials currently used for in vivo nanodiagnostics, and assess the progress made towards clinical translation. This work provides a framework for understanding both the impressive progress made thus far in the nanodiagnostics field as well as presenting challenges that must be overcome to obtain widespread clinical adoption.

Detection of Stem Cell Transplant Rejection with Ferumoxytol MR Imaging: Correlation of MR Imaging Findings with Those at Intravital Microscopy

Purpose To determine whether endogenous labeling of macrophages with clinically applicable nanoparticles enables noninvasive detection of innate immune responses to stem cell transplants with magnetic resonance (MR) imaging. Materials and Methods Work with human stem cells was approved by the institutional review board and the stem cell research oversight committee, and animal experiments were approved by the administrative panel on laboratory animal care. Nine immunocompetent Sprague-Dawley rats received intravenous injection of ferumoxytol, and 18 Jax C57BL/6-Tg (Csf1r-EGFP-NGFR/FKBP1A/TNFRSF6) 2Bck/J mice received rhodamine-conjugated ferumoxytol. Then, 48 hours later, immune-matched or mismatched stem cells were implanted into osteochondral defects of the knee joints of experimental rats and calvarial defects of Jax mice. All animals underwent serial MR imaging and intravital microscopy (IVM) up to 4 weeks after surgery. Macrophages of Jax C57BL/6-Tg (Csf1r-EGFP-NGFR/FKBP1A/TNFRSF6) 2Bck/J mice express enhanced green fluorescent protein (GFP), which enables in vivo correlation of ferumoxytol enhancement at MR imaging with macrophage quantities at IVM. All quantitative data were compared between experimental groups by using a mixed linear model and t tests. Results Immune-mismatched stem cell implants demonstrated stronger ferumoxytol enhancement than did matched stem cell implants. At 4 weeks, T2 values of mismatched implants were significantly lower than those of matched implants in osteochondral defects of female rats (mean, 10.72 msec for human stem cells and 11.55 msec for male rat stem cells vs 15.45 msec for sex-matched rat stem cells; P = .02 and P = .04, respectively) and calvarial defects of recipient mice (mean, 21.7 msec vs 27.1 msec, respectively; P = .0444). This corresponded to increased recruitment of enhanced GFP- and rhodamine-ferumoxytol-positive macrophages into stem cell transplants, as visualized with IVM and histopathologic examination. Conclusion Endogenous labeling of macrophages with ferumoxytol enables noninvasive detection of innate immune responses to stem cell transplants with MR imaging. ^© RSNA, 2017 Online supplemental material is available for this article.

Harnessing the potential of noninvasive in vivo preclinical imaging of the immune system: challenges and prospects

Preclinical imaging has become a powerful method for investigation of in vivo processes such as pharmacokinetics of therapeutic substances and visualization of physiologic and pathophysiological mechanisms. These are important aspects to understand diseases and develop strategies to modify their progression with pharmacologic interventions. One promising intervention is the application of specifically tailored nanoscale particles that modulate the immune system to generate a tumor targeting immune response. In this complex interaction between immunomodulatory therapies, the immune system and malignant disease, imaging methods are expected to play a key role on the way to generate new therapeutic strategies. Here, we summarize examples which demonstrate the current potential of imaging methods and develop a perspective on the future value of preclinical imaging of the immune system.

Emerging imaging techniques after cardiac transplantation

Improvements in survival after cardiac transplantation have in part been driven by improved graft surveillance. Graft surveillance relies mainly on 3 techniques: coronary angiography, endomyocardial biopsy and echocardiography. Developments in invasive and non-invasive imaging technology have revolutionized assessment of the heart in both health and disease, offering new insights into tissue composition and myocardial metabolism. Herein we aim to review the strengths and weaknesses of these techniques, and summarize the evidence in the following 5 fields of cardiac imaging after transplantation: cardiovascular magnetic resonance; computed tomography; positron emission tomography; single-photon emission computed tomography; and optical coherence tomography and molecular imaging techniques.

Remote magnetic targeting of iron oxide nanoparticles for cardiovascular diagnosis and therapeutic drug delivery: where are we now?

Magnetic resonance imaging (MRI) allows for an accurate assessment of both functional and structural cardiac parameters, and thereby appropriate diagnosis and validation of cardiovascular diseases. The diagnostic yield of cardiovascular MRI examinations is often increased by the use of contrast agents that are almost exclusively based on gadolinium compounds. Another clinically approved contrast medium is composed of superparamagnetic iron oxide nanoparticles (IONs). These particles may expand the field of contrast-enhanced cardiovascular MRI as recently shown in clinical studies focusing on acute myocardial infarction (AMI) and atherosclerosis. Furthermore, IONs open up new research opportunities such as remote magnetic drug targeting (MDT). The approach of MDT relies on the coupling of bioactive molecules and magnetic nanoparticles to form an injectable complex. This complex, in turn, can be attracted to and retained at a desired target inside the body with the help of applied magnetic fields. In comparison to common systemic drug applications, MDT techniques promise both higher concentrations at the target site and lower concentrations elsewhere in the body. Moreover, concurrent or subsequent MRI can be used for noninvasive monitoring of drug distribution and successful delivery to the desired organ in vivo. This review does not only illustrate the basic conceptual and biophysical principles of IONs, but also focuses on new research activities and achievements in the cardiovascular field, mainly in the management of AMI. Based on the presentation of successful MDT applications in preclinical models of AMI, novel approaches and the translational potential of MDT are discussed.

Combinatorial hydrogel library enables identification of materials that mitigate the foreign body response in primates

The foreign body response is an immune-mediated reaction that can lead to the failure of implanted medical devices and discomfort for the recipient. There is a critical need for biomaterials that overcome this key challenge in the development of medical devices. Here we use a combinatorial approach for covalent chemical modification to generate a large library of variants of one of the most widely used hydrogel biomaterials, alginate. We evaluated the materials in vivo and identified three triazole-containing analogs that substantially reduce foreign body reactions in both rodents and, for at least 6 months, in non-human primates. The distribution of the triazole modification creates a unique hydrogel surface that inhibits recognition by macrophages and fibrous deposition. In addition to the utility of the compounds reported here, our approach may enable the discovery of other materials that mitigate the foreign body response.

Thrombospondin-1 levels correlate with macrophage activity and disease progression in dysferlin deficient mice

Dysferlinopathy is associated with accumulation of thrombospondin (TSP)-1 and macrophages, both of which may contribute to the pathogenesis of the disease. The purpose of this study was to determine whether TSP-1 levels can predict macrophage activity and disease progression in dysferlin deficient BlaJ mice, focusing on the early disease process. In 3 month-old BlaJ mice, muscle TSP-1 levels exhibited strong positive correlations with both accumulation of F4/80hi macrophages and with their in vivo phagocytic activity in psoas muscles as measured by magnetic resonance imaging and flow cytometry. Muscle TSP-1 levels also exhibited a strong negative correlation with muscle mass and strong positive correlations with histological measurements of muscle fiber infiltration and regeneration. Over the course of disease progression from 3 to 12 months of age, muscle TSP-1 levels showed more complicated relationships with macrophage activity and an inverse relationship with muscle mass. Importantly, blood TSP-1 levels showed strong correlations with macrophage activity and muscle degeneration, particularly early in disease progression in BlaJ mice. These data indicate that TSP-1 may contribute to a destructive macrophage response in dysferlinopathy and pose the intriguing possibility that TSP-1 levels may serve as a biomarker for disease progression.

The Role of MR Enterography in Assessing Crohn's Disease Activity and Treatment Response

MR enterography (MRE) has become the primary imaging modality in the assessment of Crohn's disease (CD) in both children and adults at many institutions in the United States and worldwide, primarily due to its noninvasiveness, superior soft tissue contrast, and lack of ionizing radiation. MRE technique includes distention of the small bowel with oral contrast media with the acquisition of T2-weighted, balanced steady-state free precession, and multiphase T1-weighted fat suppressed gadolinium contrast-enhanced sequences. With the introduction of molecule-targeted biologic agents into the clinical setting for CD and their potential to reverse the inflammatory process, MRE is increasingly utilized to evaluate disease activity and response to therapy as an imaging complement to clinical indices or optical endoscopy. New and emerging MRE techniques, such as diffusion-weighted imaging (DWI), magnetization transfer, ultrasmall superparamagnetic iron oxide- (USPIO-) enhanced MRI, and PET-MR, offer the potential for an expanded role of MRI in detecting occult disease activity, evaluating early treatment response/resistance, and differentiating inflammatory from fibrotic strictures. Familiarity with MR enterography is essential for radiologists and gastroenterologists as the technique evolves and is further incorporated into the clinical management of CD.

Imaging Granzyme B Activity Assesses Immune-Mediated Myocarditis

RATIONALE

The development of molecular imaging approaches that assess specific immunopathologic mechanisms can advance the study of myocarditis.

OBJECTIVE

This study validates a novel molecular imaging tool that enables the in vivo visualization of granzyme B activity, a major effector of cytotoxic CD8+ T lymphocytes.

METHODS AND RESULTS

We synthesized and optimized a fluorogenic substrate capable of reporting on granzyme B activity and examined its specificity ex vivo in mice hearts with experimental cytotoxic CD8+ T lymphocyte-mediated myocarditis using fluorescence reflectance imaging, validated by histological examination. In vivo experiments localized granzyme B activity in hearts with acute myocarditis monitored by fluorescent molecular tomography in conjunction with coregistered computed tomography imaging. A model anti-inflammatory intervention (dexamethasone administration) in vivo reduced granzyme B activity (vehicle versus dexamethasone: 504±263 versus 194±77 fluorescence intensities in hearts; P=0.002).

CONCLUSIONS

Molecular imaging of granzyme B activity can visualize T cell-mediated myocardial injury and monitor the response to an anti-inflammatory intervention.

Iron-based superparamagnetic nanoparticle contrast agents for MRI of infection and inflammation

OBJECTIVE. In this article, we summarize the progress to date on the use of superparamagnetic iron oxide nanoparticles (SPIONs) as contrast agents for MRI of inflammatory processes. CONCLUSION. Phagocytosis by macrophages of injected SPIONs results in a prolonged shortening of both T2 and T2* leading to hypointensity of macrophage-infiltrated tissues in contrast-enhanced MR images. SPIONs as contrast agents are therefore useful for the in vivo MRI detection of macrophage infiltration, and there is substantial research and clinical interest in the use of SPION-based contrast agents for MRI of infection and inflammation. This technique has been used to identify active infection in patients with septic arthritis and osteomyelitis; importantly, the MRI signal intensity of the tissue has been found to return to its unenhanced value on successful treatment of the infection. In SPION contrast-enhanced MRI of vascular inflammation, animal studies have shown decreased macrophage uptake in atherosclerotic plaques after treatment with statin drugs. Human studies have shown that both coronary and carotid plaques that take up SPIONs are more prone to rupture and that abdominal aneurysms with increased SPION uptake are more likely to grow. Studies of patients with multiple sclerosis suggest that MRI using SPIONs may have increased sensitivity over gadolinium for plaque detection. Finally, SPIONs have enabled the tracking and imaging of transplanted stem cells in a recipient host.

The perfect storm: HLA antibodies, complement, FcγRs, and endothelium in transplant rejection

The pathophysiology of antibody-mediated rejection (AMR) in solid organ transplants is multifaceted and predominantly caused by antibodies directed against polymorphic donor human leukocyte antigens (HLAs). Despite the clearly detrimental impact of HLA antibodies (HLA-Abs) on graft function and survival, the prevention, diagnosis, and treatment of AMR remain a challenge. The histological manifestations of AMR reflect the signatures of HLA-Ab-triggered injury, specifically endothelial changes, recipient leukocytic infiltrate, and complement deposition. We review the interconnected mechanisms of HLA-Ab-mediated injury that might synergize in a 'perfect storm' of inflammation. Characterization of antibody features that are critical for effector functions may help to identify HLA-Abs that are more likely to cause rejection. We also highlight recent advances that may pave the way for new, more effective therapies.

Stem cell imaging: from bench to bedside

Although cellular therapies hold great promise for the treatment of human disease, results from several initial clinical trials have not shown a level of efficacy required for their use as a first line therapy. Here we discuss how in vivo molecular imaging has helped identify barriers to clinical translation and potential strategies that may contribute to successful transplantation and improved outcomes, with a focus on cardiovascular and neurological diseases. We conclude with a perspective on the future role of molecular imaging in defining safety and efficacy for clinical implementation of stem cell therapies.

Superparamagnetic iron oxide-enhanced magnetic resonance for imaging cardiac inflammation. A minireview

BACKGROUND

Advances in nanotechnology have lead to the development of a novel contrast media for Magnetic Resonance Imaging (MRI) - the superparamagnetic iron oxide nanoparticle (SPIO). SPIO nanoparticles are used to image inflammation on the cellular level in various settings. This review covers the physicochemical characteristics of SPIO particles as well as relevant animal and clinical studies and discusses the potential of SPIO particles to image cardiac inflammation including cardiac graft rejection.

METHODS

We searched the scientific biomedical databases Medline/PubMed, BioMedCentral, Google Scholar, Ovid and, ProQuest from to 2000 to 2013 for publications relevant to the topic.

CONCLUSIONS

SPIO nanoparticles due to their unique properties could become a useful tool in imaging cardiac inflammation. However, the task is to find a suitable particle size and coating with corresponding pharmacokinetics, establish the right dose and MRI scan timing for individual applications.

Imaging macrophages with nanoparticles

Nanomaterials have much to offer, not only in deciphering innate immune cell biology and tracking cells, but also in advancing personalized clinical care by providing diagnostic and prognostic information, quantifying treatment efficacy and designing better therapeutics. This Review presents different types of nanomaterial, their biological properties and their applications for imaging macrophages in human diseases, including cancer, atherosclerosis, myocardial infarction, aortic aneurysm, diabetes and other conditions. We anticipate that future needs will include the development of nanomaterials that are specific for immune cell subsets and can be used as imaging surrogates for nanotherapeutics. New in vivo imaging clinical tools for noninvasive macrophage quantification are thus ultimately expected to become relevant to predicting patients' clinical outcome, defining treatment options and monitoring responses to therapy.

Cardioprotective C-kit⁺ bone marrow cells attenuate apoptosis after acute myocardial infarction in mice - in-vivo assessment with fluorescence molecular imaging

Cardiomyocyte loss via apoptosis plays a crucial role in ventricular remodeling following myocardial infarction (MI). Cell-based therapy approaches using bone marrow derived c-kit⁺ pluripotent cells may attenuate apoptosis following ischemic injury. We therefore thought to examine the early course of apoptosis following myocardial infarction - in-vivo - and non-invasively determine the effect of c-kit⁺ bone marrow cells on post-MI remodeling. We studied apoptosis in wild-type Kit(+/+) , c-kit mutant Kit(W)/Kit(W-v) and Kit(W)/Kit(W-v) mice after cell therapy with bone-marrow derived c-kit⁺ cells after ischemia-reperfusion injury. Mice were followed by hybrid Fluorescence Molecular Tomography/X-ray Computed Tomography (FMT-XCT) at 6h, 24h and 7 days after ischemia-reperfusion injury using an Annexin V-based fluorescent nanosensor targeting phosphatidylserine. Kit(W)/Kit(W-v) mice showed increased and prolonged apoptosis compared to control Kit(+/+) mice while c-kit cell therapy was able to attenuate the altered apoptosis rates. Increased apoptosis was accompanied by severe decline in heart function, determined by cardiac Magnetic Resonance Imaging, and cell therapy was able to rescue the animals from deleterious heart failure. Post-mortem cryoslicing and immunohistochemistry localized the fluorescence signal of the Annexin V sensor within the infarcted myocardium. Flow cytometry of digested infarct specimens identified apoptotic cardiomyocytes as the major source for the in-vivo Annexin V signal. In-vivo molecular imaging using hybrid FMT-XCT reveals increased cardiomyocyte apoptosis in Kit(W)/Kit(W-v) mice and shows that c-kit⁺ cardioprotective cells are able to attenuate post-MI apoptosis and rescue mice from progressive heart failure.

Chondrogenic differentiation increases antidonor immune response to allogeneic mesenchymal stem cell transplantation

Allogeneic mesenchymal stem cells (allo-MSCs) have potent regenerative and immunosuppressive potential and are being investigated as a therapy for osteoarthritis; however, little is known about the immunological changes that occur in allo-MSCs after ex vivo induced or in vivo differentiation. Three-dimensional chondrogenic differentiation was induced in an alginate matrix, which served to immobilize and potentially protect MSCs at the site of implantation. We show that allogeneic differentiated MSCs lost the ability to inhibit T-cell proliferation in vitro, in association with reduced nitric oxide and prostaglandin E2 secretion. Differentiation altered immunogenicity as evidenced by induced proliferation of allogeneic T cells and increased susceptibility to cytotoxic lysis by allo-specific T cells. Undifferentiated or differentiated allo-MSCs were implanted subcutaneously, with and without alginate encapsulation. Increased CD3(+) and CD68(+) infiltration was evident in differentiated and splenocyte encapsulated implants only. Without encapsulation, increased local memory T-cell responses were detectable in recipients of undifferentiated and differentiated MSCs; however, only differentiated MSCs induced systemic memory T-cell responses. In recipients of encapsulated allogeneic cells, only differentiated allo-MSCs induced memory T-cell responses locally and systemically. Systemic alloimmune responses to differentiated MSCs indicate immunogenicity regardless of alginate encapsulation and may require immunosuppressive therapy for therapeutic use.

Molecular imaging of inflammation in atherosclerosis

Acute rupture of vulnerable plaques frequently leads to myocardial infarction and stroke. Within the last decades, several cellular and molecular players have been identified that promote atherosclerotic lesion formation, maturation and plaque rupture. It is now widely recognized that inflammation of the vessel wall and distinct leukocyte subsets are involved throughout all phases of atherosclerotic lesion development. The mechanisms that render a stable plaque unstable and prone to rupture, however, remain unknown and the identification of the vulnerable plaque remains a major challenge in cardiovascular medicine. Imaging technologies used in the clinic offer minimal information about the underlying biology and potential risk for rupture. New imaging technologies are therefore being developed, and in the preclinical setting have enabled new and dynamic insights into the vessel wall for a better understanding of this complex disease. Molecular imaging has the potential to track biological processes, such as the activity of cellular and molecular biomarkers in vivo and over time. Similarly, novel imaging technologies specifically detect effects of therapies that aim to stabilize vulnerable plaques and silence vascular inflammation. Here we will review the potential of established and new molecular imaging technologies in the setting of atherosclerosis, and discuss the cumbersome steps required for translating molecular imaging approaches into the clinic.

Multimodal iron oxide nanoparticles for hybrid biomedical imaging

Iron oxide core nanoparticles are attractive imaging agents because their material properties allow the tuning of pharmacokinetics as well as the attachment of multiple moieties to their surface. In addition to affinity ligands, these include fluorochromes and radioisotopes for detection with optical and nuclear imaging. As the iron oxide core can be detected by MRI, options for combining imaging modalities are manifold. Already, preclinical imaging strategies have combined noninvasive imaging with higher resolution techniques, such as intravital microscopy, to gain unprecedented insight into steady-state biology and disease. Going forward, hybrid iron oxide nanoparticles will help to merge modalities, creating a synergy that will enable imaging in basic research and, potentially, also in the clinic.

Nanoparticle PET-CT detects rejection and immunomodulation in cardiac allografts

BACKGROUND

Macrophages predominate among the inflammatory cells in rejecting allografts. These innate immune cells, in addition to allospecific T cells, can damage cardiomyocytes directly.

METHODS AND RESULTS

We explored whether sensitive positron emission tomography-computed tomography (PET-CT) imaging of macrophages-avid nanoparticles detects rejection of heart allografts in mice. In addition, we used the imaging method to follow the immunomodulatory impact of angiotensin-converting enzyme inhibitor therapy on myeloid cells in allografts. Dextran nanoparticles were derivatized with the PET isotope copper-64 and imaged 7 days after transplantation. C57BL/6 recipients of BALB/c allografts displayed robust positron emission tomography signal (standard uptake value allograft, 2.8±0.3; isograft control, 1.7±0.2; P<0.05). Autoradiography and scintillation counting confirmed the in vivo findings. We then imaged the effects of angiotensin-converting enzyme inhibitor (5 mg/kg enalapril). Angiotensin-converting enzyme inhibitor significantly decreased nanoparticle signal (P<0.05). Histology and flow cytometry showed a reduced number of myeloid cells in the graft, blood, and lymph nodes and diminished antigen presentation (P<0.05 versus untreated allografts). Angiotensin-converting enzyme inhibitor also significantly prolonged allograft survival (12 versus 7 days; P<0.0001).

CONCLUSIONS

Nanoparticle macrophage PET-CT detects heart transplant rejection and predicts organ survival by reporting on myeloid cells.

Use of optical imaging to progress novel therapeutics to the clinic

There is an undisputed need for employment and improvement of robust technology for real-time analyses of therapeutic delivery and responses in clinical translation of gene and cell therapies. Over the past decade, optical imaging has become the in vivo imaging modality of choice for many preclinical laboratories due to its efficiency, practicality and affordability, while more recently, the clinical potential for this technology is becoming apparent. This review provides an update on the current state of the art in in vivo optical imaging and discusses this rapidly improving technology in the context of it representing a translation enabler or indeed a future clinical imaging modality in its own right.

Development of antibody-tagged nanoparticles for detection of transplant rejection using biomagnetic sensors

Organ transplantation is a life-saving procedure and the preferred method of treatment for a growing number of disease states. The advent of new immunosuppressants and improved care has led to great advances in both patient and graft survival. However, acute T-cell-mediated graft rejection occurs in a significant quantity of recipients and remains a life-threatening condition. Acute rejection is associated with decrease in long-term graft survival, demonstrating a need to carefully monitor transplant patients. Current diagnostic criteria for transplant rejection rely on invasive tissue biopsies or relatively nonspecific clinical features. A noninvasive way is needed to detect, localize, and monitor transplant rejection. Capitalizing on advances in targeted contrast agents and magnetic-based detection technology, we developed anti-CD3 antibody-tagged nanoparticles. T cells were found to bind preferentially to antibody-tagged nanoparticles, as identified through light microscopy, transmission electron microscopy, and confocal microscopy. Using mouse skin graft models, we were also able to demonstrate in vivo vascular delivery of T-cell targeted nanoparticles. We conclude that targeting lymphocytes with magnetic nanoparticles is conducive to developing a novel, noninvasive strategy for identifying transplant rejection.

Potential role of γδ T cell-derived IL-17 in acute cardiac allograft rejection

BACKGROUND

Although αβ T cells are known to participate in the development of acute cardiac allograft rejection, the role of γδ T cells remains poorly understood. We hypothesized that γδ T cells contribute to acute allograft rejection thru interleukin (IL)-17 production.

METHODS

Donor hearts from FVB mice (H-2q) were heterotopically transplanted into C57BL/6-wild type (WT) and γδ T cell-deficient (TCRδ-/-) recipient mice (H-2b). Overall graft survival was monitored. Graft infiltrating cell profile, including γδ T cell subtype, cytokine expression, and myeloperoxidase activity were measured by flow cytometry, TaqMan (Applied Biosystems, Carlsbad, CA) polymerase chain reaction, and myeloperoxidase assay, respectively, on postoperative days 3 and 6.

RESULTS

Graft survival was prolonged in TCRδ-/- recipients compared with WT controls. Graft infiltrating cells, including CD45+, CD4+, CD8+, and Gr1+ cells were significantly decreased in TCRδ-/- recipients compared with WT. Donor hearts transplanted into TCRδ-/- recipients had reduced IL-17 and IL-6 messenger RNA expression. Corroborating the gene expression, intracellular cytokine staining showed decreased IL-17 producing cells in TCRδ-/- recipients. Finally, Vγ1+ and Vγ4+ T cells did not produce IL-17, although both represent 20% to 30% total graft infiltrating γδ T cells.

CONCLUSIONS

The γδ T cells promote acute cardiac allograft rejection, presumably by producing IL-17. The γδ T cell depletion may prove beneficial in prolonging allograft survival by suppressing IL-17 production.

Genetic or pharmaceutical blockade of phosphoinositide 3-kinase p110δ prevents chronic rejection of heart allografts

Chronic rejection is the major cause of long-term heart allograft failure, characterized by tissue infiltration by recipient T cells with indirect allospecificity. Phosphoinositol-3-kinase p110δ is a key mediator of T cell receptor signaling, regulating both T cell activation and migration of primed T cells to non-lymphoid antigen-rich tissue. We investigated the effect of genetic or pharmacologic inactivation of PI3K p110δ on the development of chronic allograft rejection in a murine model in which HY-mismatched male hearts were transplanted into female recipients. We show that suppression of p110δ activity significantly attenuates the development of chronic rejection of heart grafts in the absence of any additional immunosuppressive treatment by impairing the localization of antigen-specific T cells to the grafts, while not inducing specific T cell tolerance. p110δ pharmacologic inactivation is effective when initiated after transplantation. Targeting p110δ activity might be a viable strategy for the treatment of heart chronic rejection in humans.

Functional imaging of proteases: recent advances in the design and application of substrate-based and activity-based probes

Proteases are enzymes that cleave peptide bonds in protein substrates. This process can be important for regulated turnover of a target protein but it can also produce protein fragments that then perform other functions. Because the last few decades of protease research have confirmed that proteolysis is an essential regulatory process in both normal physiology and in multiple disease-associated conditions, there has been an increasing interest in developing methods to image protease activity. Proteases are also considered to be one of the few 'druggable' classes of proteins and therefore a large number of small molecule based inhibitors of proteases have been reported. These compounds serve as a starting point for the design of probes that can be used to target active proteases for imaging applications. Currently, several classes of fluorescent probes have been developed to visualize protease activity in live cells and even whole organisms. The two primary classes of protease probes make use of either peptide/protein substrates or covalent inhibitors that produce a fluorescent signal when bound to an active protease target. This review outlines some of the most recent advances in the design of imaging probes for proteases. In particular, it highlights the strengths and weaknesses of both substrate-based and activity-based probes and their applications for imaging cysteine proteases that are important biomarkers for multiple human diseases.

Activatable Optical Probes for the Detection of Enzymes

The early detection of many human diseases is crucial if they are to be treated successfully. Therefore, the development of imaging techniques that can facilitate early detection of disease is of high importance. Changes in the levels of enzyme expression are known to occur in many diseases, making their accurate detection at low concentrations an area of considerable active research. Activatable fluorescent probes show immense promise in this area. If properly designed they should exhibit no signal until they interact with their target enzyme, reducing the level of background fluorescence and potentially endowing them with greater sensitivity. The mechanisms of fluorescence changes in activatable probes vary. This review aims to survey the field of activatable probes, focusing on their mechanisms of action as well as illustrating some of the in vitro and in vivo settings in which they have been employed.