Rainer H. Kohler

Identification of Splenic Reservoir Monocytes and Their Deployment to Inflammatory Sites

Monitoring Monocyte Reservoirs Monocytes are cells of the immune system that are recruited to sites of tissue injury and inflammation where they help to resolve the infection and are important for tissue repair. The bone marrow and blood are believed to be the primary reservoirs from which monocytes are mobilized after injury. Swirski et al. (p. 612; see the Perspective by Jia and Pamer) now demonstrate that the spleen also serves as a critical reservoir of monocytes that are recruited during ischemic myocardial injury. Monocytes in the spleen are very similar in phenotype to blood-derived monocytes and are mobilized to the injured heart, where they represent a large fraction of the total monocytes that are recruited. The chemoattractant, angiotensin II, is required for optimal monocyte mobilization from the spleen and emigration into injured tissue. A rapid deployment force of immune cells is identified in the spleen that is important for resolving inflammation. A current paradigm states that monocytes circulate freely and patrol blood vessels but differentiate irreversibly into dendritic cells (DCs) or macrophages upon tissue entry. Here we show that bona fide undifferentiated monocytes reside in the spleen and outnumber their equivalents in circulation. The reservoir monocytes assemble in clusters in the cords of the subcapsular red pulp and are distinct from macrophages and DCs. In response to ischemic myocardial injury, splenic monocytes increase their motility, exit the spleen en masse, accumulate in injured tissue, and participate in wound healing. These observations uncover a role for the spleen as a site for storage and rapid deployment of monocytes and identify splenic monocytes as a resource that the body exploits to regulate inflammation.

Osteogenesis Associates With Inflammation in Early-Stage Atherosclerosis Evaluated by Molecular Imaging In Vivo

Background— Arterial calcification is associated with cardiovascular events; however, mechanisms of calcification in atherosclerosis remain obscure. Methods and Results— We tested the hypothesis that inflammation promotes osteogenesis in atherosclerotic plaques using in vivo molecular imaging in apolipoprotein E−/− mice (20 to 30 weeks old, n=35). A bisphosphonate-derivatized near-infrared fluorescent imaging agent (excitation 750 nm) visualized osteogenic activity that was otherwise undetectable by x-ray computed tomography. Flow cytometry validated the target specifically in osteoblast-like cells. A spectrally distinct near-infrared fluorescent nanoparticle (excitation 680 nm) was coinjected to simultaneously image macrophages. Fluorescence reflectance mapping demonstrated an association between osteogenic activity and macrophages in aortas of apolipoprotein E−/− mice (R2=0.93). Intravital dual-channel fluorescence microscopy was used to further monitor osteogenic changes in inflamed carotid arteries at 20 and 30 weeks of age and revealed that macrophage burden and osteogenesis concomitantly increased during plaque progression (P<0.01 and P<0.001, respectively) and decreased after statin treatment (P<0.0001 and P<0.05, respectively). Fluorescence microscopy on cryosections colocalized near-infrared fluorescent osteogenic signals with alkaline phosphatase activity, bone-regulating protein expression, and hydroxyapatite nanocrystals as detected by electron microscopy, whereas von Kossa and alizarin red stains showed no evidence of calcification. Real-time reverse-transcription polymerase chain reaction revealed that macrophage-conditioned media increased alkaline phosphatase mRNA expression in vascular smooth muscle cells. Conclusions— This serial in vivo study demonstrates the real-time association of macrophage burden with osteogenic activity in early-stage atherosclerosis and offers a cellular-resolution tool to identify preclinical microcalcifications.

Arterial and Aortic Valve Calcification Abolished by Elastolytic Cathepsin S Deficiency in Chronic Renal Disease

Background— Clinical studies have demonstrated that 50% of individuals with chronic renal disease (CRD) die of cardiovascular causes, including advanced calcific arterial and valvular disease; however, the mechanisms of accelerated calcification in CRD remain obscure, and no therapies can prevent disease progression. We recently demonstrated in vivo that inflammation triggers cardiovascular calcification. In vitro evidence also indicates that elastin degradation products may promote osteogenesis. Here, we used genetically modified mice and molecular imaging to test the hypothesis in vivo that cathepsin S (catS), a potent elastolytic proteinase, accelerates calcification in atherosclerotic mice with CRD induced by 5/6 nephrectomy. Methods and Results— Apolipoprotein-deficient (apoE−/−)/catS+/+ (n=24) and apoE−/−/catS−/− (n=24) mice were assigned to CRD and control groups. CRD mice had significantly higher serum phosphate, creatinine, and cystatin C levels than those without CRD. To visualize catS activity and osteogenesis in vivo, we coadministered catS-activatable and calcification-targeted molecular imaging agents 10 weeks after nephrectomy. Imaging coregistered increased catS and osteogenic activities in the CRD apoE−/−/catS+/+ cohort, whereas CRD apoE−/−/catS−/− mice exhibited less calcification. Quantitative histology demonstrated greater catS-associated elastin fragmentation and calcification in CRD apoE−/−/catS+/+ than CRD apoE−/−/catS−/− aortas and aortic valves. Notably, catS deletion did not cause compensatory increases in RNA levels of other elastolytic cathepsins or matrix metalloproteinases. Elastin peptide and recombinant catS significantly increased calcification in smooth muscle cells in vitro, a process further amplified in phosphate-enriched culture medium. Conclusions— The present study provides direct in vivo evidence that catS-induced elastolysis accelerates arterial and aortic valve calcification in CRD, providing new insight into the pathophysiology of cardiovascular calcification.

Optical Visualization of Cathepsin K Activity in Atherosclerosis With a Novel, Protease-Activatable Fluorescence Sensor

Background— Cathepsin K (CatK), a potent elastinolytic and collagenolytic cysteine protease, likely participates in the evolution and destabilization of atherosclerotic plaques. To assess better the biology of CatK activity in vivo, we developed a novel near-infrared fluorescence (NIRF) probe for imaging of CatK and evaluated it in mouse and human atherosclerosis. Methods and Results— The NIRF imaging agent consists of the CatK peptide substrate GHPGGPQGKC-NH2 linked to an activatable fluorogenic polymer. In vitro, CatK produced a 2- to 14-fold activation of the agent over other cysteine and matrix metalloproteinases (P<0.0001), as well as a >8-fold activation over a control imaging agent (P<0.001). Optical imaging of atheroma revealed >100% NIRF signal increases in apolipoprotein E−/− mice in vivo (n=13; P<0.05, CatK imaging agent versus control agent) and in human carotid endarterectomy specimens ex vivo (n=14; P<0.05). Fluorescence microscopy of plaque sections demonstrated that enzymatically active CatK (positive NIRF signal) localized primarily in the vicinity of CatK-positive macrophages. Augmented NIRF signal (reflecting CatK activity) colocalized with disrupted elastin fibers within the media underlying plaques. Conclusions— Use of this novel protease-activatable NIRF agent for optical imaging in vivo demonstrated preferential localization of enzymatically active CatK to macrophages, consistent with their known greater elastinolytic capabilities compared with smooth muscle cells. Augmented CatK proteolysis in atheromata further links CatK to vascular remodeling and plaque vulnerability.

Arterial and aortic valve calcification inversely correlates with osteoporotic bone remodelling: a role for inflammation.

Aims Westernized countries face a growing burden of cardiovascular calcification and osteoporosis. Despite its vast clinical significance, the precise nature of this reciprocal relationship remains obscure. We hypothesize that cardiovascular calcification progresses with inflammation and inversely correlates with bone tissue mineral density (TMD). Methods and results Arterial, valvular, and bone metabolism were visualized using near-infrared fluorescence (NIRF) molecular imaging agents, targeting macrophages and osteogenesis. We detected significant arterial and aortic valve calcification in apoE−/− mice with or without chronic renal disease (CRD, 30 weeks old; n = 28), correlating with the severity of atherosclerosis. We demonstrated decreases in osteogenic activity in the femurs of apoE−/− mice when compared with WT mice, which was further reduced with CRD. Three-dimensional micro-computed tomography imaging of the cortical and cancellous regions of femurs quantified structural remodelling and reductions in TMD in apoE−/− and CRD apoE−/− mice. We established significant correlations between arterial and valvular calcification and loss of TMD (R2 = 0.67 and 0.71, respectively). Finally, we performed macrophage-targeted molecular imaging to explore a link between inflammation and osteoporosis in vivo. Although macrophage burden, visualized as uptake of NIRF-conjugated iron nanoparticles, was directly related to the degree of arterial and valvular inflammation and calcification, the same method inversely correlated inflammation with TMD (R2 = 0.73; 0.83; 0.75, respectively). Conclusion This study provides direct in vivo evidence that in arteries and aortic valves, macrophage burden and calcification associate with each other, whereas inflammation inversely correlates with bone mineralization. Thus, understanding inflammatory signalling mechanisms may offer insight into selective abrogation of divergent calcific phenomena.

Analysis of Mitosis and Antimitotic Drug Responses in Tumors by In Vivo Microscopy and Single-Cell Pharmacodynamics

Cancer relies upon frequent or abnormal cell division, but how the tumor microenvironment affects mitotic processes in vivo remains unclear, largely due to the technical challenges of optical access, spatial resolution, and motion. We developed high-resolution in vivo microscopy methods to visualize mitosis in a murine xenograft model of human cancer. Using these methods, we determined whether the single-cell response to the antimitotic drug paclitaxel (Ptx) was the same in tumors as in cell culture, observed the impact of Ptx on the tumor response as a whole, and evaluated the single-cell pharmacodynamics (PD) of Ptx (by in vivo PD microscopy). Mitotic initiation was generally less frequent in tumors than in cell culture, but subsequently it proceeded normally. Ptx treatment caused spindle assembly defects and mitotic arrest, followed by slippage from mitotic arrest, multinucleation, and apoptosis. Compared with cell culture, the peak mitotic index in tumors exposed to Ptx was lower and the tumor cells survived longer after mitotic arrest, becoming multinucleated rather than dying directly from mitotic arrest. Thus, the tumor microenvironment was much less proapoptotic than cell culture. The morphologies associated with mitotic arrest were dose and time dependent, thereby providing a semiquantitative, single-cell measure of PD. Although many tumor cells did not progress through Ptx-induced mitotic arrest, tumor significantly regressed in the model. Our findings show that in vivo microscopy offers a useful tool to visualize mitosis during tumor progression, drug responses, and cell fate at the single-cell level.

Single-cell and subcellular pharmacokinetic imaging allows insight into drug action in vivo

Pharmacokinetic analysis at the organ level provides insight into how drugs distribute throughout the body, but cannot explain how drugs work at the cellular level. Here we demonstrate in vivo single-cell pharmacokinetic imaging of PARP-1 inhibitors and model drug behaviour under varying conditions. We visualize intracellular kinetics of the PARP-1 inhibitor distribution in real time, showing that PARP-1 inhibitors reach their cellular target compartment, the nucleus, within minutes in vivo both in cancer and normal cells in various cancer models. We also use these data to validate predictive finite element modelling. Our theoretical and experimental data indicate that tumour cells are exposed to sufficiently high PARP-1 inhibitor concentrations in vivo and suggest that drug inefficiency is likely related to proteomic heterogeneity or insensitivity of cancer cells to DNA-repair inhibition. This suggests that single-cell pharmacokinetic imaging and derived modelling improve our understanding of drug action at single-cell resolution in vivo.

Predicting therapeutic nanomedicine efficacy using a companion magnetic resonance imaging nanoparticle

Magnetic nanoparticles predict the efficacy of drug-loaded polymeric nanoparticles in vivo, helping select for tumors more responsive to nanomedicine. Particle prediction One particle, it seems, can predict the behavior of another. Thankfully, this is not the beginning of a lesson on quantum physics; instead, it is the basis for potentially designing targeted clinical trials in nanomedicine, by knowing if a tumor is likely to respond to a particular therapeutic nanoparticle. Miller et al. hypothesized that if a tumor readily takes up magnetic nanoparticles (MNP), it will also accumulate other nanoparticles carrying a deadly payload. The authors injected MNPs and a fluorescent version of the therapeutic nanoparticles into mice and followed their biodistribution using imaging. Both types of nanoparticles had similar pharmacokinetics and uptake in tumor-associated host cells owing to the enhanced permeability and retention effect. In mice with human tumors, Miller and colleagues found that the tumors with high MNP uptake were significantly more responsive than those with medium or low uptake to nanoparticles delivering chemotherapeutics. Thus, MNPs can be used as companion imaging agents during nanomedicine trials to predict the therapeutic effect of their nanosized counterparts. Therapeutic nanoparticles (TNPs) have shown heterogeneous responses in human clinical trials, raising questions of whether imaging should be used to identify patients with a higher likelihood of NP accumulation and thus therapeutic response. Despite extensive debate about the enhanced permeability and retention (EPR) effect in tumors, it is increasingly clear that EPR is extremely variable; yet, little experimental data exist to predict the clinical utility of EPR and its influence on TNP efficacy. We hypothesized that a 30-nm magnetic NP (MNP) in clinical use could predict colocalization of TNPs by magnetic resonance imaging (MRI). To this end, we performed single-cell resolution imaging of fluorescently labeled MNPs and TNPs and studied their intratumoral distribution in mice. MNPs circulated in the tumor microvasculature and demonstrated sustained uptake into cells of the tumor microenvironment within minutes. MNPs could predictably demonstrate areas of colocalization for a model TNP, poly(d,l-lactic-co-glycolic acid)-b-polyethylene glycol (PLGA-PEG), within the tumor microenvironment with >85% accuracy and circulating within the microvasculature with >95% accuracy, despite their markedly different sizes and compositions. Computational analysis of NP transport enabled predictive modeling of TNP distribution based on imaging data and identified key parameters governing intratumoral NP accumulation and macrophage uptake. Finally, MRI accurately predicted initial treatment response and drug accumulation in a preclinical efficacy study using a paclitaxel-encapsulated NP in tumor-bearing mice. These approaches yield valuable insight into the in vivo kinetics of NP distribution and suggest that clinically relevant imaging modalities and agents can be used to select patients with high EPR for treatment with TNPs.

Ultrafluorogenic Coumarin–Tetrazine Probes for Real‐Time Biological Imaging

We have developed a series of new ultrafluorogenic probes in the blue-green region of the visible-light spectrum that display fluorescence enhancement exceeding 11,000-fold. These fluorogenic dyes integrate a coumarin fluorochrome with the bioorthogonal trans-cyclooctene(TCO)-tetrazine chemistry platform. By exploiting highly efficient through-bond energy transfer (TBET), these probes exhibit the highest brightness enhancements reported for any bioorthogonal fluorogenic dyes. No-wash, fluorogenic imaging of diverse targets including cell-surface receptors in cancer cells, mitochondria, and the actin cytoskeleton is possible within seconds, with minimal background signal and no appreciable nonspecific binding, opening the possibility for in vivo sensing.

Tumour-associated macrophages act as a slow-release reservoir of nano-therapeutic Pt(IV) pro-drug

Therapeutic nanoparticles (TNPs) aim to deliver drugs more safely and effectively to cancers, yet clinical results have been unpredictable owing to limited in vivo understanding. Here we use single-cell imaging of intratumoral TNP pharmacokinetics and pharmacodynamics to better comprehend their heterogeneous behaviour. Model TNPs comprising a fluorescent platinum(IV) pro-drug and a clinically tested polymer platform (PLGA-b-PEG) promote long drug circulation and alter accumulation by directing cellular uptake toward tumour-associated macrophages (TAMs). Simultaneous imaging of TNP vehicle, its drug payload and single-cell DNA damage response reveals that TAMs serve as a local drug depot that accumulates significant vehicle from which DNA-damaging Pt payload gradually releases to neighbouring tumour cells. Correspondingly, TAM depletion reduces intratumoral TNP accumulation and efficacy. Thus, nanotherapeutics co-opt TAMs for drug delivery, which has implications for TNP design and for selecting patients into trials.