Noninvasive assessment of localized inflammatory responses

Imaging in Chronic Wound Diagnostics

Significance: Chronic wounds affect millions of patients worldwide, placing a huge burden on health care resources. Although significant progress has been made in the development of wound treatments, very few advances have been made in wound diagnosis. Recent Advances: Standard imaging methods like computed tomography, single-photon emission computed tomography, magnetic resonance imaging, terahertz imaging, and ultrasound imaging have been widely employed in wound diagnostics. A number of noninvasive optical imaging modalities like optical coherence tomography, near-infrared spectroscopy, laser Doppler imaging, spatial frequency domain imaging, digital camera imaging, and thermal and fluorescence imaging have emerged over the years. Critical Issues: While standard diagnostic wound imaging modalities provide valuable information, they cannot account for dynamic changes in the wound environment. In addition, they lack the capability to predict the healing outcome. Thus, there remains a pressing need for more efficient methods that can not only indicate the current state of the wound but also help determine whether the wound is on track to heal normally. Future Directions: Many imaging probes have been fabricated and shown to provide real-time assessment of tissue microenvironment and inflammatory responses in vivo. These probes have been demonstrated to noninvasively detect various changes in the wound environment, which include tissue pH, reactive oxygen species, fibrin deposition, matrix metalloproteinase production, and macrophage accumulation. This review summarizes the creation of these probes and their potential implications in wound monitoring.

Design and evaluation of an imager for assessing wound inflammatory responses and bioburden in a pig model

Our work details the development and characterization of a portable luminescence imaging device for detecting inflammatory responses and infection in skin wounds. The device includes a CCD camera and close-up lens integrated into a customizable 3D printed imaging chamber to create a portable light-tight imager for luminescence imaging. The chamber has an adjustable light portal that permits ample ambient light for white light imaging. This imager was used to quantify in real time the extent of two-dimensional reactive oxygen species (ROS) activity distribution using a porcine wound infection model. The imager was used to successfully visualize ROS-associated luminescent activities in vitro and in vivo. Using a pig full-thickness cutaneous wound model, we further demonstrate that this portable imager can detect the change of ROS activities and their relationship with vasculature in the wound environment. Finally, by analyzing ROS intensity and distribution, an imaging method was developed to distinguish infected from uninfected wounds. We discovered a distinct ROS pattern between bacteria-infected and control wounds corresponding to the microvasculature. The results presented demonstrate that this portable luminescence imager is capable of imaging ROS activities in cutaneous wounds in a large animal model, indicating suitability for future clinical applications.

Ratiometric Reactive Oxygen Species Nanoprobe for Noninvasive In Vivo Imaging of Subcutaneous Inflammation/Infection

Release of reactive oxygen species (ROS) accompanied with acute inflammation and infection often results in cell death and tissue injury. Several ROS-reactive bioluminescent probes have been investigated in recent years to detect ROS activity in vivo. Unfortunately, these probes cannot be used to quantify the degree of ROS activity and inflammatory responses due to the fact that the extent of the bioluminescent signals is also probe-concentration dependent. To address this challenge, we fabricated a ratiometric ROS probe in which both ROS-sensitive chemiluminescent agents and ROS-insensitive fluorescent reference dye were conjugated to particle carriers. The bioluminescence/reference fluorescence intensity ratios was calculated to reflect the extent of localized ROS activities while circumventing the variations in bioluminescent intensities associated with the ROS probe concentrations. The physical and chemical properties of the ratiometric probes were characterized. Furthermore, we assessed the accuracy and reproducibility of the probe in detecting ROS in vitro. The ability of the ratiometric probes to detect ROS production in inflamed/infected tissues was also examined using animal models of inflammation and infection. The overall results imply that ratiometric ROS probes can rapidly and non-invasively detect and quantify the extent of inflammatory responses and bacterial infection on wounds in real time.

* A Paradigm for the Evaluation of Tissue-Engineering Biomaterials and Templates

Procedures for the evaluation of tissue-engineering processes, including those used for the testing of the relevant biomaterials, have not been developed in a logical manner. This perspectives paper discusses the limitations of testing regimes and recommends a very different approach. The main emphasis is on the existing methods for assessing the biological safety of these biomaterials, which, it is suggested, are irrelevant for evaluating materials that are intended to facilitate the generation of new tissue. An algorithm is proposed that sets out the pathway from materials design and characterization through to the production of a file that sets out full biocompatibility, functionality, and tissue incorporation data that are suitable for regulatory consideration for first-in-man experiences. Central to this algorithm is the choice of animal models and the real-time monitoring of the implanted construct performance.

Rapid repeatable in vivo detection of retinal reactive oxygen species

Oxidative injuries, such as those related to reactive oxygen species (ROS), have been implicated in various retinal and optic nerve disorders. Many ROS detection methods have been developed. Although widely utilized, many of these methods are useful only in post mortem tissues, or require relatively expensive equipment, or involve intraocular injection. In the present study, we demonstrated and characterized a chemiluminescent probe L-012 as a noninvasive, in vivo ROS detection agent in the mouse retina. Using optic nerve crush (ONC) and retinal ischemia/reperfusion (I/R) as injury models, we show that L-012 produced intensive luminescent signals specifically in the injured eyes. Histological examination showed that L-012 administration was safe to the retina. Additionally, compounds that reduce tissue superoxide levels, apocynin and TEMPOL, decreased injury-induced L-012 chemiluminescence. The decrease in L-012 signals correlated with their protective effects against retinal I/R-induced morphological and functional changes in the retina. Together, these data demonstrate the feasibility of a fast, simple, reproducible, and non-invasive detection method to monitor in vivo ROS in the retina. Furthermore, the results also show that reduction of ROS is a potential therapeutic approach for protection from these retinal injuries.

Design of a portable imager for near-infrared visualization of cutaneous wounds

A portable imager developed for real-time imaging of cutaneous wounds in research settings is described. The imager consists of a high-resolution near-infrared CCD camera capable of detecting both bioluminescence and fluorescence illuminated by an LED ring with a rotatable filter wheel. All external components are integrated into a compact camera attachment. The device is demonstrated to have competitive performance with a commercial animal imaging enclosure box setup in beam uniformity and sensitivity. Specifically, the device was used to visualize the bioluminescence associated with increased reactive oxygen species activity during the wound healing process in a cutaneous wound inflammation model. In addition, this device was employed to observe the fluorescence associated with the activity of matrix metalloproteinases in a mouse lipopolysaccharide-induced infection model. Our results support the use of the portable imager design as a noninvasive and real-time imaging tool to assess the extent of wound inflammation and infection.

Facile Assembly of Cost-Effective and Locally Applicable or Injectable Nanohemostats for Hemorrhage Control

Currently, there is still unmet demand for effective and safe hemostats to control abnormal bleeding in different conditions. With the aim to develop affordable, safe, effective, easily stored, and low-cost hemostats, we developed a series of positively charged nanoparticles by a facile one-pot assembly approach. In this strategy, nanoparticles were formed by cholic-acid-mediated self-assembly of polyethylenimine (PEI). Regardless of different structures of cholic acids and PEIs, well-defined nanoparticles could be successfully formed. The assembly process was dominated by multiple interactions between cholic acid and PEI, including electrostatic, hydrogen bonding, and hydrophobic forces. In vitro studies showed that assembled nanoparticles effectively induced aggregation and activation of platelets. Local application of aqueous solution containing nanoparticles assembled by different cholic acids and PEIs significantly reduced bleeding times in different rodent models including tail transection in mice as well as liver bleeding and femoral artery bleeding in rats or rabbits. Moreover, intravenous (i.v.) injection of this type of positively charged nanoparticles notably prevented bleeding in the femoral artery in rats by targeting the injured site via opsonization of nanoparticles with fibrinogen. By contrast, a control negatively charged nanoparticle showed no hemostatic activity after i.v. delivery. Also, preliminary evaluations in rats revealed a good safety profile after i.v. administration of assembled nanoparticles at a dose 4-fold higher than that used for hemostasis. These results demonstrated that cholic acid/PEI-assembled positive nanoparticles may function as cost-effective and locally applicable or injectable nanohemostats for hemorrhage control in the civilian setting and on the battlefield.

Methods for Implant Acceptance and Wound Healing: Material Selection and Implant Location Modulate Macrophage and Fibroblast Phenotypes

This review focuses on materials and methods used to induce phenotypic changes in macrophages and fibroblasts. Herein, we give a brief overview on how changes in macrophages and fibroblasts phenotypes are critical biomarkers for identification of implant acceptance, wound healing effectiveness, and are also essential for evaluating the regenerative capabilities of some hybrid strategies that involve the combination of natural and synthetic materials. The different types of cells present during the host response have been extensively studied for evaluating the reaction to different materials and there are varied material approaches towards fabrication of biocompatible substrates. We discuss how natural and synthetic materials have been used to engineer desirable outcomes in lung, heart, liver, skin, and musculoskeletal implants, and how certain properties such as rigidity, surface shape, and porosity play key roles in the progression of the host response. Several fabrication strategies are discussed to control the phenotype of infiltrating macrophages and fibroblasts: decellularization of scaffolds, surface coatings, implant shape, and pore size apart from biochemical signaling pathways that can inhibit or accelerate unfavorable host responses. It is essential to factor all the different design principles and material fabrication criteria for evaluating the choice of implant materials or regenerative therapeutic strategies.

Imaging Reactive Oxygen Species-Induced Modifications in Living Systems

SIGNIFICANCE

Reactive Oxygen Species (ROS) may regulate signaling, ion channels, transcription factors, and biosynthetic processes. ROS-related diseases can be due to either a shortage or an excess of ROS.

RECENT ADVANCES

Since the biological activity of ROS depends on not only concentration but also spatiotemporal distribution, real-time imaging of ROS, possibly in vivo, has become a need for scientists, with potential for clinical translation. New imaging techniques as well as new contrast agents in clinically established modalities were developed in the previous decade.

CRITICAL ISSUES

An ideal imaging technique should determine ROS changes with high spatio-temporal resolution, detect physiologically relevant variations in ROS concentration, and provide specificity toward different redox couples. Furthermore, for in vivo applications, bioavailability of sensors, tissue penetration, and a high signal-to-noise ratio are additional requirements to be satisfied.

FUTURE DIRECTIONS

None of the presented techniques fulfill all requirements for clinical translation. The obvious way forward is to incorporate anatomical and functional imaging into a common hybrid-imaging platform. Antioxid. Redox Signal. 24, 939-958.

A BODIPY-luminol chemiluminescent resonance energy-transfer (CRET) cassette for imaging of cellular superoxide

Spectroscopic and in cellulo studies are here reported on the very first BODIPY-luminol chemiluminescent resonance energy-transfer (CRET) cassette where the luminol CL agent is covalently linked to the BODIPY energy-transfer acceptor in a molecular dyad. The efficiency of intramolecular CRET investigated for the BODIPY-luminol dyad was found to be 64% resulting in a dual emissive response. Successful in cellulo biochemiluminescence via CRET was achieved in PMA activated splenocytes.

Imaging of reactive oxygen species using [(3)H]hydromethidine in mice with cisplatin-induced nephrotoxicity

Background

Reactive oxygen species (ROS) have been implicated in cisplatin-induced nephrotoxicity. The aim of this study was to investigate the potential of using [³H]-labeled N-methyl-2,3-diamino-6-phenyl-dihydrophenanthridine ([³H]hydromethidine) for ex vivo imaging of regional ROS overproduction in mouse kidney induced by cisplatin.

Methods

Male C57BL/6 J mice were intraperitoneally administered with a single dose of cisplatin (30 mg/kg). Renal function was assessed by measuring serum creatinine and blood urea nitrogen (BUN) levels and morphology by histological examination. Renal malondialdehyde levels were measured as a lipid peroxidation marker. Autoradiographic studies were performed with kidney sections from mice at 60 min after [³H]hydromethidine injection.

Results

Radioactivity accumulation after [³H]hydromethidine injection was observed in the renal corticomedullary area of cisplatin-treated mice and was attenuated by pretreatment with dimethylthiourea (DMTU), a hydroxyl radical scavenger. Cisplatin administration significantly elevated serum creatinine and BUN levels, caused renal tissue damage, and promoted renal lipid peroxidation. These changes were significantly suppressed by DMTU pretreatment.

Conclusions

The present study showed that [³H]hydromethidine was rapidly distributed to the kidney after its injection and trapped there in the presence of ROS such as hydroxyl radicals, suggesting that [³H]hydromethidine is useful for assessment of the renal ROS amount in cisplatin-induced nephrotoxicity.

Optical imaging of COX-2: studies on an autofluorescent 2,3-diaryl-substituted indole-based cyclooxygenase-2 inhibitor

This study aimed at in vivo visualization of cyclooxygenase-2 (COX-2) by optical imaging using a representative compound of a class of autofluorescent 2,3-diaryl-substituted indole-based selective COX-2 inhibitors (2,3-diaryl-indole coxibs). COX-2 was successfully visualized in mice models with phorbol myristate ester (TPA)-induced inflammation or bearing xenografted human melanoma cells by 2-[4-(aminosulfonyl)phenyl]-3-(4-methoxyphenyl)-1H-indole (C1). COX-2 protein expression in both TPA-induced inflammatory sites and human melanoma xenografts was confirmed by immunoblotting. Control experiments using surrogate markers, sham injections, and non-COX-2 expressing melanoma cells further confirmed specificity of tissue association of C1. The merging of therapeutic and diagnostic properties of 2,3-diaryl-indole coxibs may widen the range of applications of COX-2-targeted treatment, e.g., for in situ-guided surgery and ex vivo diagnostics.

Amphiphilized poly(ethyleneimine) nanoparticles: a versatile multi-cargo carrier with enhanced tumor-homing efficiency and biocompatibility

Current theranostic approaches in cancer therapy demand delivery systems that can carry multiple drugs or imaging agents in a single nanoplatform with uniform biodistribution and improved target specificity. In this study, we have developed amphiphilized poly(ethyleneimine) nanoparticles (aPEI NPs) as a versatile multi-cargo delivery platform. The aPEI NPs were engineered to have the loading capacity for both hydrophobic molecules and negatively charged hydrophilic colloidal cargos through amphiphilic modification, i.e., octadecylation and subsequent PEGylation of poly(ethyleneimine). In the aqueous phase, the resulting aPEIs underwent amphiphilic self-assembly into spherical nanoparticles whose structure is constituted of the hydrophobic core with the positively charged surface and the hydrophilic neutral corona. The high degree of PEGylation resulted in the tiny colloidal size (<15 nm in diameter) and rendered the outmost surface coated with an antifouling corona which minimizes general shortcomings of poly(ethyleneimine)-based nanocarriers (e.g., cytotoxicity and liver filtration) while keeping its advantage (loading capability for negatively charged drugs). The unique nanostructure of aPEI NPs allowed for facile loading of hydrophobic model drugs (rubrene and IR780) in the core as well as negatively charged colloids (Pdots, proteins and DNA) on the inner surface via the hydrophobic and electrostatic interactions, respectively. Fluorescence imaging experiments demonstrated that the highly PEGylated aPEI-25 NPs showed prolonged blood circulation with minimal liver filtration and efficient delivery of the loaded cargos to the tumor. These combined merits, along with negligible toxicity profiles both in vitro and in vivo, validate the potential of aPEI-25 NPs as versatile nanocarriers for multi-cargo delivery.

A thermoresponsive biodegradable polymer with intrinsic antioxidant properties

Oxidative stress in tissue can contribute to chronic inflammation that impairs wound healing and the efficacy of cell-based therapies and medical devices. We describe the synthesis and characterization of a biodegradable, thermoresponsive gel with intrinsic antioxidant properties suitable for the delivery of therapeutics. Citric acid, poly(ethylene glycol) (PEG), and poly-N-isopropylacrylamide (PNIPAAm) were copolymerized by sequential polycondensation and radical polymerization to produce poly(polyethylene glycol citrate-co-N-isopropylacrylamide) (PPCN). PPCN was chemically characterized, and the thermoresponsive behavior, antioxidant properties, morphology, potential for protein and cell delivery, and tissue compatibility in vivo were evaluated. The PPCN gel has a lower critical solution temperature (LCST) of 26 °C and exhibits intrinsic antioxidant properties based on its ability to scavenge free radicals, chelate metal ions, and inhibit lipid peroxidation. PPCN displays a hierarchical architecture of micropores and nanofibers, and contrary to typical thermoresponsive polymers, such as PNIPAAm, PPCN gel maintains its volume upon formation. PPCN efficiently entrapped and slowly released the chemokine SDF-1α and supported the viability and proliferation of vascular cells. Subcutaneous injections in rats showed that PPCN gels are resorbed over time and new connective tissue formation takes place without signs of significant inflammation. Ultimately, this intrinsically antioxidant, biodegradable, thermoresponsive gel could potentially be used as an injectable biomaterial for applications where oxidative stress in tissue is a concern.

In vivo imaging of reactive oxygen and nitrogen species in murine colitis

BACKGROUND

Traditional techniques analyzing mouse colitis are invasive, laborious, or indirect. Development of in vivo imaging techniques for specific colitis processes would be useful for monitoring disease progression and/or treatment effectiveness. The aim was to evaluate the applicability of the chemiluminescent probe L-012, which detects reactive oxygen and nitrogen species, for in vivo colitis imaging.

METHODS

Two genetic colitis mouse models were used; K8 knockout (K8(-/-)) mice, which develop early colitis and the nonobese diabetic mice, which develop a transient subclinical colitis. Dextran sulphate sodium was used as a chemical colitis model. Mice were anesthetized, injected intraperitoneally with L-012, imaged, and quantified for chemiluminescent signal in the abdominal region using an IVIS camera system.

RESULTS

K8(-/-) and nonobese diabetic mice showed increased L-012-mediated chemiluminescence from the abdominal region compared with control mice. L-012 signals correlated with the colitis phenotype assessed by histology and myeloperoxidase staining. Although L-012 chemiluminescence enabled detection of dextran sulphate sodium-induced colitis at an earlier time point compared with traditional methods, large mouse-to-mouse variations were noted. In situ and ex vivo L-012 imaging as well as [18F]FDG-PET imaging of K8(-/-) mice confirmed that the in vivo signals originated from the distal colon. L-012 in vivo imaging showed a wide variation in reactive oxygen and nitrogen species in young mice, irrespective of K8 genotype. In aging mice L-012 signals were consistently higher in K8(-/-) as compared to K8(+/+) mice.

CONCLUSIONS

In vivo imaging using L-012 is a useful, simple, and cost-effective tool to study the level and longitudinal progression of genetic and possibly chemical murine colitis.

In vivo imaging of inflammatory phagocytes

Inflammation contributes to the pathophysiology of many diseases. In this report, we present noninvasive bioluminescence imaging methods that distinguish acute and chronic inflammation in mouse models. Systemic delivery of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) enables detection of acute inflammation largely mediated by tissue-infiltrating neutrophils, whose myeloperoxidase (MPO) activity is required for luminol bioluminescence. In contrast, bioluminescence from injection of lucigenin (bis-N-methylacridinium nitrate) closely correlates with late phase and chronic inflammation. Lucigenin bioluminescence is independent of MPO and, instead, requires phagocyte NADPH oxidase (Phox) activity in macrophages. We are able to visualize tissue inflammation resulting from wound healing, bacterial infection, foreign substance implantation, and antitumor immune responses. Given the central role of inflammation in a variety of disorders, we believe these noninvasive imaging methods can help dissect the differential roles of neutrophils and macrophages in a variety of pathological conditions.

Development of optical probes for in vivo imaging of polarized macrophages during foreign body reactions

Plasticity of macrophage (MΦ) phenotypes exist in a spectrum from classically activated (M1) cells, to alternatively activated (M2) cells, contributing to both the normal healing of tissues and the pathogenesis of implant failure. Here, folate- and mannose-based optical probes were fabricated to simultaneously determine the degree of MΦ polarization. In vitro tests show the ability of these probes to specifically target M1 and M2 cells. In an in vivo murine model, they were able to distinguish between the M1-dominated inflammatory response to infection and the M2-dominated regenerative response to particle implants. Finally, the probes were used to assess the inflammatory/regenerative properties of biomaterial implants. Our results show that these probes can be used to monitor and quantify the dynamic processes of MΦ polarization and their role in cellular responses in real time.

Alternative strategies to manipulate fibrocyte involvement in the fibrotic tissue response: pharmacokinetic inhibition and the feasibility of directed-adipogenic differentiation

Fibrocytes have previously been identified as important mediators in several inflammatory and fibrotic diseases. However, there is no effective treatment thus far to reduce fibrotic tissue responses without affecting wound healing reactions. Here we investigate two strategies to alleviate fibrocyte interactions at the biomaterial interface, reducing collagen production and scar tissue formation. First, in an indirect approach, TGF-β inhibitor-SB431542 and IL-1β/TNF-α inhibitor SB203580 were locally released from scaffold implants to block their respective signaling pathways. We show that the inhibition of IL-1β/TNF-α has no influence on overall fibrotic tissue reactions to the implants. However, the reduction of localized TGF-β significantly decreases the fibrocyte accumulation and myofibroblast activation while reducing the fibrotic tissue formation. Since fibrocytes can be differentiated into non-fibrotic cell types, such as adipocytes, we further sought a more direct approach to reduce fibrocyte responses by directing fibrocyte differentiation into adipocytes. Interestingly, by initiating fibrocyte-to-adipocyte differentiation through sustained differentiation cocktail release, we find that adipogenic differentiation forces incoming fibrocytes away from the traditional myofibroblast lineage, leading to a substantial reduction in the collagen formation and fibrotic response. Our results support a novel and effective strategy to improve implant safety by reducing implant-associated fibrotic tissue reactions via directing non-fibrotic differentiation of fibrocytes.

Optical imaging of fibrin deposition to elucidate participation of mast cells in foreign body responses

Mast cell activation has been shown to be an initiator and a key determinant of foreign body reactions. However, there is no non-invasive method that can quantify the degree of implant-associated mast cell activation. Taking advantage of the fact that fibrin deposition is a hallmark of mast cell activation around biomaterial implants, a near infrared probe was fabricated to have high affinity to fibrin. Subsequent in vitro testing confirmed that this probe has high affinity to fibrin. Using a subcutaneous particle implantation model, we found significant accumulation of fibrin-affinity probes at the implant sites as early as 15 min following particle implantation. The accumulation of fibrin-affinity probes at the implantation sites could also be substantially reduced if anti-coagulant - heparin was administered at the implant sites. Further studies have shown that subcutaneous administration of mast cell activator - compound 48/80 - prompted the accumulation of fibrin-affinity probes. However, implant-associated fibrin-affinity probe accumulation was substantially reduced in mice with mast cell deficiency. The results show that our fibrin-affinity probes may serve as a powerful tool to monitor and measure the extent of biomaterial-mediated fibrin deposition and mast cell activation in vivo.

Macrophage polarization in response to wear particles in vitro

Total joint replacement is a highly successful surgical procedure for treatment of patients with disabling arthritis and joint dysfunction. However, over time, with high levels of activity and usage of the joint, implant wear particles are generated from the articulating surfaces. These wear particles can lead to activation of an inflammatory reaction, and subsequent bone resorption around the implant (periprosthetic osteolysis). Cells of the monocyte/macrophage lineage orchestrate this chronic inflammatory response, which is dominated by a pro-inflammatory (M1) macrophage phenotype rather than an anti-inflammatory pro-tissue healing (M2) macrophage phenotype. While it has been shown that interleukin-4 (IL-4) selectively polarizes macrophages towards an M2 anti-inflammatory phenotype which promotes bone healing, rather than inflammation, little is known about the time course in which this occurs or conditions in which repolarization through IL-4 is most effective. The goal of this work was to study the time course of murine macrophage polarization and cytokine release in response to challenge with combinations of polymethyl methacrylate (PMMA) particles, lipopolysaccharide (LPS) and IL-4 in vitro. Treatment of particle-challenged monocyte/macrophages with IL-4 led to an initial suppression of pro-inflammatory cytokines and inducible nitric oxide synthase (iNOS) production and subsequent polarization into an M2 anti-inflammatory phenotype. This result was optimized when IL-4 was delivered before PMMA particle challenge, to an M1 phenotype rather than to uncommitted (M0) macrophages. The effects of this polarization were sustained over a 5-day time course. Polarization of M1 macrophages into an M2 phenotype may be a strategy to mitigate wear particle associated periprosthetic osteolysis.

In vivo imaging method to distinguish acute and chronic inflammation

Inflammation is a fundamental aspect of many human diseases. In this video report, we demonstrate non-invasive bioluminescence imaging techniques that distinguish acute and chronic inflammation in mouse models. With tissue damage or pathogen invasion, neutrophils are the first line of defense, playing a major role in mediating the acute inflammatory response. As the inflammatory reaction progresses, circulating monocytes gradually migrate into the site of injury and differentiate into mature macrophages, which mediate chronic inflammation and promote tissue repair by removing tissue debris and producing anti-inflammatory cytokines. Intraperitoneal injection of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione, sodium salt) enables detection of acute inflammation largely mediated by tissue-infiltrating neutrophils. Luminol specifically reacts with the superoxide generated within the phagosomes of neutrophils since bioluminescence results from a myeloperoxidase (MPO) mediated reaction. Lucigenin (bis-N-methylacridinium nitrate) also reacts with superoxide in order to generate bioluminescence. However, lucigenin bioluminescence is independent of MPO and it solely relies on phagocyte NADPH oxidase (Phox) in macrophages during chronic inflammation. Together, luminol and lucigenin allow non-invasive visualization and longitudinal assessment of different phagocyte populations across both acute and chronic inflammatory phases. Given the important role of inflammation in a variety of human diseases, we believe this non-invasive imaging method can help investigate the differential roles of neutrophils and macrophages in a variety of pathological conditions.