Identification of splenic reservoir monocytes and their deployment to inflammatory sites

New insights into angiotensin receptor actions: from blood pressure to aging

PURPOSE OF REVIEW

The renin-angiotensin system (RAS) is critical for cardiovascular control, impacting normal physiology and disease pathogenesis. Although several biologically active peptides are generated by this system, its major actions are mediated by angiotensin II acting through its type 1 (AT1) and type 2 (AT2) receptors. Along with their effects to influence blood pressure and hemodynamics, recent studies have provided evidence that angiotensin receptors influence a range of processes independent from hemodynamic effects.

RECENT FINDINGS

This review is focused on new molecular mechanisms underlying actions of AT1 receptors to influence vasoconstriction, inflammation, immune responses, and longevity. Moreover, we also highlight new advances in understanding functions of the AT2 receptor in end-organ damage, emphasizing the AT2 receptor as a potential therapeutic target in cardiovascular diseases.

SUMMARY

Here we review recent advances in understanding the role of angiotensin receptors in normal physiology and disease states, focusing on their properties that may contribute to blood pressure regulation, end-organ damage, autoimmune disease and longevity.

Discovery of mouse spleen signaling responses to anthrax using label-free quantitative phosphoproteomics via mass spectrometry

Inhalational anthrax is caused by spores of the bacterium Bacillus anthracis (B. anthracis), and is an extremely dangerous disease that can kill unvaccinated victims within 2 weeks. Modern antibiotic-based therapy can increase the survival rate to ∼50%, but only if administered presymptomatically (within 24-48 h of exposure). To discover host signaling responses to presymptomatic anthrax, label-free quantitative phosphoproteomics via liquid chromatography coupled to mass spectrometry was used to compare spleens from uninfected and spore-challenged mice over a 72 h time-course. Spleen proteins were denatured using urea, reduced using dithiothreitol, alkylated using iodoacetamide, and digested into peptides using trypsin, and the resulting phosphopeptides were enriched using titanium dioxide solid-phase extraction and analyzed by nano-liquid chromatography-Linear Trap Quadrupole-Orbitrap-MS(/MS). The fragment ion spectra were processed using DeconMSn and searched using both Mascot and SEQUEST resulting in 252,626 confident identifications of 6248 phosphopeptides (corresponding to 5782 phosphorylation sites). The precursor ion spectra were deisotoped using Decon2LS and aligned using MultiAlign resulting in the confident quantitation of 3265 of the identified phosphopeptides. ANOVAs were used to produce a q-value ranked list of host signaling responses. Late-stage (48-72 h postchallenge) Sterne strain (lethal) infections resulted in global alterations to the spleen phosphoproteome. In contrast, ΔSterne strain (asymptomatic; missing the anthrax toxin) infections resulted in 188 (5.8%) significantly altered (q<0.05) phosphopeptides. Twenty-six highly tentative phosphorylation responses to early-stage (24 h postchallenge) anthrax were discovered (q<0.5), and ten of these originated from eight proteins that have known roles in the host immune response. These tentative early-anthrax host response signaling events within mouse spleens may translate into presymptomatic diagnostic biomarkers of human anthrax detectable within circulating immune cells, and could aid in the identification of pathogenic mechanisms and therapeutic targets.

Chemokine receptor 2-mediated accumulation of fungicidal exudate macrophages in mice that clear cryptococcal lung infection

Clearance of pulmonary infection with the fungal pathogen Cryptococcus neoformans is associated with the accumulation and activation of lung macrophages. However, the phenotype of these macrophages and the mechanisms contributing to their accumulation are not well-defined. In this study, we used an established murine model of cryptococcal lung infection and flow cytometric analysis to identify alveolar macrophages (AMs) and the recently described exudate macrophages (ExMs). Exudate macrophages are distinguished from AMs by their strong expression of CD11b and major histocompatibility complex class II and modest expression of costimulatory molecules. Exudate macrophages substantially outnumber AMs during the effector phase of the immune response; and accumulation of ExMs, but not AMs, was chemokine receptor 2 (CCR2) dependent and attributable to the recruitment and subsequent differentiation of Ly-6C(high) monocytes originating from the bone marrow and possibly the spleen. Peak ExM accumulation in wild-type (CCR2(+/+)) mice coincided with maximal lung expression of mRNA for inducible nitric oxide synthase and correlated with the known onset of cryptococcal clearance in this strain of mice. Exudate macrophages purified from infected lungs displayed a classically activated effector phenotype characterized by cryptococcal-enhanced production of inducible nitric oxide synthase and tumor necrosis factor α. Cryptococcal killing by bone marrow-derived ExMs was CCR2 independent and superior to that of AMs. We conclude that clearance of cryptococcal lung infection requires the CCR2-mediated massive accumulation of fungicidal ExMs derived from circulating Ly-6C(high) monocytes.

Improved serum-free culture conditions for spleen-derived murine fibrocytes

Both wound repair and fibrosing diseases involve circulating monocytes entering a tissue and differentiating into fibroblast-like cells called fibrocytes. Fibrocyte biology has been extensively studied in both humans and mice. However, current in vitro techniques to culture murine fibrocytes can take up to two weeks and can require multiple mice to obtain enough circulating monocytes for a single experiment. An alternative source of fibrocytes is the splenic reservoir of monocytes, where one can obtain significantly more cells compared to the peripheral blood. We found that in serum-free medium, fibrocytes differentiate from murine spleen cells within 5 days. To maximize fibrocyte yield, we found the optimal purification technique was to digest the spleen with a collagenase/DNase cocktail, pass the cells through a cell strainer, and lyse the red blood cells. We found that IL-13 and M-CSF significantly enhanced fibrocyte differentiation and that the optimal cell density to promote differentiation was 1.75×10⁶ cells/ml. Serum amyloid P (SAP) and cross-linked IgG are two factors known to inhibit the differentiation of human monocytes into fibrocytes. We found that SAP and cross-linked IgG also inhibited the differentiation of murine spleen cells into fibrocytes. These results suggest that culturing murine spleen cells in serum-free medium is a rapid and efficient system to study factors that can affect fibrocyte differentiation.

A role for spleen monocytes in post-ischemic brain inflammation and injury

Although infiltration of peripheral monocytes/macrophages is implicated in stroke pathology, in vivo data regarding the deployment of monocytes and their mobilization to the infarct area is scarce. Recent literature showed that mouse monocytes exhibit two distinct populations that represent pro-inflammatory (Ly-6Chi/CCR2+) and anti-inflammatory (Ly-6Clow/CCR2-) subsets and that spleen is a major source for monocyte deployment upon injury. By reducing post-ischemic infection with antibacterial moxifloxacin (MFX) treatment, the present study investigates the effect of the treatment on Ly-6C and CCR2 expression in the spleen following ischemia and the extent to which the effect is associated with attenuation of post-ischemic inflammation and injury. Mice subjected to a middle cerebral artery occlusion (MCAO) showed a significant reduction in their spleen weights compared to sham animals. Compared to vehicle controls, splenocytes obtained from daily MFX-treated mice 7 days after ischemia exhibited significantly reduced mean Ly-6C expression within pro-inflammatory subsets, whereas the distribution of pro- and anti-inflammatory subsets was not different between the treatment groups. Additionally, MFX treatment significantly reduced CCR2 expression in the spleen tissue and in the post-ischemic brain and attenuated infarct size. The study suggests a potential contributing role of spleen monocytes in post-ischemic inflammation and injury. The influence of peripheral inflammatory status on the primary injury in the CNS further implies that the attenuation of post-stroke infection may be beneficial in mitigating stroke-induced brain injury.

Minocycline differentially modulates viral infection and persistence in an experimental model of Japanese encephalitis

Japanese encephalitis (JE) is caused by a neurotropic flavivirus that causes CNS damage that leads to death in acute cases or permanent neuropsychiatric sequel in survivors. The course of infection of this virus is not well defined though it is clear that it evades the host's innate immune response in the periphery. The current study was designed to investigate the time-dependent changes in the spleen and lymph node, apart from the CNS that are infected by the Japanese encephalitis virus (JEV). Our previous studies have led to the identification of minocycline, a semi-synthetic antibiotic, as a protective drug in JE. In this study we have also investigated the role of minocycline on the peripheral organs that are infected by JEV. Levels of IL-12 and MCP-1 in the organs were estimated by cytometric bead array, and immunohistochemical studies were performed on cryosections of tissue to detect CD3- or CD11b-positive cells as well as JEV antigen. We found that the levels of T cell-activating cytokine IL-12 and MCP-1 levels were significantly elevated in JEV-infected tissue samples in a time-dependent manner. Corresponding to this increase was the increase in the number of CD3- and CD11b-positive cells in the tissues of infected animals. Minocycline treatment abrogated these changes. Minocycline treatment also resulted in the gradual decrease in the number of CD11b (but not CD3) positive cells in the lymph node and spleen, even though the virus persisted in these organs. We also observed structural changes in the spleen following minocycline treatment.

Umbilical cord-derived mesenchymal stromal cells modulate monocyte function to suppress T cell proliferation

Mesenchymal stromal cells (MSCs) may be derived from a variety of tissues, with human umbilical cord (UC) providing an abundant and noninvasive source. Human UC-MSCs share similar in vitro immunosuppressive properties as MSCs obtained from bone marrow and cord blood. However, the mechanisms and cellular interactions used by MSCs to control immune responses remain to be fully elucidated. In this paper, we report that suppression of mitogen-induced T cell proliferation by human UC-, bone marrow-, and cord blood-MSCs required monocytes. Removal of monocytes but not B cells from human adult PBMCs (PBMNCs) reduced the immunosuppressive effects of MSCs on T cell proliferation. There was rapid modulation of a number of cell surface molecules on monocytes when PBMCs or alloantigen-activated PBMNCs were cultured with UC-MSCs. Indomethacin treatment significantly inhibited the ability of UC-MSCs to suppress T cell proliferation, indicating an important role for PGE(2). Monocytes purified from UC-MSC coculture had significantly reduced accessory cell and allostimulatory function when tested in subsequent T cell proliferation assays, an effect mediated in part by UC-MSC PGE(2) production and enhanced by PBMNC alloactivation. Therefore, we identify monocytes as an essential intermediary through which UC-MSCs mediate their suppressive effects on T cell proliferation.

Angiotensin II overcomes strain-dependent resistance of rapid CKD progression in a new remnant kidney mouse model

The remnant kidney model in C57BL/6 mice does not develop progressive chronic kidney disease (CKD). In this study we modified the model to mimic features of human CKD and to define accelerants of disease progression using three strains of mice. Following the procedure, there was a progressive increase in albuminuria, progressive loss in renal function, severe glomerulosclerosis and interstitial fibrosis, hypertension, cardiac fibrosis, and anemia by 4 weeks in CD-1 mice and by 12 weeks in 129S3 mice. In contrast, even after 16 weeks, the C57BL/6 mice with a remnant kidney had modestly increased albuminuria without increased blood pressure and without developing CKD or cardiac fibrosis. The baseline blood pressure, determined by radiotelemetry in conscious animals, correlated with CKD progression rates in each strain. Administering angiotensin II overcame the resistance of C57BL/6 mice to CKD following renal mass reduction, displaying high blood pressure and albuminuria, severe glomerulosclerosis, and loss of renal function by 4 weeks. Decreasing blood pressure with olmesartan, but not hydralazine, in CD-1 mice with a remnant kidney reduced CKD progression and cardiac fibrosis. C57BL/6 mice with a remnant kidney and DOCA-salt hypertension developed modest CKD. Each strain had similar degrees of interstitial fibrosis in three different normotensive models of renal fibrosis. Thus, reducing renal mass in CD-1 or 129S3 mice mimics many features of human CKD. Angiotensin II can convert the C57BL/6 strain from CKD resistant to susceptible in this disease model.

Angiotensin-converting enzyme inhibition prevents the release of monocytes from their splenic reservoir in mice with myocardial infarction

RATIONALE

Monocytes recruited to ischemic myocardium originate from a reservoir in the spleen, and the release from their splenic niche relies on angiotensin (Ang) II signaling.

OBJECTIVE

Because monocytes are centrally involved in tissue repair after ischemia, we hypothesized that early angiotensin-converting enzyme (ACE) inhibitor therapy impacts healing after myocardial infarction partly via effects on monocyte traffic.

METHODS AND RESULTS

In a mouse model of permanent coronary ligation, enalapril arrested the release of monocytes from the splenic reservoir and consequently reduced their recruitment into the healing infarct by 45%, as quantified by flow cytometry of digested infarcts. Time-lapse intravital microscopy revealed that enalapril reduces monocyte motility in the spleen. In vitro migration assays and Western blotting showed that this was caused by reduced signaling through the Ang II type 1 receptor. We then studied the long-term consequences of blocked splenic monocyte release in atherosclerotic apolipoprotein (apo)E(-/-) mice, in which infarct healing is impaired because of excessive inflammation in the cardiac wound. Enalapril improved histologic healing biomarkers and reduced inflammation in infarcts measured by FMT-CT (fluorescence molecular tomography in conjunction with x-ray computed tomography) of proteolytic activity. ACE inhibition improved MRI-derived ejection fraction by 14% on day 21, despite initially comparable infarct size. In apoE(-/-) mice, ischemia/reperfusion injury resulted in larger infarct size and enhanced monocyte recruitment and was reversible by enalapril treatment. Splenectomy reproduced antiinflammatory effects of enalapril.

CONCLUSION

This study suggests that benefits of early ACE inhibition after myocardial infarction can partially be attributed to its potent antiinflammatory impact on the splenic monocyte reservoir.

Glutamine targeting inhibits systemic metastasis in the VM-M3 murine tumor model

Metastatic cancer is a major cause of morbidity and mortality. Current therapeutic options consist of chemotherapy, radiation or targeted therapies. However, these therapies are often toxic, effective over a small range of cancer types or result in drug resistance. Therefore, a more global, less toxic strategy for the management of metastatic cancer is required. Although most cancers display increased glucose metabolism, glutamine is also a major energy substrate for many cancers. We evaluated the antimetastatic potential of 6-diazo-5-oxo-L-norleucine (DON), a glutamine analog, using the new VM mouse model of systemic metastasis. We found that primary tumor growth was ∼20-fold less in DON-treated mice than in untreated control mice. We also found that DON treatment inhibited metastasis to liver, lung and kidney as detected by bioluminescence imaging and histology. Our findings provide proof of concept that metabolic therapies targeting glutamine metabolism can manage systemic metastatic cancer.

Spatial and temporal coordination of bone marrow-derived cell activity during arteriogenesis: regulation of the endogenous response and therapeutic implications

Arterial occlusive disease is the leading cause of morbidity and mortality throughout the developed world, which creates a significant need for effective therapies to halt disease progression. Despite success of animal and small-scale human therapeutic arteriogenesis studies, this promising concept for treating arterial occlusive disease has yielded largely disappointing results in large-scale clinical trials. One reason for this lack of successful translation is that endogenous arteriogenesis is highly dependent on a poorly understood sequence of events and interactions between bone marrow derived cells (BMCs) and vascular cells, which makes designing effective therapies difficult. We contend that the process follows a complex, ordered sequence of events with multiple, specific BMC populations recruited at specific times and locations. Here, we present the evidence suggesting roles for multiple BMC populations-from neutrophils and mast cells to progenitor cells-and propose how and where these cell populations fit within the sequence of events during arteriogenesis. Disruptions in these various BMC populations can impair the arteriogenesis process in patterns that characterize specific patient populations. We propose that an improved understanding of how arteriogenesis functions as a system can reveal individual BMC populations and functions that can be targeted for overcoming particular impairments in collateral vessel development.

Microbial stimulation fully differentiates monocytes to DC-SIGN/CD209(+) dendritic cells for immune T cell areas

Dendritic cells (DCs), critical antigen-presenting cells for immune control, normally derive from bone marrow precursors distinct from monocytes. It is not yet established if the large reservoir of monocytes can develop into cells with critical features of DCs in vivo. We now show that fully differentiated monocyte-derived DCs (Mo-DCs) develop in mice and DC-SIGN/CD209a marks the cells. Mo-DCs are recruited from blood monocytes into lymph nodes by lipopolysaccharide and live or dead gram-negative bacteria. Mobilization requires TLR4 and its CD14 coreceptor and Trif. When tested for antigen-presenting function, Mo-DCs are as active as classical DCs, including cross-presentation of proteins and live gram-negative bacteria on MHC I in vivo. Fully differentiated Mo-DCs acquire DC morphology and localize to T cell areas via L-selectin and CCR7. Thus the blood monocyte reservoir becomes the dominant presenting cell in response to select microbes, yielding DC-SIGN(+) cells with critical functions of DCs.

Human embryonic stem cell-derived microvascular grafts for cardiac tissue preservation after myocardial infarction

We present use of a synthetic, injectable matrix metalloproteinase (MMP)-responsive, bioactive hydrogel as an in situ forming scaffold to deliver thymosin β4 (Tβ4), a pro-angiogenic and pro-survival factor, along with vascular cells derived from human embryonic stem cells (hESC) in ischemic injuries to the heart in a rat model. The gel was found to substitute the degrading extracellular matrix in the infarcted myocardium of rats and to promote structural organization of native endothelial cells, while some of the delivered hESC-derived vascular cells formed de novo capillaries in the infarct zone. Magnetic resonance imaging (MRI) revealed that the microvascular grafts effectively preserved contractile performance 3 d and 6 wk after myocardial infarction, attenuated left ventricular dilation, and decreased infarct size as compared to infarcted rats treated with PBS injection as a control (3 d ejection fraction, + ∼7%, P < 0.001; 6 wk ejection faction, + ∼12%, P < 0.001). Elevation in vessel density was observed in response to treatment, which may be due in part to elevations in human (donor)-derived cytokines EGF, VEGF and HGF (1 d). Thus, a clinically relevant matrix for dual delivery of vascular cells and drugs may be useful in engineering sustained tissue preservation and potentially regenerating ischemic cardiac tissue.

Exercise improves the dilatation function of mesenteric arteries in postmyocardial infarction rats via a PI3K/Akt/eNOS pathway-mediated mechanism

Myocardial infarction (MI) has been shown to induce endothelial dysfunction in peripheral resistance arteries and thus increase peripheral resistance. This study was designed to investigate the underlying mechanisms of post-MI-related dysfunctional dilatation of peripheral resistance arteries and, furthermore, to examine whether exercise may restore dysfunctional dilatation of peripheral resistance arteries. Adult male Sprague-Dawley rats were divided into three groups: sham-operated, MI, and MI + exercise. Ultrastructure and relaxation function of the mesenteric arteries, as well as phosphatidylinositol-3 kinase (PI3K), Akt kinases (Akt), endothelial nitric oxide synthase (eNOS) activity, and phosphorylation of PI3K, Akt, and eNOS by ACh were determined. Post-MI rats exhibited pronounced ultrastructural changes in mesenteric artery endothelial cells and endothelial dysfunction. In addition, the activities of PI3K, Akt, and eNOS, and their phosphorylation by ACh were significantly attenuated in mesenteric arteries (P < 0.05-0.01). After 8 wk of exercise, not only did endothelial cells appeared more normal in structure, but also ameliorated post-MI-associated mesenteric arterial dysfunction, which were accompanied by elevated activities of PI3K, Akt, and eNOS, and their phosphorylation by ACh (P < 0.05-0.01). Importantly, inhibition of either PI3K or eNOS attenuated exercise-induced restoration of the dilatation function and blocked PI3K, Akt, and eNOS phosphorylation by ACh in the mesenteric arteries. These data demonstrate that MI induces dysfunctional dilation of peripheral resistance arteries by degradation of endothelial structural integrity and attenuating PI3K-Akt-eNOS signaling. Exercise may restore dilatation function of peripheral resistance arteries by protecting endothelial structural integrity and increasing PI3K-Akt-eNOS signaling cascades.

Trauma-induced inflammation and fracture healing

Fracture healing is an extremely complex interaction of cells, biologic pathways, and molecules. Certainly, the inflammatory response is one of the initiating factors for bone healing. The inflammatory phase is a critical period characterized by low oxygen tension, impaired perfusion, and the migration of a wide array of cells and release of active molecules. Systemwide inflammatory conditions also modulate the primary processes of fracture management. Osteoprogenitor cells, mesenchymal cells, osteoblasts, and chondrocytes contribute to the healing and inflammatory response at the bone level. The inflammatory process is dependent on and propagates through proinflammatory cytokines, the transforming growth factor-beta superfamily with other growth factors, and the metalloproteinases and angiogenic factors. Interference with any of these pathways or proteins either promotes or more likely decreases fracture healing. This article reviews the initial inflammatory response to trauma as it pertains to musculoskeletal healing.

Periovulatory leukocyte infiltration in the rat ovary

Ovulation is preceded by intraovarian inflammatory reactions that occur in response to the preovulatory gonadotropin surge. As a main inflammatory event, leukocytes infiltrate the ovary and release proteolytic enzymes that degrade the extracellular matrix weakening the follicular wall, a required step for follicle rupture. This study aimed to quantitatively measure the infiltrating leukocytes, determine their cell types, and localize infiltration sites in the periovulatory rat ovary. Cycling adult and gonadotropin-stimulated immature rats were used as animal models. Ovaries were collected at five different stages of estrous cycle in the adult rats (diestrus, 1700 h; proestrus, 1500 h; proestrus, 2400 h; estrus, 0600 h; and metestrus, 1700 h) and at five different time points after superovulation induction in the immature rats (pregnant mare's serum gonadotrophin, 0 h; pregnant mare's serum gonadotrophin, 48 h; human chorionic gonadotropin, 6 h; human chorionic gonadotropin, 12 h; and human chorionic gonadotropin, 24 h). The ovaries were either dissociated into a single cell suspension for flow cytometric analysis or fixed for immunohistochemical localization of the leukocytes. Similar numbers of leukocytes were seen throughout the estrous cycle (approximately 500,000/ovary), except proestrus 2400 when 2-fold higher numbers of leukocytes were found (approximately 1.1 million/ovary). A similar trend of periovulatory rise of leukocyte numbers was seen in the superovulation-induced immature rat model, recapitulating a dramatic increase in leukocyte numbers upon gonadotropin stimulation. Both macrophage/granulocytes and lymphocytes were among the infiltrating leukocytes and were localized in the theca and interstitial tissues, where platelet-endothelial cell adhesion molecule-1 and intercellular adhesion molecule-1 may play roles in the transmigration of leukocytes, because their expressions correlates spatiotemporally with the infiltrating leukocytes. In addition, a strong inverse relationship between leukocyte numbers in the ovary and spleen, as well as significant reduction of leukocyte infiltration in the splenectomized rats, were seen, indicating that the spleen may serve as an immediate supplier of leukocytes to the periovulatory ovary.

Blockade of self-reactive IgM significantly reduces injury in a murine model of acute myocardial infarction

AIMS

Coronary artery occlusion resulting in ischaemia/reperfusion (I/R) injury is a major cause of mortality in the western world. Circulating natural IgM has been shown to play a significant role in reperfusion injury, leading to the notion of a pathogenic response against self by the innate immune system. A specific self-antigen (non-muscle myosin heavy chain II) was recently identified as the major target of pathogenic natural IgM. Therefore, we hypothesized that a synthetic peptide mimetope (N2) or monoclonal antibodies directed against the self-antigen would prevent specific IgM binding to the self-antigen and reduce reperfusion injury in the heart.

METHODS AND RESULTS

We find that treatment with N2 peptide reduces infarct size by 47% and serum cardiac troponin-I levels by 69% following 1 h ischaemia and 24 h reperfusion. N2 peptide or an anti-N2 F(ab')(2) (21G6) is also effective at preventing IgM and complement deposition. Additionally, N2 peptide treatment significantly reduces monocyte and neutrophil infiltration at 24 h and collagen deposition at 5 days. Finally, we show that human IgM (hIgM) also includes specificity for the highly conserved self-antigen and that myocardial injury in antibody-deficient mice reconstituted with hIgM is blocked by treatment with N2 peptide or 21G6 F(ab')(2).

CONCLUSION

The findings in this study identify potential therapeutics [i.e. N2 peptide or 21G6 F(ab')(2)] that prevent specific IgM binding to ischaemic antigens in the heart, resulting in a significant reduction in cardiac I/R injury.

CXCR3-dependent accumulation and activation of perivascular macrophages is necessary for homeostatic arterial remodeling to hemodynamic stresses

Sustained changes in blood flow modulate the size of conduit arteries through structural alterations of the vessel wall that are dependent on the transient accumulation and activation of perivascular macrophages. The leukocytic infiltrate appears to be confined to the adventitia, is responsible for medial remodeling, and resolves once hemodynamic stresses have normalized without obvious intimal changes. We report that inward remodeling of the mouse common carotid artery after ligation of the ipsilateral external carotid artery is dependent on the chemokine receptor CXCR3. Wild-type myeloid cells restored flow-mediated vascular remodeling in CXCR3-deficient recipients, adventitia-infiltrating macrophages of Gr1^low resident phenotype expressed CXCR3, the perivascular accumulation of macrophages was dependent on CXCR3 signaling, and the CXCR3 ligand IP-10 was sufficient to recruit monocytes to the adventitia. CXCR3 also contributed to selective features of macrophage activation required for extracellular matrix turnover, such as production of the transglutaminase factor XIII A subunit. Human adventitial macrophages displaying a CD14⁺/CD16⁺ resident phenotype, but not circulating monocytes, expressed CXCR3, and such cells were more frequent at sites of disturbed flow. Our observations reveal a CXCR3-dependent accumulation and activation of perivascular macrophages as a necessary step in homeostatic arterial remodeling triggered by hemodynamic stress in mice and possibly in humans as well.

The multiple roles of monocyte subsets in steady state and inflammation

Monocytes participate importantly in immunity. Produced in the bone marrow and released into the blood, they circulate in blood or reside in a spleen reservoir before entering tissue and giving rise to macrophages or dendritic cells. Monocytes are more than transitional cells that adapt to a particular tissue environment indiscriminately. Accumulating evidence now indicates that monocytes are heterogeneous in several species and are themselves predetermined for particular function in the steady state and inflammation. Future therapeutics may harness this heterogeneity to target harmful functions while sparing those that are beneficial. Here, we review recent advances on the ontogeny and function of monocytes and their subsets in humans and mice.

Tumor-host interactions: a far-reaching relationship

Carcinomas are composed of neoplastic epithelial cells, which form the heart of the tumor, as well as a variety of mesenchymal cell types and extracellular matrix components that comprise the tumor stroma, often termed its microenvironment. The normal counterparts of some stromal cells are thought to limit tumor growth, while tumor-associated stromal cells have been convincingly shown to actively promote tumor progression via complex heterotypic interactions with the nearby carcinoma cells. More recent advances have revealed that tumor-host interactions extend well beyond the local tissue microenvironment (ie, interactions between the neoplastic cells and the nearby stroma) and that tumors not only respond to, but actively perturb host organs at distant anatomic sites. This indicates that many aspects of tumor biology can only be explained by a detailed understanding of both local and systemic interactions, yet we currently have only a fragmentary understanding of both processes. In this review, we address the recent advances in our understanding of the contributions of local and systemic environments to cancer progression, the ability of tumors to actively perturb the host environment, and current therapeutic approaches that are designed to disrupt tumor-host relationships.

Quantitative profiling of the full APOBEC3 mRNA repertoire in lymphocytes and tissues: implications for HIV-1 restriction

The human APOBEC3 proteins are DNA cytidine deaminases that impede the replication of many different transposons and viruses. The genes that encode APOBEC3A, APOBEC3B, APOBEC3C, APOBEC3D, APOBEC3F, APOBEC3G and APOBEC3H were generated through relatively recent recombination events. The resulting high degree of inter-relatedness has complicated the development of specific quantitative PCR assays for these genes despite considerable interest in understanding their expression profiles. Here, we describe a set of quantitative PCR assays that specifically measures the mRNA levels of each APOBEC3 gene. The specificity and sensitivity of each assay was validated using a full matrix of APOBEC3 cDNA templates. The assays were used to quantify the APOBEC3 repertoire in multiple human T-cell lines, bulk leukocytes and leukocyte subsets, and 20 different human tissues. The data demonstrate that multiple APOBEC3 genes are expressed constitutively in most types of cells and tissues, and that distinct APOBEC3 genes are induced upon T-cell activation and interferon treatment. These data help define the APOBEC3 repertoire relevant to HIV-1 restriction in T cells, and they suggest a general model in which multiple APOBEC3 proteins function together to provide a constitutive barrier to foreign genetic elements, which can be fortified by transcriptional induction.

Monocytic suppressive cells mediate cardiovascular transplantation tolerance in mice

One of the main unresolved questions in solid organ transplantation is how to establish indefinite graft survival that is free from long-term treatment with immunosuppressive drugs and chronic rejection (i.e., the establishment of tolerance). The failure to achieve this goal may be related to the difficulty in identifying the phenotype and function of the cell subsets that participate in the induction of tolerance. To address this issue, we investigated the suppressive roles of recipient myeloid cells that may be manipulated to induce tolerance to transplanted hearts in mice. Using depleting mAbs, clodronate-loaded liposomes, and transgenic mice specific for depletion of CD11c+, CD11b+, or CD115+ cells, we identified a tolerogenic role for CD11b+CD115+Gr1+ monocytes during the induction of tolerance by costimulatory blockade with CD40L-specific mAb. Early after transplantation, Gr1+ monocytes migrated from the bone marrow into the transplanted organ, where they prevented the initiation of adaptive immune responses that lead to allograft rejection and participated in the development of Tregs. Our results suggest that mobilization of bone marrow CD11b+CD115+Gr1+ monocytes under sterile inflammatory conditions mediates the induction of indefinite allograft survival. We propose that manipulating the common bone marrow monocyte progenitor could be a useful clinical therapeutic approach for inducing transplantation tolerance.

The expression and functions of toll-like receptors in atherosclerosis

Inflammation drives atherosclerosis. Both immune and resident vascular cell types are involved in the development of atherosclerotic lesions. The phenotype and function of these cells are key in determining the development of lesions. Toll-like receptors are the most characterised innate immune receptors and are responsible for the recognition of exogenous conserved motifs on pathogens, and, potentially, some endogenous molecules. Both endogenous and exogenous TLR agonists may be present in atherosclerotic plaques. Engagement of toll-like receptors on immune and resident vascular cells can affect atherogenesis as signalling downstream of these receptors can elicit proinflammatory cytokine release, lipid uptake, and foam cell formation and activate cells of the adaptive immune system. In this paper, we will describe the expression of TLRs on immune and resident vascular cells, highlight the TLR ligands that may act through TLRs on these cells, and discuss the consequences of TLR activation in atherosclerosis.

Myeloperoxidase-rich Ly-6C+ myeloid cells infiltrate allografts and contribute to an imaging signature of organ rejection in mice

Rates of graft rejection are high among recipients of heart transplants. The onset and progression of clinically significant heart transplant rejection are currently monitored by serial biopsy, but this approach is highly invasive and lacks sensitivity. Here, we have developed what we believe to be a new technique to measure organ rejection noninvasively that involves the exploration of tissue-infiltrating leukocytes as biomarker sources for diagnostic imaging. Specifically, we profiled the myeloid response in a murine model of heart transplantation with the aim of defining and validating an imaging signature of graft rejection. Ly-6Chi monocytes, which promote inflammation, accumulated progressively in allografts but only transiently in isografts. Ly-6Clo monocytes, which help resolve inflammation, did not accumulate, although they composed the majority of the few remaining monocytes in isografts. The persistence of Ly-6Chi monocytes in allografts prompted us to screen for a Ly-6Chi monocyte-associated imaging marker. Low-density array data revealed that Ly-6Chi monocytes express 10-fold higher levels of myeloperoxidase (MPO) than Ly-6Clo monocytes. Noninvasive magnetic resonance imaging of MPO with an MPO-activatable Gd-chelate revealed a spatially defined T1-weighted signal in rejected allografts but not in isografts or MPO-deficient allograft recipients. Flow cytometry, enzymography, and histology validated the approach by mapping MPO activity to Ly-6Chi monocytes and neutrophils. Thus, MPO imaging represents a potential alternative to the current invasive clinical standard by which transplants are monitored.

Cellular association and assembly of a multistage delivery system

The realization that blood-borne delivery systems must overcome a multiplicity of biological barriers has led to the fabrication of a multistage delivery system (MDS) designed to temporally release successive stages of particles or agents to conquer sequential barriers, with the goal of enhancing delivery of therapeutic and diagnostic agents to the target site. In its simplest form, the MDS comprises stage-one porous silicon microparticles that function as carriers of second-stage nanoparticles. Cellular uptake of nontargeted discoidal silicon microparticles by macrophages is confirmed by electron and atomic force microscopy (AFM). Using superparamagnetic iron oxide nanoparticles (SPIONs) as a model of secondary nanoparticles, successful loading of the porous matrix of silicon microparticles is achieved, and retention of the nanoparticles is enhanced by aminosilylation of the loaded microparticles with 3-aminopropyltriethoxysilane. The impact of silane concentration and reaction time on the nature of the silane polymer on porous silicon is investigated by AFM and X-ray photoelectron microscopy. Tissue samples from mice intravenously administered the MDS support co-localization of silicon microparticles and SPIONs across various tissues with enhanced SPION release in spleen, compared to liver and lungs, and enhanced retention of SPIONs following silane capping of the MDS. Phantom models of the SPION-loaded MDS display negative contrast in magnetic resonance images. In addition to forming a cap over the silicon pores, the silane polymer provides free amines for antibody conjugation to the microparticles, with both VEGFR-2- and PECAM-specific antibodies leading to enhanced endothelial association. This study demonstrates the assembly and cellular association of a multiparticle delivery system that is biomolecularly targeted and has potential for applications in biological imaging.

Age-dependent alterations of monocyte subsets and monocyte-related chemokine pathways in healthy adults

Background

Recent experimental approaches have unraveled essential migratory and functional differences of monocyte subpopulations in mice. In order to possibly translate these findings into human physiology and pathophysiology, human monocyte subsets need to be carefully revisited in health and disease. In analogy to murine studies, we hypothesized that human monocyte subsets dynamically change during ageing, potentially influencing their functionality and contributing to immunosenescence.

Results

Circulating monocyte subsets, surface marker and chemokine receptor expression were analyzed in 181 healthy volunteers (median age 42, range 18-88). Unlike the unaffected total leukocyte or total monocyte counts, non-classical CD14⁺CD16⁺ monocytes significantly increased with age, but displayed reduced HLA-DR and CX₃CR1 surface expression in the elderly. Classical CD14⁺⁺CD16^- monocyte counts did not vary dependent on age. Serum MCP-1 (CCL2), but not MIP1α (CCL3), MIP1β (CCL4) or fractalkine (CX₃CL1) concentrations increased with age. Monocyte-derived macrophages from old or young individuals did not differ with respect to cytokine release in vitro at steady state or upon LPS stimulation.

Conclusions

Our study demonstrates dynamic changes of circulating monocytes during ageing in humans. The expansion of the non-classical CD14⁺CD16⁺ subtype, alterations of surface protein and chemokine receptor expression as well as circulating monocyte-related chemokines possibly contribute to the preserved functionality of the monocyte pool throughout adulthood.

Stem cells in the spleen: therapeutic potential for Sjogren's syndrome, type I diabetes, and other disorders

The view of the spleen as an unnecessary organ has been shattered. The evidence shows the spleen to be a source of naturally-occurring multipotent stem cells with possibly pluripotent potential. The stem cells are sequestered in the spleen of not only of animals but also of normal human adults. The reservoir of cells is set for differentiation and they need not be manipulated in vitro or ex vivo before autologous or heterologous use. Splenic stem cells, of Hox11 lineage, have been found in disease or injury to differentiate into pancreatic islets, salivary epithelial cells and osteoblast-like cells, cranial neurons, cochlea, lymphocytes, and more differentiated immune cells that repair injured heart cells. Injury or disease in target tissues induces these stem cells, still in the spleen, to upregulate the same embryonic transcription factors artificially introduced into induced pluripotent stem cells (iPS). Splenic stem cells may have broad pluripotent potential, but unlike iPS cells, possess low oncogenic risk.

Phagocyte partnership during the onset and resolution of inflammation

Neutrophils, monocytes and macrophages are closely related phagocytic cells that cooperate during the onset, progression and resolution of inflammation. This Review highlights the mechanisms involved in the intimate partnership of phagocytes during each progressive phase of the inflammatory response. We describe how tissue-resident macrophages recognize tissue damage to promote the recruitment of neutrophils and the mechanisms by which infiltrating neutrophils can then promote monocyte recruitment. Furthermore, we discuss the phagocyte-derived signals that abrogate neutrophil recruitment and how the uptake of apoptotic neutrophils by macrophages leads to termination of the inflammatory response. Finally, we highlight the potential therapeutic relevance of these interactions.

Technical advance: autofluorescence as a tool for myeloid cell analysis

Cellular AF is usually considered a hindrance to flow cytometric analysis. Here, we incorporate AF into analysis of complex mixtures of leukocytes. Using a mouse model, we examined cellular AF at multiple excitation and emission wavelengths, and populations with discrete patterns were gated and examined for surface marker expression. In the spleen, all major myeloid populations were identified. In particular, the approach allowed simultaneous characterization of RPM and resident monocytes. When monocytes and RPM were compared, RPM exhibited a phenotype that was consistent with involvement in physiological processes, including expression of genes involved in lipid and iron metabolism. The presence of large amounts of stored ferric iron within RPM enabled purification of these cells using a magnetic-based approach. When adapted for use on leukocytes isolated from a range of other organs, incorporation of AF into analysis allowed identification and isolation of biologically important myeloid populations, including subsets that were not readily identifiable by conventional cytometric analysis.

Unravelling mononuclear phagocyte heterogeneity

When Ralph Steinman and Zanvil Cohn first described dendritic cells (DCs) in 1973 it took many years to convince the immunology community that these cells were truly distinct from macrophages. Almost four decades later, the DC is regarded as the key initiator of adaptive immune responses; however, distinguishing DCs from macrophages still leads to confusion and debate in the field. Here, Nature Reviews Immunology asks five experts to discuss the issue of heterogeneity in the mononuclear phagocyte system and to give their opinion on the importance of defining these cells for future research.

Is Endothelium the Origin of Endothelial Progenitor Cells?

Endothelial cells provide the dynamic lining of blood vessels throughout the body and provide many tissue-specific functions, in addition to providing a nonthrombogenic surface for blood cells and conduit for oxygen and nutrient delivery. As might be expected, some endothelial cells are injured or become senescent and are sloughed into the bloodstream, and most circulating endothelial cells display evidence of undergoing apoptosis or necrosis. However, there are rare viable circulating endothelial cells that display properties consistent with those of a progenitor cell for the endothelial lineage. This article reviews historical and current literature to present some evidence that the endothelial lining of blood vessels may serve as a source for rare endothelial colony-forming cells that display clonal proliferative potential, self-renewal, and in vivo vessel forming ability. The article also discusses the current gaps in our knowledge to prove whether the colony-forming cells are in fact derived from vascular endothelium.

Differentiation and function of mouse monocyte-derived dendritic cells in steady state and inflammation

Although monocytes were originally described as precursors to all the different subpopulations of macrophages found in the steady state and formed under inflammatory and infectious conditions, recent data have demonstrated conclusively that monocytes can also differentiate into dendritic cells (DCs). Monocytes are the precursors to different subsets of DCs, such as Langerhans cells and DCs found in the lamina propria of the gastrointestinal, respiratory, and urogenital tracts. In addition, monocyte-derived DCs (moDCs), newly formed during inflammatory reactions, appear to fulfill an essential role in defense mechanisms against pathogens by participating in the induction of both adaptive and innate immune responses. In this regard, moDCs have the capacity to activate antigen-specific CD4(+) T-cell responses and to cross-prime CD8(+) T cells, during viral, bacterial, and parasitic infections. In addition, monocytes have been recently described as the precursors to a subset of DCs specialized in innate immunity against pathogens, named TipDCs [for TNF-alpha (tumor necrosis factor-alpha)-iNOS (inducible nitric oxide synthase)-producing DCs] that display a remarkable microbicidal activity and also provide iNOS-dependent help for antibody production by B cells. Importantly, in contrast to DCs developing in the steady state, moDCs formed during inflammatory and infectious processes are subjected to diverse soluble mediators that determine the multiple functional specificities displayed by moDCs, as a result of the remarkable developmental plasticity of monocytes. In this review, we discuss recent findings dealing with the differentiation and functional relevance of moDCs that have widened the frontiers of DC immunobiology in relation to innate and adaptive immunity and the etiology of chronic inflammatory diseases.

Platelet factor 4 regulation of monocyte KLF4 in experimental cerebral malaria

Cerebral malaria continues to be a difficult to treat complication of Plasmodium falciparum infection in children. We have shown that platelets can have major deleterious immune functions in experimental cerebral malaria (ECM). One of the platelet derived mediators we have identified as particularly important is platelet factor 4/CXCL4. Our prior work demonstrated that PF4(-/-) mice are protected from ECM, have reduced plasma cytokines, and have reduced T-cell trafficking to the brain. We now show that PF4 drives monocyte cytokine production in a Kruppel like factor 4 (KLF4) dependent manner. Monocyte depleted Plasmodium berghei infected mice have improved survival, and KLF4 is greatly increased in control, but not monocyte depleted mice. PF4(-/-) mice have less cerebral monocyte trafficking and no change in KLF4 expression. These data indicate that PF4 induction of monocyte KLF4 expression may be an important step in the pathogenesis of ECM.

Astrocytes initiate inflammation in the injured mouse spinal cord by promoting the entry of neutrophils and inflammatory monocytes in an IL-1 receptor/MyD88-dependent fashion

CNS injury stimulates the expression of several proinflammatory cytokines and chemokines, some of which including MCP-1 (also known as CCL2), KC (CXCL1), and MIP-2 (CXCL2) act to recruit Gr-1(+) leukocytes at lesion sites. While earlier studies have reported that neutrophils and monocytes/macrophages contribute to secondary tissue loss after spinal cord injury (SCI), recent work has shown that depletion of Gr-1(+) leukocytes compromised tissue healing and worsened functional recovery. Here, we demonstrate that astrocytes distributed throughout the spinal cord initially contribute to early neuroinflammation by rapidly synthesizing MCP-1, KC, and MIP-2, from 3 up to 12h post-SCI. Chemokine expression by astrocytes was followed by the infiltration of blood-derived immune cells, such as type I "inflammatory" monocytes and neutrophils, into the lesion site and nearby damaged areas. Interestingly, astrocytes from mice deficient in MyD88 signaling produced significantly less MCP-1 and MIP-2 and were unable to synthesize KC. Analysis of the contribution of MyD88-dependent receptors revealed that the astrocytic expression of MCP-1, KC, and MIP-2 was mediated by the IL-1 receptor (IL-1R1), and not by TLR2 or TLR4. Flow cytometry analysis of cells recovered from the spinal cord of MyD88- and IL-1R1-knockout mice confirmed the presence of significantly fewer type I "inflammatory" monocytes and the almost complete absence of neutrophils at 12h and 4days post-SCI. Together, these results indicate that MyD88/IL-1R1 signals regulate the entry of neutrophils and, to a lesser extent, type I "inflammatory" monocytes at sites of SCI.

Macrophages and renal fibrosis

Renal fibrosis is a key determinant of the progression of renal disease irrespective of the original cause and thus can be regarded as a final common pathway that dictates eventual outcome. The development of renal fibrosis involves many cellular and molecular mediators including leukocytes, myofibroblasts, cytokines, and growth factors, as well as metalloproteinases and their endogenous inhibitors. Study of experimental and human renal disease has shown the involvement of macrophages in renal fibrosis resulting from diverse disease processes. Recent work exploring the nature of both circulating monocytes and tissue macrophages has highlighted their multifaceted phenotype and this impacts their role in renal fibrosis in vivo. In this review we outline the key players in the fibrotic response of the injured kidney and discuss the role of monocytes and macrophages in renal scarring.

Monocyte and macrophage biology: an overview

This review provides an overview of the current understanding of the biology of monocytes and macrophages. It focuses on four rapidly advancing areas that underpin recent conceptual advances, namely: (1) the bone marrow origins of monocytes and macrophages, (2) monocyte heterogeneity, (3) the early inflammatory consequences of tissue injury, and (4) current concepts of macrophage activation and their limitations.

Myeloid cells in cancer progression: unique subtypes and their roles in tumor growth, vascularity, and host immune suppression

Leukocytic infiltrates, particularly myeloid cells, can stimulate an anti-tumor immune response, but more often they stimulate tumor development, including promoting invasion, tumor growth, angiogenesis, and metastasis. Distinct myeloid phenotypes are being characterized that have been shown to promote tumor growth, angiogenesis, and metastasis. This review provides an overview of myeloid differentiation and spotlights specific pro-tumorogenic myeloid populations and their role in cancer progression. Efforts to characterize these pro-tumorogenic myeloid cell immunophenotypes may lead to novel targets for cancer therapy.

Increased monocyte turnover from bone marrow correlates with severity of SIV encephalitis and CD163 levels in plasma

Cells of the myeloid lineage are significant targets for human immunodeficiency virus (HIV) in humans and simian immunodeficiency virus (SIV) in monkeys. Monocytes play critical roles in innate and adaptive immunity during inflammation. We hypothesize that specific subsets of monocytes expand with AIDS and drive central nervous system (CNS) disease. Additionally, there may be expansion of cells from the bone marrow through blood with subsequent macrophage accumulation in tissues driving pathogenesis. To identify monocytes that recently emigrated from bone marrow, we used 5-bromo-2′-deoxyuridine (BrdU) labeling in a longitudinal study of SIV-infected CD8+ T lymphocyte depleted macaques. Monocyte expansion and kinetics in blood was assessed and newly migrated monocyte/macrophages were identified within the CNS. Five animals developed rapid AIDS with differing severity of SIVE. The percentages of BrdU+ monocytes in these animals increased dramatically, early after infection, peaking at necropsy where the percentage of BrdU+ monocytes correlated with the severity of SIVE. Early analysis revealed changes in the percentages of BrdU+ monocytes between slow and rapid progressors as early as 8 days and consistently by 27 days post infection. Soluble CD163 (sCD163) in plasma correlated with the percentage of BrdU+ monocytes in blood, demonstrating a relationship between monocyte activation and expansion with disease. BrdU+ monocytes/macrophages were found within perivascular spaces and SIVE lesions. The majority (80–90%) of the BrdU+ cells were Mac387+ that were not productively infected. There was a minor population of CD68+BrdU+ cells (<10%), very few of which were infected (<1% of total BrdU+ cells). Our results suggest that an increased rate of monocyte recruitment from bone marrow into the blood correlates with rapid progression to AIDS, and the magnitude of BrdU+ monocytes correlates with the severity of SIVE.

Human immunodeficiency virus (HIV) and the closely related simian immunodeficiency virus (SIV) can infect monocyte/macrophages, which enter and accumulate in the brain leading to neuronal dysfunction. Monocyte/macrophages exit the bone marrow, transit through the blood and enter the central nervous system (CNS). What triggers these cells to traffic is undefined, but it occurs in normal non-infected conditions at a rate that is accelerated with viral infection. Here, we used 5-bromo-2′-deoxyuridine (BrdU) injection and incorporation into the DNA of monocytes prior to their departure from the bone marrow. We found that the percentage of BrdU+ monocytes leaving the bone marrow 24 hours after injection increased in animals that rapidly succumbed to AIDS and correlated with the severity of SIV encephalitis (SIVE). Differences in BrdU labeled monocytes in slow and rapid progressors were revealed as early as 8 days and were consistent by 27 days post infection. Soluble CD163, shed by activated monocyte/macrophages, directly correlated with BrdU+ monocyte expansion. Our study provides new insights into the development of HIV-related CNS disease and underscores the importance of monocyte/macrophage recruitment from the bone marrow as an AIDS defining event.

The vascular biology of atherosclerosis and imaging targets

The growing worldwide health challenge of atherosclerosis, together with advances in imaging technologies, have stimulated considerable interest in novel approaches to gauging this disease. The last several decades have witnessed a burgeoning in understanding of the molecular pathways involved in atherogenesis, lesion progression, and the mechanisms underlying the complications of human atherosclerotic plaques. The imaging of atherosclerosis is reaching beyond anatomy to encompass assessment of aspects of plaque biology related to the pathogenesis and complication of the disease. The harnessing of these biologic insights promises to provide a plethora of new targets for molecular imaging of atherosclerosis. The goals for the years to come must include translation of the experimental work to visualization of these appealing biologic targets in humans.

Hybrid PET-optical imaging using targeted probes

Fusion imaging of radionuclide-based molecular (PET) and structural data [x-ray computed tomography (CT)] has been firmly established. Here we show that optical measurements [fluorescence-mediated tomography (FMT)] show exquisite congruence to radionuclide measurements and that information can be seamlessly integrated and visualized. Using biocompatible nanoparticles as a generic platform (containing a (18)F isotope and a far red fluorochrome), we show good correlations between FMT and PET in probe concentration (r(2) > 0.99) and spatial signal distribution (r(2) > 0.85). Using a mouse model of cancer and different imaging probes to measure tumoral proteases, macrophage content and integrin expression simultaneously, we demonstrate the distinct tumoral locations of probes in multiple channels in vivo. The findings also suggest that FMT can serve as a surrogate modality for the screening and development of radionuclide-based imaging agents.

Nonresolving inflammation

Nonresolving inflammation is a major driver of disease. Perpetuation of inflammation is an inherent risk because inflammation can damage tissue and necrosis can provoke inflammation. Nonetheless, multiple mechanisms normally ensure resolution. Cells like macrophages switch phenotypes, secreted molecules like reactive oxygen intermediates switch impact from pro- to anti-inflammatory, and additional mediators of resolution arise, including proteins, lipids, and gasses. Aside from persistence of initiating stimuli, nonresolution may result from deficiencies in these mechanisms when an inflammatory response begins either excessively or subnormally. This greatly complicates the development of anti-inflammatory therapies. The problem calls for conceptual, organizational, and statistical innovations.