Carbon monoxide-treated dendritic cells decrease β1-integrin induction on CD8⁺ T cells and protect from type 1 diabetes

A carbon monoxide releasing metal organic framework nanoplatform for synergistic treatment of triple-negative breast tumors

BACKGROUND

Carbon monoxide (CO) is an important signaling molecule participating in multiple biological functions. Previous studies have confirmed the valuable roles of CO in cancer therapies. If the CO concentration and distribution can be controlled in tumors, new cancer therapeutic strategy may be developed to benefit the patient survival.

RESULTS

In this study, a UiO-67 type metal-organic framework (MOF) nanoplatform was produced with cobalt and ruthenium ions incorporated into its structure (Co/Ru-UiO-67). Co/Ru-UiO-67 had a size range of 70-90 nm and maintained the porous structure, with cobalt and ruthenium distributed uniformly inside. Co/Ru-UiO-67 was able to catalyze carbon dioxide into CO upon light irradiation in an efficient manner with a catalysis speed of 5.6 nmol/min per 1 mg Co/Ru-UiO-67. Due to abnormal metabolic properties of tumor cells, tumor microenvironment usually contains abundant amount of CO₂. Co/Ru-UiO-67 can transform tumor CO₂ into CO at both cellular level and living tissues, which consequently interacts with relevant signaling pathways (e.g. Notch-1, MMPs etc.) to adjust tumor microenvironment. With proper PEGylation (pyrene-polyacrylic acid-polyethylene glycol, Py-PAA-PEG) and attachment of a tumor-homing peptide (F3), functionalized Co/Ru-UiO-67 could accumulate strongly in triple-negative MDA-MB-231 breast tumors, witnessed by positron emission tomography (PET) imaging after the addition of radioactive zirconium-89 (⁸⁹Zr) into Co-UiO-67. When applied in vivo, Co/Ru-UiO-67 could alter the local hypoxic condition of MDA-MB-231 tumors, and work synergistically with tirapazamine (TPZ).

CONCLUSION

This nanoscale UiO-67 MOF platform can further our understanding of CO functions while produce CO in a controllable manner during cancer therapeutic administration.

Optimal Tolerogenic Dendritic Cells in Type 1 Diabetes (T1D) Therapy: What Can We Learn From Non-obese Diabetic (NOD) Mouse Models?

Tolerogenic dendritic cells (tolDCs) are explored as a promising standalone or combination therapy in type 1 diabetes (T1D). The therapeutic application of tolDCs, including in human trials, has been tested also in other autoimmune diseases, however, T1D displays some unique features. In addition, unlike in several disease-induced animal models of autoimmune diseases, the prevalent animal model for T1D, the NOD mouse, develops diabetes spontaneously. This review compares evidence of various tolDCs approaches obtained from animal (mainly NOD) models of T1D with a focus on parameters of this cell-based therapy such as protocols of tolDC preparation, antigen-specific vs. unspecific approaches, doses of tolDCs and/or autoantigens, application schemes, application routes, the migration of tolDCs as well as their preventive, early pre-onset intervention or curative effects. This review also discusses perspectives of tolDC therapy and areas of preclinical research that are in need of better clarification in animal models in a quest for effective and optimal tolDC therapies of T1D in humans.

Genetic Restoration of Heme Oxygenase-1 Expression Protects from Type 1 Diabetes in NOD Mice

Antigen-presenting cells (APCs) including dendritic cells (DCs) play a critical role in the development of autoimmune diseases by presenting self-antigen to T-cells. Different signals modulate the ability of APCs to activate or tolerize autoreactive T-cells. Since the expression of heme oxygenase-1 (HO-1) by APCs has been associated with the tolerization of autoreactive T-cells, we hypothesized that HO-1 expression might be altered in APCs from autoimmune-prone non-obese diabetic (NOD) mice. We found that, compared to control mice, NOD mice exhibited a lower percentage of HO-1-expressing cells among the splenic DCs, suggesting an impairment of their tolerogenic functions. To investigate whether restored expression of HO-1 in APCs could alter the development of diabetes in NOD mice, we generated a transgenic mouse strain in which HO-1 expression can be specifically induced in DCs using a tetracycline-controlled transcriptional activation system. Mice in which HO-1 expression was induced in DCs exhibited a lower Type 1 Diabetes (T1D) incidence and a reduced insulitis compared to non-induced mice. Upregulation of HO-1 in DCs also prevented further increase of glycemia in recently diabetic NOD mice. Altogether, our data demonstrated the potential of induction of HO-1 expression in DCs as a preventative treatment, and potential as a curative approach for T1D.

An Overview of the Potential Therapeutic Applications of CO-Releasing Molecules

Carbon monoxide (CO) has long been known as the “silent killer” owing to its ability to form carboxyhemoglobin—the main cause of CO poisoning in humans. Its role as an endogenous neurotransmitter, however, was suggested in the early 1990s. Since then, the biological activity of CO has been widely examined via both the direct administration of CO and in the form of so-called “carbon monoxide releasing molecules (CORMs).” This overview will explore the general physiological effects and potential therapeutic applications of CO when delivered in the form of CORMs.

Carbon monoxide ameliorates murine T-cell-dependent colitis through the inhibition of Th17 differentiation

Recent studies have identified carbon monoxide (CO) as a potential therapeutic molecule for the treatment of inflammatory diseases including intestinal inflammation. In the present study, we explored the efficacy and the mechanisms of action of CO-releasing molecule (CORM)-A1 in T-cell transfer induced colitis model in mice. In addition, the impact of CORM-A1 on the T helper (Th) cell differentiation was evaluated using naïve CD4⁺ T cells isolated from the spleens in Balb/c mice. The results showed that CORM-A1 conferred protection against the development of intestinal inflammation and attenuated Th17 cell differentiation. Hence, the observed immunomodulatory effects of CORM-A1 could be useful for developing novel therapeutic approaches for managing intestinal inflammation through the regulation of Th17 differentiation.

Carbon monoxide in lung cell physiology and disease

Carbon monoxide (CO) is an endogenously produced gas that has gained recognition as a biological signal transduction effector with properties similar, but not identical, to that of nitric oxide (NO). CO, which binds primarily to heme iron, may activate the hemoprotein guanylate cyclase, although with lower potency than NO. Furthermore, CO can modulate the activities of several cellular signaling molecules such as p38 MAPK, ERK1/2, JNK, Akt, NF-κB, and others. Emerging studies suggest that mitochondria, the energy-generating organelle of cells, represent a key target of CO action in eukaryotes. Dose-dependent modulation of mitochondrial function by CO can result in alteration of mitochondrial membrane potential, mitochondrial reactive oxygen species production, release of proapoptotic and proinflammatory mediators, as well as the inhibition of respiration at high concentration. CO, through modulation of signaling pathways, can impact key biological processes including autophagy, mitochondrial biogenesis, programmed cell death (apoptosis), cellular proliferation, inflammation, and innate immune responses. Inhaled CO is widely known as an inhalation hazard due to its rapid complexation with hemoglobin, resulting in impaired oxygen delivery to tissues and hypoxemia. Despite systemic and cellular toxicity at high concentrations, CO has demonstrated cyto- and tissue-protective effects at low concentration in animal models of organ injury and disease. These include models of acute lung injury (e.g., hyperoxia, hypoxia, ischemia-reperfusion, mechanical ventilation, bleomycin) and sepsis. The success of CO as a candidate therapeutic in preclinical models suggests potential clinical application in inflammatory and proliferative disorders, which is currently under evaluation in clinical trials.

Carbon monoxide regulates glycolysis-dependent NLRP3 inflammasome activation in macrophages

Low dose of carbon monoxide (CO) has anti-inflammatory role through various signaling pathways. Cellular metabolism has been implicated in the activation of inflammation in immune cells. However, the mechanisms by which CO-dependent metabolic regulation affect the immune response remain unclear. Here we show that CO-dependent metabolic pathway regulates the activation of the nucleotide-binding domain, leucine-rich-repeat-containing receptor (NLR), pyrin-domain-containing 3 (NLRP3) inflammasome. CO-releasing molecule-3 (CORM-3) resulted in reduced glycolysis-dependent NLRP3 inflammasome activation in macrophages. The reduced mTORC1 activation by CORM-3 resulted in less glycolysis during NLRP3 inflammasome activation. CORM-3 suppressed caspase-1 activation and the secretion of interleukin (IL)-1β and IL-18 in macrophages in response to lipopolysaccharide (LPS) and ATP. Moreover, CORM-3 inhibits the oligomerization of the adaptor protein apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), which is required for NLRP3-dependent caspase-1 activation. Furthermore, CORM-3-treated mice showed substantial reduction in IL-1β production by hyperglycemia in a mouse model of streptozotocin (STZ)-induced diabetes. Our results suggest that CO regulates glycolysis-dependent NLRP3 inflammasome activation and may provide a therapeutic approach for inflammation in metabolic diseases.

Inhibition of effector antigen-specific T cells by intradermal administration of heme oxygenase-1 inducers

Developing protocols aimed at inhibiting effector T cells would be key for the treatment of T cell-dependent autoimmune diseases including type 1 autoimmune diabetes (T1D) and multiple sclerosis (MS). While heme oxygenase-1 (HO-1) inducers are clinically approved drugs for non-immune-related diseases, they do have immunosuppressive properties when administered systemically in rodents. Here we show that HO-1 inducers inhibit antigen-specific effector T cells when injected intradermally together with the T cell cognate antigens in mice. This phenomenon was observed in both a CD8⁺ T cell-mediated model of T1D and in a CD4⁺ T cell-dependent MS model. Intradermal injection of HO-1 inducers induced the recruitment of HO-1⁺ monocyte-derived dendritic cell (MoDCs) exclusively to the lymph nodes (LN) draining the site of intradermal injection. After encountering HO-1⁺MoDCs, effector T-cells exhibited a lower velocity and a reduced ability to migrate towards chemokine gradients resulting in impaired accumulation to the inflamed organ. Intradermal co-injection of a clinically approved HO-1 inducer and a specific antigen to non-human primates also induced HO-1⁺ MoDCs to accumulate in dermal draining LN and to suppress delayed-type hypersensitivity. Therefore, in both mice and non-human primates, HO-1 inducers delivered locally inhibited effector T-cells in an antigen-specific manner, paving the way for repositioning these drugs for the treatment of immune-mediated diseases.

Impacts of cigarette smoking on immune responsiveness: Up and down or upside down?

Cigarette smoking is associated with numerous diseases and poses a serious challenge to the current healthcare system worldwide. Smoking impacts both innate and adaptive immunity and plays dual roles in regulating immunity by either exacerbation of pathogenic immune responses or attenuation of defensive immunity. Adaptive immune cells affected by smoking mainly include T helper cells (Th1/Th2/Th17), CD4+CD25+ regulatory T cells, CD8+ T cells, B cells and memory T/B lymphocytes while innate immune cells impacted by smoking are mostly DCs, macrophages and NK cells. Complex roles of cigarette smoke have resulted in numerous diseases, including cardiovascular, respiratory and autoimmune diseases, allergies, cancers and transplant rejection etc. Although previous reviews have described the effects of smoking on various diseases and regional immunity associated with specific diseases, a comprehensive and updated review is rarely seen to demonstrate impacts of smoking on general immunity and, especially on major components of immune cells. Here, we aim to systematically and objectively review the influence of smoking on major components of both innate and adaptive immune cells, and summarize cellular and molecular mechanisms underlying effects of cigarette smoking on the immune system. The molecular pathways impacted by cigarette smoking involve NFκB, MAP kinases and histone modification. Further investigations are warranted to understand the exact mechanisms responsible for smoking-mediated immunopathology and to answer lingering questions over why cigarette smoking is always harmful rather than beneficial even though it exerts dual effects on immune responses.

Targeting heme oxygenase-1 and carbon monoxide for therapeutic modulation of inflammation

The heme oxygenase-1 (HO-1) enzyme system remains an attractive therapeutic target for the treatment of inflammatory conditions. HO-1, a cellular stress protein, serves a vital metabolic function as the rate-limiting step in the degradation of heme to generate carbon monoxide (CO), iron, and biliverdin-IXα (BV), the latter which is converted to bilirubin-IXα (BR). HO-1 may function as a pleiotropic regulator of inflammatory signaling programs through the generation of its biologically active end products, namely CO, BV and BR. CO, when applied exogenously, can affect apoptotic, proliferative, and inflammatory cellular programs. Specifically, CO can modulate the production of proinflammatory or anti-inflammatory cytokines and mediators. HO-1 and CO may also have immunomodulatory effects with respect to regulating the functions of antigen-presenting cells, dendritic cells, and regulatory T cells. Therapeutic strategies to modulate HO-1 in disease include the application of natural-inducing compounds and gene therapy approaches for the targeted genetic overexpression or knockdown of HO-1. Several compounds have been used therapeutically to inhibit HO activity, including competitive inhibitors of the metalloporphyrin series or noncompetitive isoform-selective derivatives of imidazole-dioxolanes. The end products of HO activity, CO, BV and BR may be used therapeutically as pharmacologic treatments. CO may be applied by inhalation or through the use of CO-releasing molecules. This review will discuss HO-1 as a therapeutic target in diseases involving inflammation, including lung and vascular injury, sepsis, ischemia-reperfusion injury, and transplant rejection.

Haeme oxygenase signalling pathway: implications for cardiovascular disease

Evidence now points to the haeme oxygenase (HO) pathway as a possible actor in modulating risk for cardiovascular disease (CVD). In particular, the HO pathway may represent a key endogenous modulator of oxidative, inflammatory, and cytotoxic stress while also exhibiting vasoregulatory properties. In this review, we summarize the accumulating experimental and emerging clinical data indicating how activity of the HO pathway and its products may play a role in mechanisms underlying the development of CVD. We also identify gaps in the literature to date and suggest future directions for investigation. Because HO pathway activity can be influenced not only by genetic traits and environmental stimuli but also by a variety of existing pharmacologic interventions, the pathway could serve as a prime target for reducing the overall burden of CVD. Further work is needed to determine the role of HO pathway products as possible prognostic markers of risk for clinical CVD events and the extent to which therapeutic augmentation or inhibition of HO pathway activity could serve to modify CVD risk.

Anti-diabetic actions of carbon monoxide-releasing molecule (CORM)-A1: Immunomodulation and regeneration of islet beta cells

We have recently shown that carbon monoxide releasing molecule (CORM)-A1 prevents type 1 diabetes induced in C57BL/6 mice with multiple low doses of streptozotocin (MLDS) by shifting the Th1/Th17/M1 balance towards a Th2/M2 response. In the present work we tested the hypothesis that CORM-A1 might influence regulatory arm of the immune response, as well as beta cell regeneration. CORM-A1 (2 mg/kg/day) was administered for 10 days to mice induced with MLDS and/or depleted of low dose cyclophosphamide (CY)-sensitive FoxP3+ T regulatory (Treg) cells. Besides monitoring hyperglycaemia, ex vivo analysis of spleen, pancreatic lymph nodes (PLN) and pancreas was performed at the end of treatment. In CORM-A1-treated MLDS-induced mice the improvement of hyperglycaemia was observed only without depletion of CY-sensitive FoxP3+ Treg cells. This was accompanied by decreased levels of interleukin (IL)-12, IL-2 and early activation marker CD25 in the spleen and PLN and increased transforming growth factor (TGF)-β, resulting in reduced lymphocyte proliferation in both organs. In parallel, decreased transcript levels of IL-2, but increased mRNA expression of TGF-β, accompanied with up-regulation of Ki-67 protein expression was observed within pancreas. Together, the data suggested that besides the immunomodulatory potential, CORM-A1 probably induces beta cell regeneration.

Skewing dendritic cell differentiation towards a tolerogenic state for recovery of tolerance in rheumatoid arthritis

To date, the available options to treat autoimmune diseases such as rheumatoid arthritis (RA) include traditional corticoids and biological drugs, which are not exempt of adverse effects. The development of cellular therapies based on dendritic cells with tolerogenic functions (TolDCs) has opened a new possibility to efficiently eradicate symptoms and control the immune response in the field of autoimmunity. TolDCs are an attractive tool for antigen-specific immunotherapy to restore self-tolerance in RA and other autoimmune disorders. A promising strategy is to inject autologous self-antigen-loaded TolDCs, which are able to delete or reprogram autoreactive T cells. Different protocols for the generation of stable human TolDCs have been established and the therapeutic effect of TolDCs has been investigated in multiple rodent models of arthritis. Pilot studies in humans confirmed that TolDC application is safe, encouraging clinical trials using self-antigen-loaded TolDCs in RA patients. Although an abundance of molecular regulators of DC functions has been discovered in the last decade, no master regulator of tolerogenicity has been identified yet. Further research is required to define biomarkers or key regulators of tolerogenicity that might facilitate the induction and monitoring of TolDCs.

Carbon monoxide down-regulates α4β1 integrin-specific ligand binding and cell adhesion: a possible mechanism for cell mobilization

Background

Carbon monoxide (CO), a byproduct of heme degradation, is attracting growing attention from the scientific community. At physiological concentrations, CO plays a role as a signal messenger that regulates a number of physiological processes. CO releasing molecules are under evaluation in preclinical models for the management of inflammation, sepsis, ischemia/reperfusion injury, and organ transplantation. Because of our discovery that nitric oxide signaling actively down-regulates integrin affinity and cell adhesion, and the similarity between nitric oxide and CO-dependent signaling, we studied the effects of CO on integrin signaling and cell adhesion.

Results

We used a cell permeable CO releasing molecule (CORM-2) to elevate intracellular CO, and a fluorescent Very Late Antigen-4 (VLA-4, α₄β₁-integrin)-specific ligand to evaluate the integrin state in real-time on live cells. We show that the binding of the ligand can be rapidly down-modulated in resting cells and after inside-out activation through several Gα_i-coupled receptors. Moreover, cell treatment with hemin, a natural source of CO, resulted in comparable VLA-4 ligand dissociation. Inhibition of VLA-4 ligand binding by CO had a dramatic effect on cell-cell interaction in a VLA-4/VCAM-1-dependent cell adhesion system.

Conclusions

We conclude that the CO signaling pathway can rapidly down-modulate binding of the VLA-4 -specific ligand. We propose that CO-regulated integrin deactivation provides a basis for modulation of immune cell adhesion as well as rapid cell mobilization, for example as shown for splenic monocytes in response to surgically induced ischemia of the myocardium.

Targeting dendritic cell function during systemic autoimmunity to restore tolerance

Systemic autoimmune diseases can damage nearly every tissue or cell type of the body. Although a great deal of progress has been made in understanding the pathogenesis of autoimmune diseases, current therapies have not been improved, remain unspecific and are associated with significant side effects. Because dendritic cells (DCs) play a major role in promoting immune tolerance against self-antigens (self-Ags), current efforts are focusing at generating new therapies based on the transfer of tolerogenic DCs (tolDCs) during autoimmunity. However, the feasibility of this approach during systemic autoimmunity has yet to be evaluated. TolDCs may ameliorate autoimmunity mainly by restoring T cell tolerance and, thus, indirectly modulating autoantibody development. In vitro induction of tolDCs loaded with immunodominant self-Ags and subsequent cell transfer to patients would be a specific new therapy that will avoid systemic immunosuppression. Herein, we review recent approaches evaluating the potential of tolDCs for the treatment of systemic autoimmune disorders.

Pharmacological application of carbon monoxide ameliorates islet-directed autoimmunity in mice via anti-inflammatory and anti-apoptotic effects

AIMS/HYPOTHESIS

Recent studies have identified carbon monoxide (CO) as a potential therapeutic molecule for the treatment of autoimmune diseases owing to its anti-inflammatory and anti-apoptotic properties. We explored the efficacy and the mechanisms of action of the CO-releasing molecule (CORM)-A1 in preclinical models of type 1 diabetes.

METHODS

The impact of CORM-A1 on diabetes development was evaluated in models of spontaneous diabetes in NOD mice and in diabetes induced in C57BL/6 mice by multiple low-dose streptozotocin (MLDS). Ex vivo analysis was performed to determine the impact of CORM-A1 both on T helper (Th) cell and macrophage differentiation and on their production of soluble mediators in peripheral tissues and in infiltrates of pancreatic islets. The potential effect of CORM-A1 on cytokine-induced apoptosis in pancreatic islets or beta cells was evaluated in vitro.

RESULTS

CORM-A1 conferred protection from diabetes in MLDS-induced mice and reduced diabetes incidence in NOD mice as confirmed by preserved insulin secretion and improved histological signs of the disease. In MLDS-challenged mice, CORM-A1 attenuated Th1, Th17, and M1 macrophage response and facilitated Th2 cell differentiation. In addition, CORM-A1 treatment in NOD mice upregulated the regulatory arm of the immune response (M2 macrophages and FoxP3(+) regulatory T cells). Importantly, CORM-A1 interfered with in vitro cytokine-induced beta cell apoptosis through the reduction of cytochrome c and caspase 3 levels.

CONCLUSIONS/INTERPRETATION

The ability of CORM-A1 to protect mice from developing type 1 diabetes provides a valuable proof of concept for the potential exploitation of controlled CO delivery in clinical settings for the treatment of autoimmune diabetes.

Dendritic cell subsets in type 1 diabetes: friend or foe?

Type 1 diabetes (T1D) is a T cell mediated autoimmune disease characterized by immune mediated destruction of the insulin-producing β cells in the islets of Langerhans. Dendritic cells (DC) have been implicated in the pathogenesis of T1D and are also used as immunotherapeutic agents. Plasmacytoid (p)DC have been shown to have both protective and pathogenic effects and a newly described merocytic DC population has been shown to break tolerance in the mouse model of T1D, the non-obese diabetic (NOD) mouse. We have used DC populations to prevent the onset of T1D in NOD mice and clinical trials of DC therapy in T1D diabetes have been initiated. In this review we will critically examine the recent published literature on the role of DC subsets in the induction and regulation of the autoimmune response in T1D.

Carbon monoxide exposure improves immune function in lupus-prone mice

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by multiple alterations affecting the normal function of immune cells, such as lymphocytes, dendritic cells (DCs) and monocytes. Although the understanding of autoimmunity has significantly increased, the breakthrough in effective therapies has been modest, making necessary the development of new therapeutic strategies. Here we propose that a new potential target for therapy is haem oxygenase-1 (HO-1), an enzyme that catalyses the degradation of the haem group into biliverdin, carbon monoxide (CO) and Fe(2+) . These products exhibit immunosuppressive and anti-inflammatory effects, which can contribute to improving tolerance during organ transplantation. Because HO-1 is highly expressed by immune cells involved in SLE pathogenesis, such as monocytes and DCs, we evaluated whether induction of HO-1 expression or the administration of CO could ameliorate disease in the FcγRIIb knockout (KO) mouse model for SLE. We found that CO administration decreased the expansion of CD11b(+) cells, prevented the decline of regulatory T cells and reduced anti-histone antibodies observed in untreated FcγRIIb KO mice. Furthermore, CO-treated animals and HO-1 induction showed less kidney damage compared with untreated mice. These data suggest that HO-1 modulation and CO administration can ameliorate autoimmunity and prevent the lupus symptoms shown by FcγRIIb KO mice, highlighting HO-1 as a potential new target for autoimmune therapy.

Carbon monoxide decreases endosome-lysosome fusion and inhibits soluble antigen presentation by dendritic cells to T cells

Heme oxygenase-1 (HO-1) inhibits immune responses and inflammatory reactions via the catabolism of heme into carbon monoxide (CO), Fe(2+) , and biliverdin. We have previously shown that either induction of HO-1 or treatment with exogenous CO inhibits LPS-induced maturation of dendritic cells (DCs) and protects in vivo and in vitro antigen-specific inflammation. Here, we evaluated the capacity of HO-1 and CO to regulate antigen presentation on MHC class I and MHC class II molecules by LPS-treated DCs. We observed that HO-1 and CO treatment significantly inhibited the capacity of DCs to present soluble antigens to T cells. Inhibition was restricted to soluble OVA protein, as no inhibition was observed for antigenic OVA-derived peptides, bead-bound OVA protein, or OVA as an endogenous antigen. Inhibition of soluble antigen presentation was not due to reduced antigen uptake by DCs, as endocytosis remained functional after HO-1 induction and CO treatment. On the contrary, CO significantly reduced the efficiency of fusion between late endosomes and lysosomes and not by phagosomes and lysosomes. These data suggest that HO-1 and CO can inhibit the ability of LPS-treated DCs to present exogenous soluble antigens to naïve T cells by blocking antigen trafficking at the level of late endosome-lysosome fusion.

Carbon monoxide: present and future indications for a medical gas

Gaseous molecules continue to hold new promise in molecular medicine as experimental and clinical therapeutics. The low molecular weight gas carbon monoxide (CO), and similar gaseous molecules (e.g., H2S, nitric oxide) have been implicated as potential inhalation therapies in inflammatory diseases. At high concentration, CO represents a toxic inhalation hazard, and is a common component of air pollution. CO is also produced endogenously as a product of heme degradation catalyzed by heme oxygenase enzymes. CO binds avidly to hemoglobin, causing hypoxemia and decreased oxygen delivery to tissues at high concentrations. At physiological concentrations, CO may have endogenous roles as a signal transduction molecule in the regulation of neural and vascular function and cellular homeostasis. CO has been demonstrated to act as an effective anti-inflammatory agent in preclinical animal models of inflammation, acute lung injury, sepsis, ischemia/reperfusion injury, and organ transplantation. Additional experimental indications for this gas include pulmonary fibrosis, pulmonary hypertension, metabolic diseases, and preeclampsia. The development of chemical CO releasing compounds constitutes a novel pharmaceutical approach to CO delivery with demonstrated effectiveness in sepsis models. Current and pending clinical evaluation will determine the usefulness of this gas as a therapeutic in human disease.

The immunomodulatory role of carbon monoxide during transplantation

The number of organ and tissue transplants has increased worldwide in recent decades. However, graft rejection, infections due to the use of immunosuppressive drugs and a shortage of graft donors remain major concerns. Carbon monoxide (CO) had long been regarded solely as a poisonous gas. Ultimately, physiological studies unveiled the endogenous production of CO, particularly by the heme oxygenase (HO)-1 enzyme, recognizing CO as a beneficial gas when used at therapeutic doses. The protective properties of CO led researchers to develop uses for it, resulting in devices and molecules that can deliver CO in vitro and in vivo. The resulting interest in clinical investigations was immediate. Studies regarding the CO/HO-1 modulation of immune responses and their effects on various immune disorders gave rise to transplantation research, where CO was shown to be essential in the protection against organ rejection in animal models. This review provides a perspective of how CO modulates the immune system to improve transplantation and suggests its use as a therapy in the field.