Carbon Monoxide Inhibits TLR-Induced Dendritic Cell Immunogenicity

Abstract

Heme oxygenase-1 (HO-1) exerts its functions via the catabolism of heme into carbon monoxide (CO), Fe 2؉ , and biliverdin, as well as by depletion of free heme. We have recently described that overexpression of HO-1 is associated with the tolerogenic capacity to dendritic cells (DCs) stimulated by LPS. In this study, we demonstrate that treatment of human monocytederived DCs with CO blocks TLR3 and 4-induced phenotypic maturation, secretion of proinflammatory cytokines, and alloreactive T cell proliferation, while preserving IL-10 production. Treatment of DCs with biliverdin, bilirubin, and deferoxamine or replenishing intracellular heme stores had no effect on DC maturation. HO-1 and CO inhibited LPS-induced activation of the IFN regulatory factor 3 pathway and their effects were independent of p38, ERK, and JNK MAPK. H eme oxygenases are the rate-limiting enzymes in the catabolism of heme, yielding equimolar amounts or carbon monoxide (CO), 5 free iron, and biliverdin (BV) (1), which is subsequently reduced into bilirubin (BL). Heme oxygenase 1 (HO-1), the inducible form of heme oxygenases, has protective effects in a variety of experimental inflammatory models (reviewed in Ref.2). The physiological importance of HO-1 has been demonstrated in both mice and humans, where HO-1 deficiency resulted in a progressive and chronic inflammation and a reduced cellular resistance to oxidative stress (3-5). Induction of HO-1 expression by pharmacological activators or gene transfer have therapeutic effects in a variety of conditions or disorders involving inflammation and immune responses, including organ transplantation and autoimmunity (6 -12). In several models, CO mimics the effects of HO-1 (reviewed in Ref. 13), indicating that HO-1 acts via the generation of CO. However, other end products of HO-1 activity, such as BV (14), free iron depletion by increased H chain ferritin expression (15), or cellular efflux pumps (16), or heme depletion (17) can also mediate the effects of HO-1.Dendritic cells (DCs) play a major role in the initiation and regulation of the immune response. They have distinct stages of cell development, activation, and maturation and have the potential to induce both immunity and tolerance (reviewed in Ref. 18). In the absence of inflammation, immature DCs (iDCs) located in peripheral tissues continuously capture innocuous and cell-associated self-Ags and migrate to draining lymph nodes where they can induce tolerance (19). In the presence of danger and TLR signals, DCs mature, acquiring the ability to stimulate differentiation of naive T cells into effector cells. In certain conditions, phenotypically mature DCs have tolerogenic functions (18).We previously showed that human and rat iDCs express HO-1, that this expression is restricted to certain DC populations, and that HO-1 expression drastically decreases upon DC maturation (20). We and others have demonstrated that overexpression of HO-1 in DCs inhibits their LPS-induced maturation and proinflammatory functions (20,21), and it has been recently...

The Mononuclear Phagocyte System in Homeostasis and Disease: A Role for Heme Oxygenase-1

“…A role for CD8 + DC in antigen cross presentation to CD8 + T cells is further supported by the work of Remy and colleagues using a mouse model of DC-induced diabetes in Inshemagglutinin (HA) mice (136), which express the HA antigen on islet cells of the pancreas. In these mice, co-adoptive transfer of naïve anti-HA CD8 + T cells with HA peptideloaded, LPS-matured DC results in islet cell destruction and diabetes development in 6-9 days (113).…”

“…However, when DC are co-treated with either cobalt protoporphyrin (CoPP; to induce HO-1 overexpression) or CO releasing molecule 2 (CORM2; tricarbonyldichlororuthenium II), the induction of diabetes in Ins-HA mice is prevented indefinitely. Thus, HO-1 and its byproduct, CO, inhibit antigen cross-presentationmediated immunogenicity of DC in vivo, which appears to result from HO-1/CO-mediated inhibition of DC maturation and IRF-3-dependent cytokine production (39,136). Although HO-1 induction blocks LPS-induced phenotypic maturation, pro-inflammatory cytokine secretion, and alloreactive T-cell proliferation, the ability to produce anti-inflammatory IL-10 is retained (39).…”

Immunoregulatory Effects of Stored Red Blood Cells

“…17 HO-1 is considered immunosuppressive because it was shown to block the maturation of dendritic cells and to inhibit proinflammatory and allogeneic immune responses. 18,19 Dendritic cells treated with HO-1 and CO can inhibit ROS and the production of proinflammatory cytokines such as IL-12, IL-6, TNF-␣, and IFN type I. 19,20 Conversely, the antiinflammatory IL-10 cytokine is increased after HO-1 overexpression or CO, and CO alone has been shown to inhibit T-lymphocyte proliferation.…”

“…18,19 Dendritic cells treated with HO-1 and CO can inhibit ROS and the production of proinflammatory cytokines such as IL-12, IL-6, TNF-␣, and IFN type I. 19,20 Conversely, the antiinflammatory IL-10 cytokine is increased after HO-1 overexpression or CO, and CO alone has been shown to inhibit T-lymphocyte proliferation. 21 This has led to the proposal that HO-1 and CO may mediate immune tolerance by favoring the induction of master immunosuppressor cells, the so-called regulatory T cells (Tregs).…”

Development of inhibitory antibodies to therapeutic factor VIII in severe hemophilia A is associated with microsatellite polymorphisms in the HMOX1 promoter

“…34 It is possible that, in hemophilia A patients with shorter (GT) n repeats in the promoter region of the HMOX1 gene, mild inflammatory signals at the sites of injury are sufficient to induce HO-1, thus reducing the maturation of antigen-presenting cells and decreasing the probability of initiating an anti-FVIII immune response. 39,40 Conversely, similar recurrent joint bleeding in patients with long (GT) n repeats would fail to induce sufficient levels of HO-1, and would leave the patients at a greater risk of developing FVIII inhibitors. The present set of data suggests that the capacity of patients with hemophilia A to cope with inflammation, whether induced by recurrent hemorrhages or by other yet uncharacterized stimuli, may be one of the critical parameters that control the development of the immune response to FVIII.…”