Heme oxygenase-1 (HO-1) is an intracellular enzyme that degrades heme and inhibits immune responses and inflammation in vivo. In most cell types, HO-1 is inducible by inflammatory stimuli and oxidative stress. Here we demonstrate that human monocyte-derived immature dendritic cells (iDCs) and several but not all freshly isolated rat splenic DC subsets and rat bone marrow-derived iDCs, spontaneously express HO-1. HO-1 expression drastically decreases during human and rat DC maturation induced in vitro. In IntroductionHeme oxygenases (HOs) are the rate-limiting intracellular enzymes that degrade heme to biliverdin, free divalent iron, and CO (for a review, see Otterbein and Choi 1 ). Three distinct HO enzymes have been identified: HO-1, HO-2, and HO-3. 1 HO-1 is a stress responsive gene whose expression is induced by a variety of stimuli including heme, heavy metals, inflammatory cytokines, and nitric oxide. 1 HO-1 is known for its cytoprotective effect against oxidative injuries and inflammation. 1 Induction of HO-1 expression by pharmacologic activators or gene transfer has had therapeutic effects in a variety of conditions or disorders involving the immune system, including transplantation and inflammatory disorders. [2][3][4][5][6][7][8] Biliverdin and its metabolite, bilirubin, are known for their antioxidant 9 and immunosuppressive effects. 10 HO-1 and CO have been shown to inhibit lipopolysaccharide (LPS)-induced expression of proinflammatory cytokines and to increase LPS-induced expression of interleukin 10 (IL-10) in macrophages. 11,12 Moreover, IL-10 induces HO-1 expression in macrophages. [13][14][15] We previously reported that overexpression of HO-1, obtained with an HO-1-encoding adenovirus in rats having heart transplants, results in long-term allograft survival associated with an inhibition of cellular allogeneic immune responses, which could be mediated by adenoviral transduction of dendritic cells (DCs). 6 DCs play a central role in the induction of immunity and tolerance (for a review, see Steinman et al 16 ). In the absence of inflammation, immature DCs (iDCs) located in peripheral tissues specialize in taking up innocuous and cell-associated self antigens.They continuously capture antigens and migrate to draining lymph nodes where they can induce tolerance. 16 In the presence of danger signals, DCs undergo maturation, a process involving upregulation of surface major histocompatibility complex (MHC) class II and costimulatory molecules, secretion of proinflammatory and anti-inflammatory cytokines, and the acquired ability to stimulate differentiation of naive T cells into effector cells.Our working hypothesis was that DCs can express HO-1, which can regulate DC functions. In this study, we demonstrate that human and rat iDCs express HO-1 and that HO-1 expression is down-regulated by maturation stimuli. Our results also demonstrate that induction of HO-1 expression renders DCs refractory to LPS-induced maturation, but preserves IL-10 secretion, suggesting that HO-1 may be used to regulate DC f...

The mannose receptor (MR) is a C-type lectin expressed by dendritic cells (DCs). We have investigated the ability of MR to recognize glycosylated allergens. Using a gene silencing strategy, we have specifically inhibited the expression of MR on human monocyte-derived DCs. We show that MR mediates internalization of diverse allergens from mite (Der p 1 and Der p 2), dog (Can f 1), cockroach (Bla g 2), and peanut (Ara h 1) through their carbohydrate moieties. All of these allergens bind to the C-type lectin-like carbohydrate recognition domains 4–7 of MR. We have also assessed the contribution of MR to T cell polarization after allergen exposure. We show that silencing MR expression on monocyte-derived DCs reverses the Th2 cell polarization bias, driven by Der p 1 allergen exposure, through upregulation of IDO activity. In conclusion, our work demonstrates a major role for MR in glycoallergen recognition and in the development of Th2 responses.

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...