Essential role of program death 1-ligand 1 in regulatory T-cell-afforded protection against blood-brain barrier damage after stroke

BACKGROUND AND PURPOSE

Our recent research revealed that adoptively transferred regulatory T cells (Tregs) reduced acute ischemic brain injury by inhibiting neutrophil-derived matrix metalloproteinase-9 (MMP-9) and protecting against blood-brain barrier damage. The mechanisms underlying Treg interactions with neutrophils remain elusive. This study evaluates the contribution of program death 1-ligand 1 (PD-L1) to Treg-mediated neutrophil inhibition and neuroprotection after cerebral ischemia.

METHODS

In vitro experiments were performed using a transwell system or a coculture system allowing cell-to-cell contact. Focal cerebral ischemia was induced in mice for 60 minutes. Tregs (2×10(6)) isolated from donor animals (wild-type or PD-L1-/-) were intravenously injected into ischemic recipients 2 hours after middle cerebral artery occlusion (MCAO). MMP-9 production, blood-brain barrier permeability, and brain infarct were assessed at 1 or 3 days after MCAO.

RESULTS

In vitro experiments reveal that Treg-mediated inhibition of neutrophil MMP-9 required direct cell-to-cell contact. The suppression of MMP-9 was abolished when Tregs were pretreated with PD-L1 neutralizing antibodies or when neutrophils were pretreated with PD-1 antibodies. In vivo studies confirmed that intravenous administration of Tregs pretreated with PD-L1 antibodies or Tregs isolated from PD-L1-deficient mice failed to inhibit MMP-9 production by blood neutrophils 1 day after 60 minutes MCAO. Furthermore, the blood-brain barrier damage after MCAO was greatly ameliorated in PD-L1-competent Treg-treated mice but not in PD-L1-compromised Treg-treated mice. Consequently, PD-L1 dysfunction abolished Treg-mediated brain protection and neurological improvements 3 days after MCAO.

CONCLUSIONS

PD-L1 plays an essential role in the neuroprotection afforded by Tregs against cerebral ischemia by mediating the suppressive effect of Tregs on neutrophil-derived MMP-9.

IL-12hi rapamycin-conditioned dendritic cells mediate IFN-γ-dependent apoptosis of alloreactive CD4+ T cells in vitro and reduce lethal graft-versus-host disease

Rapamycin (RAPA) inhibits the mechanistic target of rapamycin (mTOR), a crucial immune system regulator. Dendritic cells (DC) generated in RAPA (RAPA-DC) enrich for CD4(+) forkhead box p3 (FoxP3(+)) regulatory T cells and induce T cell apoptosis by an unknown mechanism. RAPA-DC also promote experimental allograft survival, yet paradoxically secrete increased IL-12, crucial for the generation of IFN-γ(+) CD4(+) T cells. However, IFN-γ is pro-apoptotic and IL-12-driven IFN-γ inhibits experimental graft-versus-host disease (GVHD). We hypothesized that IL-12(hi) RAPA-DC would facilitate IFN-γ-mediated apoptosis of alloreactive T cells and, unlike control (CTR)-DC, would reduce lethal GVHD. Following LPS stimulation, RAPA-DC exhibited decreased MHCII and co-stimulatory molecules and contained a significant population of CD86(lo) IL-12(hi) cells. Consistent with our hypothesis, both unstimulated and LPS-stimulated RAPA-DC enhanced alloreactive CD4(+) T cell apoptosis in culture. Augmented T cell apoptosis was ablated by IFN-γ neutralization or using T cells lacking the IFN-γ receptor, and it was associated with increased expression of Fas and cleaved caspase 8. DC production or responses to IFN-γ were not important to increased apoptotic functions of RAPA-DC. LPS-stimulated IL-12p40(-/-) RAPA-DC induced lower levels of T cell apoptosis in culture, which was further decreased with addition of anti-IFN-γ. Finally, whereas CTR-DC accelerated mortality from GVHD, LPS-treated RAPA-DC significantly prolonged host survival. In conclusion, increased apoptosis of allogeneic CD4(+) T cells induced by LPS-stimulated IL-12(hi) RAPA-DC is mediated in vitro through IFN-γ and in part by increased IL-12 expression. Enhanced production of IL-12, the predominant inducer of IFN-γ by immune cells, is a probable mechanism underlying the capacity of LPS-treated RAPA-DC to reduce GVHD.

CD39 expression by hepatic myeloid dendritic cells attenuates inflammation in liver transplant ischemia-reperfusion injury in mice

Hepatic innate immune cells, in particular, interstitial dendritic cells (DCs), regulate inflammatory responses and may promote inherent liver tolerogenicity. After tissue injury, adenosine triphosphate (ATP) is released and acts as a damage-associated molecular pattern that activates innate immune cells by pattern recognition receptors. CD39 (ectonucleoside triphosphate diphosphohydrolase-1) rapidly hydrolyzes extracellular ATP to maintain physiological levels. We hypothesized that CD39 expression on liver DCs might contribute to regulation of their innate immune functions. Mouse liver conventional myeloid DCs (mDCs) were hyporesponsive to ATP, compared with their splenic counterparts. This disparity was ascribed to more efficient hydrolysis of ATP by higher expression of CD39 on liver mDCs. Human liver mDCs expressed greater levels of CD39 than those from peripheral blood. The comparatively high expression of CD39 on liver mDCs correlated strongly with both ATP hydrolysis and adenosine production. Notably, CD39(-/-) mouse liver mDCs exhibited a more mature phenotype, greater responsiveness to Toll-like receptor 4 ligation, and stronger proinflammatory and immunostimulatory activity than wild-type (WT) liver mDCs. To investigate the role of CD39 on liver mDCs in vivo, we performed orthotopic liver transplantation with extended cold preservation using CD39(-/-) or WT donor mouse livers. Compared to WT liver grafts, CD39(-/-) grafts exhibited enhanced interstitial DC activation, elevated proinflammatory cytokine levels, and more-severe tissue injury. Moreover, portal venous delivery of WT, but not CD39(-/-) liver mDCs, to donor livers immediately post-transplant exerted a protective effect against graft injury in CD39(-/-) to CD39(-/-) liver transplantation.

CONCLUSIONS

These data reveal that CD39 expression on conventional liver mDCs limits their proinflammatory activity and confers protective properties on these important innate immune cells against liver transplant ischemia/reperfusion injury.

Hepatic B cells are readily activated by Toll-like receptor-4 ligation and secrete less interleukin-10 than lymphoid tissue B cells

B cells perform various immunological functions that include production of antibody, presentation of antigens, secretion of multiple cytokines and regulation of immune responses mainly via their secretion of interleukin (IL)-10. While the liver is regarded both as an important immune organ and a tolerogenic environment, little is known about the functional biology of hepatic B cells. In this study we demonstrate that, following lipopolysaccharide (LPS) stimulation in vivo, normal mouse hepatic B cells rapidly increase their surface expression of CD39, CD40, CD80 and CD86, and produce significantly elevated levels of proinflammatory interferon (IFN)-γ, IL-6 and tumour necrosis factor (TNF)-α compared with splenic B cells. Moreover, LPS-activated hepatic B cells produce very low levels of IL-10 compared with activated splenic B cells that produce comparatively high levels of this immunosuppressive cytokine. Splenic, but not hepatic, B cells inhibited the activation of liver conventional myeloid dendritic cells (mDCs). Furthermore, compared with the spleen, the liver exhibited significantly smaller proportions of B1a and marginal zone-like B cells, which have been shown to produce IL-10 upon LPS stimulation. These data suggest that, unlike in the spleen, IL-10-producing regulatory B cells in the liver are not a prominent cell type. Consistent with this, when compared with liver conventional mDCs from B cell-deficient mice, those from B cell-competent wild-type mice displayed enhanced expression of the cell surface co-stimulatory molecule CD86, greater production of proinflammatory cytokines (IFN-γ, IL-6, IL-12p40) and reduced secretion of IL-10. These findings suggest that hepatic B cells have the potential to initiate rather than regulate inflammatory responses.

Post-transplant repopulation of naïve and memory T cells in blood and lymphoid tissue after alemtuzumab-mediated depletion in heart-transplanted cynomolgus monkeys

Repopulation of memory T cells (Tmem) in allograft recipients after lymphodepletion is a major barrier to transplant tolerance induction. Ineffective depletion of naïve T cells (Tn) and Tmem may predispose to repopulation of Tmem after transplantation. Cynomolgus macaque monkeys given heart allografts were lymphodepleted using Alemtuzumab (Campath-1H; anti-CD52). Peripheral blood (PB) and lymph nodes (LN) were analyzed for CD95(-) (Tn) and CD95(+) cells (Tmem), one day, one month and up to three months after Alemtuzumab infusion. CD52 expression, susceptibility to Alemtuzumab cytotoxicity and pro-apoptotic caspase-3 were evaluated in Tn and Tmem. In vivo, Alemtuzumab induction profoundly depleted lymphocytes in PB (99% reduction) but exerted a lesser effect in LN (70% reduction), with similar depletion of Tn and Tmem subsets. After transplantation, Tmem comprised the majority of lymphocytes in PB and LN. In vitro, LN T cells were more resistant to Alemtuzumab-mediated cytotoxicity than PB lymphocytes. CD4(+) Tn and Tmem were equally susceptible to Alemtuzumab-mediated cytotoxicity, whereas CD8(+) Tn were more resistant than CD8(+) Tmem. However, no significant differences in CD52 expression between lymphocyte subsets in PB and LN were observed. Caspase-3 expression was higher in PB than LN T cells. CD4(+) and CD8(+) Tn expressed lower levels of Caspase-3 than Tmem, in both PB and LN. Thus, after Alemtuzumab infusion, residual Tn in secondary lymphoid tissue may predispose to rapid recovery of Tmem in allograft recipients.

All-trans retinoic acid and rapamycin synergize with transforming growth factor-β1 to induce regulatory T cells but confer different migratory capacities

Tregs play important roles in maintaining immune homeostasis, and thus, therapies based on Treg are promising candidates for the treatment for a variety of immune-mediated disorders. These therapies, however, face the significant challenge of obtaining adequate numbers of Tregs from peripheral blood that maintains suppressive function following extensive expansion. Inducing Tregs from non-Tregs offers a viable alternative. Different methods to induce Tregs have been proposed and involve mainly treating cells with TGF-β-iTreg. However, use of TGF-β alone is not sufficient to induce stable Tregs. ATRA or rapa has been shown to synergize with TGF-β to induce stable Tregs. Whereas TGF-β plus RA-iTregs have been well-described in the literature, the phenotype, function, and migratory characteristics of TGF-β plus rapa-iTreg have yet to be elucidated. Herein, we describe the phenotype and function of mouse rapa-iTreg and reveal that these cells differ in their in vivo homing capacity when compared with mouse RA-iTreg and mouse TGF-β-iTreg. This difference in migratory activity significantly affects the therapeutic capacity of each subset in a mouse model of colitis. We also describe the characteristics of iTreg generated in the presence of TGF-β, RA, and rapa.

Adoptive regulatory T-cell therapy protects against cerebral ischemia

OBJECTIVE

Recent evidence suggests that functional deficiency in regulatory T cells (Tregs), an innate immunomodulator, exacerbates brain damage after cerebral ischemia. We therefore evaluated the effect of Treg transfer in rodent models of ischemic stroke and further investigated the mechanism underlying Treg-afforded neuroprotection.

METHODS

We examined the therapeutic potential of Tregs and the mechanisms of neuroprotection in vivo in 2 rodent models of ischemic stroke and in vitro in Treg-neutrophil cocultures using a combined approach including cell-specific depletion, gene knockout mice, and bone marrow chimeras.

RESULTS

Systemic administration of purified Tregs at 2, 6, or even 24 hours after middle cerebral artery occlusion resulted in a marked reduction of brain infarct and prolonged improvement of neurological functions lasting out to 4 weeks. Treg-afforded neuroprotection was accompanied by attenuated blood-brain barrier (BBB) disruption during early stages of ischemia, decreased cerebral inflammation, and reduced infiltration of peripheral inflammatory cells into the lesioned brain. Surprisingly, Tregs exerted early neuroprotection without penetrating into the brain parenchyma or inhibiting the activation of residential microglia. Rather, both in vivo and in vitro studies demonstrated that Tregs suppressed peripheral neutrophil-derived matrix metallopeptidase-9 production, thus preventing proteolytic damage of the BBB. In addition to its potent central neuroprotection, Treg treatment was shown to ameliorate poststroke lymphopenia, suggesting a beneficial effect on immune status.

INTERPRETATION

Our study suggests that Treg adoptive therapy is a novel and potent cell-based therapy targeting poststroke inflammatory dysregulation and neurovascular disruption.

Plasmacytoid dendritic cells: no longer an enigma and now key to transplant tolerance?

Plasmacytoid (p) dendritic cells (DC) are a specialized subset of DC whose primary role was initially defined by the production of type I interferons in response to viral infection. They are now known to also possess a repertoire of functions capable of determining T cell fate and activation. Under homeostatic conditions, non-lymphoid tissue-resident pDC play a critical role in the regulation of mucosal immunity, as well as the development of central and peripheral tolerance. Although these cells display a number of characteristics that differ from conventional DC, particularly altered costimulatory molecule expression and poor allostimulatory capacity when interacting with T cells, this phenotype favors the generation of alloantigen-specific regulatory CD4(+) or CD8(+) T cells critical to the development of graft tolerance. In this minireview, we discuss pDC ontogeny, functional biology and the emerging data that demonstrate the importance of pDC in the induction of tolerance, as well as recent studies that define mechanisms underlying pDC-mediated tolerance to both solid organ and haematopoietic stem cell transplants. We also highlight their use in clinical settings and the potential of pDC both as targets and cellular therapeutic agents to improve the outcome of organ transplantation.

Roles of dendritic cells in murine hepatic warm and liver transplantation-induced cold ischemia/reperfusion injury

Dendritic cells (DCs) induce and regulate both innate and adaptive immune responses; however, their in vivo functional importance in hepatic ischemia/reperfusion (IR) injury is perplexing. We hypothesized that liver-resident DC and locally recruited blood-borne DC might have distinctive roles in hepatic IR injury. We tested this hypothesis by using DC-deficient, fms-like tyrosine kinase 3 ligand (Flt3L) knockout (KO) mice in hepatic warm (70% partial clamping for 60 minutes) and cold IR injury (liver transplant [LTx] with 24-hour cold storage). Flt3L KO liver and lymphoid organs contained virtually no CD11c+ F4/80- DC. Hepatic warm IR injury was significantly lower in Flt3L KO than in wildtype (WT) mice with lower alanine aminotransferase (ALT) levels, reduced hepatic necrosis, and lower neutrophil infiltration. Hepatic messenger RNA (mRNA) and protein levels for inflammatory cytokines (tumor necrosis factor alpha [TNFα], interleukin [IL]-6) and chemokines (CCL2, CXCL2) were also significantly lower in Flt3L KO than in WT mice, indicating that lack of both liver-resident and blood-borne DC ameliorated hepatic warm IR injury. Adoptive transfer of splenic or hepatic WT DC into Flt3L KO or WT mice increased hepatic warm IR injury, suggesting injurious roles of DC infusion. When Flt3L KO liver was transplanted into WT mice, ALT levels were significantly higher than in WT to WT LTx, with enhanced hepatic necrosis and neutrophil infiltration, indicating a protective role of liver-resident DC.

CONCLUSION

Using both warm and cold hepatic IR models, this study suggests differential roles of liver-resident versus blood-borne DC, and points to the importance of the local microenvironment in determining DC function during hepatic IR injury.

Spermatogonial stem cell transplantation into rhesus testes regenerates spermatogenesis producing functional sperm

Spermatogonial stem cells (SSCs) maintain spermatogenesis throughout a man's life and may have application for treating some cases of male infertility, including those caused by chemotherapy before puberty. We performed autologous and allogeneic SSC transplantations into the testes of 18 adult and 5 prepubertal recipient macaques that were rendered infertile with alkylating chemotherapy. After autologous transplant, the donor genotype from lentivirus-marked SSCs was evident in the ejaculated sperm of 9/12 adult and 3/5 prepubertal recipients after they reached maturity. Allogeneic transplant led to donor-recipient chimerism in sperm from 2/6 adult recipients. Ejaculated sperm from one recipient transplanted with allogeneic donor SSCs were injected into 85 rhesus oocytes via intracytoplasmic sperm injection. Eighty-one oocytes were fertilized, producing embryos ranging from four-cell to blastocyst with donor paternal origin confirmed in 7/81 embryos. This demonstration of functional donor spermatogenesis following SSC transplantation in primates is an important milestone for informed clinical translation.

Activated CD47 regulates multiple vascular and stress responses: implications for acute kidney injury and its management

Ischemia-reperfusion injury (IRI) remains a significant source of early and delayed renal transplant failure. Therapeutic interventions have yet to resolve this ongoing clinical challenge although the reasons for this remain unclear. The cell surface receptor CD47 is widely expressed on vascular cells and in tissues. It has one known soluble ligand, the stress-released matricellular protein thrombospondin-1 (TSP1). The TSP1-CD47 ligand receptor axis controls a number of important cellular processes, inhibiting survival factors such as nitric oxide, cGMP, cAMP, and VEGF, while activating injurious pathways such as production of reactive oxygen species. A role of CD47 in renal IRI was recently revealed by the finding that the TSP1-CD47 axis is induced in renal tubular epithelial cells (RTEC) under hypoxia and following IRI. The absence of CD47 in knockout mice increases survival, mitigates RTEC damage, and prevents subsequent kidney failure. Conversely, therapeutic blockade of TSP1-CD47 signaling provides these same advantages to wild-type animals. Together, these findings suggest an important role for CD47 in renal IRI as a proximate promoter of injury and as a novel therapeutic target.

Hepatic stellate cells undermine the allostimulatory function of liver myeloid dendritic cells via STAT3-dependent induction of IDO

Hepatic stellate cells (HSCs) are critical for hepatic wound repair and tissue remodeling. They also produce cytokines and chemokines that may contribute to the maintenance of hepatic immune homeostasis and the inherent tolerogenicity of the liver. The functional relationship between HSCs and the professional migratory APCs in the liver, that is, dendritic cells (DCs), has not been evaluated. In this article, we report that murine liver DCs colocalize with HSCs in vivo under normal, steady-state conditions, and cluster with HSCs in vitro. In vitro, HSCs secrete high levels of DC chemoattractants, such as MΙP-1α and MCP-1, as well as cytokines that modulate DC activation, including TNF-α, IL-6, and IL-1β. Culture of HSCs with conventional liver myeloid (m) DCs resulted in increased IL-6 and IL-10 secretion compared with that of either cell population alone. Coculture also resulted in enhanced expression of costimulatory (CD80, CD86) and coinhibitory (B7-H1) molecules on mDCs. HSC-induced mDC maturation required cell-cell contact and could be blocked, in part, by neutralizing MΙP-1α or MCP-1. HSC-induced mDC maturation was dependent on activation of STAT3 in mDCs and, in part, on HSC-secreted IL-6. Despite upregulation of costimulatory molecules, mDCs conditioned by HSCs demonstrated impaired ability to induce allogeneic T cell proliferation, which was independent of B7-H1, but dependent upon HSC-induced STAT3 activation and subsequent upregulation of IDO. In conclusion, by promoting IDO expression, HSCs may act as potent regulators of liver mDCs and function to maintain hepatic homeostasis and tolerogenicity.

Immunoregulatory properties of rapamycin-conditioned monocyte-derived dendritic cells and their role in transplantation

In efforts to minimize the chronic administration of immunosuppression (IS) drugs in transplantation and autoimmune disease, various cell-based tolerogenic therapies, including the use of regulatory or tolerogenic dendritic cells (tolDC) have been developed. These DC-based therapies aim to harness the inherent immunoregulatory potential of these professional antigen-presenting cells. In this short review, we describe both the demonstrated tolerogenic properties, and current limitations of rapamycin-conditioned DC (RAPA-DC). RAPA-DC are generated through inhibition of the integrative kinase mammalian target of rapamycin (mTOR) by the immunosuppressive macrolide rapamycin during propagation of monocyte-derived DC. Consistent with the characteristics of tolDC, murine RAPA-DC display resistance to phenotypic maturation induced by pro-inflammatory stimuli; exhibit the ability to migrate to secondary lymphoid tissue (important for 'cross-presentation' of antigen to T cells), and enrich for naturally-occurring CD4+ regulatory T cells. In rodent models, delivery of recipient-derived RAPA-DC pulsed with donor antigen prior to organ transplantation can prolong allogeneic heart-graft survival indefinitely, especially when combined with a short course of IS. These encouraging data support ongoing efforts to develop RAPA-DC for clinical testing. When compared to murine RAPA-DC however, human RAPA-DC have proven only partially resistant to maturation triggered by pro-inflammatory cytokines, and display heterogeneity in their impact on effector T-cell expansion and function. In total, the evidence suggests the need for more in-depth studies to better understand the mechanisms by which mTOR controls human DC function. These studies may facilitate the development of RAPA-DC therapy alone or together with agents that preserve/enhance their tolerogenic properties as clinical immunoregulatory vectors.

Dendritic cells promote macrophage infiltration and comprise a substantial proportion of obesity-associated increases in CD11c+ cells in adipose tissue and liver

Obesity-associated increases in adipose tissue (AT) CD11c(+) cells suggest that dendritic cells (DC), which are involved in the tissue recruitment and activation of macrophages, may play a role in determining AT and liver immunophenotype in obesity. This study addressed this hypothesis. With the use of flow cytometry, electron microscopy, and loss-and-gain of function approaches, the contribution of DC to the pattern of immune cell alterations and recruitment in obesity was assessed. In AT and liver there was a substantial, high-fat diet (HFD)-induced increase in DC. In AT, these increases were associated with crown-like structures, whereas in liver the increase in DC constituted an early and reversible response to diet. Notably, mice lacking DC had reduced AT and liver macrophages, whereas DC replacement in DC-null mice increased liver and AT macrophage populations. Furthermore, delivery of bone marrow-derived DC to lean wild-type mice increased AT and liver macrophage infiltration. Finally, mice lacking DC were resistant to the weight gain and metabolic abnormalities of an HFD. Together, these data demonstrate that DC are elevated in obesity, promote macrophage infiltration of AT and liver, contribute to the determination of tissue immunophenotype, and play a role in systemic metabolic responses to an HFD.

Dendritic cell activation and memory cell development are impaired among mice administered medroxyprogesterone acetate prior to mucosal herpes simplex virus type 1 infection

Epidemiological studies indicate that the exogenous sex steroid medroxyprogesterone acetate (MPA) can impair cell-mediated immunity, but mechanisms responsible for this observation are not well defined. In this study, MPA administered to mice 1 wk prior to HSV type 1 (HSV-1) infection of their corneal mucosa impaired initial expansion of viral-specific effector and memory precursor T cells and reduced the number of viral-specific memory T cells found in latently infected mice. MPA treatment also dampened expression of the costimulatory molecules CD40, CD70, and CD80 by dendritic cells (DC) in lymph nodes draining acute infection, whereas coculture of such DC with T cells from uninfected mice dramatically impaired ex vivo T cell proliferation compared with the use of DC from mice that did not receive MPA prior to HSV-1 infection. In addition, T cell expansion was comparable to that seen in untreated controls if MPA-treated mice were administered recombinant soluble CD154 (CD40L) concomitant with their mucosal infection. In contrast, the immunomodulatory effects of MPA were infection site dependent, because MPA-treated mice exhibited normal expansion of virus-specific T cells when infection was systemic rather than mucosal. Taken together, our results reveal that the administration of MPA prior to viral infection of mucosal tissue impairs DC activation, virus-specific T cell expansion, and development of virus-specific immunological memory.

Activation of parenchymal CD47 promotes renal ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) contributes to decreased allograft function and allograft rejection in transplanted kidneys. Thrombospondin-1 is a stress protein typically secreted in response to hypoxia and the ligand activator for the ubiquitously expressed receptor CD47. The function of activated CD47 in IRI remains completely unknown. Here, we found that both CD47 and its ligand thrombospondin-1 were upregulated after renal IRI in mice. CD47-knockout mice were protected against renal dysfunction and tubular damage, suggesting that the development of IRI requires intact CD47 signaling. Chimeric CD47-knockout mice engrafted with wild-type hematopoietic cells had significantly lower serum creatinine and less tubular damage than wild-type controls after IRI, suggesting that CD47 signaling in parenchymal cells predominantly mediates renal damage. Treatment with a CD47-blocking antibody protected mice from renal dysfunction and tubular damage compared with an isotype control. Taken together, these data imply that CD47 on parenchymal cells promotes injury after renal ischemia and reperfusion. Therefore, CD47 blockade may have therapeutic potential to prevent or suppress ischemia-reperfusion-mediated damage.

IL-27 production and STAT3-dependent upregulation of B7-H1 mediate immune regulatory functions of liver plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) are highly specialized APCs that, in addition to their well-recognized role in anti-viral immunity, also regulate immune responses. Liver-resident pDCs are considerably less immunostimulatory than those from secondary lymphoid tissues and are equipped to promote immune tolerance/regulation through various mechanisms. IL-27 is an IL-12 family cytokine that regulates the function of both APCs and T cells, although little is known about its role in pDC immunobiology. In this study, we show that mouse liver pDCs express higher levels of IL-27p28 and EBV-induced protein 3 (Ebi3) compared with those of splenic pDCs. Both populations of pDCs express the IL-27Rα/WSX-1; however, only liver pDCs significantly upregulate expression of the coregulatory molecule B7 homolog-1 (B7-H1) in response to IL-27. Inhibition of STAT3 activation completely abrogates IL-27-induced upregulation of B7-H1 expression on liver pDCs. Liver pDCs treated with IL-27 increase the percentage of CD4(+)Foxp3(+) T cells in MLR, which is dependent upon expression of B7-H1. pDCs from Ebi3-deficient mice lacking functional IL-27 show increased capacity to stimulate allogeneic T cell proliferation and IFN-γ production in MLR. Liver but not spleen pDCs suppress delayed-type hypersensitivity responses to OVA, an effect that is lost with Ebi3(-/-) and B7-H1(-/-) liver pDCs compared with wild-type liver pDCs. These data suggest that IL-27 signaling in pDCs promotes their immunoregulatory function and that IL-27 produced by pDCs contributes to their capacity to regulate immune responses in vitro and in vivo.

Selective expansion of allogeneic regulatory T cells by hepatic stellate cells: role of endotoxin and implications for allograft tolerance

Hepatic stellate cells (HSCs) may play an important role in hepatic immune regulation by producing numerous cytokines/chemokines and expressing Ag-presenting and T cell coregulatory molecules. Due to disruption of the endothelial barrier during cold-ischemic storage and reperfusion of liver grafts, HSCs can interact directly with cells of the immune system. Endotoxin (LPS), levels of which increase in liver diseases and transplantation, stimulates the synthesis of many mediators by HSCs. We hypothesized that LPS-stimulated HSCs might promote hepatic tolerogenicity by influencing naturally occurring immunosuppressive CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs). Following their portal venous infusion, allogeneic CD4(+) T cells, including Tregs, were found closely associated with HSCs, and this association increased in LPS-treated livers. In vitro, both unstimulated and LPS-stimulated HSCs upregulated Fas (CD95) expression on conventional CD4(+) T cells and induced their apoptosis in a Fas/Fas ligand-dependent manner. By contrast, HSCs induced Treg proliferation, which required cell-cell contact and was MHC class II-dependent. This effect was augmented when HSCs were pretreated with LPS. LPS increased the expression of MHC class II, CD80, and CD86 and stimulated the production of IL-1α, IL-1β, IL-6, IL-10 and TNF-α by HSCs. Interestingly, production of IL-1α, IL-1β, IL-6, and TNF-α was strongly inhibited, but that of IL-10 enhanced in LPS-pretreated HSC/Treg cocultures. Adoptively transferred allogeneic HSCs migrated to the secondary lymphoid tissues and induced Treg expansion in lymph nodes. These data implicate endotoxin-stimulated HSCs as important immune regulators in liver transplantation by inducing selective expansion of tolerance-promoting Tregs and reducing inflammation and alloimmunity.