Transfusion-related immunomodulation by platelets is dependent on their expression of MHC Class I molecules and is independent of white cells

BACKGROUND: Transfusion‐related immunomodulation (TRIM) has been correlated with the presence of white cells (WBCs) in blood transfusions, but the role of components such as platelets (PLTs) in mediating TRIM has not been extensively examined. We designed a murine PLT transfusion model to study whether leukoreduced PLTs mediate TRIM effects. STUDY DESIGN AND METHODS: CBA recipient mice were administered four weekly transfusions of either fresh (4 hr) or aged (24 and 72 hr) donor leukoreduced PLTs from allogeneic BALB/c mice and then transplanted with skin grafts from donor‐matched mice. TRIM was measured by comparing the times to graft rejection and these were correlated with immunoglobulin G (IgG) antibody development measured by flow cytometry. RESULTS: Compared with nontransfused control recipients, four transfusions of fresh, extremely leukoreduced (<0.05 WBCs/mL), allogeneic PLTs significantly (p < 0.002) reduced the recipient's ability to reject donor‐matched skin grafts (survival >49 days compared with <14 days in nontransfused controls) despite the presence of high‐titered serum IgG donor antibodies. In contrast, however, aged PLTs or fresh PLTs devoid of MHC Class I molecules were unable to affect skin graft survival nor stimulate antibody production. The PLT age‐related inability to induce TRIM was shown to be due to loss of PLT‐associated MHC Class I molecules; soluble supernatant MHC molecules that were transfused were unable to induce TRIM. CONCLUSION: These results suggest that fresh PLTs can induce TRIM independently of WBCs due to their MHC antigen expression whereas aging results in loss of MHC and ability to mediate TRIM. The findings support the concept that either active MHC removal from fresh PLTs or passive removal by, for example, storage, may reduce any deleterious effects of TRIM in transfusion recipients.     

A prospective, randomized study of the use of platelet concentrates irradiated with ultraviolet-B light in patients with hematologic malignancy

BACKGROUND: Irradiation of platelet concentrates (PCs) with ultraviolet- B (UVB) light inactivates the contaminating white cells and might be an alternative to filtration for the prevention of alloimmunization to HLA antigens and subsequent refractoriness to further platelet transfusions in multiply transfused patients with bone marrow failure. STUDY DESIGN AND METHODS: Patients with hematologic malignancy, mainly acute myeloid leukemia, were prospectively assigned in a random manner to receive either UVB-irradiated or control, nonirradiated PCs. All patients were given red cells that were white cell reduced by filtration. Transfusion efficacy and alloimmunization were assessed by means of corrected count increments, requirement for red cells and PCs, and measurement of lymphocyte-reactive antibodies. RESULTS: UVB-irradiated PCs had a clinical efficacy similar to controls as judged by corrected count increments at 1 to 6 and 12 to 24 hours and by the median requirement for red cell and platelet transfusions. Alloimmunization determined by measurements of lymphocyte-reactive antibodies using both conventional and antiglobulin-augmented lymphocytotoxicity techniques was not abolished in recipients of UVB-irradiated PCs (4/30, 13%) but was less than that in controls (5/20, 25%; p = NS). The mean number of platelet transfusion episodes prior to the occurrence of alloimmunization was greater in the control group (27 vs. 10; p = 0.017). CONCLUSION: In this trial, UVB irradiation did not diminish the clinical efficacy of platelet transfusions. There was a small but nonsignificant reduction alloimmunization, but no difference in refractoriness of the two groups was observed. Larger prospective randomized studies are required to confirm these findings and to compare UVB irradiation with white cell reduction.     

Immune responsiveness against allogeneic platelet transfusions is determined by the recipient's major histocompatibility complex class II phenotype

BACKGROUND: Immunoglobulin G (IgG) anti-platelet (PLT) immunity has been shown to be initiated by indirect allorecognition where recipient T cells recognize donor PLT antigens presented by class II molecules encoded by the major histocompatibility complex (MHC) on recipient antigen-presenting cells. To understand how the recipient's MHC class II molecules may influence PLT alloimmunity, immune responsiveness against transfused PLTs was tested in different mouse strains. STUDY DESIGN AND METHODS: Various inbred and mutant mouse strains were transfused with allogeneic PLTs and IgG donor antibodies were measured by flow cytometry. RESULTS: When recipient mice, expressing both MHC class II I-A and MHC class II I-E molecules, were transfused weekly with allogeneic PLTs, high titers of IgG donor antibodies were generated. In comparison, however, recipient mice expressing only MHC class II I-A molecules had significantly (p < 0.001) reduced IgG antibody responsiveness against PLT transfusions. The low IgG responder status against allogeneic PLT transfusions was rescued in transgenic mice expressing I-E molecules and in mice genetically deficient in either beta2-microglobulin or CD8+ T cells. CONCLUSION: IgG immune responsiveness against allogeneic PLT transfusions is dependent on recipient expression of I-E MHC class II molecules, whereas I-A expression is linked with CD8-mediated suppression of PLT immunity. The data suggest that strategies to modify recipient MHC class II presentation of donor PLT antigens would be effective in eliminating PLT alloimmunity.     

Characterization of the defective interaction between a subset of natural killer cells and dendritic cells in HIV-1 infection

In this study, we demonstrate that the in vitro interactions between a CD56(neg)/CD16(pos) (CD56(neg)) subset of natural killer (NK) cells and autologous dendritic cells (DCs) from HIV-1-infected viremic but not aviremic individuals are markedly impaired and likely interfere with the development of an effective immune response. Among the defective interactions are abnormalities in the process of reciprocal NK-DC activation and maturation as well as a defect in the NK cell-mediated editing or elimination of immature DCs (iDCs). Notably, the lysis of mature DCs (mDCs) by autologous NK cells was highly impaired even after the complete masking of major histocompatibility complex I molecules, suggesting that the defective elimination of autologous iDCs is at the level of activating NK cell receptors. In this regard, the markedly impaired expression/secretion and function of NKp30 and TNF-related apoptosis-inducing ligand, particularly among the CD56(neg) NK cell subset, largely accounts for the highly defective NK cell-mediated lysis of autologous iDCs. Moreover, mDCs generated from HIV-1 viremic but not aviremic patients are substantially impaired in their ability to secrete interleukin (IL)-10 and -12 and to prime the proliferation of neighboring autologous NK cells, which, in turn, fail to secrete adequate amounts of interferon-gamma.     

Ultraviolet B light-exposed human platelets mediate acute lung injury in a two-event mouse model of transfusion

BACKGROUND: Ultraviolet B (UVB) light has been used alone on platelet (PLT) transfusion products to prevent alloimmunization or with chemical sensitizers to reduce pathogens. Such processing can damage PLTs and potentiate their storage lesion. Transfusion‐related acute lung injury (ALI) has occurred in patients whose underlying condition led to an inflamed endothelium and who were transfused with products that contained either HLA or HNA antibodies or biologic modifiers such as lipids or antigens from stored cells. Clinical trials of UV‐treated PLTs in patients with thrombocytopenia generated controversy regarding association of these cells with respiratory distress. We evaluated whether UVB PLTs could mediate ALI in an animal model of ALI. STUDY DESIGN AND METHODS: We used a two‐event animal model where the sensitizing event was lipopolysaccharide (LPS) and the second event was infusion of human PLTs or UVB human PLTs (2.4 J/cm²). Infused human PLTs were followed with whole animal imaging, lung histology, confocal microscopy, lung water, and changes in bronchoalveolar lavage fluid (BALF) related to ALI. RESULTS: In LPS‐treated mice UVB human PLTs accumulated in the lungs and were associated with ALI manifested by increased protein and white blood cells (WBCs) in BALF. Untreated human PLTs did not accumulate in the lungs or increase BALF protein or WBC counts. CONCLUSIONS: We provide a proof of principle that UVB human PLTs can accumulate in lungs of LPS‐primed animals and mediate ALI. PLTs exposed to high doses of UVB could potentially mediate similar effects in patients predisposed with sepsis or other causes of endothelial cell inflammation.     

Effects of pentoxifylline on differentiation, maturation, and function of human CD14+ monocyte-derived dendritic cells

Pentoxifylline (PTX) is a nonspecific phosphodiesterase inhibitor which has potent immunoregulatory and antiinflammatory effects. Although its immunomodulation property has been recognized, it is not clear whether PTX could affect dendritic cells (DCs), the most efficient antigen-presenting cells. The purpose of this study was to determine whether PTX could suppress DC differentiation, maturation, and its associated functions. Immature DCs (iDCs) were generated from human peripheral blood mononuclear cell CD14+ monocytes cultured with granulocyte macrophage colony stimulating factor and interleukin-4 for 5 days. PTX concentration-dependently suppressed the expression of iDC differentiation markers including CD54, CD80, CD86, and human leukocyte antigen-DR. In addition, PTX also inhibited DC maturation marker CD83 expression after stimulating DCs with lipopolysaccharide. Furthermore, PTX inhibited the antigen-uptake ability of DCs when tested by fluorescein isothiocyanate-dextran endocytosis assay. PTX significantly reduced the production of TNF-alpha and IFN-gamma in mature DCs (mDCs). Consequently, PTX-treated mDCs showed a reduced activity of mDC-induced T-cell allostimulation and proliferation by mixed-lymphocyte reaction (MLR) assay. Therefore, PTX significantly inhibits CD14+ monocyte-derived DC differentiation, maturation, antigen-uptake ability of iDCs, and antigen-presentation ability of mDCs possibly due to the suppression of TNF-alpha and IFN-gamma production. These results suggested that inhibitory effects of PTX on DCs may contribute its antiinflammatory and immunoregulatory functions.     

Induction of classical transplantation tolerance in the adult

Transplantation tolerance across histoincompatibilities in multiple non-H-2 minors (B10.BR into CBA/Ca) and "minor" plus H-2D (B10.A into CBA/Ca) antigens has been achieved successfully by combined adult bone marrow transplantation and treatment with CD4 and CD8 mAbs. The tolerant state was confirmed by permanent acceptance of donor strain skin grafts, and in vitro unresponsiveness to donor cells. Tolerance was associated with partial donor chimerism to various degrees. Tolerance to minor transplantation antigens induced in this manner was restricted to recipient-type MHC. The possibility was raised that tolerance resulted, at least in part, from clonal anergy rather than deletion.     

Reduction of the immune response to factor VIII mediated through tolerogenic factor VIII presentation by immature dendritic cells

Summary. Background: The development of neutralizing antibodies to factor FVIII (FVIII) represents the most serious complication in the treatment of hemophilia A. Objective: We have explored the potential of using immature dendritic cells (iDCs) to present FVIII in a tolerogenic manner to T cells. Methods: The iDCs were isolated from hemophilic murine bone marrow and pulsed with canine cFVIII (cFVIII‐iDCs) in the presence or absence of the NFκB pathway blocking compound Andrographolide (Andro‐cFVIII‐iDCs). Three weekly intravenous infusions of one million cFVIII pulsed‐iDCs were administered to a group of five hemophilic Balb/c mice. Anti‐FVIII antibody levels were monitored by functional Bethesda assay after four weekly intravenous challenges with 2 IU of cFVIII. Results: We have shown that cFVIII in the presence or absence of Andro is efficiently taken up by iDCs and that this process does not result in the maturation of DCs or the activation of co‐cultured T cells. Following repeated infusion of the cFVIII‐iDCs and Andro‐cFVIII‐iDCs into hemophilic mice, which were subsequently challenged with cFVIII, long‐term reductions of FVIII inhibitors of 25% and 40%, respectively, were documented. Studies of cytokine release and T‐cell phenotypes indicate that the mechanisms responsible for reducing immunologic responsiveness to cFVIII appear to involve an expansion of Foxp3 T regulatory cells in the case of cFVIII‐iDC infusion and the elaboration of the immunosuppressive cytokines IL‐10 and TGF‐β following andrographolide‐treated cFVIII‐iDCs. Conclusions: This study shows that tolerogenic presentation of cFVIII to the immune system can significantly reduce immunogenicity of the protein.     

Tumoricidal activity of TLR7/8-activated inflammatory dendritic cells

Imiquimod (IMQ), a synthetic agonist to Toll-like receptor (TLR) 7, is being successfully used for the treatment of certain skin neoplasms, but the exact mechanisms by which this compound induces tumor regression are not yet understood. While treating basal cell carcinoma (BCC) patients with topical IMQ, we detected, by immunohistochemistry, sizable numbers of both myeloid dendritic cells (mDCs) and plasmacytoid DCs (pDCs) within the inflammatory infiltrate. Surprisingly, peritumoral mDCs stained positive for perforin and granzyme B, whereas infiltrating pDCs expressed tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). The biological relevance of this observation can be deduced from our further findings that peripheral blood-derived CD11c(+) mDCs acquired antiperforin and anti-granzyme B reactivity upon TLR7/8 stimulation and could use these molecules to effectively lyse major histocompatibility complex (MHC) class I(lo) cancer cell lines. The same activation protocol led pDCs to kill MHC class I-bearing Jurkat cells in a TRAIL-dependent fashion. While suggesting that mDCs and pDCs are directly involved in the IMQ-induced destruction of BCC lesions, our data also add a new facet to the functional spectrum of DCs, ascribing to them a major role not only in the initiation but also in the effector phase of the immune response.     

Human platelets target dendritic cell differentiation and production of proinflammatory cytokines

BACKGROUND: Dendritic cells (DCs) are the most potent antigen-presenting cells that initiate and regulate immune responses. They are unique in their feature to produce bioactive interleukin (IL)-12, a major proinflammatory cytokine connecting innate and adaptive immunity. Platelets (PLTs) are highly reactive components of the circulatory system with fundamental importance in hemostasis and innate immunity. Recently, immunomodulatory capacities of single specific human PLT-derived products on DC effector functions were identified. To improve the understanding of PLT-DC interactions, this study investigates the influence of intact resting and activated PLTs on DC phenotype and key functions. STUDY DESIGN AND METHODS: Magnetic beads sorted CD14+ cells were expanded in the presence and absence of resting or activated PLTs. DC differentiation, maturation, allostimulatority capacity, antigen uptake, and cytokine profile were estimated to control group. RESULTS: Activated PLTs potently impaired DC differentiation according to CD1a expression (mean reduction, 62%; p < 0.05). Production of IL-12p70 and tumor necrosis factor-alpha was reduced in the presence of resting (mean reduction, 46 and 55%, respectively; p < 0.05) as well as activated PLTs (mean reduction, 63 and 49%, respectively; p < 0.05). In contrast to the suppression of proinflammatory cytokines, activated PLTs increased production of the immunoregulatory cytokine IL-10 by DCs (mean increase, 52%; p < 0.05). DC allostimulatority capacity, antigen uptake, and phenotypic maturation remained unaffected. CONCLUSION: It is proposed that intact PLTs connect immunity and hemostasis by interfering with DC differentiation and cytokine production. This interference might be of importance in clinical settings, such as DC therapy and PLT transfusions.     

Donor platelets stored for at least 3 days can elicit activation marker expression by the recipient's blood mononuclear cells: an in vitro study

BACKGROUND: Recent studies have demonstrated that infused platelets (PLTs) can promote inflammation. The objective of this study was to evaluate the impact of storage of transfusion-grade PLTs on the peripheral blood mononuclear cells (PBMNCs) of the recipient.
STUDY DESIGN AND METHODS: An in vitro cell model system was established to measure the degree of activation of donor PLTs during 5 days of their storage and then to measure immune cell activation by detecting marker expression in coculture experiments.
RESULTS: The level of soluble CD62p increased significantly by Day 3, and membrane expression of CD62p increased significantly from Day 2, indicating some degree of PLT activation over time during storage (p < 0.05). Donor PLTs and PBMNC subsets (monocytes, B cells, and T cells) from recipients were cocultured for 48 hours. The number of PLT-PBMNC subset doublets detected by flow cytometry was correlated with the PLT storage time after Day 3 (p < 0.05), indicating consistent binding of PLTs to PBMNCs. The results of these experiments showed that there was a consistent and significant increase in expression of conventional activation markers of T cells, B cells, and monocytes compared with appropriate controls (p < 0.05 to <0.01).
CONCLUSION: The results of this study indicate that, from Day 3 onward, activation markers are consistently expressed on PLTs. From these results, we conclude that activated PLTs may affect PBMNC interactions in recipients.     

Activation of immature monocyte-derived dendritic cells after transduction with high doses of lentiviral vectors

Dendritic cells (DCs) are an attractive tool for immunomodulation, targeting mature DCs (mDCs) for immunization or immature/semimature DCs (iDCs) for tolerization. Therefore, introducing antigens into DCs has become a prime topic in various immunological disciplines. Numerous studies have shown that lentiviruses are an efficient vehicle for this purpose. This study evaluates the effects of lentiviral transduction on iDC activation. Immature DCs are efficiently transduced with increasing doses of lentivirus without affecting cell viability. Transduction at low multiplicities of infection (MOIs) did not result in phenotypical or functional maturation. Higher doses of lentivirus, however, resulted in upregulation of adhesion, costimulatory, and HLA molecules, as well as in increased allostimulatory capacity and secretion of interleukin (IL)-6, IL-8, and tumor necrosis factor-alpha. Production of IL-12 p70, IL-10, and interferon-alpha was observed only at extremely high doses. Protein kinase R phosphorylation on transduction at an MOI of 150 was demonstrated by Western blotting. A Toll-like receptor (TLR)-driven luciferase reporter assay showed dose-dependent activation of TLR2, TLR3, and TLR8, which was independent of the pseudotype, production, or transduction protocol and was abrogated on heat inactivation. These data show that lentiviral vectors provide not only the antigen but also appropriate activation signals to iDCs, favoring their use for immunotherapy and vaccine development.     

Ultraviolet-B irradiation of leukapheresis products: Dose-response relationship with the mixed lymphocyte reaction

Ultraviolet-B (UVB) irradiation of blood constituents intensifies their anti-rejection effect in pretransplant donor-specific transfusions. UVB-induced inhibition of the mixed lymphocyte reaction (MLR) between UVB-irradiated donor cells and prospective recipient cells is a predicator of this anti-rejection effect. In order to define the dose-response relationship between the incident UVB irradiation on leukocyte concentrates and subsequent inhibition of their MLR responses, we collected 4 ± 2 × 10⁹ leukocytes (93 ± 7% lymphocytes) in 200 ml plasma from each of three volunteers by leukapheresis and exposed them to rapid, serial doses of UVB irradiation which was delivered by a blood product irradiator (4R4440 UVB Irradiator, Baxter, Inc) with aliquots removed between doses. Lymphocytes from each aliquot were placed in MLR with panel donors and studied in three groups: 1) the panel donor cells were γ-irradiated (1,500 rads) (i.e., only the UVB-irradiated cells could proliferate), 2) the UVB-irradiated cells were γ-irradiated (i.e., only the panel lymphocytes could proliferate), and 3) no γ-irradiation (i.e., both cell populations could proliferate). Each group had a similar UVB dose-related diminution in the MLR (p = .79, ANOVA). A single dose of 6 J/cm² extinguished the MLR to baseline in all groups. This dose should theoretically prevent transfused cells from producing either graft-versus-host disease or allosensitization, and might heighten their tolerogenic effect. This dose will be employed in our study of donor-specific leukocyte transfusion in clinical renal transplantation. © 1996 Wiley-Liss, Inc.     

Constitutive versus activation-dependent cross-presentation of immune complexes by CD8(+) and CD8(-) dendritic cells in vivo

Murine splenic dendritic cells (DCs) can be divided into two subsets based on CD8alpha expression, but the specific role of each subset in stimulation of T cells is largely unknown. An important function of DCs is the ability to take up exogenous antigens and cross-present them in the context of major histocompatibility complex (MHC) class I molecules to CD8(+) T cells. We previously demonstrated that, when cell-associated ovalbumin (OVA) is injected into mice, only the CD8(+) DC subset cross-presents OVA in the context of MHC class I. In contrast to this selectivity with cell-associated antigen, we show here that both DC subsets isolated from mice injected with OVA/anti-OVA immune complexes (OVA-IC) cross-present OVA to CD8(+) T cells. The use of immunoglobulin G Fc receptor (Fc(gamma)R) common gamma-chain-deficient mice revealed that the cross-presentation by CD8(-) DCs depended on the expression of gamma-chain-containing activating FcgammaRs, whereas cross-presentation by CD8(+) DCs was not reduced in gamma-chain-deficient mice. These results suggest that although CD8(+) DCs constitutively cross-present exogenous antigens in the context of MHC class I molecules, CD8(-) DCs only do so after activation, such as via ligation of Fc(gamma)Rs. Cross-presentation of immune complexes may play an important role in autoimmune diseases and the therapeutic effect of antitumor antibodies.     

NK cell activation in visceral leishmaniasis requires TLR9, myeloid DCs, and IL-12, but is independent of plasmacytoid DCs

Natural killer (NK) cells are sentinel components of the innate response to pathogens, but the cell types, pathogen recognition receptors, and cytokines required for their activation in vivo are poorly defined. Here, we investigated the role of plasmacytoid dendritic cells (pDCs), myeloid DCs (mDCs), Toll-like receptors (TLRs), and of NK cell stimulatory cytokines for the induction of an NK cell response to the protozoan parasite Leishmania infantum. In vitro, pDCs did not endocytose Leishmania promastigotes but nevertheless released interferon (IFN)-alpha/beta and interleukin (IL)-12 in a TLR9-dependent manner. mDCs rapidly internalized Leishmania and, in the presence of TLR9, produced IL-12, but not IFN-alpha/beta. Depletion of pDCs did not impair the activation of NK cells in L. infantum-infected mice. In contrast, L. infantum-induced NK cell cytotoxicity and IFN-gamma production were abolished in mDC-depleted mice. The same phenotype was observed in TLR9(-/-) mice, which lacked IL-12 expression by mDCs, and in IL-12(-/-) mice, whereas IFN-alpha/beta receptor(-/-) mice showed only a minor reduction of NK cell IFN-gamma expression. This study provides the first direct evidence that mDCs are essential for eliciting NK cell cytotoxicity and IFN-gamma release in vivo and demonstrates that TLR9, mDCs, and IL-12 are functionally linked to the activation of NK cells in visceral leishmaniasis.     

Cytokine-conditioned dendritic cells induce humoral tolerance to protein therapy in mice

A major obstacle in the genetic therapy of inherited metabolic disease is host immune responses to the therapeutic protein. This is best exemplified by inhibitor formation in the protein therapy for hemophilia A. An approach to overcoming this is induction of immunological tolerance to the therapeutic protein. Tolerogenic dendritic cells (DCtols) have been reported to induce tolerance. In addition, cytokines such as interleukin (IL)-10 and transforming growth factor (TGF)-β(1) are known to induce tolerance. To model protein therapy, we used ovalbumin (OVA) as antigen in BALB/c mice and their transgenic derivative, DO11.10 mice. In this study we show that adoptive transfer of antigen-pulsed dendritic cells (DCs) treated with a combination of IL-10 and TGF-β(1) can suppress the antibody response in mice. Adoptive transfer of cytokine-conditioned DCs in preimmunized mice results in reduction of antibody response in the mice. Furthermore, the effect is antigen specific, as the recipient mice were able to mount a potent antibody response to the control antigen. Last, we show that TGF-β(1) and IL-10-conditioned DCs are able to inhibit anti-FVIII antibody responses in FVIII knockout (KO) mice. Analysis of the contribution of IL-10 and TGF-β(1) to the DCtol phenotype shows that IL-10 treatment of DCs is sufficient for inducing OVA-specific tolerance in BALB/c mice, but we observed a requirement for treatment with both human TGF-β(1) and human IL-10 to significantly inhibit anti-FVIII antibody responses in FVIII KO mice. This paper demonstrates that autologous cell therapy for antigen-targeted immune suppression may be developed to facilitate long-term therapy.     

A novel immunosuppressive pathway involving peroxynitrite-mediated [corrected] nitration of platelet antigens within antigen-presenting cells

BACKGROUND: Studies have demonstrated that immunity against platelet (PLT) transfusions is dependent on recipient antigen‐presenting cells (APCs) and their ability to produce nitric oxide (NO). To further analyze this, we focused on NO's major metabolite peroxynitrite (ONOO^‐) and its ability to affect PLT immunity. STUDY DESIGN AND METHODS: To address how NO and its major metabolite may mediate PLT immunity, GP91^PHOX knockout (KO) mice that lack the ability to produce the ONOO^‐ were transfused weekly with allogeneic BALB/c PLTs, and donor antibody development was analyzed. RESULTS: Compared with controls, GP91^PHOX KO mice developed significantly (p < 0.0001) higher‐titered immunoglobulin G (IgG) donor antibodies by two transfusions, and this immune response could be inhibited by treating the recipient mice with aminoguanidine, a relatively selective inhibitor of inducible nitric oxide synthase. In vitro nitration of PLTs did not alter PLT antibody binding but significantly inhibited the transfused PLT's ability to stimulate IgG immunity in either wild‐type or KO mice. The lack of nitrated PLT immunity correlated with an inability of APCs to mediate phagocytosis of nitrated PLTs. The lack of nitrated PLT immunity could only be restored when normal PLTs were mixed with the nitrated PLTs and transfused. CONCLUSION: The results identify a dual role for NO metabolism within APCs that significantly modulates PLT immunity; nitration of PLT antigens leads to lack of immunity due to an inability of APCs to move PLT antigens intracellularly whereas there exists an NO‐dependent pathway that stimulates anti‐PLT immunity.     

Tolerogenic dendritic cells: the ins and outs of outcome

Recent studies point to an important role for dendritic cells (DCs) in the induction of peripheral tolerance, revealing that the maturation and/or activation state of DCs might be a control point for the induction of peripheral tolerance. Recent progress in our understanding of the mechanisms mediating immune tolerance indicates them to be far more complex than hitherto anticipated. Factors deciding the outcome of vaccination with autologous DCs to prevent and treat diseases with an autoimmune background include maturation state of DCs, their administration route, long-term effects, antigen loading, and in vivo microenvironment. DC vaccination, although promising, is far from standardized. In this review, we discuss the ins and outs of DC-mediated immune tolerance and the need for careful experimental design to unequivocally prove the efficacy and reach the goal of optimized use of DCs in autoimmune diseases.