CD2 engagement induces dendritic cell activation: implications for immune surveillance and T-cell activation

We have shown previously that primary dendritic cells and monocytes express equal levels of CD14 but are distinguishable by the presence of CD2 on dendritic cells. CD2 is known to mediate the activation of T and natural killer (NK) cells through its interaction with CD58. CD2 epitopes recognized by anti-T111, -T112, and -T113 monoclonal antibodies (mAbs) are present on dendritic cells. Here we show that CD2 engagement significantly increases class II, costimulatory (CD40, CD80, CD86), adhesion (CD54, CD58), and CCR7 molecule expression on primary dendritic cells. Conversely, minimal or no change in the expression of the above antigens occurs on monocyte-derived dendritic cells, because these molecules are already maximally expressed. However, both kinds of dendritic cells release interleukin-1beta (IL-1beta) and IL-12 after CD2 engagement. Lastly, interference with dendritic cell CD2-T-cell CD58 engagement decreases naive CD4+CD45RA+ T-cell proliferation. Collectively, our results suggest another role of the CD2-CD58 pathway that allows nonimmune and immune cells to interact directly with dendritic cells and initiate innate and adaptive immune responses.     

Plasmacytoid dendritic cells: from the plasmacytoid T-cell to type 2 dendritic cells CD4+CD56+ malignancies

Recent identification of CD4(+)CD56(+) malignancies as pathological counterparts of the precursors of type 2 dendritic cells (DC2) has shed new light on a leukocyte lineage that long remained elusive. This review retraces how knowledge evolved, through careful examination and analysis of both normal lymphoid tissue and rare proliferative diseases, from plasmacytoid T cells to plasmacytoid dendritic cells (pDC) and then DC2. The functions of these cells and their key role at the crossroads of innate and cognitive immunity are also discussed. The major characteristics of DC2 malignancies are summarized and compared to natural killer cell (NK) lymphomas, another type of proliferative disease sharing the expression of CD56.     

Factor VIII inhibitors: role of von Willebrand factor on the uptake of factor VIII by dendritic cells

Summary  In patients with haemophilia A, factor VIII (FVIII) therapy leads to the development of anti-FVIII alloantibodies that inhibit FVIII pro-coagulant activity, in up to 25% of the cases. At a time when efficient viral screening procedures are at place, development of inhibitors poses the greatest threat to haemophilia A patients. Various risk factors, both patient and product-related, are responsible for the development of inhibitory antibodies. The role of FVIII-specific CD4+ T lymphocytes in the initiation of the humoral immune response to exogenous FVIII has been well. In view of their capacity to stimulate naïve T cells, dendritic cells (DCs) play a central role in the initiation of the primary immune response. Thus, in the context of a primary alloimmunization against FVIII, i.e. when FVIII-specific B lymphocytes are not there to take up FVIII from the circulation and to serve as antigen presenting cells (APCs), DCs are the only cell type that internalize FVIII, leading to activation of FVIII-specific CD4+ T lymphocytes. von Willebrand factor (VWF) present in plasma-derived FVIII therapeutic concentrates, is known to act as a chaperone molecule for procoagulant FVIII. In addition to its role in reducing the 'antigenicity' of FVIII, the role of VWF in the reduction of the 'immunogenicity' of therapeutic FVIII in patients with haemophilia A has also been suggested. We have recently demonstrated that VWF protects FVIII from being endocytosed by human DCs and subsequently being presented to FVIII-specific T cells. We propose that VWF may reduce the immunogenicity of FVIII by preventing, upstream from the activation of immune effectors, the entry of FVIII in professional antigen presenting cells.     

Anti-idiotypic T-cell activation in multiple myeloma induced by M-component fragments presented by dendritic cells

The monoclonal immunoglobulin (Ig) (M-component) secreted by the tumour plasma cells in multiple myeloma (MM) has specific antigenic determinants (idiotypes; id) that can serve as tumour-specific antigens. The intact Ig molecule is a weak antigen, and small fragments of id protein might be more immunogenic for the induction of id-specific immunity. Dendritic cells (DC) have attracted attention as the most efficient antigen-presenting cells and promising adjuvants for immunotherapy in tumours. In this study the in vitro T-cell response against F(ab')₂ and Fab fragments, heavy and light chains of the M-component was examined in five patients with MM clinical stage I. All fragments were able to stimulate T cells but F(ab')₂ or Fab fragments and heavy chains induced a stronger response than light chains. DC induced a significantly stronger id-specific immune response than monocytes. Moreover, with DC as antigen-presenting cells, a predominant interferon (IFN)-γ (type-1 T-cell) response was seen in all patients. Both IFN-γ and interleukin (IL)-4 (type-1 and type-2 T-cell) responses were noted when monocytes were used. Our study suggests that DC pulsed with idiotypic fragments such as F(ab')₂ fragment and heavy chain can be used for the induction of type-1 anti-idiotypic T-cell response for immunotherapy in MM.     

Cytokine-dependent bystander hepatitis due to intrahepatic murine CD8 T-cell activation by bone marrow-derived cells

BACKGROUND & AIMS: Intrahepatic accumulation of CD8+ T cells following antigen-specific activation has been demonstrated in a number of transgenic models and also in extrahepatic viral infections. In some transgenic models, intrahepatic accumulation of cytotoxic T lymphocytes is associated with hepatitis. This observation suggests that hepatocellular damage may occur in some forms of immune-mediated hepatitis on the basis of a "bystander injury," whereby cytotoxic T lymphocytes accumulating in the liver mediate injury to hepatocytes in a nonspecific manner. Mouse transgenic models were therefore developed to investigate whether bystander damage to non-antigen-bearing hepatocytes occurs in vivo. METHODS: T cell receptor transgenic T cells were adoptively transferred into transgenic mice ubiquitously expressing the specific antigen, or into bone marrow radiation chimeras in which hepatocytes did not express the antigen. RESULTS: Selective accumulation of transgenic CD8+ T cells in the liver of intact recipients could be detected within 2 hours of transfer, despite ubiquitous antigenic expression. T cells retained in the liver were activated and induced hepatitis. Similar results were obtained using bone marrow chimeras, suggesting that antigen expression by hepatocytes was not required either for intrahepatic accumulation or for subsequent hepatitis. This "bystander hepatitis" was dependent on tumor necrosis factor alpha and interferon gamma. CONCLUSIONS: Intrahepatic accumulation of activated CD8+ T cells and subsequent hepatitis can result from primary activation of CD8+ T cells by liver resident bone marrow-derived cells, inducing bystander damage to non-antigen-bearing hepatocytes. This mechanism may play a role in some forms of biologically significant hepatitis, including autoimmune hepatitis and hepatitis associated with extrahepatic diseases.     

FoxP3+ regulatory T cells essentially contribute to peripheral CD8+ T-cell tolerance induced by steady-state dendritic cells

Peripheral T-cell tolerance is thought to significantly contribute to the prevention of autoimmunity, and it has been shown that antigen-presenting steady-state dendritic cells efficiently induce peripheral tolerance. We previously showed that dendritic-cell–induced tolerance is a T-cell–intrinsic process that depends on coinhibitory molecules such as programmed death-1. Here we specifically analyze the involvement of FoxP3⁺ regulatory T cells, which are known to be important for maintenance of self-tolerance. We show that antigen presentation by steady-state dendritic cells failed to induce peripheral tolerance in the absence of FoxP3⁺ regulatory T cells but induced protective CD8⁺ T-cell–mediated immunity instead. Regulatory T-cell–depleted mice had massively increased numbers of dendritic cells in lymph nodes. Dendritic cells isolated from mice without regulatory T cells had up-regulated costimulatory molecules and showed stronger T-cell stimulatory capacity ex vivo, suggesting that regulatory T cells contribute to peripheral tolerance by keeping the dendritic cells in an immature state. Using blocking antibodies, we demonstrate that CTLA-4 but not IL-10 is necessary for control of dendritic cells by regulatory T cells.     

HIV inhibits CD4+ T-cell proliferation by inducing indoleamine 2,3-dioxygenase in plasmacytoid dendritic cells

Infection with the human immunodeficiency virus type-1 (HIV) results in acute and progressive numeric loss of CD4(+) T-helper cells and functional impairment of T-cell responses. The mechanistic basis of the functional impairment of the surviving cells is not clear. Indoleamine 2,3-dioxygenase (IDO) is an immunosuppressive enzyme that inhibits T-cell proliferation by catabolizing the essential amino acid tryptophan (Trp) into the kynurenine (kyn) pathway. Here, we show that IDO mRNA expression is elevated in peripheral blood mononuclear cells (PBMCs) from HIV(+) patients compared with uninfected healthy controls (HCs), and that in vitro inhibition of IDO with the competitive blocker 1-methyl tryptophan (1-mT) results in increased CD4(+) T-cell proliferative response in PBMCs from HIV-infected patients. We developed an in vitro model in which exposure of PBMCs from HCs to either infectious or noninfectious, R5- or X4-tropic HIV induced IDO in plasmacytoid dendritic cells (pDCs). HIV-induced IDO was not inhibited by blocking antibodies against interferon type I or type II, which, however, induced IDO in pDCs when added to PBMC cultures. Blockade of gp120/CD4 interactions with anti-CD4 Ab inhibited HIV-mediated IDO induction. Thus, induction of IDO in pDCs by HIV may contribute to the T-cell functional impairment observed in HIV/AIDS by a non-interferon-dependent mechanism.     

VWF protects FVIII from endocytosis by dendritic cells and subsequent presentation to immune effectors

Von Willebrand factor (VWF) is a chaperone molecule for procoagulant factor VIII (FVIII). Its role in the reduction of the immunogenicity of therapeutic FVIII in patients with hemophilia A has been evoked but lacks clear cellular and molecular rationale. Here, we demonstrate that VWF protects FVIII from being endocytosed by human dendritic cells (DCs) and subsequently presented to FVIII-specific T cells. The immunoprotective effect of VWF requires a physical interaction with FVIII because the endocytosis of FVIII was significantly restored on hindering the formation of the VWF-FVIII complex. Interestingly, VWF had no direct inhibitory effect either on the ability of DCs to present antigenic peptides or on the activation potency of CD4+ T cells. We thus propose that VWF may reduce the immunogenicity of FVIII by preventing, upstream from the activation of immune effectors, the entry of FVIII in professional antigen-presenting cells.     

Failure of HIV-exposed CD4+ T cells to activate dendritic cells is reversed by restoration of CD40/CD154 interactions

Because interactions between activated CD4+ T cells and antigen-presenting cells (APCs) are crucial for optimal APC function, defective CD4+ T-cell activation may contribute to APC dysregulation in HIV infection. Here, we show that CD4+ T cells exposed during stimulation to noninfectious HIV having functional envelope glycoproteins failed to provide activation signals to autologous dendritic cells (DCs). Consequently, important DC functions, including production of immunoregulatory cytokines (interleukin-12 p40 and interleukin-10) and up-regulation of costimulatory molecules (CD86, CD40, CD83), as well as the capacity to stimulate naive allogeneic T cells, were all adversely affected. The blunted up-regulation of CD154 in CD4+ T cells that were activated in the presence of noninfectious viruses is likely to be the major underlying mechanism for these defects. Addition of recombinant trimeric CD154 could restore production of cytokines by DCs cocultured with HIV-exposed T cells. Moreover, the functional defects mediated by coculture with HIV-exposed T cells were similar to those following antibody blockade of CD40-CD154 interactions. HIV-mediated blunted CD154 expression may thus play an important role in the suppression of cell-mediated immunity seen in HIV infection.     

Steady-state dendritic cells expressing cognate antigen terminate memory CD8+ T-cell responses

Antigen stimulation of naive T cells in conjunction with strong costimulatory signals elicits the generation of effector and memory populations. Such terminal differentiation transforms naive T cells capable of differentiating along several terminal pathways in response to pertinent environmental cues into cells that have lost developmental plasticity and exhibit heightened responsiveness. Because these cells exhibit little or no need for the strong costimulatory signals required for full activation of naive T cells, it is generally considered memory and effector T cells are released from the capacity to be inactivated. Here, we show that steady-state dendritic cells constitutively presenting an endogenously expressed antigen inactivate fully differentiated memory and effector CD8(+) T cells in vivo through deletion and inactivation. These findings indicate that fully differentiated effector and memory T cells exhibit a previously unappreciated level of plasticity and provide insight into how memory and effector T-cell populations may be regulated.     

Mesenchymal stem cells impair in vivo T-cell priming by dendritic cells

Dendritic cells (DC) are highly specialized antigen-presenting cells characterized by the ability to prime T-cell responses. Mesenchymal stem cells (MSC) are adult stromal progenitor cells displaying immunomodulatory activities including inhibition of DC maturation in vitro. However, the specific impact of MSC on DC functions, upon in vivo administration, has never been elucidated. Here we show that murine MSC impair Toll-like receptor-4 induced activation of DC resulting in the inhibition of cytokines secretion, down-regulation of molecules involved in the migration to the lymph nodes, antigen presentation to CD4(+) T cells, and cross-presentation to CD8(+) T cells. These effects are associated with the inhibition of phosphorylation of intracellular mitogen-activated protein kinases. Intravenous administration of MSC decreased the number of CCR7 and CD49dβ1 expressing CFSE-labeled DC in the draining lymph nodes and hindered local antigen priming of DO11.10 ovalbumin-specific CD4(+) T cells. Upon labeling of DC with technetium-99m hexamethylpropylene amine oxime to follow their in vivo biodistribution, we demonstrated that intravenous injection of MSC blocks, almost instantaneously, the migration of subcutaneously administered ovalbumin-pulsed DC to the draining lymph nodes. These findings indicate that MSC significantly affect DC ability to prime T cells in vivo because of their inability to home to the draining lymph nodes and further confirm MSC potentiality as therapy for immune-mediated diseases.     

Murine plasmacytoid dendritic cells induce effector/memory CD8+ T-cell responses in vivo after viral stimulation

Like their human counterparts, mouse plasmacytoid dendritic cells (pDCs) play a central role in innate immunity against viral infections, but their capacity to prime T cells in vivo remains unknown. We show here that virus-activated pDCs differentiate into antigen-presenting cells able to induce effector/memory CD8(+) T-cell responses in vivo against both epitopic peptides and endogenous antigen, whereas pDCs activated by synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine motifs (CpG) acquire only the ability to recall antigen-experienced T-cell responses. We also show that immature pDCs are unable to induce effector or regulatory CD8(+) T-cell responses. Thus, murine pDCs take part in both innate and adaptive immune responses by directly priming naive CD8(+) T cells during viral infection.     

Role in T-cell activation for HLA class I molecules from accessory cells: further distinction between activation signals delivered to T cells via CD2 and CD3 molecules

The immunological function of major histocompatibility complex molecules, including HLA class I molecules, is to present antigens and/or their processed peptides to various lymphocyte subpopulations. Thus, they play a pivotal role in regulatory interactions between cells of the immune system, which can result in the activation and function of T cells. We looked for a role of major histocompatibility complex molecules during T-cell activation induced by monoclonal antibody (mAb) or combinations of mAb recognizing the two well-characterized T-cell surface molecules CD3 and CD2. To activate T-cell peripheral blood lymphocytes, we used a CD3 mAb or two different pairs of CD2 mAb, CD2 "GT2 + T11(1)" and CD2 "D66 + T11(1)," which, as we have previously shown, deliver different signals of activation to T cells. Anti-HLA class I mAb blocked the activation induced by CD3 mAb or by CD2 GT2 + T11(1), but it did not block activation induced by CD2 D66 + T11(1). We observed this pattern of inhibition according to the stimulus used to activate T cells both when the anti-HLA class I mAbs were added to cultures of whole peripheral blood mononuclear cells and when they were fixed to monocytes only. In the latter case, purified monocytes were first incubated with the anti-HLA mAb (whether whole immunoglobulin or Fab fragment) and then fixed with paraformaldehyde before culture with autologous purified T cells. Anti-HLA class I fixed on monocytes prevented both interleukin 2 (IL-2) receptor expression and IL-2 synthesis on T cells. The inhibitory effects of anti-class I mAb bound to monocytes were not reversed by adding large amounts of recombinant IL-2 or recombinant IL-1, a finding consistent with the observations that accessory cells surface components can fully complement the signals directly delivered to T cells by CD2 or CD3 mAb. We conclude that HLA class I from accessory cells plays an important role in the early phase of T-cell activation when direct contacts between accessory cells and T cells are required.