Plasmacytoid dendritic cells activated by influenza virus and CD40L drive a potent TH1 polarization

Flt3 ligand and granulocyte-macrophage colony-stimulating factor preferentially expand and stimulate different dendritic and T-cell subsets

OBJECTIVE

Mechanisms of T-cell stimulation by Flt3 ligand (Flt3L) and granulocyte-macrophage colony-stimulating factor (GM-CSF) remain unclear. Herein, we compared the effects of Flt3L and GM-CSF on the expansion of dendritic cells (DC) and T-cell subsets and cytokine expression.

METHODS

Naïve and effector/memory T cells were analyzed by flow cytometry (FC). CD4(+) and CD8(+) T cells and CD11c(+)CD11b(dull/-)(DC1) and CD11c(+)CD11b(+) (DC2) subsets were isolated and the frequency of IFN-gamma-, IL-12- (type 1) and IL-4-, IL-10 (type 2)-producing cells and cytokine mRNA expression evaluated.

RESULTS

Flt3L expanded both DC1 and DC2 subsets with a significantly higher percentage and number of DC1 than DC2, while GM-CSF preferentially expanded the DC2 subset. Isolated DC1 from Flt3L-injected mice had significantly higher levels of IL-12 (p40) than IL-10, while the converse occurred with DC2. The numbers of naïve and memory T cells were elevated in mice that received Flt3L or GM-CSF. However, the number of memory CD4(+) and CD8(+) T cells was significantly increased in Flt3L as compared to GM-CSF cohorts. While GM-CSF increased the frequency of both type 1 and type 2 cytokine-producing cells, Flt3L significantly augmented the frequency of type 1 T cells.

CONCLUSIONS

In contrast to GM-CSF, Flt3L preferentially induces the expansion of type 1 T cells. The mechanism of Flt3L-induced T-cell stimulation is associated with the expansion of the IL-12 (p40)-producing DC1 and memory T cells.

Dendritic cells and tumor immunity

Researchers and clinicians have tried for decades to use the mechanisms of immunity for the fight against cancer. Early attempts aimed at the instrumentation of soluble immune mediators such as antibodies or cytotoxic proteins for the therapy of malignancies. Major improvements in understanding the induction and regulation of cellular immunity have now made it possible to generate effector cells in cancer patients which are specific for the neoplastic disease. At the beginning of every cellular immune reaction against cancers tumor antigens have to be presented to T cells in order to activate them and drive them into clonal expansion. This is done by antigen presenting cells, the most powerful of which is the dendritic cell (DC). While DC were hard to isolate initially, they can be generated in large numbers in vitro today and manipulated in multiple ways before given back to a patient to induce tumor immunity. Thus, a great amount of hope lies in the use of DC as inducers of tumor immunity. However, the first clinical studies, which have now been completed with only limited success make clear, that still a lot of open questions remain to be answered. This review tries to give an overview of this rapidly developing field, mentioning the major conceptual approaches and techniques, but also discussing important caveats. The next years will show whether we can improve our understanding of DC biology and the mechanisms of immune induction strongly enough to effectively employ DC for immunotherapy of cancer.

Dendritic cells: their significance in health and disease

Dendritic cells (DCs) represent a heterogeneous population of professional antigen-presenting cells. Precursor cells move via the blood to peripheral tissues. These immature DCs can take up invading pathogens and then rapidly migrate to the draining secondary lymphoid organs. Converted into antigen-presenting mature DCs, these cells are able to prime naive T cells and to initiate an adoptive immune response. The extraordinary functional profile suggests that, under certain preconditions, DCs may represent an ideal vector in the immunotherapy of cancer and infectious diseases

Dendritic cell subsets and the regulation of Th1/Th2 responses

Cells of the dendritic family are suited to perform two distinct functions at two discrete locations. In the peripheral tissues, dendritic cells (DC) act as sentinels for "dangerous" antigens. They then migrate into the lymphoid organ, where they initiate activation of T lymphocytes which are specific for these antigens. During their migration, DC shift from an antigen-capturing mode to a T cell sensitizing mode. In addition to switching on the immune response, subtypes of DC appear to influence the character of T cell differentiation, i.e. the Th1/Th2 balance. We will review the cellular and molecular bases of Th1-Th2 development by DC subsets, and will focus primarily, although not exclusively, on mouse DC.

CD40 and CD86 upregulation with divergent CMRF44 expression on blood dendritic cells in inflammatory bowel diseases

OBJECTIVE

Dendritic cells (DC) are the only antigen-presenting cells that can activate naïve T lymphocytes and initiate a primary immune response. They are also thought to have a role in immune tolerance. DC traffic from the blood to peripheral tissue where they become activated. They then present antigen and the costimulating signals necessary to initiate an immune response. In this study, we investigated the number, subsets, and activation pattern of circulating and intestinal DC from patients with clinically mild ulcerative colitis (UC) or Crohn's disease.

METHODS

Patients were recruited, if they were not taking immunosuppressive therapy, and were assessed for clinical severity of their disease using for UC, the Clinical Activity Index, and for Crohn's disease, the Crohn's Disease Activity Index. Blood CD11c+ and CD11c- DC subsets, expression of costimulatory antigens, CD86 and CD40, and the early differentiation/activation antigen, CMRF44, were enumerated by multicolor flow cytometry of lineage negative (lin- = CD3-, CD19-, CD14-, CD16-) HLA-DR+ DC. These data were compared with age-matched healthy and the disease control groups of chronic noninflammatory GI diseases (cGI), acute noninflammatory GI diseases (aGI), and chronic non-GI inflammation (non-GI). In addition, cryostat sections of colonoscopic biopsies from healthy control patients and inflamed versus noninflamed gut mucosa of inflammatory bowel disease (IBD) patients were examined for CD86+ and CD40+ lin- cells.

RESULTS

Twenty-one Crohn's disease and 25 UC patients, with mean Crohn's Disease Activity Index of 98 and Clinical Activity Index of 3.1, and 56 healthy controls, five cGI, five aGI, and six non-GI were studied. CD11c+ and CD11c- DC subsets did not differ significantly between Crohn's, UC, and healthy control groups. Expression of CD86 and CD40 on freshly isolated blood DC from Crohn's patients appeared higher (16.6%, 31%) and was significantly higher in UC (26.6%, 46.3%) versus healthy controls (5.5%, 25%) (p = 0.004, p = 0.012) and non-GI controls (10.2%, 22.8%) (p = 0.012, p = 0.008), but not versus cGI or aGI controls. CD86+ and CD40+ DC were also present in inflamed colonic and ileal mucosa from UC and Crohn's patients but not in noninflamed IBD mucosa or normal mucosa. Expression of the CMRF44 antigen was low on freshly isolated DC, but it was upregulated after 24-h culture on DC from all groups, although significantly less so on DC from UC versus Crohn's or healthy controls (p = 0.024). The CMRF44+ antigen was mainly associated with CD11c+ DC, and in UC was inversely related to the Clinical Activity Index (r = -0.69, p = 0.0002).

CONCLUSIONS

There is upregulation of costimulatory molecules on blood DC even in very mild IBD but surprisingly, there is divergent expression of the differentiation/activation CMRF44 antigen. Upregulation of costimulatory molecules and divergent expression of CMRF44 in blood DC was also apparent in cGI and aGI but not in non-GI or healthy controls, whereas intestinal CD86+ and CD40+ DC were found only in inflamed mucosa from IBD patients. Persistent or distorted activation of blood DC or divergent regulation of costimulatory and activation antigens may have important implications for gut mucosal immunity and inflammation.

Dendritic cells signal T cells in the absence of exogenous antigen

Interactions with self-major histocompatibility complex molecules on dendritic cells (DCs) are important for the survival of mature CD4+ T cells. We have followed the DC-mediated signal from the T cell surface to the nucleus and identified a pattern of activation that correlates with increased in vitro survival. This response is induced exclusively by DCs and is likely associated with a modulation of the T cell activation threshold. We have also found that DC-mediated activation results in antigen-independent cytokine gene expression, which points to a new role for DCs in shaping the cytokine milieu. Such antigen-independent activation of T cells may play a role in protective immunity, but may also induce and perpetuate autoimmune states such as multiple sclerosis.

Dendritic cells and viral immunity: friends or foes?

Dendritic cells (DC) residing in epithelial tissues of various mucosae and the skin are characterized by the unique ability to capture antigens and migrate to draining lymph nodes, where they can activate naive and memory T cells. Although DC play a pivotal role in inducing protective immunity to viral infection, they can also be exploited by viruses to evade the host immune response, induce immune suppression, or serve as latent viral reservoirs. Thus, virus interactions with DC may lead to an immune response that can be protective, but does not necessarily lead to complete virus elimination, resulting in immunopathology.

A pivotal role of Rho GTPase in the regulation of morphology and function of dendritic cells

Dendritic cell (DC) is the most potent activator of CD4+ T cells and has unique dendrites and veils. To explore the function of Rho in DC, exoenzyme C3 from Clostridium botulinum was used as a specific inhibitor of Rho. Treatment of DC with C3 (DC/C3) resulted in profound morphological changes by losing dendrites and emerging of shrunk membrane processes that were in parallel with marked reduction of polymerized actin in the marginal area. Inactivation of Rho-associated coiled coil-containing kinase (p160ROCK) by a specific ROCK inhibitor Y-27632 also led to disappearance of dendrites of DC with retaining large membrane expansions. In scanning electron microscopy, untreated DCs interacted with CD4+ T cells more efficiently than DC/C3. Conjugate formation assay showed that the number of DCs associated with CD4+ T cells was 2-fold higher in untreated DCs than that of DC/C3. Alloantigen-presenting capacity of DC/C3 was significantly suppressed in a dose-dependent manner. Because C3 treatment did not affect the surface expression of HLA, costimulatory, and adhesion molecules of DC, we examined cytokine production of DC and naive CD4+ T cells to further elucidate the inhibitory mechanism of MLR. Unexpectedly, DC/C3 increased IL-12 production after LPS stimulation. Naive CD4+ T cells cocultured with DC/C3 produced the increased percentage of IFN-gamma-producing cells, whereas the percentage of IL-2-producing T cells was decreased. These results demonstrate that Rho GTPase in DC controls both characteristic shape and immunogenic capacity.

Plasmacytoid monocytes/T cells: a dendritic cell lineage?

Plasmacytoid monocytes/T cells were first described in 1958, yet their origin and function have remained enigmatic. Recently a series of publications brought these cells to the forefront of immunological research. Indeed, plasmacytoid monocytes/T-cells contain natural type-I interferon producing cells and can differentiate in vitro into dendritic cells (DC). It has been proposed that plasmacytoid monocytes/T-cells represent a distinct lineage of cells whose fate it is to differentiate into dendritic cells. Herein we will review recent advances in our understanding of plasmacytoid monocytes/T cells and highlight arguments in favor or against this lineage hypothesis. We propose that plasmacytoid monocytes/T cells represent a composite group of both myeloid and lymphoid early-committed cells that are characterized by their ability to differentiate in vitro into DC.

Subsets of human dendritic cell precursors express different toll-like receptors and respond to different microbial antigens

Toll-like receptors (TLRs) are ancient microbial pattern recognition receptors highly conserved from Drosophila to humans. To investigate if subsets of human dendritic cell precursors (pre-DC), including monocytes (pre-DC1), plasmacytoid DC precursors (pre-DC2), and CD11c(+) immature DCs (imDCs) are developed to recognize different microbes or microbial antigens, we studied their TLR expression and responses to microbial antigens. We demonstrate that whereas monocytes preferentially express TLR 1, 2, 4, 5, and 8, plasmacytoid pre-DC strongly express TLR 7 and 9. In accordance with these TLR expression profiles, monocytes respond to the known microbial ligands for TLR2 (peptidoglycan [PGN], lipoteichoic acid) and TLR4 (lipopolysaccharide), by producing tumor necrosis factor (TNF)-alpha and interleukin (IL)-6. In contrast, plasmacytoid pre-DCs only respond to the microbial TLR9-ligand, CpG-ODNs (oligodeoxynucleotides [ODNs] containing unmethylated CpG motifs), by producing IFN-alpha. CD11c(+) imDCs preferentially express TLR 1, 2, and 3 and respond to TLR 2-ligand PGN by producing large amounts of TNF-alpha, and to viral double-stranded RNA-like molecule poly I:C, by producing IFN-alpha and IL-12. The expression of distinct sets of TLRs and the corresponding difference in reactivity to microbial molecules among subsets of pre-DCs and imDCs support the concept that they have developed through distinct evolutionary pathways to recognize different microbial antigens.

Deregulation of the expression of the fractalkine/fractalkine receptor complex in HIV-1-infected patients

Fractalkine is the only member of the CX3C chemokine family. Polymorphism of the fractalkine receptor gene may influence the prognosis of human immunodeficiency virus (HIV) infection, but the nature of the cells expressing fractalkine or its receptor in HIV-infected patients remains unknown. We show that, in contrast to HIV-uninfected individuals, a large number of cells expressed fractalkine in T-cell zones of lymph nodes from HIV-infected patients. CD83(+) mature and CD123(+) plasmacytoid dendritic cells as well as plasma cells are involved in this increased expression of fractalkine. Increased numbers of plasmacytoid dendritic cells and plasma cells were present in T-cell zones of HIV-infected patients. CD83(+) dendritic cells were present in similar number in HIV-infected patients and controls, but an increased fraction of these cells produced fractalkine in HIV-infected patients. Many plasma cells in the gut-associated lymphoid tissue from HIV-infected patients also produced fractalkine, whereas few cells produced fractalkine in the gut of controls. The fraction of CD45RO(+) and CD45RO(-) T helper (Th) cells expressing the fractalkine receptor CX3CR1 was higher in HIV-infected patients than in healthy individuals, and these cells were abnormally sensitive to fractalkine stimulation. This increased response correlated with HIV viremia, and it returned to normal levels in patients successfully treated with antiretroviral drugs. The increased expression of the fractalkine/fractalkine receptor complex associated with HIV infection may affect adhesion and migration of Th lymphocytes and their interaction with dendritic cells. Thus, it may influence the equilibrium between depletion and renewal of the Th lymphocyte compartment.

Interferon-alpha generation and immune reconstitution during antiretroviral therapy for human immunodeficiency virus infection

OBJECTIVES

To quantify the effect of HIV infection and HIV-suppressive therapy on interferon-alpha (IFN-alpha) production by human blood mononuclear cells; to compare, in parallel, effects on CD4+ T-cell numbers; and to ascertain the relationship of these interferon and CD4 parameters to resistance to opportunistic infections.

DESIGN

Serial studies of 294 unselected patients with HIV infection during therapy, with outcomes analysis.

METHODS

Determination of IFN generation by blood mononuclear cells via bioassay, and T-lymphocyte subset analysis via flow cytometry; serial studies of individual patients; linear regression and chi2 contingency table analysis.

RESULTS

HIV burden is inversely related to interferon-alpha generation, much as it is to CD4+ T-cell counts. Both of these recover during HIV-suppressive therapy. Reconstitution of IFN-alpha generation to levels commensurate with protection against opportunistic infection occurs prior to similar restoration of CD4 counts. In the outcomes analyses, such immune reconstitution was associated with protection from recurrent or new opportunistic infection. Conversely, viral suppression without such immunologic recovery was not protective against opportunistic infection.

CONCLUSIONS

Rapidly responding IFN-alpha generating cells appear to participate in resistance to opportunistic intracellular infection. Recovery of IFN-alpha generation may be an early marker of immune reconstitution in AIDS.

Corticosteroids depress IFN-alpha-producing plasmacytoid dendritic cells in human blood

Glucocorticoids are strongly immunosuppressive and are associated with reactivation of some intracellular infections. The plasmacytoid dendritic cell is a rare blood mononuclear cell detected through its production of IFN-alpha in response to herpes simplex virus and by surface immunophenotyping. We here report that steroid administration results in a decrease of IFN-alpha generation of approximately 25-fold, accompanied by reduction in circulating plasmacytoid dendritic cell numbers. Both parameters return to normal within days after steroid cessation.

Processing and presentation of antigens by dendritic cells: implications for vaccines

Dendritic cells (DCs) are potent antigen-presenting cells that are specialized in initiation of T-cell immunity. DCs induce promising anti-tumor T-cell and clinical responses, apparently without significant toxicity. Under certain conditions, DCs even silence T-cell immune responses in vivo. This dual capacity to modulate the immune system uniquely positions DCs for the treatment of autoimmunity, cancer and chronic viral infections.

Induction of antigen-specific human CD4(+) T cell anergy by peripheral blood DC2 precursors

Dendritic cells (DC) are antigen (Ag)-presenting cells that are essential for initiation of T cell-dependent immunity, and distinct DC subsets are known to direct different classes of immune responses. DC2 precursors (pDC2) or plasmacytoid DC were recently identified as a Th2-skewing and IFN-alpha-producing human DC subset. Here, we demonstrate that pDC2 enriched from human peripheral blood have a capacity to induce an anergic state in human Ag-specific CD4(+) T cell lines. Tetanus toxoid-specific T cell lines incubated with tetanus toxoid-pulsed autologous pDC2 failed to proliferate in secondary cultures with optimal Ag stimulation. T cell anergy induction required TCR engagement with Ag/MHC complex presented on pDC2. T cells rendered anergic lost IL-2 production but produced IFN-gamma and IL-10 upon stimulation. The pDC2-induced unresponsiveness was completely or partially reversible when a high concentration of exogenous IL-2 was added in the secondary cultures. Autoreactive CD4(+) T cell clones specific for topoisomerase I derived from a patient with scleroderma were also rendered anergic after co-culture with topoisomerase I-pulsed autologous pDC2,resulting in failure to proliferate or provide help to B cells. These results suggest that pDC2 are involved in maintenance of peripheral T cell tolerance and have potential for use in the suppression of pathogenic T cell responses in autoimmune diseases and organ transplantation.

Myeloid dendritic cells stimulate both Th1 and Th2 immune responses depending on the nature of the antigen

It has been shown that different types of pathogens induce different immune responses. Recovery from intracellular bacterial and viral infection is dependent on the secretion of Th1 cytokines, such as interferon-gamma (IFN-gamma), and on the generation of cytotoxic T cells. In contrast, responses to some parasitic invaders are of the Th2 type, characterized by secretion of interleukin-4 (IL-4). At present, it is not clear what directs this choice, and the most prevalent hypotheses are based on the dendritic cells (DC). In this work, we studied the immune responses generated in mice to a number of antigens, both replicating and nonreplicating, using bone marrow-derived DC as vehicles for immunization. We demonstrate that DC infected with influenza virus prime for a pure Th1 response in vivo devoid of IL-4 induction. This immune response correlates with the induction of DC maturation by the virus. In contrast, nonreplicating antigens, such as fetal bovine serum (FBS), beta-galactosidase, or inactivated influenza virus, do not mature the DC and prime for responses characterized by the secretion of large amounts of IL-4. These data support the hypothesis that myeloid DC are capable of eliciting both types of responses depending on the nature of the antigen.

Myeloperoxidase expression by histiocytes in Kikuchi's and Kikuchi-like lymphadenopathy

Forty-five examples of Kikuchi's lymphadenitis (KL), 5 Kikuchi-like lupus erythematosus lymphadenopathies, 25 nonnecrotizing lymphadenitidies (5 toxoplasmic, 5 sarcoid-like, 6 dermatopathic, 4 suppurative, 3 tubercular, 2 with sinus histiocytosis), 4 examples of hyaline-vascular Castleman disease (CD), 2 plasmacytoid monocyte tumors (PM-Ts), and 61 accessory cell neoplasms were studied by a panel of antibodies, including the PG-M1 (against a macrophage-restricted CD68 epitope) and a polyclonal anti-myeloperoxidase (MPO). In KL and Kikuchi-like lupus erythematosus lymphadenopathies, 25 to 75% of CD68(+) histiocytes co-expressed MPO. This did not occur in nonnecrotizing lymphadenitidies and accessory cell neoplasms. MPO(+)/CD68(+) elements corresponded to nonphagocytosing mononuclear cells and some crescentic macrophages and phagocytosing histiocytes. Typical PMs were MPO(-)/CD68(+) in all cases, including CD and PM-T. Our observations suggest that in KL and KL-like lymphadenopathies: 1) MPO(+)/CD68(+) blood monocytes might be attracted into tissues because of the lack or paucity of granulocytes and the need of MPO for oxidative processes; 2) PMs are more likely to be involved in the cytotoxic immune reaction than in phagocytic phenomena; 3) the peculiar phenotype of the histiocytic component can be usefully used for the differentiation from malignant lymphoma and PM-T.

Mature dendritic cells prime functionally superior melan-A-specific CD8+ lymphocytes as compared with nonprofessional APC

Priming of melan-A(26/27-35)-specific CTL occurs only in a fraction of late stage melanoma patients, whereas during the early stages of the disease and in healthy volunteers, melan-A CTL have functional and phenotypic markers consistent with a naive phenotype. To study the requirements for expansion of naive melan-A CTL from healthy donors, we set up an in vitro priming protocol and, using tetramer assays, we demonstrate that the activity and phenotype of the expanded melan-A CTL are profoundly influenced by the type of APC used. Priming by nonprofessional APC leads to expansion of melan-A CTL with reduced cytolytic activity and low level of IFN-gamma secretion. In contrast, mature dendritic cells (DC) expand cytolytic and IFN-gamma-producing melan-A CTL. Priming by mature DC is also efficient at low peptide concentration and requires only one round of stimulation. Finally, we observed that a significant fraction of CD45RO(+) melan-A CTL primed by mature DC expresses high levels of the homing receptor CD62L, whereas CTL primed by nonprofessional APC express CD62L in lower percentages and at lower levels. These results suggest that suboptimal priming by nonprofessional APC could account for the presence in vivo of dysfunctional cells and strongly support the immunotherapeutic use of mature DC for expansion of effector and memory Ag-specific CTL.

An etiopathogenic role for the type I IFN system in SLE

The type I interferon (IFN) system plays a pivotal role in the etiopathogenesis of systemic lupus erythematosus (SLE). The initial appearance of autoantibody-producing B cells can be precipitated by infection-induced type I IFNs, but the further, significant generation of autoimmune T and B cells is caused by the prolonged production of IFN-alpha, which is maintained by a vicious circle mechanism. This involves the activation of immature dendritic cells, known as natural IFN-producing cells, by continuously formed endogenous IFN-alpha inducers. These IFN-alpha inducers consist of complexes of autoantibodies with nucleic-acid-containing autoantigens derived from apoptotic cells.

Th1- and Th2-cell commitment during infectious disease: asymmetry in divergent pathways

The development of T helper 1 (Th1) versus Th2 cells is a major branch point in the immune response. It is an important determinant of whether the response to an infectious pathogen will lead to protection of the host or dissemination of the disease. Recent studies have suggested that this process is governed by distinct sets of signals provided by dendritic cells upon interactions with specific infectious agents. A model is proposed that links together the pathogen, the innate response and Th-cell polarization.

IFN-γ-Inducible Protein-10 Is Essential for the Generation of a Protective Tumor-Specific CD8 T Cell Response Induced by Single-Chain IL-12 Gene Therapy

The successful induction of T cell-mediated protective immunity against poorly immunogenic malignancies remains a major challenge for cancer immunotherapy. Here, we demonstrate that the induction of tumor-protective immunity by IL-12 in a murine neuroblastoma model depends entirely on the CXC chemokine IFN-gamma-inducible protein 10 (IP-10). This was established by in vivo depletion of IP-10 with mAbs in mice vaccinated against NXS2 neuroblastoma by gene therapy with a linearized, single-chain (sc) version of the heterodimeric cytokine IL-12 (scIL-12). The efficacy of IP-10 depletion was indicated by the effective abrogation of scIL-12-mediated antiangiogenesis and T cell chemotaxis in mice receiving s.c. injections of scIL-12-producing NXS2 cells. These findings were extended by data demonstrating that IP-10 is directly involved in the generation of a tumor-protective CD8+ T cell-mediated immune response during the early immunization phase. Four lines of evidence support this contention: First, A/J mice vaccinated with NXS2 scIL-12 and depleted of IP-10 by two different anti-IP-10 mAbs revealed an abrogation of systemic-protective immunity against disseminated metastases. Second, CD8+ T cell-mediated MHC class I Ag-restricted tumor cell lysis was inhibited in such mice. Third, intracellular IFN-gamma expressed by proliferating CD8+ T cells was substantially inhibited in IP-10-depleted, scIL-12 NXS2-vaccinated mice. Fourth, systemic tumor protective immunity was completely abrogated in mice depleted of IP-10 in the early immunization phase, but not if IP-10 was depleted only in the effector phase. These findings suggest that IP-10 plays a crucial role during the early immunization phase in the induction of immunity against neuroblastoma by scIL-12 gene therapy.

The instructive role of dendritic cells on T cell responses: lineages, plasticity and kinetics

Dendritic cells are responsible for directing different types of T cell responses, from thymic negative selection to the generation of effector and memory cells and the induction of peripheral tolerance. Recent studies indicate that the dendritic cell lineage, the extent of recruitment into inflamed tissues and migration to lymph nodes, the nature of maturation stimuli and the kinetics of activation have a quantitative and qualitative impact on T cell stimulation, thus exerting an instructive control on T cell responses.

Advances in transplant immunobiology

The present review briefly addresses the most recent knowledge acquired in the field of transplant immunology. A particular emphasis is placed on articles published during the past 12-18 months that have focused on allorecognition, dendritic cells and tolerance.

Migration and function of antigen-primed nonpolarized T lymphocytes in vivo

Upon antigenic stimulation, naive T lymphocytes proliferate and a fraction of the activated cells acquire a T helper cell type 1 (Th1) or Th2 phenotype as well as the capacity to migrate to inflamed tissues. However, the antigen-primed T cells that receive a short T cell receptor (TCR) stimulation do not acquire effector function and remain in a nonpolarized state. Using TCR transgenic CD4(+) T cells in an adoptive transfer system, we compared the in vivo migratory capacities of naive, nonpolarized, Th1 or Th2 cells. Although all cell types migrated to the spleen, only naive and nonpolarized T cells efficiently migrated to lymph nodes. In addition Th1, but not Th2, migrated to inflamed tissues. In the lymph nodes, nonpolarized T cells proliferated and acquired effector function in response to antigenic stimulation, displaying lower activation threshold and faster kinetics compared with naive T cells. These results suggest that nonpolarized T cells are in an intermediate state of differentiation characterized by lymph node homing capacity and increased responsiveness that allows them to mount a prompt and effective secondary response.

Human thymus contains IFN-alpha-producing CD11c(-), myeloid CD11c(+), and mature interdigitating dendritic cells

Three distinct dendritic cell (DC) subsets capable of stimulating allogeneic naive T cells were isolated from human thymus. The most abundant subset was represented by plasmacytoid DCs (pDCs), which secreted high amounts of IFN-alpha upon stimulation with inactivated influenza virus and thus likely correspond to the recently identified peripheral blood natural IFN-alpha/beta-producing cells (IPCs). Like those latter cells, thymic pDCs had distinctive phenotypic features (i.e., Lin(-), HLA-DR(int), IL-3R alpha(hi), CD45RA(hi), CD11c(-), CD13(-), and CD33(lo)) and developed into mature DCs upon culture in IL-3 and CD40L. Of the two other DC subsets, one displayed a phenotype of immature myeloid DCs (imDCs) (HLA-DR(int), CD11c(+), CD13(+), CD33(+)), and the other represented HLA-DR(hi) CD11c(+) mature DCs (mDCs). Since they also expressed DC-LAMP, these mDCs appear to correspond to interdigitating dendritic cells (IDCs). Thymic pDCs, but not myeloid imDCs, strongly expressed lymphoid-specific transcripts such as pre-T alpha, lambda-like, and Spi-B, thereby suggesting a possible lymphoid origin. The detection of Spi-B mRNA, not only upon in vitro maturation of pDCs, but also in freshly purified IDCs, suggests that in vivo pDCs may differentiate into IDCs.

Two subsets of dendritic cells are present in human cerebrospinal fluid

Little is known about the presence of dendritic cells in the human CNS. To investigate the occurrence of dendritic cells in the CSF, paired blood/CSF samples from patients with multiple sclerosis, acute optic neuritis, Lyme neuroborreliosis, other inflammatory neurological diseases and non-inflammatory neurological diseases were examined using flow cytometry. Almost all CSF samples contained myeloid (lin-CD11c+HLA-DR++CD123(dim)) and plasmacytoid (lin-CD11c-HLA-DR+CD123(high)) dendritic cells. In non-inflammatory neurological diseases, dendritic cells of either subset only constituted up to 1% of CSF mononuclear cells. Myeloid CSF dendritic cells were elevated in optic neuritis, neuroborreliosis and other inflammatory neurological disorders, while plasmacytoid dendritic cells were elevated in all neuroinflammatory conditions studied, with especially high numbers in neuroborreliosis. Numbers of CSF dendritic cells correlated with the common parameters of CNS inflammation. The myeloid dendritic cells in CSF expressed higher levels of HLA-DR, CD86, CD80 and CD40 than those in blood, whereas expression of these molecules by plasmacytoid dendritic cells was equal in blood and CSF. Both CSF and blood dendritic cells expressed the chemokine receptor CCR5. This is the first demonstration that dendritic cells are present in human CSF and that plasmacytoid dendritic cells are present in a non-lymphoid compartment. Myeloid and plasmacytoid dendritic cells in CSF may contribute to orchestration of the local immune responses.

Chemokines and disease

We examine here several diseases that are associated with inappropriate activation of the chemokine network. Detailed comment has been restricted to pathological states for which there are compelling data either from clinical observations or animal models. These include cardiovascular disease, allergic inflammatory disease, transplantation, neuroinflammation, cancer and HIV-associated disease. Discussion focuses on therapeutic directions in which the rapidly evolving chemokine field appears to be headed.

Kinetics of dendritic cell activation: impact on priming of TH1, TH2 and nonpolarized T cells

To prime immune responses, dendritic cells (DCs) need to be activated to acquire T cell stimulatory capacity. Although some stimuli trigger interleukin 12 (IL-12) production that leads to T helper cell type I (TH1) polarization, others fail to do so and favor TH2 polarization. We show that after activation by lipopolysaccharide, DCs produced IL-12 only transiently and became refractory to further stimulation. The exhaustion of cytokine production impacted the T cell polarizing process. Soon after stimulation DCs primed strong TH1 responses, whereas at later time points the same cells preferentially primed TH2 and nonpolarized T cells. These findings indicate that during an immune response, T cell priming conditions may change in the lymph nodes, suggesting another mechanism for the regulation of effector and memory T cells.