Generation of dendritic cell-like antigen-presenting cells in long-term mixed leucocyte culture: phenotypic and functional studies

Myeloid-derived suppressor cells and tolerogenic dendritic cells are distinctively induced by PI3K and Wnt signaling pathways

Imbalanced immune responses are a prominent hallmark of cancer and autoimmunity. Myeloid cells can be overly suppressive, inhibiting protective immune responses or inactive not controlling autoreactive immune cells. Understanding the mechanisms that induce suppressive myeloid cells, such as myeloid-derived suppressor cells (MDSCs) and tolerogenic dendritic cells (TolDCs), can facilitate the development of immune-restoring therapeutic approaches. MDSCs are a major barrier for effective cancer immunotherapy by suppressing antitumor immune responses in cancer patients. TolDCs are administered to patients to promote immune tolerance with the intent to control autoimmune disease. Here, we investigated the development and suppressive/tolerogenic activity of human MDSCs and TolDCs to gain insight into signaling pathways that drive immunosuppression in these different myeloid subsets. Moreover, monocyte-derived MDSCs (M-MDSCs) generated in vitro were compared to M-MDSCs isolated from head-and-neck squamous cell carcinoma patients. PI3K-AKT signaling was identified as being crucial for the induction of human M-MDSCs. PI3K inhibition prevented the downregulation of HLA-DR and the upregulation of reactive oxygen species and MerTK. In addition, we show that the suppressive activity of dexamethasone-induced TolDCs is induced by β-catenin-dependent Wnt signaling. The identification of PI3K-AKT and Wnt signal transduction pathways as respective inducers of the immunomodulatory capacity of M-MDSCs and TolDCs provides opportunities to overcome suppressive myeloid cells in cancer patients and optimize therapeutic application of TolDCs. Lastly, the observed similarities between generated- and patient-derived M-MDSCs support the use of in vitro-generated M-MDSCs as powerful model to investigate the functionality of human MDSCs.

Mobilizing dendritic cells for tolerance by engagement of immune inhibitory receptors for HLA-G

The presence of dendritic cells (DC) in the maternal decidua has pointed to a biologic role of antigen-presenting cell in maternal-fetal interaction. The expression of immune inhibitory receptors on DC opens the intriguing possibility that these types of receptors are directly involved in maturation/activation of DC and modulate their function. We show that the triggering of the murine inhibitory receptor paired immunoglobulin-like receptor-B by cross-linking or by human leukocyte antigen (HLA)-G tetramer resulted in the modulation of DC function and prolongation of allogeneic graft survival. In addition, we found that the engagement of human inhibitory receptor ILT4 by its natural ligand, HLA-G, alters maturation of human DC. In this study, we examined the mechanisms for the modulation of antigen-presenting cells by HLA-G. These findings have established an important link between HLA-G and immune inhibitory receptor regulation in vivo and in vitro, thereby placing HLA-G in the inhibitory pathway.

Differentiation of monocytic cell clones into CD8 alpha+ dendritic cells (DC) suggests that monocytes can be direct precursors for both CD8 alpha+ and CD8 alpha- DC in the mouse

Dendritic cells (DC) are the professional APCs that initiate T cell immune responses. DC can develop from both myeloid and lymphoid progenitors. In the mouse, the CD8alpha(+) DC had been designated as "lymphoid" DC, and CD8alpha(-) DC as "myeloid" DC until recently when it was demonstrated that common myeloid progenitors can also give rise to CD8alpha(+) DC in bone marrow chimera mice. However, it is still not clear which committed myeloid lineages differentiate into CD8alpha(+) DC. Because monocytes can differentiate into DC in vivo, the simplest hypothesis is that the CD8alpha(+) DC can be derived from the monocyte/macrophage. In this study we show that cell clones, isolated from CD8alpha(+) DC lymphoma but with a monocytic phenotype (CD11c(low/-)D11b(high)CD8alpha(-)I-A(low)), can redifferentiate into CD8alpha(+) DC either when stimulated by LPS and CD40L or when they migrate into the lymphoid organs. Maturation of DC in vivo correlated with strong priming of allogeneic T cells. Moreover, the monocytes from cultured splenocytes or peritoneal exudates macrophages of wild-type mice are also capable of differentiating into CD11c(+)CD8alpha(+) DC after their migration into the draining lymph nodes. Our results suggest that monocytes can be direct precursors for CD11c(+)CD8alpha(+) DC in vivo. In addition, the monocyte clones described in this study may be valuable for studying the differentiation and function of CD8alpha(+) DC that mediate cross-presentation of Ag to CD8 T cells specific for cell-associate Ags.

Appearance and phenotype of murine follicular dendritic cells expressing VCAM-1

The architecture of lymphoid follicles is determined by a series of interactions between lymphoid and follicular stromal cells. A cardinal population in the non-lymphoid compartment is the follicular dendritic cell (FDC), whose communication with resting and activated B cells involves various adhesive interactions. The FDC phenotype variably includes the display of vascular cell adhesion molecule (VCAM-1). In this report we investigated the appearance and follicular tissue distribution of VCAM-1 in murine peripheral lymphoid tissues, and compared VCAM-1 with other FDC markers using immunohistochemistry. Correlating the appearance of VCAM-1 with other murine FDC-associated markers (CR1.2 [complement receptor 1.2 or CD35/21] and FDC-M1) revealed that the display of VCAM-1 is restricted to a subset of CR1.2-positive FDCs. We found that the expression of VCAM-1 antigen in the spleen or peripheral lymph nodes on FDCs requires antigenic stimulus, and that it coincides with germinal center formation. The VCAM-1 expression is associated with the appearance of mucosal addressin cell adhesion molecule (MAdCAM-1), with some slight differences in occurrence. The appearance of VCAM-1 and MAdCAM-1 antigens on FDCs may serve as indicators of FDC activation.

Immunologic mechanisms of extracorporeal photochemotherapy in chronic graft-versus-host disease

Extracorporeal photochemotherapy (ECP) has been shown to be an effective therapy for patients with acute and chronic graft-versus-host disease (GVHD) following allogeneic bone marrow transplantation, but its biologic mechanism is not understood. We reported that clinical response to ECP was associated not only with normalization of skewed CD4/CD8 ratios but also with an increase in CD3(-)/CD56(+) natural killer cells and a decrease in the number of CD80(+) and CD123(+) circulating dendritic cells (DCs). To further elucidate the effects of ECP on activated lymphocyte subpopulations and the interaction between effector lymphocytes and antigen-presenting DCs, we isolated and characterized DC populations from patients with chronic GVHD undergoing ECP therapy. Antigen-presenting activity of DCs was measured as proliferation of antigen-stimulated autologous and allogeneic T cells by mixed-lymphocyte reaction (MLR). In MLR assays the proliferation of T cells was decreased in all 10 patients by a mean of 84% (range, 75%-95%; P < or =.002) after a 2-day cycle of ECP and longitudinally over the 12-month course of therapy. Immunophenotypic analysis of DC populations revealed a preponderance of DC1 monocytic dendritic cells in all patients before the initiation of ECP. Nine of 10 patients demonstrated a shift from DC1 to DC2 and as a concordant shift from a predominantly Th1 (interleukin-2 [IL-2], interferon-gamma) to Th2 (IL-4, IL-10) cytokine profile after ECP, and 8 of 10 had a clinical response to ECP. Our results suggest that ECP alters alloreactivity by affecting allo-targeted effector T cells and antigen-presenting DCs.

Vascular cell adhesion molecule-1 (VCAM-1) blockade in collagen-induced arthritis reduces joint involvement and alters B cell trafficking

Vascular cell adhesion molecule-1 (VCAM-1 or CD106) is important in leucocyte trafficking and its increased expression is associated with a number of chronic inflammatory diseases, including rheumatoid arthritis (RA). We used a neutralizing monoclonal antibody (M/K-2.7) to investigate the role of VCAM-1 in collagen-induced arthritis (CIA), an autoimmune model of RA. A single injection of M/K-2.7 (0.5 mg) into naive mice caused leucocytosis within 20 h, due to increased numbers of circulating B cells and macrophages, as well as neutrophils. The most marked effect was on the numbers of immature B cells (B220loIgM+) which were increased approximately fourfold. CIA was elicited in DBA/1 mice by immunization with chick type II collagen (CII) in Freund's complete adjuvant, followed by a repeat injection 21 days later. Repeated M/K-2.7 administration from the time of primary CII immunization reduced the clinical severity, but not the incidence, of CIA compared to isotype-control monoclonal antibody-treated mice. Histological assessment showed fewer arthritic joints in M/K-2.7-treated mice; however, affected joints showed the same range of severity as those of control mice. Anti-CII IgG1 levels were reduced in anti-VCAM-1-treated mice but the cellular immune response to CII was unaffected. In contrast, VCAM-1 blockade from the onset of clinical features of CIA did not prevent disease progression. These results establish a role for VCAM-1 in promoting polyarticular involvement in CIA, most probably via an effect on B cells.

Control of the differentiation state and function of human epidermal Langerhans cells by cytokines in vitro

Langerhans cells can originate in vitro from immature precursors stimulated with granulocyte macrophage-colony-stimulating factor (GM-CSF), tumour necrosis factor (TNF)-alpha and stem cell factor (SCF). We asked whether these cytokines also control the differentiation state of Langerhans cells within the epidermis and upon leaving this tissue. We harvested sheets of human epidermis by controlled dispase hydrolysis of keratomes, cultured them in RPMI and 10% fetal calf serum for 48 h and analysed the sheets and the cells migrated spontaneously into the medium, most of which were Langerhans cells containing Birbeck granules. By flow cytometry, the intensity of CD1a expression was reduced quite evenly among Langerhans cells migrated from sheets within 48 h. The cells in the sheets underwent loss of dendrites, with a significant reduction in the cell perimeter that was prevented by GM-CSF and TNF-alpha together. Either of these cytokines induced expression of CD18 by cells in the sheets and those in the medium. Moreover, TNF-alpha induced expression of CD54 by cells in the medium, but not by those retained in the sheets, whereas human SCF induced, dose dependently, expression of CD54 by cells in the sheets, but not from those in the medium. The proliferation of allogeneic lymphocytes was much higher when stimulating Langerhans cells were harvested from cultures with any cytokine, rather than from cultures without cytokines. We conclude the following: (i) GM-CSF and TNF-alpha help to maintain full differentiation of Langerhans cells within the epidermis; (ii) cytokine influence on Langerhans cells adhesiveness is in part context dependent; and (iii) pretreatment with cytokines influences positively the number or accessory activity of Langerhans cells on lymphocytes during subsequent mixed leucocyte reaction.

Efficient expression of the tumor-associated antigen MAGE-3 in human dendritic cells, using an avian influenza virus vector

Dendritic cells (DCs) are the most potent inducers of immune reactions. Genetically modified DCs, which express tumor-associated antigens (TAA), can efficiently induce antitumor immunity and thus have a high potential as tools in cancer therapy. The gene delivery is most efficiently achieved by viral vectors. Here, we explored the capacity of influenza virus vectors to transduce TAA genes. These viruses abortively infect DCs without interfering with their antigen-presenting capacity. In contrast to other viruses used for DC transduction, influenza viruses can be efficiently controlled by antiviral pharmaceuticals, lack the ability to integrate into host chromosomes, and fail to establish persistent infections. Genes encoding a melanoma-derived TAA (MAGE-3), or the green fluorescence protein (GFP), were introduced into a high-expression avian influenza virus vector. Monocyte-derived mature DCs infected by these recombinants efficiently produced GFP or MAGE-3. More than 90% of the infected DCs can express a transduced gene. Importantly, these transduced DCs retained their characteristic phenotype and their potent allogeneic T cell stimulatory capacity, and were able to stimulate MAGE-3-specific CD8(+) cytotoxic T cells. Thus influenza virus vectors provide a highly efficient gene delivery system in order to transduce human DCs with TAA, which consequently stimulate TAA-specific T cells.

Taking lessons from dendritic cells: multiple xenogeneic ligands for leukocyte integrins have the potential to stimulate anti-tumor immunity

Expression of large numbers of different costimulatory integrin ligands (CILs) attributes dendritic cells with an ability to induce primary anti-tumor immune responses. Here, we show that optimized gene transfer of the xenogeneic (human) CILs VCAM-1, MAdCAM-1 and ICAM-1 causes rapid and complete rejection of established mouse EL-4 tumors, and generates prolonged systemic anti-tumor immunity; whereas human E-cadherin weakly slows tumor growth. In each case the immune response was mediated by CD8+ T cells and NK cells, accompanied by augmented tumor-specific cytolytic T cell (CTL) activity involving both the perforin and Fas-ligand pathways. Adoptive transfer of splenocytes from cured mice rapidly cleared established tumors in recipients. The mechanism for CIL-mediated immunity is unknown, but may involve CTL-facilitated tumor lysis, since CTLs were generally twice as efficient at killing CIL-transfected tumor cells than parental tumor cells. Optimized CIL-based gene therapy may provide an approach to complement or replace conventional DC adoptive cell therapy for suppressing tumor growth.

CD40-deficient dendritic cells producing interleukin-10, but not interleukin-12, induce T-cell hyporesponsiveness in vitro and prevent acute allograft rejection

The induction of an immune response or tolerance is mediated by corresponding subsets of dendritic cells (DC). However, the property of tolerogenic DC is not clear. Recently, we have characterized a population of CD11c+ splenic DC derived from long-term mixed leucocyte culture (LT-MLC), which are able to proliferate upon stimulation and have a strong primary mixed leucocyte reaction (MLR)-stimulating activity in conventional MLR. In this study, we show that, in contrast to the irradiated ones, non-irradiated LT-MLC-derived DC induce polyclonal antigen-specific T-cell hyporesponsiveness when cocultured with allogeneic splenocytes for 3-11 days. The degree of the hyporesponsiveness increased with the length of coculture. Although these DC expressed major histocompatibility complex class II and B7 costimulatory molecules, which are down-regulated during coculture, they expressed very low or undetectable CD40 before and after coculture, respectively. The CD40-deficient DC spontaneously produce interleukin-10 (IL-10), but not IL-12. The skewed balance between IL-10 and IL-12 is associated with their capability to induce T-cell hyporesponsiveness, because a neutralizing antibody to IL-10, exogenous recombinant IL-12 or lipopolysaccharide (LPS) significantly blocked the hyporesponsiveness. Accordingly, infusion of a small number of non-irradiated LT-MLC-derived DC (5x105) significantly prolonged the survival of a vascularized heterotopic murine heart transplant, whereas irradiated DC accelerated graft rejection. These data suggest that CD40-deficient DC producing IL-10, but not IL-12 can induce T-cell hyporesponsiveness in vitro and in vivo.

CpG DNA and LPS induce distinct patterns of activation in human monocytes

A specific set of immune functions is switched on in response to DNA containing unmethylated CpG dinucleotides in particular base contexts ('CpG motifs'). Plasmids, viral vectors and antisense oligodeoxynucleotides used for DNA vaccination, gene replacement or gene blockade contain immunostimulatory CpG motifs which may have independent biological activity. Although the immune stimulatory effects of CpG motifs on murine cells are well established, the evaluation of their possible effects on human cells is complicated by the higher LPS sensitivity of human leukocytes compared with those in mice. To address this issue, we analyzed CpG- and LPS-mediated immune activation of human PBMC. The biologic activity of LPS could be detected within 4 h using intracellular TNF staining of monocytes with flow cytometry at concentrations just one-twentieth (0.0014 Eu/ml) of the lower detection limit for the routinely used LAL assay (0.03 EU/ml). In contrast to the rapid LPS response, CpG DNA-stimulated TNF and IL-6 synthesis in human monocytes was not detectable until 18 h. E. coli DNA induced IL-6 synthesis in a concentration-dependent manner (30 micrograms/ml E. coli DNA; 409 pg/ml +/- 75 pg/ml, n = 7, IL-6 ELISA), but calf thymus DNA did not (< 10 pg/ml). Likewise, the CpG oligodeoxynucleotides 1760 (phosphorothioate) and 2059 (unmodified) induced IL-6 synthesis, but the corresponding control oligonucleotides 1908 and 2077 did not CpG DNA and LPS enhanced IL-6 synthesis synergistically. ICAM-1-expression of monocytes was increased 4.6-fold by E. coli DNA, 3.5-fold by 1760 and three-fold by 2059, compared with 3.6-fold by a maximal LPS stimulus and no change with non-CpG DNA. In conclusion, CpG-motifs induce TNF, IL-6 and ICAM-1 expression in human monocytes, but the kinetics of this differ from that induced by LPS, which makes it possible to distinguish immune activation by these agents. These results have important implications for the clinical development of therapeutic DNA in humans.

Distinct roles for transforming growth factor-beta2 and tumour necrosis factor-alpha in immune deviation elicited by hapten-derivatized antigen-presenting cells

The role of antigen-presenting cells (APC) in the induction of antigen-specific unresponsiveness was examined, using two functionally distinct murine macrophage hybridomas, #59 and #63 cells. Derivatized with the hapten (dinitrofluorobenzene; DNFB), #59 cells induced contact hypersensitivity (CH) in mice. Hapten-derivatized #63 cells failed to induce CH. Instead, they prevented recipients from acquiring CH when exposed subsequently to a sensitizing dose of the hapten. Similarly, hapten-derivatized #59 cells, pretreated in vitro with transforming growth factor-beta2 (TGF-beta2) lost their capacity to evoke CH, and induced tolerance. Hapten-derivatized #63 cells and TGF-beta2-treated #59 cells eliminated CH in mice sensitized to hapten. Reverse transcription-polymerase chain reaction analysis of mRNAs for various accessory molecules important in T-cell activation revealed that #63 and TGF-beta2-treated #59 cells differed only in their expression of tumour necrosis factor-alpha (TNF-alpha) mRNA. The latter expressed higher levels of TNF-alpha mRNA than did untreated #59 cells. As a consequence, #63 and TGF-beta2-treated #59 cells, both of which induce tolerance, secrete TNF-alpha protein unlike untreated #59 cells, which do not induce tolerance to hapten. Since neutralizing anti-TNF-alpha antibodies abrogated the tolerogenic potential of #63 cells in vivo, we conclude that TGF-beta2 equips hapten-bearing APC with the capacity to evoke systemic immune deviation in which CH is selectively silenced. We speculate that one effect of TGF-beta2 is to cause APC to up-regulate TNF-alpha production. In turn, this cytokine biases the functional property of responding hapten-specific T cells in a direction that not only interferes with acquisition, but suppresses induction of CH.