The novel deubiquitinase inhibitor b-AP15 induces direct and NK cell-mediated antitumor effects in human mantle cell lymphoma.

The first therapeutic proteasome inhibitor bortezomib has clinical efficacy in mantle cell lymphoma (MCL) which resulted in its incorporation in treatment algorithms for this disease. Impairment of proteasomal function by bortezomib is mediated via inhibition of the 20S core particle. However, proteasome function can also be modified by targeting upstream components of the ubiquitin-proteasome system. Recently, b-AP15 has been identified as a small molecule achieving proteasome inhibition by targeting the deubiquitinase (DUB) activity of the 19S regulatory subunit and was found to inhibit cancer cell growth in preclinical analyses. In the present study, both direct antitumor effects and the possibility to induce natural killer group 2 member D ligands (NKG2DL) to reinforce NK cell immunity with b-AP15 were investigated to provide a rational basis for clinical evaluation of this novel DUB inhibitor in MCL. Treatment with b-AP15 resulted in reduced viability as well as induction of apoptosis in a time- and dose-dependent manner, which could be attributed to caspase activation in MCL cells. In addition, treatment with b-AP15 differentially induced NKG2DL expression and subsequent NK cell lysis of MCL cells. These results indicate that the DUB inhibitor b-AP15 displays substantial antitumor activity in human MCL and suggest that b-AP15 might be a novel therapeutic option in the treatment of MCL that warrants clinical investigation.

The BCR-ABL inhibitor nilotinib influences phenotype and function of monocyte-derived human dendritic cells.

In chronic myeloid leukemia (CML), the translocation t(9;22) results in the fusion protein BCR-ABL (breakpoint cluster region-abelson murine leukemia), a tyrosine kinase mediating oncogenic signaling which is successfully targeted by treatment with BCR-ABL inhibitors like imatinib. However, BCR-ABL inhibitors may also affect antitumor immunity. For instance, it was reported that imatinib impairs the function of dendritic cells (DCs) that play a central role in initiating and sustaining T cell responses. Meanwhile, second generation BCR-ABL inhibitors like nilotinib, which inhibits BCR-ABL with enhanced potency have become standard of treatment, at least in patients with BCR-ABL kinase domain mutations. In this study we analyzed the influence of therapeutic concentrations of nilotinib on human monocyte-derived DCs and compared its effects to imatinib. We found that both tyrosine kinase inhibitors (TKI) comparably and significantly impaired differentiation of monocytes to DCs as revealed by curtated downregulation of CD14 and reduced upregulation of CD1a and CD83. This was only partially restored after withdrawal of the TKI. Moreover, both TKI significantly reduced activation-induced IL-12p70 and C-C motif chemokine ligand (CCL) 3 secretion, while divergent TKI effects for CCL2 and CCL5 were observed. In contrast, only nilotinib significantly impaired the migratory capacity of DCs and their capacity to induce T-cell immune responses in MLRs. Our results indicate that imatinib and nilotinib may differ significantly with regard to their influence on antitumor immunity. Thus, for future combinatory approaches and particularly stop studies in CML treatment, choice of the most suitable BCR-ABL inhibitor requires careful consideration.

The transcription factor MITF is a critical regulator of GPNMB expression in dendritic cells.

BACKGROUND: Dendritic cells (DC) are the most potent antigen-presenting cells (APC) with the unique ability to activate naïve T cells and to initiate and maintain primary immune responses. Immunosuppressive and anti-inflammatory stimuli on DC such as the cytokine IL-10 suppress the activity of the transcription factor NF-κB what results in downregulation of costimulatory molecules, MHC and cytokine production. Glycoprotein NMB (GPNMB) is a transmembrane protein, which acts as a coinhibitory molecule strongly inhibiting T cell responses if present on APC. Interestingly, its expression on human monocyte-derived dendritic cells (moDC) is dramatically upregulated upon treatment with IL-10 but also by the BCR-ABL tyrosine kinase inhibitors (TKI) imatinib, nilotinib or dasatinib used for the treatment of chronic myeloid leukemia (CML). However, the molecular mechanisms responsible for GPNMB overexpression are yet unknown. RESULTS: The immunosuppressive cytokine IL-10 and the BCR-ABL TKI imatinib or nilotinib, that were examined here, concordantly inhibit the PI3K/Akt signaling pathway, thereby activating the downstream serine/threonine protein kinase GSK3ß, and subsequently the microphthalmia-associated transcription factor (MITF) that is phosphorylated and translocated into the nucleus. Treatment of moDC with a small molecule inhibitor of MITF activity reduced the expression of GPNMB at the level of mRNA and protein, indicating that GPNMB expression is in fact facilitated by MITF activation. In line with these findings, PI3K/Akt inhibition was found to result in GPNMB overexpression accompanied by reduced stimulatory capacity of moDC in mixed lymphocyte reactions (MLR) with allogeneic T cells that could be restored by addition of the GPNMB T cell ligand syndecan-4 (SD-4). CONCLUSIONS: In summary, imatinib, nilotinib or IL-10 congruently inhibit the PI3K/Akt signaling pathway thereby activating MITF in moDC, resulting in a tolerogenic phenotype. These findings extend current knowledge on the molecular mechanisms balancing activating and inhibitory signals in human DC and may facilitate the targeted manipulation of T cell responses in the context of DC-based immunotherapeutic interventions.

Comprehensive analysis of NKG2D ligand expression and release in leukemia: implications for NKG2D-mediated NK cell responses.

Ligands of the prototypical activating NK receptor NKG2D render cancer cells susceptible to NK cell-mediated cytolysis if expressed at sufficiently high levels. However, malignant cells employ mechanisms to evade NKG2D-mediated immunosurveillance, such as NKG2D ligand (NKG2DL) shedding resulting in reduced surface expression levels. In addition, systemic downregulation of NKG2D on NK cells of cancer patients has been observed in many studies and was attributed to soluble NKG2DL (sNKG2DL), although there also are conflicting data. Likewise, relevant expression of NKG2DL in leukemia has been reported by some, but not all studies. Hence, we comprehensively studied expression, release, and function of the NKG2D ligands MHC class I chain-related molecules A and B and UL16-binding proteins 1-3 in 205 leukemia patients. Leukemia cells of most patients (75%) expressed at least one NKG2DL at the surface, and all investigated patient sera contained elevated sNKG2DL levels. Besides correlating NKG2DL levels with clinical data and outcome, we demonstrate that sNKG2DL in patient sera reduce NKG2D expression on NK cells, resulting in impaired antileukemia reactivity, which also critically depends on number and levels of surface-expressed NKG2DL. Together, we provide comprehensive data on the relevance of NKG2D/NKG2DL expression, release, and function for NK reactivity in leukemia, which exemplifies the mechanisms underlying NKG2D-mediated tumor immunosurveillance and escape.

The immune inhibitory receptor osteoactivin is upregulated in monocyte-derived dendritic cells by BCR-ABL tyrosine kinase inhibitors.

Multiple approaches presently aim to combine targeted therapies using tyrosine kinase inhibitors with immunotherapy. Ex vivo-generated dendritic cells are frequently used in such strategies due to their unique ability to initiate primary T-cell immune responses. Besides governing tumor cell growth, many kinases targeted by tyrosine kinase inhibitors are involved in the development and function of dendritic cells and thus tyrosine kinase inhibitor therapy may cause immunoinhibitory side effects. We here report that exposure of developing human monocyte-derived dendritic cells to the BCR-ABL inhibitors imatinib, dasatinib, and nilotinib results in profound upregulation of the transmembrane glycoprotein osteoactivin that has recently been characterized as a negative regulator of T-cell activation. Thus, in line with osteoactivin upregulation, exposure to tyrosine kinase inhibitors resulted in significantly reduced stimulatory capacity of dendritic cells in mixed lymphocyte reactions that could be restored by the addition of blocking anti-osteoactivin antibody. Our data demonstrate that tyrosine kinase inhibitor-mediated inhibition of dendritic cell function is, at least in great part, mediated by upregulation of the immune inhibitory molecule osteoactivin.

CD137 ligand mediates opposite effects in human and mouse NK cells and impairs NK-cell reactivity against human acute myeloid leukemia cells.

Natural killer (NK) cells play an important role in the immunosurveillance of leukemia. Their reactivity is governed by a balance of activating and inhibitory receptors including various members of the tumor necrosis factor receptor (TNFR) family. Here we report that human NK cells acquire expression of the TNFR family member CD137 upon activation, and NK cells of acute myeloid leukemia (AML) patients display an activated phenotype with substantial CD137 expression. CD137 ligand (CD137L) was detectable on leukemic cells in 35% of 65 investigated AML patients, but not on healthy CD34(+) cells, and expression was associated with monocytic differentiation. Bidirectional signaling following CD137-CD137L interaction induced the release of the immunomodulatory cytokines interleukin-10 and TNF by AML cells and directly diminished granule mobilization, cytotoxicity, and interferon-gamma production of human NK cells, which was restored by blocking CD137. Cocultures of NK cells with CD137L transfectants confirmed that human CD137 inhibits NK-cell reactivity, while activating signals were transduced by its counterpart on NK cells in mice. Our data underline the necessity to study the function of seemingly analog immunoregulatory molecules in mice compared with men and demonstrate that CD137-CD137L interaction enables immune evasion of AML cells by impairing NK-cell tumor surveillance in humans.

Aberrant expression of the homeobox gene CDX2 in pediatric acute lymphoblastic leukemia.

Members of the caudal (cdx) family of homeobox proteins are essential regulators of embryonic blood development in zebrafish. Previously, we reported that the murine homologues (Cdx1, Cdx2, and Cdx4) affect formation and differentiation of embryonic stem cell (ESC)-derived hematopoietic progenitor cells. Consistent with the notion that embryonic pathways can reactivate during adult oncogenesis, recent studies suggest involvement of CDX2 in human acute myeloid leukemia (AML). Here we study CDX2 in healthy and leukemic human lymphoid cells, and show that a majority of leukemic samples display various degrees of aberrant CDX2 expression. Analysis of a cohort of 37 childhood acute lymphoblastic leukemia (ALL) patients treated in our hospital reveals that high CDX2 expression levels at diagnosis correlate with persistence of minimal residual disease (MRD) during the course of treatment. Thus, CDX2 expression levels may serve as a marker for adverse prognosis in pediatric ALL.

The BCR/ABL-inhibitors imatinib, nilotinib and dasatinib differentially affect NK cell reactivity.

In chronic myeloid leukemia (CML), BCR/ABL-mediated oncogenic signaling can be targeted with the BCR/ABL-inhibitors Imatinib, Nilotinib and Dasatinib. However, these agents may also affect anti-tumor immunity. Here, we analyzed the effects of the 3 BCR/ABL-inhibitors on natural killer (NK) cell reactivity. Exposure of CML cells (K562, Meg-01) to pharmacological concentrations of Imatinib, Nilotinib and Dasatinib diminished expression of ligands for the activating immunoreceptor NKG2D to a similar extent. This resulted in comparably reduced NK cell cytotoxicity and IFN-gamma production. When direct effects on NK cell responses to K562 and primary CML cells as well as activating cytokines were studied, Dasatinib was found to abrogate NK cytotoxicity and cytokine production. Nilotinib did not alter cytotoxicity but, at high levels, impaired NK cytokine production, while Imatinib had no direct influence on NK cell reactivity. Of note, Nilotinib, but not the other BCR/ABL-inhibitors increased cell death within the preferentially cytokine-secreting CD56(bright)CD16(-) NK cell subset, which may, at least in part, serve to explain the effect of Nilotinib on NK cytokine production. Analysis of NK cell signaling revealed that Dasatinib inhibited proximal signaling events leading to decreased phosphorylation of PI3K and ERK that are crucial for NK cell reactivity. Imatinib and Nilotinib, in contrast, showed no relevant effect on NK cell PI3K or ERK activity. In light of the potential role of NK cells in the immunesurveillance of residual leukemia and for future combinatory immunotherapeutic approaches, our data indicate that choice and dosing of the most suitable BCR/ABL-inhibitor for a given patient require careful consideration.

Immunomodulation by semi-mature dendritic cells: a novel role of Toll-like receptors and interleukin-6.

Dendritic cells (DCs) are key players in activation of the adaptive immune system by their ability of antigen presentation to and priming of T cells. An increasing body of evidence suggests that DCs may also play an important role in induction of tolerance, predominantly by induction of regulatory T cells (T(reg)). More recently, data have been published on how Toll-like receptor (TLR) ligands and cytokines affect DC differentiation, and how DC subsets might be involved in immunoregulation and tolerance rather than in T cell activation. The most important features of tolerance-inducing DCs appear to be their maturation state and their cytokine secretion pattern. The following types of tolerance-inducing DCs have been reported: immature DCs (DCs(im)) or DCs in the steady state (DCs(st)), DCs(IL-10), semi-mature DCs(TNF-alpha), semi-mature DCs(IL-6). With this review article we would like to discuss the aforementioned types of tolerogenic DCs with a focus on semi-mature DCs(IL-6) and discuss their potential role in maintenance of (hepatic or intestinal) immune homeostasis and inflammatory diseases such as inflammatory bowel disease.

Matrilysin (MMP-7) is a novel broadly expressed tumor antigen recognized by antigen-specific T cells.

PURPOSE: A prerequisite for the development of vaccination strategies is the identification and characterization of relevant tumor-associated antigen. Using microarray and reverse transcription-PCR analysis, we found matrix metalloproteinase (MMP)-7 to be extensively up-regulated in renal cell carcinomas and expressed in a broad variety of malignant cells. MMP-7 can promote cancer invasion and angiogenesis by proteolytic cleavage of extracellular matrix and basement membrane proteins, thus making it a promising target in the context of immunotherapies. EXPERIMENTAL DESIGN: To analyze the possible use of MMP-7 as a tumor-associated antigen, specific CTLs were induced using monocyte-derived dendritic cells electroporated with MMP-7-mRNA. In addition, to better characterize the fine specificity of these CTLs, MMP-7 MHC class I ligands were isolated and characterized in renal cell carcinoma tissue, which overexpressed MMP-7, by mass spectrometry-based peptide sequencing. Using this approach, we identified a novel HLA-A3-binding antigenic MMP-7 peptide. CTLs generated from healthy donors by in vitro priming with dendritic cells, pulsed with the novel peptide, were used as effectors in (51)Cr-release assays. RESULTS: The induced CTLs elicited an antigen-specific and HLA-restricted cytolytic activity against tumor cells endogenously expressing the MMP-7 protein. Furthermore, we were able to induce MMP-7-specific CTLs using peripheral blood mononuclear cells from a patient with acute lymphoblastic leukemia capable of recognizing the autologous leukemic blasts while sparing nonmalignant cells. CONCLUSIONS: Our study describes the identification of a novel broadly expressed T-cell epitope derived from the MMP-7 protein that represents an interesting candidate to be applied in immunotherapies of human malignancies targeting both tumor cells and neovascularization.

BCR-ABL activity is critical for the immunogenicity of chronic myelogenous leukemia cells.

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder caused by excessive granulopoiesis due to the formation of the constitutively active tyrosine kinase BCR-ABL. An effective drug against CML is imatinib mesylate, a tyrosine kinase inhibitor acting on Abl kinases, c-KIT, and platelet-derived growth factor receptor. Recently, a study revealed that patients treated with imatinib showed impaired CTL responses compared with patients treated with IFN-alpha, which might be due to a treatment-induced reduction in immunogenicity of CML cells or immunosuppressive effects. In our study, we found that inhibition of BCR-ABL leads to a down-regulation of immunogenic antigens on the CML cells in response to imatinib treatment, which results in the inhibition of CML-directed immune responses. By treating CML cells with imatinib, we could show that the resulting inhibition of BCR-ABL leads to a decreased expression of tumor antigens, including survivin, adipophilin, hTERT, WT-1, Bcl-x(L), and Bcl-2 in correlation to a decreased development of CML-specific CTLs. In contrast, this reduction in immunogenicity was not observed when a CML cell line resistant to the inhibitory effects of imatinib was used, but could be confirmed by transfection with specific small interfering RNA against BCR-ABL or imatinib treatment of primary CML cells.

The Deubiquitinase Inhibitor b-AP15 and Its Effect on Phenotype and Function of Monocyte-Derived Dendritic Cells.

The ubiquitin-proteasome system is elementary for cellular protein degradation and gained rising attention as a new target for cancer therapy due to promising clinical trials with bortezomib, the first-in class proteasome inhibitor meanwhile approved for multiple myeloma and mantle cell lymphoma. Both bortezomib and next-generation proteasome inhibitors mediate their effects by targeting the 20S core particle of the 26S proteasome. The novel small molecule inhibitor b-AP15 affects upstream elements of the ubiquitin-proteasome cascade by suppressing the deubiquitinase activity of both proteasomal regulatory 19S subunits and showed promising anticancer activity in preclinical models. Nonetheless, effects of inhibitors on the ubiquitin-proteasome system are not exclusively restricted to malignant cells: alteration of natural killer cell-mediated immune responses had already been described for drugs targeting either 19S or 20S proteasomal subunits. Moreover, it has been shown that bortezomib impairs dendritic cell (DC) phenotype and function at different levels. In the present study, we comparatively analyzed effects of bortezomib and b-AP15 on monocyte-derived DCs. In line with previous results, bortezomib exposure impaired maturation, antigen uptake, migration, cytokine secretion and immunostimulation, whereas treatment with b-AP15 had no compromising effects on these DC features. Our findings warrant the further investigation of b-AP15 as an alternative to clinically approved proteasome inhibitors in the therapy of malignancies, especially in the context of combinatorial treatment with DC-based immunotherapies.

The use of dendritic cells in cancer immunotherapy.

Cancer immunotherapy aims at eliciting an immune response directed against tumor antigens to help fight off residual tumor cells and thereby improve survival and quality of life of cancer patients. Different immunotherapeutic approaches share the use of dendritic cells (DCs) to present tumor-associated antigens to T-lymphocytes. Ex vivo generated DCs can be loaded with antigens and re-infused to the patients, or they can be used for ex vivo expansion of antitumor lymphocytes. Alternatively, methods exist to target antigens to DCs in vivo without need for ex vivo cell manipulations. The clinical studies have shown that DC administration to patients is safe and induces antigen-specific immunity. However, it seldom elicits objective clinical responses in patients with advanced-stage malignancies. Novel insights into DC and lymphocyte regulation are expected to lead to more effective vaccines in the near future. Meanwhile, efforts are directed at identifying the most appropriate clinical targets for active specific immunotherapies. Data suggests that vaccinations may indeed be beneficial when given in the adjuvant setting rather than to treat metastatic cancers. These issues are discussed here together with an overview of the DC-based antitumor immunotherapy studies.

Histone deacetylase inhibitors affect dendritic cell differentiation and immunogenicity.

PURPOSE: Histone deacetylases (HDAC) modulate gene transcription and chromatin assembly by modifying histones at the posttranscriptional level. HDAC inhibitors have promising antitumor activity and are presently explored in clinical studies. Cumulating evidence in animal models of immune disorders also suggests immunosuppressive properties for these small molecules, although the underlying mechanisms remain at present poorly understood. Here, we have evaluated the effects of two HDAC inhibitors currently in clinical use, sodium valproate and MS-275, on human monocyte-derived DCs. EXPERIMENTAL DESIGN: DCs were generated from monocytes through incubation with granulocyte macrophage colony-stimulating factor and interleukin-4. DC maturation was induced by addition of polyinosinic-polycytidylic acid. DC phenotype, immunostimulatory capacity, cytokine secretion, and migratory capacity were determined by flow cytometry, mixed leukocyte reaction, ELISA, and Transwell migration assay, respectively. Nuclear translocation of RelB, IFN regulatory factor (IRF)-3, and IRF-8 were determined by immunoblotting. RESULTS: HDAC inhibition skews DC differentiation by preventing the acquisition of the DC hallmark CD1a and by affecting the expression of costimulation and adhesion molecules. In addition, macrophage inflammatory protein-3beta/chemokine, motif CC, ligand 19-induced migration, immunostimulatory capacity, and cytokine secretion by DCs are also profoundly impaired. The observed defects in DC function on exposure to HDAC inhibitors seem to reflect the obstruction of signaling through nuclear factor-kappaB, IRF-3, and IRF-8. CONCLUSIONS: HDAC inhibitors exhibit strong immunomodulatory properties in human DCs. Our results support the evaluation of HDAC inhibitors in inflammatory and autoimmune disorders.

Identification and characterization of T-cell epitopes deduced from RGS5, a novel broadly expressed tumor antigen.

PURPOSE: Identification of tumor-associated antigens and advances in tumor immunology resulted in the development of vaccination strategies to treat patients with malignant diseases. In a novel experimental approach that combined comparative mRNA expression analysis of defined cell types with the characterization of MHC ligands by mass spectrometry, we found that regulator of G protein signaling 5 (RGS5) is extensively up-regulated in a broad variety of malignant cells, and we identified two HLA-A2- and HLA-A3-binding peptides derived from the RGS5 protein. Interestingly, RGS5 was recently shown to be involved in tumor angiogenesis. EXPERIMENTAL DESIGN: We used monocyte-derived dendritic cells pulsed with these novel antigenic peptides or transfected with RGS5-mRNA for the in vitro induction of CTLs, generated from healthy donors, to analyze the presentation of RGS5-deduced epitopes by malignant cells. RESULTS: The generated CTL lines elicited an antigen-specific and HLA-restricted cytolytic activity against tumor cells endogenously expressing the RGS5 protein. Furthermore, we were able to induce RGS5-specific CTLs using peripheral blood mononuclear cells from a patient with acute myeloid leukemia capable of recognizing the autologous leukemic blasts while sparing nonmalignant cells. CONCLUSIONS: These results indicate that the RGS5 peptides represent interesting candidates for the development of cancer vaccines designed to target malignant cells and tumor vessels.

BCR-ABL is not an immunodominant antigen in chronic myelogenous leukemia.

In the present study, we analyzed the involvement of the BCR-ABL protein in the induction of antigen-specific CTL in order to develop an immunotherapeutic approach in patients with chronic myelogenous leukemia (CML). To accomplish this, we generated dendritic cells (DC) in vitro and electroporated them with various sources of RNA harboring the chimeric bcr-abl transcript. These genetically engineered DCs were used as antigen-presenting cells for the induction of CTLs. By applying this approach, we found that the CTLs induced by DCs transfected with RNA extracted from bcr-abl-positive K-562 cells or CML blasts lysed DCs transfected with the corresponding RNA, but failed to recognize epitopes derived from the chimeric BCR-ABL fusion protein in (51)Cr-release assays. In contrast, they were able to lyse autologous DCs electroporated with RNA isolated from patients with acute myeloid leukemia, indicating that antigens shared among these malignant cells are involved and recognized by these CTLs. In patients with CML in complete cytogenetic remission during IFN-alpha treatment, we detected some reactivity of CD8(+) T cells against BCR-ABL in IFN-gamma ELISPOT assays, which was weaker as compared with proteinase 3 (PR3)- or prame-directed responses, suggesting that the BCR-ABL protein is less immunogenic as compared with other CML-derived antigens.

The proteasome and its inhibitors in immune regulation and immune disorders.

The ubiquitin-proteasome pathway is a well-characterized mechanism deputed to the degradation of intracellular proteins. Proteasomal degradation intervenes in the regulation of numerous cellular functions including signal transduction, apoptosis, cell cycle, and antigen presentation. In vitro and in vivo studies have shown that both normal and malignant cells of the immune system are exquisitely affected by inhibition of proteasome activity. This property is currently exploited in the treatment of multiple myeloma and mantle cell lymphoma, two B-cell malignancies that respond to treatment with the proteasome inhibitor bortezomib. Pharmacological inhibitors of the proteasome also affect function and survival of B and T lymphocytes and of dendritic cells and were shown to reduce autoimmune and inflammatory manifestations in several models of immune-mediated disorders. The present review offers an overview of the mechanisms implicated in the immunomodulatory effects of proteasome inhibitors and discusses prospective future applications for these small molecules in immune and inflammatory diseases.

Effects of imatinib on monocyte-derived dendritic cells are mediated by inhibition of nuclear factor-kappaB and Akt signaling pathways.

Dendritic cells are the most powerful antigen-presenting cells playing a decisive role for the initiation and maintenance of primary immune responses. However, signaling pathways involved in the differentiation of these cells have not been fully determined. Imatinib is a novel tyrosine kinase inhibitor effective against Abl kinases, c-Kit, and platelet-derived growth factor receptor. Using this compound, we show that human monocyte-derived dendritic cells generated in the presence of therapeutic concentrations of imatinib show a reduced expression of CD1a, MHC class I and II, and costimulatory molecules as well as decreased secretion of chemokines and cytokines resulting in an impaired capacity of dendritic cells to elicit primary T-cell responses. Using Western blot analyses, we found that these effects are mediated by inhibition of phosphatidylinositol 3-kinase/Akt pathways and a pronounced down-regulation of nuclear localized protein levels of nuclear factor-kappaB family members. Importantly, using blocking antibodies and tyrosine kinase inhibitors, we show that the inhibitory effects of imatinib on dendritic cell differentiation are not mediated via platelet-derived growth factor receptor and c-Kit. Taken together, our study reveals that imatinib inhibits dendritic cell differentiation and function via Akt and nuclear factor-kappaB signal transduction. Importantly, we show that imatinib can inhibit the function of normal, nonmalignant cells that may result in immunosuppression of these patients.

Activated CD8+ T lymphocytes induce differentiation of monocytes to dendritic cells and restore the stimulatory capacity of interleukin 10-treated antigen-presenting cells.

We analyzed the role of CD8+ T cells on dendritic cells (DCs). Incubation of monocytes with activated but not resting CD8+ T cells induced their differentiation into DCs with a CD1a+ and CD14- phenotype. Activated CD8+ lymphocytes induced down-regulation of CD14 expression in interleukin (IL)-10-treated macrophages and resulted in the expression of DC-related CD1a and up-regulation of mRNA transcripts for RelB, IL-12p40, MCP-2, MIP-1alpha, and ELC. These DCs were potent stimulatory cells in mixed lymphocyte reaction and induced primary CTL responses. This interaction was independent of a direct cell-to-cell contact. Monoclonal antibodies against IFN-gamma completely abolished the CD1a induction on IL-10-treated antigen-presenting cells. These results demonstrate that CD8+ cells can provide stimulatory signals to antigen-presenting cells to induce their differentiation in DC and to increase their immunostimulatory capacity.