Flt3 ligand antitumor activity in a murine breast cancer model: a comparison with granulocyte-macrophage colony-stimulating factor and a potential mechanism of action

High FLT3 expression indicates favorable prognosis and correlates with clinicopathological parameters and immune infiltration in breast cancer

Purpose: Breast cancer is a highly heterogeneous malignancy, seriously threatening female health worldwide and inducing higher mortalities. Few have the studies evaluated Fms-like TyrosineKinase-3 (FLT3) in prognostic risk, immunotherapy or any other treatment of breast cancer. Our study focused on investigating the function of FLT3 in breast cancer.

Patients and methods: Based on transcriptome and methylation data mined from The Cancer Gene Atlas (TCGA), we explored the clinical features of FLT3 expression in 1079 breast cancer samples. RT-qPCR in cell lines and tissue samples was used to verify the expression difference of FLT3. Kaplan–Meier survival analysis and cox regression models were employed for screening of FLT3 with potential prognostic capacity. Subsequently, functional analysis of the co-expressed genes was conducted using Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and gene-set enrichment analysis (GSEA). The correlation between FLT3 expression and tumor immune infiltration was jointly analyzed with estimate, ssGSEA, TIMER, and TISIDB. Then we employed checkpoint-related molecules, immunophenoscore (IPS), and tumor mutation burden (TMB) to assess the efficacy of immuno-checkpoint inhibitors (ICIs). Pearson correlation coefficient was employed to exam the association between DNA methylation and FLT3 expression.

Results: FLT3 displays an elevated expression in breast cancer than normal pairs and is significantly associated with multiple clinical characteristics like age, menopause status, histological type, pathological stage, and molecular subtype as well as increased overall survival (OS). Additionally, FLT3 is a favorable independent prognostic factor. GO, KEGG, and GSEA suggested that FLT3 was associated with diversified immune-related features. FLT3 expression is correlated with the abundance of various immune cells namely CD4+T cell, CD8+ T cell, myeloid dendritic cell, and neutrophil as well as immune inhibitors especially CTLA4, which is positively correlated with FLT3 expression. Moreover, TMB displayed a negative correlation with FLT3 expression while IPS showed adverse tendency. Ultimately, the methylation of FLT3 downregulates the gene expression and closely binds to a few clinical parameters.

Conclusion: FLT3 can be used for prognostic prediction and is relevant to immune infiltration in breast cancer. FLT3 may pave the way for future novel immunotherapies.

Human Placental-Derived Adherent Stromal Cells Co-Induced with TNF-α and IFN-γ Inhibit Triple-Negative Breast Cancer in Nude Mouse Xenograft Models

Culturing 3D-expanded human placental-derived adherent stromal cells (ASCs) in the presence of tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ) transiently upregulated the secretion of numerous anti-proliferative, anti-angiogenic and pro-inflammatory cytokines. In a 3D-spheroid screening assay, conditioned medium from these induced-ASCs inhibited proliferation of cancer cell lines, including triple-negative breast cancer (TNBC) lines. In vitro co-culture studies of induced-ASCs with MDA-MB-231 human breast carcinoma cells, a model representing TNBC, supports a mechanism involving immunomodulation and angiogenesis inhibition. In vivo studies in nude mice showed that intramuscular administration of induced-ASCs halted MDA-MB-231 cell proliferation, and inhibited tumor progression and vascularization. Thirty percent of treated mice experienced complete tumor remission. Murine serum concentrations of the tumor-supporting cytokines Interleukin-6 (IL-6), Vascular endothelial growth factor (VEGF) and Granulocyte-colony stimulating factor (G-CSF) were lowered to naïve levels. A somatic mutation analysis identified numerous genes which could be screened in patients to increase a positive therapeutic outcome. Taken together, these results show that targeted changes in the secretion profile of ASCs may improve their therapeutic potential.

Peripheral FLT-3 ligand levels as a pathobiological parameter duringthe clinical course of acute myeloid leukemia

BACKGROUND/AIM

FLT-3 ligand is a growth factor affecting the hematopoietic lineage. The aim of this study was to evaluate the variability of peripheral FLT-3 ligand during the clinical course of acute myeloid leukemia (AML) patients.

MATERIALS AND METHODS

Twenty-four patients were enrolled in this study in order to assess alterations in the circulating levels of FLT-3 ligand during the clinical course of AML.

RESULTS

We studied the association in the diagnostic period between the FLT-3 ligand and peripheral blood cells together with serum electrolytes. FLT-3 ligand levels (pg/mL) during the aplastic period due to remission induction and consolidation were higher than the levels at initial diagnosis. On the other hand, the diagnostic and remission induction values of leukocytes and FLT-3 ligand showed an inverse association. These results indicate to us that higher white cell counts are associated with lower FLT-3 ligand levels. We also found a reversed association between FLT-3 ligand and serum lactate dehydrogenase level. However, there was no association between FLT-3 ligand and other serum electrolyte levels. We also found higher FLT-3 ligand levels in male patients.

CONCLUSION

Our study demonstrates the inverse proliferative action of FLT-3 ligand on the early myeloid lineage. In addition, this study showed us that FLT-3 receptor inhibition during chemotherapy-induced aplasia causes a compensative ligand overexpression.

Evolution of oncolytic viruses: novel strategies for cancer treatment

Many viruses have documented oncolytic activity, with the first evidence observed clinically over a decade ago. In recent years, there has been a resurgence of interest in the field of oncolytic viruses. Viruses may be innately oncotropic, lacking the ability to cause disease in people or they may require engineering to allow selective tumor targeting and attenuation of pathogenicity. Following infection of a neoplastic cell, several events may occur, including direct viral oncolysis, apoptosis, necrotic cell death and autophagic cellular demise. Of late, a large body of work has recognized the ability of oncolytic viruses (OVs) to activate the innate and adaptive immune system, as well as directly killing tumors. The production of viruses expressing transgenes encoding for cytokines, colony-stimulating factors, costimulatory molecules and tumor-associated antigens has been able to further incite immune responses against target tumors. Multiple OVs are now in the advanced stages of clinical trials, with several individual viruses having completed their respective trials with positive results. This review introduces the multiple mechanisms by which OVs are able to act as an antineoplastic therapy, either on their own or in combination with other more traditional treatment modalities. The full benefit and the place where OVs will be integrated into standard-of-care therapies will be determined with ongoing studies ranging from the laboratory to the patient. With various different viruses now in the clinic this therapeutic option is beginning to prove its worth, and the versatility of these agents means further innovative and novel applications will continue to be developed.

Selective expansion of merocytic dendritic cells and CD8DCs confers anti-tumour effect of Fms-like tyrosine kinase 3-ligand treatment in vivo

Vaccination with autologous cancer cells aims to enhance adaptive immune responses to tumour-associated antigens. The incorporation of Fms-like tyrosine kinase 3-ligand (FLT3L) treatment to the vaccination scheme has been shown previously to increase the immunogenicity of cancer vaccines, thereby enhancing their therapeutic potential. While evidence has been provided that FLT3L confers its effect through the increase of absolute dendritic cell (DC) numbers, it is currently unknown which DC populations are responsive to FLT3L and which effect FLT3L treatment has on DC functions. Here we show that the beneficial effects of FLT3L treatment resulted predominantly from a marked increase of two specific DC populations, the CD8 DCs and the recently identified merocytic DC (mcDC). These two DC populations (cross)-present cell-associated antigens to T cells in a natural killer (NK)-independent fashion. FLT3L treatment augmented the absolute numbers of these DCs, but did not change their activation status nor their capacity to prime antigen-specific T cells. While both DC populations effectively primed CD8(+) T cell responses to cell-associated antigens, only mcDC were capable to prime CD4(+) T cells to cell-associated antigens. Consequentially, the transfer of tumour vaccine-pulsed mcDC, but not of CD8 DCs, protected mice from subsequent tumour challenge in a vaccination model and resulted in eradication of established tumours in a therapeutic approach. These results show that the beneficial effect of FLT3L is associated with the induction of mcDC and suggests that selective targeting to mcDC or instilling mcDC 'characteristics' into conventional DC populations could significantly enhance the efficacy of tumour vaccines.

RNA interference screen in primary human T cells reveals FLT3 as a modulator of IL-10 levels

Functional studies of human primary immune cells have been hampered by the lack of tools to silence gene functions. In this study, we report the application of a lentiviral RNA interference library in primary human T cells. Using a subgenomic short hair RNA library targeting approximately 1000 signaling genes, we identified novel genes that control the levels of IL-10 produced. IL-10 is a potent anti-inflammatory cytokine secreted by several cell types, including T regulatory type 1 cells, a subset of T regulatory cells that exert their suppressive activity through IL-10 secretion. FLT3, a known hematopoeitic growth factor, was found to be a negative regulator of IL-10 levels in activated T cells. This was based on several observations. First, FLT3 and its ligand (FL) were both induced by T cell activation. Second, silencing of FLT3 led to increased IL-10 levels, whereas addition of FL suppressed IL-10 secretion and increased FLT3 surface levels. Third, engagement of CD46, a known inducer of T regulatory type 1 cells, upregulated surface FLT3, and secreted FL, which then inhibited IL-10 production by T cells. Hence, FL and FLT3 form a novel regulatory feedback loop that limits IL-10 production in T cells. Our results identified FLT3 as a new regulator of T cell function and offer a strategy to genetically dissect specific pathways in T cells.

Human Flt-3-ligand-mobilized dendritic cells require additional activation to drive effective immune responses

OBJECTIVE

Dendritic cells (DCs) play a pivotal role in the induction of immunity in response to pathogenic challenge or vaccination. As such, the fms-like tyrosine kinase 3-ligand (Flt-3L) has been used to increase DC populations in vivo, with contrasting outcomes, which include an increase in immunity, tolerance induction, or expansion of regulatory cells. This study examines the adjuvant role that human Flt-3L (hFL) administration has in generating immune responses upon immunization with a poorly immunogenic and soluble protein antigen.

MATERIALS AND METHODS

Mice were immunized with the nominal antigen, ovalbumin, alone or with antigen emulsified in complete Freund's adjuvant (CFA), with or without prior hFL-mediated expansion of DC subsets. The maturation of DC subsets and activation status of antigen-specific T cells were analyzed by flow cytometry, with effector function assessed in cytolytic T-lymphocyte assays.

RESULTS

hFL treatment expanded both conventional DC and plasmacytoid DC in vivo, resulting in increased antigen presentation by both direct and cross-presentation pathways. However, it was only in the context of CFA that antigen immunization could mature DCs and subsequently fully activate antigen-specific T cells with enhanced cytolytic activity.

CONCLUSIONS

Our studies reveal that hFL essentially acts as a coadjuvant, as hFL augments the size of an immune response but requires further adjuvant activation to alter the quality of the response.

Effect of Flt3 ligand gene transfer in experimental pancreatic cancer

BACKGROUND

Fms-like tyrosine kinase 3 receptor (Flt3) is an important receptor expressed on the cell membrane of immature antigen-presenting cells. The binding of Flt3 to its ligand (FL) activates the proliferation of dendritic cells (DCs). This mechanism is currently being evaluated in the therapy of malignant tumors. The aim of the present study was to study the effect of FL gene transfer on the immune response and tumor growth in experimental pancreatic cancer.

MATERIALS AND METHODS

The rat FL was sequenced and cloned from total mRNA extract of the spleen. Transfection efficiency of subcutaneously growing rat duct-like pancreatic cancer (DSL6A) with DOTAP-/cholesterol-based liposomes was tested using a pcDNA3.1-lacZ construct. Flt3 ligand production of in vitro transfected tumor cells and in vivo transfected tumors was measured by enzyme-linked immunosorbent assay. Tumor induction was achieved in Lewis rats by a subcutaneous inoculation of syngeneic pancreatic tumor cells (DSL6A). The animals were allocated into three groups: control, mock treatment, and treatment with FL plasmid. The plasmid was injected intratumorally three times per week for 2 weeks. The total observation time was 6 weeks.

RESULTS

The tumor volume was significantly lower in the FL-transfected group during the first 3 weeks. The number of responders was significantly higher in the FL group compared with control and mock treatment. The number of CD80+ DCs in the spleen was significantly higher after FL gene transfer. The responders showed a significantly higher number of splenic natural killer (NK) cells. There were no differences of infiltrating lymphocytes, proliferation, and tumor blood vessels between the groups.

CONCLUSION

Intratumoral gene transfer of FL in rats activated proliferation of DCs and NK cells, which causes a moderate reduction of tumor growth. This improvement of local tumor control during the first weeks could be explained by an improved antigen presentation.

Genetic vaccination with Flt3-L and GM-CSF as adjuvants: Enhancement of cellular and humoral immune responses that results in protective immunity in a murine model of hepatitis C virus infection

AIM: To investigate whether transfection of plasmid DNA encoding these cytokines enhances both humoral and cellular immune responses to hepatitis C virus (HCV) in a murine model.

METHODS: We established a tumor model of HCV infection using syngenic mouse myeloma cells stably transfected with NS5. Co-vaccination of DNA encoding granulocyte macrophage colony-stimulating factor (GM-CSF) and Flt-3 ligand together with a plasmid encoding for the HCV NS5 protein was carried out. Mice were sacrificed 14 d after the last immunization event with collection of spleen cells and serum to determine humoral and cellular immune responses.

RESULTS: Co-vaccination of DNA encoding GM-CSF and Flt-3 ligand together with a plasmid encoding for the HCV NS5 protein induced increased antibody responses and CD4+ T cell proliferation to this protein. Vaccination with DNA encoding GM-CSF and Flt-3L promoted protection against tumor formation and/or reduction in mice co-immunized with cytokine-encoding DNA constructs. This suggests this strategy is capable of generating cytotoxic T lymphocyte activity in vivo. Following inoculation with plasmid DNA encoding Flt-3L, no increase in spleen size or in dendritic cell (DC) and natural killer cell numbers was observed. This was in contrast to a dramatic increase of both cell types after administration of recombinant Flt3-L in vivo. This suggests that vaccination with plasmid DNA encoding cytokines that regulate DC generation and mobilization may not promote unwanted side effects, such as autoimmunity, splenic fibrosis or hematopoietic malignancies that may occur with administration of recombinant forms of these proteins.

CONCLUSION: Our data support the view that plasmid DNA vaccination is a promising approach for HCV immunization, and may provide a general adjuvant vaccination strategy against malignancies and other pathogens.

Perioperative immunomodulation with Flt3 kinase ligand or a whole tumor cell vaccine is associated with a reduction in lung metastasis formation after laparotomy in mice

INTRODUCTION

Laparotomy has been associated with temporary postoperative immunosuppression and accelerated tumor growth in experimental models. In a previous murine study, a whole cell vaccine plus the adjuvant monophosphoryl-lipid A was shown to be effective in decreasing the number of lung metastases that develop after laparotomy. This study was conducted to assess the impact of the adjuvant fetal liver tyrosine kinase 3 (Flt3) ligand on perioperative tumor growth when used alone or with a tumor cell vaccine.

METHODS

An intravenous tumor cell injection lung metastases model was used. Sixty female A/J mice were divided into six equal groups designated (1) anesthesia control (AC), (2) AC with Flt3 ligand (ACFlt3), (3) sham laparotomy (OP), (4) OP with Flt3 ligand (OPFlt3), (5) OP with vaccine (OPVac), and (6) OP with Flt3 ligand and vaccine (OPFlt3Vac). Groups 2, 4, and 6 received daily intraperitoneal injections of Flt3 ligand (10 microg/dose with carrier) for 5 days before and 5 days after surgery. Groups 1 and 3 received similar injections of saline on the same schedule. Groups 5 and 6 were vaccinated with irradiated whole Ta3Ha tumor cells intraperitoneally three times before and twice after surgery. Immediately after surgery, all mice were injected with 10(5) Ta3Ha tumor cells via a tail vein. After 14 days, the mice were sacrificed and their lungs and tracheas were excised en bloc. Specimens were stained and counterstained with India ink and Fekete solution, and surface metastases were counted by a blinded observer. Differences between study groups were determined by analysis of variance. The peritumoral inflammatory cell infiltrate of some Flt3 and control specimens was also assessed.

RESULTS

Regarding laparotomy, Flt3 ligand (mean, 1.22 metastases), whole cell vaccine (1.12 metastases), and the combination of these two agents (0.1 metastases) were each effective in significantly decreasing the number of surface lung metastases compared with surgery alone (9.88 metastases, P < .05 for all comparisons). There were no differences between the various treatment groups in regards to number of metastases. Only the combination of Flt3 and the vaccine significantly lowered the incidence of tumors (number of mice with > or =1 tumors). Histologic analysis revealed that the Flt3-treated mice demonstrated increased numbers of antigen-presenting cells surrounding the tumors compared with controls.

CONCLUSIONS

Perioperative treatment with either Flt3 ligand or a whole cell tumor vaccine significantly reduced the number of lung metastases after laparotomy. The combination of the Flt3 ligand and the vaccine also decreased the incidence of metastases and was the most effective treatment. Further studies regarding perioperative immune modulation in the setting of cancer appear warranted.

Functional expression, purification, and characterization of human Flt3 ligand in the Pichia pastoris system

Flt3 ligand (FL) is a potent hematopoietic cytokine that affects the growth and differentiation of hematopoietic progenitor and stem cells both in vivo and in vitro. Pichia pastoris transformants secreting high-level rhFL were obtained using 'yeastern blotting' method and the expression level in liquid was about 30 mg/L. rhFL was purified to about 95% purity with overnight dialysis, filtration and an anion-exchange step. Further purification steps employing Sephacryl S-200 and reverse-phase HPLC raised the purity to over 99%. The purified rhFL possessed correct N-terminal amino acid sequence and positive Western blotting bands. SDS-PAGE and mass spectrometry analysis showed molecular weight of rhFL was about 21 and 34 kDa, suggesting that rhFL was glycosylated. The result of capillary electrophoresis showed that its pI is 3.12-4.72. Endo H deglycosylation analysis indicated that there was O-glycosylation besides N-glycosylation in rhFL secreted from P. pastoris. Bioactivity assay showed that the purified rhFL had dose-dependent expansion activity on bone marrow nucleated cells.

Cerebrospinal fluid Flt3 ligand level in patients with amyotrophic lateral sclerosis

OBJECTIVES

The Flt3 ligand (FL) is a cytokine with a neurotrophic and antiapoptotic activity in the central nervous system that induces the survival of neurons. The aim of the study was to measure levels of FL in amyotrophic lateral sclerosis (ALS) patients.

MATERIALS AND METHODS

The study involved 23 ALS patients and 23 people in the control group. The measurement of FL in the cerebrospinal fluid (CSF) and serum was performed by the enzyme-linked immunosorbent method.

RESULTS

Results showed that CSF FL levels were significantly increased in ALS patients compared with the controls (P < 0.05) but the serum levels of this cytokine did not differ from the controls (P > 0.05). There was no significant correlation between CSF and serum FL levels and clinical parameters of ALS (P > 0.05). The difference in CSF/serum ratio of FL between ALS patients and controls was not statistically significant (P > 0.05).

CONCLUSION

An increase in CSF FL levels in ALS patients, observed in this study, could be a compensative response for neurodegeneration but may also reflect increased diffusion of this cytokine into the central nervous system caused by blood-CSF barrier dysfunction.

Significant reduction of laparotomy-associated lung metastases and subcutaneous tumors after perioperative immunomodulation with flt3 ligand in mice

Laparotomy has been associated with increased rates of tumor establishment and metastasis formation postoperatively in animal models. The purpose of this study was to determine the impact on postoperative tumor growth of perioperative upregulation of immune function via fetal liver tyrosine kinase 3 (Flt3 ligand). Two murine studies were carried out: the first utilized a lung metastases model, and the second involved a subcutaneous tumor model. Each study included four groups: anesthesia control (AC), AC plus Flt3 ligand (ACFlt3), sham laparotomy (OP), and OP plus Flt3 ligand (OPFlt3). Flt3 ligand was administered by daily intraperitoneal injection (10 mug/dose) beginning 5 days preoperatively and continuing for 1 week postoperatively. In study 1, A/J mice were given tail vein injections of 1.5 x 10(5) TA3Ha cancer cells on the day of surgery. The mice were sacrificed 14 days after surgery, the lungs processed, and the surface metastases counted by a blinded observer. In study 2 C3H/He mice were given a dorsal subcutaneous injection of 10(4) MC-2 cancer cells on the day of surgery. The mice were sacrificed 31 days after surgery, and the injection sites were evaluated for subcutaneous tumors grossly and histologically. In study 1, the median number of surface lung metastases per mouse was 166 in the OP group and 38 in the OPFlt3 (P = .021). Mice in the AC group developed a median 50 lung metastases per animal compared with mice in the ACFlt3 group who had a median of 10 metastases per mouse (P = .001). The OP group had significantly more metastases than the AC group (P = .048). In study 2, the percentage of animals that developed tumors in the AC, OP, ACFlt3, and OPFlt3 groups was 43, 80, 0, and 20, respectively. The incidence of tumors in the OPFLt3 group and the ACFlt3 group was significantly less than their respective control groups (P < .01). The difference between the OP and AC groups was not significant (P > .05). Perioperatively administered Flt3 ligand was associated with significantly fewer lung metastases and a lower incidence of subcutaneous tumor formation after laparotomy and anesthesia alone. Perioperative immunomodulation may limit untoward surgery-related oncologic effects.

Treatment of hepatocellular carcinoma with adenoviral vector-mediated Flt3 ligand gene therapy

Fms-like tyrosine kinase 3 ligand (Flt3L) plays an important role in development and activation of dendritic cells (DCs) and natural killer cells (NK). It has been shown that administration of either tumor cells transfected in vitro with Flt3L vectors or soluble Flt3L fusion protein in a high dose can enhance host antitumor immunity in animal model systems. In this study, we developed a recombinant defective adenovirus with an insert of gene encoding extracellular domain of mouse Flt3L (Ad-mFlt3L) under control of cytomegalovirus promoter and investigated its biological efficacy in eliciting tumor-specific immune response against hepatocellular carcinoma in mouse hepatoma model. The constructed Ad-mFlt3L efficiently infected hepa 1-6 hepatoma cells both in vitro and in vivo, leading to a high production of mFlt3L proteins in association with accumulation of DCsNK cells and lymphocytes in local tumor tissues. Tumor cells infected with Ad-mFlt3L lost tumorigenicity and became more immunogenic in syngeniec animal models. Intratumoral injection of Ad-mFlt3L (10(9) expression-forming unit) x 3 significantly inhibited tumor growth with elicitation of long-lasting antitumor immunity, which is both preventive and curative. The tumor-specific immunity can be partially abrogated by depletion of either CD3+CD4+ T cells or NK cells and can be also re-established in naïve animals by adoptive transfer of splenocytes from treated mice. The results suggest that adenovirus-mediated Flt3L gene therapy may provide a useful strategy for treatment of cancers.

Assessment of a combined, adenovirus-mediated oncolytic and immunostimulatory tumor therapy

In this study, we identified murine breast cancer cell lines that support DNA replication of E1-deleted adenovirus vectors and which can be killed by an oncolytic adenovirus expressing adenovirus E1A and tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) in a replication-dependent manner (Ad.IR-E1A/TRAIL). We showed that systemic or intratumoral (i.t.) injection of adenovirus vectors into mice increases plasma levels of proinflammatory cytokines and chemokines, including TNF-alpha, INF-gamma, and MCP-1, which are potent inducers of dendritic cell maturation. Furthermore, we showed that in vivo expression of Flt3L from an adenovirus vector increases the number of CD11b+ and CD11c+ cells (populations that include dendritic cells) in the blood circulation. Based on these findings, we tested whether Ad.IR-E1A/TRAIL induced killing of tumor cells in combination with dendritic cell mobilization by Ad.Flt3L or, for comparison, Ad.GM-CSF would have an additive antitumor effect. As a model, we used immunocompetent C3H mice with syngeneic s.c. tumors derived from C3L5 cells. We found that vaccination of mice with C3L5 cells that underwent viral oncolysis in combination with Flt3L or granulocyte-macrophage colony-stimulating factor (GM-CSF) expression induces a systemic antitumor immune response. I.t. injection of the oncolytic and Flt3L expressing vectors into established tumors delayed tumor growth but did not cause efficient tumor elimination. This study shows the effectiveness of a combined oncolytic/immunostimulatory tumor therapy approach.

Murine mammary adenocarcinoma cells transfected with p53 and/or Flt3L induce antitumor immune responses

Transfection of tumors with tumor-associated antigens (Ags) or cytokines can increase immunogenicity and slow down tumor growth. However, the effect of cotransfection with genes that encode a tumor-associated Ag, such as the tumor suppressor gene p53, and a cytokine has been rarely investigated. We report that transfection of 4T1 mammary tumor cells (p53-null) with the dendritic cell (DC) growth factor, fms-like tyrosine kinase 3 ligand (Flt3L), significantly delayed their growth in vivo, resulting in the rejection of 100% of the tumors formed by injection of tumor cells cotransfected with Flt3L and p53. Immunization with irradiated 4T1 cells transfected with Flt3L induced DC infiltration of the immunization site and significantly increased the antitumor T-cell responses. Further, immunization with irradiated 4T1 cells cotransfected with p53 and Flt3L significantly increased p53-specific immune responses, as compared to vaccination with 4T1 cells transfected with either Flt3L or p53 alone. These responses included increased activity against clone 66 (Cl-66), a sister tumor to 4T1 with high murine mutant p53 expression levels. Challenge with Cl-66 revealed that immunization with irradiated 4T1-Flt3L-p53 cells significantly slowed growth, prolonged survival, and resulted in complete remissions. Further, immunization with irradiated 4T1-Flt3L also slowed Cl-66 growth, although to a lesser extent than 4T1-Flt3L-p53. We suggest that immunization with DCs transfected with the Flt3L transgene and a tumor Ag may potentially heighten T-cell responses and therapeutic activity.

Flt-3 ligand as adjuvant for DNA vaccination augments immune responses but does not skew TH1/TH2 polarization

Since transfection of dendritic cells (DC) plays a key role in DNA vaccination, in vivo expansion of DC might be a tool to increase vaccine efficacy. We asked whether Fms-like tyrosine kinase-3 ligand (Flt-3L), a growth factor for DC, can be used as an adjuvant for DNA vaccination. Beta-galactosidase (beta-gal) was used as a model antigen in C57BL/6 mice. Mice were immunized i.m. with DNA coding for beta-gal with or without additional injection of Flt-3L. In both cases, antigen-specific CD4+ and CD8+ T cells were detectable after vaccination. Compared with DNA alone, additional administration of Flt-3L led to a significant increase in the antigen-specific proliferative response. However, increased cytotoxicity by T cells was not observed. The cytokines secreted by splenocytes of immunized mice upon in vitro stimulation with antigen had a TH2 profile. Humoral responses against beta-gal preferentially consisted of IgG1 antibodies. Analysis of DC from Flt-3L-treated mice revealed an immature phenotype with low or absent expression levels of CD80, CD86 and CD40. We conclude that Flt-3L does not generally skew immune responses towards a TH1 type. More likely, factors determined by the antigen and/or the vaccination procedure itself are crucial for the resulting type of immune response. Flt-3L - under circumstances such as the one we have investigated - can also lead to suppression of TH1 T cell immunity, possibly by expansion of immature/unactivated DC.

Prostate Tumor Microenvironment Alters Immune Cells and Prevents Long-Term Survival in an Orthotopic Mouse Model Following flt3-Ligand/CD40-Ligand Immunotherapy

A novel orthotopic metastatic model of mouse prostate cancer was developed using MHC-negative TRAMP-C1P3 (transgenic adenocarcinoma of mouse prostate) cells derived by serial passage of the parental TRAMP-C1 line in mouse prostate glands. TRAMP-C1P3 cells grew efficiently in mouse prostate glands and reproducibly metastasized to draining lymph nodes. Using this model, we show that Fms-like tyrosine kinase-3 ligand (flt3-L) dramatically inhibited growth of preexisting orthotopic TRAMP-C1P3 tumors and the development of metastatic disease. Mice remained in remission for several months following termination of flt3-L treatment but eventually relapsed and died of progressive disease. flt3-ligand treatment induced a pronounced mixed inflammatory cell infiltrate that consisted of CD8alpha-CD4- dendritic cells (CD11c+), macrophages, granulocytes (Gr-1+) and to a lesser extent T cells (CD4+ and CD8+). Dendritic cells isolated from TRAMP-C1P3 tumors were phenotypically immature (CD11c+ B7.2-I-A-CD40-), and this phenotype was also predominant in peripheral organs of mice treated with flt3-L alone or in combination with the DC maturation factor, CD40-L. Diminished expression of TCR-beta, CD3-epsilon, and CD3-zeta was also observed on intratumoral T cells, although these signaling proteins were reexpressed following in vitro culture with IL-2. The TCR/CD3 complex remained intact on peripheral T cells except in mice treated with flt3-L where CD3-zeta loss was observed. In contrast to alphabeta-T cells, tumor-infiltrating gammadelta-T cells maintained expression of their antigen receptors but not CD3epsilon. Thus, TRAMP-C1P3 tumors quickly establish a microenvironment that profoundly diminishes expression of molecules critical for normal dendritic cell and T cell function, thus limiting the efficacy of flt3-L and CD40-L immunotherapy. Overall, these data suggest that long-term cures of established MHC-negative tumors may not be achieved until therapeutic interventions are engineered to overcome this immunosuppressive microenvironment.

Regional, but not systemic recruitment/expansion of dendritic cells by a pluronic-formulated Flt3-ligand plasmid with vaccine adjuvant activity

Regional recruitment of dendritic cells (DCs) by the local administration of granulocyte macrophage-colony stimulating factor (GM-CSF) or Flt3-ligand (Flt3L) has vaccine adjuvant activity. However, Flt3L, with its DC growth factor activity, has not been extensively studied as a vaccine adjuvant, particularly as a plasmid vector. We report that the intramuscular (IM) injection of a Flt3L plasmid (pNGVL-hFlex), when formulated in a pluronic carrier (SP1017, Supratek Pharma, Inc., Laval, Que., Canada), recruits DC to the injection site and regional lymph nodes (LNs) and augments immune responses to a p17 HIV plasmid vaccine to a greater extent than the injection of a naked DNA vaccine alone. Following IM administration of pNGVL-hFlex, Flt3L mRNA, Flt3L protein and infiltrating DC accumulate at the injection site. The number of DC in the draining LNs are also significantly increased with the greatest increase observed following injection of 2.5 microg of pNGVL-hFlex formulated in 0.01% SP1017. Flow cytometric studies demonstrate that the LN-infiltrating DC is mainly of the CD11c(+)CD11b(-) phenotype (IL-12 producing). Further, the co-injection of pNGVL3-hFlex and p17 HIV plasmids, formulated in SP1017, significantly increases the immune responses to the plasmid vaccine (pVAX-gag). The co-injection of pVAX-gag and pNGVL3-hFlex, formulated in SP1017, significantly increase delayed-type hypersensitivity responses and the numbers of antigen (Ag)-specific interferon-gamma secreting T cells in the spleen (Enzyme Linked Immune Spot (ELISpot) assay), compared to mice immunized with pVAX-gag formulated in SP1017 alone. We conclude that the IM injection of pNGVL-hFlex with SP1017 can increase the number of DC in draining LN and at the site of injection, thereby providing adjuvant activity for a plasmid vaccine resulting in a significantly increased, Ag-specific T cell response.

Antitumor effect of secreted Flt3-ligand can act at distant tumor sites in a murine model of head and neck cancer

The Flt3 ligand (Flt3-L) manifests antitumor activity, presumably due to its capacity to recruit dendritic cells and cause their proliferation. To assess whether local production of Flt3-L can mediate a "distant bystander" effect, murine B4B8 squamous cell carcinoma cells were transfected with a plasmid encoding a secretory form of Flt3-L to produce B4B8FL cells. Similarly, B4B8FL and B4B8 cells were transfected with herpes simplex virus thymidine kinase (HSVTK) to produce B4B8TK and B4B8FL/TK cells, which should be sensitive to ganciclovir (GCV), to know whether the effects of Flt3-L and HSVTK/GCV would be synergistic. To test for a distant bystander effect in vivo, B4B8FL, B4B8TK, and B4B8FL/TK cells were injected subcutaneously into the left flank of syngeneic Balb/c mice, and naïve B4B8 cells were injected into the right flank. The formation of tumors derived from B4B8FL and B4B8FL/TK cells was significantly delayed in both flanks compared with naïve B4B8 and B4B8TK cells. Growth of B4B8TK tumors in the ipsilateral flank was retarded following GCV treatment, but in contrast to B4B8FL and B4B8FL/TK cells, no distant bystander effect in the contralateral flank was observed. Immunohistochemistry showed lymphocytic infiltrates in both flanks of the B4B8FL and B4B8FL/TK groups. The data indicate that in these cells, local secretion of Flt3-L is sufficient to evoke a distant bystander effect but that expression of HSVTK, even after GCV administration, is not. Furthermore, the combination of local Flt3-L and HSVTK production, together with GCV administration, does not enhance the distant bystander effect produced by Flt3-L alone.

Adjuvant effect of a Flt3 ligand (FL) gene-transduced xenogeneic cell line in a murine colon cancer model

BACKGROUND

Flt3 Ligand (FL) has been shown to elicit antitumor responses induced by tumor antigen stimulation. Allogeneic and xenogeneic cell lines transduced with cytokine genes may be used to augment the antitumor efficacy of tumor antigens.

OBJECTIVES

The objective was to evaluate the augmentation of tumor lysate-induced immunity by a more clinically applicable FL gene-transduced xenogeneic cell line in combination with interleukin-2 (IL-2) in a CC-36 murine colon cancer model.

METHODS

Human 143B osteosarcoma tumor cells were transduced with full-length FL cDNA (143B-FL). Secretion of FL from 143B-FL was analyzed in vivo in normal BALB/c mice transplanted with 143B-FL, and expansion of dendritic cells (DC) was also analyzed in the same mice by flow cytometry. Eight-week-old, male BALB/c mice were used in a prophylactic vaccination protocol utilizing tumor lysate (CLy), 143B-FL, and soluble IL-2. Prophylactic group designations (n = 10/group) were as follows: ten million 143B-FL cells (alone, with tumor lysate, or with tumor lysate and IL-2), IL-2 with tumor lysate, IL-2 alone, or a no treatment control. The tumor lysate (200 microg of protein) and IL-2 (100,000 IU) injections were administered intraperitoneally. Mice were challenged subcutaneously with 10(3) CC-36 tumor cells. Tumor protection and tumor burden (TB), as mean tumor diameter, were determined. Peripheral blood lymphocytes (PBLs) from the 143B-FL + IL-2 + tumor lysate vaccinated group were analyzed for cytolytic activity in 4-h chromium release assays. In addition, plasma cytokine concentrations of interleukin-12 (IL-12) and interferon gamma (IFN-gamma) were assessed by ELISA. Student's t tests were used for all statistical comparisons.

RESULTS

In vivo expression of FL was observed 24 h following the inoculation of 143B-FL, and a four fold increase in DCs was observed in the peripheral blood of these mice. Mice immunized with a combination of 143B-FL, tumor lysate and IL-2 showed statistically significant protection against tumor development (10%) for 100 days after tumor challenge; incidences in other groups ranged from 40 to 100% (P < 0.05). Moreover, this immunization protocol produced the lowest TB at 3- and 6-week time points (0, 1.6 mm) when compared to all other groups (TB between 7.2 and 15.9 mm) (P < 0.05). In addition, PBLs from vaccinated mice showed increased cytolytic activity against CC-36 target cells. This corresponded to increased levels of IL-12 and IFN-g in the plasma of mice following vaccination.

CONCLUSIONS

These data suggest that FL gene-transduced xenogeneic tumor cells may augment the immunity induced by tumor antigens and systemic IL-2 through the activation of dendritic cells and T-cell-mediated mechanisms.

Generation of potent and specific cellular immune responses via in vivo stimulation of dendritic cells by pNGVL3-hFLex plasmid DNA and immunogenic peptides

Dendritic cells (DC) are the most potent professional antigen-presenting cells with exquisite capacity to interact with T cells to initiate strong primary cellular immune responses. The antigen-presenting capability of DC makes them attractive vehicles for the delivery of therapeutic cancer vaccines. Recently, we have demonstrated that the introduction of a recombinant gene encoding the human Flt3L gene into mice could result in the expansion of the DC population in vivo. In this report, we have introduced the human Flt-3L gene via naked DNA-based immunization in combination with the muc-1 tumor peptide to immunize mice. We demonstrated that the population of DC expanded following stimulation with the human Flt-3L gene in vivo is functional and they are able to elicit potent muc-1 peptide-specific cellular responses. The strategy described here allows the efficient generation of antigen-specific CTL immunity in vivo and has the potential to be applied in developing efficient protocols for antigen-specific immunotherapy of human malignancies.

Flt3 ligand augmentation of T cell mitogenesis and expansion of type 1 effector/memory T cells

Herein we report mechanisms whereby Flt3 ligand (FL) augments steady state T cell activity in addition to the expansion of dendritic cells (DCs). We demonstrate that in vivo administration of FL increases the frequency and absolute number of effector/memory T cells and preferentially expands T cells that express a type-1 cytokine phenotype. In addition, FL enhances T cell proliferative responses to Concanavalin A that directly correlated with increased frequencies in effector/memory T cells and expansion of lymphoid-derived (type 1) DCs (DC1s). Together, these data demonstrate that mechanisms of FL-induced T cell regulation include not only the expansion of DC subsets, but also the preferential expansion of type 1 -effector/memory T cell populations, and suggest multiple mechanisms of action for FL as a vaccine adjuvant and as a therapeutic modality.

Blood stem cell transplantation for breast cancer: new approaches using pre- peri- post-transplant immunotherapy

Autologous peripheral blood stem cell transplantation (auto-PBSCT) after high dose chemotherapy is usually offered to breast cancer patients carrying a high risk of relapse or having chemosensitive metastatic disease. Whether progression free and overall survival of such patients is improved after auto-PBSCT compared to conventional chemotherapy is a matter of debate. Currently available results of randomised trials could not uniformly prove or disprove auto-PBSCT being advantageous. Yet such studies have not employed any manipulation of the stem cell graft or any post-transplant immunomodulation exploiting the unique immunological environment for tumour eradication which exists only after auto-PBSCT. Preliminary data have discussed the ex vivo and in vivo generation of cytotoxic effector cells employing IL-2 and/or IFN-alpha/gamma in the auto-PBSCT setting. Other cytokines such as IL-12, IL-15 and prolactin have likewise been considered. Several anticancer vaccine protocols after auto-PBSCT are ongoing using monovalent vaccines or anti-idiotypic antibodies. Polyvalent anticancer vaccines, cytokine secreting tumour cells, tumour pulsed or hybridised dendritic cells (DC) enhanced with cytokines are studied. Monoclonal antibodies (mAb) could assist: unlabelled for pretransplant exvivo purging, post-transplant for enhancing antibody-dependent cell mediated cytotoxicity (ADCC) or radioimmunoconjugated as an additive cytotoxic part of the conditioning regimen. Autologous graft versus host induction and allogeneic stem cell transplantation (probably with non-myeloablative conditioning followed by donor lymphocyte infusions) are other approaches. Evaluation of successful combinations, optimal dosages and appropriate timing schedules is the subject of future investigations. Since breast cancer patients belong to countless subgroups, a large number of protocols need to be addressed in order to avoid over treatment and prevent relapse.

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.

Preoperative mobilization of circulating dendritic cells by Flt3 ligand administration to patients with metastatic colon cancer

PURPOSE

To evaluate preoperative dendritic cell (DC) mobilization and tumor infiltration after administration of Flt3 ligand (Flt3L) to patients with metastatic colon cancer.

PATIENTS AND METHODS

Twelve patients with colon cancer metastatic to the liver or lung received Flt3L (20 microg/kg/d subcutaneously for 14 days for one to three cycles at monthly intervals) before attempted metastasectomy. The number and phenotype of DCs mobilized into peripheral-blood mononuclear cells (PBMCs) were evaluated by flow cytometry. After surgical resection, metastatic tumor tissue was evaluated for DC infiltration. In vivo immune responses to recall antigens were measured.

RESULTS

After Flt3L administration, on average, the total number of leukocytes in the peripheral blood increased from 5.9 +/- 1.0 x 10(3)/mm(3) to 11.2 +/- 3.8 x 10(3)/mm(3) (mean +/- SD, P: =. 0001). The percentage of CD11c(+)CD14(-) DCs in PBMCs increased from 2.4% +/- 1.8% to 8.8% +/- 4.7% (P: =.004). Delayed-type hypersensitivity (DTH) responses to recall antigens (CANDIDA:, mumps, and tetanus) showed marginally significant increases in reactivity after Flt3L administration (P: =.06, P: =.03, and P: =.08, respectively). An increase in the number of DCs was observed at the periphery of the tumors of patients who received Flt3L compared with those of patients who had not.

CONCLUSION

Flt3L is capable of mobilizing DCs into the peripheral blood of patients with metastatic colon cancer and may be associated with increases in DC infiltration in the peritumoral regions. Flt3L mobilization is associated with a trend toward increased DTH responses to recall antigens in vivo. The use of Flt3L to increase circulating DCs for cancer immunotherapy should be considered.

The CC chemokine CK beta-11/MIP-3 beta/ELC/Exodus 3 mediates tumor rejection of murine breast cancer cells through NK cells

CK beta-11 chemoattracts T cells, B cells, dendritic cells, macrophage progenitors, and NK cells and facilitates dendritic cell and T cell interactions in secondary lymphoid tissues. We hypothesized that expression of CK beta-11 in tumor cells may generate antitumor immunity through these interactions. After transduction with the retroviral vector L(CK beta 11)SN, the murine breast cancer cell line C3L5 (C3L5-CK beta 11) showed expression of retroviral mRNA by Northern analysis and production of functional CK beta-11 by chemotaxis of human NK cells to C3L5-CK beta 11 supernatant. Only 10% of mice injected with C3L5-CK beta 11 developed tumors, compared with 100% of mice injected with a transduced control C3L5 line (C3L5-G1N). Importantly, the in vitro growth characteristics of the CK beta-11-transduced cell line were unaffected, suggesting the difference in growth in vivo was a result of chemokine production. Vaccination with C3L5-CK beta 11 partially protected animals from parental C3L5 challenge. Immunodepletion with anti-asialo-GM1 or anti-CD4 during C3L5-CK beta 11 vaccination significantly reduced CK beta-11 antitumor activity compared with control and anti-CD8-treated groups. Splenocytes from NK-depleted animals transferred the acquired immunity generated with C3L5-CK beta 11 vaccination, while splenocytes from the CD4-depleted animals did not. These results indicate, for the first time, that expression of CK beta-11 in a breast cancer cell line mediates rejection of the transduced tumor through a mechanism involving NK and CD4+ cells. Furthermore, CK beta-11-transduced tumor cells generate long-term antitumor immunity that requires CD4+ cells. These studies demonstrate the potential role of CK beta-11 as an adjuvant in stimulating antitumor responses.