Flt3 ligand enhances the immunogenicity of a gag-based HIV-1 vaccine

Dendritic cell-targeting DNA-based nasal adjuvants for protective mucosal immunity to Streptococcus pneumoniae

To develop safe vaccines for inducing mucosal immunity to major pulmonary bacterial infections, appropriate vaccine antigens (Ags), delivery systems and nontoxic molecular adjuvants must be considered. Such vaccine constructs can induce Ag-specific immune responses that protect against mucosal infections. In particular, it has been shown that simply mixing the adjuvant with the bacterial Ag is a relatively easy means of constructing adjuvant-based mucosal vaccine preparations; the resulting vaccines can elicit protective immunity. DNA-based nasal adjuvants targeting mucosal DCs have been studied in order to induce Ag-specific mucosal and systemic immune responses that provide essential protection against microbial pathogens that invade mucosal surfaces. In this review, initially a plasmid encoding the cDNA of Flt3 ligand (pFL), a molecule that is a growth factor for DCs, as an effective adjuvant for mucosal immunity to pneumococcal infections, is introduced. Next, the potential of adding unmethylated CpG oligodeoxynucleotide and pFL together with a pneumococcal Ag to induce protection from pneumococcal infections is discussed. Pneumococcal surface protein A has been used as vaccine for restoring mucosal immunity in older persons. Further, our nasal pFL adjuvant system with phosphorylcholine-keyhole limpet hemocyanin (PC-KLH) has also been used in pneumococcal vaccine development to induce complete protection from nasal carriage by Streptococcus pneumoniae. Finally, the possibility that anti-PC antibodies induced by nasal delivery of pFL plus PC-KLH may play a protective role in prevention of atherogenesis and thus block subsequent development of cardiovascular disease is discussed.

Novel vaccine development strategies for inducing mucosal immunity

To develop protective immune responses against mucosal pathogens, the delivery route and adjuvants for vaccination are important. The host, however, strives to maintain mucosal homeostasis by responding to mucosal antigens with tolerance, instead of immune activation. Thus, induction of mucosal immunity through vaccination is a rather difficult task, and potent mucosal adjuvants, vectors or other special delivery systems are often used, especially in the elderly. By taking advantage of the common mucosal immune system, the targeting of mucosal dendritic cells and microfold epithelial cells may facilitate the induction of effective mucosal immunity. Thus, novel routes of immunization and antigen delivery systems also show great potential for the development of effective and safe mucosal vaccines against various pathogens. The purpose of this review is to introduce several recent approaches to induce mucosal immunity to vaccines, with an emphasis on mucosal tissue targeting, new immunization routes and delivery systems. Defining the mechanisms of mucosal vaccines is as important as their efficacy and safety, and in this article, examples of recent approaches, which will likely accelerate progress in mucosal vaccine development, are discussed.

A single nasal dose of fms-like tyrosine kinase receptor-3 ligand, but not peritoneal application, enhances nontypeable Haemophilus influenzae-specific long-term mucosal immune responses in the nasopharynx

Nasal vaccination is an effective therapeutic regimen for preventing otitis media. In the development of nasal vaccine, an appropriate adjuvant is required. In the present study, we examined the efficacy of fms-like tyrosine kinase receptor-3 ligand (Flt3L) as a mucosal adjuvant. Flt3L was administered intranasally or peritoneally to mice, which were then immunized intranasally with P6 protein of nontypeable Haemophilus influenzae (NTHi), and P6-specific immune responses were examined. In addition, NTHi challenges were performed and the level of NTHi was quantified in nasal washes. Nasal application of Flt3L induced an increase in the number of dendritic cells in nasal-associated lymphoid tissue. P6-specific nasal wash immunoglobulin (Ig)A and serum IgG titers were elevated significantly after nasal immunization. Enhanced NTHi clearance from the nasopharynx was also observed. The effect of nasal vaccination with P6 combined with nasal Flt3L application was prolonged. These results indicate the potential of Flt3L as an effective mucosal adjuvant and suggest that nasal vaccination with P6 in combination with nasal Flt3L might be an effective regimen for the induction of NTHi-specific protective immunity.

Dendritic cell-targeting DNA-based mucosal adjuvants for the development of mucosal vaccines

In order to establish effective mucosal immunity against various mucosal pathogens, vaccines must be delivered via the mucosal route and contain effective adjuvant(s). Since mucosal adjuvants can simply mix with the antigen, it is relatively easy to adapt them for different types of vaccine development. Even in simple admixture vaccines, the adjuvant itself must be prepared without any complications. Thus, CpG oligodeoxynucleotides or plasmids encoding certain cDNA(s) would be potent mucosal adjuvant candidates when compared with other substances that can be used as mucosal adjuvants. The strategy of a DNA-based mucosal adjuvant facilitates the targeting of mucosal dendritic cells, and thus is an effective and safe approach. It would also provide great flexibility for the development of effective vaccines for various mucosal pathogens.

A novel adenovirus expressing Flt3 ligand enhances mucosal immunity by inducing mature nasopharyngeal-associated lymphoreticular tissue dendritic cell migration

Previously, we showed that nasal administration of a naked cDNA plasmid expressing Flt3 ligand (FL) cDNA (pFL) enhanced CD4(+) Th2-type, cytokine-mediated mucosal immunity and increased lymphoid-type dendritic cell (DC) numbers. In this study, we investigated whether targeting nasopharyngeal-associated lymphoreticular tissue (NALT) DCs by a different delivery mode of FL, i.e., an adenovirus (Ad) serotype 5 vector expressing FL (Ad-FL), would provide Ag-specific humoral and cell-mediated mucosal immunity. Nasal immunization of mice with OVA plus Ad-FL as mucosal adjuvant elicited high levels of OVA-specific Ab responses in external secretions and plasma as well as significant levels of OVA-specific CD4(+) T cell proliferative responses and OVA-induced IFN-gamma and IL-4 production in NALT, cervical lymph nodes, and spleen. We also observed higher levels of OVA-specific CTL responses in the spleen and cervical lymph nodes of mice given nasal OVA plus Ad-FL than in mice receiving OVA plus control Ad. Notably, the number of CD11b(+)CD11c(+) DCs expressing high levels of costimulatory molecules was preferentially increased. These DCs migrated from the NALT to mucosal effector lymphoid tissues. Taken together, these results suggest that the use of Ad-FL as a nasal adjuvant preferentially induces mature-type NALT CD11b(+)CD11c(+) DCs that migrate to effector sites for subsequent CD4(+) Th1- and Th2-type cytokine-mediated, Ag-specific Ab and CTL responses.

Transgenic mice expressing Tel-FLT3, a constitutively activated form of FLT3, develop myeloproliferative disease

Evidence is continuing to accumulate that the FMS-like tyrosine kinase 3 (FLT3) receptor plays an important role in acute leukemias. Acute myeloid leukemia patients often express constitutive active mutant forms of the receptor in their leukemic cells. A t(12;13)(p13;q12) translocation between Tel and the FLT3 receptor was recently described in a patient with myeloproliferative disease (MPD). Here a Tel-FLT3 construct mimicking this fusion protein was used to generate transgenic mice. The fusion protein was previously found to constitutively activate FLT3 signaling and transform Ba/F3 cells. Expression of the fusion protein in the transgenic mice was found in all tissues assayed including spleen, bone marrow (BM), thymus and liver. These mice developed splenomegaly and had a high incidence of MPD with extramedullary hematopoiesis in the liver and lymph nodes. Spleens also had increased dendritic and natural killer cell populations. In vitro analysis of the hematopoietic progenitor cells derived from Tel-FLT3 transgenic mice showed a significant increase in the number of CFU-GM in the BM, and CFU-GM, BFU-E and CFU-GEMM in the spleen. BM also showed significant increases of in vivo CFU-S colonies. Thus, transgenic mice expressing constitutively activated Tel-FLT3 develop MPD with a long latency and also result in the expansion of the hematopoietic stem/progenitor cells.

T cells recognize PD(N/T)R motif common in a variable number of tandem repeat and degenerate repeat sequences of MUC1

The tumor-associated antigen MUC1 is a transmembrane glycoprotein, which is overexpressed in human carcinomas. Peptide epitopes, containing the PDTR fragment from the variable number of tandem repeat (VNTR) domains of MUC1 have been found to be immunodominant in T-cell and B-cell responses. However, little is known about the immunogenicity and specificity of T-cell epitopes from other regions of MUC1 that may also participate in immune responses against tumors. In this study, the combination of immunoinformatics, molecular modeling and a vaccine adjuvant strategy were used to predict and describe a novel T-cell epitope, SAPDNRPAL, located within the degenerate tandem repeat of MUC1. This peptide possesses structural similarity to both VNTR-derived SAPDTRPAP and Sendai virus peptide FAPGNYPAL, which are known to induce cytotoxic T lymphocytes (CTL). We found that SAPDNRPAL had a higher affinity for mouse H-D(b), H-2K(b) and human HLA-A2 molecules than SAPDTRPAP. A chimeric peptide (CP) containing SAPDNRPAL and an adjuvant C5a-derived decapeptide induced epitope-specific type 1 T cells in human MUC1 transgenic mice (ELISPOT). Mice that received dendritic cells (DC) pulsed with the CP or a 25-mer peptide containing the SAPDNRPAL sequence showed increased frequencies of SAPDNRPAL- and SAPDTRPAP-specific interferon-gamma producing T cells. PDTR-specific antibody 214D4 reacted with both SAPDNRPAL and SAPDTRPAP (ELISA). Altogether, our data suggest that the degenerate MUC1 repeat sequence contains the immunogenic T-cell epitope SAPDNRPAL, which is cross-reactive with the VNTR-derived peptide SAPDTRPAP. We suggest that the use of immunogenic PDNR-containing epitope(s) in vaccine strategies could be beneficial for developing increased, PD(N/T)R motif-specific T-cell responses against tumors expressing MUC1.

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.

Nasal Flt3 ligand cDNA elicits CD11c+CD8+ dendritic cells for enhanced mucosal immunity

Nasal immunization is an effective way to induce both mucosal and systemic immune responses. In this study, we assessed a cDNA vector for Flt3 ligand (FL) for its potential to enhance mucosal immunity or tolerance. Interestingly, tolerance was avoided and elevated levels of OVA-specific Ab responses were induced in nasal washes, fecal extracts, and saliva as well as in plasma when compared with mice given nasal OVA plus DNA plasmid without the FL gene. In addition, significant levels of OVA-specific CD4+ T cell proliferative responses and OVA-induced IL-4 and IL-2 production were noted in spleen and cervical lymph nodes. Further, marked increases in FL protein occurred in the nasal lamina propria and submandibular glands and the frequencies of CD11c+CD8+ dendritic cells (DCs) significantly increased in the mucosal tissues. Moreover, these DCs expressed high levels of CD40, CD80, CD86, and MHC class II molecules. Nasal delivery of plasmid FL with OVA resulted in FL expression in both mucosal inductive and effector sites and resulted in expanded activated lymphoid DCs. Thus, nasal plasmid FL prevents mucosal tolerance and enhances active immunity when given by a mucosal route.

Plasma Flt-3 ligand concentration correlated with radiation-induced bone marrow damage during local fractionated radiotherapy

PURPOSE

To determine whether variations in the plasma Flt-3 ligand (FL) concentration after radiotherapy (RT) may serve as a biomarker for radiation-induced bone marrow damage.

METHODS AND MATERIALS

Twenty-seven patients were followed during RT. The irradiated bone marrow volume was determined. The blood cell counts and plasma FL concentrations were evaluated before and after RT. The expression of membrane-bound FL and mRNA expression were also defined in circulating blood cells.

RESULTS

We found a negative correlation between the plasma FL concentration and the number of circulating white blood cells and platelets during RT. Moreover, the overall amount of FL in the blood of patients during RT correlated directly with both the cumulated radiation dose and the proportion of irradiated bone marrow.

CONCLUSIONS

We demonstrated that the variations in plasma FL concentration directly reflect the radiation-induced bone marrow damage during fractionated local RT. We suggest a possible use for FL monitoring as a means to predict the occurrence of Grade 3-4 leukopenia or thrombocytopenia during the course of RT.

Comparative analysis of effects of cytokine gene adjuvants on DNA vaccination against Mycobacterium tuberculosis heat shock protein 65

DNA-based vaccines generate potent cellular immunity as well as humoral immunity. It seems evident that cytokines play a crucial role in generation of effector T cell subsets and in determining the magnitude of the response by DNA vaccines. In this study, we compared the effects of several TH1 cytokine genes as adjuvant in DNA vaccination using mycobacterial Hsp65 as a model antigen. Our results demonstrated that although the overall immune response to Hsp65 was enhanced by co-injection of Hsp65 DNA with cytokine genes, each cytokine gene was shown to affect different immune response elements. Co-injection of Hsp65 DNA with IL-12 or GM-CSF led to an increase in IFN-gamma production and represented potent protections against Mycobacterium tuberculosis challenge, while that with Eta-1, IL-12 or IL-18 gene led to an elevated IgG2a/IgG1 ratio. Interestingly, co-administration of Flt3L gene was shown to enhance the Ag-specific CTL response. These results show that the direction and magnitude of immune response in DNA vaccination against Hsp65 of M. tuberculosis could be modulated in different ways by co-injection of an appropriate cytokine gene as adjuvant.

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.

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.

Flt3 ligand and conjugation to IL-1beta peptide as adjuvants for a type 1, T-cell response to an HIV p17 gag vaccine

The adjuvant activity of Flt3 ligand (Flt3L) and conjugation to an interleukin (IL)-1beta bioactive fragment were compared, either alone or in combination, for their ability to induce T- and B-cell responses to the HGP-30 peptide sequence (amino acids 86-115 of human immunodeficiency virus (HIV) gag p17). The efficiency of HGP-30/IL-1beta conjugation, Flt3L administration or both as adjuvants was examined and all were found to augment similar levels of delayed type hypersensitivity (DTH) responses. In contrast, significant antigen (Ag)-specific types 1 and 2 T-cell ELISPOT responses were induced only by the combination of adjuvants. Further, in vitro sensitization with HGP-30 selectively increased Ag-specific, type 1 T-cell and cytotoxic T lymphocyte (CTL) responses to HGP-30-derived nonapeptide epitopes, while type 2 responses declined as measured in the ELISPOT assay. No serum antibodies to HGP-30 were induced unless HGP-30 was conjugated to keyhole-limpet hemocyanin. This suggests that a combination adjuvant strategy using Flt3L and conjugation to a biologically active IL-1beta fragment may be used to preferentially increase type 1 T-cell and CTL responses to HIV-1 gag antigenic epitopes.

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.