Autophagy regulates long-term cross-presentation by murine dendritic cells.

Autophagy has been reported to be involved in supporting antigen cross-presentation by dendritic cells (DCs). We have shown that DCs have the ability to store antigen for a prolonged time in endolysosomal compartments and thereby sustain MHCI antigen cross-presentation to CD8+ T cells. In the current study, we investigated the role of autophagy in long-term antigen presentation. We show that the autophagy machinery has a negative impact on storage of antigen in DCs. Atg5-/- DCs which are deficient in autophagy or DCs treated with common autophagy inhibitors showed enhanced antigen storage and antigen cross-presentation. This augmented antigen cross-presentation effect is independent of altered proteasome enzyme activity or MHCI surface expression on DCs. We visualized that the storage compartments are in close proximity to LC3 positive autophagosomes. Our results indicate that autophagosomes disrupt antigen storage in DCs and thereby regulate long-term MHCI cross-presentation.

Upconversion nanoparticle platform for efficient dendritic cell antigen delivery and simultaneous tracking.

Upconversion nanoparticles (UCNPs) represent a group of NPs that can convert near-infrared (NIR) light into ultraviolet and visible light, thus possess deep tissue penetration power with less background fluorescence noise interference, and do not induce damage to biological tissues. Due to their unique optical properties and possibility for surface modification, UCNPs can be exploited for concomitant antigen delivery into dendritic cells (DCs) and monitoring by molecular imaging. In this study, we focus on the development of a nano-delivery platform targeting DCs for immunotherapy and simultaneous imaging. OVA 254-267 (OVA24) peptide antigen, harboring a CD8 T cell epitope, and Pam3CysSerLys4 (Pam3CSK4) adjuvant were chemically linked to the surface of UCNPs by amide condensation to stimulate DC maturation and antigen presentation. The OVA24-Pam3CSK4-UCNPs were thoroughly characterized and showed a homogeneous morphology and surface electronegativity, which promoted a good dispersion of the NPs. In vitro experiments demonstrated that OVA24-Pam3CSK4-UCNPs induced a strong immune response, including DC maturation, T cell activation, and proliferation, as well as interferon gamma (IFN-γ) production. In vivo, highly sensitive upconversion luminescence (UCL) imaging of OVA24-Pam3CSK4-UCNPs allowed tracking of UCNPs from the periphery to lymph nodes. In summary, OVA24-Pam3CSK4-UCNPs represent an effective tool for DC-based immunotherapy.

Distinct antigen uptake receptors route to the same storage compartments for cross-presentation in dendritic cells.

An exclusive feature of dendritic cells (DCs) is their capacity to present exogenous antigens by MHC class I molecules, called cross-presentation. Here, we show that protein antigen can be conserved in mature murine DCs for several days in a lysosome-like storage compartment, distinct from MHC class II and early endosomal compartments, as an internal source for the supply of MHC class I ligands. Using two different uptake routes via Fcγ receptors and C-type lectin receptors, we could show that antigens were routed towards the same endolysosomal compartments after 48 h. The antigen-containing compartments lacked co-expression of molecules involved in MHC class I processing and presentation including TAP and proteasome subunits as shown by single-cell imaging flow cytometry. Moreover, we observed the absence of cathepsin S but selective co-localization of active cathepsin X with protein antigen in the storage compartments. This indicates cathepsin S-independent antigen degradation and a novel but yet undefined role for cathepsin X in antigen processing and cross-presentation by DCs. In summary, our data suggest that these antigen-containing compartments in DCs can conserve protein antigens from different uptake routes and contribute to long-lasting antigen cross-presentation.

A Restricted Role for FcγR in the Regulation of Adaptive Immunity.

By their interaction with IgG immune complexes, FcγR and complement link innate and adaptive immunity, showing functional redundancy. In complement-deficient mice, IgG downstream effector functions are often impaired, as well as adaptive immunity. Based on a variety of model systems using FcγR-knockout mice, it has been concluded that FcγRs are also key regulators of innate and adaptive immunity; however, several of the model systems underpinning these conclusions suffer from flawed experimental design. To address this issue, we generated a novel mouse model deficient for all FcγRs (FcγRI/II/III/IV-/- mice). These mice displayed normal development and lymphoid and myeloid ontogeny. Although IgG effector pathways were impaired, adaptive immune responses to a variety of challenges, including bacterial infection and IgG immune complexes, were not. Like FcγRIIb-deficient mice, FcγRI/II/III/IV-/- mice developed higher Ab titers but no autoantibodies. These observations indicate a redundant role for activating FcγRs in the modulation of the adaptive immune response in vivo. We conclude that FcγRs are downstream IgG effector molecules with a restricted role in the ontogeny and maintenance of the immune system, as well as the regulation of adaptive immunity.

FcγR interaction is not required for effective anti-PD-L1 immunotherapy but can add additional benefit depending on the tumor model.

Immunomodulatory antibodies blocking interactions of coinhibitory receptors to their ligands such as CTLA-4, PD1 and PD-L1 on immune cells have shown impressive therapeutic efficacy in clinical studies. The therapeutic effect of these antibodies is mainly mediated by reactivating antitumor T cell immune responses. Detailed analysis of anti-CTLA4 antibody therapy revealed that an optimal therapeutic efficacy also requires binding to Fc receptors for IgG, FcγR, mediating depletion of intratumoral regulatory T cells. Here, we investigated the role of Fc binding in anti-PD-L1 antibody therapy in the MC38 C57BL/6 and CT26 BALB/c colon adenocarcinoma tumor models. In the MC38 tumor model, all IgG subclasses anti-PD-L1 showed similar therapeutic efficacy when compared to each other in either wild-type mice or in mice deficient for all FcγR. In contrast, in the CT26 tumor model, anti-PD-L1 mIgG2a, the IgG subclass with the highest affinity for activating FcγR, showed stronger therapeutic efficacy than other IgG subclasses. This was associated with a reduction of a myeloid cell subset with high expression of PD-L1 in the tumor microenvironment. This subclass preference for mIgG2a was lost in C57BL/6 × BALB/c F1 mice, indicating that the genetic background of the host may determine the additional clinical benefit of the high affinity antibody subclasses. Based on these data, we conclude that FcγR are not crucial for anti-PD-L1 antibody therapy but might play a role in some tumor models.

Sustained cross-presentation capacity of murine splenic dendritic cell subsets in vivo.

An exclusive feature of dendritic cells (DCs) is their ability to cross-present exogenous antigens in MHC class I molecules. We analyzed the fate of protein antigen in antigen presenting cell (APC) subsets after uptake of naturally formed antigen-antibody complexes in vivo. We observed that murine splenic DC subsets were able to present antigen in vivo for at least a week. After ex vivo isolation of four APC subsets, the presence of antigen in the storage compartments was visualized by confocal microscopy. Although all APC subsets stored antigen for many days, their ability and kinetics in antigen presentation was remarkably different. CD8α+ DCs showed sustained MHC class I-peptide specific CD8+ T-cell activation for more than 4 days. CD8α- DCs also presented antigenic peptides in MHC class I but presentation decreased after 48 h. In contrast, only the CD8α- DCs were able to present antigen in MHC class II to specific CD4+ T cells. Plasmacytoid DCs and macrophages were unable to activate any of the two T-cell types despite detectable antigen uptake. These results indicate that naturally occurring DC subsets have functional antigen storage capacity for prolonged T-cell activation and have distinct roles in antigen presentation to specific T cells in vivo.

C1q-Dependent Dendritic Cell Cross-Presentation of In Vivo-Formed Antigen-Antibody Complexes.

Dendritic cells (DCs) are specialized in Ag engulfment via a wide variety of uptake receptors on their cell surface. In the present study we investigated Ag uptake and presentation of in vivo-formed Ag-Ab complexes by i.v. injecting mice with Ag-specific Abs followed by the cognate Ag. We show by this natural Ab-mediated Ag targeting system that uptake by splenic APC subsets is severely hampered in mice lacking complement factor C1q (C1qa-/-). Moreover, no detectable Ag cross-presentation by CD8α+ DCs from C1qa-/- mice was found. On the contrary, Ag uptake was not hampered by APCs in FcγRI/II/III/IV-deficient (FcγR quadruple-/-) mice, and the cross-presentation ability of CD8α+ DCs was not affected. In conclusion, we show that C1q rather than FcγRs controls the Ab-mediated Ag uptake and its presentation by spleen APC subsets to T cells.

Dendritic cells process synthetic long peptides better than whole protein, improving antigen presentation and T-cell activation.

The efficiency of antigen (Ag) processing by dendritic cells (DCs) is vital for the strength of the ensuing T-cell responses. Previously, we and others have shown that in comparison to protein vaccines, vaccination with synthetic long peptides (SLPs) has shown more promising (pre-)clinical results. Here, we studied the unknown mechanisms underlying the observed vaccine efficacy of SLPs. We report an in vitro processing analysis of SLPs for MHC class I and class II presentation by murine DCs and human monocyte-derived DCs. Compared to protein, SLPs were rapidly and much more efficiently processed by DCs, resulting in an increased presentation to CD4⁺ and CD8⁺ T cells. The mechanism of access to MHC class I loading appeared to differ between the two forms of Ag. Whereas whole soluble protein Ag ended up largely in endolysosomes, SLPs were detected very rapidly outside the endolysosomes after internalization by DCs, followed by proteasome- and transporter associated with Ag processing-dependent MHC class I presentation. Compared to the slower processing route taken by whole protein Ags, our results indicate that the efficient internalization of SLPs, accomplished by DCs but not by B or T cells and characterized by a different and faster intracellular routing, leads to enhanced CD8⁺ T-cell activation.

Fcγ receptor IIb strongly regulates Fcγ receptor-facilitated T cell activation by dendritic cells.

FcγR ligation by Ag-Ab immune complexes (IC) not only mediates effective Ag uptake, but also strongly initiates dendritic cell (DC) maturation, a requirement for effective T cell activation. Besides the activating FcγRI, FcγRIII, and FcγRIV, the inhibitory FcγRIIb is expressed on DCs. It is unclear how the ratio between signals from the activating FcγR and the inhibitory FcγRIIb determines the outcome of FcγR ligation on DCs. By microarray analysis, we compared the transcriptomes of steady state and IC-activated bone marrow-derived wild-type (WT) DCs expressing all FcγR or DCs expressing only activating FcγR (FcγRIIb knockout [KO]) or only the inhibitory FcγRIIb (FcR γ-chain KO). In WT DCs, we observed a gene expression profile associated with effective T cell activation, which was absent in FcR γ-chain KO, but strikingly more pronounced in FcγRIIb KO bone marrow-derived DCs. These microarray results, confirmed at the protein level for many cytokines and other immunological relevant genes, demonstrate that the transcriptome of IC-activated DCs is dependent on the presence of the activating FcγR and that the modulation of the expression of the majority of the genes was strongly regulated by FcγRIIb. Our data suggest that FcγRIIb-deficient DCs have an improved capacity to activate naive T lymphocytes. This was confirmed by their enhanced FcγR-dependent Ag presentation and in vivo induction of CD8(+) T cell expansion compared with WT DCs. Our findings underscore the potency of FcγR ligation on DCs for the effective induction of T cell immunity by ICs and the strong regulatory role of FcγRIIb.

Neoantigen-specific T cell help outperforms non-specific help in multi-antigen DNA vaccination against cancer.

CD4+ T helper antigens are essential components of cancer vaccines, but the relevance of the source of these MHC class II-restricted antigens remains underexplored. To compare the effectiveness of tumor-specific versus tumor-unrelated helper antigens, we designed three DNA vaccines for the murine MC-38 colon carcinoma, encoding CD8+ T cell neoantigens alone (noHELP) or in combination with either "universal" helper antigens (uniHELP) or helper neoantigens (neoHELP). Both types of helped vaccines increased the frequency of vaccine-induced CD8+ T cells, and particularly uniHELP increased the fraction of KLRG1+ and PD-1low effector cells. However, when mice were subsequently injected with MC-38 cells, only neoHELP vaccination resulted in significantly better tumor control than noHELP. In contrast to uniHELP, neoHELP-induced tumor control was dependent on the presence of CD4+ T cells, while both vaccines relied on CD8+ T cells. In line with this, neoHELP variants containing wild-type counterparts of the CD4+ or CD8+ T cell neoantigens displayed reduced tumor control. These data indicate that optimal personalized cancer vaccines should include MHC class II-restricted neoantigens to elicit tumor-specific CD4+ T cell help.

Circulating specific antibodies enhance systemic cross-priming by delivery of complexed antigen to dendritic cells in vivo.

Increasing evidence suggests that antibodies can have stimulatory effects on T-cell immunity. However, the contribution of circulating antigen-specific antibodies on MHC class I cross-priming in vivo has not been conclusively established. Here, we defined the role of circulating antibodies in cross-presentation of antigen to CD8(+) T cells. Mice with hapten-specific circulating antibodies, but naϊve for the T-cell antigen, were infused with haptenated antigen and CD8(+) T-cell induction was measured. Mice with circulating hapten-specific antibodies showed significantly enhanced cross-presentation of the injected antigen compared with mice that lacked these antibodies. The enhanced cross-presentation in mice with circulating antigen-specific antibodies was associated with improved antigen capture by APCs. Importantly, CD11c(+) APCs were responsible for the enhanced and sustained cross-presentation, although CD11c(-) APCs had initially captured a significant amount of the injected antigen. Thus, in vivo formation of antigen-antibody immune complexes improves MHC class I cross-presentation, and CD8(+) T-cell activation, demonstrating that humoral immunity can aid the initiation of systemic cellular immunity. These findings have important implications for the understanding of the action of therapeutic antibodies against tumor-associated antigens intensively used in the clinic nowadays.

Efficient ex vivo induction of T cells with potent anti-tumor activity by protein antigen encapsulated in nanoparticles.

Protein antigen (Ag)-based immunotherapies have the advantage to induce T cells with a potentially broad repertoire of specificities. However, soluble protein Ag is generally poorly cross-presented in MHC class I molecules and not efficient in inducing robust cytotoxic CD8(+) T cell responses. In the present study, we have applied poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NP) which strongly improve protein Ag presentation by dendritic cells (DC) in the absence of additional Toll-like receptor ligands or targeting devices. Protein Ag-loaded DC were used as antigen presenting cells to stimulate T cells in vitro and subsequently analyzed in vivo for their anti-tumor effect via adoptive transfer, a treatment strategy widely studied in clinical trials as a therapy against various malignancies. In a direct comparison with soluble protein Ag, we show that DC presentation of protein encapsulated in plain PLGA-NP results in efficient activation of CD4(+) and CD8(+) T cells as reflected by high numbers of activated CD69(+) and CD25(+), interferon (IFN)-γ and interleukin (IL)-2-producing T cells. Adoptive transfer of PLGA-NP-activated CD8(+) T cells in tumor-bearing mice displayed good in vivo expansion capacity, potent Ag-specific cytotoxicity and IFN-γ cytokine production, resulting in curing mice with established tumors. We conclude that delivery of protein Ag through encapsulation in plain PLGA-NP is a very efficient and simple procedure to stimulate potent anti-tumor T cells.

The inhibiting Fc receptor for IgG, FcγRIIB, is a modifier of autoimmune susceptibility.

FcγRIIB-deficient mice generated in 129 background (FcγRIIB(129)(-/-)) if back-crossed into C57BL/6 background exhibit a hyperactive phenotype and develop lethal lupus. Both in mice and humans, the Fcγr2b gene is located within a genomic interval on chromosome 1 associated with lupus susceptibility. In mice, the 129-derived haplotype of this interval, named Sle16, causes loss of self-tolerance in the context of the B6 genome, hampering the analysis of the specific contribution of FcγRIIB deficiency to the development of lupus in FcγRIIB(129)(-/-) mice. Moreover, in humans genetic linkage studies revealed contradictory results regarding the association of "loss of function" mutations in the Fcγr2b gene and susceptibility to systemic lupus erythematosis. In this study, we demonstrate that FcγRIIB(-/-) mice generated by gene targeting in B6-derived ES cells (FcγRIIB(B6)(-/-)), lacking the 129-derived flanking Sle16 region, exhibit a hyperactive phenotype but fail to develop lupus indicating that in FcγRIIB(129)(-/-) mice, not FcγRIIB deficiency but epistatic interactions between the C57BL/6 genome and the 129-derived Fcγr2b flanking region cause loss of tolerance. The contribution to the development of autoimmune disease by the resulting autoreactive B cells is amplified by the absence of FcγRIIB, culminating in lethal lupus. In the presence of the Yaa lupus-susceptibility locus, FcγRIIB(B6)(-/-) mice do develop lethal lupus, confirming that FcγRIIB deficiency only amplifies spontaneous autoimmunity determined by other loci.

Selective cytotoxic T-lymphocyte targeting of tumor immune escape variants.

Defects in major histocompatibility complex (MHC) class I-restricted antigen presentation are frequently observed in human cancers and result in escape of tumors from cytotoxic T lymphocyte (CTL) immune surveillance in mice. Here, we show the existence of a unique category of CTLs that can prevent this escape. The CTLs target an alternative repertoire of peptide epitopes that emerge in MHC class I at the surface of cells with impaired function of transporter associated with antigen processing (TAP), tapasin or the proteasome. These peptides, although derived from self antigens such as the commonly expressed Lass5 protein (also known as Trh4), are not presented by normal cells. This explains why they act as immunogenic neoantigens. The newly discovered epitopes can be exploited for immune intervention against processing-deficient tumors through adoptive T-cell transfer or peptide vaccination.

Genome-wide CRISPR screens define determinants of epithelial-mesenchymal transition mediated immune evasion by pancreatic cancer cells.

The genetic circuits that allow cancer cells to evade immune killing via epithelial mesenchymal plasticity remain poorly understood. Here, we showed that mesenchymal-like (Mes) KPC3 pancreatic cancer cells were more resistant to cytotoxic T lymphocyte (CTL)-mediated killing than the parental epithelial-like (Epi) cells and used parallel genome-wide CRISPR screens to assess the molecular underpinnings of this difference. Core CTL-evasion genes (such as IFN-γ pathway components) were clearly evident in both types. Moreover, we identified and validated multiple Mes-specific regulators of cytotoxicity, such as Egfr and Mfge8. Both genes were significantly higher expressed in Mes cancer cells, and their depletion sensitized Mes cancer cells to CTL-mediated killing. Notably, Mes cancer cells secreted more Mfge8 to inhibit proliferation of CD8+ T cells and production of IFN-γ and TNFα. Clinically, increased Egfr and Mfge8 expression was correlated with a worse prognosis. Thus, Mes cancer cells use Egfr-mediated intrinsic and Mfge8-mediated extrinsic mechanisms to facilitate immune escape from CD8+ T cells.

Effective combination of liposome-targeted chemotherapy and PD-L1 blockade of murine colon cancer.

Therapeutic cancer drug efficacy can be limited by insufficient tumor penetration, rapid clearance, systemic toxicity and (acquired) drug resistance. The poor therapeutic index due to inefficient drug penetration and rapid drug clearance and toxicity can be improved by using a liposomal platform. Drug resistance for instance against pemetrexed, can be reduced by combination with docetaxel. Here, we developed a specific liposomal formulation to simultaneously deliver docetaxel and pemetrexed to enhance efficacy and safety. Hydrophobic docetaxel and hydrophilic pemetrexed were co-encapsulated into pH-sensitive liposomes using a thin-film hydration method with high efficiency. The physicochemical properties, toxicity, and immunological effects of liposomes were examined in vitro. Biodistribution, anti-tumor efficacy, and systemic immune response were evaluated in vivo in combination with PD-L1 immune checkpoint therapy using two murine colon cancer models. In cellular experiments, the liposomes exhibited strong cytotoxicity and induced immunogenic cell death. In vivo, the treatment with the liposome-based drug combination inhibited tumor development and stimulated immune responses. Liposomal encapsulation significantly reduced systemic toxicity compared to the delivery of the free drug. Tumor control was strongly enhanced when combined with anti-PDL1 immunotherapy in immunocompetent mice carrying syngeneic MC38 or CT26 colon tumors. We showed that treatment with liposome-mediated chemotherapy of docetaxel and pemetrexed combined with anti-PD-L1 immunotherapy is a promising strategy for the treatment of colon cancers.

Multiantigen pan-sarbecovirus DNA vaccines generate protective T cell immune responses.

SARS-CoV-2 is the third zoonotic coronavirus to cause a major outbreak in humans in recent years, and many more SARS-like coronaviruses with pandemic potential are circulating in several animal species. Vaccines inducing T cell immunity against broadly conserved viral antigens may protect against hospitalization and death caused by outbreaks of such viruses. We report the design and preclinical testing of 2 T cell-based pan-sarbecovirus vaccines, based on conserved regions within viral proteins of sarbecovirus isolates of human and other carrier animals, like bats and pangolins. One vaccine (CoVAX_ORF1ab) encoded antigens derived from nonstructural proteins, and the other (CoVAX_MNS) encoded antigens from structural proteins. Both multiantigen DNA vaccines contained a large set of antigens shared across sarbecoviruses and were rich in predicted and experimentally validated human T cell epitopes. In mice, the multiantigen vaccines generated both CD8+ and CD4+ T cell responses to shared epitopes. Upon encounter of full-length spike antigen, CoVAX_MNS-induced CD4+ T cells were responsible for accelerated CD8+ T cell and IgG Ab responses specific to the incoming spike, irrespective of its sarbecovirus origin. Finally, both vaccines elicited partial protection against a lethal SARS-CoV-2 challenge in human angiotensin-converting enzyme 2-transgenic mice. These results support clinical testing of these universal sarbecovirus vaccines for pandemic preparedness.

OX40 agonism enhances PD-L1 checkpoint blockade by shifting the cytotoxic T cell differentiation spectrum.

Immune checkpoint therapy (ICT) has the power to eradicate cancer, but the mechanisms that determine effective therapy-induced immune responses are not fully understood. Here, using high-dimensional single-cell profiling, we interrogate whether the landscape of T cell states in the peripheral blood predict responses to combinatorial targeting of the OX40 costimulatory and PD-1 inhibitory pathways. Single-cell RNA sequencing and mass cytometry expose systemic and dynamic activation states of therapy-responsive CD4+ and CD8+ T cells in tumor-bearing mice with expression of distinct natural killer (NK) cell receptors, granzymes, and chemokines/chemokine receptors. Moreover, similar NK cell receptor-expressing CD8+ T cells are also detected in the blood of immunotherapy-responsive cancer patients. Targeting the NK cell and chemokine receptors in tumor-bearing mice shows the functional importance of these receptors for therapy-induced anti-tumor immunity. These findings provide a better understanding of ICT and highlight the use and targeting of dynamic biomarkers on T cells to improve cancer immunotherapy.

Antigen storage compartments in mature dendritic cells facilitate prolonged cytotoxic T lymphocyte cross-priming capacity.

Dendritic cells (DCs) are crucial for priming of naive CD8(+) T lymphocytes to exogenous antigens, so-called "cross-priming." We report that exogenous protein antigen can be conserved for several days in mature DCs, coinciding with strong cytotoxic T lymphocyte cross-priming potency in vivo. After MHC class I peptide elution, protein antigen-derived peptide presentation is efficiently restored, indicating the presence of an intracellular antigen depot. We characterized this depot as a lysosome-like organelle, distinct from MHC class II compartments and recently described early endosomal compartments that allow acute antigen presentation in MHC class I. The storage compartments we report here facilitate continuous supply of MHC class I ligands. This mechanism ensures sustained cross-presentation by DCs, despite the short-lived expression of MHC class I-peptide complexes at the cell surface.

Pyroptosis-inducing active caspase-1 as a genetic adjuvant in anti-cancer DNA vaccination.

Pyroptosis is a recently discovered form of inflammatory programmed necrosis characterized by caspase-1-mediated and gasdermin D-dependent cell death leading to the release of pro-inflammatory cytokines such as Interleukin-1 beta (IL-1ß). Here, we evaluated whether pyroptosis could be exploited in DNA vaccination by incorporating a constitutively active variant of caspase-1 to the antigen-expressing DNA. In vitro, transfection with constitutively active caspase-1 DNA induced pro-IL-1ß maturation and IL-1ß release as well as gasdermin D-dependent cell death. To test active caspase-1 as a genetic adjuvant for the induction of antigen-specific T cell responses, mice were vaccinated intradermally with a DNA vaccine consisting of the active caspase-1 plasmid together with a plasmid encoding an ovalbumin-derived CD8 T cell epitope. Active caspase-1 accelerated and amplified antigen-specific CD8 T cell responses when administered simultaneously with the DNA vaccine at an equimolar dose. Moreover, upon challenge with melanoma cells expressing ovalbumin, mice vaccinated with the antigen vaccine adjuvanted with active caspase-1 showed significantly better survival compared to the non-adjuvanted group. In conclusion, we have developed a novel genetic adjuvant that for the first time employs the pyroptosis pathway to improve DNA vaccination against cancer.