Endogenously Expressed Antigens Bind Mammalian RNA via Cationic Domains that Enhance Priming of Effector CD8 T Cells by DNA Vaccination.

Hepatitis B virus (HBV) core (HBV-C) antigens with homologous or heterologous HIV-tat48-57-like (HBV-C149tat) cationic domains non-specifically bind cellular RNA in vector-transfected cells. Here, we investigated whether RNA-binding to cationic domains influences the immunogenicity of endogenously expressed antigens delivered by DNA vaccination. We initially evaluated induction of HBV-C (Kb/C93)-specific CD8+ T cell responses in C57BL/6J (B6) and 1.4HBV-Smut transgenic (tg) mice that harbor a replicating HBV genome in hepatocytes by DNA immunization. RNA-binding HBV-C and HBV-C149tat antigens moderately enhanced Kb/C93-specific CD8+ T cells in B6 mice as compared with RNA-free HBV-C149 antigen (lacking cationic domains). However, only the RNA-binding antigens elicited Kb/C93-specific CD8+ T cells that inhibited HBV replication in 1.4HBV-Smut tg mice. Moreover, RNA-binding to designer antigens, which express a Kb/p15E epitope from an endogenous murine leukemia virus-derived tumor-specific gp70 protein, was crucial to prime tumor-rejecting effector CD8+ T cells in B6 mice. Antigen-bound endogenous RNAs function as a Toll-like receptor 7 (TLR-7) ligand and stimulated priming of Kb/p15E-specific CD8+ T cells in B6, but not TLR-7-/-, mice. Antigen-bound cellular RNAs thus function as an endogenous natural adjuvant in in vivo vector-transfected cells, and thus are an attractive tool to induce and/or enhance effector CD8+ T cell responses directed against chronic viral infections or tumor self-antigens by DNA vaccination.

Differential presentation of endogenous and exogenous hepatitis B surface antigens influences priming of CD8(+) T cells in an epitope-specific manner.

Little is known about whether presentation of endogenous and exogenous hepatitis B virus (HBV) surface antigens on APCs targeted by vaccination and/or virus-harboring hepatocytes influences de novo priming of CD8(+) T cells. We showed that surface antigen-expressing transfectants exclusively display a K(b) /S190 epitope, whereas cells pulsed with recombinant surface particles (rSPs) exclusively present a K(b) /S208 epitope to CD8(+) T cells. The differential presentation of these epitopes largely reflects the selective, but not exclusive, priming of K(b) /S190- and K(b) /S208-specific T cells in C57BL/6 mice by endogenous/DNA- or exogenous/protein-based vaccines, respectively. Silencing the K(b) /S190 epitope (K(b) /S190V194F ) in antigen-expressing vectors rescued the presentation of the K(b) /S208 epitope in stable transfectants and significantly enhanced priming of K(b) /S208-specific T cells in C57BL/6 mice. A K(b) /S190-mediated immunodominance operating in surface antigen-expressing cells, but not in rSP-pulsed cells, led to an efficient suppression in the presentation of the K(b) /S208 epitope and a consequent decrease in the priming of K(b) /S208-specific T cells. This K(b) /S190-mediated immunodominance also operated in 1.4HBV-S(mut) transgenic (tg) hepatocytes selectively expressing endogenous surface antigens and allowed priming of K(b) /S208- but not K(b) /S190-specific T cells in 1.4HBV-S(mut) tg mice. However, IFN-γ(+) K(b) /S208-specific T cells could not inhibit HBV replication in the liver of 1.4HBV-S(mut) tg mice. These results have practical implications for the design of T-cell-stimulating therapeutic vaccines.

IFN-γ treatment protocol for MHC-Ilo/PD-L1+ pancreatic tumor cells selectively restores their TAP-mediated presentation competence and CD8 T-cell priming potential.

BACKGROUND: Many cancer cells express a major histocompatibility complex class I low/ programmed cell death 1 ligand 1 positive (MHC-Ilo/PD-L1+) cell surface profile. For immunotherapy, there is, thus, an urgent need to restore presentation competence of cancer cells with defects in MHC-I processing/presentation combined with immune interventions that tackle the tumor-initiated PD-L1/PD-1 signaling axis. Using pancreatic ductal adenocarcinoma cells (PDACCs) as a model, we here explored if (and how) expression/processing of tumor antigens via transporters associated with antigen processing (TAP) affects priming of CD8 T cells in PD-1/PD-L1-competent/-deficient mice. METHODS: We generated tumor antigen-expressing vectors, immunized TAP-competent/-deficient mice and determined de novo primed CD8 T-cell frequencies by flow cytometry. Similarly, we explored the antigenicity and PD-L1/PD-1 sensitivity of PDACCs versus interferon-γ (IFN-γ)-treated PDACCs in PD-1/PD-L1-competent/deficient mice. The IFN-γ-induced effects on gene and cell surface expression profiles were determined by microarrays and flow cytometry. RESULTS: We identified two antigens (cripto-1 and an endogenous leukemia virus-derived gp70) that were expressed in the Endoplasmic Reticulum (ER) of PDACCs and induced CD8 T-cell responses either independent (Cripto-1:Kb/Cr16-24) or dependent (gp70:Kb/p15E) on TAP by DNA immunization. IFN-γ-treatment of PDACCs in vitro upregulated MHC-I- and TAP- but also PD-L1-expression. Mechanistically, PD-L1/PD-1 signaling was superior to the reconstitution of MHC-I presentation competence, as subcutaneously transplanted IFN-γ-treated PDACCs developed tumors in C57BL/6J and PD-L1-/- but not in PD-1-/- mice. Using PDACCs, irradiated at day 3 post-IFN-γ-treatment or PD-L1 knockout PDACCs as vaccines, we could selectively bypass upregulation of PD-L1, preferentially induce TAP-dependent gp70:Kb/p15E-specific CD8 T cells associated with a weakened PD-1+ exhaustion phenotype and reject consecutively injected tumor transplants in C57BL/6J mice. CONCLUSIONS: The IFN-γ-treatment protocol is attractive for cell-based immunotherapies, because it restores TAP-dependent antigen processing in cancer cells, facilitates priming of TAP-dependent effector CD8 T-cell responses without additional check point inhibitors and could be combined with genetic vaccines that complement priming of TAP-independent CD8 T cells.

A tumor-specific neoepitope expressed in homologous/self or heterologous/viral antigens induced comparable effector CD8+ T-cell responses by DNA vaccination.

Somatic mutations in tumors often generate neoproteins that contain MHC-I-binding neoepitopes. Little is known if and how efficient tumor-specific neoantigens activate CD8+ T cells. Here, we asked whether a de novo generated neoepitope, encoded either within an otherwise conserved and ubiquitously expressed self-antigen or in a chimeric HBV core antigen expression platform, providing heterologous helper functions, induces CD8+ T cells in C57Bl/6J mice by DNA immunization. For it, we chose an established Db/Sp244-252/R251H neoepitope generated in the murine Endophilin-B2/SH3GLB2 (EndoB2-Sp) protein by a single amino acid exchange. We showed that a single injection of EndoB2-Sp expression vectors efficiently primed dimer/pentamer+, IFN-γ+ and cytolytic Db/Sp244-252/R251H-specific effector CD8+ T cells in C57Bl/6J mice. Priming of Db/Sp244-252/R251H-specific CD8+ T cells proceeded independent from CD4+ T-cell help in MHC-II-deficient Aα-/- mice. As compared to the homologous EndoB2-Sp vaccine, the selective expression of the Db/Sp244-252/R251H neoepitope in chimeric particle-forming and assembly-deficient HBV core antigens induced comparable frequencies Db/Sp244-252/R251H-specific CD8+ T cells with the same cytolytic effector phenotype. The homologous EndoB2 carrier, but not the nine-residue neoepitope presented on chimeric HBV core particles, induced EndoB2-specific IgG antibody responses. The HBV core expression platform is thus an attractive option to selectively induce neoepitope-specific effector CD8+ T cells by DNA vaccination. These novel findings have practical implications for the design of heterologous/self and heterologous/viral cancer vaccines that prime and/or activate neoepitope-specific CD8+ T cells.

Preproinsulin Designer Antigens Excluded from Endoplasmic Reticulum Suppressed Diabetes Development in NOD Mice by DNA Vaccination.

DNA vaccines against autoimmune type 1 diabetes (T1D) contain a nonpredictable risk to induce autoreactive T cell responses rather than a protective immunity. Little is known if (and how) antigen expression and processing requirements favor the induction of autoreactive or protective immune responses by DNA immunization. Here, we analyzed whether structural properties of preproinsulin (ppins) variants and/or subcellular targeting of ppins designer antigens influence the priming of effector CD8+ T cell responses by DNA immunization. Primarily, we used H-2b RIP-B7.1 tg mice, expressing the co-stimulator molecule B7.1 in beta cells, to identify antigens that induce or fail to induce autoreactive ppins-specific (Kb/A12-21 and/or Kb/B22-29) CD8+ T cell responses. Female NOD mice, expressing the diabetes-susceptible H-2g7 haplotype, were used to test ppins variants for their potential to suppress spontaneous diabetes development. We showed that ppins antigens excluded from expression in the endoplasmic reticulum (ER) did not induce CD8+ T cells or autoimmune diabetes in RIP-B7.1 tg mice, but efficiently suppressed spontaneous diabetes development in NOD mice as well as ppins-induced CD8+ T cell-mediated autoimmune diabetes in PD-L1 -/- mice. The induction of a ppins-specific therapeutic immunity in mice has practical implications for the design of immune therapies against T1D in individuals expressing different major histocompatibility complex (MHC) I and II molecules.

Cationic domains in particle-forming and assembly-deficient HBV core antigens capture mammalian RNA that stimulates Th1-biased antibody responses by DNA vaccination.

The HBV core protein self-assembles into particles and encapsidates immune-stimulatory bacterial RNA through a cationic COOH-terminal (C150-183) domain. To investigate if different cationic domains have an impact on the endogenous RNA-binding of HBV-C antigens in mammalian cells, we developed a strep-tag (st) based expression/purification system for HBV-C/RNA antigens in vector-transfected HEK-293 cells. We showed that HBV-stC but not HBV-stC149 particles (lacking the cationic domain) capture low amounts of mammalian RNA. Prevention of specific phosphorylation in cationic domains, either by exchanging the serine residues S155, S162 and S170 with alanines (HBV-stCAAA) or by exchanging the entire cationic domain with a HIV-tat48-57-like sequence (HBV-stC149tat) enhanced the encapsidation of RNA into mutant core particles. Particle-bound mammalian RNA functioned as TLR-7 ligand and induced a Th1-biased humoral immunity in B6 but not in TLR-7-/- mice by exogenous (protein) and endogenous (DNA) vaccines. Compared to core particles, binding of mammalian RNA to freely exposed cationic domains in assembly-deficient antigens was enhanced. However, RNA bound to non-particulate antigens unleash its Th1-stimulating adjuvant activity by DNA- but not protein-based vaccination. Mammalian RNAs targeted by an endogenously expressed antigen thus function as a natural adjuvant in the host that facilitates priming of Th1-biased immune responses by DNA-based immunization.