Lymphocytic Choriomeningitis Virus Challenge Research Articles

Gene editing of hematopoietic stem cells restores T-cell response in familial hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disorder characterized by a life-threatening cytokine storm and immunopathology. Familial HLH type 3 (FHL3) accounts for approximately 30% of all inborn HLH cases worldwide. It is caused by mutations in the UNC13D gene that result in impaired degranulation of cytotoxic vesicles and hence compromised T-cell- and natural killer-cell-mediated killing. Current treatment protocols, including allogeneic hematopoietic stem cell (HSC) transplantation, still show high mortality. We sought to develop and evaluate a curative genome editing strategy in the preclinical FHL3 Jinx mouse model. Jinx mice harbor a cryptic splice donor site in Unc13d intron 26 and develop clinical symptoms of human FHL3 upon infection with lymphocytic choriomeningitis virus (LCMV). We employed clustered regularly interspaced short palindromic repeats (CRISPR)-Cas technology to delete the disease-causing mutation in HSCs and transplanted Unc13d-edited stem cells into busulfan-conditioned Jinx recipient mice. Safety studies included extensive genotyping and chromosomal aberrations analysis by single targeted linker-mediated PCR sequencing (CAST-Seq)-based off-target analyses. Cure from HLH predisposition was assessed by LCMV infection. Hematopoietic cells isolated from transplanted mice revealed efficient gene editing (>95%), polyclonality of the T-cell receptor repertoire, and neither signs of off-target effects nor leukemogenesis. Unc13d transcription levels of edited and wild-type cells were comparable. While LCMV challenge resulted in acute HLH in Jinx mice transplanted with mock-edited HSCs, Jinx mice grafted with Unc13d-edited cells showed rapid virus clearance and protection from HLH. Our study demonstrates that transplantation of CRISPR-Cas edited HSCs supports the development of a functional polyclonal T-cell response in the absence of genotoxicity-associated clonal outgrowth.

Abstract B121: Checkpoint inhibitor vaccine stimulates antitumor T-cells that block tumor growth

Abstract Checkpoint inhibitor therapy augments pre-existing T-cell responses to tumor antigens, resulting in tumor control in 20-40% of patients with melanoma and other cancers. As a means to increase the effectiveness of this approach, we developed a dendritic cell (DC)-based vaccine strategy that specifically induces anti-tumor T-cell responses and prevents checkpoint activation. The approach is based on lentiviral vectors that express checkpoint inhibitor and tumor antigen and transduce DCs at high efficiency as a result of virion-packaging of the lentiviral accessory protein Vpx. In addition, the vectors express CD40 ligand (CD40L), a cell surface protein that strongly activates DCs, causing the cells to mature and secrete immunostimulatory cytokines, including TNFα, IL-6, and IL-12, that potentiate T-cell responses. The therapy harnesses the ability of DCs to prime T-cell responses against tumors, resulting in long-term antigen expression and continuous, localized release of checkpoint inhibitor during antigen-presentation. In the lymphocytic choriomeningitis virus (LCMV) mouse model, immunization with DCs transduced with lentiviral vectors that express soluble programmed cell death protein 1 (PD-1), CD40L and LCMV epitopes induced a 10-fold expansion of LCMV-specific CD8 T-cells and protected the mice against high-titer LCMV and a lethal intracranial LCMV challenge. Protection was 100-fold greater than that achieved by vaccination with LCMV peptide epitope-pulsed DCs. The strategy also appeared to cure mice infected with clone 13 LCMV, a chronic form of disease that results from T-cell exhaustion induced by checkpoint activation. In the B16-OVA mouse melanoma model, immunization with DCs transduced with vectors that express soluble PD-1, CD40L and OVA epitopes or a tumor antigen epitope from tyrosinase-related protein (TRP-1) protected mice from tumor formation. DC vaccination post-B16 challenge reduced tumor growth and prolonged survival. Mice immunized four days after intravenous injection of B16 cells showed no evidence of lung melanoma nodules three weeks later. Remarkably, immunization by direct injection of lentiviral vector, a method that obviates the need to harvest patient DCs, proved equally effective as ex vivo DC transduction in suppressing tumor growth. In addition, direct injection resulted in a much longer half-life of transduced cells. These studies demonstrate the ability of this approach to induce antigen-specific responses and overcome CTL exhaustion. Moreover, the lentiviral vector approach provides a means to express tumor neoantigens for the development of personalized immunotherapies that stimulate tumor-specific T-cell responses by clones present at low level or to antigens that the host has not responded. Citation Format: Takuya Tada, Nathaniel R. Landau, Thomas David Norton. Checkpoint inhibitor vaccine stimulates antitumor T-cells that block tumor growth [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr B121.

Adenovirus Vector Vaccination Impacts NK Cell Rheostat Function following Lymphocytic Choriomeningitis Virus Infection.

ABSTRACTNatural killer (NK) cells respond rapidly as a first line of defense against infectious pathogens. In addition, NK cells may provide a “rheostat” function and have been shown to reduce the magnitude of antigen-specific T cell responses following infection to avoid immunopathology. However, it remains unknown whether NK cells similarly modulate vaccine-elicited T cell responses following virus challenge. We used the lymphocytic choriomeningitis virus (LCMV) clone 13 infection model to address whether NK cells regulate T cell responses in adenovirus vector-vaccinated mice following challenge. As expected, NK cell depletion in unvaccinated mice resulted in increased virus-specific CD4+ and CD8+ T cell responses and immunopathology following LCMV challenge. In contrast, NK cell depletion had minimal to no impact on antigen-specific T cell responses in mice that were vaccinated with an adenovirus serotype 5 (Ad5)-GP vector prior to LCMV challenge. Moreover, NK cell depletion in vaccinated mice prior to challenge did not result in immunopathology and did not compromise protective efficacy. These data suggest that adenovirus vaccine-elicited T cells may be less sensitive to NK cell rheostat regulation than T cells primed by LCMV infection.IMPORTANCE Recent data have shown that NK cell depletion leads to enhanced virus-elicited T cell responses that can result in severe immunopathology following LCMV infection in mice. In this study, we observed that NK cells exerted minimal to no impact on vaccine-elicited T cells following LCMV challenge, suggesting that adenovirus vaccine-elicited T cells may be less subject to NK cell regulation. These data contribute to our understanding of NK cell regulatory functions and T cell-based vaccines.

Replication-incompetent rabies virus vector harboring glycoprotein gene of lymphocytic choriomeningitis virus (LCMV) protects mice from LCMV challenge.

BackgroundLymphocytic choriomeningitis virus (LCMV) causes a variety of diseases, including asymptomatic infections, meningitis, and congenital infections in the fetus of infected mother. The development of a safe and effective vaccine against LCMV is imperative. This study aims to develop a new candidate vaccine against LCMV using a recombinant replication-incompetent rabies virus (RV) vector.Methodology/Principal findingsIn this study, we have generated a recombinant deficient RV expressing the LCMV glycoprotein precursor (GPC) (RVΔP-LCMV/GPC) which is lacking the RV-P gene. RVΔP-LCMV/GPC is able to propagate only in cells expressing the RV-P protein. In contrast, the LCMV-GPC can be expressed in general cells, which do not express RV-P protein. The ability of RVΔP-LCMV/GPC to protect mice from LCMV infection and induce cellular immunity was assessed. Mice inoculated intraperitoneally with RVΔP-LCMV/GPC showed higher survival rates (88.2%) than those inoculated with the parental recombinant RV-P gene-deficient RV (RVΔP) (7.7%) following a LCMV challenge. Neutralizing antibody (NAb) against LCMV was not induced, even in the sera of surviving mice. CD8+ T-cell depletion significantly reduced the survival rates of RVΔP-LCMV/GPC-inoculated mice after the LCMV challenge. These results suggest that CD8+ T cells play a major role in the observed protection against LCMV. In contrast, NAbs against RV were strongly induced in sera of mice inoculated with either RVΔP-LCMV/GPC or RVΔP. In safety tests, suckling mice inoculated intracerebrally with RVΔP-LCMV/GPC showed no symptoms.Conclusions/SignificanceThese results show RVΔP-LCMV/GPC might be a promising candidate vaccine with dual efficacy, protecting against both RV and LCMV.

High Level of Perforin Expression Is Required for Effective Correction of Hemophagocytic Lymphohistiocytosis.

Perforin-1 mutations result in a potentially fatal hemophagocytic lymphohistiocytosis (HLH) with heightened immune activation, hypercytokinemia, pancytopenia, and end-organ damage. At present, hematopoietic stem cell (HSC) transplantation is curative, but limited by donor availability and associated mortality, making gene therapy an attractive alternative approach for HLH. We reported that perforin expression driven by cellular promoters in lentiviral (LV) vectors resulted in significant, albeit partial, correction of the inflammatory features in a murine model of HLH. We hypothesized that the level of perforin expression achieved per cell from ectopic moderate-strength cellular promoters (phosphoglycerate kinase gene/perforin-1 gene) is inadequate and thus engineered an LV vector using a viral promoter (MND; a modified Moloney murine leukemia virus long terminal repeat with myeloproliferative sarcoma virus enhancer) containing microRNA126 target sequences to restrict perforin expression in HSCs. We show here that the MND-LV vector restored perforin expression to normal levels in a perforin-deficient human natural killer cell line and perforin gene-corrected Perforin1-/- transplant recipients, whereas cellular promoters drove only partial correction. On lymphocytic choriomeningitis virus challenge, the clinical scores and survival improved only with the MND-LV vector, but inflammatory markers and cytotoxicity were improved with all LV vectors. Our studies suggest that although moderate levels of expression can result in partial amelioration of the HLH phenotype, high levels of perforin expression per cell are required for complete correction of HLH.

237. Genetic Therapy for Perforin Deficiency Associated Hemophagocytic Lymphohistiocytosis Requires High Level Expression of the Perforin Gene for Adequate Correction

Hemophagocytic lymphohistiocytosis (HLH) is a potentially fatal immune regulatory disorder, whereby heightened immune activation following viral infections causes hypercytokinemia, pancytopenia and end-organ damage. The majority of familial HLH is caused by Perforin-1 (prf1) gene mutations. Perforin is expressed in CD8+ T cells and natural killer (NK) cells and is critical for lymphocyte cytotoxicity. Currently, immunosuppressive therapy followed by allogeneic hematopoietic stem cell (HSC) transplant is the only curative option for HLH, but is restricted by availability of matched donors and a high (20-50%) transplant related mortality. We posited that perforin gene transfer into autologous HSC may be curative, have lower mortality, and not be limited by donor availability. We recently reported significant, but partial, correction of HLH symptoms in prf1-/- mice using lentivirus vectors (LV) expressing perforin from a ubiquitous phosphoglycerate kinase (PGK) promoter or a tissue specific perforin gene promoter (PRF). We hypothesized that negative selection of high perforin expressing HSC and the perforin gene dosage delivered by the LV contributes to the partial correction. We redesigned and developed a series of LV and compared prf1 expression from the PGK/PRF promoters to that from the MND promoter/enhancer; additionally we restricted perforin expression from PGK and MND in HSC with 4 repeats of miR126 (4T) target sequence, a miRNA specifically expressed in HSCs but not in lymphocytes, thus, circumventing the negative selection of high perforin expressing HSC. Improved perforin expression and cytotoxicity was observed with the ‘HSC-restricted’ MND4TLV in the KHYG1 human NK cell line, and in prf1-/- P14 mice, transgenic for a T-cell receptor that recognizes a lymphocytic choriomeningitis virus (LCMV) glycoprotein. These LV were compared in prf1-/- mice that were challenged with LCMV following transplant of gene-modified HSC. High gene marked NK chimerism was achieved: 87.6±1.7%, 83.1±2.4% and 58.1±4.6% with the MND4T, PGK4T and PRF0T LV. Following LCMV challenge, γ-IFN levels in MND4T, PRF0T and PGK4T mice were 1.7±0.3, 1.5±0.5 and 5.9±2.1 ng/ml, respectively, compared to 1.7±0.5 ng/ml in prf1-/- mice receiving WT HSC. Similarly, the degree of anemia in WT and MND4T mice was comparable 15 days after LCMV challenge: hemoglobins declined by 4.3±0.3, 3.7±0.3, 5.6±0.3 and 5.8±0.7 g/dL in WT, MND4T, PGK4T and PRF0T mice, respectively. Most importantly, 66.7% of the mice in the MND4T group survived the LCMV challenge, while only 10% and no mice survived in PGK4T and PRF0T groups, suggesting that perforin expression from PGK4T and PRF0T were insufficient to prevent fatal HLH in the majority of animals. Overall, we show that perforin deficient HLH requires a robust perforin expression in cytotoxic T and NK cells for adequate correction of the disease phenotype.

Reduced type I interferon production by dendritic cells and weakened antiviral immunity in patients with Wiskott-Aldrich syndrome protein deficiency

BackgroundWiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency caused by absence of Wiskott-Aldrich syndrome protein (WASP) expression, resulting in defective function of many immune cell lineages and susceptibility to severe bacterial, viral, and fungal infections. Despite a significant proportion of patients with WAS having recurrent viral infections, surprisingly little is known about the effects of WASP deficiency on antiviral immunity.ObjectiveWe sought to evaluate the antiviral immune response in patients with WASP deficiency in vivo.MethodsViral clearance and associated immunopathology were measured after infection of WASP-deficient (WAS KO) mice with lymphocytic choriomeningitis virus (LCMV). Induction of antiviral CD8+ T-cell immunity and cytotoxicity was documented in WAS KO mice by means of temporal enumeration of total and antigen-specific T-cell numbers. Type I interferon (IFN-I) production was measured in serum in response to LCMV challenge and characterized in vivo by using IFN-I reporter mice crossed with WAS KO mice.ResultsWAS KO mice showed reduced viral clearance and enhanced immunopathology during LCMV infection. This was attributed to both an intrinsic CD8+ T-cell defect and defective priming of CD8+ T cells by dendritic cells (DCs). IFN-I production by WAS KO DCs was reduced both in vivo and in vitro.ConclusionsThese studies use a well-characterized model of persistence-prone viral infection to reveal a critical deficiency of CD8+ T-cell responses in murine WASP deficiency, in which abrogated production of IFN-I by DCs might play an important contributory role. These findings might help us to understand the immunodeficiency of WAS.

Memory T cells are uniquely resistant to melanoma-induced suppression

We have previously observed that in vivo exposure to growing melanoma tumors fundamentally alters activated T cell homeostasis by suppressing the ability of naïve T cells to undergo antigen-driven proliferative expansion. We hypothesized that exposure of T cells in later stages of differentiation to melanoma would have similar suppressive consequences. C57BL/6 mice were inoculated with media or syngeneic B16F10 melanoma tumors 8 or 60 days after infection with lymphocytic choriomeningitis virus (LCMV), and splenic populations of LCMV-specific T cells were quantified using flow cytometry 18 days after tumor inoculation. Inoculation with melanoma on post-infection day 8 potentiated the contraction of previously activated T cells. This enhanced contraction was associated with increased apoptotic susceptibility among T cells from tumor-bearing mice. In contrast, inoculation with melanoma on post-infection day 60 did not affect the ability of previously established memory T cells to maintain themselves in stable numbers. In addition, the ability of previously established memory T cells to respond to LCMV challenge was unaffected by melanoma. Following adoptive transfer into melanoma-bearing mice, tumor-specific memory T cells were significantly more effective at controlling melanoma growth than equivalent numbers of tumor-specific effector T cells. These observations suggest that memory T cells are uniquely resistant to suppressive influences exerted by melanoma on activated T cell homeostasis; these findings may have implications for T cell-based cancer immunotherapy.

Epitope specificity of memory CD8+ T cells dictates vaccination-induced mortality in LCMV-infected perforin-deficient mice.

Perforin-deficient (PKO) mice serve as models for familial hemophagocytic lympho-histiocytosis, a uniformly fatal disease associated with viral infection of perforin-deficient humans. Naïve perforin-deficient BALB/c mice survive while vaccinated PKO mice containing virus-specific memory CD8(+) T cells rapidly succumb to lymphocytic choriomeningitis virus (LCMV) infection. Thus, vaccination converts a nonlethal persistent infection into a fatal disease mediated by virus-specific memory CD8(+) T cells. Here, we determine the extent to which vaccination-induced mortality in PKO mice following LCMV challenge is due to differences in vaccine modalities, the quantity or epitope specificity of memory CD8(+) T cells. We show that LCMV-induced mortality in immune PKO mice is independent of vaccine modalities and that the starting number of memory CD8(+) T cells specific to the immunodominant epitope NP(118-126) dictates the magnitude of secondary CD8(+) T-cell expansion, the inability to regulate production of CD8(+) T-cell-derived IFN-γ, and mortality in the vaccinated PKO mice. Importantly, mortality is determined by the epitope specificity of memory CD8(+) T cells and the associated degree of functional exhaustion and cytokine dysregulation but not the absolute magnitude of CD8(+) T-cell expansion. These data suggest that deeper understanding of the parameters that influence the outcome of vaccine-induced diseases would aid rational vaccine design to minimize adverse outcomes after infection.

Quantifying Antigen-Specific CD4 T Cells during a Viral Infection: CD4 T Cell Responses Are Larger Than We Think

The number of virus-specific CD8 T cells increases substantially during an acute infection. Up to 90% of CD8 T cells are virus specific following lymphocytic choriomeningitis virus (LCMV) infection. In contrast, studies identifying virus-specific CD4 T cell epitopes have indicated that CD4 T cells often recognize a broader array of Ags than CD8 T cells, consequently making it difficult to accurately quantify the total magnitude of pathogen-specific CD4 T cell responses. In this study, we show that CD4 T cells become CD11a(hi)CD49d(+) after LCMV infection and retain this expression pattern into memory. During the effector phase, all the LCMV-specific IFN-γ(+) CD4 T cells display a CD11a(hi)CD49d(+) cell surface expression phenotype. In addition, only memory CD11a(hi)CD49d(+) CD4 T cells make IFN-γ after stimulation. Furthermore, upon secondary LCMV challenge, only CD11a(hi)CD49d(+) memory CD4 T cells from LCMV-immune mice undergo proliferative expansion, demonstrating that CD11a(hi)CD49d(+) CD4 T cells are truly Ag specific. Using the combination of CD11a and CD49d, we demonstrate that up to 50% of the CD4 T cells are virus specific during the peak of the LCMV response. Our results indicate that the magnitude of the virus-specific CD4 T cell response is much greater than previously recognized.

A highly optimized DNA vaccine confers complete protective immunity against high-dose lethal lymphocytic choriomeningitis virus challenge

Protection against infection is the hallmark of immunity and the basis of effective vaccination. For a variety of reasons there is a great demand to develop new, safer and more effective vaccine platforms. In this regard, while ‘first-generation’ DNA vaccines were poorly immunogenic, new genetic ‘optimization’ strategies and the application of in vivo electroporation (EP) have dramatically boosted their potency. We developed a highly optimized plasmid DNA vaccine that expresses the lymphocytic choriomeningitis virus (LCMV) nucleocapsid protein (NP) and evaluated it using the LCMV challenge model, a gold standard for studying infection and immunity. When administered intramuscularly with EP, robust NP-specific cellular and humoral immune responses were elicited, the magnitudes of which approached those following acute LCMV infection. Furthermore, these responses were capable of providing 100% protection against a high-dose, normally lethal virus challenge. This is the first non-infectious vaccine conferring complete protective immunity up to 8 weeks after vaccination and demonstrates the potential of ‘next-generation’ DNA vaccines.

Protection against autoimmune diabetes by viral infection

Type 1 Diabetes (T1D) is an autoimmune disease that results from the selective destruction of insulin‐producing beta cells in the pancreas. While it is generally believed that viral infections are responsible for inducing autoimmunity in individuals at risk, experimental evidence indicates that viruses can efficiently prevent T1D. We have identified two mechanisms by which acute lymphocytic choriomeningitis virus (LCMV) infection hampers autoimmune diabetes development in the mouse.The first mechanism is the direct consequence of the early release of inflammatory mediators following LCMV challenge which rapidly confer leukocytes from infected mice the capacity to diminish T1D. The second mechanism involves longer‐term modification of CD4+CD25+ regulatory T cells which become capable of preventing both spontaneous and virally induced T1D as a consequence of TGF‐beta production. Importantly, hampering of autoimmunity by viral infection does not appear to impair antiviral immunity and viral clearance.These findings provide novel mechanistic insight into the ability of viral infection to abort autoimmune diabetes. Induction of such mechanisms during infection may account for the capacity of viruses to prevent T1D as well as other autoimmune disorders.

Novel Plant Virus-Based Vaccine Induces Protective Cytotoxic T-Lymphocyte-Mediated Antiviral Immunity through Dendritic Cell Maturation

Currently used vaccines protect mainly through the production of neutralizing antibodies. However, antibodies confer little or no protection for a majority of chronic viral infections that require active involvement of cytotoxic T lymphocytes (CTLs). Virus-like particles (VLPs) have been shown to be efficient inducers of cell-mediated immune responses, but administration of an adjuvant is generally required. We recently reported the generation of a novel VLP system exploiting the self-assembly property of the papaya mosaic virus (PapMV) coat protein. We show here that uptake of PapMV-like particles by murine splenic dendritic cells (DCs) in vivo leads to their maturation, suggesting that they possess intrinsic adjuvant-like properties. DCs pulsed with PapMV-like particles displaying the lymphocytic choriomeningitis virus (LCMV) p33 immunodominant CTL epitope (PapMV-p33) efficiently process and cross-present the viral epitope to p33-specific transgenic T cells. Importantly, the CTL epitope is also properly processed and presented in vivo, since immunization of p33-specific T-cell receptor transgenic mice with PapMV-p33 induces the activation of large numbers of specific CTLs. C57BL/6 mice immunized with PapMV-p33 VLPs in the absence of adjuvant develop p33-specific effector CTLs that rapidly expand following LCMV challenge and protect vaccinated mice against LCMV infection in a dose-dependent manner. These results demonstrate the efficiency of this novel plant virus-based vaccination platform in inducing DC maturation leading to protective CTL responses.

The L Antigen of the LCM Virus is a Target of Dominant CD8+ Responses (43.47)

Abstract CD8+ T cell responses control lymphocytic choriomeningitis virus (LCMV) infection in H-2b mice. Although antigen-specific responses against LCMV infection are well studied, we found that up to one third of the CD8+CD44hi response to LCMV in H-2b mice was not accounted for by known epitopes. We screened peptides predicted to bind MHC Class I, and overlapping 15-mer peptides spanning the complete LCMV proteome for their capacity to induce interferon-γ (IFNγ) production from CD8+ T cells derived from LCMV-infected H-2b mice. We identified thirteen novel epitopes. These together with the 7 previously known epitopes account for the total CD8+CD44hi response, diminishing the possible role for bystander T cell activation. Strikingly, 9 out of the 13 new epitopes were derived from the viral L polymerase, which, until now, was not recognized as a target of the cellular response induced by LCMV infection. The L epitopes induced significant levels of in vivo cytotoxicity and conferred protection against LCMV challenge. Interestingly, protection from viral challenge was best correlated with the cytolytic potential of CD8+ T cells, whereas IFNγ production and peptide avidity appear to play a lesser role. Taken together, these findings illustrate that the LCMV-specific CD8+T cell response is more complex than previously appreciated.

Lentiviral Vectors Encoding HIV-1 Polyepitopes Induce Broad CTL Responses In Vivo

Lentiviral Vectors Encoding HIV-1 Polyepitopes Induce Broad CTL Responses In Vivo

The CD8+T-Cell Response to Lymphocytic Choriomeningitis Virus Involves the L Antigen: Uncovering New Tricks for an Old Virus

CD8(+) T-cell responses control lymphocytic choriomeningitis virus (LCMV) infection in H-2(b) mice. Although antigen-specific responses against LCMV infection are well studied, we found that a significant fraction of the CD8(+) CD44(hi) T-cell response to LCMV in H-2(b) mice was not accounted for by known epitopes. We screened peptides predicted to bind major histocompatibility complex class I and overlapping 15-mer peptides spanning the complete LCMV proteome for gamma interferon (IFN-gamma) induction from CD8(+) T cells derived from LCMV-infected H-2(b) mice. We identified 19 novel epitopes. Together with the 9 previously known, these epitopes account for the total CD8(+) CD44(hi) response. Thus, bystander T-cell activation does not contribute appreciably to the CD8(+) CD44(hi) pool. Strikingly, 15 of the 19 new epitopes were derived from the viral L polymerase, which, until now, was not recognized as a target of the cellular response induced by LCMV infection. The L epitopes induced significant levels of in vivo cytotoxicity and conferred protection against LCMV challenge. Interestingly, protection from viral challenge was best correlated with the cytolytic potential of CD8(+) T cells, whereas IFN-gamma production and peptide avidity appear to play a lesser role. Taken together, these findings illustrate that the LCMV-specific CD8(+) T-cell response is more complex than previously appreciated.

CD4(+) T cells induced by a DNA vaccine: immunological consequences of epitope-specific lysosomal targeting.

Our previous studies have shown that targeting DNA vaccine-encoded major histocompatibility complex class I epitopes to the proteasome enhanced CD8(+) T-cell induction and protection against lymphocytic choriomeningitis virus (LCMV) challenge. Here, we expand these studies to evaluate CD4(+) T-cell responses induced by DNA immunization and describe a system for targeting proteins and minigenes to lysosomes. Full-length proteins can be targeted to the lysosomal compartment by covalent attachment to the 20-amino-acid C-terminal tail of lysosomal integral membrane protein-II (LIMP-II). Using minigenes encoding defined T-helper epitopes from lymphocytic choriomeningitis virus, we show that the CD4(+) T-cell response induced by the NP(309-328) epitope of LCMV was greatly enhanced by addition of the LIMP-II tail. However, the immunological consequence of lysosomal targeting is not invariably positive; the CD4(+) T-cell response induced by the GP(61-80) epitope was almost abolished when attached to the LIMP-II tail. We identify the mechanism which underlies this marked difference in outcome. The GP(61-80) epitope is highly susceptible to cleavage by cathepsin D, an aspartic endopeptidase found almost exclusively in lysosomes. We show, using mass spectrometry, that the GP(61-80) peptide is cleaved between residues F(74) and K(75) and that this destroys its ability to stimulate virus-specific CD4(+) T cells. Thus, the immunological result of lysosomal targeting varies, depending upon the primary sequence of the encoded antigen. We analyze the effects of CD4(+) T-cell priming on the virus-specific antibody and CD8(+) T-cell responses which are mounted after virus infection and show that neither response appears to be accelerated or enhanced. Finally, we evaluate the protective benefits of CD4(+) T-cell vaccination in the LCMV model system; in contrast to DNA vaccine-induced CD8(+) T cells, which can confer solid protection against LCMV challenge, DNA vaccine-mediated priming of CD4(+) T cells does not appear to enhance the vaccinee's ability to combat viral challenge.

Enhancing T Cell Activation and Antiviral Protection by Introducing the HIV-1 Protein Transduction Domain into a DNA Vaccine

Protein transduction domains (PTD), which can transport proteins or peptides across biological membranes, have been identified in several proteins of viral, invertebrate, and vertebrate origin. Here, we evaluate the immunological and biological consequences of including PTD in synthetic peptides and in DNA vaccines that contain CD8(+) T cell epitopes from lymphocytic choriomeningitis virus (LCMV). Synthetic PTD-peptides did not induce detectable CD8(+) T cell responses. However, fusion of an open reading frame encoding a PTD to an epitope minigene caused transfected tissue culture cells to stimulate epitope-specific T cells much more effectively. Kinetic studies indicated that the epitope reached the surface of transfected cells more rapidly and that the number of transfected cells needed to stimulate T cell responses was reduced by 35- to 50-fold when compared to cells transfected with a standard minigene plasmid. The mechanism underlying the effect of PTD linkage is not clear, but transit of the PTD-attached epitope from transfected cells to nontransfected cells (cross presentation) seemed to play, at most, a minimal role. Mice immunized once with the plasmid encoding the PTD-linked epitope showed a markedly accelerated CD8(+) T cell response and, unlike mice immunized with a standard plasmid, were completely protected against a normally lethal LCMV challenge administered only 8 days post-immunization.

In vivo induction of a high-avidity, high-frequency cytotoxic T-lymphocyte response is associated with antiviral protective immunity.

Many approaches are currently being developed to deliver exogenous antigen into the major histocompatibility complex class I-restricted antigen pathway, leading to in vivo priming of CD8(+) cytotoxic T cells. One attractive possibility consists of targeting the antigen to phagocytic or macropinocytic antigen-presenting cells. In this study, we demonstrate that strong CD8(+) class I-restricted cytotoxic responses are induced upon intraperitoneal immunization of mice with different peptides, characterized as CD8(+) T-cell epitopes, bound to 1-microm synthetic latex microspheres and injected in the absence of adjuvant. The cytotoxic response induced against a lymphocytic choriomeningitis virus (LCMV) peptide linked to these microspheres was compared to the cytotoxic T-lymphocyte (CTL) response obtained upon immunization with the nonreplicative porcine parvovirus-like particles (PPV:VLP) carrying the same peptide (PPV:VLP-LCMV) previously described (C. Sedlik, M. F. Saron, J. Sarraseca, I. Casal, and C. Leclerc, Proc. Natl. Acad. Sci. USA 94:7503-7508, 1997). We show that the induction of specific CTL activity by peptides bound to microspheres requires CD4(+) T-cell help in contrast to the CTL response obtained with the peptide delivered by viral pseudoparticles. Furthermore, PPV:VLP are 100-fold more efficient than microspheres in generating a strong CTL response characterized by a high frequency of specific T cells of high avidity. Moreover, PPV:VLP-LCMV are able to protect mice against a lethal LCMV challenge whereas microspheres carrying the LCMV epitope fail to confer such protection. This study demonstrates the crucial involvement of the frequency and avidity of CTLs in conferring antiviral protective immunity and highlights the importance of considering these parameters when developing new vaccine strategies.

Plasmid DNA Vaccines Are Effective in the Absence of IFNγ

Intramuscular injection of bacterially derived plasmid DNA results in the development of both humoral and cellular immune responses against plasmid-encoded antigens. Immunostimulatory CpG sequences within bacterial DNA are thought to enhance this process by stimulating the secretion of proinflammatory cytokines such as interferon γ (IFNγ) by cells of the innate immune system. Although IFNγ induction by CpG elements within plasmid DNA has been documented in vitro and more recently in vivo, and coimmunization with plasmids expressing IFNγ has been shown to enhance DNA-immunization-induced immune responses, it is unclear if IFNγ is necessary for successful DNA immunization. To address this issue, we compared humoral and cellular immune responses in wild-type and IFNγ-deficient mice vaccinated with a plasmid (pCMVNP) expressing the nucleoprotein gene from the arenavirus lymphocytic choriomeningitis virus (LCMV). IFNγ-positive (BALB/c) and IFNγ-negative (GKO) mice responded to DNA vaccination by the development of antigen-specific CD8+ T cells, which were detectable directly ex vivo by intracellular cytokine staining and comprised 0.7–2.5% of all CD8+ T cells in the vaccinee. DNA vaccines also induced virus-specific cytotoxic T lymphocytes (CTL), even in the absence of IFNγ. DNA vaccination of both mouse strains also was associated with a significant reduction in viral titers after LCMV challenge, indicating that, at least in the presence of other immune effector mechanisms, IFNγ is not required for induction of protective anti-viral immunity by DNA immunization. No quantitative differences were observed in antiviral IgG levels among GKO and BALB/c vaccinees, although GKO mice did exhibit a significant reduction of the IgG2a:IgG1 ratio, in agreement with the previously documented requirement for IFNγ in isotype switching to IgG2a. Immunized BALB/c mice produced similar levels of both IgG1 and IgG2a, indicating a mixed Th1/Th2 response to intramuscular immunization with pCMVNP. These results show that IFNγ induction by bacterially derived plasmid DNA does not contribute to the magnitude of the antibody response and is not required for the induction or short-term maintenance of DNA-induced CTL. However, IFNγ is necessary for the development of IgG2a antibodies that may be crucial for protection against some pathogens.