Proceedings of the 2023 Viral Clearance Symposium, Session 2: Viral Clearance Strategy and Case Studies.

This session deals with the rational design of viral clearance studies for biopharmaceuticals including recombinant proteins such as monoclonal antibodies and, as new in scope of the symposium, also viral clearance for adeno-associated viral (AAV) vectors. For recombinant proteins, large datasets were accumulated over the last decades and are intended to be used for accelerated product process development and streamlining of viral clearance studies. How to utilize prior knowledge in viral clearance validation and how it can be used in a risk assessment tool to decide whether additional virus clearance studies are necessary during product development is being addressed by three of the presentations of this session. This also includes an a priori intended design and generation of validation data for a new kind of detergent such as CG-110, to build up a platform dataset to be used as prior knowledge in future marketing application. Another presentation investigates the virus removal mechanism of a newly developed hydrophobic interaction chromatography (HIC) resin and demonstrates for highly hydrophobic antibodies appropriate reduction for a retrovirus and impurities in a defined process range in contrast to the moderate to poor virus reduction of recent HIC resins. The last two presentations deal with virus clearance approaches for AAV, which will become mandatory with approval of the ICH Q5A revision. Appropriate virus removal and virus inactivation procedures can be implemented into the manufacturing processes of AAV vectors including viral filtration, viral inactivation (e.g., heat inactivation), affinity chromatography, and anion-exchange chromatography with which it seems possible to achieve a good clearance for helper and also adventitious viruses. The heat treatment step can be even a robust step for adenovirus helper inactivation for AAV products when product characteristics and process conditions are understood.

Proceedings of the 2019 Viral Clearance Symposium, Session 1: Viral Clearance Strategies and Case Studies.

Session 1 of the 2019 Viral Clearance Symposium proposed strategies to use prior knowledge for demonstrating robust viral clearance for biotechnologically produced recombinant proteins, such as monoclonal antibodies, and introduced methods for virus reduction applicable to recombinant adeno-associated virus (rAAV) vectors. Proposals for generic virus clearance claims were made for detergent inactivation with Triton CG-110, low pH inactivation, anion-exchange membrane chromatography, and virus filtration, and acceptance of these proposals were discussed. Furthermore, the development of an ASTM standard for retrovirus removal by small virus filters was presented, and it was found that updating and refining of conditions is still warranted. The current approach for log reduction value calculation by using the lower value from duplicate runs as worst case was questioned, and a modified concept was proposed. Finally, for rAAV vectors, several options for reduction of adventitious viruses were demonstrated such as 35 nm virus filtration, detergent inactivation, heat inactivation, and anion-exchange or affinity chromatography and were found to be feasible in order to satisfy the principles of ICH Q5A.

Proceedings of the 2019 Viral Clearance Symposium, Session 2: New Modalities in Chromatography and Adsorptive Filters.

In Session 2 of the 2019 Viral Clearance Symposium, new outcomes of viral clearance performance of polishing chromatography and depth filters utilized in the purification processes of biotechnologically produced recombinant proteins such as monoclonal antibodies were presented. It was shown that flow through both multimodal anion-exchange and anion-exchange chromatography are effective tools for removal of retroviruses and, under defined conditions, also for parvoviruses. Cation-exchange chromatography, when operated in bind/elute mode, hydrophobic interaction chromatography, and depth filtration can also be potent for virus removal, but are less robust for parvoviruses. Despite these data provide further understanding of virus removal mechanisms by chromatography and depth filtration operations, more investigation is required for a clear understanding of the molecular mechanisms of virus removal and the relevance of different moleculés interactions including resin, virus, and product.

Proceedings of the 2017 Viral Clearance Symposium, Session 4: Submission Strategies.

Appropriate performance of virus validation studies and testing of unprocessed bulk harvests for retrovirus particle count are procedures in the demonstration of an acceptable level of viral safety for cell-derived biotechnology products. Product-specific validation studies on virus reduction with two model viruses [usually murine leukemia virus (MuLV) and a parvovirus] performed in duplicate runs are standard for clinical trial applications. For the retroviral safety margin, a 6 log reduction is normally expected. Retroviral particle counts are measured traditionally by transmission electron microscopy (TEM) and are commonly performed at contract laboratories. These procedures are quite time-consuming and can be associated with significant costs. In particular, the time factor is a hurdle for companies that want to quickly bring their new products to the clinic. In this session, several strategies on how to lower time, cost, and workload in the evaluation of viral safety for early clinical trial applications, while still ensuring sufficient level of viral safety of the product, were presented. In addition, virus reduction strategies for molecules that do not have the standard antibody structure are presented. Also presented in this session is the feasibility of the use of retrovirus-like particle (RVLP) in the prevalidation of virus removal and the use of quantitative polymerase chain reaction (qPCR) as an alternative to infectivity assays in virus validation studies as well as its use as an alternative to quantitative TEM analysis for determining RVLP count in the bulk harvest of a perfusion bioreactor.LAY ABSTRACT: In this session, several strategies on how to lower time, cost, and workload in the evaluation of viral safety for early clinical trial applications of cell-derived biotechnology products, while still ensuring sufficient level of viral safety of the product, were presented. In addition, virus reduction strategies for molecules that do not have the standard antibody structure are presented. Also presented in this session is the feasibility of the use of retrovirus-like particle (RVLP) in the prevalidation of virus removal and the use of quantitative polymerase chain reaction (qPCR) as an alternative to infectivity assays in virus validation studies as well as its use as an alternative to quantitative TEM analysis for determining RVLP count in the bulk harvest of a perfusion bioreactor.

Interleukin-1ß receptor expressed by modified vaccinia virus Ankara interferes with interleukin-1ß activity produced in various virus-infected antigen-presenting cells.

BACKGROUND: Modified vaccinia virus Ankara (MVA) is a highly attenuated virus and a promising vaccine vector with potent immune stimulating properties. Deletion of the gene encoding the viral interleukin-1beta receptor (vIL-1ßR) in MVA (MVAΔIL-1ßR) was previously shown to enhance memory T cell function. Here, we investigated the influence of vIL-1ßR on blocking interleukin-1beta (IL-1ß) upon MVA infection in various antigen presenting cells of murine and human origin, and analyzed whether inflammasome function contributes to IL-1ß production in different cell types. FINDINGS: Extending previous studies, immunizing mice with low doses of MVAΔIL-1ßR still showed enhanced memory CD8(+) T cell activation compared to MVA wild-type (MVAwt) immunization. In vitro, murine myeloid dendritic cells, and activated, but not naive primary macrophages were identified as potent producers of IL-1ß upon infection with MVA. Importantly, free IL-1ß was only detected in the absence of vIL-1ßR. Moreover, MVAΔIL-1ßR increased amounts of bioactive IL-1ß compared to MVAwt after infection of human THP-1 cells, as detected using a reporter system that only responds to active and free IL-1ß. The MVA-mediated induction of IL-1ß was confirmed to depend on inflammasome function in human and murine cells, however in murine cells this apparently involves caspase-1-independent pathways. CONCLUSIONS: MVA lacking IL-1ß blocking activity leads to increased concentrations of free IL-1ß upon infection of murine and human antigen presenting cells; this is likely responsible for enhanced memory T cell activation upon MVAΔIL-1ßR immunization of mice. Moreover, our results suggest that MVA-mediated IL-1ß induction is a multifactorial process.

Cytosolic DNA triggers mitochondrial apoptosis via DNA damage signaling proteins independently of AIM2 and RNA polymerase III.

A key host response to limit microbial spread is the induction of cell death when foreign nucleic acids are sensed within infected cells. In mouse macrophages, transfected DNA or infection with modified vaccinia virus Ankara (MVA) can trigger cell death via the absent in melanoma 2 (AIM2) inflammasome. In this article, we show that nonmyeloid human cell types lacking a functional AIM2 inflammasome still die in response to cytosolic delivery of different DNAs or infection with MVA. This cell death induced by foreign DNA is independent of caspase-8 and carries features of mitochondrial apoptosis: dependence on BAX, APAF-1, and caspase-9. Although it does not require the IFN pathway known to be triggered by infection with MVA or transfected DNA via polymerase III and retinoid acid-induced gene I-like helicases, it shows a strong dependence on components of the DNA damage signaling pathway: cytosolic delivery of DNA or infection with MVA leads to phosphorylation of p53 (serines 15 and 46) and autophosphorylation of ataxia telangiectasia mutated (ATM); depleting p53 or ATM with small interfering RNA or inhibiting the ATM/ATM-related kinase family by caffeine strongly reduces apoptosis. Taken together, our findings suggest that a pathway activating DNA damage signaling plays an important independent role in detecting intracellular foreign DNA, thereby complementing the induction of IFN and activation of the AIM2 inflammasome.

N1L is an ectromelia virus virulence factor and essential for in vivo spread upon respiratory infection.

The emergence of zoonotic orthopoxvirus infections and the threat of possible intentional release of pathogenic orthopoxviruses have stimulated renewed interest in understanding orthopoxvirus infections and the resulting diseases. Ectromelia virus (ECTV), the causative agent of mousepox, offers an excellent model system to study an orthopoxvirus infection in its natural host. Here, we investigated the role of the vaccinia virus ortholog N1L in ECTV infection. Respiratory infection of mice with an N1L deletion mutant virus (ECTVΔN1L) demonstrated profound attenuation of the mutant virus, confirming N1 as an orthopoxvirus virulence factor. Upon analysis of virus dissemination in vivo, we observed a striking deficiency of ECTVΔN1L spreading from the lungs to the livers or spleens of infected mice. Investigating the immunological mechanism controlling ECTVΔN1L infection, we found the attenuated phenotype to be unaltered in mice deficient in Toll-like receptor (TLR) or RIG-I-like RNA helicase (RLH) signaling as well as in those missing the type I interferon receptor or lacking B cells. However, in RAG-1(-/-) mice lacking mature B and T cells, ECTVΔN1L regained virulence, as shown by increasing morbidity and virus spread to the liver and spleen. Moreover, T cell depletion experiments revealed that ECTVΔN1L attenuation was reversed only by removing both CD4(+) and CD8(+) T cells, so the presence of either cell subset was still sufficient to control the infection. Thus, the orthopoxvirus virulence factor N1 may allow efficient ECTV infection in mice by interfering with host T cell function.

Evaluation of a modified vaccinia virus Ankara (MVA)-based candidate pandemic influenza A/H1N1 vaccine in the ferret model.

The zoonotic transmissions of highly pathogenic avian influenza viruses of the H5N1 subtype that have occurred since 1997 have sparked the development of novel influenza vaccines. The advent of reverse genetics technology, cell-culture production techniques and novel adjuvants has improved the vaccine strain preparation, production process and immunogenicity of the vaccines, respectively, and has accelerated the availability of pandemic influenza vaccines. However, there is still room for improvement, and alternative vaccine preparations can be explored, such as viral vectors. Modified vaccinia virus Ankara (MVA), originally developed as a safe smallpox vaccine, can be exploited as a viral vector and has many favourable properties. Recently, we have demonstrated that an MVA-based vaccine could protect mice and macaques against infection with highly pathogenic influenza viruses of the H5N1 subtype. In the present study, recombinant MVA expressing the haemagglutinin (HA) gene of pandemic influenza A/H1N1 virus was evaluated in the ferret model. A single immunization induced modest antibody responses and afforded only modest protection against the development of severe disease upon infection with a 2009(H1N1) strain. In contrast, two immunizations induced robust antibody responses and protected ferrets from developing severe disease, confirming that MVA is an attractive influenza vaccine production platform.

Functional F11L and K1L genes in modified vaccinia virus Ankara restore virus-induced cell motility but not growth in human and murine cells.

Modified vaccinia virus Ankara (MVA) was generated by serial passaging in chicken embryo fibroblasts. During this attenuation, MVA lost the capacity to productively grow in human and most other mammalian cell lines, as well as acquiring a multitude of deletions and mutations in the MVA genome. This means that the precise molecular basis for the MVA host-range restriction is still unknown. The vaccinia virus (VACV) genes F11L and K1L are mutated or truncated in MVA. F11L was previously implicated in VACV-induced cell motility and virion maturation. Here, we demonstrate that the restoration of F11L gene expression in MVA rescued virus-induced cell motility, but had no impact on MVA virion maturation and host-range restriction. Additional insertion of the K1L gene, which restores MVA replication in RK-13 cells, was not sufficient to extend MVA growth capacity to other mammalian cells.

Viral host-range factor C7 or K1 is essential for modified vaccinia virus Ankara late gene expression in human and murine cells, irrespective of their capacity to inhibit protein kinase R-mediated phosphorylation of eukaryotic translation initiation factor 2alpha.

Vaccinia virus (VACV) infection induces phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha), which inhibits cellular and viral protein synthesis. In turn, VACV has evolved the capacity to antagonize this antiviral response by expressing the viral host-range proteins K3 and E3. This study revealed that the host-range genes K1L and C7L also prevent eIF2alpha phosphorylation in modified VACV Ankara (MVA) infection of several human and murine cell lines. Moreover, C7L-deleted MVA (MVA-DeltaC7L) lacked late gene expression, which could be rescued by the function of host-range factor K1 or C7. It was demonstrated that viral gene expression was blocked after viral DNA replication and that it was independent of apoptosis induction. Furthermore, it was found that eIF2alpha phosphorylation in MVA-DeltaC7L-infected cells is mediated by protein kinase R (PKR) as shown in murine embryonic fibroblasts lacking PKR function, and it was shown that this was not due to reduced E3L gene expression. The block of eIF2alpha phosphorylation by C7 could be complemented by K1 in cells infected with MVA-DeltaC7L encoding a reinserted K1L gene (MVA-DeltaC7L-K1L). Importantly, these data illustrated that eIF2alpha phosphorylation by PKR is not responsible for the block of late viral gene expression. This suggests that other mechanisms targeted by C7 and K1 are essential for completing the MVA gene expression cycle and probably also for VACV replication in a diverse set of cell types.

MVA-based H5N1 vaccine affords cross-clade protection in mice against influenza A/H5N1 viruses at low doses and after single immunization.

Human infections with highly pathogenic avian influenza viruses of the H5N1 subtype, frequently reported since 2003, result in high morbidity and mortality. It is feared that these viruses become pandemic, therefore the development of safe and effective vaccines is desirable. MVA-based H5N1 vaccines already proved to be effective when two immunizations with high doses were used. Dose-sparing strategies would increase the number of people that can be vaccinated when the amount of vaccine preparations that can be produced is limited. Furthermore, protective immunity is induced ideally after a single immunization. Therefore the minimal requirements for induction of protective immunity with a MVA-based H5N1 vaccine were assessed in mice. To this end, mice were vaccinated once or twice with descending doses of a recombinant MVA expressing the HA gene of influenza virus A/Vietnam/1194/04. The protective efficacy was determined after challenge infection with the homologous clade 1 virus and a heterologous virus derived from clade 2.1, A/Indonesia/5/05 by assessing weight loss, virus replication and histopathological changes. It was concluded that MVA-based vaccines allowed significant dose-sparing and afford cross-clade protection, also after a single immunization, which are favorable properties for an H5N1 vaccine candidate.

The orthopoxvirus 68-kilodalton ankyrin-like protein is essential for DNA replication and complete gene expression of modified vaccinia virus Ankara in nonpermissive human and murine cells.

Modified vaccinia virus Ankara (MVA) is a highly attenuated and replication-deficient vaccinia virus (VACV) that is being evaluated as replacement smallpox vaccine and candidate viral vector. MVA lacks many genes associated with virulence and/or regulation of virus tropism. The 68-kDa ankyrin-like protein (68k-ank) is the only ankyrin repeat-containing protein that is encoded by the MVA genome and is highly conserved throughout the Orthopoxvirus genus. We showed previously that 68k-ank is composed of ankyrin repeats and an F-box-like domain and forms an SCF ubiquitin ligase complex together with the cellular proteins Skp1a and Cullin-1. We now report that 68k-ank (MVA open reading frame 186R) is an essential factor for completion of the MVA intracellular life cycle in nonpermissive human and murine cells. Infection of mouse NIH 3T3 and human HaCaT cells with MVA with a deletion of the 68k-ank gene (MVA-Delta68k-ank) was characterized by an extensive reduction of viral intermediate RNA and protein, as well as late transcripts and drastically impaired late protein synthesis. Furthermore, infections with MVA-Delta68k-ank failed to induce the host protein shutoff that is characteristic of VACV infections. Although we demonstrated that proteasome function in general is essential for the completion of the MVA molecular life cycle, we found that a mutant 68k-ank protein with a deletion of the F-box-like domain was able to fully complement the deficiency of MVA-Delta68k-ank to express all classes of viral genes. Thus, our data demonstrate that the 68k-ank protein contains another critical domain that may function independently of SCF ubiquitin ligase complex formation, suggesting multiple activities of this interesting regulatory protein.

Postexposure immunization with modified vaccinia virus Ankara or conventional Lister vaccine provides solid protection in a murine model of human smallpox.

BACKGROUND: Decades after the cessation of smallpox vaccination, the potential of the deliberate release of pathogenic orthopoxviruses has forced a reconsideration of using these extremely efficient human vaccines. Scenarios of sudden biothreats have prompted demand for rapidly protective vaccination. However, the feasibility of short-term vaccination (i.e., vaccination shortly before exposure) with vaccinia virus (VACV) is uncertain. METHODS: We tested the rapid protective capacity of vaccines based on VACV strain Lister (VACV-Lister) and on modified VACV Ankara (MVA) in different mouse models, comparing lethal infections with VACV strain Western Reserve (VACV-WR) or ectromelia virus (ECTV). RESULTS: In contrast to VACV-WR challenge, we found extended incubation periods after ECTV challenge, allowing successful therapeutic immunization with VACV-Lister and MVA when applied 2-3 days after exposure. Rapid protection from respiratory tract ECTV infection was significantly affected by vaccine dose and was associated with occurrence of poxvirus-specific antibodies. Vaccinations in type I interferon receptor-deficient mice were protective, whereas recombination activating gene 1-deficient mice lacking mature T and B cells failed to mount immunity after short-term vaccination, confirming an essential role of adaptive immune responses. CONCLUSIONS: ECTV infection in mice models the course of human smallpox. Our data provide evidence to substantiate historical data on the usefulness of postexposure vaccination with conventional VACV and the new candidate MVA to protect against fatal orthopoxvirus infections.

Vaccinia virus-mediated inhibition of type I interferon responses is a multifactorial process involving the soluble type I interferon receptor B18 and intracellular components.

Poxviruses such as virulent vaccinia virus (VACV) strain Western Reserve encode a broad range of immune modulators that interfere with host responses to infection. Upon more than 570 in vitro passages in chicken embryo fibroblasts (CEF), chorioallantois VACV Ankara (CVA) accumulated mutations that resulted in highly attenuated modified vaccinia virus Ankara (MVA). MVA infection of mice and of dendritic cells (DC) induced significant type I interferon (IFN) responses, whereas infection with VACV alone or in combination with MVA did not. These results implied that VACV expressed an IFN inhibitor(s) that was functionally deleted in MVA. To further characterize the IFN inhibitor(s), infection experiments were carried out with CVA strains isolated after 152 (CVA152) and 386 CEF passages (CVA386). Interestingly, neither CVA152 nor CVA386 induced IFN-alpha, whereas the latter variant did induce IFN-beta. This pattern suggested a consecutive loss of inhibitors during MVA attenuation. Similar to supernatants of VACV- and CVA152-infected DC cultures, recombinantly expressed soluble IFN decoy receptor B18, which is encoded in the VACV genome, inhibited MVA-induced IFN-alpha but not IFN-beta. In the same direction, a B18R-deficient VACV variant triggered only IFN-alpha, confirming B18 as the soluble IFN-alpha inhibitor. Interestingly, VACV infection inhibited IFN responses induced by a multitude of different stimuli, including oligodeoxynucleotides containing CpG motifs, poly(I:C), and vesicular stomatitis virus. Collectively, the data presented show that VACV-mediated IFN inhibition is a multistep process involving secreted factors such as B18 plus intracellular components that cooperate to efficiently shut off systemic IFN-alpha and IFN-beta responses.

The highly conserved orthopoxvirus 68k ankyrin-like protein is part of a cellular SCF ubiquitin ligase complex.

The 68k ankyrin-like protein (68k-ank) of unknown function is highly conserved among orthopoxviruses and contains ankyrin repeats and an F-box-like domain. We performed a yeast-two-hybrid screen with 68k-ank to find interacting proteins. From a human and a murine cDNA library, 99% of the interaction partners were S-phase kinase-associated protein 1a (Skp1a), a part of the SCF ubiquitin ligase complex. 68k-ank co-immunoprecipitated with components of the endogenous, mammalian SCF ubiquitin ligase. This interaction was F-box domain dependent and could also be observed in infected cells, indicating that SCF complex formation might be important for the viral life cycle.

Vaccinia virus double-stranded RNA-binding protein E3 does not interfere with siRNA-mediated gene silencing in mammalian cells.

Vaccinia virus (VACV) evolved several strategies to evade antiviral cellular defence. The vaccinia virus E3 protein for example binds and sequesters double stranded RNA (dsRNA) and counteracts interferon action. We were interested to find out whether and to what extend E3 interferes with RNA silencing mediated by short interfering RNA (siRNA) in mammalian cells. We could show that the expression of a VACV-encoded marker gene can be efficiently inhibited by siRNA independently of the presence of the E3 protein. In addition, expression of E3 had no impact on RNA polymerase III promoter-derived shRNA-induced silencing of a cellular gene in human cells. Both VACV early and late gene expression could be inhibited by siRNA. Furthermore, downregulation of the expression of the E3L gene itself by siRNA in VACV infected cells produced the previously described phenotype of a knock-out virus, which illustrates the power of siRNA for vaccinia virus gene function analysis.

The kinase insert domain-containing receptor is an angiogenesis-associated antigen recognized by human cytotoxic T lymphocytes.

Antigen-specific cancer immunotherapy directed toward tumor-nourishing angiogenic blood vessels holds the promise of high efficacy, low toxicity, and ease of application. To evaluate whether the human angiogenic kinase insert domain-containing receptor (KDR) can serve as a target for cellular immunotherapy, 19 peptide sequences with HLA-A*0201 motifs were selected by computer-based algorithms. Five peptides (KDR82-90, KDR288-297, KDR766-774, KDR1093-1101, KDR1035-1044) stimulated specific cytotoxic T lymphocytes (CTLs) from peripheral-blood mononuclear cells (PBMCs) of 3 HLA-A*0201 donors. The decapeptide KDR288-297 was efficient in sensitizing target cells for recognition by a CTL clone at a concentration of 10 nM. More important, KDR288-297-specific CTLs lysed target cells transfected with HLA-A2/KDR cDNAs and a range of HLA-matched KDR+ angiogenic endothelial cells (aECs) and also recognized CD34+ endothelial progenitor cells. The specificity of CTLs was further confirmed by tetramer assay and cold-target inhibition assay. In addition, ex vivo exposure of aECs to the inflammatory cytokines enhanced CTL reactivity, which is in keeping with up-regulated KDR and HLA class 1 expression. In Matrigel assays, recognition of aECs by specific CTLs triggered an antivascular effect. These findings provide the first proof of the antigenic property of KDR protein and may be useful for devising new immunotherapeutic approaches to human cancers.

Features of the Env leader protein and the N-terminal Gag domain of feline foamy virus important for virus morphogenesis.

Previous studies have shown that foamy virus (FV) particle budding, especially the involvement of the viral env glycoprotein is different from that of other (ortho) retroviruses: the N-terminal Env leader protein Elp is a constituent of released FV particles. A defined sequence in Elp required for particle budding binds to the MA domain of Gag. To extend these findings, we show that feline FV Elp is a membrane-anchored protein with the N-terminus located inside the particle. Thus, the internal/cytoplasmic domain of Elp has the correct topology for interacting with Gag during budding. In addition to Elp, an Elp-related protein of about 9 kDa was shown to be virion associated and is probably generated by cellular signal peptidases. Besides the function of Elp binding, the N-terminal domain of Gag was shown to be required for proper localization of feline FV Gag to the cytoplasm and the perinuclear/nuclear region.

Application of chimeric feline foamy virus-based retroviral vectors for the induction of antiviral immunity in cats.

In order to define the potential and applicability of replication-competent foamy virus-based vaccine vectors, recombinant feline foamy virus (FFV) vectors encoding defined segments of the feline calicivirus (FCV) capsid protein E domain were constructed. In cell cultures, these FFV-FCV vectors efficiently transduced and expressed a hybrid fusion protein consisting of the essential FFV Bet protein and the attached FCV E domains. The stability of the vectors in vitro was inversely correlated to the size of the heterologous insert. The deletion of a part of the FFV U3 sequence in these FFV-FCV vectors did not interfere with replication and titer in cell cultures but increased the genetic stability of the hybrid vectors. Selected chimeric vectors were injected into immunocompetent cats and persisted in the transduced host concomitant with a strong and specific humoral immune response against vector components. In a substantial number of cats, antibodies directed against the FCV E domain were induced by the FFV-FCV vectors, but no FCV-neutralizing activities were detectable in vitro. When the vaccinated cats were challenged with a high-titer FCV dose, sterile immunity was not induced by any of the hybrid FFV-FCV vectors. However, the FFV-FCV vector with a truncated U3 region of the long terminal repeat promoter significantly reduced the duration of FCV shedding after challenge and suppressed the appearance of FCV-specific ulcers. Possible mechanisms contributing to the partial protection will be discussed.