Removal of cryptic poxvirus transcription termination signals from the human immunodeficiency virus type 1 envelope gene enhances expression and immunogenicity of a recombinant vaccinia virus

Evaluation of T cell responses to naturally processed variant SARS-CoV-2 spike antigens in individuals following infection or vaccination

Most existing studies characterizing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific T cell responses are peptide based. This does not allow evaluation of whether tested peptides are processed and presented canonically. In this study, we use recombinant vaccinia virus (rVACV)-mediated expression of SARS-CoV-2 spike protein and SARS-CoV-2 infection of angiotensin-converting enzyme (ACE)-2-transduced B cell lines to evaluate overall T cell responses in a small cohort of recovered COVID-19 patients and uninfected donors vaccinated with ChAdOx1 nCoV-19. We show that rVACV expression of SARS-CoV-2 antigen can be used as an alternative to SARS-CoV-2 infection to evaluate T cell responses to naturally processed spike antigens. In addition, the rVACV system can be used to evaluate the cross-reactivity of memory T cells to variants of concern (VOCs) and to identify epitope escape mutants. Finally, our data show that both natural infection and vaccination could induce multi-functional T cell responses with overall T cell responses remaining despite the identification of escape mutations.

The Brighton Collaboration standardized template for collection of key information for risk/benefit assessment of a Modified Vaccinia Ankara (MVA) vaccine platform

The Brighton Collaboration Viral Vector Vaccines Safety Working Group (V3SWG) was formed to evaluate the safety and characteristics of live, recombinant viral vector vaccines. The Modified Vaccinia Ankara (MVA) vector system is being explored as a platform for development of multiple vaccines. This paper reviews the molecular and biological features specifically of the MVA-BN vector system, followed by a template with details on the safety and characteristics of an MVA-BN based vaccine against Zaire ebolavirus and other filovirus strains. The MVA-BN-Filo vaccine is based on a live, highly attenuated poxviral vector incapable of replicating in human cells and encodes glycoproteins of Ebola virus Zaire, Sudan virus and Marburg virus and the nucleoprotein of the Thai Forest virus. This vaccine has been approved in the European Union in July 2020 as part of a heterologous Ebola vaccination regimen. The MVA-BN vector is attenuated following over 500 serial passages in eggs, showing restricted host tropism and incompetence to replicate in human cells. MVA has six major deletions and other mutations of genes outside these deletions, which all contribute to the replication deficiency in human and other mammalian cells. Attenuation of MVA-BN was demonstrated by safe administration in immunocompromised mice and non-human primates. In multiple clinical trials with the MVA-BN backbone, more than 7800 participants have been vaccinated, demonstrating a safety profile consistent with other licensed, modern vaccines. MVA-BN has been approved as smallpox vaccine in Europe and Canada in 2013, and as smallpox and monkeypox vaccine in the US in 2019. No signal for inflammatory cardiac disorders was identified throughout the MVA-BN development program. This is in sharp contrast to the older, replicating vaccinia smallpox vaccines, which have a known risk for myocarditis and/or pericarditis in up to 1 in 200 vaccinees. MVA-BN-Filo as part of a heterologous Ebola vaccination regimen (Ad26.ZEBOV/MVA-BN-Filo) has undergone clinical testing including Phase III in West Africa and is currently in use in large scale vaccination studies in Central African countries. This paper provides a comprehensive picture of the MVA-BN vector, which has reached regulatory approvals, both as MVA-BN backbone for smallpox/monkeypox, as well as for the MVA-BN-Filo construct as part of an Ebola vaccination regimen, and therefore aims to provide solutions to prevent disease from high-consequence human pathogens.

One or two injections of MVA-vectored vaccine shields hACE2 transgenic mice from SARS-CoV-2 upper and lower respiratory tract infection

Modified vaccinia virus Ankara (MVA) is a replication-restricted smallpox vaccine, and numerous clinical studies of recombinant MVAs (rMVAs) as vectors for prevention of other infectious diseases, including COVID-19, are in progress. Here, we characterize rMVAs expressing the S protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Modifications of full-length S individually or in combination included two proline substitutions, mutations of the furin recognition site, and deletion of the endoplasmic retrieval signal. Another rMVA in which the receptor binding domain (RBD) is flanked by the signal peptide and transmembrane domains of S was also constructed. Each modified S protein was displayed on the surface of rMVA-infected cells and was recognized by anti-RBD antibody and soluble hACE2 receptor. Intramuscular injection of mice with the rMVAs induced antibodies, which neutralized a pseudovirus in vitro and, upon passive transfer, protected hACE2 transgenic mice from lethal infection with SARS-CoV-2, as well as S-specific CD3+CD8+IFNγ+ T cells. Antibody boosting occurred following a second rMVA or adjuvanted purified RBD protein. Immunity conferred by a single vaccination of hACE2 mice prevented morbidity and weight loss upon intranasal infection with SARS-CoV-2 3 wk or 7 wk later. One or two rMVA vaccinations also prevented detection of infectious SARS-CoV-2 and subgenomic viral mRNAs in the lungs and greatly reduced induction of cytokine and chemokine mRNAs. A low amount of virus was found in the nasal turbinates of only one of eight rMVA-vaccinated mice on day 2 and none later. Detection of low levels of subgenomic mRNAs in turbinates indicated that replication was aborted in immunized animals.

Injection site vaccinology of a recombinant vaccinia-based vector reveals diverse innate immune signatures

Poxvirus systems have been extensively used as vaccine vectors. Herein a RNA-Seq analysis of intramuscular injection sites provided detailed insights into host innate immune responses, as well as expression of vector and recombinant immunogen genes, after vaccination with a new multiplication defective, vaccinia-based vector, Sementis Copenhagen Vector. Chikungunya and Zika virus immunogen mRNA and protein expression was associated with necrosing skeletal muscle cells surrounded by mixed cellular infiltrates. The multiple adjuvant signatures at 12 hours post-vaccination were dominated by TLR3, 4 and 9, STING, MAVS, PKR and the inflammasome. Th1 cytokine signatures were dominated by IFNγ, TNF and IL1β, and chemokine signatures by CCL5 and CXCL12. Multiple signatures associated with dendritic cell stimulation were evident. By day seven, vaccine transcripts were absent, and cell death, neutrophil, macrophage and inflammation annotations had abated. No compelling arthritis signatures were identified. Such injection site vaccinology approaches should inform refinements in poxvirus-based vector design.

Poxvirus vector systems have been widely developed for vaccine applications. Despite considerable progress, so far only one recombinant poxvirus vectored vaccine has to date been licensed for human use, with ongoing efforts seeking to enhance immunogenicity whilst minimizing reactogenicity. The latter two characteristics are often determined by early post-vaccination events at the injection site. We therefore undertook an injection site vaccinology approach to analyzing gene expression at the vaccination site after intramuscular inoculation with a recombinant, multiplication defective, vaccinia-based vaccine. This provided detailed insights into inter alia expression of vector-encoded immunoregulatory genes, as well as host innate and adaptive immune responses. We propose that such injection site vaccinology can inform rational vaccine vector design, and we discuss how the information and approach elucidated herein might be used to improve immunogenicity and limit reactogenicity of poxvirus-based vaccine vector systems.

MVA Vector Vaccines Inhibit SARS CoV-2 Replication in Upper and Lower Respiratory Tracts of Transgenic Mice and Prevent Lethal Disease

Replication-restricted modified vaccinia virus Ankara (MVA) is a licensed smallpox vaccine and numerous clinical studies investigating recombinant MVAs (rMVAs) as vectors for prevention of other infectious diseases have been completed or are in progress. Two rMVA COVID-19 vaccine trials are at an initial stage, though no animal protection studies have been reported. Here, we characterize rMVAs expressing the S protein of CoV-2. Modifications of full length S individually or in combination included two proline substitutions, mutations of the furin recognition site and deletion of the endoplasmic retrieval signal. Another rMVA in which the receptor binding domain (RBD) flanked by the signal peptide and transmembrane domains of S was also constructed. Each modified S protein was displayed on the surface of rMVA-infected human cells and was recognized by anti-RBD antibody and by soluble hACE2 receptor. Intramuscular injection of mice with the rMVAs induced S-binding and pseudovirus-neutralizing antibodies. Boosting occurred following a second homologous rMVA but was higher with adjuvanted purified RBD protein. Weight loss and lethality following intranasal infection of transgenic hACE2 mice with CoV-2 was prevented by one or two immunizations with rMVAs or by passive transfer of serum from vaccinated mice. One or two rMVA vaccinations also prevented recovery of infectious CoV-2 from the lungs. A low amount of virus was detected in the nasal turbinates of only one of eight rMVA-vaccinated mice on day 2 and none later. Detection of subgenomic mRNA in turbinates on day 2 only indicated that replication was abortive in immunized animals.

Recombinant Fowlpox Virus Expressing gB Gene from Predominantly Epidemic Infectious Larygnotracheitis Virus Strain Demonstrates Better Immune Protection in SPF Chickens

Background: Infectious laryngotracheitis (ILT) is a highly contagious acute respiratory disease of chickens. Antigenic mutation of infectious laryngotracheitis virus (ILTV) may result in a vaccination failure in the poultry industry and thus a protective vaccine against predominant ILTV strains is highly desirable. Methods: The full-length glycoprotein B (gB) gene of ILTV with the two mutated synonymous sites of fowlpox virus (FPV) transcription termination signal sequence was cloned into the insertion vector p12LS, which was co-transfected with wild-type (wt) FPV into chicken embryo fibroblast (CEF) to develop a recombinant fowlpox virus-gB (rFPV-gB) candidate vaccine strain. Furthermore, its biological and immunological characteristics were evaluated. Results: The results indicated that gB gene was expressed correctly in the rFPV by indirect immunofluorescent assay and Western blot, and the rFPV-gB provided a 100% protection in immunized chickens against the challenge of predominant ILTV strains that were screened by pathogenicity assay when compared with the commercialized rFPV vaccine, which only provided 83.3%. Conclusion: rFPV-gB can be used as a potential vaccine against predominant ILTV strains.

Expression of the Vaccinia Virus Antiapoptotic F1 Protein Is Blocked by Protein Kinase R in the Absence of the Viral E3 Protein

Poxviruses encode many proteins with the ability to regulate cellular signaling pathways. One such protein is the vaccinia virus innate immunity modulator E3. Multiple functions have been ascribed to E3, including modulating the cellular response to double-stranded RNA, inhibiting the NF-κB and IRF3 pathways, and dampening apoptosis. Apoptosis serves as a powerful defense against damaged and unwanted cells and is an effective defense against viral infection; many viruses therefore encode proteins that prevent or delay apoptosis. Here, we present data indicating that E3 does not directly inhibit the intrinsic apoptotic pathway; instead, it suppresses apoptosis indirectly by stimulating expression of the viral F1 apoptotic inhibitor. Our data demonstrate that E3 promotes F1 expression by blocking activation of the double-stranded RNA-activated protein kinase R (PKR). F1 mRNA is present in cells infected with E3-null virus, but the protein product does not detectably accumulate, suggesting a block at the translational level. We also show that two 3' coterminal transcripts span the F1 open reading frame (ORF), a situation previously described for the vaccinia virus mRNAs encoding the J3 and J4 proteins. One of these is a conventional monocistronic transcript of the F1L gene, while the other arises by read-through transcription from the upstream F2L gene and does not give rise to appreciable levels of F1 protein.IMPORTANCE Previous studies have shown that E3-deficient vaccinia virus triggers apoptosis of infected cells. Our study demonstrates that this proapoptotic phenotype stems, at least in part, from the failure of the mutant virus to produce adequate quantities of the viral F1 protein, which acts at the mitochondria to directly block apoptosis. Our data establish a regulatory link between the vaccinia virus proteins that suppress the innate response to double-stranded RNA and those that block the intrinsic apoptotic pathway.

Role of CD4 T cell helper subsets in immune response and deviation of CD8 T cells in mice

The ability of different CD4⁺ T cell subsets to help CD8⁺ T-cell response is not fully understood. Here, we found using the murine system that Th17 cells induced by IL-1β, unlike Th1, were not effective helpers for antiviral CD8 responses as measured by IFNγ-producing cells or protection against virus infection. However, they skewed CD8 responses to a Tc17 phenotype. Thus, the apparent lack of help was actually immune deviation. This skewing depended on both IL-21 and IL-23. To overcome this effect, we inhibited Th17 induction by blocking TGF-β. Anti-TGF-β allowed the IL-1β adjuvant to enhance CD8⁺ T-cell responses without skewing the phenotype to Tc17, thereby providing an approach to harness the benefit of common IL-1-inducing adjuvants like alum without immune deviation.

The evolution of poxvirus vaccines

After Edward Jenner established human vaccination over 200 years ago, attenuated poxviruses became key players to contain the deadliest virus of its own family: Variola virus (VARV), the causative agent of smallpox. Cowpox virus (CPXV) and horsepox virus (HSPV) were extensively used to this end, passaged in cattle and humans until the appearance of vaccinia virus (VACV), which was used in the final campaigns aimed to eradicate the disease, an endeavor that was accomplished by the World Health Organization (WHO) in 1980. Ever since, naturally evolved strains used for vaccination were introduced into research laboratories where VACV and other poxviruses with improved safety profiles were generated. Recombinant DNA technology along with the DNA genome features of this virus family allowed the generation of vaccines against heterologous diseases, and the specific insertion and deletion of poxvirus genes generated an even broader spectrum of modified viruses with new properties that increase their immunogenicity and safety profile as vaccine vectors. In this review, we highlight the evolution of poxvirus vaccines, from first generation to the current status, pointing out how different vaccines have emerged and approaches that are being followed up in the development of more rational vaccines against a wide range of diseases.

The neutralizing capacity of antibodies elicited by parainfluenza virus infection of African Green Monkeys is dependent on complement

The African Green Monkey (AGM) model was used to analyze the role of complement in neutralization of parainfluenza virus. Parainfluenza virus 5 (PIV5) and human parainfluenza virus type 2 were effectively neutralized in vitro by naïve AGM sera, but neutralizing capacity was lost by heat-inactivation. The mechanism of neutralization involved formation of massive aggregates, with no evidence of virion lysis. Following inoculation of the respiratory tract with a PIV5 vector expressing HIV gp160, AGM produced high levels of serum and tracheal antibodies against gp120 and the viral F and HN proteins. However, in the absence of complement these anti-PIV5 antibodies had very poor neutralizing capacity. Virions showed extensive deposition of IgG and C1q with post- but not pre-immune sera. These results highlight the importance of complement in the initial antibody response to parainfluenza viruses, with implications for understanding infant immune responses and design of vaccine strategies for these pediatric pathogens.

Using an Agent-Based Model to Examine the Role of Dynamic Bacterial Virulence Potential in the Pathogenesis of Surgical Site Infection

OBJECTIVE

Despite clinical advances, surgical site infections (SSIs) remain a problem. The development of SSIs involves a complex interplay between the cellular and molecular mechanisms of wound healing and contaminating bacteria, and here, we utilize an agent-based model (ABM) to investigate the role of bacterial virulence potential in the pathogenesis of SSI.

APPROACH

The Muscle Wound ABM (MWABM) incorporates muscle cells, neutrophils, macrophages, myoblasts, abstracted blood vessels, and avirulent/virulent bacteria to simulate the pathogenesis of SSIs. Simulated bacteria with virulence potential can mutate to possess resistance to reactive oxygen species and increased invasiveness. Simulated experiments (t=7 days) involved parameter sweeps of initial wound size to identify transition zones between healed and nonhealed wounds/SSIs, and to evaluate the effect of avirulent/virulent bacteria.

RESULTS

The MWABM reproduced the dynamics of normal successful healing, including a transition zone in initial wound size beyond which healing was significantly impaired. Parameter sweeps with avirulent bacteria demonstrated that smaller wound sizes were associated with healing failure. This effect was even more pronounced with the addition of virulence potential to the contaminating bacteria.

INNOVATION

The MWABM integrates the myriad factors involved in the healing of a normal wound and the pathogenesis of SSIs. This type of model can serve as a useful framework into which more detailed mechanistic knowledge can be embedded.

CONCLUSION

Future work will involve more comprehensive representation of host factors, and especially the ability of those host factors to activate virulence potential in the microbes involved.

Reflections on the early development of poxvirus vectors

Poxvirus expression vectors were described in 1982 and quickly became widely used for vaccine development as well as research in numerous fields. Advantages of the vectors include simple construction, ability to accommodate large amounts of foreign DNA and high expression levels. Numerous poxvirus-based veterinary vaccines are currently in use and many others are in human clinical trials. The early reports of poxvirus vectors paved the way for and stimulated the development of other viral vectors and recombinant DNA vaccines.

Recombinant MVA vaccines: dispelling the myths

Diseases such as HIV/AIDS, tuberculosis, malaria and cancer are prime targets for prophylactic or therapeutic vaccination, but have proven partially or wholly resistant to traditional approaches to vaccine design. New vaccines based on recombinant viral vectors expressing a foreign antigen are under intense development for these and other indications. One of the most advanced and most promising vectors is the attenuated, non-replicating poxvirus MVA (modified vaccinia virus Ankara), a safer derivative of the uniquely successful smallpox vaccine. Despite the ability of recombinant MVA to induce potent humoral and cellular immune responses against transgenic antigen in humans, especially when used as the latter element of a heterologous prime-boost regimen, doubts are occasionally expressed about the ultimate feasibility of this approach. In this review, five common misconceptions over recombinant MVA are discussed, and evidence is cited to show that recombinant MVA is at least sufficiently genetically stable, manufacturable, safe, and immunogenic (even in the face of prior anti-vector immunity) to warrant reasonable hope over the feasibility of large-scale deployment, should useful levels of protection against target pathogens, or therapeutic benefit for cancer, be demonstrated in efficacy trials.

Genome-wide analysis of the 5' and 3' ends of vaccinia virus early mRNAs delineates regulatory sequences of annotated and anomalous transcripts

Poxviruses are large DNA viruses that encode a multisubunit RNA polymerase, stage-specific transcription factors, and enzymes that cap and polyadenylate mRNAs within the cytoplasm of infected animal cells. Genome-wide microarray and RNA-seq technologies have been used to profile the transcriptome of vaccinia virus (VACV), the prototype member of the family. Here, we adapted tag-based methods in conjunction with SOLiD and Illumina deep sequencing platforms to determine the precise 5' and 3' ends of VACV early mRNAs and map the putative transcription start sites (TSSs) and polyadenylation sites (PASs). Individual and clustered TSSs were found preceding 104 annotated open reading frames (ORFs), excluding pseudogenes. In the majority of cases, a 15-nucleotide consensus core motif was present upstream of the ORF. This motif, however, was also present at numerous other locations, indicating that it was insufficient for transcription initiation. Further analysis revealed a 10-nucleotide AT-rich spacer following functional core motifs that may facilitate DNA unwinding. Additional putative TSSs occurred in anomalous locations that may expand the functional repertoire of the VACV genome. However, many of the anomalous TSSs lacked an upstream core motif, raising the possibility that they arose by a processing mechanism as has been proposed for eukaryotic systems. Discrete and clustered PASs occurred about 40 nucleotides after an UUUUUNU termination signal. However, a large number of PASs were not preceded by this motif, suggesting alternative polyadenylation mechanisms. Pyrimidine-rich coding strand sequences were found immediately upstream of both types of PASs, signifying an additional feature of VACV 3'-end formation and polyadenylation.

Viral double-stranded RNAs from vaccinia virus early or intermediate gene transcripts possess PKR activating function, resulting in NF-kappaB activation, when the K1 protein is absent or mutated

PKR is a potent antiviral molecule that can terminate infection by inhibiting protein synthesis and stimulating NF-κB activation and apoptosis. Originally, it was thought that only intermediate and late gene transcription produced double-stranded (ds) RNA to activate PKR during vaccinia virus (VACV) infection. The VACV E3 or K3 proteins squelch this effect by binding to either dsRNA or PKR. However, in the absence of the K1 protein, VACV infection activates PKR at very early times post-infection and despite the presence of E3 and K3. These data suggest that VACV infection induces PKR activation by a currently unknown mechanism. To determine this mechanism, cells were infected with K1L-containing or -deficient VACVs. By using conditions that limited the progression of the poxvirus replication cycle, we observed that early gene transcripts activated PKR in RK13 cells, identifying a new PKR-activating mechanism of poxvirus infection. Using a similar approach for HeLa cells, intermediate gene transcription was sufficient to activate PKR. RNA isolated from infected RK13 or HeLa cells maintained PKR-activating properties only when dsRNA was present. Moreover, viral dsRNA was directly detected in infected cells either by RT-PCR or immunofluorescent microscopy. Interestingly, dsRNA levels were higher in infected cells in which the K1 protein was nonfunctional. Only K1 proteins with PKR inhibitory function prevented downstream NF-κB activation. These results reveal a new PKR activation pathway during VACV infection, in which the K1 protein reduces dsRNA levels early in VACV infection to directly inhibit PKR and several of its downstream antiviral effects, thereby enhancing virus survival.

Simultaneous high-resolution analysis of vaccinia virus and host cell transcriptomes by deep RNA sequencing

Deep RNA sequencing was used to simultaneously analyze vaccinia virus (VACV) and HeLa cell transcriptomes at progressive times following infection. VACV, the prototypic member of the poxvirus family, replicates in the cytoplasm and contains a double-stranded DNA genome with approximately 200 closely spaced open reading frames (ORFs). The acquisition of a total of nearly 500 million short cDNA sequences allowed construction of temporal strand-specific maps of the entire VACV transcriptome at single-base resolution and analysis of over 14,000 host mRNAs. Before viral DNA replication, transcripts from 118 VACV ORFs were detected; after replication, transcripts from 93 additional ORFs were characterized. The high resolution permitted determination of the precise boundaries of many mRNAs including read-through transcripts and location of mRNA start sites and adjacent promoters. Temporal analysis revealed two clusters of early mRNAs that were synthesized in the presence of inhibitors of protein as well as DNA synthesis, indicating that they do not correspond to separate immediate- and delayed-early classes as defined for other DNA viruses. The proportion of viral RNAs reached 25-55% of the total at 4 h. This rapid change, resulting in a relative decrease of the vast majority of host mRNAs, can contribute to the profound shutdown of host protein synthesis and blunting of antiviral responses. At 2 h, however, a minority of cellular mRNAs was increased. The overrepresented functional categories of the up-regulated RNAs were NF-kappaB cascade, apoptosis, signal transduction, and ligand-mediated signaling, which likely represent the host response to invasion.

Using 3 TLR ligands as a combination adjuvant induces qualitative changes in T cell responses needed for antiviral protection in mice

TLR ligands are promising candidates for the development of novel vaccine adjuvants that can elicit protective immunity against emerging infectious diseases. Adjuvants have been used most frequently to increase the quantity of an immune response. However, the quality of a T cell response can be more important than its quantity. Stimulating certain pairs of TLRs induces a synergistic response in terms of activating dendritic cells and eliciting/enhancing T cell responses through clonal expansion, which increases the number of responding T cells. Here, we have found that utilizing ligands for 3 TLRs (TLR2/6, TLR3, and TLR9) greatly increased the protective efficacy of vaccination with an HIV envelope peptide in mice when compared with using ligands for only any 2 of these TLRs; surprisingly, increased protection was induced without a marked increase in the number of peptide-specific T cells. Rather, the combination of these 3 TLR ligands augmented the quality of the T cell responses primarily by amplifying their functional avidity for the antigen, which was necessary for clearance of virus. The triple combination increased production of DC IL-15 along with its receptor, IL-15Ralpha, which contributed to high avidity, and decreased expression of programmed death-ligand 1 and induction of Tregs. Therefore, selective TLR ligand combinations can increase protective efficacy by increasing the quality rather than the quantity of T cell responses.

Design and evaluation of multi-gene, multi-clade HIV-1 MVA vaccines

Recombinant modified vaccinia virus Ankara (rMVA) expressing HIV-1 genes are promising vaccine candidates. Toward the goal of conducting clinical trials with one or a cocktail of recombinant viruses, four rMVAs expressing env and gag-pol genes from primary HIV-1 isolates representing predominant subtypes from Kenya, Tanzania, Uganda, and Thailand (A, C, D, and CRF01_AE, respectively) were constructed. Efficient expression, processing, and function of Env and Gag were demonstrated. All inserted genes were shown to be genetically stable after repeated passage in cell culture. Strong HIV-specific cellular and humoral immune responses were elicited in mice immunized with each individual vaccine candidate. The MVA/CMDR vaccine candidate expressing CRF01_AE genes has elicited HIV-specific T-cell responses in two independent Phase I clinical trials. Further testing of the other rMVA is warranted.

Elucidating and minimizing the loss by recombinant vaccinia virus of human immunodeficiency virus gene expression resulting from spontaneous mutations and positive selection

While characterizing modified vaccinia virus recombinants (rMVAs) containing human immunodeficiency virus env and gag-pol genes, we detected nonexpressing mutants by immunostaining individual plaques. In many cases, the numbers of mutants increased during successive passages, indicating strong selection pressure. This phenomenon provided an opportunity to investigate the formation of spontaneous mutations in vaccinia virus, which encodes its own cytoplasmic replication system, and a challenge to reduce the occurrence of mutations for vaccine production. Analysis of virus from individual plaques indicated that loss of expression was due to frameshift mutations, mostly by addition or deletion of a single nucleotide in runs of four to six Gs or Cs, and large deletions that included MVA DNA flanking the recombinant gene. Interruption of the runs of Gs and Cs by silent codon alterations and moving the recombinant gene to a site between essential, highly conserved MVA genes eliminated or reduced frameshifts and viable deletion mutants, respectively. The rapidity at which nonexpressing mutants accumulated depended on the individual env and gag-pol genes and their suppressive effects on virus replication. Both the extracellular and transmembrane domains contributed to the selection of nonexpressing Env mutants. Stability of an unstable Env was improved by swapping external or transmembrane domains with a more stable Env. Most dramatically, removal of the transmembrane and cytoplasmic domains stabilized even the most highly unstable Env. Understanding the causes of instability and taking preemptive actions will facilitate the development of rMVA and other poxviruses as human and veterinary recombinant vaccines.

IMVAMUNE: modified vaccinia Ankara strain as an attenuated smallpox vaccine

Smallpox vaccines based on replicating vaccinia virus are known to elicit rare yet serious adverse events, particularly in human populations with immune deficiency, atopic dermatitis and at the extremes of age. A vaccine that induces protective immune responses equivalent to first-generation smallpox vaccines while reducing the risk for severe adverse events is critical for a national stockpile of smallpox vaccines. Modified vaccinia Ankara (MVA) has been proposed as an immediate solution for vaccination of high-risk individuals. Bavarian Nordic's vaccine MVA-BN (IMVAMUNE) is a MVA strain that is replication incompetent in mammalian cell lines. IMVAMUNE has been administered to more than 1900 human subjects to date, including high-risk populations (e.g., people diagnosed with atopic dermatitis or infected with HIV) in which standard replicating vaccines are contraindicated. We review the Phase I clinical trial safety profile and immune responses and compare them with other smallpox vaccines, including ACAM2000 and Dryvax.

Overview of the vaccinia virus expression system

The vaccinia virus expression system differs from others in that transcription occurs in the cytoplasm of mammalian cells rather than in the nucleus. As a vector, vaccinia virus has a number of useful characteristics, including a capacity that permits cloning large fragments of foreign DNA (20+ kbp) with retention of infectivity, a wide host range, a relatively high level of protein synthesis, and "appropriate" transport, secretion, processing, and posttranslational modifications as dictated by the primary structure of the expressed protein and the cell type used. This overview discusses the life cycle of the vaccinia virus along with effects of vaccinia infection. The vaccinia vector expression system is described along with specific steps for expressing genes using these vectors. Important safety considerations are also presented.

Design, construction, and characterization of a multigenic modified vaccinia Ankara candidate vaccine against human immunodeficiency virus type 1 subtype C/B'

The rapid spread of HIV-1 underscores the urgent need to develop an effective vaccine. Using modified vaccinia Ankara (MVA) as a vector, we designed and constructed a multigenic candidate vaccine against a recombinant C/B' subtype of HIV-1 that is dominant in southwest China. Five HIV-1 genes (gag, pol, DeltaV2env, tat, and nef) were introduced into 2 separate regions of the MVA genome using modified single- and dual-promoter insertion vectors. Recombinant MVA was selected by immunofluorescence double-staining and foci purification. The end product is a single recombinant MVA, termed ADMVA, that expresses HIV-1 DeltaV2Env and fusion proteins Gag-Pol and Nef-Tat. By in vitro analyses, all expected HIV-1 proteins were expressed in infected chicken embryo fibroblasts and various human cell lines. Additionally, 2 sequential intramuscular injections of 10(6) 50% tissue infectious culture dose (TCID50) of ADMVA into BALB/c and B6 x B10 mice elicited broad cell-mediated immune responses against all 5 viral proteins as determined by interferon-gamma enzyme immunospot assays. The number of spot-forming cells was in the range of 200 to 800 per million splenocytes, and both CD4 and CD8 T-cell responses were detected. Moreover, high serum titers (>1:20,000) of antibodies against HIV-1 gp120 were also elicited. The magnitude of immune responses correlated with the dose of ADMVA, and the vaccine caused no overt adverse consequences, up to 10(7) TCID50 per injection. ADMVA has since been advanced into clinical trials. A phase 1 study has been completed, and a prime-boost with ADVAX (see accompanying article) is now underway.

IL-15 as a mediator of CD4+ help for CD8+ T cell longevity and avoidance of TRAIL-mediated apoptosis

CD4+ helper T cells contribute to the induction and maintenance of antigen-specific CD8+ T cells. Their absence results in short-lived antigen-specific CD8+ T cells and defective secondary CD8+ T cell responses because of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis. Here, we show that IL-15 codelivered with vaccines can overcome CD4+ T cell deficiency for promoting longevity of antigen-specific CD8+ T cells and avoidance of TRAIL-mediated apoptosis. In both priming and secondary responses, IL-15 down-regulates proapoptotic Bax, an intermediate in TRAIL-mediated apoptosis, and increases anti-apoptotic Bcl-X(L) in CD8+ T cells. Thus, IL-15 is sufficient to mimic CD4+ T cell help. Antigen-specific CD4+ T cells induce dendritic cells (DCs) to produce IL-15. IL-15 is also necessary for optimal help, because helper cells do not deliver effective help through IL-15-/- DCs. Therefore, IL-15 codelivered with vaccines can overcome CD4+ helper T cell deficiency for induction of functionally efficient CD8+ T cells and maintenance of CD8+ cytotoxic T lymphocytes (CTLs), and IL-15 is probably one of the natural mediators of help. These findings suggest new vaccine strategies against infections and cancers, especially in individuals with CD4-deficiency.

Phenotypic analysis of a temperature sensitive mutant in the large subunit of the vaccinia virus mRNA capping enzyme

The heterodimeric vaccinia virus mRNA capping enzyme is a multifunctional enzyme, encoded by genes D1R and D12L. Published biochemical experiments demonstrate that, in addition to mRNA capping, the enzyme is involved in early viral gene transcription termination and intermediate viral gene transcription initiation. This paper presents the phenotypic characterization of Dts36, a temperature sensitive mutant in the large subunit of the mRNA capping enzyme (G705D), encoded by gene D1R. At the non-permissive temperature, Dts36 displays decreased steady state levels of some early RNAs, suggesting a defect in mRNA capping. Mutant infections also show decreased steady state levels of some early proteins, while DNA replication and post-replicative gene expression are absent. Under non-permissive conditions, the mutant directs synthesis of longer-than-normal early mRNAs from some genes, demonstrating that early gene transcription termination is defective. If mutant infections are initiated at the permissive temperature and shifted to the non-permissive temperature late during infection, steady state levels of intermediate gene transcripts decrease while the levels of late gene transcripts remain constant, consistent with a defect in intermediate gene transcription initiation. In addition to its previously described role in mRNA capping, the results presented in this study provide the first in vivo evidence that the vaccinia virus mRNA capping enzyme plays a role in early gene transcription termination and intermediate gene transcription.

Estimation of low frequency antigen-presenting cells with a novel RELISPOT assay

Adequate presentation of self and foreign antigens is a key factor for efficient T-cell immunosurveillance against pathogens and tumors. Cells presenting foreign antigens usually comprise a rare population and are difficult to detect even at the peak of infection. Here we demonstrate a CD8(+) T-cell-based approach that allows detection of specific antigen-presenting cells (APC) at a frequency of less than 0.0005%. When T cells are in excess, they form rosettes with rare APCs, which appear as single spots in an IFN-gamma ELISPOT assay. Using this RELISPOT (Rosette ELISPOT) method we demonstrate the dynamic interplay between CD8 T cells and professional and non-professional APCs following virus challenge.

Enhanced cell surface expression, immunogenicity and genetic stability resulting from a spontaneous truncation of HIV Env expressed by a recombinant MVA

During propagation of modified vaccinia virus Ankara (MVA) encoding HIV 89.6 Env, a few viral foci stained very prominently. Virus cloned from such foci replicated to higher titers than the parent and displayed enhanced genetic stability on passage. Sequence analysis showed a single nucleotide deletion in the 89.6 env gene of the mutant that caused a frame shift and truncation of 115 amino acids from the cytoplasmic domain. The truncated Env was more highly expressed on the cell surface, induced higher antibody responses than the full-length Env, reacted with HIV neutralizing monoclonal antibodies and mediated CD4/co-receptor-dependent fusion. Intramuscular (i.m.), intradermal (i.d.) needleless, and intrarectal (i.r.) catheter inoculations gave comparable serum IgG responses. However, intraoral (i.o.) needleless injector route gave the highest IgA in lung washings and i.r. gave the highest IgA and IgG responses in fecal extracts. Induction of CTL responses in the spleens of individual mice as assayed by intracellular cytokine staining was similar with both the full-length and truncated Env constructs. Induction of acute and memory CTL in the spleens of mice immunized with the truncated Env construct by i.d., i.o., and i.r. routes was comparable and higher than by the i.m. route, but only the i.r. route induced CTL in the gut-associated lymphoid tissue. Thus, truncation of Env enhanced genetic stability as well as serum and mucosal antibody responses, suggesting the desirability of a similar modification in MVA-based candidate HIV vaccines.

Role of alpha3 domain of class I MHC molecules in the activation of high- and low-avidity CD8+ CTLs

CD8 can serve as a co-receptor or accessory molecule on the surface of CTL. As a co-receptor, CD8 can bind to the alpha3 domain of the same MHC class I molecules as the TCR to facilitate TCR signaling. To evaluate the role of the MHC class I molecule alpha3 domain in the activation of CD8(+) CTL, we have produced a soluble 227 mutant of H-2D(d), with a point mutation in the alpha3 domain (Glu227 --> Lys). 227 mutant class I-peptide complexes were not able to effectively activate H-2D(d)-restricted CD8 T cells in vitro, as measured by IFN-gamma production by an epitope-specific CD8(+) CTL line. However, the 227 mutant class I-peptide complexes in the presence of another MHC class I molecule (H-2K(b)) (that cannot present the peptide) with a normal alpha3 domain can induce the activation of CD8(+) CTL. Therefore, in order to activate CD8(+) CTL, the alpha3 domain of MHC class I does not have to be located on the same molecule with the alpha1 and alpha2 domains of MHC class I. A low-avidity CD8(+) CTL line was significantly less sensitive to stimulation by the 227 mutant class I-peptide complexes in the presence of the H-2K(b) molecule. Thus, low-avidity CTL may not be able to take advantage of the interaction between CD8 and the alpha3 domain of non-presenting class I MHC molecules, perhaps because of a shorter dwell time for the TCR-MHC interaction.

Control of mucosal virus infection by influenza nucleoprotein-specific CD8+ cytotoxic T lymphocytes

Background

MHC class I-restricted CD8⁺ cytotoxic T lymphocytes (CTL) are thought to play a major role in clearing virus and promoting recovery from influenza infection and disease. This has been demonstrated for clearance of influenza virus from the lungs of infected mice. However, human influenza infection is primarily a respiratory mucosal infection involving the nasopharynx and tracheobronchial tree. The role of CD8⁺ CTL directed toward the influenza nucleoprotein (NP) in defense against influenza virus infection at the respiratory mucosa was evaluated in two separate adoptive transfer experiments.

Methods

Influenza nucleoprotein (NP)-specific CD8⁺ CTL were generated from splenocytes obtained from Balb/c mice previously primed with influenza A/Taiwan/1/86 (H1N1) infection or with influenza A/PR/8/34 (H1N1)-derived NP plasmid DNA vaccine followed by infection with A/Hong Kong/68 (H3N2) virus. After in vitro expansion by exposure to an influenza NP-vaccinia recombinant, highly purified CD8⁺ T cells exhibited significant lysis in vitro of P815 target cells infected with A/Hong Kong/68 (H3N2) virus while the CD8^- fraction (CD4⁺ T cells, B cells and macrophages) had no CTL activity. Purified CD8⁺ and CD8^- T cells (1 × 10⁷) were injected intravenously or interperitoneally into naive mice four hours prior to intranasal challenge with A/HK/68 (H3N2) virus.

Results

The adoptively transferred NP-vaccinia-induced CD8⁺ T cells caused significant reduction of virus titers in both the lungs and nasal passages when compared to CD8^- cells. Neither CD8⁺ nor CD8^- T cells from cultures stimulated with HIV gp120-vaccinia recombinant reduced virus titers.

Conclusion

The present data demonstrate that influenza NP-specific CD8⁺ CTL can play a direct role in clearance of influenza virus from the upper respiratory mucosal surfaces.

Recombinant modified vaccinia virus Ankara provides durable protection against disease caused by an immunodeficiency virus as well as long-term immunity to an orthopoxvirus in a non-human primate

Recombinant and non-recombinant modified vaccinia virus Ankara (MVA) strains are currently in clinical trials as human immunodeficiency virus-1 (HIV) and attenuated smallpox vaccines, respectively. Here we tested the ability of a recombinant MVA delivered by alternative needle-free routes (intramuscular, intradermal, or into the palatine tonsil) to protect against immunodeficiency and orthopoxvirus diseases in a non-human primate model. Rhesus macaques were immunized twice 1 month apart with MVA expressing 5 genes from a pathogenic simian human immunodeficiency virus (SHIV)/89.6P and challenged intrarectally 9 months later with the pathogenic SHIV/89.6P and intravenously 2.7 years later with monkeypox virus. Irrespective of the route of vaccine delivery, binding and neutralizing antibodies and CD8 responses to SHIV and orthopoxvirus proteins were induced and the monkeys were successively protected against the diseases caused by the challenge viruses in unimmunized controls as determined by viral loads and clinical signs. These non-human primate studies support the clinical testing of recombinant MVA as an HIV vaccine and further demonstrate that MVA can provide long-term poxvirus immunity, essential for use as an alternative smallpox vaccine.

Need for tripeptidyl-peptidase II in major histocompatibility complex class I viral antigen processing when proteasomes are detrimental

CD8(+) T lymphocytes recognize infected cells that display virus-derived antigenic peptides complexed with major histocompatibility complex class I molecules. Peptides are mainly byproducts of cellular protein turnover by cytosolic proteasomes. Cytosolic tripeptidyl-peptidase II (TPPII) also participates in protein degradation. Several peptidic epitopes unexpectedly do not require proteasomes, but it is unclear which proteases generate them. We studied antigen processing of influenza virus nucleoprotein epitope NP(147-155), an archetype epitope that is even destroyed by a proteasome-mediated mechanism. TPPII, with the assistance of endoplasmic reticulum trimming metallo-aminopeptidases, probably ERAAP (endoplasmic reticulum aminopeptidase associated with antigen processing), was crucial for nucleoprotein epitope generation both in the presence of functional proteasomes and when blocked by lactacystin, as shown with specific chemical inhibitors and gene silencing. Different protein contexts and subcellular targeting all allowed epitope processing by TPPII as well as trimming. The results show the plasticity of the cell's assortment of proteases for providing ligands for recognition by antiviral CD8(+) T cells. Our observations identify for the first time a set of proteases competent for antigen processing of an epitope that is susceptible to destruction by proteasomes.

Active treatment of murine tumors with a highly attenuated vaccinia virus expressing the tumor associated antigen 5T4 (TroVax) is CD4+ T cell dependent and antibody mediated

5T4 is a tumor associated antigen that is expressed on the surface of a wide spectrum of human adenocarcinomas. The highly attenuated virus, modified vaccinia Ankara, has been engineered to express human 5T4 (h5T4). In a pre-clinical murine model, the recombinant virus (TroVax) induces protection against challenge with CT26-h5T4 (a syngeneic tumor line expressing h5T4). Anti-tumor activity is long lived, with protection still evident 6 months after the final vaccination. In a therapeutic setting, injection of mice with TroVax results in a reduction in tumor burden of >90%. Depletion of CD8+ T cells has no effect upon therapy in the active treatment model, whereas depletion of CD4+ T cells completely abrogates anti-tumor activity. In a prophylactic setting, depletion of CD4+ and CD8+ T cells after the induction of a h5T4 immune response has no deleterious effect on protection following challenge with CT26-h5T4. In light of these studies, the role of antibodies in protection against tumor challenge was investigated. 5T4 specific polyclonal serum decreased tumor burden by approximately 70%. Thus, we conclude that CD4+ T cells are essential for the induction of a protective immune response and that antibodies are the likely effector moiety in this xenogeneic murine tumor model.

Targeted infection of HIV-1 Env expressing cells by HIV(CD4/CXCR4) vectors reveals a potential new rationale for HIV-1 mediated down-modulation of CD4

Background

Efficient targeted gene transfer and cell type specific transgene expression are important for the safe and effective expression of transgenes in vivo. Enveloped viral vectors allow insertion of exogenous membrane proteins into their envelopes, which could potentially aid in the targeted transduction of specific cell types. Our goal was to specifically target cells that express the T cell tropic HIV-1 envelope protein (Env) using the highly specific interaction of Env with its cellular receptor (CD4) inserted into the envelope of an HIV-1-based viral vector.

Results

To generate HIV-1-based vectors carrying the CD4 molecule in their envelope, the CD4 ectodomain was fused to diverse membrane anchors and inserted together with the HIV-1 coreceptor CXCR4 into the envelopes of HIV-1 vector particles. Independent of the type of CD4 anchor, all chimeric CD4 proteins inserted into HIV-1 vector envelopes and the resultant HIV(CD4/CXCR4) particles were able to selectively confer neomycin resistance to cells expressing the fusogenic T cell tropic HIV-1 Env protein. Unexpectedly, in the absence of Env on the target cells, all vector particles carrying the CD4 ectodomain anchored in their envelope adhered to various cell types without infecting these cells. This cell adhesion was very avid. It was independent of the presence of Env on the target cell, the type of CD4 anchor or the presence of CXCR4 on the particle. In mixed cell populations with defined ratios of Env⁺/Env^- cells, the targeted transduction of Env⁺ cells by HIV(CD4/CXCR4) particles was diminished in proportion to the number of Env^- cells.

Conclusion

Vector diversion caused by a strong, non-selective cell binding of CD4⁺-vector particles effectively prevents the targeted transduction of HIV-1 Env expressing cells in mixed cell populations. This Env-independent cell adhesion severely limits the effective use of targeted HIV(CD4/CXCR4) vectors designed to interfere with HIV-1 replication in vivo. Importantly, the existence of this newly described and remarkably strong CD4-dependent cell adhesion suggests that the multiple viral efforts to reduce CD4 cell surface expression may, in part, be to prevent cell adhesion to non-target cells and thereby to increase the infectivity of viral progeny. Preventing CD4 down-modulation by HIV-1 might be an effective component of a multi-faceted antiviral strategy.

Cutting edge: a single MHC class Ia is sufficient for CD8 memory T cell differentiation

Recent studies have suggested a role for MHC class Ib molecules in providing signals for memory T cell differentiation during the early phases of acute infection. To test this hypothesis, we assessed the development of effector and memory CD8 T cells in transgenic mice expressing a single chain H-2D(d)/beta2-microglobulin (beta2M) fusion protein on a beta2M-deficient background. These mice thus express a single MHC class Ia in the absence of all other beta2M-dependent class Ia and Ib molecules. Following infection with a recombinant vaccinia virus expressing a known D(d)-restricted epitope from HIV-1 gp160, the development of effector and memory cells CD8 T cells was comparable to control mice. Furthermore, these memory cells responded rapidly and robustly to antigenic restimulation. Therefore, we conclude that full CD8 memory differentiation requires only a single MHC class Ia chain, ruling out a requirement for MHC class Ib molecules in this process.

Fusion of HIV-1 envelope-expressing cells to human glomerular endothelial cells through an CXCR4-mediated mechanism

A central question in the pathogenesis of HIV-associated thrombotic microangiopathic (HIV-TMA) lesions is whether the HIV-1 envelope glycoprotein (HIV-1 Env) can interact directly with human glomerular endothelial cells (HGECs) through specific HIV-1 co-receptors. The goal of this study was to determine whether cultured primary HGECs express significant levels of the major HIV-1 co-receptors CD4, CXCR4, and/or CCR5 to allow fusion interactions with HIV-1. The expression of CD4, CXCR-4 and CCR-5 was assessed in cultured HGECs by reverse transcriptase-polymerase chain reaction (RT-PCR) and flow cytometry using specific antibodies. The HIV-1 Env-mediated membrane fusion of target glomerular cells was evaluated by a fluorescent dye transfer-based cell-cell fusion microscopic method. HGECs express CXCR4 mRNA and protein as determined by RT-PCR and immunostaining with phycoerythrin-conjugated anti-CXCR4 Mab 12G5. CD4 and CCR5 were not detected in HGECs, either by RT-PCR or by surface immunostaining with specific antibodies. Incubation of HGECs with cells expressing a CD4-independent envelope strain (HIV-1IIIB-8x) and the CD4-dependent envelope strain (HIV-1IIIB) resulted in transfer of fluorescent dyes of approximately 20% after 8-16 h incubation at 37 degrees C. Incubation in the presence of inhibitors (C34, which blocks six-helix bundle formation, and AMD3100, which interacts with CXCR4) reduced dye transfer by 60%-80%, confirming that the dye transfer was specific with respect to gp120-gp41-mediated fusion. Cultured primary HGECs express CXCR4 but not CD4 or CCR5. The ability of HGECs to promote fusion by a CD4-independent HIV-1 envelope glycoprotein suggests that these cells may become a potential direct target of certain HIV-1 isolates.

Synergy of IL-21 and IL-15 in regulating CD8+ T cell expansion and function

Interleukin (IL)-21 is the most recently recognized of the cytokines that share the common cytokine receptor γ chain (γ_c), which is mutated in humans with X-linked severe combined immunodeficiency. We now report that IL-21 synergistically acts with IL-15 to potently promote the proliferation of both memory (CD44^high) and naive (CD44^low) phenotype CD8⁺ T cells and augment interferon-γ production in vitro. IL-21 also cooperated, albeit more weakly, with IL-7, but not with IL-2. Correspondingly, the expansion and cytotoxicity of CD8⁺ T cells were impaired in IL-21R^−/− mice. Moreover, in vivo administration of IL-21 in combination with IL-15 boosted antigen-specific CD8⁺ T cell numbers and resulted in a cooperative effect on tumor regression, with apparent cures of large, established B16 melanomas. Thus, our studies reveal that IL-21 potently regulates CD8⁺ T cell expansion and effector function, primarily in a synergistic context with IL-15.

Natural history of a recurrent feline coronavirus infection and the role of cellular immunity in survival and disease

We describe the natural history, viral dynamics, and immunobiology of feline infectious peritonitis (FIP), a highly lethal coronavirus infection. A severe recurrent infection developed, typified by viral persistence and acute lymphopenia, with waves of enhanced viral replication coinciding with fever, weight loss, and depletion of CD4+ and CD8+ T cells. Our combined observations suggest a model for FIP pathogenesis in which virus-induced T-cell depletion and the antiviral T-cell response are opposing forces and in which the efficacy of early T-cell responses critically determines the outcome of the infection. Rising amounts of viral RNA in the blood, consistently seen in animals with end-stage FIP, indicate that progression to fatal disease is the direct consequence of a loss of immune control, resulting in unchecked viral replication. The pathogenic phenomena described here likely bear relevance to other severe coronavirus infections, in particular severe acute respiratory syndrome, for which multiphasic disease progression and acute T-cell lymphopenia have also been reported. Experimental FIP presents a relevant, safe, and well-defined model to study coronavirus-mediated immunosuppression and should provide an attractive and convenient system for in vivo testing of anticoronaviral drugs.

CD4 down-regulation by HIV-1 and simian immunodeficiency virus (SIV) Nef proteins involves both internalization and intracellular retention mechanisms

Among the pleiotropic effects of Nef proteins of HIV and simian immunodeficiency virus (SIV), down-modulation of cell surface expression of CD4 is a prominent phenotype. It has been presumed that Nef proteins accelerate endocytosis of CD4 by linking the receptor to the AP-2 clathrin adaptor. However, the related AP-1 and AP-3 adaptors have also been shown to interact with Nef, hinting at role(s) for these complexes in the intracellular retention of CD4. By using genetic inhibitors of endocytosis and small interfering RNA-induced knockdown of AP-2, we show that accelerated CD4 endocytosis is not a dominant mechanism of HIV-1 (NL4-3 strain) Nef in epithelial cells, T lymphocyte cell lines, or peripheral blood lymphocytes. Furthermore, we show that both the CD4 recycling from the plasma membrane and the nascent CD4 in transit to the plasma membrane are susceptible to intracellular retention in HIV-1 Nef-expressing cells. In contrast, AP-2-mediated enhanced endocytosis constitutes the predominant mechanism for SIV (MAC-239 strain) Nef-induced down-regulation of human CD4 in human cells.

IL-15/IL-15Ralpha-mediated avidity maturation of memory CD8+ T cells

T cell avidity is critical to viral clearance, but mechanisms of CD8(+) T cell avidity maturation are poorly understood. Here, we find that IL-15 mediates two mechanisms of avidity maturation. (i) By selection at the population level, IL-15 promotes greater survival of high- compared with low-avidity cytotoxic T lymphocytes (CTLs). High-avidity CTLs express higher levels of IL-15Ralpha and persist longer by homeostatic proliferation. (ii) At the individual cell level, IL-15 induces higher levels of surface coreceptor CD8alphabeta, increasing functional avidity. IL-15 during priming selects or induces higher-avidity CTLs. Conversely, high-avidity CTLs are diminished in IL-15Ralpha knockout mice. These results provide an explanation of CD8+ T cell avidity maturation and may contribute to the design of novel vaccines.

Highly effective control of an AIDS virus challenge in macaques by using vesicular stomatitis virus and modified vaccinia virus Ankara vaccine vectors in a single-boost protocol

Previous studies have shown that vaccination and boosting of rhesus macaques with attenuated vesicular stomatitis virus (VSV) vectors encoding Env and Gag proteins of simian immunodeficiency virus-human immunodeficiency virus (SHIV) hybrid viruses protect rhesus macaques from AIDS after challenge with the highly pathogenic SHIV 89.6P (23). In the present study, we compared the effectiveness of a single prime-boost protocol consisting of VSV vectors expressing SHIV Env, Gag, and Pol proteins to that of a protocol consisting of a VSV vector prime followed with a single boost with modified vaccinia virus Ankara (MVA) expressing the same SHIV proteins. After challenge with SHIV 89.6P, MVA-boosted animals controlled peak challenge viral loads to less than 2 x 10(6) copies/ml (a level significantly lower than that seen with VSV-boosted animals and lower than those reported for other vaccine studies employing the same challenge). MVA-boosted animals have shown excellent preservation of CD4(+) T cells, while two of four VSV-boosted animals have shown significant loss of CD4(+) T cells. The improved protection in MVA-boosted animals correlates with trends toward stronger prechallenge CD8(+)-T-cell responses to SHIV antigens and stronger postchallenge SHIV-neutralizing antibody production.

Coadministration of HIV vaccine vectors with vaccinia viruses expressing IL-15 but not IL-2 induces long-lasting cellular immunity

Vaccine efficacy is determined largely by cellular and humoral immunity as well as long-lasting immunological memory. IL-2 and IL-15 were evaluated in vaccinia vectors expressing HIV gp160 for the establishment of an effective vaccine strategy. Both IL-2 and IL-15 in the vaccinia vector induced strong and long-lasting antibody-mediated immunity as well as a short-term cytotoxic T cell response against HIV gp120. In addition, IL-15 also supported robust CD8+ T cell-mediated long-term immunity, whereas the CD8+ T cell-mediated immunity induced by IL-2 was short-lived. Moreover, we found that the cytokine milieu at the time of priming had surprisingly persistent effects on the character of the memory CD8 T cells long afterward with respect to their fate, functional activities, cytokine receptor expression, and antigen-independent proliferation.

Induction of a strong HIV-specific CD8+ T cell response in mice using a fowlpox virus vector expressing an HIV-1 multi-CTL-epitope polypeptide

Recombinant avipoxvirus vectors are attractive candidates for use in vaccination strategies for infections such as human immunodeficiency virus type 1 (HIV-1), where induction of a CD8+ T cell response is thought to be an important component of protective immunity. Here, we report the expression of a multiepitope polypeptide (TAB9) composed of the central 15 amino acids of the V3 loop from six different isolates of HIV-1 in a fowlpox virus (FWPV) vector, and the use of this vector (FPTAB9LZ) to induce strong HIV-specific CD8+ T cell responses in mice. In animals immunized twice intravenously with FPTAB9LZ, almost 2% of the CD8+ T cells in the spleen were shown to produce IFN-gamma in response to stimulation with HIV-1 peptides 1 week after the second immunization. The most dominant response was to the HIV-1 IIIB peptide. A strong HIV-specific response was also induced by intraperitoneal immunization of mice with FPTAB9LZ, whilst subcutaneous immunization elicited a weaker response. Intraperitoneal immunization with FPTAB9LZ was also shown to provide protection against challenge with a recombinant vaccinia virus expressing antigens, including those in TAB9. These results confirm the potential of FWPV vectors for use in HIV vaccination strategies.

A push-pull approach to maximize vaccine efficacy: abrogating suppression with an IL-13 inhibitor while augmenting help with granulocyte/macrophage colony-stimulating factor and CD40L

Although a role for CD4(+) helper cells in CD8(+) cytotoxic T lymphocyte (CTL) induction by vaccines is widely recognized, much less is known about a counterbalancing role of CD4(+) T cells in down-modulating this response, or about ways to optimize vaccine responses through abrogation of this negative regulatory mechanism. Here, we discovered a synergistic enhancement of vaccine-mediated CTL induction and protection by the relief of suppression through depletion of regulatory CD4(+) cells, including CD4(+) NKT cells, or blockade of IL-13 made by these cells, combined with the cytokine granulocyte/macrophage colony-stimulating factor and the costimulatory molecule CD40L. Indeed, in the absence of helper epitopes, granulocyte/macrophage colony-stimulating factor and the helper-mimetic molecule CD40L are not sufficient to replace help to induce CTL without abrogation of CD4(+) T cell-mediated suppression, suggesting a role for T cell help in overcoming suppression. The increased CTL induction translated to striking protection against viral infection by a vaccine by using this synergistic combined approach. These results argue for a push-pull approach to maximize vaccine efficacy, especially for HIV and cancer.

Cell surface expression of the HIV-1 envelope glycoproteins is directed from intracellular CTLA-4-containing regulated secretory granules

The envelope glycoprotein (Env) of HIV-1 is incorporated into virions that bud from the cell surface of infected T cells. With immunofluorescence microscopy and subcellular membrane fractionation techniques, the intracellular fate of Env in the secretory pathway of HIV-1-infected T cells was evaluated. Rather than trafficking constitutively from the Golgi to the cell surface, Env is directed to intracellular CTLA-4-containing granules, whose recruitment to the cell surface is regulated. The use of the regulated pathway for intracellular Env storage before virion assembly holds implications for the staging of Env exposure at the cell surface of infected cells and of coordinating HIV virion assembly.

Effect of the V3 loop deletion of envelope glycoprotein on cellular responses and protection against challenge with recombinant vaccinia virus expressing gp160 of primary human immunodeficiency virus type 1 isolates

The magnitude and breadth of cytotoxic-T-lymphocyte (CTL) responses induced by human immunodeficiency virus type 1 (HIV-1) envelope protein from which the hypervariable V3 loop had been deleted (DeltaV3) were evaluated in the HLA-A2/K(b) transgenic mice. It was demonstrated that vaccines expressing the DeltaV3 mutant of either HIV-1(IIIB) or HIV-1(89.6) envelope glycoprotein induced broader CD8(+) T-cell activities than those elicited by the wild-type (WT) counterparts. Specifically, the differences were associated with higher responses to conserved HLA-A2-restricted CTL epitopes of the envelope glycoprotein and could be correlated with an increased cell surface occupancy by the epitope-HLA-A2 complexes in target cells expressing the DeltaV3 mutant. Using recombinant vaccinia virus expressing heterologous gp160 of primary HIV-1 isolates in a murine challenge system, we observed that the extent of resistance to viral transmission was higher in animals immunized with the DeltaV3 than the WT envelope vaccine. The protection was linked to the presence of envelope-specific CD8(+) T cells, since depletion of these cells by anti-CD8 antibody treatment at the time of challenge abolished the vaccine-induced protection. The results from our studies provide insights into approaches for boosting the breadth of envelope-specific CTL responses.

Comparison of vaccine strategies using recombinant env-gag-pol MVA with or without an oligomeric Env protein boost in the SHIV rhesus macaque model

Rhesus macaques were immunized with a replication-deficient vaccinia virus (MVA) expressing human immunodeficiency virus type 1 89.6 envelope (env) and SIV gagpol (MVA/SHIV89.6) with or without a protein boost consisting of soluble 89.6 env (gp140). Immunization with MVA/SHIV89.6 alone elicited binding antibodies in all animals and neutralizing antibodies in 5 of 15 animals. Both types of antibodies were enhanced by protein boosting. In addition, CD8 cells exhibiting CM9 tetramer binding were detected in the subset of animals that were Mamu-A*01 positive. Animals were challenged intravenously with either SHIV-89.6 (Study 1) or the more pathogenic derivative SHIV-89.6P (Study 2). In Study 1, all control and vaccinated animals except one became infected. However, the levels of viremia were as follows: controls > rMVA alone > rMVA + protein. The differences were statistically significant between immunized and control groups but not between the two immunized groups. In Study 2, all animals became infected; however, the vaccinated group exhibited a 5-fold reduction in peak viremia and a 10-fold reduction in the postacute phase viremia in comparison to the controls. All of the controls required euthanasia by 10 months after challenge. A relationship between vaccine-induced antibody titers and reduction in virus burden was observed in both studies. Thus, immunization with MVA/SHIV89.6 alone or with a protein boost stimulated both arms of the immune system and resulted in significant control of viremia and delayed progression to disease after challenge with SHIV-89.6P.