Cellular and molecular mechanisms of vaccine-induced protection against retroviral infections

More than 15 years after the discovery of human immunodeficiency virus (HIV), researchers are still struggling to design a protective AIDS vaccine. A remaining problem is a lack of basic knowledge about the immunological requirements for protection against retroviruses. Infection of macaque monkeys with simian immunodeficiency virus is still the best model for HIV vaccine research. However, in this model it remains difficult to determine protective immunological mechanisms because of limited numbers of experimental animals and their genetic heterogeneity. Thus, fundamental concepts in retroviral immunology have to be defined in other ways such as mouse models. This minireview summarizes new findings on cellular and molecular mechanisms in protection of mice against Friend murine retrovirus infection. It has been shown that complex immune responses, including B and T cell responses, are required for efficient protection in this model. Multiple viral antigens are necessary to elicit such broad immune reactivity. Efficacious vaccines must protect not only against acute disease, but also against the establishment of persistent infections or the host is at serious risk of virus reactivation. The minireview closes with a discussion on the relevance of findings from the mouse model on the design of a protective vaccine against HIV.

Feline leukemia virus DNA vaccine efficacy is enhanced by coadministration with interleukin-12 (IL-12) and IL-18 expression vectors

The expectation that cell-mediated immunity is important in the control of feline leukemia virus (FeLV) infection led us to test a DNA vaccine administered alone or with cytokines that favored the development of a Th1 immune response. The vaccine consisted of two plasmids, one expressing the gag/pol genes and the other expressing the env gene of FeLV-A/Glasgow-1. The genetic adjuvants were plasmids encoding the feline cytokines interleukin-12 (IL-12), IL-18, or gamma interferon (IFN-gamma). Kittens were immunized by three intramuscular inoculations of the FeLV DNA vaccine alone or in combination with plasmids expressing IFN-gamma, IL-12, or both IL-12 and IL-18. Control kittens were inoculated with empty plasmid. Following immunization, anti-FeLV antibodies were not detected in any kitten. Three weeks after the final immunization, the kittens were challenged by the intraperitoneal inoculation of FeLV-A/Glasgow-1 and were then monitored for a further 15 weeks for the presence of virus in plasma and, at the end of the trial, for latent virus in bone marrow. The vaccine consisting of FeLV DNA with the IL-12 and IL-18 genes conferred significant immunity, protecting completely against transient and persistent viremia, and in five of six kittens protecting against latent infection. None of the other vaccines provided significant protection.

Infections after stem cell transplantation in children: state of the art and recommendations

At the workshop on infections after stem cell transplantation (SCT) in children, the following topics were introduced by invited speakers and discussed with the audience: empirical antimicrobial therapy in the pre-engraftment period, early diagnosis of fungal and viral infections and possibilities to treat them and the possible role of G-CSF early post-SCT. Episodes of fever in the pre-engraftment period mostly are unexplained, and in about one quarter due to bacteremia, mostly by Gram-positive cocci. No single drug or combination of drugs used for antimicrobial therapy is superior, neither does it cover 100% of the pathogens. Close microbiological surveillance of the patients and knowledge of the local microbial epidemiology are requested for optimal therapy. Early fungal infections are reactivations of pre-SCT infections, late fungal infections mostly are associated with failure of engraftment or GvHD and its treatment. Except for suggestive ultrasound or CT-scan abnormalities, the possibilities for early diagnosis are limited c.q. not reliable. Fluconazol prophylaxis is recommended to prevent Candida albicans invasion. A number of new antifungal drugs are being tested in phase I and II studies. CMV, EBV and adenoviruses may reactivate after SCT, causing severe disease with a high mortality, especially in non-HLA-identical donor-recipient combinations. Frequent surveillance cultures for CMV and adenoviruses, pp65-CMV antigen detection in WBC and PCR techniques for CMV, EBV and adenoviruses all have their own contribution to the early diagnosis of dissemination of the viral infection. Therapeutical possibilities, except with respect to ganciclovir and foscarnet for CMV infection, are still limited. The effectiveness of cidofovir is under study. Adoptive therapy with virus-specific CTL's probably represents the new frontier. G-CSF administration early after SCT has a beneficial effect on PMN recovery, hospitalization time, use of antibiotics and total parenteral nutrition requirement in children undergoing allogeneic and autologous BMT. No benefit is observed in children undergoing peripheral blood SCT. The routine use of G-CSF in the latter group of patients is not justified.

A novel glycosylated Cu/Zn-containing superoxide dismutase: production and potential therapeutic effect

The fungal strain Humicola lutea 103 produces a naturally glycosylated Cu/Zn SOD. To improve its yield, the effect of an increased concentration of dissolved oxygen (DO) on growth and enzyme biosynthesis by the producer, cultivated in a 3 l bioreactor, was examined. Exposure to a 20% DO level caused a 1.7-fold increase of SOD activity compared to the DO-uncontrolled culture. Maximum enzyme productivity of SOD was approximately 300 x 10(3) U (kg wet biomass)(-1). The novel enzyme was purified to electrophoretic homogeneity. The presence of Cu and Zn were confirmed by atomic absorption spectrometry. The molecular mass of H. lutea Cu/Zn SOD was calculated to be 31870 Da for the whole molecule and 15936 Da for the structural subunits. The N-terminal sequence revealed a high degree of structural homology with Cu/Zn SOD from other prokaryotic and eukaryotic sources. H. lutea Cu/Zn SOD was used in an in vivo model for the demonstration of its protective effect against myeloid Graffi tumour in hamsters. Comparative studies revealed that the enzyme (i) elongated the latent time for tumour appearance, (ii) inhibited tumour growth in the early stage of tumour progression (73-75% at day 10) and (iii) increased the mean survival time of Graffi-tumour-bearing hamsters. Moreover, the fungal Cu/Zn SOD exhibited a strong protective effect on experimental influenza virus infection in mice. The survival rate increased markedly, the time of survival rose by 5.2 d and the protective index reached 86%. The H. lutea SOD protected mice from mortality more efficiently compared to the selective antiviral drug ribavirin and to commercial bovine SOD. In conclusion, our results suggest that appropriate use of the novel fungal SOD, applied as such or in combination with selective inhibitors, could outline a promising strategy for the treatment of myeloid Graffi tumour and influenza virus infection.

Packaging cell line DNA contamination of vector supernatants: implication for laboratory and clinical research

Investigators conducting retroviral gene therapy trials are required to monitor for the presence of replication-competent retrovirus (RCR). The required testing utilizes a combination of biologic assays and molecular tests using PCR. In the course of a human clinical gene therapy trial, we detected 4070A viral envelope sequences in CD34(+) peripheral blood stem cells 2 days after transduction using a PCR-based assay, suggesting the presence of RCR. The supernatant and producer cells used for vector generation had been negative in extensive screening using the extended S(+)/L(-) assay. The presence of a replication-competent virus was subsequently excluded by a combination of biologic and PCR analyses. The source of the 4070A viral envelope sequences was determined to be packaging cell line DNA in the vector supernatant. The analysis of a variety of vector supernatants by quantitative real-time PCR revealed 4070A envelope DNA sequences from the packaging cell line in concentrations equivalent to approximately 50-500 focus-forming units per milliliter of wild-type 4070A virus. When PCR was performed after reverse transcriptase treatment of supernatant (i.e., assessing both RNA and DNA content), 4070A envelope sequence concentrations ranged from 10(2) to 3.5 x 10(3) focus-forming units per milliliter of wild-type 4070A virus. Our data indicate that PCR should not be used to analyze transduced cells for RCR within the first 2 weeks of vector exposure. Furthermore, investigators using PCR to analyze transduction efficiency shortly after vector exposure may experience false-positive findings.

A milk protein lactoferrin enhances human T cell leukemia virus type I and suppresses HIV-1 infection

Human T cell leukemia virus type I (HTLV-I) and HIV-1, causative agents of adult T cell leukemia/lymphoma and AIDS, respectively, are transmitted vertically via breast milk. Here we demonstrate that lactoferrin, a milk protein that has a variety of antimicrobial and immunomodulatory activities, facilitates replication of HTLV-I in lymphocytes derived from asymptomatic HTLV-I carriers and transmission to cord blood lymphocytes in vitro. Transient expression assays revealed that lactoferrin can transactivate HTLV-I long terminal repeat promoter. In contrast, lactoferrin inhibits HIV-1 replication, at least in part, at the level of viral fusion/entry. These results suggest that lactoferrin may have different effects on vertical transmission of the two milk-borne retroviruses.

Feline leukaemia virus: protective immunity is mediated by virus-specific cytotoxic T lymphocytes

Feline leukaemia virus (FeLV) nucleic acid vaccination of domestic cats affords protection against viraemia and the development of latency without inducing antiviral antibodies.1 To determine the contribution of cell-mediated immunity to the control of virus replication and clearance from the host, FeLV-specific cytotoxic T lymphocyte (CTL) responses were compared in vaccine-protected, transiently viraemic, and persistently viraemic cats. Vaccinal immunity was associated with the detection of higher levels of virus-specific effector CTL in the peripheral blood and lymphoid organs to FeLV Gag/Pro and Env antigens than those observed in unvaccinated control, persistently viraemic cats (P<0.001). Likewise, higher levels of virus-specific CTLs were also observed in transiently viraemic cats which recovered following exposure to FeLV. In cats that controlled their infection, recognition of Gag/Pro antigens was significantly higher than the recognition of Env antigens. This is the first report highlighting the very significant role that virus-specific CTL have in determining the outcome of FeLV infection in either vaccinated cats or cats recovering naturally from FeLV exposure.

Neoplastic diseases: Marek's disease, avian leukosis and reticuloendotheliosis

The commercially important neoplastic diseases of poultry are Marek's disease, which is caused by a herpesvirus, and the avian leukoses and reticuloendotheliosis, which are caused by retroviruses. These diseases are responsible for economic loss due to both mortality and depressed performance. Marek's disease virus (MDV) and avian leukosis viruses (ALVs) are prevalent throughout the world, and new strains which arise in particular locations may spread across borders, thereby undermining national disease control measures. Reticuloendotheliosis virus (REV) is also present in many countries. Marek's disease virus is transmitted horizontally only, and international spread in hatching eggs and day-old chicks can be prevented by appropriate hygiene precautions. Transmission of ALV and REV occurs both horizontally and vertically (through the egg), and measures to prevent international spread are more demanding. Marek's disease is controlled by vaccination, whilst avian leukosis is controlled by virus eradication programmes, mainly at the primary breeding level. Similar virus control measures can be applied for reticuloendotheliosis if necessary. No strong evidence exists to suggest that these avian tumour viruses constitute a danger to public health.

Different immunological requirements for protection against acute versus persistent Friend retrovirus infections

The propensity of retroviruses to rapidly establish persistent infections poses a formidable problem in vaccination strategies. In the current study, we use a live attenuated vaccine to study protection against acute and persistent Friend virus infections in mice. Adoptive transfers of immune CD8(+) T cells combined with passive immunizations with virus-neutralizing antibodies increased protection against acute disease compared with either treatment alone, but there was no protection against the establishment of persistent infection. In addition, the protection against acute disease elicited by the combination treatment was dependent on endogenous CD4(+) T cells as no protection was achieved in CD4(+) T-cell-depleted mice. Quantitative studies showed that doubling the numbers of immune lymphocytes used in adoptive transfer experiments increased protection against acute disease depending on the type of lymphocyte subset used in the transfer. CD8(+) T cells were the most potent subset for the transfer of such protection. However, even high numbers of immune CD8(+) T cells gave no protection against the establishment of persistent infections. The data indicate that strengthening the numbers of specific immune cell subsets may have a beneficial effect on protection against acute disease, but protection from establishment of persistence requires complex immune responses involving multiple lymphocyte subsets.

Effects of a 98% solution of glycerol or sterilization with ethylene oxide on FeLV in bone allografts and effects on bone incorporation of allografts in cats

OBJECTIVES

To compare virucidal effects and bone incorporation properties of cortical bone allografts transplanted into specific-pathogen-free (SPF) cats. Allografts consisted of untreated bone from a SPF cat (negative-control group) and bone from 5 FeLV-infected cats that was subjected to sterilization with ethylene oxide (ETO), preservation with glycerol, or no treatment (positive-control group).

SAMPLE POPULATION

Bones from the aforementioned groups and twenty 8-week-old SPF cats (5 cats/group) implanted with an allograft from 1 of the aforementioned groups.

PROCEDURE

After implantation, blood samples were collected weekly to monitor FeLV p27 antigen and antibody titers. Quantification of FeLV provirus was performed on blood samples at weeks 0, 4, and 8 and donor bone samples at time of implantation. Cats were euthanatized 8 weeks after transplantation, and graft sites were evaluated.

RESULTS

All results for negative-control cats were negative. All ETO group cats had negative results for antigen and provirus in blood, whereas 1 cat had a low antibody titer. Although 3 ETO-treated allografts were positive for provirus, the DNA appeared denatured. One cat in the glycerol group had positive results for all tests in blood samples. All glycerol-preserved allografts were positive when tested for provirus. All results for positive-control group cats were positive. Differences in incorporation of bone grafts were not observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Glycerol preservation of FeLV-infected bone allografts did not eliminate transmission of retrovirus to recipients. In contrast, ETO sterilization appeared to denature DNA and prevent infection. Treatments did not affect incorporation of bone grafts in young cats.

Safe sorting of GFP-transduced live cells for subsequent culture using a modified FACS vantage

BACKGROUND

A stream-in-air cell sorter enables rapid sorting to a high purity, but it is not well suited for sorting of infectious material due to the risk of airborne spread to the surroundings.

METHODS

A FACS Vantage cell sorter was modified for safe use with potentially HIV infected cells. Safety tests with bacteriophages were performed to evaluate the potential spread of biologically active material during cell sorting. Cells transduced with a retroviral vector carrying the gene for GFP were sorted on the basis of their GFP fluorescence, and GFP expression was followed during subsequent culture.

RESULTS

The bacteriophage sorting showed that the biologically active material was confined to the sorting chamber. A failure mode simulating a nozzle blockage resulted in detectable droplets inside the sorting chamber, but no droplets could be detected when an additional air suction from the sorting chamber had been put on. The GFP transduced cells were sorted to 99% purity. Cells not expressing GFP at the time of sorting did not turn on the gene during subsequent culture. Un-sorted cells and cells sorted to be positive for GFP showed a decrease in the fraction of GFP positive cells during culture.

CONCLUSIONS

Sorting of live infected cells can be performed safely and with no deleterious effects on vector expression using the modified FACS Vantage instrument.

Kinetics of the development of protective immunity in mice vaccinated with a live attenuated retrovirus

Vaccination of mice with a live attenuated vaccine virus induces potent protection against subsequent challenge with pathogenic Friend retroviral complex. The kinetic studies presented here demonstrate protection from acute splenomegaly as early as 1 week postvaccination. At this time point virus-specific cytotoxic T lymphocytes (CTL) were demonstrable in direct chromium release assays. However, during the first 2 weeks after vaccination protection was incomplete since the mice were not protected against establishment of low-level persistent infections in the spleen. By 3 weeks postvaccination the animals were protected against the establishment of persistent virus as well as acute splenomegaly. The timing of this complete protection correlated with the presence of both virus-neutralizing antibodies and primed CTL in the immunized mice. Within 3 days of virus challenge, vaccinated mice showed high levels of activated B cells and CD4(+) and CD8(+) T cells, indicating an efficient priming of all lymphocyte subsets. Despite very limited replication of the vaccine virus, the protective effect was long lived and was still present 6 months after immunization.

Effects of Selegiline in a retroviral rat model for neurodegenerative disease

Upon inoculation into neonatal rats, murine leukemia virus (MuLV) NT40 causes a non-inflammatory degeneration of the central nervous system. While microglia cells appear to be the major target cells within the brain parenchyma for neurovirulent MuLV, degenerating neurons do not express retroviral gene products. In order to protect rats from neuronal damage we treated retrovirally infected rats once with monoamine oxidase (MAO) B inhibitor Selegiline which--under different conditions--exerts neuroprotective effects. Unexpectedly, when administered at 17 days post-infection (d.p.i.) a single intraperitoneal dose of Selegilin (1 mg/kg bodyweight) significantly shortened the incubation period for neurological disease. In contrast, Selegiline given in a lower dosage (0.05 mg/kg bodyweight) and/or at a different time point (13 d.p.i.) at the low (0.05 mg/kg bodyweight) and the high dose (1.0 mg/kg bodyweight) had no effect on the outcome of neurological disease. Animals treated with Selegiline (1.0 mg/kg bodyweight at 17 d.p.i.) contained higher amounts of viral loads in the CNS, higher numbers of brain cells expressing major histocompatibility complex class II molecules, and exhibited inhibition of MAO-B in comparison to untreated yet infected (control) animals. Supposedly, Selegiline activated the major target cell population of the CNS for MuLV-NT40, microglia, with the consequence of enhanced susceptibility for retroviral infection and triggered endogenous mechanism(s) involved in the pathogenesis of retroviral neurodegeneration.

Protection against establishment of retroviral persistence by vaccination with a live attenuated virus

Many human viruses not only cause acute diseases but also establish persistent infections. Such persistent viruses can cause chronic diseases or can reactivate to cause acute diseases in AIDS patients or patients receiving immunosuppressive therapies. While the prevention of persistent infections is an important consideration in the design of modern vaccines, surprisingly little is known about this aspect of protection. In the current study, we tested the feasibility of vaccine prevention of retroviral persistence by using a Friend virus model that we recently developed. In this model, persistent virus can be detected at very low levels by immunosuppressing the host to reactivate virus or by transferring persistently infected spleen cells into highly susceptible mice. Two vaccines were analyzed, a recombinant vaccinia virus vector expressing Friend virus envelope protein and a live attenuated Friend virus. Both vaccines reduced pathogenic virus loads to levels undetectable by infectious center assays. However, only the live, attenuated vaccine prevented immunosuppression-induced reactivation of persistent virus. Thus, even very low levels of persistent Friend virus posed a significant threat during immunosuppression. Our results demonstrate that vaccine protection against establishment of retroviral persistence is attainable.

Requirement for multiple lymphocyte subsets in protection by a live attenuated vaccine against retroviral infection

Infection by live attenuated retroviruses provides excellent protection from challenge with pathogenic viruses in several animal models, but little is known about which immune effectors are necessary for protection. We examined this using adoptive transfer experiments in the Friend virus mouse model. Transfers of immune spleen cells into naive mice conferred complete protection, and transfers of purified lymphocyte subsets demonstrated that this effect required complex immune responses involving CD4+ and CD8+ T cells and also B cells. In addition, passive immunization experiments demonstrated that antibodies alone reduced virus loads but did not prevent infection. These findings may have implications for retroviral vaccine design in general.

Protection of retrovirus-induced disease by transplantation of bone marrow cells transduced with MuLV env gene via retrovirus vector

Fv-4 is a mouse gene that dominantly confers resistance to infection by ecotropic murine leukemia virus (MuLV). We have demonstrated previously that bone marrow chimeras in which hematopoietic cells were replaced with cells expressing Fv-4 resistant (Fv-4r) gene product became refractory to Friend leukemia virus (FLV)-induced leukemogenesis. To induce in vivo resistance against retrovirus-induced diseases by retroviral vector-mediated gene transduction, we introduced Fv-4 env gene into bone marrow cells of FLV-susceptible C3H/He (C3H) mice with retroviral vector (pLSF) derived from murine Friend spleen focus forming virus (SFFV) and the cells were transplanted into lethally irradiated C3H mice. After the bone marrow transplantation, Fv-4r gene product was successfully expressed on erythroid and myeloid cells, while lymphoid cells were only weakly expressing Fv-4r gene product. The C3H mice expressing relatively higher amounts of Fv-4r gene product were rendered resistant to FLV-induced erythroleukemia, while mice expressing lower amounts of the Fv-4r gene product were still susceptible. Effective protection of FLV-induced leukemia in these mice suggested that the Fv-4r gene expression by erythroid cells that were the major target of FLV infection might be critical for resisting FLV-induced leukemia. Thus, gene therapy model by transducing Fv-4r env gene using bone marrow transplantation would provide a useful protection model system of retrovirus-induced diseases.