Protection of macaques against infection with simian type D retrovirus (SRV-1) by immunization with recombinant vaccinia virus expressing the envelope glycoproteins of either SRV-1 or Mason-Pfizer monkey virus (SRV-3)

Simian Foamy Virus Co-Infections

Foamy viruses (FVs), also known as spumaretroviruses, are complex retroviruses that are seemingly nonpathogenic in natural hosts. In natural hosts, which include felines, bovines, and nonhuman primates (NHPs), a large percentage of adults are infected with FVs. For this reason, the effect of FVs on infections with other viruses (co-infections) cannot be easily studied in natural populations. Most of what is known about interactions between FVs and other viruses is based on studies of NHPs in artificial settings such as research facilities. In these settings, there is some indication that FVs can exacerbate infections with lentiviruses such as simian immunodeficiency virus (SIV). Nonhuman primate (NHP) simian FVs (SFVs) have been shown to infect people without any apparent pathogenicity. Humans zoonotically infected with simian foamy virus (SFV) are often co-infected with other viruses. Thus, it is important to know whether SFV co-infections affect human disease.

Induction of neutralizing antibody response against koala retrovirus (KoRV) and reduction in viral load in koalas following vaccination with recombinant KoRV envelope protein

Koala retrovirus (KoRV) infects the majority of Australia’s koalas (Phascolarctos cinereus) and has been linked to several life-threatening diseases such as lymphoma and leukemia, as well as Chlamydia and thus poses a threat to the continued survival of this species. While quarantine and antiretroviral drug treatment are possible control measures, they are impractical, leaving vaccination as the only realistic option. In this study, we examined the effect of a recombinant envelope protein-based anti-KoRV vaccine in two groups of South Australian koalas: KoRV infected or KoRV free. We report a successful vaccination response in the koalas with no vaccine-associated side effects. The vaccine induced a significant humoral immune response as well as the production of neutralizing antibodies in both groups of koalas. We also identified B-cell epitopes that were differentially recognized in KoRV-infected versus KoRV-free koalas following vaccination. Importantly, we also showed that vaccination had a therapeutic effect on koalas infected exogenously with KoRV by reducing their circulating viral load. Together, this study highlights the possibility of successfully developing a vaccine against KoRV infection in koalas.

A vaccine candidate for Koala retrovirus elicits a protective antibody response and reduces the viral load in already-infected koalas. Koala retrovirus (KoRV), first identified in the last 20 years, is a life-threatening, endemic pathogen affecting Australian koalas. In pursuit of an effective KoRV vaccine, the University of the Sunshine Coast’s Peter Timms led a group of Australian scientists to develop a candidate based on the transmembrane section of the virus’ envelope protein. The six koalas vaccinated in the study all generated a strong antibody response to the envelope protein, and a strong neutralizing antibody response was reported during in vitro tests. Vaccinated koalas with pre-existing KoRV infection benefited from an average 79% reduction in viral load when measured 12 weeks after vaccination. Further research should be prioritized to provide much-needed protection to Australia’s koalas.

Facial paralysis and lymphocytic facial neuritis in a rhesus macaque (Macaca mulatta) positive for simian retrovirus type D2

Simian retrovirus type D (SRVD) is a naturally occurring betaretrovirus in nonhuman primates of the genus Macaca. Infection can lead to a variety of clinical, hematologic, and histopathologic abnormalities. We report an unusual clinical presentation of facial paralysis and histologic lymphocytic neuritis in an SRVD type 2 (SRVD2)-infected rhesus macaque (Macaca mulatta) with a catheter-associated vena caval thrombus, anemia, thrombocytopenia, and multisystemic lymphoid hyperplasia. At initial presentation, a right atrial mass was detected by echocardiography. The macaque was clinically asymptomatic but had persistent anemia, thrombocytopenia, hyperglobulinemia, and later neutropenia. It was seropositive for SRV and PCR-positive for SRVD 2. Approximately 1 mo after initial presentation, the macaque developed right facial paralysis and was euthanized. Histologic lesions included lymphoplasmacytic aggregates affecting multiple organs, consistent with SRV-related lymphoid hyperplasia. The right facial nerve showed lymphoplasmacytic inflammation. The nerve itself was negative immunohistochemically for SRV antigen, but antigen was present infrequently in pericapillary lymphoid cells within the facial nerve and abundantly within lymphoid aggregates in the adjacent parotid salivary gland, bone marrow, and soft tissue. Known neurotropic viruses could not be identified. Given the widespread inflammation in this macaque, particularly in the area surrounding the facial nerve, lymphocytic neuritis and facial paralysis likely were an indirect effect of SRV infection due to local extension of SRV-related inflammation in the surrounding tissue.

Vesicular stomatitis virus-simian retrovirus type 2 vaccine protects macaques from detectable infection and B-cell destruction

Natural infection with simian retrovirus (SRV) has long been recognized in rhesus macaques (RMs) and may result in an AIDS-like disease. Importantly, SRV infections persist as a problem in recently imported macaques. Therefore, there is a clear need to control SRV spread in macaque colonies. We developed a recombinant vesicular stomatitis virus (VSV)-SRV vaccine consisting of replication-competent hybrid VSVs that express SRV gag and env in separate vectors. The goal of this study was to assess the immunogenicity and protective efficacy of the VSV-SRV serotype 2 vaccine prime-boost approach in RMs. The VSV-SRV vector (expressing either SRV gag or env) vaccines were intranasally administered in 4 RMs, followed by a boost 1 month after the first vaccination. Four RMs served as controls and received the VSV vector alone. Two months after the boost, all animals were intravenously challenged with SRV-2 and monitored for 90 days. After the SRV-2 challenge, all four controls became infected, and viral loads (VLs) ranged from 10(6) to 10(8) SRV RNA copies/ml of plasma. Two animals in the control group developed simian AIDS within 7 to 8 weeks postinfection and were euthanized. Anemia and weight loss were observed in the remaining controls. During acute infection, severe B-cell depletion and no significant changes in T-cell population were observed in the control group. Control RMs with greater preservation of B cells and lower VLs survived longer. SRV-2 was undetectable in vaccinated animals, which remained healthy, with no clinical or biological signs of infection and preservation of B cells. Our study showed that the VSV-SRV vaccine is a strong approach for preventing clinically relevant type D retrovirus infection and disease in RMs, with protection of 4/4 RMs from SRV infection and prevention of B-cell destruction. B-cell protection was the strongest correlate of the long-term survival of all vaccinated and control RMs.

An updated review of simian betaretrovirus (SRV) in macaque hosts

This review is an updated summary of nearly 30 years of SRV history and provides new and critical findings of original research accomplished in the last 5 years including, but not limited to, the pathogenetic mechanisms underlying the origin of hematopoietic abnormalities observed in infected hosts and proposed new SRV serotypes. Despite major advances in the understanding and control of SRV disease, much more remains to be learned and SRV continues to be an exciting and attractive primate model for comparative studies of the mechanisms of retroviral immunosuppression.

Problems and emerging approaches in HIV/AIDS vaccine development

According to recent estimates, 39.5 million people have been infected with HIV and 2.9 million have already died. The effect of HIV infection on individuals and communities is socially and economically devastating. Although antiretroviral drugs have had a dramatically beneficial impact on HIV-infected individuals who have access to treatment, it has had a negligible impact on the global epidemic. Therefore, the need for an efficacious HIV/AIDS vaccine remains the highest priority of the world HIV/AIDS agenda. The generation of a vaccine against HIV/AIDS has turned out to be extremely challenging, as indicated by > 20 years of unsuccessful attempts. This review discusses the major challenges in the field and key experimental evidence providing a rationale for the use of non-structural HIV proteins, such as Rev, Tat and Nef, either in the native form or expressed by viral vectors such as a replicating adeno-vector. These non-structural proteins alone or in combination with modified structural HIV-1 Env proteins represent a novel strategy for both preventative and therapeutic HIV/AIDS vaccine development.

Persistence and dissemination of simian retrovirus type 2 DNA in relation to viremia, seroresponse, and experimental transmissibility in Macaca fascicularis

Endemic simian retrovirus (SRV) infection can cause fatal simian AIDS in Macaca fascicularis, but many individuals survive with few clinical signs. To further clarify the parameters of SRV pathogenesis, we investigated the persistence of viral DNA forms in relation to active viremia, antibody response, and transmissibility of infection. In M. fascicularis from endemically SRV-2-infected colonies, viral DNA was present in both linear and unintegrated long terminal repeat circular forms in peripheral blood mononuclear cells of all viremic and many nonviremic animals. Long-term followup of three individuals with distinct infection patterns demonstrated persistence of linear and circular forms of viral DNA in peripheral blood mononuclear cells and tissues, irrespective of viremia or antibody status, but reactivation of latent infections was not observed. The role of viral DNA in transmission and early pathogenesis of SRV-2 was investigated by inoculation of SRV-2 DNA-positive blood into groups of naïve M. fascicularis from either a viremic or nonviremic donor and subsequent analysis of the virological and serological status of the recipients. Transmission of SRV and development of anti-SRV antibodies were only observed in recipients of blood from the viremic donor; transfer of SRV provirus and unintegrated circular DNA in blood from the nonviremic donor did not lead to infection of the recipients. These results indicate that a proportion of M. fascicularis are able to effectively control the replication and infectivity of SRV despite long-term persistence of viral DNA forms in infected lymphocytes.

Protective immunity against foot-and-mouth disease virus induced by a recombinant vaccinia virus

We report the construction of a recombinant vaccinia virus expressing the precursor for the four structural proteins of FMD virus (FMDV) (P1) strain C3Arg85 using a procedure for isolation of recombinant vaccinia viruses based solely on plaque formation. Adult mice vaccinated with this recombinant vaccinia virus elicited high titers of neutralizing antibodies against both the homologous FMDV and vaccinia virus, measured by neutralization assays. Liquid phase blocking sandwich enzyme-linked immunosorbent assays (ELISAs) using whole virus as antigen showed high total antibody titers against homologous FMDV, similar to those induced by the conventional inactivated vaccine. When ELISAs were carried out with heterologous strains A79 or O1Caseros as antigens, sera from animals vaccinated with the recombinant virus cross-reacted. Mice boosted once with the recombinant vaccinia virus were protected against challenge with infectious homologous virus. These results indicate that recombinant vaccinia viruses are efficient immunogens against FMDV when used as a live vaccine in a mouse model.

Histologic lesions in cynomolgus monkeys (Macaca fascicularis) naturally infected with simian retrovirus type D: comparison of seropositive, virus-positive, and uninfected animals

Simian retrovirus (SRV) type D is a common cause of simian acquired immunodeficiency syndrome (SAIDS), a usually fatal immunosuppressive disease of macaques. Associated gross and histologic lesions have been well described for the rhesus macaque (Macaca mulatta) in experimental and natural infections. However, morphologic changes induced by this virus at the gross and light-microscopic level have not been documented in the cynomolgus macaque (Macaca fascicularis). In 1996, sporadic cases of anemia, weight loss, and diarrhea were noted in a colony of cynomolgus macaques in our research facility. Out of 28 animals, 24 tested positive for SRV by serology or virus isolation. Animals could mainly be classified into 1 of 2 categories: 1) positive for virus isolation but negative for SRV antibody and 2) negative for virus isolation but antibody positive. During the process of eliminating the virus from the colony, a complete postmortem examination was performed on the 24 infected animals that had to be culled. Twelve SRV-negative animals were available as controls. Minimal to mild follicular lymphoid infiltrates were seen in various organ systems in 75% of the negative animals, compared with moderate to marked infiltrates in 83% of infected animals. Lymphoid infiltrates were more common in the brain, bone marrow, and salivary gland of viremic animals and were rare to nonexistent in seropositive or negative animals. Lymphoid hyperplasia was present in 38% of the infected animals, whereas lymphoid depletion was seen in 47% of the infected animals. Overall, lesions were of greater severity in viremic animals than in virus-negative or seropositive animals. Overall, infected animals had lower, statistically significant hematocrit and lymphocyte values. Viremic animals had significantly lower hematocrit, white blood cell, lymphocyte, and neutrophil values than did controls. Only 1 out of 24 infected animals had clinical signs that were consistent with the definition of SAIDS, and none had evidence of opportunistic infections. Lesions were similar to those already reported in other species of macaques, but the absence of severe illness that was consistent with SAIDS in most viremic animals suggests that there may be a different manifestation of disease in the cynomolgus.

The RD114/simian type D retrovirus receptor is a neutral amino acid transporter

The RD114/simian type D retroviruses, which include the feline endogenous retrovirus RD114, all strains of simian immunosuppressive type D retroviruses, the avian reticuloendotheliosis group including spleen necrosis virus, and baboon endogenous virus, use a common cell-surface receptor for cell entry. We have used a retroviral cDNA library approach, involving transfer and expression of cDNAs from highly infectable HeLa cells to nonpermissive NIH 3T3 mouse cells, to clone and identify this receptor. The cloned cDNA, denoted RDR, is an allele of the previously cloned neutral amino acid transporter ATB0 (SLC1A5). Both RDR and ATB0 serve as retrovirus receptors and both show specific transport of neutral amino acids. We have localized the receptor by radiation hybrid mapping to a region of about 500-kb pairs on the long arm of human chromosome 19 at q13.3. Infection of cells with RD114/type D retroviruses results in impaired amino acid transport, suggesting a mechanism for virus toxicity and immunosuppression. The identification and functional characterization of this retrovirus receptor provide insight into the retrovirus life cycle and pathogenesis and will be an important tool for optimization of gene therapy using vectors derived from RD114/type D retroviruses.

Outcome of immunization of cynomolgus monkeys with recombinant Semliki Forest virus encoding human immunodeficiency virus type 1 envelope protein and challenge with a high dose of SHIV-4 virus

Infection of macaques with chimeric simian-human immunodeficiency viruses (SHIVs) allows evaluation of HIV-1 envelope vaccines. SHIV-4 is based on SIVmac239 but carries the env, tat, and rev genes of HIV-1IIIB. In this study we used Semliki Forest virus (SFV) RNA vectors to express the envelope protein gp160 of HIV-1IIIB in cynomolgus macaques. Monkeys were immunized four times with recombinant suicide SFV. Whereas two of four monkeys showed T cell-proliferative responses, only one monkey had demonstrable levels of antibodies to HIV-1 gp41 and gp120 as shown by enzyme-linked immunosorbent assay (ELISA) and Western blot. The vaccinated monkeys and four control animals were challenged with 10,000 MID100 (100% minimum infectious doses) of cell-free monkey cell-grown SHIV-4 virus. As demonstrated by virus isolation, all macaques became infected after challenge. All vaccinated monkeys showed an HIV-1-specific anamnestic T cell-proliferative response. Three of four vaccines had developed HIV-1-Env-specific antibodies 2 weeks after challenge whereas none of the four controls showed any detectable immune response at this time point. Furthermore, three of four vaccinated monkeys had no demonstrable viral antigenemia and low viral load as opposed to one of the four naive control animals.

The history of simian AIDS

Retrospective data indicate that two separate outbreaks of simian AIDS and associated lymphoma were caused by Simian Immunodeficiency Virus (SIVmac and SIVstm, respectively) in group-housed macaques at the California Regional Primate Research Center (CRPRC) in the early and mid-1970s. Because these epizootics were not then recognized as infectious in nature, surviving healthy SIV carriers were sent to other primate centers where they transmitted the viruses to resident macaques. The source of SIV at the CRPRC was by contact with co-housed seropositive sooty mangabeys. Spread of SIV via saliva and blood while fighting most likely accounted for these epizootics. Separate outbreaks of a somewhat different version of simian AIDS, caused by the simian Type D retrovirus (SRV-1), and spread from healthy carriers via saliva and blood also occurred in the late 1970s and early 1980s in group-housed macaques at the CRPRC. Initially, these SRV-1 outbreaks were also not recognized as infectious.

An HTLV-I/II vaccine: from animal models to clinical trials?

A human T-lymphotropic virus type I/II (HTLV-I/II) vaccine is necessary in view of two etiologically related, life-threatening diseases, namely, adult T-cell leukemia/lymphoma and tropical spastic paraparesis/HTLV-I-associated myelopathy. When the risk of developing autoimmune diseases such as uveitis, polymyositis, and arthritis is included, one can estimate the life-long risk of infected individuals to develop an HTLV associated pathology as approximately 10%. The populations at risk are, in a large majority, from developing countries but the epidemic of HTLV-II infection in intravenous drug users (IVDU) represents a possible reservoir for dissemination in the general population. The number of HTLV-I-infected individuals (15 to 25 million), together with the severity of associated disease, justifies the development of a vaccine. Different vaccine preparations have been developed, using mostly recombinant pox and adenoviruses, but DNA plasmid technology will soon become a feasible approach. Various animal models exist for experimental viral infections, involving rats, rabbits, or monkeys, but up to now, neither hematological nor neurological disorders have been induced by HTLV infection in such animal models. For long-term protection from HTLV-I-associated diseases, vaccination should induce both neutralizing antibodies and specific cell-mediated immunity. This will require the incorporation of both env and gag coding sequences in the vaccine preparations. Preventive clinical trials may involve different cohorts of seronegative young girls from endemic areas prior to sexual activity and IVDU in the industrialized world. In parallel, one should consider therapeutic vaccine trials in HTLV-I-positive mothers and IVDU to protect them against disease development. The observed rate of seroconversion in these different cohorts makes such trials feasible.

Transport of a lysosomally targeted Rous sarcoma virus envelope glycoprotein involves transient expression on the cell surface

The details of intracellular transport pathways for glycosylated proteins remain incompletely described. We previously described a mutant Rous sarcoma virus envelope glycoprotein (gp), mu 26, with an altered membrane-spanning domain that was targeted to lysosomes after traversing the trans-Golgi. This mutant protein was not detectable on the cell surface by immunofluorescence, but its pathway for degradation remained unclear. To investigate this we have employed a second env mutation, S19, that results in a protein which is defective for normal cleavage/activation by intracellular enzymes, but remains susceptible to cleavage by extracellular proteases. Cleavage/activation of the double mutant by trypsin, which could only occur if it was exposed on the cell surface, was observed, indicating that the plasma membrane is an intermediate destination in the transport of this mutant protein. To substantiate these results, cells expressing the mu 26 glycoprotein were incubated with an antibody specific for the native protein in the presence of chloroquine. The specific accumulation of this antibody/gp complex in vesicles, as detected by internal immunofluorescence, confirmed the trypsin cleavage results. We conclude that this rapidly degraded mutant protein is transported from the trans-Golgi to the cell surface, where it is only transiently exposed, and then rapidly endocytosed and lysosomally degraded. The relevance of these results to the targeting of lysosomal proteins is discussed.

Poxvirus vectors: cytoplasmic expression of transferred genes

During the past year there have been a number of advances in poxvirus vector design and application. These include improvements in gene expression and a direct method of inserting DNA into the poxvirus genome. Alternative methods of selecting recombinant viruses have been developed, and attenuated and non-replicating vaccinia virus and avian poxvirus vectors are now being used successfully. Field trials of an oral, wild-life rabies vaccine, and phase 1 testing of human vaccines derived from vaccinia virus are in progress. This review is an updated and modified version of a review first published in Current Opinion in Biotechnology 1992, 3:533-539.

Vaccinia and other poxvirus expression vectors

Over the past year improvements have been made in recombinant vaccinia virus gene expression and a new method for inserting DNA into the poxvirus genome has been developed, along with alternative methods for selecting recombinant viruses. Attenuated and non-replicating vaccinia virus and avian poxvirus vectors are now being used successfully. Field trials of an oral, wild-life rabies vaccine and phase 1 testing of human vaccines derived from vaccinia virus are underway.