Sabrina Rodriguez

Preventive and Therapeutic Strategies for Bovine Leukemia Virus: Lessons for HTLV

Bovine leukemia virus (BLV) is a retrovirus closely related to the human T-lymphotropic virus type 1 (HTLV-1). BLV is a major animal health problem worldwide causing important economic losses. A series of attempts were developed to reduce prevalence, chiefly by eradication of infected cattle, segregation of BLV-free animals and vaccination. Although having been instrumental in regions such as the EU, these strategies were unsuccessful elsewhere mainly due to economic costs, management restrictions and lack of an efficient vaccine. This review, which summarizes the different attempts previously developed to decrease seroprevalence of BLV, may be informative for management of HTLV-1 infection. We also propose a new approach based on competitive infection with virus deletants aiming at reducing proviral loads.

Massive depletion of bovine leukemia virus proviral clones located in genomic transcriptionally active sites during primary infection

Deltaretroviruses such as human T-lymphotropic virus type 1 (HTLV-1) and bovine leukemia virus (BLV) induce a persistent infection that remains generally asymptomatic but can also lead to leukemia or lymphoma. These viruses replicate by infecting new lymphocytes (i.e. the infectious cycle) or via clonal expansion of the infected cells (mitotic cycle). The relative importance of these two cycles in viral replication varies during infection. The majority of infected clones are created early before the onset of an efficient immune response. Later on, the main replication route is mitotic expansion of pre-existing infected clones. Due to the paucity of available samples and for ethical reasons, only scarce data is available on early infection by HTLV-1. Therefore, we addressed this question in a comparative BLV model. We used high-throughput sequencing to map and quantify the insertion sites of the provirus in order to monitor the clonality of the BLV-infected cells population (i.e. the number of distinct clones and abundance of each clone). We found that BLV propagation shifts from cell neoinfection to clonal proliferation in about 2 months from inoculation. Initially, BLV proviral integration significantly favors transcribed regions of the genome. Negative selection then eliminates 97% of the clones detected at seroconversion and disfavors BLV-infected cells carrying a provirus located close to a promoter or a gene. Nevertheless, among the surviving proviruses, clone abundance positively correlates with proximity of the provirus to a transcribed region. Two opposite forces thus operate during primary infection and dictate the fate of long term clonal composition: (1) initial integration inside genes or promoters and (2) host negative selection disfavoring proviruses located next to transcribed regions. The result of this initial response will contribute to the proviral load set point value as clonal abundance will benefit from carrying a provirus in transcribed regions.

Safety of long-term treatment of HAM/TSP patients with valproic acid

HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a neurodegenerative disease of the central nervous system induced by human T-lymphotropic virus type 1. As a potential therapeutic approach, we previously suggested reducing the proviral load by modulating lysine deacetylase activity using valproic acid (VPA) and exposing virus-positive cells to the host immune response. We conducted a single-center, 2-year, open-label trial, with 19 HAM/TSP volunteers treated with oral VPA. Proviral load, CD38/HLA-DR expression, and CD8(+) lysis efficiency were not significantly affected by VPA. Mean scores of HAM/TSP disability did not differ between baseline and final visit. Walking Time Test increased significantly (> 20%) in 3 patients and was in keeping with minor VPA side effects (drowsiness and tremor). Walking Time Test improved rapidly after VPA discontinuation. We conclude that long-term treatment with VPA is safe in HAM/TSP.

Vaccination against δ−Retroviruses: The Bovine Leukemia Virus Paradigm

Bovine leukemia virus (BLV) and human T-lymphotropic virus type 1 (HTLV-1) are closely related δ-retroviruses that induce hematological diseases. HTLV-1 infects about 15 million people worldwide, mainly in subtropical areas. HTLV-1 induces a wide spectrum of diseases (e.g., HTLV-associated myelopathy/tropical spastic paraparesis) and leukemia/lymphoma (adult T-cell leukemia). Bovine leukemia virus is a major pathogen of cattle, causing important economic losses due to a reduction in production, export limitations and lymphoma-associated death. In the absence of satisfactory treatment for these diseases and besides the prevention of transmission, the best option to reduce the prevalence of δ-retroviruses is vaccination. Here, we provide an overview of the different vaccination strategies in the BLV model and outline key parameters required for vaccine efficacy.

An efficient vaccine against bovine leukemia virus

Previous attempts to produce a vaccine against BLV faced problems of efficacy (i.e. only a fraction of animals were protected), persistence (i.e. rapid decrease of immune protection), cost (e.g. production of purified proteins) or safety (e.g. genetically modified hybrid viruses). We have designed a novel strategy based on the use of a live-attenuated BLV provirus. The rationale behind this strategy relies on the deletion of genes required to induce pathogenesis maintaining integrity of those involved in infectivity. We have identified a BLV deleted provirus that is infectious in cattle but replicates at reduced levels in cows as shown by real-time quantitative PCR. The deletant elicits a strong anti-BLV immune response as indicated by wild-type antibody titers. Vaccinated animals but not uninfected controls resist challenge by a wild type BLV virus. The deletant does not spread to uninfected sentinels maintained during 5 years in the same herd supporting biosafety of the vaccine. Passive immunity, but not viral infection, is transmitted to the newborn calves via the maternal colostrum. Assays regarding production, storage and delivery of the vaccine, as well as safety of the milk produced by vaccinated cows are currently being carried out with the aim to start a large scale trial that will be held in Argentina in real dairy conditions during this year. # Poster award winner - 3rd place

A recombinant attenuated candidate vaccine that efficiently and persistently protects against bovine leukemia virus in herds

There currently is no efficient vaccine that protects against bovine leukemia virus (BLV) infection. We now propose a novel approach based on the use of a recombinant live-attenuated BLV provirus. The rationale behind this strategy relies on the deletion of genes required to induce pathogenesis maintaining the integrity of those involved in infectivity. We have identified a BLV attenuated provirus that is infectious but replicates at reduced levels in cattle. The attenuated strain elicits a strong anti-BLV immune response and does not spread to uninfected sentinels maintained during 3 years in the same herd supporting biosafety of the vaccine. Passive antibodies are transmitted to newborn calves via maternal colostrum and persist during several months. Nevertheless, the BLV attenuated provirus does not transmit from cows to calves. Finally, vaccinated animals resist challenge by a wild-type BLV virus. In summary, we have identified a safe BLV attenuated provirus with impaired transmissibility that efficiently protects against infection in herd conditions.

Massive depletion of BLV proviral clones located in genomic transcriptionally active sites during primary infection

Only scarce data is available on early infection by human T-lymphotropic virus (HTLV). In particular, the modes of clonal selection during primary infection cannot be analyzed due to the paucity of available samples. Therefore, we addressed this question in a closely related animal model, bovine leukemia virus (BLV). As HTLV, BLV persist indefinitely into its host and is generally asymptomatic but can also lead to lymphoma or leukemia. Both viruses replicate by colonizing new lymphocytes or via clonal proliferation of infected cells. However, the modes of replication occurring soon after infection of new hosts are currently unknown. We used high-throughput sequencing to map and quantify the insertion sites of the provirus in order to monitor the clonality of the BLV-infected B-cell population (i.e. the number of distinct clones and abundance of each clone). We found that BLV propagation shifts from cell neoinfection to clonal proliferation in less than 3 months post-inoculation. Initially, BLV proviral integration significantly favors transcribed regions of the genome. Negative selection then eliminates more than 96% of the clones detected at seroconversion and disfavoring BLV-infected B-cells carrying a provirus located close to a promoter or a gene. This selection is more stringent in animals where proviral load set point is low. Among the surviving proviruses, clone abundance nevertheless positively correlates with proximity to a transcribed unit or a CpG island. We conclude that massive clone selection occurs during primary infection disfavoring proviruses located nearby genes and this selection is stronger in low proviral load animals.

A life-attenuated BLV deletant as a candidate vaccine to inhibit viral transmission in bovine herds

There are different strategies to reduce BLV prevalence. Eradication by culling of infected animals is not economically sustainable in highly infected regions such as Argentina, US or Japan. Segregation of BLV-infected cows is expensive due to duplication of facilities. Finally, several candidate vaccines based on recombinant viral proteins were unsuccessful to protect from challenge. Facing these problems, we propose a novel strategy based on the use of a live-attenuated BLV provirus. The rationale behind this strategy relies on the deletion of genes required to induce pathogenesis leaving those involved in infectivity, resulting in an attenuated mutant with impaired transmissibility. In a first set of experiments, we showed that the mutant is infectious and elicits an efficient immune response in sheep (n=3) and in cows (n=9). Lack of spread to uninfected sentinels further supports the safety of the vaccine. Based on these promising results, a validation program in herd (n=105) is ongoing to evaluate the capacity of the candidate vaccine to protect from wild-type BLV infection. The following experiments are carried out: quantification of the proviral loads, assessment of immune response efficiency (antibody titers, CTL response and cytokine profiling), measure of viral expression in vivo (qRT-PCR) and ex vivo (expression of Tax and p24gag) and determination of provirus clonality during infection. This data will be instrumental for understanding the basic mechanisms undergoing during BLV infection and for elaborating a novel vaccine.

HTLV-1 clonality during chronic infection and BLV clonality during primary infection

HTLV-1 persists by driving clonal proliferation of infected T-lymphocytes. A high proviral load predisposes to the inflammatory and malignant diseases associated with HTLV-1. Yet the reasons for the remarkable variation within and between individuals in the abundance of HTLV-1-infected clones remain unknown. We demonstrate that negative selection dominates during chronic infection, favouring establishment of proviruses integrated in transcriptionally silenced DNA: this selection is significantly stronger in asymptomatic carriers. We postulated that this selection occurred mainly during the primary infection. We are testing this hypothesis in an animal model by studying the BLV clonality during the primary infection in cows. By measuring the proviral load, the anti-BLV immune response and the BLV clonality we aim to quantify the impact of the immune response on the rate of infectious spread and on the selection of proviruses inserted in a particular genomic environment. Co-infection with Strongyloides stercoralis or Staphylococcus appears to be another risk factor for the development of HTLV-1 associated diseases. We observed that HTLV-1 clonality is altered by co-infection with these pathogens with an increase of both the number and the abundance of the infected T-cell clones. The genomic characteristics of the proviral integration sites in the most abundant clones differ significantly between co-infected individuals and those with HTLV-1 alone, implying the existence of different selection forces in co-infected patients. The rate of appearance of new clones in patients co-infected with Strongyloides stercoralis is higher than in patients with HTLV-1 alone. By comparing skin lesions and blood samples from patients with Infective Dermatitis associated with HTLV-1 (IDH), we observed a significant proportion of distinct infected clones between the two compartments. The skin lesions seem to be a site for HTLV-1 infectious spread.

Evidence for the existence of pathogenicity determinants in the Long Terminal Repeats (LTRs) of the Bovine Leukemia Virus (BLV) genome

The majority of BLV-infected animals are asymptomatic carriers (AL) while about 30% develop a benign persistent lymphocytosis (PL). Fatal lymphosarcoma (LS) occurs in 5% of infected animals. The genetic basis of these diverse outcomes of BLV infection is still unknown. Viral LTRs constitute a genetic determinant of pathogenesis for other retroviruses. However, this possibility has never been tested for BLV. Analyses to test correlation between clinical and genotypic traits across species must be corrected by including the group phylogeny. Otherwise, shared evolutionary history can jeopardize statistical independence. Thus, the influence of BLV LTR genetic variation on the clinical manifestation of the disease was investigated by employing Cladistic and Probabilistic, phylogenetic comparative methods. With this purpose, the 5’ LTR region of 40 BLV proviruses from bovines with different clinical presentations (AL, PL, LS) was sequenced. Seven polymorphic positions showing an apparent association with the clinical presentation were identified. A provirus phylogeny was obtained using env gene sequences from 28 of the 40 provirus studied in this work. Both Cladistic and Probabilistic comparative analyses based on the empirical sequence alignment and the provirus phylogeny suggested that positions 41 and 56 might be correlated to the clinical presentation. The probabilistic analysis further indicated an association with the viral pathogenesis for positions 373, 450, 494 and 505, though the corresponding statistical supports were lower in comparison to the supports obtained for positions 41 and 56. These observations indicate that the BLV LTRs might contain pathogenicity determinants.