Protective effects of vaccination with bovine leukemia virus (BLV) Tax DNA against BLV infection in sheep

BLV: lessons on vaccine development

Vaccination against retroviruses is a challenge because of their ability to stably integrate into the host genome, undergo long-term latency in a proportion of infected cells and thereby escape immune response. Since clearance of the virus is almost impossible once infection is established, the primary goal is to achieve sterilizing immunity. Besides efficacy, safety is the major issue since vaccination has been associated with increased infection or reversion to pathogenicity. In this review, we discuss the different issues that we faced during the development of an efficient vaccine against bovine leukemia virus (BLV). We summarize the historical failures of inactivated vaccines, the efficacy and safety of a live-attenuated vaccine and the economical constraints of further industrial development.

Vaccines and Vaccination

Livestock vaccines aim to increase livestock product and improve the health and welfare of livestock animals in a cost-efficient manner and prevent disease transmission. Successful livestock vaccines have been generated for pathogens including bacterial, viral, protozoan, and multicellular pathogens. These livestock vaccines have a significant effect on animal health and products and on human health through growing safe food procurement and preventing zoonotic diseases. There are successful production of biotechnological-based animal vaccines licensed for use that include virus-like particle vaccines, gene-deleted marker vaccines, subunit vaccines, DIVA vaccines, and DNA vaccines.

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.

Cationic liposomal vaccine adjuvants in animal challenge models: overview and current clinical status

Cationic liposome formulations can function as efficient vaccine adjuvants. However, due to the highly diverse nature of lipids, cationic liposomes have different physical-chemical characteristics that influence their adjuvant mechanisms and their relevance for use in different vaccines. These characteristics can be further manipulated by incorporation of additional lipids or stabilizers, and inclusion of carefully selected immunostimulators is a feasible strategy when tailoring cationic liposomal adjuvants for specific disease targets. Thus, cationic liposomes present a plasticity, which makes them promising adjuvants for future vaccines. This versatility has also led to a vast amount of literature on different experimental liposomal formulations in combination with a wide range of immunostimulators. Here, we have compiled information about the animal challenge models and administration routes that have been used to study vaccine adjuvants based on cationic liposomes and provide an overview of the applicability, progress and clinical status of cationic liposomal vaccine adjuvants.

Nanomedicine and veterinary science: the reality and the practicality

Nanomedicine is a rapidly expanding field with a promising future that is already permeating veterinary science. This review summarises the current applications for nanoparticles in human medicine and explores their potential applicability for veterinary use. The principles underlying the use of nanoparticles in drug delivery, imaging and as vaccine adjuvants are explored along with the unique issues surrounding nanoparticle toxicity and regulatory approval. A brief overview of the properties of different nanoparticle systems including, liposomes, micelles, emulsions and inorganic nanoparticles, is provided, along with a description of their current and potential future applications in veterinary medicine.

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.

Cytotoxic responses to BLV tax oncoprotein do not prevent leukemogenesis in sheep

Delta retrovirus-mediated leukemogenesis is dependent on the oncogenic potential of Tax. It is not clear, however, whether Tax-specific immune responses play a role in leukemia onset and progression. Using the BLV-associated leukemia model in sheep, we found that Tax-specific cytotoxic responses induced by DNA immunization or viral infection of naïve animals were not predictive of disease outcome and did not prevent tumor development. Furthermore, provirus and tax may be absent from blood for extended periods, emphasizing the relevance of surveying other compartments during chronic lymphoproliferative disorders. Our results support the conclusion that Tax-specific cytotoxic responses, even during the initial phase of infection, are not sufficient to prevent leukemogenesis.

Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human

In 1871, the observation of yellowish nodules in the enlarged spleen of a cow was considered to be the first reported case of bovine leukemia. The etiological agent of this lymphoproliferative disease, bovine leukemia virus (BLV), belongs to the deltaretrovirus genus which also includes the related human T-lymphotropic virus type 1 (HTLV-1). This review summarizes current knowledge of this viral system, which is important as a model for leukemogenesis. Recently, the BLV model has also cast light onto novel prospects for therapies of HTLV induced diseases, for which no satisfactory treatment exists so far.