The invention relates to recombinant bovine leukemia viruses that have an attenuated phenotype and comprise a combination of at least two specific mutations. The invention also provides recombinant nucleic acids encoding such viruses, vectors comprising such nucleic acids, and host cells comprising such nucleic acids or vectors. The recombinant attenuated BLV viruses, recombinant nucleic acids, vectors and host cells allow for the preparation of improved vaccines, in particular vaccines suitable for the prophylactic treatment of BLV-associated diseases in subjects. The invention further provides methods for treating BLV-associated diseases in subjects and pharmaceutical compositions suitable for use in these methods.

An object of the present invention is to provide a novel non-replicating bovine leukemia virus (BLV) and a producing cell thereof. According to the present invention, there is provided a bovine leukemia virus (BLV) in which at least a part of the function of a pol gene is deficient. Also, according to the present invention, there is provided a non-replicating BLV-producing cell comprising a gene of a BLV in which at least a part of the function of a pol gene is deficient. The present invention is advantageous in that it can provide a BLV vaccine which is highly immunogenic, and is highly safe without replicating in an infected subject.