Bovine leukemia virus (BLV) is a type C retrovirus infecting bovine B cells and causing enzootic bovine leukosis. Since it takes long periods to develop the disease, it is believed that BLV and host immune responses are closely related. In this review, the accumulated data showing close relationship between BLV and host immune responses are summarized in 4 sections. First, we discuss the role of cell-mediated immunity in protecting hosts from BLV infection. Second, several reports showing the

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... lationship between the disease progression and the change of cytokine profiles are summarized. In the third section, we have focused on tumor necrosis factor α (TNFα) and its two types of receptors, and the possible involvement of TNF α in the BLV-induced leukemogenesis is discussed. The expression of TNF α has been shown to be regulated by major histocompatibility complex (MHC) haplotype. The resistance to BLV infection is supposed to be established by some innate factors, which are closely related to MHC haplotype. Finally, we propose that a breeding strategy based on the MHC haplotype could be a good approach to control BLV infection. This review includes some recent data from us and other groups. KEY WORDS: bovine leukemia virus, cell-mediated immunity, cytokine profile, major histocompatibility complex, tumor necrosis factor α.