The major histocompatibility antigens (MHC antigens) are molecules involved in self-nonself differentiation in the defense mechanism of the living body against infection. They are classified into Class I molecule composed of .alpha. chain and .beta.2M, and class II molecule composed of .alpha. chain and .beta. chain. A groove for trapping an antigen peptide is present on the .alpha.1 and .alpha.2 domains, and also on the .alpha.1 and .beta.1 domains. They are featured to have the T cell receptor recognize only a fragmented peptide trapped in the groove, and thereby achieve cell death (cellular immunity) by CD8+ cells which have recognized the class I antigens, as well as induce mainly antibody production (humoral immunity) by CD4+ cells which have recognized the class II antigens.
The MHC genes constitute a gene group most full of polymorphism, and the locations of pockets, shapes, sizes and properties of the peptide trapping grooves are different among haplotypes. It is considered that association conditions of the trapped fragment peptides may vary depending on these differences, which decide immune response and disease sensitivity of each individual. The correlation between the MHC haplotypes and a resistance to a disease (disease insusceptibility) or a possibility of the onset of a disease (disease susceptibility) has been reported, for example, as to human immune deficiency virus (HIV), human T cell leukemia virus (HTLV) and malaria.
As for the bovine MHC (BoLA) class II genes, existence of DQA, DQB, DRA, DRB, DNA, DOB, DYA, and DYB genes has been estimated. DRB3, inter alia, which is one of the three genes (DRB1 to B3) identified on the DRB genetic locus, has been known to encode a functional protein, and existence of 73 alleles has been revealed so far. However, there is almost no report about correlation between bovine infectious diseases and the bovine MHC (BoLA) haplotypes.
In particular, as to the bovine leukemia virus (BLV), which has the gene pX that regulates virus proliferation in the same manner as the human immunodeficiency virus (HIV) and is a retrovirus most related to HTLV-I, a research group in the United States has reported its relationship with the bovine MHC (BoLA) haplotypes mainly focusing disease resistance; however, its relationship with possibility of onset of the leukemia has not been reported. The ratio of cattle infected by this virus (infection rate in Japan) is 10-20%, and 1-2% of the infected cattle develops extremely malignant endemic bovine leukemia to die after a long latent period of 10-15 years. Therefore, economic loss of stockbreeders caused by the virus is very serious. If a possibility of the onset of a cattle after BLV infection can be evaluated by the analysis of bovine MHC (BoLA) haplotypes, it becomes possible to preliminarily select disease resistant cattle for bleeding, and it is expected that extremely safe cattle breeding can be continued.
The inventors of the present invention analyzed the structure of DRB gene locus among the bovine MHC (BoLA) class II genes, and revealed the structures of DRB3 gene (BoLA-DRB3) and its gene product (Biochem. Biophys. Res. Commun., 209, pp.981-988, 1995). The inventors further studied the function of the gene and found that a certain portion, whose amino acid sequence is distinctly different between a cattle developing the leukemia and a cattle not developing the disease, is present in the gene product from the second exon (.beta.1 domain) of BoLA-DRB3 showing particularly noticeable polymorphism. They also found that the amino acid substitutions directly correlated with disease susceptibility to BLV. Furthermore, they found that a cattle, which has homozygous alleles comprising an amino acid sequence: Val-Asp-Thr-Thy at the position defined by the amino acid numbers of from 75 to 78 of the .beta.1 domain of the bovine MHC Class II DR.beta. chain, can be judged to have a possibility of the onset of the leukemia (the specification of Japanese patent application No. 8-190933 filed on Jul. 19, 1996; the 19th Congress of the Molecular Biology Society of Japan, Sapporo-shi, Aug. 28, 1996, Presentation No. 3-P-1124, Abstracts p.526 published on Jul. 25, 1996).
It has been known that a sheep is also infected by the bovine leukemia virus to cause ovine leukemia. The virus causes three pathologic states in bovines as natural hosts, i.e., antibody-positive healthy condition not developing the disease, persistent lymphocytosis (PL), and endemic bovine leukemia, which is B lymphoma, after a long latent period. When sheep are infected, they are featured to show only two pathological stages, i.e., healthy condition not developing the disease and symptomatic leukemia, and the onset is observed in a shorter period. Therefore, sheep can be used as animals for experimental infection by the bovine leukemia virus, and useful for experiments to evaluate effectiveness of vaccines (Aida, Y. et al., Am. J. Vet. Res., 50, pp.1946-1951, 1989; Aida, Y. et al., Jikken Igaku [Experimental Medicine], Extra Volume, "AIDS and ATL, from Molecular Level Consideration to Prevention and Treatment", 11(5), pp.547-557, 1993). In addition, canceration (malignant transformation) cannot be induced in cultured cells in vitro by bovine leukemia virus infection, whereas malignant transformation can be readily induced in sheep by the virus inoculation, and therefore, sheep are indispensable for estimation of ability of malignant transformation of the bovine leukemia virus.
However, when sheep are used for evaluation of effectiveness of vaccines against the bovine leukemia virus, a problem arises in that experimental data vary and evaluation of experimental results become difficult, which is a cause of lowering efficiency of vaccine developments. The variety may possibly be caused by MHC (OLA) haplotypes; however, correlation between the ovine haplotypes and onset of the leukemia has not been reported so far. If resistance to the onset of the leukemia caused by the bovine leukemia virus can be surely predicted in an ovine individual, and if ovine individuals having constant immune response can be selected, it may be possible to markedly increase efficiency of the development of vaccines against the bovine leukemia virus.