Evidence for BoLA-linked resistance and susceptibility to subclinical progression of bovine leukaemia virus infection

The Global Epidemiology of Bovine Leukemia Virus: Current Trends and Future Implications

Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leucosis (EBL), which is the most significant neoplastic disease in cattle. Although EBL has been successfully eradicated in most European countries, infections continue to rise in Argentina, Brazil, Canada, Japan, and the United States. BLV imposes a substantial economic burden on the cattle industry, particularly in dairy farming, as it leads to a decline in animal production performance and increases the risk of disease. Moreover, trade restrictions on diseased animals and products between countries and regions further exacerbate the problem. Recent studies have also identified fragments of BLV nucleic acid in human breast cancer tissues, raising concerns for public health. Due to the absence of an effective vaccine, controlling the disease is challenging. Therefore, it is crucial to accurately detect and diagnose BLV at an early stage to control its spread and minimize economic losses. This review provides a comprehensive examination of BLV, encompassing its genomic structure, epidemiology, modes of transmission, clinical symptoms, detection methods, hazards, and control strategies. The aim is to provide strategic information for future BLV research.

Phenotypic Selection of Dairy Cattle Infected with Bovine Leukemia Virus Demonstrates Immunogenetic Resilience through NGS-Based Genotyping of BoLA MHC Class II Genes

Characterization of the bovine leukocyte antigen (BoLA) DRB3 gene has shown that specific alleles associate with susceptibility or resilience to the progression of bovine leukemia virus (BLV), measured by proviral load (PVL). Through surveillance of multi-farm BLV eradication field trials, we observed differential phenotypes within seropositive cows that persist from months to years. We sought to develop a multiplex next-generation sequencing workflow (NGS-SBT) capable of genotyping 384 samples per run to assess the relationship between BLV phenotype and two BoLA genes. We utilized longitudinal results from milk ELISA screening and subsequent blood collections on seropositive cows for PVL determination using a novel BLV proviral load multiplex qPCR assay to phenotype the cows. Repeated diagnostic observations defined two distinct phenotypes in our study population, ELISA-positive cows that do not harbor detectable levels of provirus and those who do have persistent proviral loads. In total, 565 cows from nine Midwest dairy farms were selected for NGS-SBT, with 558 cows: 168 BLV susceptible (ELISA-positive/PVL-positive) and 390 BLV resilient (ELISA-positive/PVL-negative) successfully genotyped. Three BoLA-DRB3 alleles, including one novel allele, were shown to associate with disease resilience, *009:02, *044:01, and *048:02 were found at rates of 97.5%, 86.5%, and 90.3%, respectively, within the phenotypically resilient population. Alternatively, DRB3*015:01 and *027:03, both known to associate with disease progression, were found at rates of 81.1% and 92.3%, respectively, within the susceptible population. This study helps solidify the immunogenetic relationship between BoLA-DRB3 alleles and BLV infection status of these two phenotypic groupings of US dairy cattle.

Bovine Leukaemia Virus: Current Epidemiological Circumstance and Future Prospective

Bovine leukaemia virus (BLV) is a deltaretrovirus that is closely related to human T-cell leukaemia virus types 1 and 2 (HTLV-1 and -2). It causes enzootic bovine leukosis (EBL), which is the most important neoplastic disease in cattle. Most BLV-infected cattle are asymptomatic, which potentiates extremely high shedding rates of the virus in many cattle populations. Approximately 30% of them show persistent lymphocytosis that has various clinical outcomes; only a small proportion of animals (less than 5%) exhibit signs of EBL. BLV causes major economic losses in the cattle industry, especially in dairy farms. Direct costs are due to a decrease in animal productivity and in cow longevity; indirect costs are caused by restrictions that are placed on the import of animals and animal products from infected areas. Most European regions have implemented an efficient eradication programme, yet BLV prevalence remains high worldwide. Control of the disease is not feasible because there is no effective vaccine against it. Therefore, detection and early diagnosis of the disease are essential in order to diminish its spreading and the economic losses it causes. This review comprises an overview of bovine leukosis, which highlights the epidemiology of the disease, diagnostic tests that are used and effective control strategies.

Kinetic Study of BLV Infectivity in BLV Susceptible and Resistant Cattle in Japan from 2017 to 2019

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis. Polymorphism in bovine lymphocyte antigen (BoLA)-DRB3 alleles is related to susceptibility to BLV proviral load (PVL), which is a useful index for estimating disease progression and transmission risk. However, whether differential BoLA-DRB3 affects BLV infectivity remains unknown. In a three-year follow-up investigation using a luminescence syncytium induction assay for evaluating BLV infectivity, we visualized and evaluated the kinetics of BLV infectivity in cattle with susceptible, resistant and neutral BoLA-DRB3 alleles which were selected from 179 cattle. Susceptible cattle showed stronger BLV infectivity than both resistant and neutral cattle. The order of intensity of BLV infectivity was as follows: susceptible cattle > neutral cattle > resistant cattle. BLV infectivity showed strong positive correlation with PVL at each testing point. BLV-infected susceptible cattle were found to be at higher risk of horizontal transmission, as they had strong infectivity and high PVL, whereas BLV-infected resistant cattle were low risk of BLV transmission owing to weak BLV infection and low PVL. Thus, this is the first study to demonstrate that the BoLA-DRB3 polymorphism is associated with BLV infection.

Genomic Association between SNP Markers and Diseases in the "Curraleiro Pé-Duro" Cattle

Susceptibility to diseases is inherited and can be transmitted between populations. Single-nucleotide polymorphism (SNPs) in genes related to immune response is associated with diseases in cattle. This study investigated SNPs in the genomic region of cytokines in 702 samples of Curraleiro Pé-Duro cattle and associated them with the occurrence of antibodies in brucellosis, leptospirosis, neosporosis, leukosis, infectious bovine rhinotracheitis (IBR), and bovine viral diarrhea (BVD) tests. DNA samples were evaluated by the kompetitive allele-specific polymerase chain reaction (KASP) method to identify polymorphisms. The gametic phase and SNP haplotypes were determined with the help of PHASE 2.1.1 software. Haplotypes were associated with serological results against Brucella abortus, Leptospira sp., Neospora caninum, leukosis, infectious rhinotracheitis, and BVD using univariate analysis followed by logistic regression. Haplotype 2 of TLR2 was present in 70% of the animals that tested positive for N. caninum infection. Haplotypes of TLR10 and TLR6 and IL10RA were more common in seronegative animals. Haplotypes related to the gene IL10RA were associated with animals negative to all infections. Curraleiro Pé-Duro cattle presented polymorphisms related to resistance to bacterial, viral, and N. caninum infections.

Bovine major histocompatibility complex (BoLA) heterozygote advantage against the outcome of bovine leukemia virus infection

Bovine leukemia virus (BLV) causes enzootic bovine leucosis. Host genetic heterozygosity at the major histocompatibility complex can enhance the ability to combat infectious diseases. However, heterozygote advantage is loci specific and depends on disease type. Bovine leukocyte antigen (BoLA)-DRB3 polymorphisms are related with BLV-infection outcome; however, whether BoLA-DRB3 heterozygotes have an advantage against BLV-induced lymphoma and proviral load (PVL) remains unclear. By analyzing 1567 BLV-infected individuals, we found that BoLA-DRB3 heterozygous status was significantly associated with lymphoma resistance irrespective of cattle breeds (p < 0.0001). Similarly, decreased PVL was observed in BoLA-DRB3 heterozygotes (p = 0.0407 for Holstein cows; p = 0.0889 for Japanese Black cattle). Our report provides first evidence of BoLA-DRB3 heterozygote advantage against BLV infection outcome.

Relationship between Allelic Heterozygosity in BoLA-DRB3 and Proviral Loads in Bovine Leukemia Virus-Infected Cattle

Simple Summary

Bovine leukemia virus (BLV) caused a severe cattle neoplastic disease called enzootic bovine leukosis (EBL). EBL causes significant economic losses in farming by reducing milk production, reproductive performance, and fertility, and through cattle culling or death. The BLV proviral load (PVL) represents the quantity of BLV genome that has integrated into the host’s genome in BLV-infected cells. It has been reported that PVLs differ according to the genetic background of the host, and some studies of BLV-associated host factors have reported on polymorphisms within the bovine major histocompatibility complex (MHC), namely bovine MHC is bovine leukocyte antigen (BoLA-DRB3). However, there is a great diversity in the PVLs associated with carrying various combinations of these alleles, especially for heterozygous alleles. Therefore, this research investigated whether heterogeneity in BoLA-DRB3 allele combinations would affect PVLs during BLV infections in different ages and breeds of cattle in Japan. This is the first report where the association between heterozygous allelic combinations and BLV PVLs phenotypes (HPLs, LPLs) was analyzed. Our findings augment current understanding about the immunological role played by BoLA heterozygosity in BLV-associated PVLs and biocontrol in BLV infections.

Abstract

Enzootic bovine leukosis is a lethal neoplastic disease caused by bovine leukemia virus (BLV), belongs to family Retroviridae. The BLV proviral load (PVL) represents the quantity of BLV genome that has integrated into the host’s genome in BLV-infected cells. Bovine leukocyte antigen (BoLA) class II allelic polymorphisms are associated with PVLs in BLV-infected cattle. We sought to identify relationships between BoLA-DRB3 allelic heterozygosity and BLV PVLs among different cattle breeds. Blood samples from 598 BLV-infected cattle were quantified to determine their PVLs by real-time polymerase chain reaction. The results were confirmed by a BLV-enzyme-linked immunosorbent assay. Restriction fragment length polymorphism-polymerase chain reaction identified 22 BoLA-DRB3 alleles. Multivariate negative binomial regression modeling was used to test for associations between BLV PVLs and BoLA-DRB3 alleles. BoLA-DRB3.2*3, *7, *8, *11, *22, *24, and *28 alleles were significantly associated with low PVLs. BoLA-DRB3.2*10 was significantly associated with high PVLs. Some heterozygous allele combinations were associated with low PVLs (*3/*28, *7/*8, *8/*11, *10/*11, and *11/*16); others were associated with high PVLs (*1/*41, *10/*16, *10/*41, *16/*27, and *22/*27). Interestingly, the BoLA-DRB3.2*11 heterozygous allele was always strongly and independently associated with low PVLs. This is the first reported evidence of an association between heterozygous allelic combinations and BLV PVLs.

Current Developments in the Epidemiology and Control of Enzootic Bovine Leukosis as Caused by Bovine Leukemia Virus

Enzootic Bovine Leukosis (EBL) caused by the bovine leukemia virus (BLV) has been eradicated in over 20 countries. In contrast, the U.S. and many other nations are experiencing increasing prevalence in the absence of efforts to control transmission. Recent studies have shown that BLV infection in dairy cattle has a greater impact beyond the long-recognized lymphoma development that occurs in <5% of infected cattle. Like other retroviruses, BLV appears to cause multiple immune system disruptions, affecting both cellular and humoral immunity, which are likely responsible for increasingly documented associations with decreased dairy production and decreased productive lifespan. Realization of these economic losses has increased interest in controlling BLV using technology that was unavailable decades ago, when many nations eradicated BLV via traditional antibody testing and slaughter methods. This traditional control is not economically feasible for many nations where the average herd antibody prevalence is rapidly approaching 50%. The ELISA screening of cattle with follow-up testing via qPCR for proviral load helps prioritize the most infectious cattle for segregation or culling. The efficacy of this approach has been demonstrated in at least four herds. Breeding cattle for resistance to BLV disease progression also appears to hold promise, and several laboratories are working on BLV vaccines. There are many research priorities for a wide variety of disciplines, especially including the need to investigate the reports linking BLV and human breast cancer.

Expression-based analysis of genes related to single nucleotide polymorphism hits associated with bovine leukemia virus proviral load in Argentinean dairy cattle

In dairy cattle infected with bovine leukemia virus (BLV), the proviral load (PVL) level is directly related to the viral transmission from infected animals to their healthy herdmates. Two contrasting phenotypic groups can be identified when assessing PVL in peripheral blood of infected cows. A large number of reports point to bovine genetic variants (single nucleotide polymorphisms) as one of the key determinants underlying PVL level. However, biological mechanisms driving BLV PVL profiles and infection progression in cattle have not yet been elucidated. In this study, we evaluated whether a set of candidate genes affecting BLV PVL level according to whole genome association studies are differentially expressed in peripheral blood mononuclear cells derived from phenotypically contrasting groups of BLV-infected cows. During a 10-mo-long sampling scheme, 129 Holstein cows were phenotyped measuring anti-BLV antibody levels, PVL quantification, and white blood cell subpopulation counts. Finally, the expression of 8 genes (BOLA-DRB3, PRRC2A, ABT1, TNF, BAG6, BOLA-A, LY6G5B, and IER3) located within the bovine major histocompatibility complex region harboring whole genome association SNP hits was evaluated in 2 phenotypic groups: high PVL (n = 7) and low PVL (n = 8). The log₂ initial fluorescence value (N₀) transformed mean expression values for the ABT1 transcription factor were statistically different in high- and low-PVL groups, showing a higher expression of the ABT1 gene in low-PVL cows. The PRRC2A and IER3 genes had a significant positive (correlation coefficient = 0.61) and negative (correlation coefficient = -0.45) correlation with the lymphocyte counts, respectively. Additionally, the relationships between gene expression values and lymphocyte counts were modeled using linear regressions. Lymphocyte levels in infected cows were better explained (coefficient of determination = 0.56) when fitted a multiple linear regression model using both PRRC2A and IER3 expression values as independent variables. The present study showed evidence of differential gene expression between contrasting BLV infection phenotypes. These genes have not been previously related to BLV pathobiology. This valuable information represents a step forward in understanding the BLV biology and the immune response of naturally infected cows under a commercial milk production system. Efforts to elucidate biological mechanisms leading to BLV infection progression in cows are valuable for BLV control programs. Further studies integrating genotypic data, global transcriptome analysis, and BLV progression phenotypes are needed to better understand the BLV-host interaction.

Mapping of CD4+ T-cell epitopes in bovine leukemia virus from five cattle with differential susceptibilities to bovine leukemia virus disease progression

BACKGROUND

Bovine leukemia virus (BLV), which is closely related to human T-cell leukemia virus, is the etiological agent of enzootic bovine leukosis, a disease characterized by a highly prolonged course involving persistent lymphocytosis and B-cell lymphoma. The bovine major histocompatibility complex class II region plays a key role in the subclinical progression of BLV infection. In this study, we aimed to evaluate the roles of CD4⁺ T-cell epitopes in disease progression in cattle.

METHODS

We examined five Japanese Black cattle, including three disease-susceptible animals, one disease-resistant animal, and one normal animal, classified according to genotyping of bovine leukocyte antigen (BoLA)-DRB3 and BoLA-DQA1 alleles using polymerase chain reaction sequence-based typing methods. All cattle were inoculated with BLV-infected blood collected from BLV experimentally infected cattle and then subjected to CD4⁺ T-cell epitope mapping by cell proliferation assays.

RESULTS

Five Japanese Black cattle were successfully infected with BLV, and CD4⁺ T-cell epitope mapping was then conducted. Disease-resistant and normal cattle showed low and moderate proviral loads and harbored six or five types of CD4⁺ T-cell epitopes, respectively. In contrast, the one of three disease-susceptible cattle with the highest proviral load did not harbor CD4⁺ T-cell epitopes, and two of three other cattle with high proviral loads each had only one epitope. Thus, the CD4⁺ T-cell epitope repertoire was less frequent in disease-susceptible cattle than in other cattle.

CONCLUSION

Although only a few cattle were included in this study, our results showed that CD4⁺ T-cell epitopes may be associated with BoLA-DRB3-DQA1 haplotypes, which conferred differential susceptibilities to BLV proviral loads. These CD4⁺ T-cell epitopes could be useful for the design of anti-BLV vaccines targeting disease-susceptible Japanese Black cattle. Further studies of CD4⁺ T-cell epitopes in other breeds and using larger numbers of cattle with differential susceptibilities are required to confirm these findings.

Bovine leukemia virus proviral load is more strongly associated with bovine major histocompatibility complex class II DRB3 polymorphism than with DQA1 polymorphism in Holstein cow in Japan

Bovine leukemia virus (BLV) causes enzootic bovine leukosis and is closely related to the human T-lymphotropic virus. Bovine major histocompatibility complex (BoLAs) are used extensively as markers of disease and immunological traits in cattle. For BLV diagnosis, proviral load is a major diagnosis index for the determination of disease progression and transmission risk. Therefore, we investigated the frequency of BoLA-DRB3 alleles, BoLA-DQA1 alleles, and haplotypes of BoLA class II isolated from the heads of 910 BLV-infected cows out of 1290 cows assessed from BLV-positive farms, in a nationwide survey from 2011 to 2014 in Japan. Our aim was to identify BoLA class II polymorphisms associated with the BLV proviral load in the Holstein cow. The study examined 569 cows with a high proviral load and 341 cows with a low proviral load. Using the highest odds ratio (OR) as a comparison index, we confirmed that BoLA-DRB3 was the best marker for determining which cow spread the BLV (OR 13.9 for BoLA-DRB3, OR 11.5 for BoLA-DQA1, and OR 6.2 for BoLA class II haplotype). In addition, DRB3*002:01, *009:02, *012:01, *014:01, and *015:01 were determined as BLV provirus associated alleles. BoLA-DRB3*002:01, *009:02, and *014:01 were determined as resistant alleles (OR > 1), and BoLA-DRB3*012:01 and *015:01 were determined as susceptible alleles (OR < 1). In this study, we showed that BoLA-DRB3 was a good marker for determining which cow spread BLV, and we found not only one resistant allele (BoLA-DRB3*009:02), but also two other disease-resistant alleles and two disease-susceptible alleles. This designation of major alleles as markers of susceptibility or resistance can allow the determination of the susceptibility or resistance of most cows to disease. Overall, the results of this study may be useful in eliminating BLV from farms without having to separate cows into several cowsheds.

Association of Transfer RNA Fragments in White Blood Cells With Antibody Response to Bovine Leukemia Virus in Holstein Cattle

Bovine leukemia virus (BLV) affects cattle health and productivity worldwide, causing abnormal immune function and immunosuppression. Transfer RNA fragments (tRFs) are known to be involved in inhibition of gene expression and have been associated with stress and immune response, tumor growth, and viral infection. The objective of this study was to identify tRFs associated with antibody response to BLV in Holstein cattle. Sera from 14 animals were collected to establish IgG reactivity to BLV by ELISA. Seven animals were seropositive (positive group) and seven were seronegative (negative group) for BLV exposure. Leukocytes from each animal were collected and tRFs were extracted for sequencing. tRF5^GlnCTG, tRF5^GlnTTG, and tRF5^HisGTG, were significantly different between seropositive and seronegative groups (P < 0.0067). In all cases the positive group had a lower number of normalized sequences for tRFs when compared to the negative group. Result suggests that tRF5s could potentially be used as biomarkers to establish exposure of cattle to BLV.

Variations in the viral genome and biological properties of bovine leukemia virus wild-type strains

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis (EBL), which causes enormous economic losses in the livestock industry worldwide. To reduce the economic loss caused by BLV infection, it is important to clarify the characters associated with BLV transmissibility and pathogenesis in cattle. In this study, we focused on viral characters and examined spontaneous mutations in the virus and viral properties by analyses of whole genome sequences and BLV molecular clones derived from cows with and without EBL. Genomic analysis indicated that all 28 strains harbored limited genetic variations but no deletion mutations that allowed classification into three groups (A, B, and C), except for one strain. Some nucleotide/amino acid substitutions were specific to a particular group. On the other hand, these genetic variations were not associated with the host bovine leukocyte antigen-DRB3 allele, which is known to be related to BLV pathogenesis. The viral replication activity in vitro was high, moderate, and low in groups A, B, and C, respectively. In addition, the proviral load, which is related to BLV transmissibility and pathogenesis, was high in cows infected with group A strains and low in those infected with group B/C strains. Therefore, these results suggest that limited genetic variations could affect viral properties relating to BLV transmissibility and pathogenesis.

Genome-wide scan for commons SNPs affecting bovine leukemia virus infection level in dairy cattle

BACKGROUND

Bovine leukemia virus (BLV) infection is omnipresent in dairy herds causing direct economic losses due to trade restrictions and lymphosarcoma-related deaths. Milk production drops and increase in the culling rate are also relevant and usually neglected. The BLV provirus persists throughout a lifetime and an inter-individual variation is observed in the level of infection (LI) in vivo. High LI is strongly correlated with disease progression and BLV transmission among herd mates. In a context of high prevalence, classical control strategies are economically prohibitive. Alternatively, host genomics studies aiming to dissect loci associated with LI are potentially useful tools for genetic selection programs tending to abrogate the viral spreading. The LI was measured through the proviral load (PVL) set-point and white blood cells (WBC) counts. The goals of this work were to gain insight into the contribution of SNPs (bovine 50KSNP panel) on LI variability and to identify genomics regions underlying this trait.

RESULTS

We quantified anti-p24 response and total leukocytes count in peripheral blood from 1800 cows and used these to select 800 individuals with extreme phenotypes in WBCs and PVL. Two case-control genomic association studies using linear mixed models (LMMs) considering population stratification were performed. The proportion of the variance captured by all QC-passed SNPs represented 0.63 (SE ± 0.14) of the phenotypic variance for PVL and 0.56 (SE ± 0.15) for WBCs. Overall, significant associations (Bonferroni's corrected -log10p > 5.94) were shared for both phenotypes by 24 SNPs within the Bovine MHC. Founder haplotypes were used to measure the linkage disequilibrium (LD) extent (r2 = 0.22 ± 0.27 at inter-SNP distance of 25-50 kb). The SNPs and LD blocks indicated genes potentially associated with LI in infected cows: i.e. relevant immune response related genes (DQA1, DRB3, BOLA-A, LTA, LTB, TNF, IER3, GRP111, CRISP1), several genes involved in cell cytoskeletal reorganization (CD2AP, PKHD1, FLOT1, TUBB5) and modelling of the extracellular matrix (TRAM2, TNXB). Host transcription factors (TFs) were also highlighted (TFAP2D; ABT1, GCM1, PRRC2A).

CONCLUSIONS

Data obtained represent a step forward to understand the biology of BLV-bovine interaction, and provide genetic information potentially applicable to selective breeding programs.

Single nucleotide polymorphisms in the bovine MHC region of Japanese Black cattle are associated with bovine leukemia virus proviral load

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, a malignant B cell lymphoma that has spread worldwide and causes serious problems for the cattle industry. The BLV proviral load, which represents the BLV genome integrated into host genome, is a useful index for estimating disease progression and transmission risk. Here, we conducted a genome-wide association study to identify single nucleotide polymorphisms (SNPs) associated with BLV proviral load in Japanese Black cattle. The study examined 93 cattle with a high proviral load and 266 with a low proviral load. Three SNPs showed a significant association with proviral load. One SNP was detected in the CNTN3 gene on chromosome 22, and two (which were not in linkage disequilibrium) were detected in the bovine major histocompatibility complex region on chromosome 23. These results suggest that polymorphisms in the major histocompatibility complex region affect proviral load. This is the first report to detect SNPs associated with BLV proviral load in Japanese Black cattle using whole genome association study, and understanding host factors may provide important clues for controlling the spread of BLV in Japanese Black cattle.

Polymorphism in exon 2 of the BuLA-DRB3 gene in Indian buffalo (Bubalus bubalis var. indicus) detected by PCR-RFLP

Polymerase chain reaction (PCR) primers specific to exon 2 of the bovine lymphocyte antigen (BoLA)-DRB3 gene were used successfully to amplify the equivalent region in 34 Murrah and 36 Surti buffaloes selected at random. The 304 bp amplified product of the DRB3 gene was separately digested with BstγI, HaeIII and Rsal enzymes. Digestion with BstγI enzyme did not reveal any polymorphism and all animals showed a single restriction pattern, which corresponded exactly to the BstγI pattern ‘b’ previously described for cattle. Digestion with HaeIII enzyme resulted in five patterns, four of which corresponded to the Haelll patterns previously reported in cattle. The new HaeIII pattern was observed in both the breeds of buffaloes studied. The fragment analysis with RsaI revealed 13 different patterns. All of these RsaI patterns corresponded to the RsaI patterns previously described for cattle. The high degree of similarity in the restriction fragment length polymorphism (RFLP) patterns of cattle and buffalo observed in the present study provide evidence for the strong conservation amongst other bovine species, of restriction sites previously reported in cattle.

Mechanisms of pathogenesis induced by bovine leukemia virus as a model for human T-cell leukemia virus

Bovine leukemia virus (BLV) and human T-cell leukemia virus type 1 (HTLV-1) make up a unique retrovirus family. Both viruses induce chronic lymphoproliferative diseases with BLV affecting the B-cell lineage and HTLV-1 affecting the T-cell lineage. The pathologies of BLV- and HTLV-induced infections are notably similar, with an absence of chronic viraemia and a long latency period. These viruses encode at least two regulatory proteins, namely, Tax and Rex, in the pX region located between the env gene and the 3′ long terminal repeat. The Tax protein is a key contributor to the oncogenic potential of the virus, and is also the key protein involved in viral replication. However, BLV infection is not sufficient for leukemogenesis, and additional events such as gene mutations must take place. In this review, we first summarize the similarities between the two viruses in terms of genomic organization, virology, and pathology. We then describe the current knowledge of the BLV model, which may also be relevant for the understanding of leukemogenesis caused by HTLV-1. In addition, we address our improved understanding of Tax functions through the newly identified BLV Tax mutants, which have a substitution between amino acids 240 and 265.

Detection of bovine leukemia virus in brains of cattle with a neurological syndrome: pathological and molecular studies

Bovine leukemia virus (BLV) was investigated in the central nervous system (CNS) of cattle with neurological syndrome. A total of 269 CNS samples were submitted to nested-PCR (BLV env gene gp51), and the viral genotypes were identified. The nested-PCR was positive in 4.8% (13/269) CNS samples, with 2.7% (2/74) presenting at histological examination lesions of nonpurulent meningoencephalitis (NPME), whereas 5.6% (11/195) not presenting NPME (P > 0.05). No samples presented lymphosarcoma. The PCR products (437 bp) were sequenced and submitted to phylogenetic analysis by neighbor-joining and maximum composite likelihood methods, and genotypes 1, 5, and 6 were detected, corroborating other South American studies. The genotype 6 barely described in Brazil and Argentina was more frequently detected in this study. The identity matrices showed maximum similarity (100%) among some samples of this study and one from Argentina (FJ808582), recovered from GenBank. There was no association among the genotypes and NPME lesions.

BoLA class I polymorphism and in vitro immune response to M. bovis antigens

SUMMARY

From a sample of 119 Friesian calves, serologically typed for BoLA class I, 47 subjects were chosen expressing 9 different MHC types (A6, A6.9, A10, A11, A14, A15, A30, W16, M103) with the same age and reared in the same farm conditions. The animals were s.c. injected with a water in oil suspension of killed M. bovis and the treatment was repeated two days later. Before the treatment and 21 days later, calves were bled and on PBM (peripheral blood mononuclear leucocytes) were performed the following tests: 1. Lymphocyte Stimulation with bovine and avian PPDs (Purified protein derivative of Mycobacterium bovis and Mycobacterium avium, respectively). 2. Phagocytic activity towards M. bovis. 3. Class II molecules expression on cell surface. 4. Percentage of leucocyte populations and subpopulations. In the in vitro Lymphocyte Stimulation test, all the animals and classes were responders. Animals bearing A10 BoLA class I presented c.p.m. (counts per minute) and index values higher than the other cattle; these values were significantly positively related both to bovine and avian PPDs (P < .01). By variance analysis A14 BoLA type showed a slight positive significant correlation with more efficient phagocytic activity. BoLA class I type did not seem to significantly affect percentage of class II positive cells and leucocyte percentages on PBM. ZUSAMMENFASSUNG: Der BoLA Klasse I Polymorphismus und in vitro immunologische Antwort gegen die Antigene von M. bovis Aus einer Stichprobe von 119 für BoLA Klasse I serologisch typisierten Friesian Kälber, wurden 47 Subjekte ausgewählt, die 9 verschiedene MHC Typen ausdrückten (A6, A6.9, A10, A11, A14, A15, A30, W16, M103). Alle waren gleich alt und in gleichen Haltungsbedingungen. Die Tiere wurden mit einer Wasser-in-Ol Suspension abgetöteter M. bovis subkutan injiziert und die Behandlung nach zwei Tagen wiederholt. Vor und 21 Tage nach Behandlung wurden die folgenden Tests ausgeführt: 1. Lymphozyten-Stimulationstest mit bovinen und Geflügel PPDs. 2. Phagozyten Aktivität gegen M. bovis. 3. Zeil-Oberflächen, Expression der Klasse II Moleküle. 4. Anteile der Lymphozyten Populationen und Subpopulationen. Im in vitro Lymphozyten-Stimulationstest waren alle Tiere und Klassen responder. Tiere mit A10 BoLA I zeigten höhere c.p.m. und Indexwerte als die anderen; diese Werte waren in signifikant positiver Beziehung mit der PPD von M. bovis und auch mit M. avium (P < .01). BoLA Typ A14 zeigte leicht signifikant positive Korrelation mit wirksamerer Phagozyten Aktivität. BoLA Klasse I Typ scheint nicht den Prozentsatz der positiven Zellen der Klasse II und der Leukozyten der PBM signifikant zu beeinflussen. RESUMEN: Polimorfismo de BoLA clase I y immunidad a los antigenos del M. bovis Se escojeron 47 novillos dentro de un grupo de 119 animales que segun analisis previamente hecha tenian BoLA de clase I. Estos 47 novillos fueron escojidos de manera que tuvieran 9 distintos tipos de MHC (A6, A6.9, A10, A11, A14, A15, A30, W16, M103), la misma edad, las mismas condiciones de cria. Estos animales fueron inoculados subcutaneo con M. bovis matados en una suspension oleosa y la misma inoculacion fue repetida una secunda vez despues de dos dias. Por cada animal se tomaron muestras de sangre antes y 21 dias despues de la inoculacion de arriba. Las muestras de sangre fueron pruebaoas con: 1. Stimulacion Lymhocitaria con PPD bovina y avicola. 2. Actividad phagocitaria a M. bovis. 3. Expresion sobre la superficie celular de moleculas de clase II. 4. Porcentaje de poblaciones y de subpob-laciones de leucocitos. Todos los animales y todos los tipos de MHC dieron respuestas positivas en las pruebas de Stimulacion Lymphocitaria. Los animales que tenian la BoLA A10 presentaron valores de c.p.m. y indices mas altos de los demas animales. Estos valores se encontraron significativamente y positivamente relacionados sea a la PPD bovina que a la PPD avicola. Por medio de la analisis de varianzas se encontro que el tipo BoLA A14 muestraba una correlacion significativa y algo positiva con una mejor actividad fagocitaria. Los tipos de clase BoLA I no parecieron que influenzaran de manera appreciable el porcentaje de positividad por la clase II y el porcentaje de leucocitos en la sangre PBM.

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.

BoLA antigens are associated with increased frequency of persistent lymphocytosis in bovine leukaemia virus infected cattle and with increased incidence of antibodies to bovine leukaemia virus

The association between bovine major histocompatibility system (BoLA) type and persistent lymphocytosis in cattle with antibodies to bovine leukaemia virus was examined by comparing antigen frequencies in cattle with persistent lymphocytosis to controls matched for age, sex, breed and presence of antibodies to BLV. The cattle came from nine dairy herds in south-east Queensland, Australia; six herds were Australian Illawarra Shorthorn (AIS), two herds were Jersey and one herd was Friesian. Antigen W6 and Eu28R were more common in cattle with persistent lymphocytosis than in controls. Antigen W8 was less common in AIS cattle with persistent lymphocytosis. A study of 24 offspring from one sire, heterozygous for W10 and Eu28R, showed that offspring inheriting Eu28R from the sire were significantly more likely to have antibodies to BLV than offspring inheriting the opposing W10 haplotype.

Cloning of the bovine major histocompatibility complex class II genes

Class II genes of the bovine major histocompatibility complex (MHC) have been cloned from a genomic library. The library was constructed in the bacteriophage lambda vector EMBL3 and comprises approximately 10 times the equivalent of the haploid genome. Half the library was screened with the human DQA, DQB, DRA and DRB cDNA probes. Of the 100 positively hybridizing phage clones, 37 were eventually fully characterized and mapped by means of Southern blot analysis. The exons encoding the first, second and transmembrane domain of all different A and B genes were subcloned and mapped in more detail. These analyses showed that these 37 clones were derived from five different A and 10 different B genes. The hybridization studies indicate that we have cloned and mapped two DQA genes, one DRA gene, two other A genes, four DQB genes, three DRB genes and three other B genes. Since the library was made from a heterozygous animal, this would suggest that there are at least one DQA, one DRA one other undefined A, two DQB, two DRB and one or two other undefined B genes in the haploid genome of Holstein Friesian cattle.

Nucleotide sequence and northern analysis of a bovine major histocompatibility class II DR beta-like cDNA

A 1.2-kb bovine DR beta-like cDNA clone (BoLA-DRB3) was isolated from a peripheral blood lymphocyte cDNA library utilizing a human DR beta cDNA as a probe. BoLA-DRB3 was found to have a high degree of nucleotide sequence similarity (96.8%) with a previously sequenced bovine DR beta-like gene (A1). It is believed that BoLA-DRB3 and A1 represent distinct alleles of one of the three bovine DR beta-like loci. Sequence comparison of BoLA-DRB3 with genes representing the other two bovine DR beta-like loci resulted in moderate degrees of sequence similarities (83.1% and 86.3%, respectively). Comparison of the relative abundance of RNA transcripts of the three bovine DR beta-like loci by Northern analysis of lymphocyte RNA indicated that BoLA-DRB3 is the most actively transcribed of the three bovine DR beta-like genes. Based on these results we suggest that of the three DR beta-like loci thus far identified in the bovine, only one is actively transcribed.

Expression and genetic segregation of parental BoLA serotypes in bovine embryos

The detection of parentally derived BoLA serotypes was attempted in 68 bovine embryos. 23 bovine embryos were tested for genetic segregation of maternally derived BoLA serotypes. 45 bovine embryos were tested for genetic segregation of paternally derived BoLA serotypes. The expected parentally derived BoLA gene products were detectable on approximately 50% of the embryos tested. A 1:1 segregation ratio of expression or non-expression of parental BoLA serotypes in 7-day-old preimplantation bovine embryos, which is expected for codominant alleles, could not be rejected.

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.

Host immune responses in the course of bovine leukemia virus infection

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 relationship between the disease progression and the change of cytokine profiles are summarized. In the third section, we have focused on tumor necrosis factor alpha (TNFalpha) and its two types of receptors, and the possible involvement of TNFalpha in the BLV-induced leukemogenesis is discussed. The expression of TNFalpha 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.

Comparative organization and function of the major histocompatibility complex of domesticated cattle

This review focuses on recent advances in research on the bovine major histocompatibility complex (BoLA), with specific reference to the genetic organization, polymorphism and function of the class II genes. The BoLA region is unlike the MHC of humans and mice in that a large inversion has moved several class II genes, including the TAP/LMP cluster, close to the centromere of bovine chromosome 23. Therefore, close linkage of MHC genes and other genes associated with the MHC in humans and mice does not appear to be required for normal immunological function. In cattle, polymorphism in the class IIa genes influences both the magnitude and the epitope specificity of antigen-specific T-cell responses to foot-and-mouth disease virus peptides. Disease association studies have demonstrated that BoLA alleles affect the subclinical progression of bovine leukemia virus (BLV) infection. This association is strongly correlated with the presence of specific amino acid motifs within the DRB3 antigen-binding domain. In addition to the practical significance of these findings, the association between BoLA and BLV provides a unique model to study host resistance to retrovirus infection in a non-inbred species. These studies contribute to our understanding of the evolution of the MHC in mammals, to the development of broadly effective vaccines, and to breeding strategies aimed at improving resistance to infectious diseases.

Increased MHC class II and CD25 expression on lymphocytes in the absence of persistent lymphocytosis in cattle experimentally infected with bovine leukemia virus

We recently observed that a group of cattle experimentally infected with bovine leukemia virus (BLV) had enhanced antibody responses to recall antigens. None of the cattle in this group were classified as persistently lymphocytotic, but they did have significantly increased numbers of circulating T and B cells. In order to investigate the potential mechanisms of BLV-induced immune activation, dual-color flow cytometry was used to compare the expression of MHC class II (MHC II) molecules and the inducible IL-2 receptor alpha chain, CD25, on lymphocyte subsets in freshly isolated and cultured PBMC from these same BLV-infected cattle (n=5) with that of age-matched, uninfected controls (n=3). Freshly isolated peripheral blood mononuclear cells (PBMC) from BLV-infected cattle were found to contain a significantly higher percentage of B cells that expressed MHC II molecules (p<0.01). In addition, an increased proportion of CD4+ T cells from BLV-infected cattle expressed MHC II molecules after 20 h of Concanavalin A (Con A) stimulation (p<0.05), and MHC II expression was increased on both CD4+ (p<0.01) and CD8+ (p<0.05) T cells from BLV-infected cattle after 68 h in vitro, even in the absence of exogenous mitogen. Although CD25 expression was not increased on freshly isolated lymphocytes from BLV-infected cattle, an increased percentage of B cells from BLV-infected cattle expressed CD25 after 20 h of culture, either in the presence (p<0.05) or absence (p<0.01) of Con A. Thus, in addition to causing alterations in absolute numbers of circulating lymphocytes, BLV infection appears to cause a functional activation of both B and T cells, even in cattle that are non-lymphocytotic. It is likely that these BLV-induced alterations in lymphocyte activation status contributed to the previously observed enhancement of antibody responses in vivo.

Natural transmission of bovine leukemia virus in dairy and beef cattle

Many potential routes of bovine leukemia virus (BLV) transmission are reviewed in this article. Vertical transmission, in utero, or through colostrum and milk, accounts for a relatively small proportion of infections. Iatrogenic horizontal transmission, through procedures permitting the transfer of blood between cattle, has been shown to be a major route of transmission in most settings. Contact transmission stems from a mixture of natural sources of blood, exudates, and tissues that enter the body through mucosal surfaces or broken skin. Careful analysis of management procedures and environmental conditions present in individual dairy and beef herds affords the greatest opportunity to develop effective BLV prevention programs.

Bovine leukemia virus: early reflections in blood after an experimental infection of calves

In order to study early alterations in the blood following infection with bovine leukemia virus (BLV) in the natural host, 15 calves were inoculated with blood from a BLV-positive donor cow. The humoral immunological response was followed by ELISA for 2 months. Seroconversion to BLV was demonstrated at 4-5 weeks post-infection. Total and differential leukocyte counts were performed. Acute lymphocytosis was observed at the time of seroconversion in the majority of the experimental calves. By the aid of monoclonal antibodies (mAbs), the proportion as well as the total number of lymphoid cells were studied in four of the calves, applying analytical flow cytometry. At the time of seroconversion the percentage of B-cells increased from 19.1 +/- 7.5% to 37.9 +/- 15.8%, and the T-cells (CD2+) decreased from 36.7 +/- 7.3% to 22.7 +/- 6.0%, the latter being attributable to decreases in the percentage of CD4+ as well as CD8+ T-cells for the infected calves together. Subsequently, altered B/T ratios were observed. In one of the calves an increase in the absolute number of CD5+ cells coincided with an increase in total B-cells. The early phenotypic alterations in lymphocyte subsets, before and after seroconversion to BLV, were comparable to those of non-lymphocytotic and persistent lymphocytotic cattle, respectively. Sera from 15 calves were tested for the presence of interferon (IFN), as measured by antiviral activity. BLV does not appear to induce the production of IFN.

Identification and characterization of new BoLA-DRB3 alleles by heteroduplex analysis and direct sequencing

A sample of 52 mixed-breed dairy cattle (Holstein Friesian and Jersey) and 51 beef cattle (Hereford) from south-east Queensland was studied. The second exon of BoLA-DRB3 was amplified by polymerase chain reaction (PCR), and polymorphisms were detected by heteroduplex analysis. A large number of different heteroduplex patterns indicated extensive sequence polymorphism. Direct sequencing of PCR products from 17 homozygotes and cloning and sequencing of PCR product from two heterozygotes resulted in the identification and characterization of four novel alleles. The previously described allele BoLA-DRB3*2A is characterized by an amino acid deletion at position 65. We have identified three animals that are homozygous for this amino acid deletion, indicating that the deletion is unlikely to result in loss of function. Two of these animals had allele BoLA-DRB3*2A, and one had a novel allele with codon 65 deleted but differing from BoLA-DRB3*2A at a number of other amino acid positions. In conclusion, heteroduplex analysis allows rapid discrimination between homozygotes and heterozygotes, and enables rapid identification of new BoLA-DRB3 alleles.

Sequence and PCR-RFLP analysis of 14 novel BoLA-DRB3 alleles

The genetic diversity of the bovine class II DRB3 locus was investigated by polymerase chain reaction (PCR) amplification and DNA sequencing of the first domain exon. Studying 34 animals of various cattle breeds, 14 previously unrecognized DRB3 alleles were identified. In three alleles, amino acid substitutions were observed that had not been previously found in bovine DRB3, but occurred at the same position in bovine DQB and in the DRB alleles of other mammals. For all newly identified alleles, the restriction fragment length polymorphism (RFLP) patterns of PCR products obtained with the enzymes RsaI, BstYI, and HaeIII were compared with patterns of 38 previously described alleles. Altogether, eleven novel PCR-RFLP types were defined. Twelve out of the 42 PCR-RFLP types identified so far were not found to be fully informative because they corresponded to more than one allelic sequence. PCR-RFLP may therefore be a rapid and useful method for DRB3 typing in cattle families, but for studies on outbred populations, sequencing and hybridization techniques are required.

Detection of bovine leukemia virus proviral DNA in individual cells

We have developed a method of analyzing individual cells to detect proviral DNA of the bovine leukemia virus (BLV) using flow cytometry and PCR. Individual cells of the BL3* cell line, which contain multiple integrated copies of the BLV provirus, and the uninfected cell line BL3(0), were sorted into wells of a 96-well plate. Following cell lysis, portions of the BLV envelope (ENV) and cellular prolactin (PRL) genes were amplified simultaneously using PCR. Viral and cellular products of first-round PCR were amplified separately in a second round of PCR using "heminested" primers. Separation of the PCR products by polyacrylamide gel electrophoresis yielded distinct fragments of the predicted sizes. The operational sensitivity of this method for the detection of virus was > 90% when testing single infected cells. In addition, we were able to reliably amplify DNA from a single BL3* cell among as many as 10(5) BL3(0) cells and established that the sensitivity for detecting a single infected cell among 20, 100, or 1000 uninfected cells was at least 90%. Estimates of low percentages of infected cells were obtained by applying probability theory to results of experiments conducted on wells containing more than one cell. Using these methods, B lymphocytes obtained from the peripheral blood of BLV-infected cattle were tested for proviral DNA. BLV ENV was identified in 76.9 +/- 4.9% of single B cells tested from a seropositive animal with persistent lymphocytosis (PL), but in only 0.033 +/- 0.009% of B cells from another seropositive cow without PL.(ABSTRACT TRUNCATED AT 250 WORDS)

Disease resistance in farm animals

Genetic variations in disease resistance of farm animals can be observed at all levels of defence against infectious agents. In most cases susceptibility to infections has polygenic origins. In domestic animals only a few instances of a single genetic locus responsible for disease resistance are known. A well-examined example is the Mx1 gene product of certain mice strains conferring selective resistance to influenza virus infections. Attempts to improve disease resistance by gene transfer of different gene constructs into farm animals include the use of monoclonal antibody gene constructs, transgenes consisting of antisense RNA genes directed against viruses and Mx1 cDNA containing transgenes.

Association of class I bovine lymphocyte antigen complex alleles with in vitro blood neutrophil functions, lymphocyte blastogenesis, serum complement and conglutinin levels in dairy cattle

Ninety-eight lactating Holstein cows from two genetic lines selected for high and average milk production were used in the study. Five peripheral blood samples were collected over a 60-day period from each cow for evaluation of neutrophil function, lymphocyte blastogenesis, leukocyte count, and serum complement and conglutinin levels. Blood samples were typed for antigens encoded by alleles at the bovine major histocompatibility complex (BoLA) A locus. Alleles w14(w8), w20A, and w19(w6) were the most frequent of 14 alleles present in this herd. Association of BoLA type with immune function results was examined by using gene substitution models including and ignoring sire effects. Alleles w15(w8) and w16 were associated with greater circulating mononuclear cell and total leukocyte numbers, while w27(w10), w11, and w20A were associated with lower numbers of these cell types. Alleles EU28D and w20A were positively and negatively associated with granulocyte percentage, respectively. Allele w16 was associated with greater antibody-independent neutrophil cytotoxicity, unstimulated lymphocyte proliferation, serum conglutinin activity, and with lower antibody-dependent neutrophil cytotoxicity. Allele w19(w6) was associated with decreased conglutinin activity and decreased neutrophil iodination. Increased antibody-dependent neutrophil cytotoxicity was observed for animals bearing allele w14(w8), and decreased neutrophil iodination, serum conglutinin, and nonstimulated lymphocyte blastogenesis were observed in individuals carrying w20A or EU28D. Significance of both sire and BoLA complex effects suggests that both major histocompatibility complex genes and background genes of the sire significantly affect immune function. This research suggests BoLA-A locus genes may be major genes or markers for closely linked major genes involved in regulation of nonspecific immune function.

Bovine sire effects on daughters' in vitro blood neutrophil functions, lymphocyte blastogenesis, serum complement and conglutinin levels

Blood neutrophil functions, lymphocyte blastogenic responses, serum complement, and serum conglutinin activity of 98 lactating Holstein cows from two genetic lines were evaluated. The genetic lines were produced in a selection experiment that created and perpetuated genetic differences in milk production for up to seven generations. No significant differences between the two genetic lines of cows were found for neutrophil function, lymphocyte blastogenic responses, serum complement levels, or serum conglutinin levels. Significant differences between sire progeny groups within lines were found for unstimulated and mitogen-stimulated lymphocyte blastogenesis (P less than 0.0001), and almost all neutrophil functions (antibody independent neutrophil cytotoxicity, antibody dependent neutrophil cytotoxicity, ingestion of bacteria, iodination, chemiluminescence, chemokinesis, and chemotaxis (P less than or equal to 0.05)). Sire progeny group differences (P less than or equal to 0.0001) within lines for serum complement and conglutinin activity were also found. Neutrophil chemiluminescence activity (positive relationship; P less than or equal to 0.001), concanavalin A-stimulated lymphocyte blastogenesis (positive relationship; P less than or equal to 0.004), and serum conglutinin activity levels (negative relationship; P less than or equal to 0.01) each had small but significant associations with the total milk somatic cell count. Cows seropositive for bovine leukosis virus had increased resting and mitogen-stimulated lymphocyte blastogenic activity and were associated with increased in vitro neutrophil random migration and production of superoxide anion. Estimates of genetic parameters of various immune cell functions, of serum complement and of conglutinin levels for daughters of 11 sires with 4-6 daughters in the data set were determined. In this report, genetic variation was demonstrated for nonspecific humoral and cellular immunity.

Homologies between the major histocompatibility complex of man and cattle: consequences for disease resistance and susceptibility

The major histocompatibility complex (MHC) of mammals contains a large number of mostly duplicated genes. In the HLA system (the MHC of man), which is by far the best-studied major histocompatibility system so far, roughly 20 genes have been defined and mapped. They code for three classes of proteins: HLA-A, -B and -C (Class I), HLA-DP, -DQ and -DR (Class II) and serum complement components C2, C4 and Bf (Class III). Furthermore, the region contains genes for 21-hydroxylase (21-OH) and tumor necrosis factor (TNF). The MHC thus forms a chromosomal segment containing several clusters of genes of only partially defined biological significance, but ondoubtedly playing a role in disease susceptibility. In view of the recently obtained structural information on BoLA, the MHC of cattle, it is hypothesized that susceptibility to diseases in cattle is associated with BoLA in the same way as human diseases. Finally, new technical and conceptual developments in the field of MHC research and their application to the BoLA system are discussed.

Association of class I bovine lymphocyte antigen complex alleles with health and production traits in dairy cattle

Ninety-eight Holstein cows from the I-O-State breeding research herd were serologically typed for class I bovine lymphocyte antigens. After exclusion of animals carrying alleles that occurred at frequencies lower than 4%, records from 82 cows that had 161 lactations remained for analysis of major histocompatibility complex allelic effects. A gene substitution model was used to evaluate the additive effects of nine alleles (frequencies of 4 to 20%) at the bovine lymphocyte antigen complex A locus on general health, udder health, and production traits. Allele w14(w8) was associated with decreased quarter milk sample California Mastitis Test scores, composite milk sample California Mastitis Test and Wisconsin Mastitis Test scores, decreased total health costs, and with increased milk yield, fat yield, fat percentage, and income over feed costs. Allele w11 was associated with decreased clinical mastitis, discarded milk, and udder health costs but was also associated with decreased fat yield, fat percentage, and income over feed costs. Allele w31(w30) was associated with decreased fat percentage. These relationships suggest that alleles at the bovine lymphocyte antigen complex A locus may serve as markers for health and production traits. Thus, the potential may exist for enhancement of disease resistance or production in cattle via marker-assisted selection and genetic manipulation techniques using class I genes of the bovine major histocompatibility complex.

The relationship between bovine major histocompatibility complex class II polymorphism and disease studied by use of bull breeding values

The predictive value of class II DQ and DYA polymorphisms of the bovine major histocompatibility (MHC) complex (BoLA) for the incidence of disease in dairy cattle was estimated in a sample of 196 progeny-tested AI bulls of the Swedish Red and White breed. The BoLA DQ and DYA types of the bulls were determined by analysing restriction fragment length polymorphisms (RFLPs). Breeding values of bulls for clinical mastitis, all diseases including clinical mastitis and diseases other than clinical mastitis were used as measures of disease resistance or susceptibility. The relationship between MHC polymorphism and bull breeding values for disease resistance was evaluated statistically by linear regression analysis. A significant association between the haplotype DQ1A and susceptibility to clinical mastitis was revealed. No other DQ haplotype nor the DYA locus has a significant effect on any of the disease traits studied.

Changes in B cell and T cell subsets in bovine leukaemia virus-infected cattle

Direct immunofluorescence and fluorescence-activated cell sorter techniques were used for the detection of surface immunoglobulin positive (SIg+) cells in peripheral blood lymphocytes (PBL's) of bovine leukaemia virus (BLV) infected cattle with or without persistent lymphocytosis (PL+, PL-) and in BLV-free cattle. The percentage of SIg+ cells was more than twice as high in BLV+PL+ cattle than in BLV-free and BLV+PL- cattle. Bovine T cells, and T cell subsets were identified indirectly by the same techniques using three monoclonal antibodies (MAb's) specific for all T cells (IL-A43), T helper (BoT4) cells (IL-A12) and T cytotoxic (BoT8) cells (IL-A17). The major histocompatibility complex (MHC) determinants of both class II (BoT4) and class I (BoT8) as well as all T cells were significantly reduced in BLV+PL+ compared to BLV-free cattle. The actual decrease in the BoT8 cell subset or the dilution effect that would change effector:target cell ratio suggests that a resultant decrease in cytotoxic activity in BLV+PL+ cattle may play an important role in the progress of BLV infection in cattle.

Isoelectric focusing of bovine major histocompatibility complex class I molecules

The products of the major histocompatibility complex (MHC) loci regulate an individual's immune response to pathogens. Cattle provide an important model to study the relationship between disease susceptibility and MHC haplotype since large half-sibling families are common. The definitive demonstration, however, of a firm relationship between MHC phenotype and disease susceptibility in cattle will require a precise definition of the bovine MHC allelic products. Available reagents for serological characterization of the bovine MHC gene products have not been adequate for these purposes. We have shown that existing mouse monoclonal antibodies and rabbit anti-human antisera precipitate bovine class I molecules, that these structures separate well by one-dimensional isoelectric focusing (1-D IEF), and that immunoprecipitation followed by 1-D IEF allows the detection of bovine class I MHC allelic products. Through this technique, we have identified previously undetected class I products. This approach will facilitate a detailed characterization of the bovine MHC class I gene products.