Introduction Bovine leukaemia virus (BLV) is the retroviral causative agent of enzootic bovine leukosis, the most common neoplastic disease of cattle and a serious problem worldwide. Its diagnosis is commonly by tests for antibodies recognising the p24 capsid protein and structural glycoprotein (gp) 51. With flow cytometry recently having come to veterinary immunology, applications for it may now include BLV. The study determined BLV gp51 expression in blood and milk lymphocytes of naturally infected cows by flow cytometry. Material and Methods Nineteen Polish Black and White Lowland breed cows aged 4-9 years and naturally infected with BLV and ten uninfected counterparts had blood and milk sampled and cultured. The immunological status of the animals was confirmed with ELISA and PCR. Dual-colour flow cytometry analysis was performed with specific monoclonal antibodies for lymphocyte cluster of differentiation (CD) markers and gp51 viral envelope protein and conjugates labelled with fluorescein isothiocyanate or phycoerythrin. Bovine leukaemia virus gp51 was confirmed in lymphocytes by immunofluorescence with anti-gp51 monoclonal antibodies. Results The gp51 antigen was detected in blood and milk lymphocytes of infected cows, but the percentage of cells expressing it in milk was much lower than in blood. A depleted number of CD4+ lymphocytes, an augmented number of CD8+ lymphocytes, a lower ratio of CD4+ to CD8+ and a proliferation of CD19+ immunoglobulin M+ cells were also found. Conclusion These proliferated cells were immature, gave no sign of a tendency to differentiation and were characterised by prolonged vitality.
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