Transient B cell lymphocytosis at seroconversion of sheep infected by BLV-mutants. A. Sheep n° 4536 was injected with 100 μg of proviral DNA (wild type strain 344 cloned into plasmid pBLV344) whereas animal n° 4533 was used as a negative control. At different times post-injection, leucocytes were counted using a Coulter Counter ZN and the total numbers of lymphocytes were estimated after examination under a microscope. In parallel, Peripheral Blood Mononuclear Cells (PBMCs) were isolated by Percoll gradient centrifugation and the proportions of B, CD4+ T and CD8+ T-cells were determined by flow cytometry. The vertical line represents the detection by immunodiffusion of antibodies directed against the virus (day 28: sheep n° 4536). B. Graphic representation of absolute B cell numbers at viral inoculation (Day 0, yellow bars) and at seroconversion (grey bars) of sheep infected by the BLV wild type virus (WT) or mutants (IG4, CRX3, CRE and GP30). The sheep coordinates are indicated. C. Fold increase of the B cell population in the infected sheep. The bars correspond to the mean difference (± standard deviation) between the absolute numbers of B cells at the day of virus inoculation and those determined at seroconversion (means of three values).