Bovine leukemia virus-gp51 antigen expression is associated with CD5 and IgM markers on infected lymphocytes

Cows that develop a persistent lymphocytosis (PL) as a result of bovine leukemia virus (BLV) infection develop massive proliferation of B-lymphocytes expressing both IgM and CD5 markers. The association of these two markers on peripheral blood mononuclear cells (PBMC) derived from BLV-infected cows and also expressing BLV-gp51 antigen marker on these cells was determined by three-color cytometric analysis. After in vitro cultivation of PBMC in the presence of PHA for 24 h, the mean percentages of marker-reactive cells of five PL+ cows were as follows; 43% ± 4.5 of the PBMC expressed BLV-gp51 antigen; 90% ± 1.6 of these cells expressed both IgM and CD5 at the same time, whereas but 7.5% ± 1.9 expressed only IgM and 2.9% ± 0.4 expressed only CD5. The PBMC, IgM positive cells accounted for 77.8% ± 6.8, while both CD5 and BLV-gp51 were detected simultaneously on 52.0% ± 2.4 of the IgM+ cells, while the CD5 marker and BLV-gp51 antigen were detected independently on 35.0% ± 1.9 and in 9.0% ± 3.1, respectively of the IgM+ cells. Of the CD5+ cells (equivalent to 75.5% ± 9.0 of the PBMC), 54.7% ± 4.7 expressed simultaneously IgM and BLV-gp51, while BLV-gp51 and IgM were expressed separately by 3.0% ± 0.5 and 37.8% ± 3.3, respectively. An association between the B-cell phenotype and BLV tropism might exist. It is also possible that cells bearing both IgM and CD5 markers are the main target cells for BLV infection.