CD8+ lymphocytes have been subdivided into CD8αβ and CD8αα populations in the peripheral blood lymphocytes (PBL) of humans and in several animal species but have not yet been investigated in cats. Feline immunodeficiency virus (FIV) causes progressive immunological disorders similar to human AIDS. In this study, we analysed CD8+ cells in PBL of FIV-infected or uninfected cats by two-colour flow cytometric analysis. In specific pathogen-free adult cats, feline CD8α+β(high) cells were observed but CD8α+B- cells were not found in significant numbers. On the other hand, not only CD8α+β(high) but also CD8α+β- and CD8α+β(low) cell populations were observed in cats chronically infected with FIV. The expansion of the CD8β(low) or CD8β- subpopulations resulted in the apparent differences in CD4/CD8 ratios depending on the anti-CD8 MAb used. These findings suggest a need to reconsider the CD4/CD8 ratio in studies of FIV infection. Furthermore, we found that the CD8α+B- cell population expressed CD5 at a low level.
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