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 CD8αβhigh cells were observed but CD8αβ cells were not found in significant numbers. On the other hand, not only CD8αβhigh but also CD8αβ and CD8αβlow cell populations were observed in cats chronically infected with FIV. The expansion of the CD8βlow 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 CD8αβ cell population expressed CD5 at a low level.
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