Early events in the immunopathogenesis of feline retrovirus infections

Feline leukemia virus and feline immunodeficiency virus (FIV) are lymphotropic retroviruses that cause a wide range of diseases in domestic cats. Although it is known that both viruses are capable of infecting T lymphocytes and that infected cats are lymphopenic, it was not known how infection with either virus might alter specific lymphocyte subpopulations. Using a panel of monoclonal antibodies to feline lymphocyte subpopulations, we examined, by use of flow cytometric analysis, lymphocyte changes in cats naturally infected with FeLV or FIV and explored the early stages in the immunopathogenesis of experimentally induced infection with these viruses. Both groups of naturally infected cats had T-cell lymphopenia. In the FIV-infected cats, the T-cell decrease was principally attributable to loss of CD4+ cells, whereas CD8+ and B-cell numbers remained normal. This led to inversion of the CD4+ to CD8+ ratio in these cats. In contrast, the T-cell lymphopenia in FeLV-infected cats resulted from decrease in CD4+ and CD8+ cells, which led to a CD4+ to CD8+ ratio within normal limits. Experimentally induced infection with these 2 viruses supported these findings. Infection with FIV induced early (10 weeks after infection), chronic inversion of the CD4+ to CD8+ ratio. In contrast, infection with FeLV did not alter CD4+ to CD8+ ratio in the first 20 weeks after infection.