The identification and molecular cloning of a feline leukemia virus (FeLV) isolate (FeLV-FAIDS) that consistently produces immunodeficiency syndrome has allowed prospective investigation of events that occur in the prodromal phase of disease. Using a T-lymphocyte colony forming assay (T-CFU-Ic) we have demonstrated that a drastic depletion of circulating T-CFU-Ic prefigures the development of clinical immunodeficiency disease in inoculated cats and correlates with the appearance and replication of the FeLV-FAIDS variant genome in serially collected bone marrow samples. During the same presymptomatic time period, no significant alterations in conventional mitogen-induced lymphocyte blastogenic responses or in circulating lymphocyte numbers were evident. Thus T-CFU-Ic assay but not conventional mitogen-driven blastogenesis identified animals destined to develop immunodeficiency syndrome. The correlation among T-CFU-Ic depletion, the replication of the lymphocytopathic FeLV-FAIDS variant genome in hematopoietic and lymphoid tissues, and the onset of clinical disease, infers that ablation of a colony-forming T lymphocyte progenitor subset is important in the early pathogenesis of feline retrovirus-induced immunodeficiency syndrome.

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