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We describe the identification, experimental transmission, and pathogenesis of a naturally occurring powerfully immunosuppressive isolate of feline leukemia virus (designated here as FeLV-FAIDS) which induces fatal acquired immunodeficiency syndrome (AIDS) in 100% (25 of 25) of persistently viremic experimentally infected specific pathogen-free (SPF) cats after predictable survival periods ranging from less than 3 months (acute immunodeficiency syndrome) to greater than one year (chronic immunodeficiency syndrome), depending on the age of the cat at time of virus exposure. The pathogenesis of FeLV-FAIDS-induced feline immunodeficiency disease is characterized by: a prodromal period of largely asymptomatic viremia; progressive weight loss, lymphoid hyperplasia associated with viral replication in lymphoid follicles, lymphoid depletion associated with extinction of viral replication in lymphoid follicles, intractable diarrhea associated with necrosis of intestinal crypt epithelium, lymphopenia, suppressed lymphocyte blastogenesis, impaired cutaneous allograft rejection, hypogammaglobulinemia, and opportunistic infections such as bacterial respiratory disease and necrotizing stomatitis. The clinical onset of immunodeficiency syndrome correlates with the replication of a specific FeLV-FAIDS viral variant, detected principally as unintegrated viral DNA, in bone marrow, lymphoid tissues, and intestine. Two of seven cats with chronic immunodeficiency disease that survived greater than 1 year after inoculation developed lymphoma affecting the marrow, intestine, spleen, and mesenteric nodes. Experimentally induced feline immunodeficiency syndrome, therefore, is a rapid and consistent in vivo model for prospective studies of the viral genetic determinants, pathogenesis, prevention, and therapy of retrovirus-induced immunodeficiency disease.

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.