Evolution of Replication Efficiency following Infection with a Molecularly Cloned Feline Immunodeficiency Virus of Low Virulence

The development of an effective vaccine against human immunodeficiency virus is considered to be the most practicable means of controlling the advancing global AIDS epidemic. Studies with the domestic cat have demonstrated that vaccinal immunity to infection can be induced against feline immunodeficiency virus (FIV); however, protection is largely restricted to laboratory strains of FIV and does not extend to primary strains of the virus. We compared the pathogenicity of two prototypic vaccine challenge strains of FIV derived from molecular clones; the laboratory strain PETF14 and the primary strain GL8414. PETF14 established a low viral load and had no effect on CD4- or CD8-lymphocyte subsets. In contrast, GL8414 established a high viral load and induced a significant reduction in the ratio of CD4 to CD8 lymphocytes by 15 weeks postinfection, suggesting that PETF14 may be a low-virulence-challenge virus. However, during long-term monitoring of the PETF14-infected cats, we observed the emergence of variant viruses in two of three cats. Concomitant with the appearance of the variant viruses, designated 627W135 and 628W135, we observed an expansion of CD8 lymphocyte subpopulations expressing reduced CD8 -chain, a phenotype consistent with activation. The variant viruses both carried mutations that reduced the net charge of the V3 loop (K409Q and K409E), giving rise to a reduced ability of the Env proteins to both induce fusion and to establish productive infection in CXCR4-expressing cells. Further, following subsequent challenge of naïve cats with the mutant viruses, th