Kinetics of replication of a partially attenuated virus and of the challenge virus during a three-year intersubtype feline immunodeficiency virus superinfection experiment in cats

The effects of preinfecting cats with a partially attenuated feline immunod eficiency virus (FIV) on subsequent infection with a fully virulent FIV bel onging to a different subtype were investigated. Eight specific-pathogen-fr ee cats were preinfected with graded doses of a long-term in vitro-cultured cell-free preparation of FIV Petaluma (FIV-P, subtype A). FN-P established a low-grade or a silent infection in the inoculated animals. Seven months later, the eight preinfected cats and two uninfected cats were challenged w ith in vivo-grown FIV-M2 (subtype B) and periodically monitored for immunol ogical and virological status. FIV-P-preinfected cats were not protected fr om acute infection by FIV-M2, and the sustained replication of this virus w as accompanied by a reduction of FN-P viral loads in the peripheral blood m ononuclear cells and plasma. However, from 2 years postchallenge (p.c.) unt il 3 years p.c., when the experiment was terminated, preinfected cats exhib ited reduced total viral burdens, and some also exhibited a diminished decl ine of circulating CD4(+) T lymphocytes relative to control cats infected w ith FIV-M2 alone. Interestingly, most of the virus detected in challenged c ats at late times p.c. was of FIV-P origin, indicating that the preinfectin g, attenuated virus had become largely predominant. By the end of follow-up , two challenged cats had no FIV-M2 detectable in the tissues examined. The possible mechanisms underlying the interplay between the two viral populat ions are discussed.