We have investigated the in vivo state of feline immunodeficiency virus (FIV) transcription in peripheral blood mononuclear cells (PBMC) of chronically FIV-infected, asymptomatic cats. FIV was detected in a high percentage of PBMC but not in the plasma of these cats. By quantitative reverse transcription-PCR (RT-PCR) analysis, FIV transcriptional status in the PBMC was characterized by extremely low or undetectable levels of unspliced or singly spliced mRNAs and predominantly multiply spliced mRNAs. Upon stimulation in vitro, however, the larger mRNA species and infectious virus production were rapidly induced in the PBMC. Furthermore, we demonstrated that viral production was induced in association with differential increases in the levels of each multiply spliced mRNA coding for FIV regulatory proteins. From these results, we suggest that replication of FIV is blocked at an early stage of gene expression in vivo, as described in asymptomatic human immunodeficiency virus (HIV)-infected patients, and that FIV infection in cats may be a useful model for clinical latency of HIV infection in man. Moreover, we propose that the replication of FIV in vivo may be controlled by the differential expression of each multiply spliced mRNA.
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