To study the in vitro susceptibility of peripheral blood mononuclear cells (PBMC) to hepatitis C virus (HCV), we incubated cells from healthy donors with HCVpositive sera. Using RT-PCR and in situ hybridization, the genomic viral RNA was detected in PBMC and in their supernatants until 25 days post-incubation. The PBMC of the different donors were not all permissive to HCV, but results were more constantly positive when cells from four donors were pooled. Quantification of the genomic viral RNA by the branched-DNA assay showed a decrease in the HCV RNA concentration during the first week of culture followed by a peak during the second or third week, and also an increase in the total amount of viral RNA in the inoculated cells.Although HCV RNA could be detected in the supernatants by RT-PCR, the concentration was very low. Using a sense-specific RT-PCR method, the HCV negative-strand was also detected in the cells but not in the supernatants. In two experiments PBMC were successfully infected using HCV-positive culture supernatants, therefore suggesting that infectious particles can be produced in this system. Our findings demonstrate that PBMC are permissive for HCV replication in vitro but the replication level is very low. The HCV RNA concentration measured in PBMC of 10 chronically infected patients was not significantly higher than the maximal concentration obtained in PBMC infected in vitro.

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Infection of microglial cells by the human immunodeficiency virus (HIV) is supposed to play an important role in the pathogenesis of AIDS-related central nervous system (CNS) complications. So far, however, experimental data about interactions between HIV and ramified microglia from the adult CNS were only occasionally reported, making it difficult to understand the exact nature of pathogenic events contributing to HIV-encephalopathy. Therefore, we used the animal model of feline immunodeficiency virus (FIV) infection of domestic cats to establish an experimental system which is suitable for studying the relationships between an immunodeficiency virus and the mature ramified microglia of the central nervous system. By means of density gradient centrifugation approximately 95% pure microglial cells could be isolated from adult feline brain that were characterized by their CD45(low) phenotype. Resident microglia extracted from the CNS of experimentally infected cats harbored FIV-specific DNA and cocultivation with mitogen-activated, but uninfected peripheral blood mononuclear cells (PBMC) resulted in recovery of high-titered infectious virus. Double labeling of brain cell monocultures explanted from persistently infected animals for both microglia and FIV markers disclosed less than 1% of viral antigen expressing microglial cells. This suggests that during the subclinical phase of the infection only a small number of brain-resident macrophages are productively infected. However, interaction of FIV-infected microglia and inflammatory lymphocytes may promote viral replication, thus supporting viral spread in brain tissue.