Background: GB Virus type C (GBV-C) HIV co-infection is associated with prolonged survival or clinical benefit in 8 clinical studies, and GBV-C HIV (R5 strain) co-infection of PBMC cultures resulted in decreased HIV replication. The mechanism by which GBV-C may slow HIV disease progression is not known. In this study we characterized the effects of GBV-C infection on HIV replication and on cellular gene expression in a PBMC co-infection model.

Methods: Clinical isolates of GBV-C or mock-control preparations were used to infect PBMCs and CD4-enriched T-cell cultures. Both CCR5 and CXCR4 co-receptor usage HIV strains were used. Cell viability was determined by trypan-blue exclusion and protein synthesis determined by 35S-methionine uptake. HIV replication was determined by measuring p24 antigen in culture supernatants. GBV-C replication was determined by real-time PCR. Mock- or GBV-C infected cellular RNA was prepared, 32P labeled, and hybridized to membranes containing cytokine or chemokine probes and relative quantitation determined by phosphor-imager.

Results: Virus derived from an infectious clone and clinical isolates of GBV-C inhibited both XR4 and R5 strains of HIV. HIV inhibition required GBV-C replication. Infection of cells with GBV-C for 6–24 hrs prior to HIV resulted in increased inhibition compared to simultaneous infection, and inhibition increased further if GBV-C infection proceeded 48 hrs prior to HIV. GBV-C infection resulted in improved cell viability and increased protein synthesis when compared to mock-infected cells. Preliminary experiments found increased IL-2, IL-8, and decreased IL-13 expression in PBMCs, and increased expression of Rantes, MIP-1a, MIP-1b, and SDF-1 by GBV-C infected cells compared with mock-infected controls. Furthermore, the inhibition of an R5 HIV strain was decreased when cells were incubated in neutralizing anti-RANTES, MIP-1a, MIP-1b anti-sera.

Conclusions: GBV-C inhibits R5 and XR4 strains of HIV in vitro, with maximal inhibition occurring 48 hrs post-infection. GBV-C infection of PBMCs appears to increase expression of the CCR5 chemokines RANTES, MIP-1a, MIP-1b, and SDF-1. Survival benefits associated with GBV-C co-infection may reflect a direct inhibitory effect of GBV-C on HIV replication. Additional factors may be involved, including effects on cytokine expression and decreased lymphocyte death.