Background:  Most epidemiological studies report that GB virus C (GBV-C) co-infections are associated with an increased survival of HIV-1 patients. However, not all epidemiological studies could confirm the beneficial effect. Here, we analyze the HIV-1 inhibitory capacity of clinical GBV-C isolates and investigate the molecular basis of GBV-C mediated HIV-1 inhibition.

Methods:  Serum of 1535 healthy individuals and 324 HIV-1 patients were screened for GBV-C infection marker. Sera containing GBV-C isolates with similar high replication efficiencies in primary lymphoid cells and titers of at least 10⁶ GE/mL were tested in GBV-C/HIV-1 co-infection experiments using clinical CCR5- (92TH026, 92BR025, CA-9) and CXCR4-tropic HIV-1 (92UG024, 93BR020) isolates as well as HIV-1-derived pseudoparticles. Expression of CD4, CCR5, and CXCR4 as well as secretion of RANTES, MIP-1, MIP-1ß, and SDF was quantified at different time points post GBV-C infection. For GBV-C genotyping and sequence analysis the E1-p5.6 as well as the NS5A region was analyzed.

Results:  GBV-C RNA was detected in 42 HIV-1^– and 112 HIV-1⁺ donors. GBV-C/HIV-1 co-infection experiments using GBV-C RNA positive, but anti-GBV-C-E2 antibody negative sera revealed that clinical GBV-C isolates vary in their ability to inhibit HIV-1 and could be classified as HIV-1 inhibitory and non-inhibitory GBV-C isolates. HIV-1 suppression could be explained in part by the induction of RANTES, MIP-1, and MIP-1ß and down-regulation of the CC receptor 5. Neither GBV-C replication capacity nor the genotype of the GBV-C isolates correlates with the ability to interfere with HIV-1. However, sequence analysis revealed that 2 amino acid substitutions were found exclusively within the E2 protein of GBV-C isolates that did not interfere with HIV-1. Indeed, in contrast to recombinant E2 proteins derived from HIV-1 inhibitory GBV-C isolates, E2 proteins cloned from non-inhibitory GBV-C isolates did not impair HIV-1 replication.

Conclusions:  We could demonstrate that clinical GBV-C isolates differ in their ability to suppress HIV-1 replication in cell culture. Furthermore, we corroborate the impact of the GBV-C E2 protein on HIV-1 entry. Thereby, we identified 2 amino acids within the E2 protein that may be causative for the different HIV-1 inhibitory phenotypes of clinical GBV-C isolates. Taken together, our findings may lead to the development of innovative therapeutic strategies against HIV-1.