Determinants Of Hiv-1 Transmission Fitness

DETERMINANTS OF HIV-1 TRANSMISSION FITNESS Shilpa S. Iyer Beatrice H. Hahn HIV-1 is predominantly transmitted by mucosal routes and almost 80 percent of new infections are initiated by a single variant. The elucidation of the biological properties of transmitted viruses which distinguish them from non-transmitted variants are critical for the development of therapeutic interventions. To identify such properties, we characterized the biology of 300 limiting dilution-derived virus isolates from the plasma and genital secretions of eight HIV-1 donor and recipient transmission pairs representing the most prevalent subtypes (B and C). Recipient viruses were more infectious per viral particle as determined on a reporter cell line, replicated to higher titers and were released more efficiently from infected primary CD4+ T cells than the corresponding donor isolates. Recipient viruses were more resistant to the inhibitory effects of IFN-α2 and IFN-β evidenced as higher half-maximal inhibitory concentrations and higher replication at the maximal doses of IFN-α2 and IFN-β than corresponding donor isolates. Interestingly, pretreatment of CD4+ T cells with IFN-β, but not IFN-α2 selected donor plasma isolates that exhibited phenotypes similar to transmitted viruses. This suggests that transmitted variants are distinct and that the selective pressure imposed by type I interferons may in part be responsible for the bottleneck associated with mucosal transmission. We next wanted to assess the role of the interferon stimulated gene, tetherin in the antiviral state established by type I IFNs. Thus, we introduced mutations into the vpu gene of various HIV-1 constructs to specifically disrupt their Vpu-mediated tetherin antagonism, and determined the effect on replication and release from infected cells in the presence and absence of IFN-α2. Mutations at key residues in Vpu reduced the viral particle production and release from infected primary CD4+ T cells and this was particularly evident in IFN-α2-treated cells. Interestingly, transmitted HIV-1 variants were released to higher levels from infected cells than chronic control viruses, even in the absence of Vpu. Thus, the counteraction of tetherin resulting in efficient particle release is an important determinant of the interferon resistance of mucosally transmitted HIV-1.