Expression plasmids are only useful for the investigation of co-receptor tropism and fusion capacity of short HIV-1 envelope domains.

Expression vectors have been used widely to identify functionally important domains in HIV-1 glycoproteins. Env domains such as the V3 loop were amplified by polymerase chain reaction (PCR) and inserted into plasmids carrying the backbone of an HIV-1 reference strain like NL4-3. The hypothesis of the present approach was that cloning large domains of wild type envelopes yields constructs that are non-functional in co-receptor-expressing HeLaCD4 cells, in contrast to laboratory-adapted HIV-1 strains. The background for this assumption was that primary HIV-1 virions are frequently less infectious and lack fusion capacity in HeLaCD4 cells compared to laboratory-adapted (LA) viruses. To address this hypothesis, env domains of different length were amplified from a panel of X4-tropic HIV-1 clinical isolates cultured in peripheral blood lymphocytes (PBLs) and cloned into the backbone of NL4-3 env. Constructs bearing either the V3 loops or 312 nucleotides of the intracellular trunk (ICT) of gp41 led to a similar fusion capacity as NL4-3. In contrast, none of the plasmids carrying the 2322 N-terminal nucleotides of primary isolates led to similar syncytium formation. These results have an effect on studies that investigate pathogenic effects of Env regions with chimeric constructs in the backbone of HIV reference envelopes.