Background: Antiviral agents that target HIV co-receptors have selected for co-receptor switch mutants in some studies but not others. The conflicting studies have used different HIV-1 isolates, co-receptor inhibitors that differ in mode of action, and distinct methods of HIV propagation. Our goal was to compare the frequency of co-receptor switching of different HIV-1 isolates in a standard selection system.

Methods: A "bait and switch" culture system has been devised in which HIV-1 isolates were first propagated on target cell lines (U87 or MT-2) expressing only the preferred co-receptor, followed by graded replacement with cells expressing the alternative co-receptor. After 12–16 days of culture, progeny viruses were assayed for co-receptor switching by replication assays on cells lacking CCR5 or CXCR4. Envelope sequences of co-receptor switch variants were determined.

Results: Both R5 to X4 and X4 to R5 phenotypic switch variants were isolated. The time to switching depended on the starting HIV-1 isolate. Mutations in V3 alone or V3 and V2 were associated with co-receptor switching. From 1 to 4 mutations were associated with co-receptor switching, and some unstable intermediates were identified.

Conclusions: HIV-1 co-receptor switching can occur in the absence of co-receptor inhibitors. Some virus isolates appear to undergo co-receptor switching quite easily, whereas switching is more difficult to obtain with other isolates. The envelope sequence changes involved in co-receptor switching conform to prior predictions, but the correlation between the starting envelope sequence and the propensity for co-receptor switching is not yet clear.