FIG. 2.

Natural endogenous reverse transcription (NERT) stimulation of HIV-1. Virions increase infectivity on human oral keratinocytes. Primary human keratinocytes were grown to confluence, and then 100 ng of p24 antigen equivalents of the CXCR4-tropic HIV-1 strain, NL4-3 (A), and the CCR5-tropic HIV-1 strains YU-2 (B) and ADA (C) were treated with either medium or NERT-stimulating agents (the polyamines spermine [3 mM] and spermidine [0.1 mM] and/or dNTPs [100 nM], which are the physiological concentrations in human semen), prior to infection of 1 × 10⁶ primary human oral keratinocytes. Certain viral inputs were treated with dNTPs alone and others with polyamines alone, while still others were treated with both polyamine and dNTPs. After infection for 4 h, the cells were vigorously washed three times with medium and viral growth was monitored for 1 week. Viral outgrowth using ELISA for HIV-1 p24 antigen expression in the culture supernatants was analyzed. These data are representative of three independent experiments performed in triplicate. P.I., postinfection.