Evolution of the autologous neutralizing antibody response in the children over the course of infection. The neutralizing activity present in patients plasma was analyzed against their primary virus isolates using a luciferase reporter gene assay in TZM-bl cells; plasma from child 1 from years 1998, 1999, 2000, 2001, 2003, 2006 and 2010 were tested against autologous viruses from 2000 [CT00 (R5 tropism)–pink circles] and 2003 [CT03 (X4 tropism)–green circles] and plasma from child 2 from years 1997, 1998, 1999 and 2001 were tested against autologous virus from 2001 [SC01 (X4 tropism)–blue diamonds]. Nab titers are presented as median (symbols) and interquartile range (shades) for each serum/virus. Open symbols signal the neutralization of the contemporaneous isolate. In child 1, straight lines represent neutralization of the virus present at that time of infection and dotted lines represent neutralization of virus not present at that time of infection. The F test was used to compare IC50 values obtained for CT00 (R5) and CT03 (X4) isolates. ***P = 0.0008, ****P < 0.0001.