Background:  Peripheral immune repopulation despite virological failure is a phenomenon often observed in children under ART. The aim of this study was to investigate the predictors of virologic failure and the immune repopulation in children with or without virological response to ART.

Methods:  We analyzed 30 HIV-1-infected children, pretreated with reverse transcriptase inhibitors (RTI), who entered therapy with RTI and protease inhibitors (PI). At baseline, HIV-1 DNA and intracellular HIV-1 mRNA were quantified by real-time polymerase chain reaction (RT-PCR). Sequence analyses of RT and PR regions of plasma viral isolates were performed in all children at baseline and 14 to 24 months after ART initiation in children with detectable HIV-1 RNA in plasma. CD4 and CD8 T subsets were investigated at baseline and after 14 to 24 months by flow cytometry evaluating the expression of CD45RA, CD27 molecules, and “maturation/activation” CD38 marker.

Results:  While 17  children experienced virologic failure, 13 showed a virological response ART  (HIV-1 RNA <50 copies/mL plasma within 6 months and thereafter persistent undetectable viremia). At baseline, HIV-1 mRNA/HIV DNA was significantly higher in virological non-response than virological response; 4 virological responders and 8 virological non-responders had viral isolates with RTI-resistance mutations. During ART, 9 of 17 virological non-responders developed  viruses with PI-resistance mutations. All CD4 cell subsets increased significantly in virological response children; among the virological non-response children, only those with PI-resistance mutations had a significant increase in CD4, mainly contributed by naive CD4 cells. CD38CD4  cells increased in both groups, and the increase was correlated with the increase in naive CD4 cells. Notably, CD38CD8 T cells decreased in virological response children, but remained high in virological non-response group.

Conclusions:  Our data suggest that the high mRNA/DNA ratio and RTI-resistance mutations at baseline impair subsequent therapy effectiveness, promoting development of resistance to PI. PI-resistance mutations appeared to reduce viral fitness, allowing CD4 T-cell immuno-repopulation by naive CD4 cells. Increase in CD38CD4 cells in both virological response and virological non-response infants, and its relationship with increase in naive CD4 cells, support the notion that CD38 in CD4 cells is a marker of cell immaturity rather than cell activation. Persistence of viremia in virological non-response impaired the expansion of central memory and effector/memory CD4 cells, and sustained the  persistence of high levels of “activated” CD38CD8  cells.