Background:  ART results in 2- to 3-phased decay of HIV-1 plasma viremia. The first phase may reflect decay of productively infected T cells, whereas later phases likely mirror decline of long-lived cells with lower production rates or of reactivated latently infected cells. To verify this model, HIV⁺ peripheral blood mononuclear cells (PBMC) of different viral transcriptional activity were monitored longitudinally during ART at the single cell level.

Methods:  Limiting dilution analysis of PBMC from 5 HIV⁺ patients was combined with patient-matched polymerase chain reaction (PCR) quantification of HIV RNA. Frequencies and HIV RNA content of cells with distinct HIV RNA expression were measured during 1-year ART (baseline and at weeks 2, 4 8, 12, 24, and 48).

Results:  HIV⁺ PBMC with 4 distinct transcriptional signatures were identified, L, M, H, and H_ex+. Cell classes H and H_ex+ were presumably overlapping as implied by tight correlation of their frequencies (Spearman R = 0.76) and showed high transcriptional activity, suggestive of a virologically productive state. Upon initiation of ART, they declined rapidly to undetectable levels. Cells of L- and M-type expressed HIV RNA at levels presumably insufficient for virus production, and persisted on ART, however after substantial initial declines_. All 4 cell categories were in dynamic equilibrium as suggested by significant correlations between their frequencies.

Conclusions:  In accord with current mathematical models based on decay of plasma viremia, PBMC transcribing HIV provirus at elevated rates were rare and decayed swiftly to mostly undetectable levels. Conversely, 2 classes of HIV⁺ cells with basal HIV transcription showed rapid initial decays followed by persistence. Therefore, these cell classes might comprise subpopulations with distinct half-lives, eg, cells in a state of pre- or post-integration latency. However, comparison of HIV RNA contents before and on ART revealed no difference within infected cell categories L and M (p >0.08), suggesting homogeneity. Therefore, it remains a possibility that turnover of HIV⁺ PBMC may be shaped by changes in their cellular and humoral environment during ART. In particular, alleviation of virus-mediated immune stimulation may diminish activation rates of latently infected cells toward virus production. Further characterization of the molecular mechanisms underlying the observed rapid declines of cells with basal HIV transcription may help to identify strategies to attack latent HIV reservoirs.