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Limitation of Tat-associated transcriptional processivity in HIV-infected PBMC.

The ability of HIV to match levels of viral mRNA to the activation state of the host cell may play a role in its ability to persist as well as to replicate. This linkage depends on the function of the viral transcriptional regulatory protein, Tat, which increases the efficiency of RNA elongation (transcriptional processivity) in response to cellular activation. To quantify levels of Tat function in vivo, a quantitative competitive RT-PCR assay was developed that reflects levels of TAR leader fragments (nonprocessive transcripts) and viral mRNA (processive transcripts), indicating low or high levels of Tat function, respectively. The abundance of these RNA species was measured in peripheral blood mononuclear cells (PBMC) of 22 HIV-1-positive individuals (CD4(+) T cell counts 63-934/mm3) and in established cell line models of HIV constitutive replication (H9IIIB) and reversible latency (U1 and ACH-2). In PBMC, the level of total viral transcripts ranged over four orders of magnitude; however, nonprocessive transcription predominated: 70% of PBMC samples had a ratio of processive to total transcripts of <0.3 and none of the samples had 100% processivity. The cell line studies revealed that, even in activated H9IIIB cells, nonprocessive transcription dominates and that latently infected cells can have different transcriptional responses to activation. This is the first study that enumerates degrees of transcriptional processivity in the circulating mononuclear cell compartment and the results suggest that limitation of Tat function may be a common phenotype throughout the course of the disease.