Background: Highly active antiretroviral treatment (HAART) fails to eradicate all the infected cells from HIV-1 infected individuals. We hypothesized that, in addition to the emergence of resistant HIV strains, therapeutic failure might be the result of suboptimal effects of drugs in infected cells due to high expression of efflux pumps such as the P-glycoprotein pump (P-gp) in a subset of infected cells. Protease inhibitors are substrates of the P-gp, a member of the ABC family of transporters associated with therapeutic failure and multidrugs resistance in several diseases.

Methods: We have used Rh123 and TMRE, 2 fluorescent rhodamine derivatives that are substrates for the P-gp pump, to study by-flow cytometry the P-glycoprotein activity in different subsets of primary lymphocytes from HIV-1 infected individuals (n = 42) and healthy controls (n = 16). P-gp mRNA was measured by QC-PCR. In addition, GFP-tagged HIV-1 molecular clones were used to study P-gp activity in in vitro infected primary cells.

Results: We identified a population of CD4⁺CCR5⁺ primary cells with very high P-glycoprotein efflux activity. These cells are phenotypically heterogeneous and were recognized as T-lymphocytes and natural killer cells. The presence of these cells was found in both healthy HIV-1 seronegative controls and HIV-1 infected individuals. Functional studies demonstrated that higher concentration of protease inhibitors is required to achieve the pharmacological effects of the drugs in these cells. We found no significant difference in P-gp mRNA expression and functional activity between HIV⁺ patients (pts) and uninfected controls. Monitoring GFP-tagged, HIV-infected primary cells demonstrated that very high P-gp activity is present in a set of cells actively replicating HIV-1.

Conclusions: Our results show the presence of pharmacological cellular sanctuaries resistant to the antiviral effects of protease inhibitors. The presence of these cells most likely contributes to HIV-1 persistence in pts receiving HAART. New therapeutic interventions to eliminate these cells are required to successfully treat HIV-1 infected pts.