Background:  Efflux mechanisms restrict the brain penetration of xenobiotics at the blood–brain barrier (BBB). Breast cancer resistant protein (BCRP), ABC transporter located at luminal membrane of brain microvascular endothelial cells (BMVEC) may serve as a gatekeeper for the xenobiotics entering brain, including ART.

Methods:  To investigate BCRP expression during HIV-1 brain infection, we assessed BCRP distribution and level by immunohistochemistry and Western blot in autopsy brain tissues obtained from patients with HIV-1 encephalitis (n = 8), HIV-1⁺ patients without evidence of encephalitis (n = 7), and HIV^– age-matched controls (n = 6). Expression of BCRP was studied in human monocyte-derived macrophages (MDM) by Western blot and FACS and BMVEC by Western blot.

Results:  BMVEC demonstrated strong BCRP staining in most of the brains of HIV- controls or HIV⁺ samples without evidence of encephalitis, while HIV encephalitis featured decreased BCRP labeling of in endothelium paralleling macrophage infiltration. Microglia and perivascular macrophages were positive for BCRP in HIV encephalitis; however, few positive macrophages showed BCRP staining in HIV⁺ samples and none in HIV^– controls. Western blot analysis performed in the same human brain tissues demonstrated 4.5-fold increase in CD68 (macrophage marker) and 18-fold increase in BCRP protein levels in HIV encephalitis (p <0.02) as compared to HIV^– controls. HIV⁺ brains showed augmented levels of CD68 (1.8 times) and BCRP (6 times, p <0.03) as compared to controls. To confirm in vivo observations, we established FACS method for BCRP detection on human MDM obtained from 5 seronegative donors. HIV-1 infection led to consistent 2-fold increase of BCRP⁺ MDM (20 to 30% vs to 12 to 15% of uninfected MDM). HIV-1 infection resulted in 10-fold increase of BCRP levels in MDM assessed by Western blot in protein extracts derived from the cells analyzed by FACS. BCRP expression in human BMVEC (obtained from 3 donors) was down-regulated after co-culture in HIV-1 infected MDM or exposure to tumor necrosis factor-α.

Conclusions:  Observations in vitro and in vivo demonstrate up-regulation of BCRP expression in infected macrophages and down-regulation on BMVEC in the setting of HIV encephalitis. Since antiretroviral drugs are substrates for BCRP, these finding have important implications for the treatment of HIV-1 CNS infection.