Background: Impairment of the blood-brain barrier (BBB) has been associated with cognitive decline in many CNS diseases, including HIV-associated neurocognitive disorders (HAND). Recent research suggests an important role for the Sonic hedgehog (Shh) signaling pathway in the maintenance of BBB integrity under both physiological and pathological conditions.
Methods: In the present study, we sought to examine the expression of Shh and its downstream effectors in relation to brain pericytes and BBB integrity in HIV-infected humans and rhesus macaques infected with simian immunodeficiency virus (SIV), an animal model of HIV infection and CNS disease. Cortical brain tissues from uninfected (n = 4) and SIV-infected macaques with (SIVE, n = 6) or without encephalitis (SIVnoE, n = 4) were examined using multi-label, semi-quantitative immunofluorescence microscopy of Shh, netrin-1, tight junction protein zona occludens 1 (ZO1), glial fibrillary acidic protein, CD163, platelet-derived growth factor receptor b (PDGFRB), glucose transporter 1, fibrinogen, and SIV Gag p28.
Results: While Shh presence in the brain persisted during HIV/SIV infection, both netrin-1 immunoreactivity and the size of PDGFRB+ pericytes, a cellular source of netrin-1, were increased around non-lesion-associated vessels in encephalitis compared to uninfected brain or brain without encephalitis, but were completely absent in encephalitic lesions. Hypertrophied pericytes were strongly localized in areas of fibrinogen extravasation and showed the presence of intracellular SIVp28 and HIVp24 by immunofluorescence in all SIV and HIV encephalitis cases examined, respectively.
Conclusions: The lack of pericytes and netrin-1 in encephalitic lesions, in line with downregulation of ZO1 on the fenestrated endothelium, suggests that pericyte loss, despite the strong presence of Shh, contributes to HIV/SIV-induced BBB disruption and neuropathogenesis in HAND.
Keywords: AIDS; Blood-brain barrier; HIV encephalitis; Netrin-1; Pericytes.