Abstract
The exact cause of neurocognitive dysfunction in HIV-positive patients despite successful control of the infection in the periphery is not completely understood. One suggested mechanism is a vicious cycle of microglial activation and release of proinflammatory chemokines/cytokines that eventually leads to neuronal loss and dysfunction. However, the exact role of microglial activation in the earliest stages of the infection with high cerebrospinal fluid (CSF) viral loads (VL) is unclear. In this study, we imaged the translocator protein (TSPO), a mitochondrial membrane receptor known to be upregulated in activated microglia and macrophages, in rhesus macaques before and multiple times after inoculation with a neurotropic simian immunodeficiency virus (SIV) strain (SIVsm804E), using 18F-DPA714 positron emission tomography (PET). The whole-brain standardized uptake values of TSPO at equilibrium reflecting total binding (SUVT) and binding potentials (BPND) were calculated and correlated with CSF and serum markers of disease, and a corresponding postmortem immunostaining analysis was also performed. SUVT was found to be inversely correlated with both CSF VL and monocyte chemoattractant protein 1 (MCP-1) levels. In SIV-infected macaques with very high CSF VL at necropsy (>106 copies/ml), we found decreased TSPO binding by PET, and this was supported by immunostaining which showed glial and neuronal apoptosis rather than microglial activation. On the other hand, with only moderately elevated CSF VL (∼104 copies/ml), we found increased TSPO binding as well as focal and diffuse microglial activation on immunostaining. Our results in the SIV-infected macaque model provide insights into the relationship between HIV neuropathology and CSF VL at various stages of the disease.IMPORTANCE Neurological and cognitive problems are a common complication of HIV infection and are prevalent even in treated individuals. Although the molecular processes underlying brain involvement with HIV are not completely understood, inflammation is suspected to play a significant role. Our work presents an in vivo assessment of neuroinflammation in an animal model of HIV, the simian immunodeficiency virus (SIV)-infected rhesus macaque. Using positron emission tomography (PET) imaging, we identified changes in brain inflammation after inoculation with SIV over time. Interestingly, we found decreased binding of the PET ligand in the presence of very high cerebrospinal fluid (CSF) viral loads. These findings were supported by immunostaining which showed marked glial loss instead of inflammation. This study provides insight into glial and neuronal changes associated with very high CSF viral load and could reflect similar changes occurring in HIV-infected patients.
Highlights
The exact cause of neurocognitive dysfunction in HIV-positive patients despite successful control of the infection in the periphery is not completely understood
Neuroinflammation can be imaged in vivo using positron emission tomography (PET) ligands targeted against the translocator protein (TSPO), an outer mitochondrial membrane receptor known to be upregulated under inflammatory conditions [6]
The animals were inoculated with SIVsm804E, a neurovirulent simian immunodeficiency virus (SIV) strain capable of establishing early central nervous system (CNS) infection and causing neuropathology with very high cerebrospinal fluid (CSF) viral loads (VL) in almost 80% of susceptible animals [11]
Summary
The exact cause of neurocognitive dysfunction in HIV-positive patients despite successful control of the infection in the periphery is not completely understood. We used an animal model of HIV, the simian immunodeficiency virus (SIV)-infected monkey, to better characterize brain pathological changes associated with high CSF VL such as those seen in some patients during the early stages of HIV infection [7]. Toward this goal, we performed longitudinal PET imaging of the monkeys before and after SIV inoculation using 18F-DPA714, a commonly used TSPO ligand [8,9,10]. We performed an RNAscope assay to detect the presence of the virus in the brains of infected animals and multiplex immunofluorescence (MIF) staining of microglial, astrocytic, and neuronal cell populations postmortem in both SIV-infected and control animals
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