TSPO expression in a Zika virus murine infection model as an imaging target for acute infection-induced neuroinflammation

Abstract

Introduction Zika virus (ZIKV) is a neurotropic human pathogen that causes neuroinflammation, whose hallmark is elevated translocator protein (TSPO) expression in the brain. This study investigates ZIKV-associated changes in adult brain TSPO expression, evaluates the effectiveness of TSPO radioligands in detecting TSPO expression, and identifies cells that drive brain TSPO expression in a mouse infection model. Methods The interferon-deficient AG129 mouse infected with ZIKV was used as neuroinflammation model. TSPO expression was evaluated by tissue immunostaining. TSPO radioligands, [3H]PK11195 and [18F]FEPPA, were used for in vitro and ex vivo detection of TSPO in infected brains. [18F]FEPPA-PET was used for in vivo detection of TSPO expression. Cell subsets that contribute to TSPO expression were identified by flow cytometry. Results Brain TSPO expression increased with ZIKV disease severity. This increase was contributed by TSPO-positive microglia and infiltrating monocytes; and by influx of TSPO-expressing immune cells into the brain. [3H]PK11195 and [18F]FEPPA distinguish ZIKV-infected brains from normal controls in vitro and ex vivo. [18F]FEPPA brain uptake by PET imaging correlated with disease severity and neuroinflammation. However, TSPO expression by immune cells contributed to significant blood pool [18F]FEPPA activity which could confound [18F]FEPPA-PET imaging results. Conclusions TSPO is a biologically relevant imaging target for ZIKV neuroinflammation. Brain [18F]FEPPA uptake can be a surrogate marker for ZIKV disease and may be a potential PET imaging marker for ZIKV-induced neuroinflammation. Future TSPO-PET/SPECT studies on viral neuroinflammation and related encephalitis should assess the contribution of immune cells on TSPO expression and employ appropriate image correction methods to subtract blood pool activity.

Translocator protein (TSPO) is a biomarker of Zika virus (ZIKV) infection-associated neuroinflammation

“…The ZIKV model used in this study was established as described [ 25 , 35 ]. Briefly, male AG129 were (8–11 weeks old) infected with 10 6 plaque-forming units (pfu) virus by intraperitoneal (i.p.)…”

“…with a blocking dose of “cold” PK11195 (5 mg/kg) to assess specific binding of the tracer. Organs were harvested, and brains were micro-dissected as previously described [ 25 , 36 ]. Tissues were completely dissolved in Soluene® 350 (PerkinElmer, USA) for 2–7 days and subsequently mixed with Ultima Gold liquid scintillant (PerkinElmer, USA) prior to detection of radioactivity with TriCarb 4810TR liquid scintillation counter (PerkinElmer, USA).…”

“…In separate cohorts of mice, sagittal cryosections of mouse brains (20 µm) harvested from either sham-infected or ZIKV-infected mice were subjected to in vitro radioligand binding assay as previously described [ 25 ]. Tissues were incubated with 10 nM (∼ 0.87 MBq) [ 3 H]PK11195 for 45 min at room temperature (RT).…”

“…Chronic inflammation in neurodegenerative diseases is associated with increased brain TSPO expression in astrocytes, glia, and neurons [ 22–24 ]. In contrast, acute neuroinflammation associated with viral encephalitis is marked by increased brain infiltration of immune cells that express TSPO [ 25 ]. Elevated immune cell TSPO expression has been described as a hallmark of inflammation in a variety of viral infections, including Ebola virus (EboV) [ 26 ], human immunodeficiency virus (HIV) [ 27 , 28 ], and simian immunodeficiency virus (SIV) [ 29 ].…”

“…radioligands). Recently, elevated TSPO expression in ZIKV-infected mouse brains using both interferon-deficient [ 25 ] and fully immunocompetent mice [ 33 , 34 ] has also been non-invasively detected by PET imaging.…”