¹⁸F-FAC PET Selectively Images Liver-Infiltrating CD4 and CD8 T Cells in a Mouse Model of Autoimmune Hepatitis

Abstract

Immune cell-mediated attack on the liver is a defining feature of autoimmune hepatitis and hepatic allograft rejection. Despite an assortment of diagnostic tools, invasive biopsies remain the only method for identifying immune cells in the liver. We evaluated whether PET imaging with radiotracers that quantify immune activation (F-FDG and F-1-(2'-deoxy-2'-fluoro-arabinofuranosyl)cytosine [F-FAC]) and hepatocyte biology (F-2-deoxy-2-fluoroarabinose [F-DFA]) can visualize and quantify liver-infiltrating immune cells and hepatocyte inflammation, respectively, in a preclinical model of autoimmune hepatitis. Mice treated with concanavalin A (ConA) to induce a model of autoimmune hepatitis or vehicle were imaged withF-FDG, F-FAC, andF-DFA PET. Immunohistochemistry, digital autoradiography, and ex vivo accumulation assays were used to localize areas of altered radiotracer accumulation in the liver. For comparison, mice treated with an adenovirus to induce a viral hepatitis were imaged with F-FDG,F-FAC, and F-DFA PET.F-FAC PET was performed on mice treated with ConA and vehicle or with ConA and dexamethasone. Biopsy samples of patients with autoimmune hepatitis were immunostained for deoxycytidine kinase. Hepatic accumulation ofF-FDG and F-FAC was 173% and 61% higher, respectively, and hepatic accumulation ofF-DFA was 41% lower, in a mouse model of autoimmune hepatitis than in control mice. Increased hepatic F-FDG accumulation was localized to infiltrating leukocytes and inflamed sinusoidal endothelial cells, increased hepaticF-FAC accumulation was concentrated in infiltrating CD4 and CD8 cells, and decreased hepatic F-DFA accumulation was apparent in hepatocytes throughout the liver. In contrast, viral hepatitis increased hepaticF-FDG accumulation by 109% and decreased hepatic F-DFA accumulation by 20% but had no effect on hepaticF-FAC accumulation (nonsignificant 2% decrease). F-FAC PET provided a noninvasive biomarker of the efficacy of dexamethasone for treating the autoimmune hepatitis model. Infiltrating leukocytes in liver biopsy samples from patients with autoimmune hepatitis express high levels of deoxycytidine kinase, a rate-limiting enzyme in the accumulation ofF-FAC. Our data suggest that PET can be used to noninvasively visualize activated leukocytes and inflamed hepatocytes in a mouse model of autoimmune hepatitis.

18F-FAC PET Visualizes Brain-Infiltrating Leukocytes in a Mouse Model of Multiple Sclerosis

“…Despite studies showing that 18 F-FAC selectively accumulates in activated lymphocytes in mouse models of autoimmune hepatitis and antitumor immunity (18)(19)(20), in the EAE mouse model we do not identify selective 18 F-FAC accumulation in brain-infiltrating lymphocytes. In the mouse model of antitumor immunity, the rate of 18 F-FAC accumulation in T cells was proportional to the rate of cellular proliferation (19).…”

“…Sagittal brain sections (4 µm) were immunostained as described (20) except for the inclusion of CD45 (Clone 30-F11; 1:100; Novus Biologicals), imaged at 1X and 40X magnification, and quantified using the Ilastik software (Version 1.3.2). Boxes in the 1x magnification images outline from where the 40x magnification images were taken.…”

“…Brain leukocyte isolation was conducted similar to previously described (27), and CD4 + and CD11b + cells were isolated by FACS. Ex vivo deoxycytidine accumulation was performed as previously described (20) by incubating ~20,000 of each cell type in a 96-well filter plate with 3…”

“…The radiotracer 1-(2′-deoxy-2′- 18 F-fluoroarabinofuranosyl) cytosine ( 18 F-FAC) is a deoxynucleoside analogue that measures deoxynucleoside salvage, a biochemical pathway enriched in leukocytes; accumulates at high levels in lymphoid tissues; and is increased in these same tissues in mouse models of immune activation (18)(19)(20). Additionally, 18 F-FAC accumulates at higher levels in activated T lymphocytes than in effector-memory T cells, CD11b + innate immune cells, or B220 + B cells and at low levels in the healthy brain (18)(19)(20). Thus we hypothesized that 18 F-FAC PET could non-invasively visualize brain-infiltrating leukocytes.…”

Antibody‐Based Tracers for PET/SPECT Imaging of Chronic Inflammatory Diseases

“…[73] Imaging of Tlymphocytes with radiolabeled probesh as been performed by using various methods. For example,d irect in vivo labeling of Tlymphocytes or cytokines (e.g., 99m Tc-HMPAO [74] or 111 In-oxine-labeled Tlymphocytes, [75] 99m Tc -labeled IL-2 [76] )b yu sing nucleoside metabolism (e.g., 18 F-AraG, [77] 18 F-FAC [78] )o rt argeting transporters (e.g., 18 F-FSPG [79] ), and targeting surfacem olecules on the immune cells can be effective strategies for molecular imagingo fT lymphocytes. Detailed information about each of these approaches can be found in other reviews.…”