In Vivo PET Imaging of the Activated Immune Environment in a Small Animal Model of Inflammatory Arthritis

Abstract

Background: Evolving immune-mediated therapeutic strategies for rheumatoid arthritis (RA) may benefit from an improved understanding of the complex role that T-cell activation plays in RA. This study assessed the potential of fluorine-18-labeled 9-β-d-arabinofuranosylguanine ([¹⁸F]F-AraG) positron emission tomography (PET) imaging to report immune activation in vivo in an adjuvant-induced arthritis (AIA) small animal model.

Methods: Using positron emission tomography-computed tomography imaging, uptake of [¹⁸F]F-AraG in the paws of mice affected by arthritis at 6 (acute) and 20 (chronic) days following AIA induction in a single paw was assessed and compared to uptake in contralateral control paws. Fractions of T cells and B cells demonstrating markers of activation at the 2 time points were determined by flow cytometry.

Results: Differential uptake of [¹⁸F]F-AraG was demonstrated on imaging of the affected joint when compared to control at both acute and chronic time points with corresponding changes in markers of T-cell activation observed on flow cytometry.

Conclusion: [¹⁸F]F-AraG may serve as an imaging biomarker of T-cell activation in inflammatory arthritis. Further development of this technique is warranted and could offer a tool to explore the temporal link between activated T cells and RA as well as to monitor immune-mediated therapies for RA in clinical trials.

Keywords: animal models of disease; molecular imaging of inflammation.

Figure 1.

Coronal images of representative subjects at 6 days (top) and 20 days (bottom) following initial model preparation. For both time points: (A) PET image, (B) fused PET-CT image, (C) fused PET-CT image rescaled to demonstrate difference in uptake between paws. A = arthritic paw (adjuvant injection); C = control paw (saline injection). PET indicates positron emission tomography; PET-CT, positron emission tomography–computed tomography.

Figure 2.

Average percentage injected dose per volume in paws on PET images in control and affected animals and ratio of signals (affected/control) measured on days 6 and 20. PET indicates positron emission tomography.

Figure 3.

Summary of cell surface marker expression at 6 and 20 days. A, Percentages of T-cell populations expressing markers of activation including CD25 (only CD4⁺ cells tested), CD44 (only CD4⁺ cells tested at 6 days), and CD69 (both CD4⁺ and CD8⁺ cells populations tested). B, Percentages of T-cell populations expressing CD62 L marker of resting state (only CD4⁺ cells tested at 20 days). C, Percentage of CD4⁺ B-cell populations expressing markers of activation including CD19 and CD80. (*P < .05; **P < .01).

Figure 3.

Summary of cell surface marker expression at 6 and 20 days. A, Percentages of T-cell populations expressing markers of activation including CD25 (only CD4⁺ cells tested), CD44 (only CD4⁺ cells tested at 6 days), and CD69 (both CD4⁺ and CD8⁺ cells populations tested). B, Percentages of T-cell populations expressing CD62 L marker of resting state (only CD4⁺ cells tested at 20 days). C, Percentage of CD4⁺ B-cell populations expressing markers of activation including CD19 and CD80. (*P < .05; **P < .01).

Figure 4.

Ki67 staining at day 6 demonstrates no significant difference in mean fluorescence of CD4⁺ cells in affected versus control paws, but greater nuclear activity in CD8⁺ cells (A), *P ≤ .02. Examples of Ki67 flow cytometry data from single mouse showing fluorescence of CD4⁺ (B) and CD8⁺ (C) cells from affected (red) versus control (blue) paws.

Figure 5.

Hematoxylin–eosin stain (×200) demonstrating inflammatory cell infiltrate in cartilage of affected (A) versus control (B) paws at day 6. B = bone; DS = dorsal surface; I = inflammation; M = muscle; T = tendon.