Longitudinal 18F-FDG PET imaging in a rat model of autoimmune myocarditis

Abstract

Aims: Although mortality rate is very high, diagnosis of acute myocarditis remains challenging with conventional tests. We aimed to elucidate the potential role of longitudinal 2-Deoxy-2-18F-fluoro-D-glucose (18F-FDG) positron emission tomography (PET) inflammation monitoring in a rat model of experimental autoimmune myocarditis.

Methods and results: Autoimmune myocarditis was induced in Lewis rats by immunizing with porcine cardiac myosin emulsified in complete Freund's adjuvant. Time course of disease was assessed by longitudinal 18F-FDG PET imaging. A correlative analysis between in- and ex vivo18F-FDG signalling and macrophage infiltration using CD68 staining was conducted. Finally, immunohistochemistry analysis of the cell-adhesion markers CD34 and CD44 was performed at different disease stages determined by longitudinal 18F-FDG PET imaging. After immunization, myocarditis rats revealed a temporal increase in 18F-FDG uptake (peaked at week 3), which was followed by a rapid decline thereafter. Localization of CD68 positive cells was well correlated with in vivo18F-FDG PET signalling (R2 = 0.92) as well as with ex vivo18F-FDG autoradiography (R2 = 0.9, P < 0.001, respectively). CD44 positivity was primarily observed at tissue samples obtained at acute phase (i.e. at peak 18F-FDG uptake), while CD34-positive staining areas were predominantly identified in samples harvested at both sub-acute and chronic phases (i.e. at 18F-FDG decrease).

Conclusion: 18F-FDG PET imaging can provide non-invasive serial monitoring of cardiac inflammation in a rat model of acute myocarditis.

Keywords: 18F-FDG; PET; inflammation; myocarditis; personalized treatment.

Figure 1

Longitudinal ¹⁸F-FDG PET imaging. (A) Total ¹⁸F-FDG uptake by PET given at week 1, 2, 3, 3.5, and 4. Uptake peaked at week 3, whereas a decrease could already be visualized at week 3.5. A good discrimination compared with controls could be observed. (B) In vivo PET imaging 3 weeks after immunization (acute phase). Myocardial reference PET (¹⁸F-FDG under insulin stimulation, left), ¹⁸F-FDG PET (middle) and 3D volume rendering view (right). Clear focal ¹⁸F-FDG uptake signal in the heart can be observed (white dotted circles). (C) Serial ¹⁸F-FDG imaging 2, 3, and 4 weeks after immunization with representative short-axis and horizontal long-axis PET images of a myocarditis rat. Inflammation indicated by ¹⁸F-FDG starts at the right ventricle (week 2), whereas at week 3, the global heart is affected (red dotted circles). Grayscale images served as a myocardial reference. EAM, experimental autoimmune myocarditis.

Figure 2

Upper row: HE-, CD68-staining, and ¹⁸F-FDG autoradiography images of a myocarditis rat. Lower row: In vivo correlation of ¹⁸F-FDG (%ID/g) by PET as well as ex vivo¹⁸F-FDG uptake by autoradiography (mm²) with CD68 positive stained myocardial areas (mm²). Guided by ¹⁸F-FDG in vivo PET imaging, rats were selected at peak ¹⁸F-FDG uptake (3 weeks post-immunization, acute phase) and at time-point of ¹⁸F-FDG decrease thereafter (5 weeks post-immunization, subacute phase). A good correlation for in vivo¹⁸F-FDG (R² = 0.92) as well as for autoradiography findings (R² = 0.9) with CD68 positive areas was detected (P < 0.001, respectively). White dots indicate animals, in which myocardial tissue had been harvested at week 3 (acute phase) and black dots indicate animals, in which cardiac tissue had been harvested at week 5 (subacute phase). Wk, week.

Figure 3

Exchange of adhesion molecules CD34 and CD44, guided by longitudinal in vivo PET imaging. (A) At acute phase (3 weeks after immunization), a peak ¹⁸F-FDG uptake was recorded, with corresponding CD44 positive stained myocardial areas. (B) At subacute phase (5 weeks post-immunization) and (C) chronic phase (10 weeks post-immunization), PET revealed a decline of cardiac tracer uptake: An increase in CD34 positivity was noted, whereas a further decrease of CD44 positively stained cells could be identified. (D) Quantitative analysis of adhesion markers at different phases revealed a low CD34 and—conversely—an increased CD44 positivity (in %) at acute phase. At both subacute and chronic phases, opposite findings with CD34 positively stained areas and a further decrease in CD44 positivity were recorded.