Complementary PET and MRI of myeloperoxidase activity as a noninvasive surrogate for tissue acrolein in experimental neuroinflammation

Myeloperoxidase (MPO), a pro-inflammatory enzyme, and acrolein, a reactive aldehyde present in the environment and produced in the body, play pivotal roles in experimental autoimmune encephalomyelitis (EAE) and multiple sclerosis (MS). However, the relationship between MPO activity and acrolein levels is not well understood. The purpose of this study was to explore this relationship in different EAE mouse models utilizing molecular MRI (heMAMP) and PET ([¹⁸F]MAPP) imaging that report MPO activity.

Chronic progressive C57BL/6-myelin oligodendrocyte glycoprotein (MOG)-EAE (n = 4-7/stage) and relapsing-remitting SJL-proteolipid protein (PLP)-EAE (n = 3-8/stage) mice were imaged using heMAMP-MRI and [¹⁸F]MAPP-PET across disease stages. 4-aminobenzoic acid hydrazide (ABAH, 40mg/kg IP qd) was used to inhibit MPO in SJL mice (n = 5). MPO protein and activity, and acrolein-adduct levels were assessed ex vivo. P < 0.05 was considered statistically significant.

heMAMP-MRI and [¹⁸F]MAPP-PET showed MPO+ lesions in the brains of SJL-PLP-EAE and spinal cords of C57BL/6-MOG-EAE mice during acute disease and a significant decrease at later stages. While heMAMP-MRI allowed detection of smaller MPO+ lesions (mean lesion volume MRI vs PET; SJL: 0.35 ± 0.037 mm³ vs 4.11 ± 0.56 mm³, p < 0.001; C57BL/6: 0.49±0.069 mm³ vs 8.25 ± 1.37 mm³, p < 0.001), PET detected larger total MPO+ lesion volumes in both models. Ex vivo MPO protein/activity and tissue acrolein-adducts correlated in C57BL/6-MOG-EAE (r = 0.89, p < 0.001 and r = 0.82, p < 0.001, respectively) and acute SJL-PLP-EAE (r = 0.7, p = 0.05 and r = 0.76, p = 0.03, respectively). Serum acrolein-adduct levels showed inconsistent correlation with MPO activity imaging, ex vivo MPO assays, and tissue acrolein-adduct levels, likely due to acrolein-adducts lasting for days in tissues. Similarly, inhibiting MPO after symptom onset did not immediately decrease tissue levels of acrolein-adducts.

These findings highlight the complex interplay between MPO and acrolein, with serum acrolein-adduct levels offering limited insight into CNS tissue pathophysiology. This underscores the importance of molecular imaging to dynamically detect active inflammatory lesions and monitor oxidative stress in vivo.