Trem1-Pet Imaging Of Pro-Inflammatory Myeloid Cells Distinguishes Active Disease From Remission In Multiple Sclerosis

199 Objectives: Although multiple disease-modifying immunomodulatory therapies are available for relapsing-remitting multiple sclerosis (RR-MS)1, therapeutic selection and monitoring is hampered by the lack of sensitive central nervous system (CNS) immune biomarkers. Increasing evidence indicates that the presence, extent, and spatiotemporal dynamics of activated myeloid cells (i.e., macrophages, microglia, monocytes, neutrophils, and dendritic cells) have the potential to serve as clinically meaningful biomarkers of active MS2. Hence, there is a critical need for non-invasive molecular imaging strategies to accurately quantify and track myeloid cells and their functional phenotypes in MS patients. We previously identified triggering receptor expressed on myeloid cells 1 (TREM1) as a highly-specific marker of pro-inflammatory peripheral myeloid cells, developed the first TREM1-targeted positron emission tomography (PET) probe and demonstrated the ability of TREM1-PET to track CNS-infiltrating peripheral myeloid cells in the experimental autoimmune encephalomyelitis (EAE) mouse model of MS3,4. Here, we investigate the ability of TREM1-PET to monitor active disease in the clinically relevant relapsing-remitting (RR-EAE) mouse model of MS. Additionally, we explore the potential of TREM1 as a clinical biomarker of active MS lesions via immunostaining of rare brain biopsy tissue.\nMETHODS: Animals: SJL mice were induced with RR-EAE using PLP139-151 emulsified in immune adjuvant. Mice during active EAE disease (exhibiting paresis and/or paralysis) and remission (exhibiting complete recovery following initial EAE symptoms, similar to what occurs in RR-MS patients) were used for this study. Imaging: Anti-TREM1 monoclonal antibody (mAb) was DOTA-conjugated and radiolabeled with 64Cu. PET/CT imaging was performed 20 h post-injection of [64Cu]TREM1-mAb (95-120 μCi, \u003e99% RCP). Following PET, mice were perfused with saline to remove any unbound intravascular tracer and spinal cords were analyzed via high-resolution autoradiography. Biopsy: Brain lesion from a drug and steroid naive tumefactive MS patient was characterized using standard immunostaining including Luxol Fast Blue and Myelin Basic Protein (demyelination), neurofilament and Bielschowsky’s Silver stain (axonal degeneration), H\u0026E (general immune cells), CD3 (T cells) and CD20 (B cells). TREM1 staining was then performed on MS and control (non-MS) tissue.\nRESULTS: TREM1-PET images reveled significantly elevated signal in the spinal cords of RR-EAE mice during active disease compared to naive and EAE mice in remission (Fig. 1Ai). Ex vivo autoradiography images confirmed these findings (Fig. 1Aii). Quantification of PET images revealed markedly increased tracer binding in lumbar and thoracic spinal cords of diseased EAE versus naive mice (p\u003c0.0001, Fig.1Bi-ii). Notably, the increased signal in diseased EAE was significantly reduced in EAE mice experiencing remission (p\u003c0.0001). Increased TREM1-PET signal was also observed in the brains of EAE versus naive mice (p\u003c0.05-p\u003c0.0001, Fig. 1Biii-vi), with a significant decrease in signal in the whole brains and medulla of EAE remission mice (p\u003c0.05). Immunostaining of MS brain biopsy tissue confirmed the presence of MS pathophysiology with significant demyelination, axonal degeneration and immune cell infiltration (Fig.2). Moreover, TREM1+ cells were identified in perivascular regions and TREM1 staining was substantially increased in MS compared to non-MS control tissue.\nCONCLUSION: TREM1-PET successfully discriminated active disease and remission in RR-EAE mice. Furthermore, this is the first report of the presence of TREM1+ cells in human MS brain. Thus, TREM1-PET has high potential for clinical impact on disease and therapeutic monitoring for individual RR-MS patients.