Nehal Narayan1, David R Owen2, Harpreet Mandhair3, Erica Smyth4, Francesco Carlucci3, Azeem Saleem4, Roger N Gunn4, Eugenii A Rabiner4,5, Lisa Wells4, Stephanie G Dakin3, Afsie Sabokbar3, Peter C Taylor3. 1. Nuffield Department of Orthopaedics, Rheumatology, and Musculoskeletal Sciences, University of Oxford, Oxford, United Kingdom nehal.narayan@ndorms.ox.ac.uk. 2. Division of Brain Sciences, Imperial College, London, United Kingdom. 3. Nuffield Department of Orthopaedics, Rheumatology, and Musculoskeletal Sciences, University of Oxford, Oxford, United Kingdom. 4. Imanova Centre for Imaging Sciences, London, United Kingdom; and. 5. Centre for Neuroimaging Sciences, Institute of Psychiatry, Psychology, and Neuroscience, King's College, London, United Kingdom.
Abstract
PET radioligands targeted to translocator protein (TSPO) offer a highly sensitive and specific means of imaging joint inflammation in rheumatoid arthritis (RA). Through high expression of TSPO on activated macrophages, TSPO PET has been widely reported in several studies of RA as a means of imaging synovial macrophages in vivo. However, this premise does not take into account the ubiquitous expression of TSPO. This study aimed to investigate TSPO expression in major cellular constituents of RA pannus-monocytes, macrophages, fibroblastlike synoviocytes (FLS cells), and CD4-positive (CD4+) T lymphocytes (T cells)-to more accurately interpret TSPO PET signal from RA synovium. Methods: Three RA patients and 3 healthy volunteers underwent PET of both knees using the TSPO radioligand 11C-PBR28. Through 3H-PBR28 autoradiography and immunostaining of synovial tissue in 6 RA patients and 6 healthy volunteers, cellular expression of TSPO in synovial tissue was evaluated. TSPO messenger RNA expression and 3H-PBR28 radioligand binding was assessed using in vitro monocytes, macrophages, FLS cells, and CD4+ T cells. Results: 11C-PBR28 PET signal was significantly higher in RA joints than in healthy joints (average SUV, 0.82 ± 0.12 vs. 0.03 ± 0.004; P < 0.01). Further, 3H-PBR28-specific binding in synovial tissue was approximately 10-fold higher in RA patients than in healthy controls. Immunofluorescence revealed TSPO expression on macrophages, FLS cells, and CD4+ T cells. The in vitro study demonstrated the highest TSPO messenger RNA expression and 3H-PBR28-specific binding in activated FLS cells, nonactivated M0 macrophages, and activated M2 reparative macrophages, with the least TSPO expression being in activated and nonactivated CD4+ T cells. Conclusion: To our knowledge, this study was the first evaluation of cellular TSPO expression in synovium, with the highest TSPO expression and PBR28 binding being found on activated synovial FLS cells and M2 macrophages. TSPO-targeted PET may therefore have a unique sensitivity in detecting FLS cells and macrophage-predominant inflammation in RA, with potential utility for assessing treatment response in trials using novel FLS-cell-targeted therapies.
PET radioligands targeted to translocator protein (TSPO) offer a highly sensitive and specific means of imaging joint inflammation in rheumatoid arthritis (RA). Through high expression of TSPO on activated macrophages, TSPO PET has been widely reported in several studies of RA as a means of imaging synovial macrophages in vivo. However, this premise does not take into account the ubiquitous expression of TSPO. This study aimed to investigate TSPO expression in major cellular constituents of RA pannus-monocytes, macrophages, fibroblastlike synoviocytes (FLS cells), and CD4-positive (CD4+) T lymphocytes (T cells)-to more accurately interpret TSPO PET signal from RA synovium. Methods: Three RApatients and 3 healthy volunteers underwent PET of both knees using the TSPO radioligand 11C-PBR28. Through 3H-PBR28 autoradiography and immunostaining of synovial tissue in 6 RApatients and 6 healthy volunteers, cellular expression of TSPO in synovial tissue was evaluated. TSPO messenger RNA expression and 3H-PBR28 radioligand binding was assessed using in vitro monocytes, macrophages, FLS cells, and CD4+ T cells. Results:11C-PBR28 PET signal was significantly higher in RA joints than in healthy joints (average SUV, 0.82 ± 0.12 vs. 0.03 ± 0.004; P < 0.01). Further, 3H-PBR28-specific binding in synovial tissue was approximately 10-fold higher in RApatients than in healthy controls. Immunofluorescence revealed TSPO expression on macrophages, FLS cells, and CD4+ T cells. The in vitro study demonstrated the highest TSPO messenger RNA expression and 3H-PBR28-specific binding in activated FLS cells, nonactivated M0 macrophages, and activated M2 reparative macrophages, with the least TSPO expression being in activated and nonactivated CD4+ T cells. Conclusion: To our knowledge, this study was the first evaluation of cellular TSPO expression in synovium, with the highest TSPO expression and PBR28 binding being found on activated synovial FLS cells and M2 macrophages. TSPO-targeted PET may therefore have a unique sensitivity in detecting FLS cells and macrophage-predominant inflammation in RA, with potential utility for assessing treatment response in trials using novel FLS-cell-targeted therapies.
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