014 – A quantitative neuropathological assessment of translocator protein expression in multiple sclerosis
Erik Nutma (1) – Jodie A. Stephenson (1, 2) – Rianne P. Gorter (1) – Joy de Bruin (1) – Deirdre M. Boucherie (1) – Cornelius K. Donat (4) – Marjolein Breur (1) – Paul van der Valk (1) – Paul M. Matthews (3, 4) – David R. Owen (4) – Sandra Amor (1, 2)
Department of Pathology, Amsterdam UMC, Location VUmc, The Netherlands (1) – Centre for Neuroscience and Trauma, Blizard Institute, Barts and the London School of Medicine & Dentistry, Queen Mary University of London, United Kingdom (2) – UK Dementia Research Institute, Imperial College London, United Kingdom (3) – Division of Brain Sciences, Department of Medicine, Imperial College London, United Kingdom (4)
The 18kDa translocator protein (TSPO) is increasingly used to study brain and spinal cord inflammation in degenerative diseases of the CNS such as multiple sclerosis (MS). The enhanced TSPO PET signal that arises during disease is widely-considered to reflect activated pathogenic microglia, although quantitative neuropathological data to support this interpretation has not been available. With the increasing interest in the role of chronic microglial activation in MS, characterising TSPO expression is of clear importance for understanding the cellular and pathological processes on which TSPO PET imaging reports.
Here we have studied TSPO expression and specific binding of TSPO radioligands ([3H]PK11195 and [3H]PBR28) in tissue sections from 42 MS cases and 12 age-matched controls. Markers of homeostatic and reactive microglia, astrocytes, and lymphocytes were used to investigate the phenotypes of cells expressing TSPO. There was an approximate 20-fold increase in cells double positive for TSPO and HLA-DR in active lesions and in the rim of chronic active lesion, relative to normal appearing white matter. TSPO was uniformly expressed across myeloid cells irrespective of their intermediate, pro- or anti-inflammatory phenotype. TSPO+ astrocytes were increased up to 7-fold compared to normal appearing white matter across all lesion sub-types and accounted for 25% of the TSPO+ cells in these lesions. Specific binding of the TSPO ligands [3H]PK11195 and [3H]PBR28 was determined in the same lesions. TSPO radioligand binding was increased up to seven times for [3H]PBR28 and up to two times for [3H]PK11195 in active lesions and the centre of chronic active lesions and a strong correlation was found between the radioligand binding signal for both tracers and the number of TSPO+ cells across all of the tissues examined. In summary, in MS, TSPO expression arises from microglia of different phenotypes, rather than being restricted to microglia which express classical pro-inflammatory markers. While the majority of cells expressing TSPO in active lesions or chronic active rims are microglia/macrophages, our findings emphasise the significant contribution of activated astrocytes, as well as smaller contributions from endothelial cells. These observations establish a quantitative framework for interpretation of TSPO in MS and highlight the need for neuropathological characterisation of TSPO expression for the interpretation of TSPO PET in other neurodegenerative disorders.