63 Activated microglia do not upregulate translocator protein Introduction Multiple sclerosis (MS) is a chronic demyelinating inflammatory disease of the central nervous system (CNS). While current anti-inflammatory strategies targeting the adaptive immune response effectively reduce the frequency of relapses, they fail to limit the neurodegeneration driving disease progression and irreversible accumulation of disability. One potential explanation for this is that innate immune responses, particularly microglia activation linked to ageing of the CNS and the immune system, may be a major contributing factor in neurodegeneration and irreversible disease progression1. The 18kDa Translocator Protein (TSPO) is widely used to monitor innate immune responses in the CNS in neuroinflammatory diseases using PET imaging in vivo2-4. While such expression is commonly considered to reflect activated pathogenic microglia, the early assumption that TSPO expression increases in human microglia as they become activated was inferred from animal studies using rodents5-9. However, we recently demonstrated that TSPO expression in primary human macrophages and microglia does not increase with classical proinflammatory or anti-inflammatory activation in vitro8. Similarly, TSPO mRNA was reported to be equal among all different microglia clusters in Alzheimer’s disease10. We subsequently showed that TSPO expression in microglia co-localizes with both classical pro-inflammatory and anti-inflammatory phenotypic markers in brains from people with MS11. These studies also revealed expression of TSPO in a substantial number of brain glial cells which were negative for HLA-DR, a classical marker for activated microglia. In inactive MS lesions and in the centre of chronic active lesions, these TSPO+ cells were mainly astrocytes. However, in other regions, the identity of these TSPO+, HLA-DR negative cells was not determined. Here, we substantially extended the characterisation of TSPO+ cells in the MS brain post mortem using a range of microglia and macrophage cell markers (IBA-1, CD68, HLA-DR) to test the hypothesis that the TSPO+, HLA-DR negative cells were primarily HLA-DR negative microglia. We also investigated quantitatively the relationship between TSPO expression and microglia/macrophage phenotypes across theMS brain. We show that previously unidentified TSPO+ cells are microglia and macrophages and that in control tissue and, in brains of people with MS, in normal appearing white matter (NAWM), active lesions and the rim of chronic active lesions nearly all TSPO+ cells (approximately 95%) are microglia and macrophages. As expected, the phenotype of these cells differs across regions. In control brain tissue and NAWM in MS, microglia and macrophages predominantly express IBA-1 and CD68, while in active MS lesions HLA-DR expression appears in addition. As previously documented, P2ry12 and TMEM119 are expressed in control tissue and NAWM microglia, but expression is lost or reduced in active lesions and in the rims of chronic active lesions. Nevertheless, despite the phenotypic change in microglial cells in control and NAWM relative to active and rim of chronic active lesions, the TSPO pixel count per cell is similar across regions. We conclude that the increased TSPO expression in the MS brain can be attributed to an increase in microglia and macrophage cell density predominantly and not due to activation of these cells. This is unlike the rodent brain, where both factors can contribute substantially in inflammatory pathology.
RkJQdWJsaXNoZXIy MTk4NDMw