Activated microglia do not increase 18 kDa translocator protein (TSPO) expression in the multiple sclerosis brain

Abstract

To monitor innate immune responses in the CNS, the 18 kDa Translocator protein (TSPO) is a frequently used target for PET imaging. The frequent assumption that increased TSPO expression in the human CNS reflects pro‐inflammatory activation of microglia has been extrapolated from rodent studies. However, TSPO expression does not increase in activated human microglia in vitro. Studies of multiple sclerosis (MS) lesions reveal that TSPO is not restricted to pro‐inflammatory microglia/macrophages, but also present in homeostatic or reparative microglia. Here, we investigated quantitative relationships between TSPO expression and microglia/macrophage phenotypes in white matter and lesions of brains with MS pathology. In white matter from brains with no disease pathology, normal appearing white matter (NAWM), active MS lesions and chronic active lesion rims, over 95% of TSPO+ cells are microglia/macrophages. Homeostatic microglial markers in NAWM and control tissue are lost/reduced in active lesions and chronic active lesion rims, reflecting cell activation. Nevertheless, pixel analysis of TSPO+ cells (n = 12,225) revealed that TSPO expression per cell is no higher in active lesions and chronic active lesion rims (where myeloid cells are activated) relative to NAWM and control. This data suggests that whilst almost all the TSPO signal in active lesions, chronic active lesion rims, NAWM and control is associated with microglia/macrophages, their TSPO expression predominantly reflects cell density and not activation phenotype. This finding has implications for the interpretation of TSPO PET signal in MS and other CNS diseases, and further demonstrates the limitation of extrapolating TSPO biology from rodents to humans.

The semantics of microglia activation: neuroinflammation, homeostasis, and stress

“…Similarly, although TSPO has been used as a proxy for microglia activation, the functional consequences of increased TSPO are not entirely understood [ 209 ]. In fact, PET imaging studies conducted in patients with MS show no difference in TSPO expression between activated macrophages in white matter lesions and those in normal appearing white matter or in healthy controls [ 210 ]. Other studies have indicated additional confounds to consider in TSPO studies, namely that TSPO tracers are bound by vascular endothelia and TSPO levels show maximal intensity after the resolution of inflammation [ 211 , 212 ].…”

Analyzing microglial phenotypes across neuropathologies: a practical guide

“…For instance, it has been shown that TSPO is also expressed by other cell types such as astrocytes or glial progenitor cells and thus cannot be seen as a microglia-specific radiotracer target. Moreover, a recent study could show that the findings from animal models do not always translate to the clinical context in humans [47]. Despite the fact that existing radiotracers need further improvement, the approach holds great promise for patients with (suspected) CNS pathologies.…”

Adventures in Translocation: Studies of the Translocator Protein (TSPO) 18 kDa

“…For example, recent publications have challenged longstanding assumptions by proposing that an increase in TSPO PET signal under proinflammatory conditions correlates to increased microglial density rather than the upregulated expression of the protein in humans. [60,61] Moreover, the longstanding assertion that increased TSPO expression is correlated only with microglial activity has been repeatedly challenged, with several studies also detecting its presence in astrocytes and endothelial cells. [62] Furthermore, a key gap in our knowledge is the lack of an atomic structure of human TSPO, with many researchers relying on structures of mouse [63] or bacterial [64] TSPO to construct homology models to guide their work.…”