Barts-MS rose-tinted-odometer: zero stars or one ★ depending on your disposition
I prepared this post not knowing that Prof. Laura Airas has submitted a guest post in response to a request from the Mouse Doctor. Before reading this post please make sure you read her blog post first. It provides an important counterbalance to my contrarian view below. Thank you.
More than 20 years ago, when I was a junior researcher, doing my PhD, I started a research collaboration with Dr Richard Banati, who worked in the Queen Square Brain Bank and the Imperial College PET imaging unit. Richard was investigating activated microglia in multiple sclerosis. The collaboration was very fruitful and led to the very first study of an imaging molecule called PK11195 that could label activated microglia.
In the study (see below), we elegantly showed that people with MS (pwMS) had widespread microglial activation in their brains. Their brains, in fact, lit up like Christmas trees with so-called ‘hot’ or activated microglia. The assumption, which is now dogma, from this and other studies is that these microglia must be bad for pwMS. This has led to many research and drug discovery programmes to find treatments to switch off hot microglia.
Now dial forward 20 years and finally, a follow-up study from Finland that shows that pwMS with a lit Christmas tree in their heads do worse in the longterm, i.e. pwMS with more microglial activation as determined by PK11195 staining had more disease progression that was independent of relapses. The implication is that PK11195 is a good marker of smouldering MS and if we switch off this marker we will improve long-term outcomes.
In an email exchange with colleagues, I challenged this thinking. Is the PK11195 signal, or hot microglial response, the chicken or the egg? The microglial response may not be causal but simply associated with a worse outcome in MS. Just maybe the microglia are responding to what is causing MS and are not the primary drivers of the MS pathology. Therefore if you switch off the microglial response you may not improve MS outcomes but actually make them worse.
I even provided some early odds of this happening. I predicted that a drug that switches off the microglial response had only about a 20% chance of improving MS outcomes. I balanced this by saying that I thought that a microglial inhibitor had about a 60% chance of actually making MS worse. I was then challenged that these odds were simply a guess; like an unskilled poker player. I disagree. Firstly, poker is a game of skill and the most skilled poker players make a relatively decent living from their skills. Secondly, there is a scientific process behind making accurate predictions (see post-script), which I try to apply. Finally, we need to apply science to the microglial prediction at hand.
The Science: In the smouldering or slowly expanding MS lesion the hot microglia are lined up like soldiers fighting an enemy at the edge of the lesion. They remind me of a Greek phalanx.
These microglia are not malignant cells, which makes me think they are simply doing their job. Now what if these microglia are responding to something in the surrounding tissue, for example, a slow viral infection? Switching them off may actually make the slow viral infection worse. In addition, microglia have very important function clearing up debris in the nervous system and maintaining the health of synapses and neurons in general.
More Science: Importantly, defects in the signalling pathway of CSF-1 (colony-stimulating factor 1), which is also known as macrophage colony-stimulating factor (M-CSF), cause progressive dementia and disease of the cerebral white matter called a leukoencephalopathy. CSF-1 is a microglial stimulant. This is a warning that inhibiting microglia indiscriminately is unlikely to be good for the brain and particularly a damaged MS brain. This is why I have given greater odds to a microglial inhibitor making MS worse than making MS better.
I am also aware that there are different types of microglia, different types of microglial responses and hence we may have to be more selective in how we target microglia in MS. Despite this, I think we as an MS community need to take a step back and challenge the current dogma that the microglial response in MS is necessarily bad. If we don’t we may be unpleasantly surprised and disappointed with the outcome of clinical trials targeting hot microglia and smouldering MS.
P.S. If you are interested in reading about the science of prediction I would recommend ‘Superforecasting: The Art and Science of Prediction’ by Dan Gardner and Philip Tetlock; a remarkable book that provides important insights and lessons to avoid unconscious biases and it teaches you a little poker as well 😉
Sucksdorff et al. Brain TSPO-PET predicts later disease progression independent of relapses in multiple sclerosis. Brain, awaa275, https://doi.org/10.1093/brain/awaa275 Published: 02 October 2020.
Overactivation of microglia is associated with most neurodegenerative diseases. In this study we examined whether PET-measurable innate immune cell activation predicts multiple sclerosis disease progression. Activation of microglia/macrophages was measured using the 18-kDa translocator protein (TSPO)-binding radioligand 11C-PK11195 and PET imaging in 69 patients with multiple sclerosis and 18 age- and sex-matched healthy controls. Radioligand binding was evaluated as the distribution volume ratio from dynamic PET images. Conventional MRI and disability measurements using the Expanded Disability Status Scale were performed for patients at baseline and 4.1 ± 1.9 (mean ± standard deviation) years later. Fifty-one (74%) of the patients were free of relapses during the follow-up period. Patients had increased activation of innate immune cells in the normal-appearing white matter and in the thalamus compared to the healthy control group (P = 0.033 and P = 0.003, respectively, Wilcoxon). Forward-type stepwise logistic regression was used to assess the best variables predicting disease progression. Baseline innate immune cell activation in the normal-appearing white matter was a significant predictor of later progression when the entire multiple sclerosis cohort was assessed [odds ratio (OR) = 4.26; P = 0.048]. In the patient subgroup free of relapses there was an association between macrophage/microglia activation in the perilesional normal-appearing white matter and disease progression (OR = 4.57; P = 0.013). None of the conventional MRI parameters measured at baseline associated with later progression. Our results strongly suggest that innate immune cell activation contributes to the diffuse neural damage leading to multiple sclerosis disease progression independent of relapses.
Banati, …., Giovannoni,….et al. The peripheral benzodiazepine binding site in the brain in multiple sclerosis: quantitative in vivo imaging of microglia as a measure of disease activity. Brain 2000 Nov;123 ( Pt 11):2321-37. doi: 10.1093/brain/123.11.2321.
This study identifies by microautoradiography activated microglia/macrophages as the main cell type expressing the peripheral benzodiazepine binding site (PBBS) at sites of active CNS pathology. Quantitative measurements of PBBS expression in vivo obtained by PET and [(11)C](R)-PK11195 are shown to correspond to animal experimental and human post-mortem data on the distribution pattern of activated microglia in inflammatory brain disease. Film autoradiography with [(3)H](R)-PK11195, a specific ligand for the PBBS, showed minimal binding in normal control CNS, whereas maximal binding to mononuclear cells was found in multiple sclerosis plaques. However, there was also significantly increased [(3)H](R)-PK11195 binding on activated microglia outside the histopathologically defined borders of multiple sclerosis plaques and in areas, such as the cerebral central grey matter, that are not normally reported as sites of pathology in multiple sclerosis. A similar pattern of [(3)H](R)-PK11195 binding in areas containing activated microglia was seen in the CNS of animals with experimental allergic encephalomyelitis (EAE). In areas without identifiable focal pathology, immunocytochemical staining combined with high-resolution emulsion autoradiography demonstrated that the cellular source of [(3)H](R)-PK11195 binding is activated microglia, which frequently retains a ramified morphology. Furthermore, in vitro radioligand binding studies confirmed that microglial activation leads to a rise in the number of PBBS and not a change in binding affinity. Quantitative [(11)C](R)-PK11195 PET in multiple sclerosis patients demonstrated increased PBBS expression in areas of focal pathology identified by T(1)- and T(2)-weighted MRI and, importantly, also in normal-appearing anatomical structures, including cerebral central grey matter. The additional binding frequently delineated neuronal projection areas, such as the lateral geniculate bodies in patients with a history of optic neuritis. In summary, [(11)C](R)-PK11195 PET provides a cellular marker of disease activity in vivo in the human brain.