The more I read, think and assimilate information the more I realise that the real pathology behind MS is not the new acute lesion or relapse, but what is going on behind the scenes in the so-called slowly expanding chronic MS lesion or SEL.
MS is a smouldering disease.
In an analysis of the ocrelizumab-PPMS or ORATORIO trial, it is clear that SELs already existed in the brains of PPMSers when they started the trial and best predicted their clinical course during the trial. In contrast, brain atrophy or brain volume loss and new lesion activity did not predict disability progression. What is nice about this analysis is that it is in a PPMS population with a very low relapse rate, which excludes relapses as a confounder.
I am not that concerned about brain volume loss not predicting outcome in this population, because it is out of sync with clinical outcomes; i.e. brain volume loss today is caused by pathology 2-3 years ago and hence needs to be correlated with clinical outcomes in the past.
What is important in this study is that new MRI activity in the form of new T2 lesions did not predict disability worsening. In other words, focal inflammation is not associated with clinical outcome. In comparison, SELs or smouldering MS predicted clinical outcome. Based on basic medical philosophical principles around the definition of surrogate markers it is clear that new T2 lesions can’t be the disease we call MS, but SELs can.
It is clear to me that MS is a biological disease and not an MRIscopic disease, i.e. what you see on MRI is the tip, of the tip, of the iceberg and that most of MS pathology is hidden from view when using conventional MRI. This is why you still deteriorate despite being NEDA (with no evident new disease activity). The NEDA in this context is referring to the absence of focal MS inflammatory activity, i.e. relapse(s) and new or enlarging lesions on MRI. The biology behind the worsening despite being NEDA is driven by the delayed neuroaxonal loss from previous damage, ongoing diffuse inflammation which has become independent of focal lesions (innate activation), ageing mechanisms or focal inflammatory lesions that are too small to be detected with our monitoring tools. Of all the processes listed here, the last one is the only one that is realistically modifiable by our current DMTs.
The really important question this analysis raises is that when you treat someone with a DMT and they become NEDA how do you know they don’t have ongoing smouldering MS and hence would benefit from being escalated to a more effective DMT or should be included in add-on combination therapy trial? This is why we need to start using end-organ damage markers and more sensitive inflammatory markers to look for and define smouldering MS. Only then will we be able to start answering important questions. For example, does changing treatment in people with smouldering MS to more effective DMTs, for example, natalizumab, alemtuzumab or ocrelizumab result in them doing better? The ORATORIO analysis below would suggest the treatment effect in this situation is small. This is why we are going to need a new generation of add-on treatments that target CNS pathology, for example, hot microglia, antivirals (EBV and HERVs), CNS-penetrant anti-B-cell and plasma cell agents, neuroprotectives, etc.
I have made the point that primary progressive MS (PPMS) is simply smouldering RRMS and that all we are doing with our DMTs is converting people with RRMS to PPMS and delaying the inevitable progressive phase of the disease. I don’t buy this because a proportion of pwMS who have been treated early on with an immune reconstitution therapy or IRT, in particular, alemtuzumab or HSCT, appear to be in very longterm remission and may even be cured of their MS (see the previous post on this topic). Some would argue, I included, that this group of patients has not been followed up for long enough to be sure they have been cured. I agree and this is why we need a deep phenotyping study to assess whether or not these patients have any evidence of ongoing MS disease activity. This study would help define smouldering MS, by looking for its absence.
The MRI-centric view of MS has lulled many of us into a false sense of security and has resulted in us classifying MS as a focal inflammatory autoimmune disease of the CNS. In reality, MS is a diffuse disease of the CNS and the focal inflammatory events are simply the immune response to what causes MS. This is why the field hypothesis of MS is so relevant and fundamentally challenges our worldview of MS.
If we don’t change our worldview of MS and explore what is happening in the trenches alongside the one we currently have our heads buried in we will be letting down the next generation of MSers.
Elliott et al. Chronic white matter lesion activity predicts clinical progression in primary progressive multiple sclerosis. BRAIN 2019: 142; 2787–2799.
Chronic active and slowly expanding lesions with smouldering inflammation are neuropathological correlates of progressive multiple sclerosis pathology. T1 hypointense volume and signal intensity on T1-weighted MRI reflect brain tissue damage that may develop within newly formed acute focal inflammatory lesions or in chronic pre-existing lesions without signs of acute inflammation. Using a recently developed method to identify slowly expanding/evolving lesions in vivo from longitudinal conventional T2- and T1-weighted brain MRI scans, we measured the relative amount of chronic lesion activity as measured by change in T1 volume and intensity within slowly expanding/evolving lesions and non-slowly expanding/evolving lesion areas of baseline pre-existing T2 lesions, and assessed the effect of ocrelizumab on this outcome in patients with primary progressive multiple sclerosis participating in the phase III, randomized, placebo-controlled, double-blind ORATORIO study (n = 732, NCT01194570). We also assessed the predictive value of T1-weighted measures of chronic lesion activity for clinical multiple sclerosis progression as reflected by a composite disability measure including the Expanded Disability Status Scale, Timed 25-Foot Walk and 9-Hole Peg Test. We observed in this clinical trial population that most of total brain non-enhancing T1 hypointense lesion volume accumulation was derived from chronic lesion activity within pre-existing T2 lesions rather than new T2 lesion formation. There was a larger decrease in mean normalized T1 signal intensity and greater relative accumulation of T1 hypointense volume in slowly expanding/evolving lesions compared with non-slowly expanding/evolving lesions. Chronic white matter lesion activity measured by longitudinal T1 hypointense lesion volume accumulation in slowly expanding/ evolving lesions and in non-slowly expanding/evolving lesion areas of pre-existing lesions predicted subsequent composite disability progression with consistent trends on all components of the composite. In contrast, whole brain volume loss and acute lesion activity measured by longitudinal T1 hypointense lesion volume accumulation in new focal T2 lesions did not predict subsequent composite disability progression in this trial at the population level. Ocrelizumab reduced longitudinal measures of chronic lesion activity such as T1 hypointense lesion volume accumulation and mean normalized T1 signal intensity decrease both within regions of pre-existing T2 lesions identified as slowly expanding/evolving and in non-slowly expanding/evolving lesions. Using conventional brain MRI, T1- weighted intensity-based measures of chronic white matter lesion activity predict clinical progression in primary progressive multiple sclerosis and may qualify as a longitudinal in vivo neuroimaging correlate of smouldering demyelination and axonal loss in chronic active lesions due to CNS-resident inflammation and/or secondary neurodegeneration across the multiple sclerosis disease continuum.