Disease Progression in Multiple Sclerosis: From Pathophysiological Mechanisms to Therapeutic Targets

Multiple sclerosis (MS) is a chronic inflammatory and neurodegenerative disease of the central nervous system in which disability accumulation increasingly occurs independently of acute inflammatory activity. Progression independent of relapse activity (PIRA) has emerged as a major contributor to long-term disability, yet its biological substrates, imaging correlates, and responsiveness to disease-modifying therapies (DMTs) remain incompletely understood. This thesis aimed to characterize the structural and microstructural correlates of disease progression in MS, identify biomarkers associated with disability severity and future progression, and evaluate the impact of current therapies on neurodegenerative outcomes. To address these objectives, the thesis integrates and discusses findings from five complementary studies employing advanced MRI techniques, multimodal biomarker analyses and machine learning, real-world observational data, and meta-analytical approaches. Using diffusion tensor imaging (DTI), the first study showed that people with MS (pwMS) experiencing PIRA exhibit diffuse microstructural damage in major white matter tracts compared with matched pwMS without PIRA, even in the absence of subclinical focal inflammatory activity. These findings support the concept that silent progression is associated with diffuse tissue injury extending beyond focal lesions, consistent with mechanisms such as secondary axonal injury and Wallerian degeneration. The second study applied a multiparametric quantitative MRI (qMRI) protocol to the thalamus, revealing extensive macrostructural and microstructural alterations in pwMS, with accelerated degeneration over time and more pronounced changes in progressive phenotypes. Thalamic qMRI abnormalities were more severe close to the cerebrospinal fluid (CSF), associated with disability progression, and diffusely affected normal-appearing tissue, suggesting concurrent mechanisms of lesion- related disconnection and surface-in neurodegeneration. In the third study, a multimodal machine-learning framework integrating a wide array of conventional MRI, advanced qMRI, and serum biomarkers was applied to identify the most informative predictors of neurological disability, cognitive impairment, and future PIRA. Across two independent cohorts, measures of upper cervical spinal cord atrophy and cortical degeneration emerged as key contributors to physical disability and progression risk, while qMRI metrics and serum biomarkers provided complementary information. These findings underscore the central role of cortical and spinal cord biomarkers for progression risk stratification. The fourth study compared teriflunomide and ocrelizumab in a real-world cohort of relapsing- remitting MS, focusing on the incidence of PIRA and MRI biomarkers associated with smoldering disease activity, using propensity score matching. No differences were observed in PIRA incidence, paramagnetic rim lesion burden, or DTI-based microstructural measures, suggesting comparable effects on smoldering disease activity. These findings highlight the need for further studies specifically designed to investigate the effects of current therapies on progression-related and neurodegenerative outcomes. Finally, a network meta-analysis of randomized clinical trials showed that multiple DMTs significantly reduce brain volume loss and disability accumulation compared with placebo, and that a relationship exists between treatment effects on brain atrophy and reduced disability progression, independent of effects on MRI inflammatory activity. These findings support brain volume loss as a clinically relevant therapeutic outcome, indicating that atrophy metrics retain value as therapeutic endpoints even as inflammatory activity becomes increasingly controlled. At the same time, the incomplete overlap between treatment effects on brain volume loss and disability progression suggests that additional mechanisms contribute to disease progression beyond global atrophy. Collectively, this thesis indicates that MS progression is associated with diffuse and regionally heterogeneous patterns of neurodegeneration involving major white matter tracts, deep gray matter, cerebral cortex, and cervical spinal cord, and that these processes can be captured using both advanced and more widely available biomarkers. While current therapies partially mitigate neurodegenerative outcomes, disease progression remains only partially addressed, highlighting the need for improved biomarkers and targeted strategies to better characterize and ultimately address silent disease progression in MS.