Multiple sclerosis (MS) is a prevalent chronic inflammatory and neurodegenerative disease of the central nervous system, marked by myelin loss, oligodendrocyte and axonal damage, and progressive neurological decline (Papiri et al., 2023). Over 2.9 million people worldwide currently live with MS, which causes symptoms ranging from visual disturbances and muscle weakness to cognitive deficits and fatigue, significantly impacting quality of life. The disease predominantly affects young adults, with approximately 10 % showing onset before age 18 and a higher incidence in females (Khan and Hashim, 2025; Multiple Sclerosis International Federation 2023; Safiri et al., 2024; Sen et al., 2021). Although MS etiology remains unclear, it involves genetic susceptibility, environmental triggers, immune and proteolytic dysregulation (Fernandes de Souza et al., 2023). Proteolytic enzymes, especially serine proteases, play crucial roles in immune modulation, blood–brain barrier (BBB) integrity, and extracellular matrix (ECM) remodeling (Patel, 2017). Dysregulation of proteases systems, including metalloproteinases (MMPs) and cysteine cathepsins, contributes to neuroinflammation and neurodegeneration characteristic of MS, highlighting their potential as both biomarkers and therapeutic targets (Halenova et al., 2024; Sandi et al., 2022; Wilson and Abdelhak, 2025). In particular, increased activity of matrix metalloproteinase-9 (MMP-9) is a recognized factor mediating proteolytic disruption of the BBB during active disease phases (Behl et al., 2021). Proteomic studies confirm that widespread proteolytic dysregulation and ECM remodeling represent key mechanisms in MS pathogenesis (Kollet et al., 2024).

The COVID-19 pandemic, caused by SARS-CoV-2, poses new challenges for MS patients. SARS-CoV-2 infection can exacerbate immune responses, disrupt protease homeostasis, and trigger neurological symptoms common in both MS and post-COVID-19 (“long-COVID-19″) conditions (Al-Beltagi et al., 2022; Behl et al., 2021; MacDougall et al., 2022). Recent plasma proteomics reveal altered protein profiles linked to inflammation and tissue remodeling after COVID-19 (Halenova et al., 2024; Hanson et al., 2023), but the mechanisms in MS remain unclear.

This study aimed to compare plasma proteolytic activity and proteome changes in MS patients with and without a history of COVID-19, with particular focus on fibrinogenolytic and gelatinolytic activities as functional indicators of distinct protease systems–serine protease-mediated fibrinolysis and MMP-dependent ECM degradation, respectively–both essential to neuroinflammatory and vascular processes in MS. In parallel, plasma levels of middle-mass molecules (MMM) were evaluated as integrative markers of metabolic imbalance and proteolytic by-products, reflecting systemic alterations characteristic of MS and post-COVID-19 conditions.