Immunological and pathological characterization of fatal rebound MS activity following natalizumab withdrawal

Background: Severe rebound multiple sclerosis (MS) activity is a life-threatening complication of natalizumab (NTZ) withdrawal, for which pathogenesis and treatment are still unclear. We report the immunological and pathological characterization of a case of central nervous system (CNS) inflammatory demyelination after NTZ discontinuation. Objective: To understand the pathophysiology of this neuroinflammatory condition. Methods: Antemortem blood and cerebrospinal fluid (CSF) analysis was compared with postmortem pathological studies, as well as with novel flow cytometry characterization of immune cells isolated from the CNS parenchyma. Results: Pathological analysis of the brain revealed the presence of innumerable active inflammatory demyelinating lesions typical of immunopathological pattern II. Monocytes/macrophages and B cells were enriched in the CNS parenchyma compared to the CSF. Numerous plasma cells were present in the lesions, but CD8 T lymphocytes were predominant in the parenchyma, as opposed to CD4 in the CSF. CNS-infiltrating lymphocytes expressed high levels of adhesion molecules, granzyme B (GzB), interferon-gamma (IFN-.), and interleukin (IL)-17. Conclusions: Our results underline the differences in immune cell populations between the CSF and the CNS parenchyma, and suggest that aggressive immunosuppressive therapy targeting both T and B lymphocytes is warranted to control the overwhelming CNS inflammation.