Brain Immune Cell Composition in Multiple Sclerosis and Progressive Multifocal Leukoencephalopathy After Natalizumab: An Immunohistochemical Cohort Study.

Abstract

Background and objectives: Natalizumab therapy in multiple sclerosis (MS) increases the risk of progressive multifocal leukoencephalopathy (PML) and may alter lymphocyte distribution within the CNS. Tissue-resident memory CD8⁺ T cells (T_RM CD8⁺), which rely on activation through CD4⁺ T cells, are essential for antiviral defense and may contribute to JC virus (JCV) control. We, therefore, investigated lymphoid infiltrates in patients with MS after natalizumab treatment and in patients with natalizumab-associated PML to assess antiviral immunity and clinical outcomes.

Methods: Fluorescence immunohistochemistry was performed on brain biopsies and autopsies from patients with MS (n = 34) and those with PML (n = 27) with and without prior natalizumab exposure and from healthy controls (n = 11). The analysis included CD103+ (T_RM) CD8⁺, total CD8⁺, and CD4⁺ T cells; CD209⁺ dendritic cells; and JCV-infected cells. Cell densities were correlated with clinical outcomes using the modified Rankin Scale and Expanded Disability Status Scale.

Results: T_RM CD8⁺ T-cell numbers remained stable in the normal-appearing white matter of patients with MS previously treated with natalizumab and matched levels in healthy controls. By contrast, CD4⁺ and CD8⁺ T cells were elevated in MS irrespective of natalizumab exposure and increased further beyond 35 days after treatment cessation, suggesting immune restoration that may reduce PML risk, particularly under extended-interval dosing. In PML lesions, T_RM CD8⁺ T cells were abundant, and natalizumab-associated PML demonstrated infiltrates comparable to inflammatory PML lesions (>500 T cells/mm²), indicating strong local immune activation. Approximately 4 months after natalizumab withdrawal, marked viral clearance was observed and higher densities of T_RM CD8⁺ and total CD8⁺ T cells were associated with better clinical outcomes.

Discussion: T_RM CD8⁺ T cells appear central to CNS antiviral defense and remain largely preserved during natalizumab therapy. Although natalizumab restricts T-cell trafficking, the postwithdrawal rise in CD4⁺ and CD8⁺ T cells indicates immune reconstitution that may enhance T_RM CD8⁺ activation. In natalizumab-associated PML, the presence of strong T-cell infiltrates and declining viral loads after treatment cessation underscore the importance of restored local immunity for effective JCV control. Higher densities of T_RM CD8⁺ and total CD8⁺ T cells were linked to better clinical outcomes, highlighting their contribution to viral clearance and recovery.