Bruton Tyrosine Kinase in Lesions of Multiple Sclerosis and 3 of Its Models

The pathophysiology of multiple sclerosis (MS) is contributed by B lymphocytes, macrophages, and microglia. Bruton tyrosine kinase (BTK) is an intracellular enzyme within these cells that modulates their inflammatory properties. Thus, central nervous system-penetrant inhibitors of BTK may counter immune dysregulation, and this aspiration is highlighted by 11 phase 3 clinical trials in MS to inhibit this enzyme. Despite the keen interest, the spatial and temporal elevation of BTK in lesions of MS and its models is not well characterized.

We used quantitative fluorescence immunohistology to assess the expression of BTK and a phosphorylated activated form in different lesion types of MS and 3 of its models: inflammatory experimental autoimmune encephalomyelitis (EAE), toxin-induced demyelination of lysolecithin, and oxidized phosphatidylcholine injuries. GDC-0853 (fenebrutinib), a BTK inhibitor in phase 3 clinical trials in MS, was evaluated in EAE for its capacity to alter disease course.

We observed low expression of BTK and a phosphorylated form (pBTK) in murine spinal cord but significant upregulation in white matter lesions inflicted by oxidized phosphatidylcholine, lysolecithin, and EAE. Expression predominantly localized to microglia/macrophages shown through colocalization analysis by Imaris 3-dimensional rendering. GDC-0853 (fenebrutinib) significantly reduced clinical severity of EAE when administered prophylactically and marginally ameliorated disability when initiated from onset of clinical disability. Finally, we report the increase in BTK expression in microglia/macrophages in active plaques and in the hypercellular rim of chronic active lesions of MS. In the inactive core of chronic active MS lesions, the few remaining HLA-DR⁺ myeloid cells were still BTK immunoreactive.

Our results demonstrate that BTK immunoreactivity is normally undetectable in uninjured areas or normal-appearing white matter of human and murine CNS, but that expression becomes prominent in lesions with hypercellular aggregates of microglia and macrophages. Staining for pBTK reveals that its upregulation declines in the later stage of lysolecithin and chronic stage of EAE injury while BTK upregulation is maintained. Our collective results support the rationale of using brain-penetrant BTK inhibitors to modulate the elevation of this enzyme in microglia/macrophages within inflamed plaques of MS.