Background

Multiple Sclerosis (MS) is an immune-mediated disease of the central nervous system (CNS) characterized by cortical demyelinating lesions containing activated microglia and phagocytic macrophages. The complex dynamics between microglia and oligodendrocytes during demyelination remain to be established. Bruton’s Tyrosine Kinase (BTK) is a key regulator of microglial phagocytosis; however, it is unclear whether modulation of BTK directly affects immune-mediated demyelination.

Objectives

To visualize and manipulate in vivo microglia-oligodendrocyte interactions during cortical myelin loss and repair.

Methods

We developed a novel in vivo model of immune-mediated cortical demyelination through the application of recombinant antibodies derived from MS patients and human complement (MSrAb+huC’) onto the cortical surface. We characterized cellular interactions in real-time via longitudinal in vivo two-photon microscopy of myelinating oligodendrocytes and microglia in transgenic mice.

Results

We found that MSrAb+huC’ application resulted in robust demyelination that recapitulated MS pathology. Microglia rapidly engulfed myelin sheaths following the application of MSrAb+huC’, and subsequently increased their density, accumulating around heavily affected oligodendrocytes. Oral administration of a brain-penetrant BTK-inhibitor prior to the application of MSrAb+huC’ drastically altered microglia dynamics in the 72 hours post-surgery, notably by diminishing engulfment morphology and density changes. Moreover, BTK-inhibition prevented the loss of oligodendrocytes.

Conclusions

Inhibition of BTK signaling alters microglia-oligodendrocyte interactions and limits complement-dependent antibody-mediated demyelination. These findings provide a novel context to define glial interactions during immune-regulated demyelination and outline a crucial role for microglia in driving myelin loss.

Background

Neuroinflammation in the brain and spinal cord, driven largely by CNS-resident microglia, has been proposed as a significant contributor to disability accumulation in patients living with multiple sclerosis (MS). Bruton’s tyrosine kinase (BTK) is expressed in microglia, as well as in B lymphocytes and monocytes/macrophages found in the periphery. In B cells, this kinase is an essential component of the B-cell receptor signalling pathway regulating proliferation, maturation, antigen presentation, and production of secreted immunoglobulins. We hypothesize that in addition to its role in B cells, BTK regulates microglial deleterious inflammatory signalling; therefore, inhibiting BTK with a brain-penetrant inhibitor may provide therapeutic benefit within the CNS by targeting innate immunity associated with disease progression in MS.

Objectives

To assess the role of BTK signalling in modulating inflammatory processes in microglial cells both in vitro and in vivo.

Methods

Immunohistochemistry, Western blotting, and RNA sequencing monitored BTK or phospho-BTK in primary mouse microglial cells, the rodent model of cuprizone-mediated demyelination, and post-mortem MS brain tissues.

Results

Basal activity of BTK in murine microglial cells in vitro was enhanced by stimulation with immune complexes and silenced with a BTK inhibitor. Transcriptome analysis was used to generate a BTK-dependent transcriptional signature in microglia. In tissue derived from autopsy specimens, immunohistochemistry studies and single-nucleus RNA sequencing demonstrated that BTK was expressed in B cells as well as in microglial cells, with increased levels in MS lesion samples. To further explore the role of BTK in vivo, we identified a BTK-dependent transcriptional profile in brains from cuprizone-treated mice. Oral administration of a brain-penetrant BTK inhibitor downregulated the BTK-dependent gene expression signature in the cuprizone-treated mouse brain. Finally, using post-mortem tissue, we evaluated BTK-dependent activation signatures derived from mouse models in MS samples.

Conclusions

Using the cuprizone-induced toxicity model, we extend our previous findings on the role of BTK in microglia to show that BTK-dependent inflammatory signalling in these cells can be modulated using brain-penetrant BTK inhibitors in vivo, which could abrogate microglia-driven neuroinflammation implicated in disease progression in MS.

STUDY SUPPORT: Sanofi.

Background

Neuroinflammation in the brain and spinal cord, driven largely by CNS-resident microglia, has been proposed as a significant contributor to disability accumulation in patients living with multiple sclerosis (MS). Bruton’s tyrosine kinase (BTK) is expressed in microglia, as well as in B lymphocytes and monocytes/macrophages found in the periphery. In B cells, this kinase is an essential component of the B-cell receptor signalling pathway regulating proliferation, maturation, antigen presentation, and production of secreted immunoglobulins. We hypothesize that in addition to its role in B cells, BTK regulates microglial deleterious inflammatory signalling; therefore, inhibiting BTK with a brain-penetrant inhibitor may provide therapeutic benefit within the CNS by targeting innate immunity associated with disease progression in MS.

Objectives

To assess the role of BTK signalling in modulating inflammatory processes in microglial cells both in vitro and in vivo.

Methods

Immunohistochemistry, Western blotting, and RNA sequencing monitored BTK or phospho-BTK in primary mouse microglial cells, the rodent model of cuprizone-mediated demyelination, and post-mortem MS brain tissues.

Results

Basal activity of BTK in murine microglial cells in vitro was enhanced by stimulation with immune complexes and silenced with a BTK inhibitor. Transcriptome analysis was used to generate a BTK-dependent transcriptional signature in microglia. In tissue derived from autopsy specimens, immunohistochemistry studies and single-nucleus RNA sequencing demonstrated that BTK was expressed in B cells as well as in microglial cells, with increased levels in MS lesion samples. To further explore the role of BTK in vivo, we identified a BTK-dependent transcriptional profile in brains from cuprizone-treated mice. Oral administration of a brain-penetrant BTK inhibitor downregulated the BTK-dependent gene expression signature in the cuprizone-treated mouse brain. Finally, using post-mortem tissue, we evaluated BTK-dependent activation signatures derived from mouse models in MS samples.

Conclusions

Using the cuprizone-induced toxicity model, we extend our previous findings on the role of BTK in microglia to show that BTK-dependent inflammatory signalling in these cells can be modulated using brain-penetrant BTK inhibitors in vivo, which could abrogate microglia-driven neuroinflammation implicated in disease progression in MS.

STUDY SUPPORT: Sanofi.