Brain-homing B cells in multiple sclerosis: association with Bruton's tyrosine kinase and targeting by evobrutinib

Objective: To uncover how BTK activity is regulated in B cells during the MS course and whether it contributes to the development of brain-homing T-bet+ B cells using evobrutinib. Background: A phase II trial (NCT02975349) showed promising results for Bruton’s tyrosine kinase (BTK) inhibitor evobrutinib in the treatment of MS. Recently, we found that CXCR3+ B cells preferentially infiltrate the CSF, meninges and brain tissues from MS patients and are selectively triggered under IFN-γ- and TLR9-inducing, germinal center-like conditions. CXCR3 is the surrogate marker of T-bet, which is enriched in age-associated B cells. Design/Methods: BTK and pBTK levels in transitional, naive mature, class-switched and unswitched memory B cells were compared between CIS, RRMS, SPMS, PPMS and matched healthy control blood (n=30 per group). The impact of evobrutinib on in vitro differentiation and transmigration of B cells was studied under T-bet-, IL-21/CD40L-inducing conditions and using monolayers of human brain endothelial cells. Results: BTK was mainly expressed in unswitched memory B cells, while pBTK levels were highest in both class-switched and unswitched memory B cells. In contrast to BTK, pBTK was significantly higher in memory B cells of RRMS and SPMS versus CIS, PPMS and healthy control groups. In RRMS and SPMS, pBTK was less induced after a-IgM stimulation. These levels correlated with CXCR3 and VLA-4 expression. In vitro experiments demonstrated that pBTK was upregulated after IFN-γ and TLR9 induction and that CXCR3, T-bet and CD21 were downregulated by evobrutinib irrespective of anti-IgM triggering. Evobrutinib inhibited class-switching, the formation of IgG-producing plasmablasts and transmigration of CXCR3+ memory B cells. Conclusions: BTK is more activated in memory B cells of both RRMS and SPMS patients and functionally related to T-bet+ memory B cells. This provides key insights into how evobrutinib intervenes in pathogenic B-cell differentiation and can modulate the clinical course of MS. Disclosure: Liza Rijvers has nothing to disclose. Marie-Jose Melief has nothing to disclose. Jamie Van Langelaar has nothing to disclose. The institution of Rudi Hendriks has received research support from Acerta Pharma. Roland Grenningloh has received personal compensation for serving as an employee of EMD Serono. The institution of Dr. Smolders has received personal compensation in the range of $0-$499 for serving as a Consultant for Biogen. The institution of Dr. Smolders has received personal compensation in the range of $0-$499 for serving as a Consultant for Merck. The institution of Dr. Smolders has received research support from Merck. Marvin Van Luijn has nothing to disclose.