Lindsay E. Nyhoff, Emily S. Clark, Bridgette L. Barron, Rachel H. Bonami, Wasif N. Khan, Peggy L. Kendall

Bruton's tyrosine kinase (Btk) is a crucial regulator of B cell signaling and is a therapeutic target for lymphoma and autoimmune disease. BTK-deficient patients suffer from humoral immunodeficiency, as their B cells fail to progress beyond the bone marrow. However, the role of Btk in fully developed, mature peripheral B cells is not well understood. Analysis using BTK inhibitors is complicated by suboptimal inhibition, off-target effects, or failure to eliminate BTK's adaptor function. Therefore aBtkflox/Cre-ERT2mouse model was developed and used to exciseBtkafter B cell populations were established. Mice lackingBtkfrom birth are known to have reduced follicular (FO) compartments, with expanded transitional populations, suggesting a block in development. In adultBtkflox/Cre-ERT2mice,Btkexcision did not reduce FO B cells, which persisted for weeks. Autoimmune-prone B1 cells also survived conditionalBtkexcision, contrasting their near absence in globalBtk-deficient mice. Therefore, Btk supports BCR signaling during selection into the FO and B1 compartments, but is not needed to maintain these cell populations. B1-related natural IgM levels remained normal, contrasting globalBtkdeficiency, but B cell proliferation and T-independent type II immunization responses were blunted. Thus, B cells have nuanced signaling responses that are differentially regulated by Btk for development, survival, and function. These findings raise the possibility that Btk may also be expendable for survival of mature human B cells, therefore requiring prolonged dosing to be effective, and that success of BTK inhibitors may depend in part on off-target effects.