Animals:
All animal studies were conducted with ethical approval from the Dunn School of Pathology Local Ethical Review Committee and in accordance with the UK Home Office regulations (Guidance on the Operation of Animals, Scientific Procedures Act, 1986). Male (8–12 weeks) C57BL/6J, CBA/CaCrl mice were obtained from Charles River Laboratories (Oxfordshire, UK). XID mice (CBA/CaHN-Btk xid/J) (Lindsley et al., 2007) are an inbred strain on the CBA background purchased from The Jackson Laboratory (#009361). They have a point mutation rendering the kinase domain of BTK inactive. Specifically, there is a C to T substitution at coding nucleotide 82, which alters the amino acid sequence; substituting an arginine for cysteine. The substitution is in a conserved PH domain and blocks the activation of the kinase (Rawlings et al., 1993) preventing BTK phosphorylation at Tyr223, which is a key activating site. Importantly, ibrutinib binds irreversibly to Cys481, also in the active site of the kinase domain and inhibits auto‐phosphorylation of Tyr223, thus blocking BTK activity. All mice were then housed in the same unit under conventional housing conditions at 25 ± 2°C and had access to food and water ad libitum.