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Bruton's tyrosine kinase (Btk) plays crucial roles in B cell differentiation as well as mast cell activation through the high-affinity IgE receptor (Fc∊RI). Defects in the btk gene lead to agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. Mast cells from xid and btk null mice exhibit mild defects in degranulation and severe impairments in the production of proinflammatory cytokines upon Fc∊RI cross-linking. Recent studies demonstrated the role of Btk in a sustained increase in intracellular calcium concentrations in response to antigen receptor stimulation. Btk is also involved in the activation of stress-activated protein kinases, JNK/SAPK1/2, and thereby regulates c-Jun and other transcription factors that are important in cytokine gene activation. Regulation of the JNK/SAPK activation pathway by Btk may be related to the proapoptotic function of Btk in the programmed cell death in these hematopoietic cells. J. Leukoc. Biol. 65: 286–290; 1999.

Neutrophils showed a rapid and transient adhesion to immunoglobulin G (IgG)-coated plates compared with their adhesion to bovine serum albumin (BSA)-coated plates: the adhesion reached a peak after 15 min of incubation and then gradually returned to almost the basal state in 60 min. The addition of monomeric IgG or anti-FcγRII monoclonal antibody (mAb) (IV.3) suppressed the increase in adhesion, whereas anti-FcγRIII mAb (3G8) was hardly effective, indicating that the interaction of FcγR, especially FcγRII, with coated IgG is involved in the process. Adhesion was also blocked by cytochalasin B, suggesting that functional actin filament structures are crucial. Protein kinase inhibitors, erbstatin and genistein, inhibited the adhesion in a dose-dependent manner. The adhesion was inhibited by anti-CDllb (Ml/70) and anti-CD18 (MHM23, TS1/18) mAbs. Moreover, neutrophils from a patient with complete leukocyte adhesion deficiency syndrome did not show increased adhesion to IgG-coated plates. The adhesion of neutrophils to fibrinogen- and BSA-coated plates was also increased when FcγR was stimulated in the fluid phase with soluble aggregated IgG, which was also inhibited by anti-CDllb mAb. Stimulation of neutrophil FcγR with soluble aggregated IgG enhanced the expression of CDllb in concert with the enhanced adhesion. These data collectively suggest that stimulation via FcγR evokes a tyrosine kinase-dependent and actin filament-dependent intracellular signal that enhances the specific and nonspecific adhesive activity of neutrophils, presumably through the activation of CDllb/CD18. J. Leukoc. Biol. 55: 735–742; 1994.