The non-receptor tyrosine kinase Tec controls assembly and activity of the noncanonical caspase-8 inflammasome.

Tec family kinases are intracellular non-receptor tyrosine kinases implicated in numerous functions, including T cell and B cell regulation. However, a role in microbial pathogenesis has not been described. Here, we identified Tec kinase as a novel key mediator of the inflammatory immune response in macrophages invaded by the human fungal pathogen C. albicans. Tec is required for both activation and assembly of the noncanonical caspase-8, but not of the caspase-1 inflammasome, during infections with fungal but not bacterial pathogens, triggering the antifungal response through IL-1ß. Furthermore, we identify dectin-1 as the pathogen recognition receptor being required for Syk-dependent Tec activation. Hence, Tec is a novel innate-specific inflammatory kinase, whose genetic ablation or inhibition by small molecule drugs strongly protects mice from fungal sepsis. These data demonstrate a therapeutic potential for Tec kinase inhibition to combat invasive microbial infections by attenuating the host inflammatory response.

The tyrosine kinase Btk regulates the macrophage response to Listeria monocytogenes infection.

In this study we investigated the role of Bruton's tyrosine kinase (Btk) in the immune response to the Gram-positive intracellular bacterium Listeria monocytogenes (Lm). In response to Lm infection, Btk was activated in bone marrow-derived macrophages (BMMs) and Btk (-/-) BMMs showed enhanced TNF-α, IL-6 and IL-12p40 secretion, while type I interferons were produced at levels similar to wild-type (wt) BMMs. Although Btk-deficient BMMs displayed reduced phagocytosis of E. coli fragments, there was no difference between wt and Btk (-/-) BMMs in the uptake of Lm upon infection. Moreover, there was no difference in the response to heat-killed Lm between wt and Btk (-/-) BMMs, suggesting a role for Btk in signaling pathways that are induced by intracellular Lm. Finally, Btk (-/-) mice displayed enhanced resistance and an increased mean survival time upon Lm infection in comparison to wt mice. This correlated with elevated IFN-γ and IL-12p70 serum levels in Btk (-/-) mice at day 1 after infection. Taken together, our data suggest an important regulatory role for Btk in macrophages during Lm infection.

Btk is a positive regulator in the TREM-1/DAP12 signaling pathway.

The triggering receptor expressed on myeloid cells 1 (TREM-1) has been implicated in the production of proinflammatory cytokines and chemokines during bacterial infection and sepsis. For downstream signal transduction, TREM-1 is coupled to the ITAM-containing adaptor DAP12. Here, we demonstrate that Bruton tyrosine kinase (Btk), a member of the Tec kinases, becomes phosphorylated upon TREM-1 triggering. In U937-derived cell lines, in which expression of Btk was diminished by shRNA-mediated knockdown, phosphorylation of Erk1/2 and PLCγ1 and Ca²âº mobilization were reduced after TREM-1 stimulation. Importantly, TREM-1-induced production of the pro-inflammatory cytokines, TNF-α and IL-8, and up-regulation of activation/differentiation cell surface markers were impaired in Btk knockdown cells. Similar results were obtained upon TREM-1 stimulation of BMDCs of Btk(-/-) mice. The analysis of cells containing Btk mutants revealed that intact membrane localization and a functional kinase domain were required for TREM-1-mediated signaling. Finally, after TREM-1 engagement, TNF-α production by PBMCs was reduced in the majority of patients suffering from X-linked agammaglobulinemia (XLA), a rare hereditary disease caused by mutations in the BTK gene. In conclusion, our data identify Btk as a positive regulator in the ITAM-mediated TREM-1/DAP12 pathway and suggest its implication in inflammatory processes.

The role of Tec family kinases in mononuclear phagocytes.

Mononuclear phagocytes, including monocytcs, macrophages, and dendritic cells, play an important role in innate and adaptive immune responses and are important regulators of the inflammatory response. Among these, monocytes/macrophages are involved in virtually all aspects of inflammation, ranging from the detection and phagocytosis of pathogens up to the resolution of inflammation and repair of tissue damage. The stimulation of cell surface receptors, such as Toll-like receptors, leads to the initiation of intracellular signal transduction pathways regulating macrophagc activation and effector functions. One group of signaling molecules stimulated on macrophage activation is formed by the Tec kinasc family, which consists of five members (Bmx, Btk, Itk, Rlk, and Tec) and constitutes the second-largest family of nonreceptor protein tyrosine kinases in the immune system. Some Tec kinases have been shown to be major regulators of antigen receptor signaling in lymphocytes, and deficiencies in Tec family kinases cause several immunological defects in humans and mice. Much less is known about the role of Tec family kinases in the myeloid branch of the hematopoietic system. In this review, we discuss the current knowledge about the role of Tec family kinases in monocytes/macrophages, in dendritic cells, and in osteoclasts.