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Bacterial Lipopolysaccharide (LPS) is a strong inducer of inflammation and does so by inducing polarization of macrophages to the classic inflammatory M1 population. Given the role of Btk as a critical signal transducer downstream of TLR4, we investigated its role in M1/M2 induction. In Btk deficient (Btk (-\-)) mice we observed markedly reduced recruitment of M1 macrophages following intraperitoneal administration of LPS. Ex vivo analysis demonstrated an impaired ability of Btk(-/-) macrophages to polarize into M1 macrophages, instead showing enhanced induction of immunosuppressive M2-associated markers in response to M1 polarizing stimuli, a finding accompanied by reduced phosphorylation of STAT1 and enhanced STAT6 phosphorylation. In addition to STAT activation, M1 and M2 polarizing signals modulate the expression of inflammatory genes via differential activation of transcription factors and regulatory proteins, including NF-κB and SHIP1. In keeping with a critical role for Btk in macrophage polarization, we observed reduced levels of NF-κB p65 and Akt phosphorylation, as well as reduced induction of the M1 associated marker iNOS in Btk(-/-) macrophages in response to M1 polarizing stimuli. Additionally enhanced expression of SHIP1, a key negative regulator of macrophage polarisation, was observed in Btk(-/-) macrophages in response to M2 polarizing stimuli. Employing classic models of allergic M2 inflammation, treatment of Btk (-/-) mice with either Schistosoma mansoni eggs or chitin resulted in increased recruitment of M2 macrophages and induction of M2-associated genes. This demonstrates an enhanced M2 skew in the absence of Btk, thus promoting the development of allergic inflammation.

In addition to regulating B cell development and activation, Bruton's tyrosine kinase (Btk) functions downstream of multiple TLRs, including TLR7, to regulate innate immune responses in myeloid cells. Although critical for defense against RNA viruses such as influenza and Sendai virus, recognition of self-RNA by TLR7 also has been shown to be an important contributor to the pathophysiology of systemic lupus erythematosus. To date, the role of Btk in regulating TLR7-mediated responses is poorly understood. In the current study, we have demonstrated a hitherto undiscovered role for Btk in apoptotic cell uptake, identifying the molecular chaperone calreticulin (CRT) as a novel substrate for Btk in regulating this response. CRT together with the transmembrane receptor CD91 function at the cell membrane and regulate uptake of C1q-opsonised apoptotic cells. Our results show that Btk directly phosphorylates CRT and that in the absence of Btk, CRT fails to localize with CD91 at the cell surface and at the phagocytic cup. Critically, a blocking Ab against CRT in wild-type macrophages mimics the inability of Btk-deficient macrophages to phagocytose apoptotic cells efficiently, indicating the critical importance of Btk in regulating CRT-driven apoptotic cell uptake. Our data have revealed a novel regulatory role for Btk in mediating apoptotic cell clearance, with CRT identified as the critical component of the CRT/CD91/C1q system targeted by Btk. Given the importance of clearing apoptotic cell debris to prevent inappropriate exposure of TLRs to endogenous ligands, our results have important implications regarding the role of Btk in myeloid cell function.

This study defines a critical role for Btk in regulating TLR4-induced crosstalk between antigen presenting cells (APCs) and natural killer (NK) cells. Reduced levels of IL-12, IL-18 and IFN-γ were observed in Btk-deficient mice and ex vivo generated macrophages and dendritic cells (DCs) following acute LPS administration, whilst enhanced IL-10 production was observed. In addition, upregulation of activation markers and antigen presentation molecules on APCs was also impaired in the absence of Btk. APCs, by virtue of their ability to produce IL-12 and IL-18, are strong inducers of NK-derived IFN-γ. Co-culture experiments demonstrate that Btk-deficient DCs were unable to drive wild-type or Btk-deficient NK cells to induce IFN-γ production, whereas these responses could be restored by exogenous administration of IL-12 and IL-18. Thus Btk is a critical regulator of APC-induced NK cell activation by virtue of its ability to regulate IL-12 and IL-18 production in response to acute LPS administration., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Toll-like receptors (TLRs) are a primary surveillance system for the detection of pathogens and are crucial to the activation of host defense. TLR7 and TLR8 sense single-stranded RNA from viruses or host ribonucleoproteins and synthetic imidazoquinolines such as R848, whereas TLR9 senses unmethylated CpG motifs in viral and bacterial DNA and in host DNA. Here we report the endogenous interaction between Brutons's tyrosine kinase (Btk) and human TLR8 and TLR9 in the monocytic cell line THP1. We also show that R848, single-stranded RNA, and CpGB-DNA activate Btk in THP1 cells as shown by phosphorylation of the tyrosine 223 residue of Btk and also by increased autokinase activity. We demonstrate that Btk is required for NFkappaB activation, participating in the pathway to increased phosphorylation of p65 on serine 536 activated by TLR8 and TLR9. Finally we demonstrate that peripheral blood mononuclear cells from patients with X-linked agammaglobulinaemia (XLA) that have dysfunctional Btk are impaired in the induction of interleukin-6 by CpGB-DNA. This study therefore establishes Btk as a key signaling molecule that interacts with and acts downstream of TLR8 and TLR9. Lack of functioning Btk in XLA patients downstream of TLR8 and TLR9 might explain the susceptibility of XLA patients to viral infections.

Bruton's tyrosine kinase (Btk) has recently been shown to participate in the induction of nuclear factor kappaB (NFkappaB)-dependent gene expression by the lipopolysaccharide (LPS) receptor Toll-like receptor-4 (TLR4). In this study we have examined the mechanism whereby Btk participates in this response. Treatment of the murine monocytic cell line Raw264.7 with LFM-A13, a specific Btk inhibitor, blocked LPS-induced NFkappaB-dependent reporter gene expression but not IkappaB alpha degradation. Transient transfection of HEK293 cells with Btk had no effect on NFkappaB-dependent reporter gene expression but strongly promoted transactivation of a reporter gene by a p65-Gal4 fusion protein. IkappaB alpha degradation activated by LPS was intact in macrophages from X-linked immunodeficiency (Xid) mice, which contain inactive Btk. Transfection of cells with a dominant negative form of Btk (BtkK430R) inhibited LPS-driven p65 mediated transactivation. Additionally LFM-A13 impaired phosphorylation of serine 536 on p65 induced by LPS in HEK293-TLR4 cells, and in Xid macrophages this response was impaired. This study therefore reveals a novel function for Btk. It is required for the signaling pathway activated by TLR4, which culminates in phosphorylation of p65 on serine 536 promoting transactivation by NFkappaB.

The discovery of the Toll-like receptors (TLRs) has revolutionised the field of innate immunity. One unresolved question regarding LPS signalling is whether there is a role for tyrosine kinases downstream of the LPS receptor. Studies in mice deficient in Bruton's tyrosine kinase have previously shown that they are defective in their responses to LPS. Further investigation into the role of Btk in LPS signalling has directly implicated Btk downstream of TLR4, both with respect to p38 MAPK activation and activation of the transcription factor NFkappaB. In fact Btk is activated by LPS and has been shown to directly bind TLR4 and the key proximal signalling proteins involved in LPS-induced NFkappaB activation, MyD88, Mal and IRAK-1. These recent findings point to a direct role for Btk in LPS signal transduction and raise interesting questions regarding the mode of activation of Btk following LPS stimulation and the precise nature of the pathways activated downstream of Btk. A better understanding of how Btk functions in LPS signalling will have important implications for inflammatory and autoimmune disorders and therapies thereof.

In this study we have identified members of the Toll-like receptor (TLR) family (namely, TLRs 4, 6, 8, and 9) as proteins to which the intracellular protein tyrosine kinase, Bruton's tyrosine kinase (Btk), binds. Detailed analysis of the interaction between Btk and TLR8 demonstrates that the presence of both Box 2 and 3 motifs in the Toll/interleukin-1 receptor domain was required for the interaction. Furthermore, co-immunoprecipitation experiments revealed that Btk can also interact with key proteins involved in TLR4 signal transduction, namely, MyD88, Mal (MyD88 adapter-like protein), and interleukin-1 receptor-associated kinase-1, but not TRAF-6. The ability of Btk to interact with TLR4 and Mal suggests a role for Btk in lipopolysaccharide (LPS) signal transduction. Stimulation of the human monocytic cell line THP-1 with LPS resulted in an increase in the level of tyrosine phosphorylation of Btk (indicative of activation). The autokinase activity of Btk was also stimulated after LPS stimulation. In addition, a dominant negative form of Btk inhibited TLR4-mediated activation of a nuclear factor kappaB (NFkappaB)-dependent reporter gene in HEK293 cells as well as LPS-induced activation of NFkappaB in the astrocytoma cell line U373 and the monocytic cell line RAW264.7. Further investigation revealed that the Btk-specific inhibitor, LFM-A13, inhibited the activation of NFkappaB by LPS in THP-1 cells. Our findings implicate Btk as a Toll/interleukin-1 receptor domain-binding protein that is important for NFkappaB activation by TLR4.

The article offers information on the role of MHC molecules and Toll like receptors (TLRs) in the immune system of human body. It states that TLRs are inherited immune receptors that causes the production of inflammatory cytokines. It also mentions about the importance of a nonreceptor tyrosine kinase, Bruton's tyrosine kinase (Btk) for immune response.

Members of the Toll-like receptor (TLR) family are essential players in activating the host innate immune response against infectious microorganisms. All TLRs signal through Toll/interleukin 1 receptor domain-containing adapter proteins. MyD88 adapter-like (Mal) is one such adapter that specifically is involved in TLR2 and TLR4 signaling. When overexpressed we have found that Mal undergoes tyrosine phosphorylation. Three possible phospho-accepting tyrosines were identified at positions 86, 106, and 187, and two mutant forms of Mal in which tyrosines 86 and 187 were mutated to phenylalanine acted as dominant negative inhibitors of NF-κB activation by lipopolysaccharide (LPS). Activation of THP-1 monocytic cells with the TLR4 agonist LPS and the TLR2 agonist macrophage-activating lipopeptide-2 induced phosphorylation of Mal on tyrosine residues. We found that the Bruton's tyrosine kinase (Btk) inhibitor LFM-A13 could block the endogenous phosphorylation of Mal on tyrosine in cells treated with macrophage-activating lipopeptide-2 or LPS. Furthermore, Btk immunoprecipitated from THP-1 cells activated by LPS could phosphorylate Mal. Our study therefore provides the first demonstration of the key role of Mal phosphorylation on tyrosine during signaling by TLR2 and TLR4 and identifies a novel function for Btk as the kinase involved. [ABSTRACT FROM AUTHOR]