title: Bruton's tyrosine kinase is required for apoptotic cell uptake via regulating the phosphorylation and localization of calreticulin.
abstract: 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.
authors.affiliation: Molecular and Cellular Therapeutics and RCSI Research Institute, Royal College of Surgeons in Ireland, Dublin 2, Ireland.
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: Byrne
authors.forename: Joan
authors.initials: J
authors.lastname: Ní Gabhann
authors.forename: Kevin B
authors.initials: KB
authors.lastname: Stacey
authors.forename: Barbara M
authors.initials: BM
authors.lastname: Coffey
authors.forename: Eoghan
authors.initials: E
authors.lastname: McCarthy
authors.forename: Warren
authors.initials: W
authors.lastname: Thomas
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Jefferies
mesh_terms: D000077329:Agammaglobulinaemia Tyrosine Kinase; D000818:Animals; D019138:Antibodies, Blocking / Q000276:immunology; D017209:Apoptosis; D037282:Calreticulin / Q000378:metabolism*; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D015922:Complement C1q / Q000276:immunology*; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D008213:Lymphocyte Activation; D008264:Macrophages / Q000276:immunology; D008562:Membrane Glycoproteins / Q000378:metabolism*; D008565:Membrane Proteins; D051379:Mice; D022423:Myeloid Cells / Q000276:immunology; D010587:Phagocytosis / Q000276:immunology; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D011973:Receptors, LDL / Q000378:metabolism; D015398:Signal Transduction; D051199:Toll-Like Receptor 7 / Q000378:metabolism*; D025521:Tumor Suppressor Proteins / Q000378:metabolism
chemical_list: D019138:Antibodies, Blocking; D037282:Calreticulin; D026503:Low Density Lipoprotein Receptor-Related Protein-1; C486645:Lrp1 protein, mouse; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D011973:Receptors, LDL; C477038:Tlr7 protein, mouse; D051199:Toll-Like Receptor 7; D025521:Tumor Suppressor Proteins; D015922:Complement C1q; D011505:Protein-Tyrosine Kinases; D000077329:Agammaglobulinaemia Tyrosine Kinase; C000625951:Btk protein, mouse