title: The X-linked lymphoproliferative syndrome gene product SAP regulates B cell function through the FcgammaRIIB receptor.
abstract: X-linked lympho-proliferative (XLP) is an immunodeficiency condition caused by mutation or deletion of the gene encoding the adaptor protein SAP/SH2D1A. Besides defects in T cell and NK cell function and an absence of NKT cells, XLP can also manifest as lymphomas resulting primarily from uncontrolled B cell proliferation upon acute infection by Epstein-Barr virus. While it has been demonstrated that SAP regulates the functions of T cells and NK cells through the SLAM family of immunoreceptors, its role in B cells has not been defined. Here we show that SAP forms a ternary complex with the kinase Lyn and the inhibitory IgG Fc receptor FcgammaRIIB to regulate B cell proliferation and survival. SAP binds directly and simultaneously to the Lyn SH3 domain and an Immuno-receptor Tyrosine-based Inhibitory Motif (ITIM) in FcgammaRIIB, resulting in the activation of the latter. Moreover, SAP associates with FcgammaRIIB in mouse splenic B cells and promotes its tyrosine phosphorylation. Expression of SAP in the A20 B cell line led to a marked reduction in Blnk phosphorylation, a decrease in Akt activation, and a near-complete ablation of phosphorylation of the MAP kinases Erk1/2, p38 and JNK upon colligation of FcgammaRIIB with the B cell receptor (BCR). In contrast, an XLP-causing SAP mutant was much less efficient in eliciting these effects in B cells. Furthermore, compared to A20 cells, SAP transfectants displayed a significantly reduced rate of proliferation and an increased sensitivity to activation-induced cell death. Collectively these data identify an intrinsic function for SAP in inhibitory signaling in B cells and suggests that SAP may play an important role in balancing positive versus negative immune responses.
authors.affiliation: Department of Biochemistry and the Siebens-Drake Research Institute, Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario, Canada N6A 5C1.
authors.forename: Chengjun
authors.initials: C
authors.lastname: Li
authors.forename: Brian
authors.initials: B
authors.lastname: Chung
authors.forename: Jianping
authors.initials: J
authors.lastname: Tao
authors.forename: Cristiana
authors.initials: C
authors.lastname: Iosef
authors.forename: Ala
authors.initials: A
authors.lastname: Aoukaty
authors.forename: Yefu
authors.initials: Y
authors.lastname: Wang
authors.forename: Rusung
authors.initials: R
authors.lastname: Tan
authors.forename: Shawn Shun-Cheng
authors.initials: SS
authors.lastname: Li
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D000818:Animals; D001402:B-Lymphocytes / Q000166:cytology / Q000201:enzymology / Q000276:immunology*; D016923:Cell Death; D002460:Cell Line; D049109:Cell Proliferation; D002470:Cell Survival; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D008213:Lymphocyte Activation; D008232:Lymphoproliferative Disorders / Q000276:immunology*; D051379:Mice; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D010766:Phosphorylation; D011485:Protein Binding; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D011947:Receptors, Antigen, B-Cell / Q000378:metabolism; D017452:Receptors, IgG / Q000276:immunology*; D015398:Signal Transduction; D000071179:Signaling Lymphocytic Activation Molecule Associated Protein; D018909:src Homology Domains; D019061:src-Family Kinases / Q000737:chemistry / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C113939:B cell linker protein; C080571:Fc gamma receptor IIB; D047908:Intracellular Signaling Peptides and Proteins; D011947:Receptors, Antigen, B-Cell; D017452:Receptors, IgG; C114953:SH2D1A protein, human; C499569:Sh2d1a protein, mouse; D000071179:Signaling Lymphocytic Activation Molecule Associated Protein; C060902:lyn protein-tyrosine kinase; D019061:src-Family Kinases; D051057:Proto-Oncogene Proteins c-akt; D020928:Mitogen-Activated Protein Kinases