title: A new pathway of CD5 glycoprotein-mediated T cell inhibition dependent on inhibitory phosphorylation of Fyn kinase.
abstract: Triggering of the T cell receptor initiates a signaling cascade resulting in the activation of the T cell. These signals are integrated alongside those resulting from the triggering of other receptors whose function is to modulate the overall response. CD5 is an immunotyrosine-based inhibition motif-bearing receptor that antagonizes the overt T cell receptor activation response by recruiting inhibitory intracellular mediators such as SHP-1, RasGAP, or Cbl. We now propose that the inhibitory effects of CD5 are also mediated by a parallel pathway that functions at the level of inhibition of Fyn, a kinase generally associated with T cell receptor-mediated activation. After CD5 ligation, phosphorylation of the negative regulatory tyrosine (Tyr(531)) of Fyn increases, and this correlates with a substantial reduction in the kinase activity of Fyn and a profound inhibition of ZAP-70 activation. The effect requires the last 23 amino acids of the cytoplasmic domain of the receptor, strongly implying the involvement of a new CD5-interacting signaling or adaptor protein. Furthermore, we show that upon CD5 ligation there is a profound shift in its distribution from the bulk fluid phase to the lipid raft environment, where it associates with Fyn, Lck, and PAG. We suggest that the relocation of CD5, which we also show is capable of forming homodimers, to the proximity of raft-resident molecules enables CD5 to inhibit membrane proximal signaling by controlling the phosphorylation and activity of Fyn, possibly by interfering with the disassembly of C-terminal Src kinase (Csk)-PAG-Fyn complexes during T cell activation.
authors.affiliation: Group of Cell Activation and Gene Expression, Instituto de Biologia Molecular e Celular, Universidade do Porto, 4150-180 Porto, Portugal; Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, 4150-180 Porto, Portugal.
authors.forename: Martina
authors.initials: M
authors.lastname: Bamberger
authors.affiliation: Group of Cell Activation and Gene Expression, Instituto de Biologia Molecular e Celular, Universidade do Porto, 4150-180 Porto, Portugal; Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, 4150-180 Porto, Portugal.
authors.forename: Ana Mafalda
authors.initials: AM
authors.lastname: Santos
authors.affiliation: Group of Cell Activation and Gene Expression, Instituto de Biologia Molecular e Celular, Universidade do Porto, 4150-180 Porto, Portugal; Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, 4150-180 Porto, Portugal.
authors.forename: Carine M
authors.initials: CM
authors.lastname: Gonçalves
authors.affiliation: Group of Cell Activation and Gene Expression, Instituto de Biologia Molecular e Celular, Universidade do Porto, 4150-180 Porto, Portugal; Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, 4150-180 Porto, Portugal.
authors.forename: Marta I
authors.initials: MI
authors.lastname: Oliveira
authors.affiliation: MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, Nuffield Department of Clinical Medicine, University of Oxford, Oxford OX3 9DS, United Kingdom.
authors.forename: John R
authors.initials: JR
authors.lastname: James
authors.affiliation: Group of Cell Activation and Gene Expression, Instituto de Biologia Molecular e Celular, Universidade do Porto, 4150-180 Porto, Portugal.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Moreira
authors.affiliation: Department of Immunology, Hospital Clínic of Barcelona, Institut d'Investigacions Biomèdiques August Pi i Sunyer, and Department of Cell Biology, Immunology and Neurosciences, Faculty of Medicine, University of Barcelona, 08036 Barcelona, Spain.
authors.forename: Franscisco
authors.initials: F
authors.lastname: Lozano
authors.affiliation: MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, Nuffield Department of Clinical Medicine, University of Oxford, Oxford OX3 9DS, United Kingdom.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Davis
authors.affiliation: Group of Cell Activation and Gene Expression, Instituto de Biologia Molecular e Celular, Universidade do Porto, 4150-180 Porto, Portugal; Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, 4150-180 Porto, Portugal. Electronic address: acarmo@ibmc.up.pt.
authors.forename: Alexandre M
authors.initials: AM
authors.lastname: Carmo
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D018956:CD5 Antigens / Q000737:chemistry*; D019281:Dimerization; D006023:Glycoproteins / Q000737:chemistry*; D006801:Humans; D019169:Jurkat Cells; D007963:Leukocytes, Mononuclear / Q000166:cytology; D021962:Membrane Microdomains / Q000737:chemistry; D051379:Mice; D018345:Mice, Knockout; D010766:Phosphorylation; D011485:Protein Binding; D051076:Proto-Oncogene Proteins c-fyn / Q000378:metabolism*; D015398:Signal Transduction; D013601:T-Lymphocytes / Q000378:metabolism*
chemical_list: D018956:CD5 Antigens; D006023:Glycoproteins; D051076:Proto-Oncogene Proteins c-fyn