title: LICOS, a primordial costimulatory ligand?
abstract: In mammals, the classical B7 molecules expressed on antigen-presenting cells, B7-1 (CD80) and B7-2 (CD86), bind the structurally related glycoproteins CD28 and CTLA-4 (CD152), generating costimulatory signals that regulate the activation state of T cells. A recently identified human CD28-like protein, ICOS, also induces costimulatory signals in T cells when crosslinked with antibodies, but it is unclear whether ICOS is part of a B7-mediated regulatory pathway of previously unsuspected complexity, or whether it functions independently and in parallel. Here, we report that, rather than binding B7-1 or B7-2, ICOS binds a new B7-related molecule of previously unknown function that we call LICOS (for ligand of ICOS). At 37 degrees C, LICOS binds only to ICOS but, at lower, non-physiological temperatures, it also binds weakly to CD28 and CTLA-4. Sequence comparisons suggest that LICOS is the homologue of a molecule expressed by avian macrophages and of a murine protein whose expression is induced in non-lymphoid organs by tumour necrosis factor alpha (TNFalpha). Our results define the components of a distinct and novel costimulatory pathway and raise the possibility that LICOS, rather than B7-1 or B7-2, is the contemporary homologue of a primordial vertebrate costimulatory ligand.
authors.affiliation: Sir William Dunn School of Pathology, The University of Oxford, Oxford, OX1 3RE, UK.
authors.forename: D
authors.initials: D
authors.lastname: Brodie
authors.forename: A V
authors.initials: AV
authors.lastname: Collins
authors.forename: A
authors.initials: A
authors.lastname: Iaboni
authors.forename: J A
authors.initials: JA
authors.lastname: Fennelly
authors.forename: L M
authors.initials: LM
authors.lastname: Sparks
authors.forename: X N
authors.initials: XN
authors.lastname: Xu
authors.forename: P A
authors.initials: PA
authors.lastname: van der Merwe
authors.forename: S J
authors.initials: SJ
authors.lastname: Davis
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015703:Antigens, CD; D000945:Antigens, Differentiation, T-Lymphocyte / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D002461:Cell Line, Transformed; D018076:DNA, Complementary; D006801:Humans; D060945:Inducible T-Cell Co-Stimulator Ligand; D060889:Inducible T-Cell Co-Stimulator Protein; D008024:Ligands; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D011948:Receptors, Antigen, T-Cell / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D020349:Surface Plasmon Resonance / Q000379:methods
chemical_list: D015703:Antigens, CD; D000945:Antigens, Differentiation, T-Lymphocyte; D018076:DNA, Complementary; C556692:ICOS protein, human; C498514:ICOSLG protein, human; C556694:Icos protein, mouse; C400230:Icosl protein, mouse; D060945:Inducible T-Cell Co-Stimulator Ligand; D060889:Inducible T-Cell Co-Stimulator Protein; D008024:Ligands; D008562:Membrane Glycoproteins; D011506:Proteins; D011948:Receptors, Antigen, T-Cell