Porcine CD58: cDNA cloning and molecular dissection of the porcine CD58–human CD2 interface
Résumé
10 The porcine ligands of human CD2 remain unknown in xenotransplantation despite being an important pathway of T cell 11 costimulation. Of the two main candidates, i.e., CD48 and CD58, the cDNA of the most likely ligand poCD58 was cloned from 12 CD48-negative endothelial cells costimulating human CD4 þ T cells through the CD2 pathway. The deduced protein sequence is 244 13 residues long and is 43% homologous to the human sequence. Based on similarity between porcine and human CD58 external V-set 14 Ig-type domains, a structural model of poCD58-huCD2 interaction was built. Most of the charged residues located at the interface 15 with huCD2 are highly conserved. Six putative hydrogen bonds between poCD58 and huCD2 were identified; five involve the same 16 residues as in the syngeneic combination while the sixth is formed between an additional tyrosine in poCD58 and Arg48 in huCD2, 17 increasing the complementarity between the two molecules. These structural data will help us to develop poCD58 blocking agents 18 for xenotransplantation. 19 21 When pig to human xenotransplantation was con-22 sidered in the early 1990s, the problem of hyperacute 23 rejection was first investigated and recently solved by 24 using organs from pig transgenic for complement regu-25 latory proteins [3]. Cellular rejection phenomena were 26 initially underestimated, mainly because previous mur-27 ine/human xenogeneic experiments suggested that phy-28 logenetic divergence prevented the recognition of ligand/ 29 receptor and receptor/counter-receptor pairs in xenoge-30 neic systems. In particular, human T cell responses to 31 xenogeneic (murine) antigen presenting cells (APCs) 32 were considered low due to inability of costimulatory 33 receptors (CD28, CD2, and LFA-1) to be engaged by 34 the costimulatory molecules on murine APC. However, 35 phylogenetic divergence between pig and human is less 36 than between murine and human. Indeed, strong xeno-37 geneic T cell responses to pig APC were observed and 38 costimulation blocking experiments clearly demon-39 strated that human CD28 [5], CD2, and LFA-1 [9] could 40 be engaged by the putative porcine counter-receptors. 41 Characterization of porcine costimulatory molecules 42 was therefore initiated with the description of poCD86 43 (also known as B7-2) [7] and then poCD80 [19]. Sub-44 sequently, molecular, functional, and structural char-45 acterization of porcine CTLA4, one of the poCD86 46 ligands, led to the unexpected finding that this molecule 47 bound weakly to human CD86 and CD80 [18] and failed 48 to inhibit human T cell responses, when used as a 49 CTLA4-Ig fusion protein. Porcine ICAM-1 was recently 50 characterized [14] and demonstrated a low degree of 51 conservation with human ICAM-1 (41% identity at the 52 protein level). However, despite being divergent, porcine 53 ICAM-1 has kept the ability to bind human LFA-1 and 54 to transmit costimulatory signals to human T cells [9]. 55 Up to now, in xenogeneic pig-to-primate models, por-56 cine cells have demonstrated their capacity to