Key words: circulating CD5; soluble receptor; SRCR family Acknowledgments: The authors wish to thank R. Vilella, A. Collado and C. Milstein for kindly providing valuable reagents; J. Mila ´ and J. Barcelo ´ for technical assistance; A. Gaya `, M. Isamat, M.A. Bowen and J.J. Pe ´rez-Villar for helpful discussions and critically reviewing the manuscript. This work was supported by grants from Fondo de Investigacio ´n Sanitaria (FIS 95/1102, FIS 97/0669) and Comisio ´n Interministerial de Ciencia y Tecnologı ´a (SAF 98/045). J.C., G.E., J.M.V. and O.P. are recipients of fellowships from Fundacio ´n LAIR, Hospital Clı ´nic, IDIBAPS (330017) and CIRIT (1998FI/839), respectively. Received 1 April, revised, accepted for publication 2 June 1999 Copyright c Munksgaard 1999 Tissue Antigens . ISSN 0001-2815 Tissue Antigens 1999: 54: 128–137 Printed in Denmark . All rights reserved 128 J. Calvo Identification of a natural soluble form of L. Places human CD5 G. Espinosa O. Padilla J.M. Vila ` N. Villamor M. Ingelmo T. Gallart J. Vives J. Font F. Lozano Abstract: CD5 is a 67 kDa type I glycoprotein which belongs to the Scaven- ger Receptor Cysteine-Rich (SRCR) family of receptors. This family includes either cell-surface (e.g. CD6) or secreted (e.g. Spa) proteins implicated in the development of the immune system and the regulation of immune responses. In this study, we purified and characterised a circulating natural soluble CD5 form (nsCD5) which is indistinguishable (in apparent molecular mass, glycosylation pattern, and antibody reactivity) from a recombinant soluble CD5 form (rsCD5) composed of the three extracellular SCRC domains. The nsCD5 is a N-glycosylated 52 kDa molecule present in normal human serum and in supernatants of in vitro phorbol ester- and CD3-stimulated peripheral blood mononuclear cells. The nsCD5 concentration in sera from healthy donors is relatively low (median 1.75 ng/ml, n=166) and is similar to that found in sera from patients suffering of various autoimmune (systemic lu- pus erythematosus, primary Sjo ¨gren syndrome, rheumatoid arthritis) and non-autoimmune (chronic renal failure, B-cell chronic lymphocytic leukem- ia) disorders. In vitro experiments indicate that nsCD5 is released by proteo- lytic cleavage of the membrane form. These results represent the first evi- dence of proteolytic release of a transmembrane SRCR family member follow- ing cell activation. The membrane-associated CD5 glycoprotein is a 67 kDa co-recep- tor found on thymocytes, mature T lymphocytes, B1a normal B- cell subset and B-cell chronic lymphocytic leukemias (B-CLL) (1). Functional studies implicate CD5 in lymphocyte activation, T-cell development and T cell-antigen presenting cell interactions (2–4). CD5 is physically associated with the antigen-specific receptor present on both T (TCR) and B (BCR) cells (5, 6). Ligation of CD5 by specific monoclonal antibodies (mAbs) delivers different intra- cellular signals (7–10) which result in proliferative responses of lymphocytes in the presence of monocytes (11, 12), phorbol es- ters (13) or mAbs against CD3 (14) or CD28 (15). More Authors’ affiliations: J. Calvo 1 , L. Places 1 , G. Espinosa 2 , O. Padilla 1 , J.M. Vila ` 1 , N. Villamor 3 , M. Ingelmo 2 , T. Gallart 1 , J. Vives 1 , J. Font 2 , F. Lozano 1 1 Servei d’Immunologia, Institut d’Investigacions Biome `diques August Pi i Sunyer (IDIBAPS), Hospital Clı ´nic, Barcelona, Spain, 2 Servei de Malalties Autoimmunes Siste `miques, Institut d’Investigacions Biome `diques August Pi i Sunyer (IDIBAPS), Hospital Clı ´nic, Barcelona, Spain, 3 Servei d’Hematologia, Institut d’Investigacions Biome `diques August Pi i Sunyer (IDIBAPS), Hospital Clı ´nic, Barcelona, Spain Correspondence to: Francisco Lozano Servei d’Immunologia Hospital Clı ´nic Villarroel 170 08036 Barcelona Spain Tel: π34 93 4544920 Fax: π34 93 4518038 e-mail: lozano/medicina.ub.es