Identification of a natural soluble form of human CD5

CD5 is a 67 kDa type I glycoprotein which belongs to the Scavenger Receptor Cysteine-Rich (SRCR) family of receptors. This family includes either cell-surface (e.g. CD6) or secreted (e.g. Spalpha) 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, rn=166) and is similar to that found in sera from patients suffering of various autoimmune (systemic lupus erythematosus, primary Sjogren syndrome, rheumatoid arthritis) and non-autoimmune (chronic renal failure, B-cell chronic lymphocytic leukemia) disorders. In vitro experiments indicate that nsCD5 is released by proteolytic cleavage of the membrane form. These results represent the first evidence of proteolytic release of a transmembrane SRCR family member following cell activation.