DEVELOPMENT OF AN ELISA ASSAY FOR SOLUBLE CD35 (C3B/C4B RECEPTOR) - HIGH-LEVELS OF SOLUBLE CD35 IN LE-POSITIVE PATIENTS WITH HEMATOLOGICAL MALIGNANCIES

Malignant cells usually lack CD35 (complement receptor type 1, C3b/C4b receptor), a differentiation surface antigen. We measured soluble forms of CD35 in the plasma of normal subjects and patients with various malignant diseases. A microassay for the determination of CD35 was established based on a sandwich enzyme immunoassay using 2 monoclonal antibodies that recognize different epitopes. Soluble CD35 was not detected in any plasma samples from normal subjects or from patients with a variety of solid cancers: i.e., levels were below 20 ng/ml. On the other hand, 3 of 70 patients with hematological malignancies showed high levels of plasma CD35. The molecular mass of the soluble form was about 200 kDa, which is similar in size to membrane forms of CD35. Although the clinical conditions differed in these patients, they had high transaminase titers and detectable autoantibody. Complement titers (CH50) and the levels of membrane complement regulatory proteins were within the normal range in these patients. Although the mechanism by which it is produced remains unknown, soluble CD35 is present in significant amounts in association with immunological disorders secondary to hematological malignancies.