A glycoprotein secreted by lung cancer cells is present in human serum as an immunoglobulin-binding protein.

The 6B3-Ag recognized by a monoclonal antibody 6B3 to human large cell lung carcinoma cell line (HLC-2) is a high-molecular-weight glycoprotein of 1,000,000. Its serum level is increased in various adenocarcinoma patients. When a patient's serum with a high concentration of 6B3.Ag (54 micrograms/ml) or concentrated 6B3.Ag from normal human serum was analyzed by immunoelectrophoresis, 6B3.Ag showed a long bimodal precipitin line extending from the per-beta to beta globulin region. However, the precipitin line of 6B3.Ag in the HLC-2 culture medium was formed only in the pre-beta globulin region. The 6B3.Ag was purified from pooled patients' serum by salting out, precipitation by acidification at pH 4.5 and Sepharose 4B and immunoaffinity chromatographies. Western blotting indicated that the 6B3.Ag from human serum contained IgG and/or IgM. The 6B3.Ag from human serum showed a dose-dependent reaction in a sandwich enzyme-linked immunosorbent assay with anti-6B3.Ag antibody as a solid-phase antibody and anti-human IgG or anti-human IgM antibody labeled with alkaline phosphatase. The 6B3.Ag was concluded to be partly present as a complex with IgG and/or IgM in human serum, and this complex showed a precipitin line in the beta globulin region on immunoelectrophoresis.