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Radioimmunoassay for the measurement of serum IL‐6 and its correlation with tumour cell mass parameters in multiple myeloma

Interleukin‐6 (IL‐6) was demonstrated to be a strong autocrine or paracrine plasmocytoma cell growth factor in humans. Using a bioassay, high serum IL‐6 (S‐IL‐6) levels were correlated with disease severity in plasma cell dyscrasias. Since other cytokines could interfere with the bioassays, we developed a specific radioimmunoassay to study S‐IL6 levels in 102 patients with monoclonal gammopathy (MG). S‐IL‐6 level was studied by a double antibody radioimmunoassay using a rabbit polyclonal anti‐IL‐6 antibody and a human recombinant IL‐6 as the standard. The lowest value of the standard significantly different from zero was found to be 78 pg/ml. Within‐run and between‐run precisions were characterized by a mean coefficient of variation of 3.72 and 5.5%, respectively. The mean analytical recovery was found to be 113% and the immunochemical identity of IL‐6 standard and S‐IL‐6 was shown by dilution tests. IL‐6 was detected in all tested sera. Sera from 66 healthy volunteers and 43 patients with acute leukemia or malignant lymphoma were tested as controls. In healthy subjects, S‐IL‐6 values were 294 ± 86 pg/ml. MG were classified as multiple myeloma (MM), macroglobulinemia, and MG of undetermined significance (MGUS). The distribution of S‐IL‐6 levels in patients with MG was significantly higher than in healthy subjects but lower than in patients with acute leukemia or Hodgkin's lymphoma. Results obtained in 55 patients with MM were related to other biological parameters. S‐IL‐6 levels correlated with bone‐marrow plasmacytosis (P < .0005), serum‐lactate dehydrogenase (S‐LDH; P < .005), serum β2 microglobulin (S −β2m; P < .01), and serum calcium (S‐Ca; P < .025) and inversely correlated with haemoglobin (P < .025). Our results indicate that 1 radioimmunoassay is suitable for the measurement of human IL‐6 in serum; 2 high S‐IL‐6 levels are observed in a small number of patients with MG; and 3 S‐IL‐6 level correlates with tumour cell mass in patients with overt MM.