Serum levels of interleukin-6 in multiple myeloma and other hematological disorders: correlation with disease activity and other prognostic parameters.

Interleukin-6 (IL-6) is a multifunctional cytokine involved in the regulation of the terminal differentiation pathway of B lymphocytes. Recent reports revealed its potential role in the in vitro and in vivo growth of human multiple myeloma cells. The mechanism, however, by which IL-6 triggers proliferation of malignant plasma cells remains controversial. Using the very sensitive 7TD 1 bioassay we quantified endogenous circulating IL-6 levels in serum samples of 104 patients suffering from monoclonal gammopathies and other hematological disorders [47 with multiple myeloma (MM), 24 with monoclonal gammopathy of unknown significance (MGUS), 8 with myeloproliferative disease, and 25 suffering from low-grade non-Hodgkin's lymphoma (NHL)]. Elevated serum levels of IL-6 (greater than 5 pg/ml) were detected in 42% of the patients with MM, in 13% with MGUS, in 15% with low-grade B-NHL, and in 1 patient with T-NHL. In patients suffering from chronic myeloproliferative diseases, IL-6 levels were within the normal range. In patients with myeloma, IL-6 levels were significantly higher at advanced stages (II/III) or with progressive disease than in patients with MM stage I, MGUS, or at the plateau phase (P less than 0.01). In patients with monoclonal gammopathies including MGUS, serum IL-6 levels correlated with neopterin, tumor necrosis factor alpha and beta 2-microglobulin. An inverse correlation was found with hemoglobin levels. From these results, we propose that in myeloma patients serum IL-6 levels may reflect disease activity and tumor cell mass. The correlation with serum neopterin, a macrophage product, also suggests its origin in an activated immune system.