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Abstract
A series of murine interleukin 3 (IL-3)-dependent hemopoietic cell lines was studied for the capacity to produce interleukin 6 (IL-6) in vitro. These included a bone marrow-derived mast cell line (L138.8A) and several early myeloid cell lines described in the literature (DA-1, DA-3, NFS-60, NFS-78, FDC-P1, FDC-P2, FDC-PmixA4, and 32Dcl.23). All of these cell lines produced growth factor activity for IL-6-dependent hybridoma cells (7TD1), which was completely neutralized by the monoclonal anti-IL-6-antibody 6B4. IL-6 expression was also evident at the mRNA level using a murine IL-6-specific cDNA probe. In 32Dcl.23 cells (2 × 105/ml) stimulated for 24 hr with serial dilutions of purified murine IL-3, a positive correlation was found between the IL-3 dose and the amount of IL-6 measured in the conditioned media. At 24 hr this correlation was not evident at the mRNA level. However, prolonged exposure of 32Dcl.23 cells (up to 72 hr) to either a high (60 U/ml) or a low IL-3 concentration (1 U/ml) revealed a time-dependent increase and decrease, respectively, of IL-6 mRNA levels. At both IL-3 concentrations 32Dcl.23 cells remained in a fully viable and proliferative state. The influence of IL-3 on IL-6 release could be specifically counteracted by anti-IL-3-antiserum. IL-6 added alone or in concert with IL-3 did not stimulate 32Dcl.23 proliferation. Moreover, neutralizing doses of a monoclonal anti-IL-6 antibody (6B4) were unable to suppress IL-3-induced proliferation suggesting that secreted IL-6 in this particular cell line is not active as an autocrine growth factor.