One project is focused on the regulation of Interleukin-4 receptor (IL-4R) expression on hematopoietic progenitor cells. Using the murine myeloid leukemia M1 cell line and mature, bond marrow-derived macrophages, neither of which express IL-4R, we have characterized the ability of IL-6 and/or Interferon-gamma (IFN-gamma) to induce the expression of IL-4R. Determining the kinetics of this induction at both the protein and the molecular level is currently being accomplished, giving insight into the regulatory role of these two cytokines in inflammation. The role of this induced IL-4R is also under investigation. The induction of the IL-4R on these cells appears to impart a sensitivity to IL-4 previously absent in these cells. This may explain the previously reported need for cooperation between IL-4 and other cytokines (such as G-CSF, GM-CSF, IL-3 or IL-6) before an effect of IL-4 could be observed. We are also involved in examining the function and regulation of the soluble form of the IL-4R. Using a cDNA probe that is specific for the soluble IL-4 receptor, we have shown that resting macrophages (both bone marrow-derived and M1 cells) do not express detectable levels of soluble IL-4R message. However, upon stimulation of these cells with IL-6, they undergo a rapid up-regulation of soluble IL-4R message that parallels that of the membrane-bound form of the receptor. Using an ELISA system for detecting and quantitating the expression of IL-4R protein, we have confirmed these findings and noted that murine macrophages produce large (microgram) amounts of soluble IL-4R in overnight culture after stimulation with IL-6. Until now, no cellular source for these soluble products had been identified, and the ability to regulate their production was unknown.
