Granulocyte-macrophage colony-stimulating factor elevates invariant chain expression in immature myelomonocytic cell lines.

Invariant chain (Ii) plays an important role in major histocompatibility complex (MHC) class II antigen processing and presentation and is constitutively synthesized in B lymphocytes, in macrophages, dendritic cells and in some epithelial cells. It has been shown that interferon-gamma, tumour necrosis factor-alpha and interleukin-4 co-regulate Ii and MHC class II expression in various cell types. We describe here a novel regulation of Ii expression in macrophages. Treatment of the premature monocytic cell lines WEHI 265.1, M1 and WEHI-3B with recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF) strongly enhances Ii expression while class II expression is not induced. In contrast, GM-CSF did not enhance Ii in mature macrophage cell lines. The increase of Ii expression in WEHI 265.1 cells takes several days. This long induction time, and a difference in activity between GM-CSF-conditioned medium and GM-CSF, together suggest that GM-CSF stimulates WEHI 265.1 cells to secrete a factor that modulates Ii expression. These results may imply a class II-independent function of Ii, which we discuss in this paper.