IFN-gamma-induced MHC class II gene expression is suppressed in endothelial cells by dextran sulfate

IFN-gamma-activated endothelial cells actively participate in initiating immune responses by interacting with immunocompetent cells via class II MHC proteins. In this study, dextran sulfate, a synthetic heparin analogue, was shown to selectively inhibit IFN-gamma-induced surface expression of HLA-DR molecules by human umbilical cord vascular endothelial cells, but not other cytokine-induced molecules such as ELAM-1 or ICAM-1. Inhibition occurred regardless of whether dextran sulfate was added 24 h before, at the same time as, or 24 h after IFN-gamma stimulation of cells. In addition, both high (500 kDa) and low (5 kDa) molecular mass dextran sulfate molecules were able to block class II expression, whereas treating cells with naturally occurring polysulfated glycosaminoglycans such as heparin, heparan, and chondroitin sulfate did not produce any suppressive effects. Radiolabeling of cells with [35S]methionine followed by radioimmunoprecipitation using anti-HLA-DR alpha mAb demonstrated that biosynthesis of class II proteins was specifically blocked. RT-PCR and Southern blotting were utilized to examine transcription of the HLA-DR alpha gene and demonstrated an absence of HLA-DR alpha mRNA from dextran sulfate-treated and IFN-gamma-induced cells. Dextran sulfate also prevented transcription of the gene encoding CIITA, a transactivator protein required for IFN-gamma-inducible expression of class II genes. Thus, dextran sulfate apparently inhibited this step or an earlier one in the intracellular signaling pathway for IFN-gamma in human endothelial cells, subsequent to IFN-gamma binding to its cell surface receptor.