Regulation of major histocompatibility gene expression in human vascular smooth muscle cells

Human atheromata, but not normal blood vessels, contain numerous smooth muscle cells (SMC) that bear class II major histocompatibility (MHC) antigens. These lesions also contain leukocytes that can secrete cytokines, which may modulate SMC functions. Because of morphologic evidence for immune-activated (class II+) SMC in vascular lesions, we studied the regulation by cytokines of MHC gene expression in SMC cultured from human vessels. Under basal conditions, these SMC contained mRNA for class I MHC (detected by Northern blotting with a cDNA probe for HLA-B7) and expressed surface class I MHC product determined by enzyme-linked immunoassay with monoclonal antibody (MAb) W6/32. Unstimulated SMC contained little or no class II MHC mRNA (probed with HLA-DR alpha cDNA) or surface antigen (examined using MAb I2). Secretory products of activated human leukocytes (the cell-free supernatant of a mixed leukocyte reaction) induced class II MHC antigen expression by SMC after 3 days. Treatment of SMC with interferon (IFN)-alpha or -beta (1000 U/ml for 72 hours) increased class I MHC mRNA content and surface antigen but did not alter class II expression. Immune IFN (IFN-gamma), a leukocyte product known to induce class II MHC expression in classical antigen presenting cells as well as epithelial and endothelial cells, not only increased class I MHC expression by SMC but also induced substantial levels of class II MHC mRNA and surface antigen. IFN-gamma (ED50 approximately 10 U/ml) increased class II MHC mRNA maximally after 2 to 3 days and surface expression linearly from 1 to 4 days. Immunohistochemical study demonstrated few class II+ SMC in cultured human SMC under basal conditions but homogeneous expression of high levels of DR antigen after exposure to IFN-gamma for 3 days. Neither interleukin-1 (IL-1 alpha or beta), tumor necrosis factor alpha (TNF), nor endotoxin altered class II expression by SMC. Local secretion of IFN-gamma by activated leukocytes may account for the presence of HLA-DR+ SMC in the human atheroma. Immune activation of SMC might participate in the pathogenesis of vasculitis and arteriosclerosis, particularly in the form found in the coronary arteries of transplanted hearts.