Phorbol esters potentiate the induction of class I HLA expression by interferon alpha

Insights Into the Emergence of Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal Nocturnal Hemoglobinuria (PNH) is a disease as simple as it is complex. PNH patients develop somatic loss-of-function mutations in phosphatidylinositol N-acetylglucosaminyltransferase subunit A gene (PIGA), required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors. Ubiquitous in eukaryotes, GPI anchors are a group of conserved glycolipid molecules responsible for attaching nearly 150 distinct proteins to the surface of cell membranes. The loss of two GPI-anchored surface proteins, CD55 and CD59, from red blood cells causes unregulated complement activation and hemolysis in classical PNH disease. In PNH patients, PIGA-mutant, GPI (-) hematopoietic cells clonally expand to make up a large portion of patients’ blood production, yet mechanisms leading to clonal expansion of GPI (-) cells remain enigmatic. Historical models of PNH in mice and the more recent PNH model in rhesus macaques showed that GPI (-) cells reconstitute near-normal hematopoiesis but have no intrinsic growth advantage and do not clonally expand over time. Landmark studies identified several potential mechanisms which can promote PNH clonal expansion. However, to what extent these contribute to PNH cell selection in patients continues to be a matter of active debate. Recent advancements in disease models and immunologic technologies, together with the growing understanding of autoimmune marrow failure, offer new opportunities to evaluate the mechanisms of clonal expansion in PNH. Here, we critically review published data on PNH cell biology and clonal expansion and highlight limitations and opportunities to further our understanding of the emergence of PNH clones.

IFN-alpha super-induction of HLA class I expression by a variant thymoma cell line involves nuclear translocation of Rel complexes

Variant thymoma lines have been described which exhibit a substantially increased level of HLA class I induction by IFN-alpha, but not by IFN-gamma, and an unchanged response of other IFN-alpha-stimulated genes (Burrone et al., EMBO J. 1985. 4: 2855-2860). We report that their amplified response correlates with the nuclear translocation of Rel transcription factors upon prolonged treatment with IFN-alpha. The variant cells contain an IkappaBalpha subset with a significantly shortened half-life, and a constitutively active form of IkappaBalpha efficiently blocks HLA class I induction. Therefore, in addition to STAT-mediated induction, prolonged exposure to IFN-alpha can affect transcription involving Rel factors, which are implicated in the regulation of numerous immune response and viral genes.

Effects of cytokines on HLA class I antigen expression on Huh6 and HB611 cells

This study investigated the expression of HLA class I antigens on Huh6 and HB611 cells induced by interferon (IFN)-alpha, IFN-gamma, tumor necrosis factor (TNF)-alpha and interleukin (IL)-1 beta. All of these cytokines induced the antigens on both cells in a dose-dependent manner, with IFNs inducing much more expression than TNF-alpha or IL-1 beta. We have already reported that protein kinase C (PKC) is involved in the antigen expression induced by IFN-gamma on Huh6 cells. The antigen expression induced by IFN-alpha was also blocked by a PKC inhibitor, H-7. However, the antigen expression by TNF-alpha or IL-1 beta was not inhibited by H-7, by a protein kinase A inhibitor, HA1004, nor by a calmodulin antagonist, W-7. These results suggested that PKC, Ca(2+)-calmodulin, and cAMP are not involved in the induction of HLA class I antigens on both cells by TNF-alpha and IL-1 beta. We concluded that TNF-alpha and IL-1 beta induced much less expression of HLA class I antigens on both cells than IFNs and that this might be because the signaling pathway by TNF-alpha and IL-1 beta differed from that by IFNs.

Interferon-alpha/beta, pentoxifylline, and caffeine synergize with interferon-gamma to induce major histocompatibility complex class I expression on a constitutively class I-negative murine tumor cell line

The constitutively class I-negative tumor cell line, Kgv, expresses H-2Dk in response to interferon-gamma (IFN-gamma), but not in response to IFN-alpha/beta, tumor necrosis factor, or lymphotoxin. H-2Dk expression was not induced on Kgv cells by the methylxanthines, pentoxifylline (PTX) and caffeine, which modulate class I expression on cells that constitutively express class I molecules. Treatment of Kgv cells with either IFN-alpha/beta, PTX, caffeine, or dibutyryl cAMP and a concentration of IFN-gamma insufficient by itself to induce Dk expression resulted in the induction of Dk expression. Since PTX and caffeine are cAMP-specific phosphodiesterase inhibitors, it is possible that the effects of PTX, caffeine, and dibutyryl cAMP involve a cAMP-dependent mechanism. We conclude that concentrations of IFN-gamma insufficient to induce Dk expression on Kgv cells may be capable of rendering the Dk gene responsive to signals that, in the absence of IFN-gamma treatment, have no effect on Dk expression.

Characterization of two independent mechanisms by which interferon-induced gene expression is down regulated

Interferons (IFNs) induce the expression of a variety of cellular RNAs. Phorbol esters can inhibit IFN-induced expression of some of these RNAs, including ISG-54K. The actions of phorbol esters on IFN-activated ISG-54K transcription are cell specific and are reversed by inhibitors of protein synthesis. In those cell lines in which phorbol esters inhibit IFN-induced ISG-54K transcription, prolonged IFN exposure also induces a "desensitized state" such that further IFN exposure no longer induces ISG-54K expression. IFN-induced desensitization is also reversed by inhibitors of protein synthesis. Experiments are described to determine whether the mechanism by which phorbol esters inhibit IFN-activated ISG-54K expression is the same as the mechanism by which prolonged exposure to IFN makes cells refractory to further induction of ISG-54K expression. Cultured cells treated with 12-O-tetradecanoylphorbol 13-acetate (TPA) for 72 h are desensitized to phorbol esters such that further addition of phorbols does not inhibit IFN-induced ISG-54K expression. In both naive and TPA-desensitized human fibroblasts or WISH cells, prolonged IFN treatment induced a desensitized state that was reversible by cycloheximide. This observation suggests that the mechanisms by which prolonged IFN treatment and phorbol esters inhibit ISG-54K expression are independent.