Induction of macrophage Ia antigen expression by rIFN-gamma and down-regulation by IFN-alpha/beta and dexamethasone are mediated by changes in steady-state levels of Ia mRNA

In previous studies, the induction of Ia antigens on murine peritoneal exudate macrophages by recombinant IFN-gamma (rIFN-gamma) and the antagonism of rIFN-gamma-induced Ia expression by the inhibitors IFN-alpha/beta and glucocorticoids have been examined. In this report, these findings have been extended to an analysis of total or cytoplasmic mRNA from macrophage cultures treated with rIFN-gamma in the absence or presence of these two inhibitors. Recombinant IFN-gamma induced a 5.7- to 6.5-fold increase in steady-state levels of Ia (A alpha-specific) mRNA. Coordinate increases in steady-state mRNA for A beta, and E alpha were observed in response to rIFN-gamma. Maximum induction occurred 24 hr post-treatment and required the continued presence of rIFN-gamma. Induction of A alpha-specific mRNA was sensitive to the protein synthesis inhibitor cycloheximide. Simultaneous treatment of macrophage cultures with rIFN-gamma and IFN-alpha/beta or the glucocorticoid dexamethasone (DEX) resulted in a significant decrease in steady-state, A alpha-specific mRNA levels compared with treatment with rIFN-gamma alone. This analysis suggests that both the induction of Ia expression by rIFN-gamma, and the antagonism of rIFN-gamma-induced Ia gene expression by IFN-alpha/beta and DEX, are regulated by cognate changes in Ia mRNA.     

Differential regulation of class II MHC determinants on macrophages by IFN-gamma and IL-4

Initiation of an immune response depends upon expression of class II MHC determinants on plasma membranes of APC. Murine peritoneal macrophages treated with either rIFN-gamma or rIL-4 display significantly more class II MHC determinants than untreated control cells. Analysis of the induction of macrophage Ia Ag by these cytokines showed considerable quantitative and qualitative differences. Maximal levels of Ia Ag induced in macrophages and detected by ELISA after IL-4 treatment at 48 h was about 80% of that induced by IFN-gamma. However, the frequency of Ia+ cells in replicate macrophage populations cultured for 48 h in excess concentrations of cytokine was 60 to 80% with IFN-gamma, 30 to 40% with IL-4, and 5% with medium alone. Thus, the subpopulation of macrophages able to respond to IL-4 for induction of Ia Ag expression was less than that able to respond to IFN-gamma. Expression of Ia Ag on macrophages continuously exposed to IFN-gamma was maximal at 48 h and remained at this high level through 6 days. Maximal Ia Ag expression for IL-4-treated cells was also detected at 48 h, but was not sustained with time in culture, and returned to base line by 4 days. A similar time course for levels of Ia-specific message in macrophages at various times after IFN-gamma and IL-4 treatment was detected by Northern dot blot analysis. Loss of Ia mRNA and Ag with time in culture in the IL-4 treated cells was not due to macrophage cell death, depletion of active cytokine, or presence of fluid-phase inhibitors. IL-4 unresponsive cells were fully capable of maximal response to IFN-gamma for Ia Ag induction. These findings suggest that IL-4 and IFN-gamma induce class II MHC determinants through different mechanisms which may provide discrete regulatory control of APC function.     

Inhibition of IFN-gamma-induced Ia antigen expression on synovial fibroblasts by IL-1

Naturally occurring substances capable of the negative regulation of class II molecules on synovial fibroblasts may play an important role in controlling the sustained immune processes ongoing in the rheumatoid joint. We report here that rIL-1 is capable of such a negative regulatory process. The simultaneous addition of rIL-1 and rIFN-gamma to rat synovial fibroblasts resulted in decreased Ia Ag and mRNA expression when compared with synovial fibroblasts treated with IFN-gamma alone. Both rIL-1 alpha and rIL-beta inhibited to a similar degree with the level of inhibition being dependent on both the concentration of IL-1 and IFN-gamma. Other cytokines, including IFN-alpha/beta, IL-2, and TNF, had no antagonistic effect on IFN-gamma-induced Ia expression. Time course experiments showed that IL-1 inhibited when present immediately before addition of IFN-gamma or when added during the first 24 h of IFN-gamma stimulation but not at later time points. Indomethacin failed to reverse the IL-1-mediated inhibition, despite the fact that exogenously added PGE2 also inhibited IFN-gamma-induced Ia expression. IL-1 treatment of synovial cells did not alter the ability of IFN-gamma to bind to the cells. These findings provide evidence for a negative regulatory role for IL-1 on synovial fibroblasts independent of PGE2 production and thus suggest that IL-1 is capable of both pro- and antiinflammatory actions within the rheumatoid joint.     

Down-regulation of macrophage Ia mRNA expression by interferon (IFN)-alpha and IFN-beta mediated by de novo synthesized protein

We investigated the regulation of class I and class II major histocompatibility complex (MHC) antigen expression of murine peritoneal macrophages (M phi) by interferons (IFNs) at the mRNA level. Enhancement of class I antigen expression by IFNs (IFN-alpha, beta, and gamma), induction of class II antigen expression by IFN-gamma, and inhibition of class II antigen expression by IFN-alpha or IFN-beta all corresponded to steady-state levels of these MHC-specific mRNAs. Cycloheximide (CHX), a protein synthesis inhibitor, was used to elucidate whether IFN regulation of MHC mRNA expression depends on the newly synthesized proteins. CHX concentration was carefully chosen so that M phi viability was not decreased, total protein synthesis was considerably but not completely inhibited, and suppression of surface class II expression was virtually perfect. Under these conditions CHX did not affect the levels of either class I or class II mRNA, but it prevented IFN-beta from interfering with class II mRNA induction by IFN-gamma. These results indicate that the augmentation of induction and/or accumulation of MHC mRNA by IFNs is not dependent on the de novo synthesis of protein, but the down-regulation of class II mRNA level by IFN-beta is mediated by some newly synthesized protein(s).     

Regulation of class II MHC molecules on human endothelial cells. Effects of IFN and dexamethasone

Human vascular endothelial cells normally do not express class II MHC molecules in culture. IFN-gamma has been shown to induce expression of class I and class II MHC molecules on endothelial cell cultures from umbilical cord. We could detect these Ag by FACS analysis when endothelial cells were cultured for 3 days in the presence of 200 to 1000 U/ml of rIFN-gamma. Among the class II MHC molecules, HLA-DR and -DP but not -DQ were consistently induced. Addition of rIFN-alpha-D/A to IFN-gamma-treated cells inhibited the expression of class II MHC but not class I MHC molecules. Furthermore, the inhibition was more pronounced when IFN-alpha-D/A was added before or simultaneously as IFN-gamma. Natural IFN-alpha also exhibited similar inhibition and its suppressive effect was abolished in the presence of anti-IFN-alpha antibody. On the contrary, dexamethasone, a known inhibitor of class II MHC molecules on murine macrophages, showed a slight enhancing effect on class II MHC Ag. These results suggest an immunoregulatory role for IFN-alpha on non-lymphoid cells and that controlling elements for expression of class II MHC molecules may be different on various cell types as well as species.