Nitric oxide inhibits INF gamma-induced increases in CIITA mRNA abundance and activation of CIITA dependent genes - Class II MHC, Ii and H-2M

Background: Nitric oxide (NO) has been recently implicated as a powerful in hibitor of immune responses during allograft rejection, and some autoimmune and infectious diseases. We previously showed that one potential regulator y effect of NO is inhibition of IFN gamma-stimulated expression of Class a MHC on macrophages. Activation of this gene is mediated by the "Class II Tr ansActivator" (CIITA). We now ask whether NO inhibits CIITA and thus the fa mily of genes regulated by CIITA-Class II MHC, Ii, and H-2M. The latter two genes participate in antigen processing and formation of the cell-surface peptide-Class II MHC complex. Methods: Murine macrophages-both peritoneal m acrophages and the RAW264.7 macrophage line-were stimulated in vitro with I FN gamma. NO production was measured by the Greiss reagent. Transcription o f Class II MHC was measured by nuclear run-on assay. mRNA abundance of Clas s II MHC, Ii, H-2M, and CIITA was measured by Northern blotting and RT-PCR. Results: NO inhibits IFN gamma-induced increases in the abundance and tran scription of the Class II MHC Ab gene. The increases in mRNA abundance of C IITA, Ii, and H-2M are also inhibited. Pls a control, we found that NO did not inhibit LPS-induce increases in TNF alpha mRNA abundance. Conclusions: NO inhibits IFN gamma-induced increases in CIITA, and thus inhibits the CII TA-regulated genes: Class II MHC, Ii, and H-2M. Early during rejection, NO production by macrophages may result after stimulation by IFN gamma produce d by CD4+ T cells, and be an effector of allograft damage. High concentrati ons of NO may then act as a feedback inhibitor which decreases antigen pres entation by macrophages and thus decreases CD4 T cell activation.