Block of Stat-1 activation in macrophages phagocytosing bacteria causes reduced transcription of CIITA and consequent impaired antigen presentation.

The IFN-gamma-induced HLA class II expression in human macrophages was drastically reduced after phagocytosis of Escherichia coli. HLA class II down-modulation depended on phagocytosis of bacteria and could not be reproduced by phagocytosis of inert particles or by treatment with lipopolysaccharide. Study of the kinetics and molecular analysis showed that class II molecules and corresponding mRNA were up-regulated at 6 h after phagocytosis of bacteria. Subsequently, a progressive reduction of mRNA occurred, and, at 72 h, as little as 25% of the class II mRNA level of IFN-gamma-treated control cells was found. This was due to reduced transcription of the class II transcriptional activator CIITA, as a consequence of reduced immediate-early inducible factor (IRF-1) and particularly of reduced phosphorylated Stat-1 homodimers, nuclear factors both necessary for optimal triggering of the CIITA promoter. Failure to sustain IFN-gamma-induced CIITA up-modulation during phagocytosis of bacteria had functional implications, as human macrophages could not adequately process and present antigenic peptides to HLA-DR-restricted antigen-specific T cells. This is the first evidence that phagocytosis of bacteria can down-modulate HLA class II expression in normal human macrophages by acting at the level of expression of CIITA.