PRIMING OF HUMAN MONOCYTES FOR ENHANCED LIPOPOLYSACCHARIDE RESPONSES - EXPRESSION OF ALPHA-INTERFERON, INTERFERON REGULATORY FACTORS, AND TUMOR-NECROSIS-FACTOR

Culture of human monocytes with either granulocyte-macrophage colony-s timulating factor or gamma interferon (EFN-gamma) results in a primed state, during which these cells express heightened responses to bacter ial lipopolysaccharide (LPS). The production of IFN-alpha in response to LPS by human monocytes has an absolute requirement for priming. Tum or necrosis factor (TNF) expression is also greatly enhanced in primed monocytes after LPS stimulation, but unlike IFN-alpha, TNF is readily expressed in unprimed monocytes as well. In an effort to determine th e molecular events associated with IFN-alpha induction in this system, freshly isolated human monocytes were primed by culture with either I FN-gamma or granulocyte-macrophage colony-stimulating factor and then treated with LPS; expression of IFN-alpha subtype 2 (IFN-alpha2), IFN regulatory factors (IRFs), and TNF was assessed by Northern (RNA blot) analysis. IRF-1 mRNA is expressed at high levels in monocytes and is regulated by both LPS and priming cytokines, but its expression alone does not correlate with the induction of IFN-alpha2 expression. IRF-2 mRNA is expressed in a more gradual manner following LPS stimulation, implying a possible feedback mechanism for inhibiting IFN-alpha expres sion. However, nuclear run-on analysis indicates that EFN-alpha2 is no t transcriptionally modulated in this system, in striking contrast to TNF, which is clearly regulated at the transcriptional level. In addit ion, IFN-alpha2 mRNA accumulation is superinduced when primed monocyte s are treated with LPS plus cycloheximide, while TNF mRNA is relativel y unaffected. The results demonstrate that priming can affect subseque nt LPS-induced gene expression at different levels in human monocytes.