DIFFERENTIAL EXPRESSION OF INTERFERON REGULATORY FACTOR-1 (IRF-1), IRF-2, AND INTERFERON CONSENSUS SEQUENCE BINDING-PROTEIN GENES IN LIPOPOLYSACCHARIDE (LPS)-RESPONSIVE AND LPS-HYPORESPONSIVE MACROPHAGES

Macrophages secrete interferon (IFN), as well as other cytokines, foll owing lipopolysaccharide (LPS) stimulation. The interferon regulatory factors (IRFs) comprise a family of DNA-binding proteins that have bee n implicated in the transcriptional regulation of IFN and certain IFN- inducible genes. We therefore characterized basal and LPS-inducible le vels of IRF-1, IRF-2, and interferon consensus sequence binding protei n (ICSBP) mRNA in LPS-responsive macrophages and compared the expressi on of these genes in macrophages that typify two murine models of LPS hyporesponsiveness. In the first model, the LPS-hyporesponsive phenoty pe of the C3H/HeJ mouse is genetically determined and maps to the Lps locus on mouse chromosome 4. In the second model, normally LPS-respons ive macrophages acquire a transient LPS-hyporesponsive phenotype follo wing a prior exposure to LPS, a phenomenon referred to as ''endotoxin tolerance.'' Using reverse transcription PCR, we detected basal levels of IRF-1 mRNA in LPS-responsive (Lps(R)) macrophages that were approx imately 15 times higher than those found in LPS-hyporesponsive (Lps(R) ) macrophages. Conversely, Lps(d) macrophages expressed basal levels o f IRF-2 mRNA that were approximately 18 times higher than those expres sed in Lps(n) macrophages. LPS stimulation resulted in a dose- and tim e-dependent accumulation of IRF-1, IRF-2, and ICSBP mRNA only in Lps(n ) macrophages. Cycloheximide inhibited the accumulation of LPS stimula ted IRF-2 and ICSBP mRNA, but not IRF-1 mRNA, thus designating IRF-1 a n immediate-early, LPS-inducible gene. Finally, macrophages rendered t olerant to endotoxin expressed elevated but nonmaximal mRNA levels for all three transcription factors that are not reinduced upon secondary challenge with LPS. Thus, the IRPs may represent yet an additional mo lecular pathway in the complex response to LPS.