Tolerance to lipopolysaccharide in human blood monocytes

When monocytes are stimulated with Lipopolysaccharide (LPS), they efficiently produce cytokines like tumor necrosis factor (TNF). Upon secondary stimulation, this response is only minimal, and there is little TNF mRNA transcription, mRNA accumulation, and protein production. Studies with the monocytic cell line Mono Mac 6 have shown that in such tolerant cells the CD14 LPS receptor is still present, and the transcription factor NF-kB is still efficiently mobilized. This NF-kB complex has, however, a different composition, that does not transactivate TNF promoter reportergene constructs. We now show that similar mechanisms apply to primary blood monocytes. After primary stimulation these cells also produce high levels of TNF and then develop tolerance in that upon secondary challenge little TNF is produced. CD14 cell surface expression is unchanged or even increased in tolerant cells and NF-kB mobilization does still occur. The complex mobilized in such tolerant monocytes is, however, composed mainly of high mobility binding proteins. This indicates that p50 homodimers predominate in NF-kB complex of tolerant blood monocytes, similar to what has been reported for Mono Mac 6 cells. The data add to the notion that p50 binding to the cognate -kB DNA motif in the TNF promoter may be responsible for the unresponsiveness in LPS tolerance.