Molecular mechanisms underlying the synergistic induction of CXCL10 by LPS and IFN-γ in human neutrophils

The CXCL10 chemokine is a critical chemoattractant for the recruitment of activated Th1 and NK cells into inflammatory sites. CXCL10 is typically produced by myeloid cells in response to IFN-gamma, as well as by neutrophils, though the latter require a costimulation with IFN-gamma and LPS. In this study, we investigated the molecular mechanism(s) whereby IFN-gamma and TLR4 ligation synergize to induce CXCL10 expression in neutrophils. By primary transcript real-time PCR analysis, we demonstrate that the CXCL10 gene is transcriptionally induced by the LPS plus IFN-gamma combination in neutrophils, consistent with previous studies showing that increased CXCL10 gene expression does not reflect enhanced mRNA stability. The IFN-gamma-induced STAT1 activation and the lipopolysaccharide (LPS)-induced NF-kappaB activation were not enhanced if neutrophils were exposed to both stimuli, whereas both transcription factors were activated by IFN-gamma or LPS in monocytes. Finally, pharmacological inhibitors of NF-kappaB demonstrated its role in the induction of CXCL10 expression by LPS plus IFN-gamma in neutrophils, and by LPS or IFN-gamma in monocytes. Together, these results suggest that in neutrophils, the synergy observed between LPS and IFN-gamma toward CXCL10 gene expression likely reflects the cooperative induction of the NF-kappaB and STAT1 transcription factors by LPS and IFN-gamma, respectively.