Type I interferon regulates cytokine-delayed neutrophil apoptosis, reactive oxygen species production and chemokine expression.

Interferons (IFNs) are key regulators of a number of inflammatory conditions in which neutrophils play an important role in pathology, such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE), where type I IFNs are implicated in disease pathology. However, IFNs are usually generated in vivo together with other cytokines that also have immunoregulatory functions, but such interactions are poorly defined experimentally. We measured the effects of type I (IFN-α) IFN, elevated in both RA and SLE, on the functions of healthy neutrophils incubated in vitro in the absence and presence of proinflammatory cytokines typically elevated in inflammatory diseases [tumour necrosis factor (TNF-α), granulocyte-macrophage colony-stimulating factor (GM-CSF)]. IFN-α alone had no effect on neutrophil apoptosis; however, it abrogated the anti-apoptotic effect of GM-CSF (18 h, P < 0·01). The enhanced stability of the anti-apoptotic protein myeloid cell leukaemia 1 (Mcl-1) and delayed activation of caspase activation normally regulated by GM-CSF were blocked by IFN-α: this effect was mediated, in part, by activation of p38 mitogen-activated protein kinase (MAPK). IFN-α alone also primed reactive oxygen species (ROS) production and maintained the transient priming effect of TNF-α for up to 4 h: it also down-regulated GM-CSF- and TNF-α-activated expression of chemokine (C-X-C motif) ligand (CXCL)1, CXCL2, CXCL3, CXCL8, CCL3 and CCL4 but, in contrast, increased the expression of CXCL10. These novel data identify complex regulatory signalling networks in which type I IFNs profoundly alter the response of neutrophils to inflammatory cytokines. This is likely to have important consequences in vivo and may explain the complexity and heterogeneity of inflammatory diseases such as RA, in which multiple cytokine cascades have been activated.

The CDK inhibitor purvalanol A induces neutrophil apoptosis and increases the turnover rate of Mcl-1: potential role of p38-MAPK in regulation of Mcl-1 turnover.

Human neutrophils are terminally differentiated cells that do not replicate and yet express a number of enzymes, notably cell cycle-dependent kinases (CDKs), that are associated normally with control of DNA synthesis and cell cycle progression. In neutrophils, CDKs appear to function mainly to regulate apoptosis, although the mechanisms by which they regulate this process are largely unknown. Here we show that the CDK2 inhibitor, purvalanol A, induces a rapid decrease in myeloid cell leukaemia factor-1 (Mcl-1) levels in human neutrophils and peripheral blood mononuclear cells (PBMCs), but only induces apoptosis in neutrophils which are dependent upon expression on this protein for survival. This rapid decrease in cellular Mcl-1 protein levels was due to a purvalanol A-induced decrease in stability, with the half-life of the protein decreasing from approximately 2 h in control cells to just over 1 h after addition of the CDK2 inhibitor: it also blocked the granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent stabilization of Mcl-1. Purvanalol A blocked GM-CSF-stimulated activation of extracellular-regulated kinase (Erk) and signal transducer and activator of transcription (STAT)-3, and stimulated an additive activation of protein kinase B (Akt) with GM-CSF. Purvalanol A alone stimulated a rapid and sustained activation of p38-mitogen-activated protein kinase (MAPK) and the pan p38-MAPK inhibitor, BIRB796, partly blocked the purvalanol A-induced apoptosis and Mcl-1 loss. These novel effects of purvalanol A may result, at least in part, from blocking GM-CSF-mediated Erk activation. In addition, we propose that purvalanol A-induced activation of p38-MAPK is, at least in part, responsible for its rapid effects on Mcl-1 turnover and acceleration of neutrophil apoptosis.