Novel lineage- and stage-selective effects of retinoic acid on mouse granulopoiesis: Blockade by dexamethasone or inducible NO synthase inactivation.

Despite the close relationship of eosinophils and neutrophils, these granulocyte lineages respond to distinct cytokines and play unique roles in immune responses. They nevertheless respond to shared physiological/pharmacological regulators, including glucocorticoids and retinoids, and to ubiquitous mediators, including NO. Others showed that, in humans, all-trans retinoic acid (ATRA) suppresses eosinophil differentiation, but promotes neutrophil differentiation. Mechanisms of dual co-regulation of physiological granulopoiesis were here examined in murine bone-marrow, a model system suitable for exploration of immunopharmacological mechanisms, given the availability of experimental resources, including mutant/knockout mouse strains. We examined the effects of ATRA on mouse eosinophil and neutrophil production, using wild-type (BALB/c, C57BL/6) and mutant (iNOS-, CD95L-, or CD95-KO) bone-marrow cultures, further assessing the modification of ATRA activity by dexamethasone and iNOS blockade. ATRA (10-10M) significantly decreased eosinophil production relative to IL-5 controls. This effect was iNOS-independent, but CD95L- and caspase-dependent, and prevented by dexamethasone (10M in vitro; 1-20mg·kg in vivo). In myeloid colony formation assays, ATRA markedly suppressed GM-CSF-responsive progenitors, through an iNOS-dependent, CD95-independent, dexamethasone-sensitive mechanism. By contrast, ATRA potently enhanced GM-CSF-dependent neutropoiesis in liquid culture from BALB/c or C57BL/6 bone-marrow. This novel stimulatory effect was resistant to dexamethasone and abolished in iNOS-KO bone-marrow. ATRA injections also induced lineage- and stage-selective effects on granulopoiesis in vivo. ATRA therefore co-regulates eosinophil and neutrophil production in murine bone-marrow through multiple lineage- and stage-selective mechanisms.