The Need to Consider Context in the Evaluation of Anti-infectious and Immunomodulatory Effects of Vitamin A and its Derivatives.

Vitamin A and its derivatives (retinoids) act as potent regulators in many aspects of mammalian reproduction, development, repair, and maintenance of differentiated tissue functioning. Unlike other vitamins, Vitamin A and retinoids, which have hormonal actions, present significant toxicity, which plays roles in clinically relevant situations, such as hypervitaminosis A and retinoic acid ("differentiation") syndrome. Although clinical presentation is conspicuous in states of insufficient or excessive Vitamin A and retinoid concentration, equally relevant effects on host resistance to specific infectious agents, and in the general maintenance of immune homeostasis, may go unnoticed, because their expression requires either pathogen exposure or the presence of inflammatory co-morbidities. There is a vast literature on the roles played by retinoids in the maintenance of a tolerogenic, noninflammatory environment in the gut mucosa, which is considered by many investigators representative of a general role played by retinoids as anti-inflammatory hormones elsewhere. However, in the gut mucosa itself, as well as in the bone marrow and inflammatory sites, context determines whether one observes an anti-inflammatory or proinflammatory action of retinoids. Both interactions between specialized cell populations, and interactions between retinoids and other classes of mediators/regulators, such as cytokines and glucocorticoid hormones, must be considered as important factors contributing to this overall context. We review evidence from recent studies on mucosal immunity, granulocyte biology and respiratory allergy models, highlighting the relevance of these variables as well as their possible contributions to the observed outcomes.

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 (10-6-10-8M) significantly decreased eosinophil production relative to IL-5 controls. This effect was iNOS-independent, but CD95L- and caspase-dependent, and prevented by dexamethasone (10-7M in vitro; 1-20mg·kg-1 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.

Essential roles of PKA, iNOS, CD95/CD95L, and terminal caspases in suppression of eosinopoiesis by PGE2 and other cAMP-elevating agents.

Up- and downregulation of eosinopoiesis control pulmonary eosinophilia in human asthma. In mice, eosinopoiesis is suppressed in vitro by prostaglandin E2 (PGE2) and in vivo by diethylcarbamazine, through a proapoptotic mechanism sequentially requiring inducible NO synthase (iNOS) and the ligand for death receptor CD95 (CD95L). We examined the roles of iNOS, cAMP-mediated signaling, caspases, and CD95L/CD95 in suppression of eosinopoiesis by PGE2 and other agents signaling through cAMP. Bone-marrow collected from BALB/c mice, or from iNOS-, CD95-, or CD95L-deficient mutants (and wild-type controls), was cultured with interleukin-5 (IL-5), alone or associated with PGE2, cAMP-inducing/mimetic agents, caspase inhibitor zVAD-fmk, iNOS inhibitor aminoguanidine, or combinations thereof, and eosinopoiesis was evaluated at various times. PGE2, added up to 24 hours of culture, dose-dependently suppressed eosinopoiesis, by inducing apoptosis. This effect was (a) paralleled by induction of iNOS in eosinophils; (b) duplicated by sodium nitroprusside, isoproterenol, and cAMP-inducing/mimetic agents; (c) prevented by protein kinase A inhibition. NO was produced through iNOS by dibutyryl-cAMP-stimulated bone-marrow. Overall, PGE2 and isoproterenol shared a requirement for four effector elements (iNOS, CD95L, CD95, and terminal caspases), which together define a pathway targeted by several soluble up- and downmodulators of eosinopoiesis, including drugs, mediators of inflammation, and cytokines.

Inducible nitric oxide synthase/CD95L-dependent suppression of pulmonary and bone marrow eosinophilia by diethylcarbamazine.

RATIONALE: The mechanism of action of diethylcarbamazine (DEC), an antifilarial drug effective against tropical pulmonary eosinophilia, remains controversial. DEC effects on microfilariae depend on inducible NO synthase (iNOS). In eosinophilic pulmonary inflammation, its therapeutic mechanism has not been established. We previously described the rapid up-regulation of bone marrow eosinophilopoiesis in ovalbumin (OVA)-sensitized mice by airway allergen challenge, and further evidenced the down-regulation of eosinophilopoiesis by iNOS- and CD95L-dependent mechanisms. OBJECTIVES: We investigated whether: (1) DEC can prevent the effects of airway challenge of sensitized mice on lungs and bone marrow, and (2) its effectiveness depends on iNOS/CD95L. METHODS: OVA-sensitized BALB/c mice were intranasally challenged for 3 consecutive days, with DEC administered over a 12-, 3-, or 2-day period, ending at the day of the last challenge. We evaluated: (1) airway resistance, cytokine (IFN-gamma, IL-4, IL-5, and eotaxin) production, and pulmonary eosinophil accumulation; and (2) bone marrow eosinophil numbers in vivo and eosinophil differentiation ex vivo. MEASUREMENTS AND MAIN RESULTS: DEC effectively prevented the effects of subsequent challenges on: (1) airway resistance, Th1/Th2 cytokine production, and pulmonary eosinophil accumulation; and (2) eosinophilopoiesis in vivo and ex vivo. Recovery from unprotected challenges included full responses to DEC during renewed challenges. DEC directly suppressed IL-5-dependent eosinophilopoiesis in naive bone marrow. DEC was ineffective in CD95L-deficient gld mice and in mice lacking iNOS activity because of gene targeting or pharmacological blockade. CONCLUSIONS: DEC has a strong impact on pulmonary eosinophilic inflammation in allergic mice, as well as on the underlying hemopoietic response, suppressing the eosinophil lineage by an iNOS/CD95L-dependent mechanism.