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

Pro-inflammatory effects of all-trans retinoic acid in experimental acute inflammation - insights into eosinophil and neutrophil dynamics

CONTEXT

All-trans retinoic acid (ATRA), a metabolite of vitamin A, regulates embryogenesis, regeneration, hematopoiesis, differentiation, and apoptosis. It also exerts immunomodulatory effects and is used in inflammatory disease models.

OBJECTIVE

This study aimed to investigate the paradoxical pro-inflammatory effects of ATRA on eosinophil and neutrophil recruitment and activation.

MATERIALS AND METHODS

We used thioglycolate- and zymosan-induced peritonitis models in mice to evaluate leukocyte recruitment following ATRA treatment. The roles of inducible nitric oxide synthase (iNOS), tumor necrosis factor (TNF), and the 5-lipoxygenase (5-LO) pathway were assessed using genetically deficient mice and pharmacological inhibitors.

RESULTS AND DISCUSSION

ATRA increased total leukocyte, eosinophil, and neutrophil counts in peritoneal exudates, enhancing the response to both thioglycolate and zymosan. The effects were microenvironment-dependent and likely mediated by local release of pro-inflammatory cytokines and chemokines. iNOS was required for eosinophil recruitment, while TNF contributed to both eosinophil and neutrophil recruitment. The 5-LO pathway was essential for eosinophil involvement. These findings suggest that ATRA can paradoxically enhance inflammation by modulating innate immune cell responses.

CONCLUSIONS

ATRA promotes inflammation through iNOS, TNF, and 5-LO-dependent pathways, revealing complex mechanisms of immune modulation with potential relevance for inflammatory disease management.

Coordinated regulation of eosinophil production and migration by glucocorticoids, prostaglandins, and cysteinyl-leukotrienes

INTRODUCTION

The spectrum of eosinophil functions has expanded from fighting helminths to multiple novel roles in malignancy, infection, cancer, and metabolism. In asthma, glucocorticoids, prostaglandins (PG), and cysteinyl-leukotrienes (LT) regulate eosinophil biology through separate signaling pathways. Here we've evaluated the complex interplay between Dexa, PGE2, and CysLTs in eosinopoiesis and eosinophil biology in an allergic asthma model.

METHODOLOGY

We used different inbred mouse strains to probe the interactions between these agents in eosinophil differentiation and maturation in bone marrow culture. Flow cytometry and histological analyses evaluated eosinophil precursor proliferation, maturation, and VLA-4 expression. The in vivo function of eosinophils was assessed by their in vivo migration into allergen-challenged sites.

RESULTS

Eosinophil production in IL-5-stimulated bone marrow cultures is enhanced by dexamethasone but suppressed by PGE2, which triggers eosinophil apoptosis via inducible NO synthase (iNOS). Dexamethasone-primed cultures contain mostly immature eosinophils; by contrast, dexamethasone associated with PGE2 leads to the production of mature eosinophils (without inducing eosinophil apoptosis), by a mechanism independent of iNOS. Interaction between dexamethasone and LTD4 in culture produces mostly mature eosinophils expressing VLA-4, capable of migration into the lungs in ovalbumin-sensitized and -challenged mice.

DISCUSSION

The combination of dexamethasone and either PGE2 or LTD4 - both mediators of allergic inflammation - supports the maturation of eosinophils (overcoming the maturation blockade observed with dexamethasone alone), which are functional in an in vivo model of asthma.

nNOS induction and NOSIP interaction impact granulopoiesis and neutrophil differentiation by modulating nitric oxide generation

Nitric oxide (NO), a versatile free radical and a signalling molecule, plays an important role in the haematopoiesis, inflammation and infection. Impaired proliferation and differentiation of myeloid cells lead to malignancies and Hematopoietic deficiencies. This study was aimed to define the role of nNOS derived NO in neutrophil differentiation (in-vitro) and granulopoiesis (in-vivo) using multipronged approaches. The results obtained from nNOS over-expressing K562 cells revealed induction in C/EBPα derived neutrophil differentiation as evident by an increase in the expression of neutrophil specific cell surface markers, genes, transcription factors and functionality. nNOS mediated response also involved G-CSFR-STAT-3 axis during differentiation. Consistent increase in NO generation was observed during neutrophil differentiation of mice and human CD34⁺ HSPCs. Furthermore, granulopoiesis was abrogated in the nNOS inhibitor treated mice, depicting a decrease in the numbers of BM mature and progenitor neutrophils. Likewise, in vitro inhibition of nNOS in human CD34⁺ HSPCs indicated an indispensable role of nNOS in neutrophil differentiation. Expression of nNOS inhibitory protein, NOSIP was significantly and consistently decreased during the final stage of differentiation and was linked with the augmentation in NO release. Moreover, neutrophils from CML patients had more NOSIP and less NO generation as compared to the PMNs from healthy individuals. The present study thus indicates a critical role of nNOS, and its interaction with NOSIP during neutrophil differentiation. The study also highlights the importance of nNOS in the neutrophil progenitor proliferation and differentiation warranting investigations to assess its role in the haematopoiesis-related disorders.

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.