Eosinophilopoiesis at the cross-roads of research on development, immunity and drug discovery.

Eosinophils are a minority subpopulation of leukocytes whose roles in host defense against infection remain controversial, but which have been implicated in the pathogenesis of both acute allergic inflammation and the chronic bronchopulmonary remodelling in asthma. Eosinophilia, a hallmark of both helminth infections and atopic diseases, is maintained through upregulation of eosinophilopoiesis by means of increased production and effectiveness of Interleukin-5 (IL-5), a major Th2 cytokine. These mechanisms are further modulated by a wide variety of agents, including glucocorticoids, nonsteroidal anti-inflammatory drugs and mediators of inflammation. We review recent progress made by different groups in the study of eosinophilopoiesis that led to the identification of the heterogeneous targets for developmental regulation by IL-5 and other agents, and to the ongoing characterization of the molecular mechanisms that ensure their commitment to the eosinophil lineage. We argue that the study of eosinophilopoiesis provides insight into basic developmental processes, and especially into how modulators influence the constitutive rate of eosinophil production by controlling the rates of apoptosis and terminal differentiation. The mechanisms underlying the apparently paradoxical effects of dexamethasone, a drug widely employed to control inflammation, as well as the role of specific molecular targets (including inducible NO synthase and CD95/Fas) in developmental regulation, are discussed in detail. We further argue that eosinophilopoiesis offers unique insights of how immune and endocrine effector loops interact to control both the steady-state responses to IL-5 and the susceptibility to modulation of these responses by drugs and cytokines. We also review the existing evidence on the recruitment of circulating stem cells and progenitors into inflammatory sites, and on a critical role for IL-5 in the accumulation of eosinophil lineage-committed progenitors in lungs of allergic mice. Finally, we review recent progress in the study of the regulatory T cell populations present in bone-marrow, and discuss alternative mechanisms through which cellular immunity may influence eosinophilopoiesis.

Cells isolated from bone-marrow and lungs of allergic BALB/C mice and cultured in the presence of IL-5 are respectively resistant and susceptible to apoptosis induced by dexamethasone.

We have previously reported that, in IL-5-stimulated bone-marrow cultures, dexamethasone upregulates eosinophil differentiation and protects developing eosinophils from apoptosis induced by a variety of agents. Recently developed procedures for the isolation of hemopoietic cells from allergic murine lungs have enabled us to evaluate how these cells respond to dexamethasone in IL-5-stimulated cultures, when compared with bone-marrow-derived cells isolated from the same donors, and whether differences in response patterns were linked to apoptosis. Ovalbumin challenge of sensitized mice increased significantly the numbers of mature leukocytes as well as hemopoietic cells recovered from digested lung fragments, relative to saline-challenged, sensitized controls. Both mature eosinophils and cells capable of differentiating into eosinophils in the presence of IL-5 were present in lungs from sensitized mice 24 h after airway challenge. Dexamethasone strongly inhibited eosinophil differentiation in IL-5-stimulated cultures of lung hemopoietic cells. By contrast, dexamethasone enhanced eosinophil differentiation in cultures of allergic bone-marrow cells, in identical conditions. Hemopoietic cells from lungs and bone-marrow were respectively susceptible and resistant to induction of apoptosis by dexamethasone. The dexamethasone-sensitive step was the response to IL-5 in culture, while accumulation of IL-5 responsive cells in allergen-challenged lungs was dexamethasone-resistant. Cells from lungs and bone-marrow, cultured for 3 days with IL-5 in the absence of dexamethasone, did not respond to a subsequent exposure to dexamethasone in the presence of IL-5. These findings confirm that IL-5-responsive hemopoietic cells found in challenged, sensitized murine lungs differ from those in bone-marrow, with respect to the cellular responses induced by dexamethasone, including apoptosis.

The effects of allergen and anti-allergic drugs on murine hemopoietic cells: moving targets, unusual mechanisms, and changing paradigms.

We used a variety of techniques to evaluate the effects of airway allergen exposure in mice on the responses of hemopoietic cells to cytokines and drugs in vitro and in vivo. Initial studies have shown that allergen exposure of sensitized mice leads to release of circulating mediators, that induce rapid upregulation of bone-marrow responses to IL-5 and GM-CSF. This may be related to glucocorticoids, because exogenous dexamethasone has similar effects on cultured murine bone-marrow, and because stress-induced glucocorticoids, in naïve or sensitized mice, have effects indistinguishable from those of allergen challenge in sensitized animals. Upregulation of eosinophil production is associated with an increased expression of alpha4 integrins, which may contribute to retention of these cells in the bone-marrow. Glucocorticoids regulate the adhesiveness, maturation and survival of eosinophils in murine bone-marrow culture, partly by counteracting the actions of Prostaglandin E2 and possibly other prostanoids. Allergen exposure of sensitized mice leads to accumulation of hemopoietic progenitors in the lungs, which differ from those in bone-marrow in growth properties and sensitivity to glucocorticoids. Lung transplantation has been used to demonstrate that the lung acts as a source of endocrine factors that promote hemopoietic cell accumulation, independently of damage caused by local allergic inflammation.

Allergenic sensitization prevents upregulation of haemopoiesis by cyclo-oxygenase inhibitors in mice.

1. We evaluated whether immunization affects bone-marrow responses to indomethacin, because allergenic sensitization and challenge upregulate responses to haemopoietic cytokines (including IL-5-driven eosinopoiesis) in murine bone-marrow, while indomethacin upregulates haemopoiesis and protects bone-marrow from radiation damage. 2. Progenitor (semi-solid) and/or precursor (liquid) cultures were established from bone-marrow of: (a) normal mice; (b) ovalbumin-sensitized mice, with or without intranasal challenge. Cultures were established with GM-CSF (2 ng ml(-1)) or IL-5 (1 ng ml(-1)), respectively, alone or associated with indomethacin (10(-7) - 10(-11) M) or aspirin (10(-7) - 10(-8) M). Total myeloid colony numbers and numbers of eosinophil-peroxidase-positive cells were determined at day 7. 3. In naïve BALB/c mice, indomethacin (10(-7) - 10(-9) M) increased GM-CSF-stimulated myeloid colony formation (P=0.003 and P=0.009, respectively). In contrast, it had no effect on bone-marrow of ovalbumin-sensitized and challenged mice. Indomethacin (10(-7) - 10(-9) M) also increased eosinophil precursor responses to IL-5 in bone-marrow of naïve (P<0.001 and P=0.002 respectively), but not sensitized-challenged mice. Aspirin (10(-7) M) had similar effects, equally abolished by sensitization. Enhancement of haemopoiesis by indomethacin required adherent cells from naïve bone-marrow. Nonadherent cells responded to IL-5 but not to indomethacin. Indomethacin was effective on bone-marrow from sham-sensitized, ovalbumin-challenged, but not from sensitized, saline-challenged mice. Plasma transfer from immune mice abolished eosinophil precursor responses to indomethacin in bone-marrow of naïve recipients. This was not prevented by previous removal of antibody from immune plasma. 4. COX inhibitors enhance haemopoiesis in naïve but not allergic mice. Responsiveness to indomethacin can be abolished either by active sensitization or by immune plasma transfer. Specific antibody is not involved.

Prostaglandin E2 and dexamethasone regulate eosinophil differentiation and survival through a nitric oxide- and CD95-dependent pathway.

Apoptosis, involving both CD95/CD95L interactions and their modulation by nitric oxide (NO), is central to regulation of mature eosinophil numbers. However, its role in regulating eosinophil production from bone-marrow precursors is unknown. We examined the effects of prostaglandin E2 (PGE2) and dexamethasone on eosinophil differentiation and survival in murine bone-marrow cultures, and their relationship to: NO production as well as CD95/CD95L-dependent apoptosis. Bone-marrow cultures were established with IL-5, alone or in association with PGE2, dexamethasone or both. PGE2 (10(-7)M) inhibited eosinophil differentiation by selectively inducing apoptosis in developing eosinophils. Dexamethasone (10(-7)M) protected developing eosinophils from PGE2-induced apoptosis. Since dexamethasone prevents induction of nitric oxide synthase (NOS), we evaluated the role of NO in the effects of both PGE2 and dexamethasone. NO donors (SNAP and SNP) down-modulated eosinophil precursor responses to IL-5. SNAP induced apoptosis through a dexamethasone-resistant mechanism. The NOS inhibitors, Nomega-nitro-L-arginine and aminoguanidine, blocked the effects of PGE2 on developing eosinophils. PGE2 was ineffective in bone-marrow from knockout mice lacking inducible NOS. PGE2 up-regulated CD95 and CD95L expression in developing eosinophils. Neither PGE2 nor SNAP were effective in cultures from CD95L-deficient gld mice. These data suggest that PGE2 induces apoptosis in developing eosinophils through inducible NOS, leading to NO-dependent activation of the CD95L/CD95 pathway, while dexamethasone antagonizes the effects of PGE2 on the same targets.