Contribution of basophil/mast cell and eosinophil growth and differentiation to the allergic tissue inflammatory response

Studies of the role of basophils in aspirin-exacerbated respiratory disease pathogenesis

BACKGROUND

Aspirin-exacerbated respiratory disease (AERD) is characterized by the triad of chronic rhinosinusitis with nasal polyps (CRSwNP), asthma, and intolerance to cyclooxygenase-1 enzyme inhibitors. The underlying mechanisms contributing to AERD pathogenesis are not fully understood, but AERD is characterized by an enhanced type 2 inflammatory phenotype. Basophils are potent type 2 effector cells, but their involvement in AERD pathophysiology remains unclear.

OBJECTIVE

We sought to characterize the systemic and local basophil responses in patients with AERD compared with patients with CRSwNP.

METHODS

Sinonasal tissues including inferior turbinate and/or nasal polyps (NPs) and peripheral blood were collected from controls, patients with AERD, and patients with CRSwNP. Expression of cell surface (CD45, FcεRI, CD203c), activation (CD63), and intracellular (2D7) markers associated with basophils was characterized using flow cytometry. Clinical data including Lund-Mackay scores and pulmonary function were obtained.

RESULTS

The mean number of basophils (CD45⁺CD203c⁺FcεRI⁺CD117^-) detected in AERD NPs (147 ± 28 cells/mg tissue) was significantly elevated compared with that detected in CRSwNP NPs (69 ± 20 cells/mg tissue; P = .01). The number of circulating basophils was significantly elevated in patients with AERD (P = .04). Basophils in NPs had significantly higher CD203c and CD63 mean fluorescence intensity compared with blood in both conditions (P < .01). Basophils from AERD NPs had lower expression of the granule content marker 2D7 compared with those from matched blood (P < .01) or NPs of patients with CRSwNP (P = .06), suggesting ongoing degranulation. Basophil 2D7 mean fluorescence intensity significantly correlated with pulmonary function (r = 0.62; P = .02) and inversely correlated with sinonasal inflammation (r = -0.56; P = .004).

CONCLUSIONS

Increased basophil numbers and extent of ongoing degranulation in NPs of patients with AERD compared with patients with CRSwNP may contribute to the exaggerated disease pathogenesis and severity unique to AERD.

Stability of Syk protein and mRNA in human peripheral blood basophils

In human basophils, Syk expression is 10-fold lower than most other types of leukocytes. There are indirect studies that suggest that Syk protein is highly unstable (a calculated half-life less than 15 min) in human peripheral blood basophils. Therefore, in these studies, Syk stability was directly examined. Purified basophils were metabolically labeled and a pulse-chase experimental design showed Syk protein to be stable in the time frame of 12 h (95% likelihood that half-life is more than 12 h). However, its synthetic rate was very slow (∼10-fold slower) compared with CD34-derived basophils, which have been shown to express levels of Syk consistent with other mature circulating leukocytes. Syk mRNA expression was found to be 5-30-fold lower than other cell types (CD34-derived basophils, peripheral blood eosinophils, and plasmacytoid dendritic cells). Syk protein and mRNA levels, across cell types, were relatively concordant. Syk mRNA in basophils showed a half-life of 3.5 h, which was greater than that of interleukin-4 or Fc epsilon receptor I-α mRNA (∼2 h), but somewhat shorter than Fc epsilon receptor I-β mRNA (8 h). A comparison of miR expression between CD34-derived and peripheral blood basophils demonstrated only 1 significant increase, in miR-150 (77-fold). Transfection in human embryonic kidney cells of a stabilized form of miR-150 showed that it modified expression of c-Myb mRNA but not of Syk mRNA or protein. These results suggest that low Syk expression in basophils results, not from protein instability and perhaps not from mRNA stability. Instead, the results point to the transcriptional nature of an important point of regulation.

Role of eosinophils and their clinical significance in allergic inflammation

Eosinophils are believed to play roles in the pathophysiology of allergic inflammation, such as bronchial asthma. However, recent studies on anti-interleukin-5 monoclonal antibody treatment of asthmatic patients raised the possibility that eosinophils may play only a limited role. More recent studies established that eosinophils are essentially involved in the development of airway remodeling. Moreover, it is theoretically conceivable that eosinophils are a cellular source of lipid mediators, such as cysteinyl leukotrienes or platelet-activating factor in asthma. Even in the absence of interleukin-5, it is likely that the 'T-helper Type 2 network', including a cascade of vascular cell adhesion molecule-1, intercellular cell adhesion molecule-1, CC chemokines, granulocyte-macrophage colony-stimulating factor, for example, can maintain sufficient eosinophilic infiltration and effector functions, such as superoxide anion generation and degranulation. Long-term studies, wherein tissue eosinophils are eliminated effectively will be required to establish the exact roles of these cells in asthma. Finally, the authors will demonstrate that eosinophils have the potential for not only playing detrimental roles but also beneficial ones.

Cysteinyl leukotrienes: multi-functional mediators in allergic rhinitis

Cysteinyl leukotrienes (CysLTs) are a family of inflammatory lipid mediators synthesized from arachidonic acid by a variety of cells, including mast cells, eosinophils, basophils, and macrophages. This article reviews the data for the role of CysLTs as multi-functional mediators in allergic rhinitis (AR). We review the evidence that: (1) CysLTs are released from inflammatory cells that participate in AR, (2) receptors for CysLTs are located in nasal tissue, (3) CysLTs are increased in patients with AR and are released following allergen exposure, (4) administration of CysLTs reproduces the symptoms of AR, (5) CysLTs play roles in the maturation, as well as tissue recruitment, of inflammatory cells, and (6) a complex inter-regulation between CysLTs and a variety of other inflammatory mediators exists.

Allergen-induced traffic of bone marrow eosinophils, neutrophils and lymphocytes to airways

We evaluated whether bone marrow (BM) inflammatory cells have capacity to traffic into the airways following allergen exposure in a mouse model of allergen-induced airway inflammation. We also evaluated the effect of IL-5 overexpression on (i) the production of eosinophils in BM, (ii) the accumulation of eosinophils, neutrophils and lymphocytes in blood and airways and (iii) the changes in CD34+ cell numbers in BM, blood and airways. Bromodeoxyuridine (BrdU) was used to label cells produced during the exposure period. Furthermore, CD3 splenocytes were adoptively transferred to investigate the BM inflammatory response. Allergen exposure induced traffic of BM eosinophils, neutrophils and lymphocytes to the airways and increased the number of BrdU+ eosinophils, neutrophils, lymphocytes and CD34+ cells in BALf. IL-5 overexpression enhanced the eosinophilopoiesis and increased the presence of BrdU+ eosinophils and CD34+ cells in airways and enhanced the number of CD34+ cells in BM and blood after allergen exposure. Adoptive transfer of CD3 lymphocytes overexpressing IL-5 caused increased BM eosinophilia. In conclusion, allergen exposure induces traffic of not only newly produced eosinophils but also newly produced neutrophils and lymphocytes into the airways.

Links between rhinitis and asthma

There is compelling evidence of a close relationship between the upper and lower airways in asthma and rhinitis. Rhinitis is present in the majority of patients with asthma, and a significant minority of patients with rhinitis have concomitant asthma. Similarities between the two conditions occur in the nature of the inflammation present in the target tissues. A common initiating step in the inflammatory process of allergic airways disease is the presence of immunoglobulin E providing an adaptor molecule between the offending allergen and inflammatory cell activation and mediator release. Differences in the two conditions arise largely from the structural differences between the nose and the lungs. In an asthmatic, concomitant allergic rhinitis increases healthcare costs and further impairs quality of life. The presence of rhinitis should always be investigated in children and young adults with asthma. Subjects with allergic rhinitis have an increased risk of developing asthma and may form a suitable population for secondary intervention to interrupt the 'allergic march'.

Mediators in nasal polyposis

Nasal polyposis (NP) is a chronic inflammatory disease of the sinuses often associated with asthma. Although we have not yet achieved a full understanding of the precise mechanisms underlying the pathogenesis of NP, recent insights have been acquired into the regulation of eosinophil chemotaxis, activation, and survival, in addition to their possible link to gross histopathologic changes such as pseudocyst formation. Interleukin (IL)-5, transforming growth factor-beta(1), and eotaxin seem to be crucial players in the regulation of eosinophilic inflammation and extracellular matrix breakdown. The cytokine pattern in NP assumes neither a T helper 1 (Th1) nor Th2 type predominance, because IL-4, IL-5, IL-12, and interferon-gamma have all been shown to be upregulated in NP tissue, without influence of the atopic status. However, recent studies have demonstrated a strong local upregulation of the immunoglobulin E (IgE) synthesis with the formation of specific IgE to Staphylococcus aureus enterotoxins, suggesting a possible role of superantigens in these pathologic processes.

Expression of interferon-gamma, interleukin-4 and interleukin-5 mRNA in the nasal mucosal membrane of rats with allergic rhinitis

The production of immunoglobulin E (IgE) antibody is largely dependent on the ratio between interleukin-4 (IL-4) (a T helper 2 (Th2)-type cytokine) and interferon-gamma (IFN-gamma) (a T helper 1 (Th1)-type cytokine). Interleukin-5 (IL-5) (also a Th2-type cytokine) is an important eosinophil differentiation factor and also co-stimulates B-cell growth and differentiation. The present study was designed to evaluate and compare the expression of IFN-gamma, IL-4 and IL-5 mRNA in the nasal mucosal membrane of sensitized Brown-Norway (BN) rats. Fourteen BN rats were divided into two groups: non-sensitized (control) and sensitized. The sensitized group was injected with ovalbumin (OA) intraperitoneally on three consecutive days. Twenty-one days later, rats were exposed to 1% OA aerosol. Twenty-four hours after exposure to aerosol, nasal mucosa was extracted from both groups and reverse transcriptase-polymerase chain reaction (RT-PCR) was performed. The densities of the bands of IL-4, IL-5 and IFN-gamma mRNA were expressed as percentages against beta-actin mRNA. Our results showed that the mean values for IL-4 and IL-5 mRNA were increased significantly in sensitized rats compared with control rats. In contrast, the mean value for IFN-gamma mRNA was significantly lower in sensitized rats compared with those of the control group. Our data therefore suggest that sensitization of rat nasal mucous membranes results in the predominant expression of Th2-type cytokines.

Quantitative cytology of nasal secretions and scrapings in Japanese cedar pollinosis patients

To explore the dynamics of the cellular response to natural allergen exposure, the authors of this study performed quantitative cytology in both the nasal secretions and nasal scrapings of 16 normal control subjects and 23 Japanese cedar pollinosis patients before and during the pollen season. In nasal scrapings, the number of metachromatic cells increased significantly during the pollen season, although there was no significant difference between the normal subjects and the preseason pollinosis patients. The cell differential count in the patients with pollinosis showed that lymphocytes were predominant in scrapings; however, the majority of inflammatory cells in the nasal secretions were neutrophils before the season and eosinophils during the season. The study findings suggest that the appearance of metachromatic cells in nasal scrapings is specific to the pollen exposure and that the cytology in nasal secretions is significantly different from that in nasal scrapings.

Effect of terfenadine and budesonide on nasal symptoms, olfaction, and nasal airway patency following allergen challenge

The study investigated the effect of the oral H1-blocker terfenadine on allergen challenge in subjects with nasal allergy in comparison with the topical steroid, budesonide. A randomized, placebo-controlled, double-blind, crossover study with 3 experimental days was performed outside the pollen season. Seventeen nonsmokers with hay fever (symptoms, positive skin prick test, and RAST against timothy) were treated for 14 days before each experimental day, where the response to nasal challenge with four different concentrations of timothy was measured every 15 min for 6 h. The nasal cavity dimensions were measured by acoustic rhinometry and the olfactory function as the threshold for the sense of smell of butanol. Nasal symptoms were determined by questionnaires. Both terfenadine and budesonide dry powder had an effect on the hay fever symptoms during nasal pollen challenge. Terfenadine was more efficient than budesonide against histamine-mediated symptoms such as sneezing and itching. Budesonide increased nasal airway dimensions better than terfenadine (P < 0.01). A marked effect of budesonide was seen 1-2 h after challenge, suggesting an effect on "early late phase" reaction in the nose. In 7/17 subjects, a significant (P < 0.05) improvement of olfactory function after budesonide treatment was seen. In conclusion, topical steroid (budesonide) is superior to antihistamine (terfenadine) in treatment of nasal congestion in hay fever, especially for the postchallenge reaction, and may, in some cases, relieve the decreased sense of smell during pollen challenge.

Lipopolysaccharide-induced pleural neutrophil accumulation depends on marrow neutrophils and platelet-activating factor

The involvement of platelet-activating factor (PAF) in lipopolysaccharide (LPS)-induced leukocyte accumulation in the rat pleural cavity was investigated. Intrathoracic (i.t.) injection of LPS (250 ng/cavity) induced a marked increase in the number of neutrophils at 1 h, which was maximum within 6-12 h, reducing after 24 h. In parallel, an increase in blood neutrophil counts within 1-6 h, concomitantly with a reduction in the number of these cells in the bone marrow, was observed. The number of eosinophils recovered from LPS-injected pleural cavity increased at 12 h and was maximum within 24-48 h. No change in blood or bone marrow eosinophil counts was detected. The pretreatment with WEB 2086 or PCA 4248 (20 mg/kg) significantly inhibited pleural neutrophil accumulation, blood neutrophilia and the decrease in the marrow neutrophil content, but not eosinophil accumulation. The blood neutrophilia and the decrease in marrow neutrophil counts induced by the intravenous (i.v.) injection of LPS (250 ng) were significantly lower than those observed after i.t. injection. Furthermore, WEB 2086 and PCA 4248 were ineffective against the systemic alteration induced by i.v. LPS. it was concluded that LPS-induced neutrophil, but not eosinophil, accumulation in the pleural cavity is related to the mobilization of neutrophils from the bone marrow and involves PAF dependent mechanisms.

Lymphocyte infiltration and thickness of the nasal mucous membrane in perennial and seasonal allergic rhinitis

We have used immunocytochemical techniques to study infiltration by lymphocytes in biopsy specimens of the nasal mucosal membrane in 24 atopic patients and 10 normal volunteers. Twelve patients had perennial rhinitis and 12 had seasonal allergic rhinitis (SR) to grass pollen. Biopsy specimens were taken both in and out of the pollen season in patients with SR. Biopsy specimens were strained with the indirect immunoperoxidase technique and monoclonal antibodies to CD3, CD4, CD8, CD22, and CD25. T helper cells (CD4+) and CD24+ cells were significantly more numerous in patients exposed to allergen (those with perennial rhinitis and SR in season) compared with normal volunteers, whereas values for SR out of season were intermediate. The thickness of the nasal epithelium was significantly (p < 0.05) greater in biopsy specimens from patients with perennial rhinitis (mean, 51.43 microns) than in those from patients with SR in season (median, 32.44 microns). These results suggest that in allergic rhinitis, natural exposure to allergen is accompanied by increased infiltration of the nasal mucous membrane by T-helper and CD25+ cells.

Eosinophils: a review

The eosinophil was discovered by Jones in 1846 (Dessein and David, 1982) but its proclivity to stain with aniline dyes was first described by Paul Ehrlich in 1879 (Hirsch and Hirsch, 1980). Recognized and named for this quality, eosinophils possess an abundance of highly basic proteins within their granules which confer their affinity for acidic dyes (Gleich and Loegering, 1984). Eosinophils are traditionally viewed as killer-effector cells in parasitic infestations and as modulators of Type I hypersensitivity reactions (Butterworth and David, 1981; Kay, 1985). The eosinophils' reserve of cationic proteins and enzymes which imparts their profound parasiticidal effects (Butterworth and David, 1981) contrasts with this leukocyte's purported regulatory function in inflammation (Kay, 1985; Fechter et al., 1986). The opposing functions possessed by this leukocyte exemplify the enigma of the eosinophil. Recent evidence suggests that although the eosinophil does posses some regulatory capabilities, its presence is, in fact, a harbinger of tissue destruction (Gleich and Adolphoson, 1986, Wardlaw and Kay, 1987; Spry, 1988). Nor does the presence of the eosinophil automatically infer IgE mediated hypersensitivity, as evidenced by studies examining the interaction of the eosinophil with the cellular arm of the immune system (Basten and Beeson, 1970; Ruscetti et al., 1976; Beeson and Bass, 1977; Raghavachar et al., 1987; Ohnishi et al., 1988). The purpose of this review is to provide a brief overview of the structure and biology of the mammalian eosinophil and to emphasize the fact that eosinophils fulfil a paradoxical role as effectors of tissue damage and as benign modulators of inflammation.

Interleukin-5-induced granulocyte activation in atopic patients

The effect of recombinant human and murine interleukin-5 (IL-5) on granulocytes was investigated in patients with atopic dermatitis (AD) and allergic rhinitis and compared with those from patients with plaque psoriasis and normal non-atopic controls. Granulocyte activation was measured as lucigenin-dependent chemiluminescence (CL) and release of eosinophil cationic protein (ECP) as well as by scanning- and transmission-electron microscopy (EM) and the ultrastructural detection of production of H2O2. A significant direct effect of both human and murine IL-5 on granulocyte oxidative metabolism could only be detected in those patients with AD and allergic rhinitis. Also, as compared to normal controls, significantly increased CL responses were observed in these patients with stimulation with other granulocyte-activating cytokines, particularly GM-CSF. In patients with psoriasis there was no significant increase in response to stimulation with TNF alpha, TNF beta or GM-CSF, but IL-5 induced slight but significant CL responses in the granulocytes. None of the cytokines tested significantly stimulated the release of ECP in any of the groups. Ultrastructural studies showed that stimulation with human as well as murine IL-5 produced significant morphological changes in both eosinophils and polymorphonuclear neutrophilic granulocytes from the patients with AD. Production of H2O2 was visualized at the luminal part of the intracytoplasmic vesicles of the granulocytes and at the points of contact between the cells. On morphometric analysis, almost all the polymorphonuclear neutrophils in the patients with AD appeared to be activated, whereas only some of these cells in the normal controls showed signs of activation.

Interleukin 5 modifies histamine release and leukotriene generation by human basophils in response to diverse agonists

Human interleukin 5 (IL-5), known as a selective colony-stimulating factor of the eosinophil lineage and activator of mature eosinophils, also profoundly influences the mediator release profile of human basophils. IL-5 by itself triggers neither granule release nor de novo synthesis of lipid mediators. However, at low concentrations (0.1-10 ng/ml), IL-5 rapidly primes basophils for enhanced histamine release and leukotriene C4 (LTC4) generation in response to all established basophil agonists. LTC4 generation is more strongly affected by IL-5 than histamine release. In particular, IL-5 renders basophils capable of producing large quantities of LTC4 in response to C5a, which, without the cytokine, induces histamine release only. Finally, IL-5 renders basophils responsive to agonists (neutrophil-activating peptide 1 and C3a), which are otherwise inefficient triggers for basophil mediator release. The effects are similar to the recently established bioactivity of IL-3 on basophils, with the exception of its influence on IgE-dependent basophil activation, which is less pronounced. Thus, IL-5 strongly modulates the function not only of eosinophils but also of basophils, the two major effector leukocyte types involved in allergic inflammatory processes, e.g., in asthma.

Inflammation in organic dust-induced lung disease: new approaches for research into underlying mechanisms

The application of new technology to the study of lung inflammation is providing fresh insight into the nature of the underlying reactions. This review highlights the impact of modern cell biological and molecular biological techniques on current research in this area.

Cellular characteristics of sputum from patients with asthma and chronic bronchitis

The reproducibility of sputum cell counts was examined and the cell counts in patients with asthma were compared with those in patients with chronic bronchitis. Three groups of subjects were studied. Sputum from eight patients with chronic asthma and with sputum production were studied to determine the reproducibility of sputum cell counts. The findings in 10 non-smokers with asthma uncomplicated by other airway disease examined at the time of an exacerbation with sputum (group 2) were compared with those from eight smokers with chronic cough and sputum but no features of asthma (group 3). Sputum plugs were selected by microscopy to ensure their origin from the lower respiratory tract. A total cell count was performed on a trypsinised suspension, and differential and metachromatic cell counts were performed on undiluted plugs. The within specimen and test-retest reproducibility of these measurements was high (reliability coefficient, R, = 0.99 and 0.89). The sputum of the asthmatic patients was characterised by eosinophilia (69%, range 46-92%) and the presence of formaldehyde blockable metachromatic cells (1.5%, range 0.6-2.8%). In comparison, the sputum of the patients with chronic bronchitis had few eosinophils (0.5%) or metachromatic cells (0.14%); the dominant cell type was the macrophage (83%). It is concluded that sputum cell counts are reproducible in the short term, the inflammation of asthma is characterised by eosinophilia and metachromatic cells in sputum, and sputum may provide a useful source of cells for investigating the cellular characteristics of airway inflammation.

Nerve growth factor promotes human hemopoietic colony growth and differentiation

Nerve growth factor (NGF) is a neurotropic polypeptide necessary for the survival and growth of some central neurons, as well as sensory afferent and sympathetic neurons. Much is now known of the structural and functional characteristics of NGF, whose gene has recently been cloned. Since it is synthesized in largest amounts by the male mouse submandibular gland, its role exclusively in nerve growth is questionable. NGF also causes histamine release from rat peritoneal mast cells in vitro, and we have shown elsewhere that it causes significant, dose-dependent, generalized mast cell proliferation in the rat in vivo when administered neonatally. Our experiments now indicate that NGF causes a significant stimulation of granulocyte colonies grown from human peripheral blood in standard hemopoietic methylcellulose assays. Further, NGF appears to act in a relatively selective fashion to induce the differentiation of eosinophils and basophils/mast cells. Depletion experiments show that the NGF effect may be T-cell dependent and that NGF augments the colony-stimulating effect of supernatants from the leukemic T-cell (Mo) line. The hemopoietic activity of NGF is blocked by polyclonal and monoclonal antibodies to NGF. We conclude that NGF may indirectly act as a local growth factor in tissues other than those of the nervous system by causing T cells to synthesize or secrete molecules with colony-stimulating activity. In view of the synthesis of NGF in tissue injury, the involvement of basophils/mast cells and eosinophils in allergic and other inflammatory processes, and the association of mast cells with fibrosis and tissue repair, we postulate that NGF plays an important biological role in a variety of repair processes.

Human nasal polyp epithelial basophil/mast cell and eosinophil colony-stimulating activity. The effect is T-cell-dependent

We have previously reported highly potent basophil/mast cell (BMC) and basophil/eosinophil (Eo) colony-stimulating activities (CSA) in conditioned medium derived from cultured human nasal polyp epithelial scrapings (NP-CM). We now have examined the involvement of peripheral blood T-cells in the NP-CM stimulation of colony-forming units (cfu) from the blood of atopic and nonatopic subjects. Because the number of BMC- and Eo-cfu was significantly higher in cultures of peripheral blood from subjects with out-of-season ragweed allergic rhinitis than from control subjects (23.8 +/- 4.1 versus 9.0 +/- 2.4, p less than 0.01), we asked whether the observed colony stimulation could be a T-cell-dependent effect. Indeed, peripheral blood target cells consisting of a reconstituted mixture of T-cells and T-cell-depleted peripheral blood mononuclear cells (non-T-cells) yielded a significantly higher number of colonies in the presence of NP-CM than the non-T-cells alone. NP-CM did not stimulate colony formation by isolated T-cells. These observations point to interactions among nasal epithelial growth and differentiation factors, blood-borne progenitors and T-cells in the local accumulation of basophils, mast cells, and eosinophils in nasal polyps.

Metachromatic cell progenitors and specific growth and differentiation factors in human nasal mucosa and polyps

We previously demonstrated that fluctuations in circulating metachromatic cell progenitors were inversely related to nasal metachromatic cell (NMC) counts and nasal symptoms in allergic rhinitis. Now, we have quantitated NMC progenitors and lineage-specific growth factors using a hemopoietic colony assay. Cell suspensions from excised collagenase-treated nasal polyps (n = 7) contained 3.8 +/- 1.1 granulocyte colony-forming cells per 10(6) cells plated, compared to less than or equal to 0.5 in human tonsil suspensions, less than or equal to 0.5 in nasal mucosal epithelial scrapings, and 33 +/- 8 in peripheral blood of patients with ragweed allergic rhinitis (p less than 0.01). The percentage of metachromatic cells in nasal-polyp-derived colonies was 47 +/- 10 compared with 3.0 +/- 0.7 in peripheral blood colonies (p less than 0.005). Highly potent metachromatic cell colony-stimulating activity (CSA) was detected in supernatants from cultured human nasal epithelial scrapings from both polyps and atopic nasal mucosa, but not from nonatopic nasal mucosa. Supernatants from polyp mononuclear cells stimulated with phytohemagglutinin also contained metachromatic cell-CSA, which had an approximate molecular size of 25 to 70,000 daltons on column chromatography. An IL-3-like activity was also detected in these supernatants. These observations provide further evidence for in situ hemopoietic mechanisms in human nasal mucosa, involving epithelium-derived stimulation of local metachromatic cell progenitor growth and differentiation in allergic rhinitis.