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Eosinophils, along with mast cells are key cells involved in the innate immune response against parasitic infection whereas the adaptive immune response is largely dependent on lymphocytes. In chronic parasitic disease and in chronic allergic disease, IL-5 is predominantly a T cell derived cytokine which is particularly important for the terminal differentiation, activation and survival of committed eosinophil precursors. The human IL-5 gene is located on chromosome 5 in a gene cluster that contains the evolutionary related IL-4 family of cytokine genes. The human IL-5 receptor complex is a heterodimer consisting of a unique a subunit (predominantly expressed on eosinophils) and a beta subunit which is shared between the receptors for IL-3 & GM-CSF (more widely expressed). The a subunit is required for ligand-specific binding whereas association with the beta subunit results in increased binding affinity. The alternative splicing of the alphaIL-5R gene which contains 14 exons can yield several alphaIL-5R isoforms including a membrane-anchored isoform (alphaIL-5Rm) and a soluble isoform (alphaIL-5Rs). Cytokines such as IL-5 produce specific and non-specific cellular responses through specific cell membrane receptor mediated activation of intracellular signal transduction pathways which, to a large part, regulate gene expression. The major intracellular signal transduction mechanism is activation of non-receptor associated tyrosine kinases including JAK and MAP kinases which can then transduce signals via a novel family of transcriptional factors named signal transducers and activators of transcription (STATS). JAK2, STAT1 and STAT 5 appear to be particularly important in IL-5 mediated eosinophil responses. Asthma is characterized by episodic airways obstruction, increased bronchial responsiveness, and airway inflammation. Several studies have shown an association between the number of activated T cells and eosinophils in the airways and abnormalities in FEV1, airway reactivity and clinical severity in asthma. It has now been well documented that IL-5 is highly expressed in the bronchial mucosa of atopic and intrinsic asthmatics and that the increased IL-5 mRNA present in airway tissues is predominantly T cell derived. Immunocytochemical staining of bronchial biopsy sections has confirmed that IL-5 mRNA transcripts are translated into protein in asthmatic subjects. Furthermore, the number of activated CD 4 + T cells and IL-5 mRNA positive cells are increased in asthmatic airways following antigen challenge and studies that have examined IL-5 expression in asthmatic subjects before and after steroids have shown significantly decreased expression following oral corticosteroid treatment in steroid-sensitive asthma but not in steroid resistant and chronic severe steroid dependent asthma. The link between T cell derived IL-5 and eosinophil activation in asthmatic airways is further strengthened by the demonstration that there is an increased number of alphaIL-5R mRNA positive cells in the bronchial biopsies of atopic and non-atopic asthmatic subjects and that the eosinophil is the predominant site of this increased alphaIL-5R mRNA expression. We have also shown that the subset of activated eosinophils that expressed mRNA for membrane bound alpha IL5r inversely correlated with FEV1, whereas the subset of activated eosinophils that expressed mRNA for soluble alphaIL5r directly correlated with FEV1. Hence, not only does this data suggest that the presence of eosinophils expressing alphaIL-5R mRNA contribute towards the pathogenesis of bronchial asthma, but also that the eosinophil phenotype with respect to alphaIL-5R isoform expression is of central importance. Finally, there are several animal, and more recently in vitro lung explant, models of allergen induced eosinophilia, late airway responses(LARS), and bronchial hyperresponsiveness(BHR) - all of which support a link between IL-5 and airway eosinophila and bronchial hyperresponsiveness. The most direct demonstration of T cell involvement in LARS is the finding that these physiological responses can be transferred by CD4+ but not CD8+ T cells in rats. The importance of IL-5 in animal models of allergen induced bronchial hyperresponsiveness has been further demonstrated by a number of studies which have indicated that IL-5 administration is able to induce late phase responses and BHR and that anti-IL-5 antibody can block allergen induced late phase responses and BHR. In summary, activated T lymphocytes, IL5 production and eosinophil activation are particularly important in the asthmatic response. Human studies in asthma and studies in allergic animal models have clearly emphasised the unique role of IL-5 in linking T lymphocytes and adaptive immunity, the eosinophil effector cell, and the asthma phenotype. The central role of activated lymphocytes and eosinophils in asthma would argue for the likely therapeutic success of strategies to block T cell and eosinophil activation (eg steroids). Importantly, more targeted therapies may avoid the complications associated with steroids. Such therapies could target key T cell activation proteins and cytokines by various means including blocking antibodies (eg anti-CD4, anti-CD40, anti-IL-5 etc), antisense oligonucleotides to their specific mRNAs, and/or selective inhibition of the promoter sites for these genes. Another option would be to target key eosinophil activation mechanisms including the aIL5r. As always, the risk to benefit ratio of such strategies await the results of well conducted clinical trials.

CB.17 severe combined immunodeficient (SCID) mice were used to establish a model of allergic pulmonary inflammation. SCID mice were intraperitoneally reconstituted with 10(7) peripheral blood mononuclear cells (PBMC) from patients sensitive to Dermatophagoides pteronyssinus (Dpt) and 2 weeks later were exposed to Dpt aerosols. After Dpt nebulization, SCID mice engrafted with PBMC from Dpt-sensitive patients developed a specific human ISE response as well as an intense pulmonary infiltrate of human cells. In contrast, SCID mice reconstituted with PBMC from patients allergic to grass pollen or from non-allegic donors failed to produce IgE or to exhibit a similar inflammatory response. The level of the IgE production was dependent on the IgE level of the allergic donor In the lungs of the same allergic SCID mice, 2 months after Dpt inhalation, the cell infiltrate contained CD45(+), CD45RO(-), CD20(+) and HLA-DR(+) human cells. They were located in perivascular and peribronchial areas and disseminated in the mouse lung parenchyma. Moreover mRNA IL-5(+) cells and eosinophils were found in peribronchial infiltrates. The observations indicate that humanized allergic SCID mice may develop, after nebulization with the relevant allergen, immune reactions similar to those observed in man and suggest that SCID mice may represent a useful model to analyze the regulatory mechanisms of IgE-associated allergic diseases. Eur J Immunol

Allergen-induced late nasal responses are associated with recruitment and activation of T lymphocytes and eosinophils and preferential mRNA expression for T-helper type 2 (Th2) cytokines. We tested the hypothesis that topical corticosteroids may inhibit late responses by inhibiting cells expressing mRNA for Th2 cytokines. A randomized double-blind placebo-controlled trial of topical corticosteroid (fluticasone propionate) was performed in 48 adult grass pollen-sensitive patients. Nasal biopsies were taken at baseline and repeated 24 hr after local nasal allergen provocation following 6 weeks treatment with either fluticasone propionate 200 micrograms or placebo nasal spray twice daily. Baseline mRNA expression for interleukin-4 (IL-4) (P = 0.01) and IL-5 (P = 0.002) was higher in the patients than in normal controls. Topical corticosteroid treatment significantly inhibited immediate nasal symptoms, with almost complete inhibition of the late response following allergen challenge. This was associated with a marked decrease in the allergen-induced increases in cells expressing mRNA for IL-4 (P = 0.002) but not for IL-5. Inhibition of the late response was also accompanied by decreases in CD25+ cells, presumed T lymphocytes and eosinophils. A significant correlation was observed between the decreases in IL-4 mRNA+ cells and in eosinophils after treatment (r = 0.46, P < 0.05). These results suggest that prolonged treatment with topical corticosteroid inhibits allergen-induced early and late nasal responses and the associated tissue eosinophilia, and that, at least in part, this may result from inhibition of cells expressing mRNA for IL-4.

We have tested the hypothesis that the bronchial epithelium has the capacity to generate and release cytokines that could contribute to inflammatory events associated with inflammatory lung diseases. Messenger RNA (mRNA) for interleukin-6 (IL-6), IL-8 and granulocyte-macrophage colony-stimulating factor (GM-CSF) was identified in human bronchial epithelial cell primary cultures, characterized on the basis of staining for cytokeratin, using both in situ hybridization and Northern blotting. Using in situ hybridization we have shown that the majority of the cells expressed mRNA for IL-6 and IL-8, whereas fewer than 20% of cells expressed message for GM-CSF. The numbers of cells expressing message were increased by culture with tumour necrosis factor-alpha (TNF-alpha) (20 ng/ml, 24 hr). These observations were substantiated by Northern blotting, which showed that both TNF-alpha and IL-1 beta were able to induce a dose-dependent increase in IL-8-specific mRNA. Immunoreactive IL-6 and GM-CSF were detected and quantified in the culture supernatants by ELISA, and IL-8 by radioimmunoassay. The levels of immunoreactivity were increased by incubation of epithelial cells with either IL-1 beta or TNF-alpha for 24 hr. A transformed tracheal epithelial cell line (9HTEo-) expressed mRNA for IL-6, IL-8 and GM-CSF but, whereas levels of immunoreactive IL-6 in culture supernatants were comparable with those in primary cell cultures, levels of IL-8 were low and GM-CSF trivial. These observations indicate that the bronchial epithelium has the potential to be a major source of IL-8 and a number of other cytokines, and that production can be amplified substantially by IL-1 beta and TNF-alpha. The bronchial epithelium is ideally situated to modulate inflammatory and immunological events in and around the airways, and these observations suggest that it could contribute to promote and sustain inflammatory and immunological processes in inflammatory lung diseases such asthma.