It is now generally accepted that inflammatory cells play an important role in the clinical expression and pathogenesis of asthma1-3. Investigation of respiratory secretions and biopsy samples from asthmatics has shown a correlation between the presence of activated inflammatory cells in the airways and changes in airways morphology and the existence of airways hyperreactivity4-15. The inflammatory infiltrate consists of eosinophils, mast cells, monocytes and neutrophils. The recruitment and activation of these cells appears to be controlled by T-lymphocytes through the secretion of cytokine and chemotactic agents following antigenic stimulation. The precise roles that particular inflammatory cells and mediators play in the complex inflammatory cascade leading to the morphological and functional changes in the airways in asthma remain to be defined.
A number of investigations have shown a correlation between the accumulation of activated CD4+ T cells and eosinophils and disease severity4-9, 11-17. There is also increasing evidence of the role of T-lymphocytes and eosinophils in asthma from the studies with animal models18-21. Of the cytokines produced by allergen-specific T-lymphocytes, IL-4 and IL-5 may be particularly important because of their potential to promote the recruitment and activation of airways mast cells and eosinophils13, 22. Increased levels of IL-4 and IL-5 are found in respiratory secretions from atopic asthmatics13 and increases in the number of cells in the BALF expressing elevated mRNA levels for these cytokines has been shown to correlate with bronchial hyperresponsiveness23, 24.
IL-5 is a key regulator of the eosinophil lineage, promoting the growth, differentiation and activation of eosinophils25-28. IL-4 is believed to regulate commitment to the CD4+ Th2 lineage and to regulate IgE isotype switching in B lymphocytes29-33. IgE and allergen-specific immunoglobulins may be important in mast cell activation and eosinophil degranulation34-37. Both IL-4 and IL-5 may also regulate eosinophil recruitment by activating adhesion systems at the vascular endothelium38, 39.
Recently, mouse models which mimic late-phase asthmatic responses have been used as an investigative tool for exploring the pathophysiology of allergic airways disease. Systemic ovalbumin sensitisation followed by aerosol challenge has been shown to induce pronounced airways disease with enhanced bronchial hyperreactivity and pathological changes similar to those observed in severe asthma40. The pathological changes include epithelial detachment, plugging of the airways and a pronounced infiltration of inflammatory cells, particularly eosinophils and lymphocytes. The use of cytokine-deficient mice in conjunction with the aero-allergen model provides a new approach for investigating the role of cytokines in airways inflammation. Using this model we have studied mice deficient in IL-5 and IL-4 to determine the role played by these two cytokines in allergic airways disease.
Role of IL-5 in allergic airways disease
We have made a major study of the involvement of IL-5 in allergic airways inflammation and in the recruitment of eosinophils using IL-5 deficient mice. IL-5 deficient mice41 make no detectable IL-5 protein or IL-5 mRNA. They are healthy and fertile and no indirect effects on other related cytokine levels have been detected (for an overview see42). The mice are unable to mount the characteristic helminth-induced eosinophilia41, 43 but initial studies have shown no effects on conventional B cells or T cell-dependent antibody responses41.
Although IL-5 deficient mice cannot elicit a pronounced eosinophilia in response to inflammatory stimulation following parasite infection or aeroallergen challenge they still produce basal levels of eosinophils that appear morphologically and functionally normal42-44. However, IL-5 deficiency, presumably through the inability to mount an pronounced eosinophilia compromises host defence against several helminth parasites (reviewed in42). More recent studies have suggested that IL-5 deficient mice have functional deficiencies in peritoneal B1 lymphocytes45 and in IgA production46. Therefore, although the major effect of IL-5 deficiency is on the eosinophil lineage, caution needs to be observed in interpreting all effects seen in IL-5 deficient mice in terms of eosinophils.
Dramatic effects were obtained when IL-5-deficient mice were examined in the ovalbumin aeroallergen model. In IL-5-/- mice, the airways hyperreactivity, pulmonary eosinophilia and the gross morphological changes normally resulting from aeroallergen challenge were absent40. Accumulation of lymphocytes in the lungs still occurred, although not to the same extent as in IL-5+/+ animals. However, ova-specific IgE levels were comparable in IL-5-/- and IL-5+/+ aeroallergen-challenged mice. When IL-5 production was restored using a recombinant vaccinia virus expressing IL-5, eosinophilic inflammation, lung pathology and airways hyperreactivity were fully restored40.
The role of T-lymphocytes in this model was confirmed by adoptive transfer experiments. Allergic airways disease in IL-5-/- mice could be reconstituted by transfer of CD4+ Th2 type cells from ovalbumin-sensitised and aerosol-challenged IL-5+/+ mice to IL-5-/- mice which had been challenged but not presensitised with ovalbumin47. Clearly, the IL-5 produced by the transferred T-lymphocytes from the IL-5+/+ mouse was sufficient to generate airways disease in the IL-5-/- mice. Other experiments using vaccinia virus-derived IL-5 have indicated that although IL-5 is necessary, it is not sufficient on its own to induce pronounced lung eosinophilia or airways disease and that other signals derived from activated T-lymphocytes are also required40. Overall these investigations implicate eosinophils as the primary inflammatory cell involved in the changes of pulmonary structure and function observed in the aeroallergen model.
Role of IL-4 in allergic airways disease
An analysis of the role of IL-4 in allergic inflammation of the airways was also carried out using IL-4 deficient mice48. These mice are defective in IgE production and have impaired CD4+ Th2 cell responses33. Surprisingly, in view of the established role of IL-4 and IgE as regulators of allergic disease, no major role for IL-4 in airways inflammation was identified. When the IL-4 deficient mice were tested in the ovalbumin model of allergic airways disease, it was found that recruitment of eosinophils to the lungs was impaired but the characteristic lung damage and airways hyperreactivity was not attenuated48. Airways disease in IL-4-/- mice could, however, be inhibited by pretreatment with anti-IL-5 mAb or anti-CD4+ mAb both of which abolished blood and airways eosinophilia. These findings provide further support for a central role of IL-5 and eosinophils in allergic airways disease and indicate the IL-4 and IgE do not play an obligatory role in this model.
The IL-4-/- mice, therefore, show a disease pathology analogous to intrinsic asthmatics where there is no correlation between disease and elevated IgE levels, although eosinophilia is still present.
In recent years, animal models have been used to study the role of IL-4 and IL-5 in the induction of allergic airways disease but the findings have been controversial. The ovalbumin-sensitisation / aerosol-challenge model used in our experiments generated many of the features of lung pathology seen in severe asthma including epithelial detachment, airway plugging and a pronounced inflammatory infiltrate consisting mainly of eosinophils and lymphocytes. Airways hyperresponsiveness is also induced. Our results with cytokine-deficient mice which are summarized above have consistently indicated a central role of IL-5 and eosinophils in the development of allergic airways disease and no obligatory role for IL-4. The role of IL-5 is also supported by other animal studies and clinical investigations4-15,18-21.
However, there are other studies where a correlation between increased eosinophil numbers and the development of airways hypperreactivity was not found and which have indicated that IL-4 and IgE-dependent mechanisms may be important for both cytokine production by Th2 cells and for eosinophil-associated airways dysfunction50. Although the ovalbumin aeroallergen model is in common use it is important to note that the protocols used vary widely and this should not be overlooked when comparing the findings of different laboratories. For example, Brusselle et al., (1994) have also examined IL-4-/- mice using an ovalbumin aeroallergen model. Analogous to our results, they found that eosinophil accumulation was significantly reduced in IL-4 deficient mice but their protocol produced much milder pathology in the lung; inflammation was localised, pathological changes to the airways were not pronounced and the presence of hyperreactivity was not established.
Results may also be somewhat dependent on the strain of mice used. Such an explanation51 has been put forward to try and reconcile the findings that we have reported for a central role for IL-5 using C57BL/6 mice40 with the conclusion that IL-4 is the key cytokine in allergic airways disease derived from experiments with BALB/c mice50. Interestingly, we have recently described a novel CD4+ T-cell pathway in BALB/c mice that modulates allergen-induced airways hyperreactivity independently of the collective actions of IL-4 and IL-552. In this investigation by using IL-5-/- and IL-4-/- mice of the BALB/c strain in combination with inhibitory mAbs for these cytokines, we have identified two pathways are which critically regulated by CD4+ T-cells that operate independently and in parallel in the development of allergic disease. IL-5 regulated eosinophilia was critical for the induction of aeroallergen-induced lung damage in these mice and was shown, in part, to contribute to the development of airways hyperreactivity. However, a second T-cell pathway which predominantly regulates airways hyperreactivity and was not associated with pronounced morphological changes to the airways was also observed. This pathway appears primarily to operate in BALB/c mice as we have shown that inhibition of the actions of IL-5, or in combination with IL-4, abolishes the development of enhanced airways hyperreactivity to spasmogens in other strains40, 48. Hence, it is likely that the full contribution of the IL-5/eosinophil pathway to the mechanisms underlying airways hyperreactivity is masked in the BALB/c strain. Thus, results in IL-5-/- BALB/c mice support observations that in this strain airways hyperreactivity can develop independently of IL-5 and eosinophilia50. Data indicates that CD4+ T-cells operate multiple pathways that can act independently to induce airways hyperreactivity. The co-existence of parallel pathways may account for the dissociation of airways eosinophilia from the development of airways hyperreactivity in some cases of asthma and in some studies with animal models of this disease.
Despite some apparently contradictory findings in the literature therefore, the consistency of our own findings lead us to conclude that there is a specific role of IL-5 in regulating blood and airways eosinophilia and in the induction of lung damage and airways hyperresponsiveness, thereby defining IL-5 as a potential therapeutic target for the relief of airways dysfunction in asthma. However other T-cell regulated pathways can modulate airways hyperreactivity independently of this cytokine.
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|Foster, PS; Hogan, SP; Young, IG; Matthaei, KI; (1998). The Modulation of Allergic Airways Disease by Interleukin-4 and -5: Studies using Cytokine Deficient Mice. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Invited Symposium. Available at URL http://www.mcmaster.ca/inabis98/|
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