Cell biological mechanisms and activity markers of eosinophils in relation to allergic inflammation

It is becoming increasingly apparent that the pathogenesis of respiratory allergic diseases, including asthma and allergic rhinitis, is closely linked to the presence of chronic inflammation. One of the leading actresses in the allergic inflammatory process is the eosinophil granulocyte. This thesis focuses on cell biological mechanisms and different activity markers of eosinophils in relation to the allergic inflammation. An important medical and cell biological problem is that the recruitment of cells to the inflammatory lesion, and the following tissue reactions, usually start long before patient and treating physician are aware of any symptoms. In this early phase, during the subclinical inflammation, it is of importance to enable identification of relevant risk markers for the development of allergy and asthma. Today there is not one marker alone, measured in the laboratory, that can be a useful and reliable tool in the clinic for monitoring the inflammatory process in asthma. Since the eosinophil is recruited from the circulation to the inflammatory lesion, it would be of practical use if measurements of the early activation of the eosinophil ("priming") in the blood stream could reflect the inflammation in local tissue, e.g. the bronchial mucosa.

We have demonstrated that CD9 is a reliable marker for identification of eosinophils. The existence of a preformed intracellular pool of CD9 in eosinophils was demonstrated, which is translocated upon in vitro activation. In the absence of platelets, which coexpress CD9, a decreased expression of CD9 on the surface of eosinophils was observed, probably caused by the release or shedding of a soluble form of CD9. This is in contrast to the increased expression of CD9 on eosinophils in the presence of platelets, which is probably due to adherent platelets rather than only a translocation from the intracellular pool of CD9 to the cell surface.

Our patient study during the birch pollen seasons 1993 and 1994, indicates that a combination of the three peripheral blood eosinophil markers, EG2, CD9 and CD11b, could be a useful tool to mirror events such as "priming" of eosinophils, and allergen exposure in sensitised individuals, and also to improve the characterisation of the inflammatory status in asthmatics.

Using a skin chamber model, we demonstrated that local allergen challenge of patients with allergic rhinitis and mild asthma, results in an increased recruitment of eosinophils, as compared to controls. Recruited eosinophils were activated as shown by the expression of CD9, CD11b, and the EG2-epitope on intracellular ECP. We suggest that the use of allergen challenge in skin chambers is a useful in vivo model for further studies of eosinophil recruitment, their state of activation and the involvement of different allergic inflammatory mediators, such as chemokines and cytokines.

The results from the adhesion and transmigration in vitro assays are in accordance with the results from the in vivo study with skin chambers, demonstrating activated eosinophils by means of CD9, CD11b, and the EG2-epitope on intracellular ECP expression. Together our data indicate that these assays are useful for studying eosinophil recruitment to the extracellular matrix, in vitro. In the adhesion assay we also demonstrated that antibodies to CD9 decreased the adhesion property; we therefore suggest that CD9 might be involved in the adhesion of eosinophils to extracellular matrix-proteins.

Taken together, this thesis indicates that CD9 could be used as an eosinophil activity marker, and that CD9 might also play a role in the actual adhesion process of eosinophils to extracellular matrix proteins. The skin chamber model, together with the adhesion and transmigration models, are useful tools to study mechanisms in the allergic inflammation.