Eosinophils and eosinophil-associated cytokines in allergic inflammation

Eosinophils are major effector cells in allergic tissue reactions. Their capacity to synthesize and store cytokines, particularly Th2-type cytokines, which may have autocrine effects such as increased cell survival in tissues, is of particular current interest. We have shown that eosinophils infiltrating the site of human cutaneous late-phase reactions (LPR) express mRNA and protein product for IL-4 and IL-5. Stimulation of peripheral blood eosinophils in vitro with either IgG or sIgA produces time-dependent induction of IL-4 and IL-5 transcription, indicating that mature cells may undergo local cytokine synthesis. We also demonstrated that bronchial biopsies from patients with both atopic and non-atopic (intrinsic) asthma express IL-4 and IL-5 mRNA which mainly co-localized to T cells, although clear hybridization signals were also obtained with eosinophils and mast cells. On the other hand, immunoreactivity for the corresponding protein was detectable predominantly in eosinophils and mast cells; a finding believed to reflect granule storage of cytokines. We also studied the resolution of the cutaneous LPR. Apoptosis of eosinophils persisted slightly longer than neutrophils in the tissues. The eventual decline in both neutrophil and eosinophil numbers followed on from the peak of the LPR. This was associated with terminal-deoxynucteotidyl-transferase-mediated nick and labelling positive (TUNEL+) (apoptotic) cells with the subsequent removal of apoptotic neutrophils and eosinophils by tissue macrophages. Thus, IL-4 and IL-5 production by eosinophils may amplify local allergic inflammatory responses and, in the case of IL-5, may prolong local tissue survival. However, in the allergen-induced LPR, inflammation rapidly resolves, possibly through the initiation of programmed cell death and phagocytosis of apoptotic cells by macrophages.