Summary: The gut-associated lymphoid tissue (GALT) is composed of lymphocytes residing in Peyer's patches, lamina propria, and intraepithelial compartments. In addition to these features which distinguish GALT from other peripheral sites of the immune system, the gastrointestinal immune system is also composed of resident eosinophils. Eosinophils are generally considered to be peripheral blood leukocytes that have an important pro-inflammatory role in various immune disorders. Although most research concerning this cell has focused on understanding its trafficking and function in the blood and lung, recent studies have also started to elucidate its regulation and function in the gastrointestinal tract. Interestingly, eosinophil numbers in the gastrointestinal tract are substantially higher than in other tissues. At baseline (healthy conditions), most eosinophils reside in the lamina propria in the stomach and intestine. Eosinophil homing to these sites occurs during embryonic development and their levels in perinatal mice are comparable to those in adults, indicating that their homing is not dependent upon the presence of intestinal flora. Furthermore, eosinophil localization to the lamina propria at baseline is critically regulated by eotaxin, a chemokine constitutively expressed throughout the gastrointestinal tract. Although eotaxin is required for eosinophil homing, its expression in the esophagus is not sufficient for eosinophil accumulation, since this organ is devoid of eosinophils at baseline. During Th2-associated inflammatory conditions (e.g. interleukin (IL)-5 overexpression or oral allergen challenge), marked increases of eosinophils occur not only in the lamina propria but also in Peyer's patches. The accumulation of Peyer's patch eosinophils, which mainly occurs in the outer cortex and interfollicular regions, is critically regulated by IL-5 and less significantly by eotaxin, suggesting the involvement of other eosinophil chemokines in this lymphoid compartment. Preliminary investigations have shown that gastrointestinal eosinophils express the α4β7 integrin and that this molecule is responsible, in part, for eosinophil homing. In summary, eosinophils are resident cells of the gastrointestinal immune system whose levels can be induced by antigen exposure under Th2 conditions, in a manner that is critically regulated by eotaxin and IL-5. We propose that eosinophils are integral members of the gastrointestinal immune system and are likely to be important in innate, regulatory and inflammatory immune responses.
This work was supported in part by the National Health Medical Research Council (Australia) C.J. Martin Post-doctoral Fellowship (S.P.H.), the Jaffe Family Fund of the American Academy of Allergy, Asthma, and Immunology (S.P.H.), NIH grant R01 AI45898 (M.E.R.) and the Human Frontier Science Program (M.E.R.). The authors wish to thank Drs. K. Frank Austen, Mitchell Cohen, Paul Foster, Glenn Furuta, and Nives Zimmermann for helpful discussions, as well as numerous other colleagues.