Eosinophilia in antigen-induced airways inflammation.

Airways eosinophilia is a hallmark of asthmatic inflammation. Accumulation of eosinophils in the airways of asthmatic patients is a terminal event of a process that involves complex cellular and molecular interactions. In the past five years, murine models of experimental asthmatic inflammation have provided insight into its complexity and regulation. The three main steps involved in the development of airways eosinophilia are discussed. The first step is the elicitation of an allergen-specific immune response whereby the allergen, captured and processed by antigen-presenting cells, is presented to T lymphocytes to initiate a specific immunological response. In addition to the class II major histocompatibility T cell receptor interaction, this process critically requires costimulation through two independent pathways. The second step is eosinopoiesis in the bone marrow. Under normal conditions, eosinophils represent only 1% to 3% of the white blood cell pool. Therefore, an eosinopoietic event must precede peripheral blood and tissue eosinophilia. The third step is the recruitment of eosinophils from the vascular compartment into the airways. Migration through the endothelium is an active process that involves a number of molecules such as integrins and adhesion molecules. Understanding these three key steps in the development of airways eosinophilia will help to identify new targets and unveil novel strategies for an even more effective treatment for asthma.