Immunoregulation of asthma: control of T-lymphocyte activation in the respiratory tract

Allergic asthma results ultimately from inappropriate responses to non-pathogenic airborn antigens by the respiratory tract T-cell system. In order to understand how sensitization to inhaled antigens develops, it is necessary to precisely define the tissue microenvironments within which T-cells occur in the respiratory tract, and to analyse the factor(s) which regulate their local activation. This review focuses upon recent data on the distribution of T-cells and antigen presenting cells within the lung and airway tissues of man and experimental animals, and examines the available information on their responses to antigenic stimulation. We argue that in the steady state, respiratory tract T-cells are tightly regulated by a series of inherent and acquired immunosuppressive control mechanisms, which normally limit local T-cell activation to situations where antigenic exposure is accompanied by an inflammatory stimulus, such as the case with incoming pathogenic microorganisms.