Investigating T cell activation and tolerance in vivo: peptide challenge in allergic asthmatics

In the atopic allergic individual, challenge with allergen elicits manifestations of both the humoral (IgE-mediated or early-phase reaction) and the cell-mediated immune response (late-phase reaction). Detailed study of late-phase cell-mediated events is confounded by the effects of the earlier IgE-mediated response which results in mast cell and basophil activation. Thus the relative contributions to allergic inflammation of individual cell types, such as T cells and eosinophils, are difficult to define. In this review we describe experiments, largely from our own group, in which we have attempted to dissociate early and late allergic reactions. To this end, cell-mediated responses were induced in the absence of preceding IgE-mediated events, by the delivery of synthetic peptides representing T cell epitopes of the allergen. Activation of T cells resulted in airway narrowing and an increase in airway reactivity to non-specific stimuli. Furthermore, we describe the induction of antigen-specific hyporesponsiveness or "tolerance" following intradermal, but not mucosal, peptide delivery. The induction of peptide-induced hyporesponsiveness could be temporally dissociated from the initial T cell activation resulting in bronchoconstriction and likely occurred through a different mechanism. Analysis of in vitro allergen responses of peripheral blood cells revealed that hyporesponsiveness was associated with reductions in both Th1 and Th2 cytokines, together with a concomitant increase in the regulatory cytokine IL-10. We conclude that activation of T cells in vivo may result in manifestations of chronic allergic inflammation including bronchoconstriction and hyperreactivity. Additionally, when administered systemically at low dose, peptides may induce long-lasting hyporesponsiveness in the T cell compartment, through a mechanism that is associated with induction of IL-10.