The immunoglobulin E-Toll-like receptor network.
Allergens and microbial antigens impact on effector cells and antigen-presenting cells in allergic diseases. Allergens bind specifically to immunoglobulin E (IgE) linked to the high-affinity receptor for IgE (FcepsilonRI) and stimulate a cascade of cellular events. This leads to the release of mediators of allergic reactions by effector cells on the one hand and antigen uptake, presentation and T cell priming by antigen-presenting cells on the other hand. In contrast, microbial antigens are recognized by pattern-recognition receptors (PRRs) of the innate immune system, to which Toll-like receptors (TLRs) belong. In view of the high number of microbial antigens, allergens and other soluble ligands in the cellular microenvironment in vivo, it is very likely that not only separate, but also concomitant stimulation of both receptor types, i.e. FcepsilonRI and TLRs, occurs frequently under physiological conditions and in particular in the context of allergic and infectious disorders. Thus, interaction of TLRs with FcepsilonRI and regulation of the IgE synthesis is of critical immunological importance, since it might profoundly modify the activation state of cells and the nature of the evolving immune responses. Current knowledge about the cross talk of TLRs with FcepsilonRI- and IgE-related immune responses is discussed herein.