Eosinophils and Th2 immunity: contemporary insights

Abstract

Eosinophils, innate immune leukocytes elicited by Th2 cells, have long been associated with the effector arm of Th2 immune responses. However, accumulating data over the past decade reveal a much more dynamic picture of Th2 immunity, where eosinophils are present very early in response to Th2-inducing agents and function in the initiation of Th2 immunity. Here we discuss recent data showing immune functions of eosinophils distinct from their previously appreciated tissue- and helminth-destructive capacities, providing strong evidence for a new paradigm of Th2 immunity defined by a dynamic interplay between eosinophils and T cells.

Figure 1

Eosinophils and Th2 immunity. (a) Eosinophils are one of the first cell types elicited to tissue sites of localized helminths or allergic inflammation, and secrete Th2-inducing cytokines (that is, interleukin (IL)-4) and alarmins (that is, eosinophil-derived neurotoxin, EDN), which exerts it effect on resident dendritic cells (DCs), enhancing DC activation and promoting a Th2-inducing phenotype. (b) Eosinophils traffic from tissue sites to draining lymph nodes, where they localize to T-cell zones. Within lymph nodes eosinophil-derived cytokines promote DC initiation of Th2 polarization, and eosinophils function as antigen-presenting cells to directly instruct Th2 polarization of naive, antigen-specific T cells. (c) Eosinophils within the tissues participate in the effector phases of Th2 immunity by secreting factors that inhibit Th1 lymphocytes (indoleamine 2,3-dioxygenase, IDO) and enhance activities of Th2 memory cells (IL-25). Eosinophils also secrete Th2 chemoattractants required for efficient recruitment of Th2 effector cells into tissue sites. BV, blood vessel.