A Role for Eosinophils in Adaptive Humoral Immunity

This review describes eosinophils in the context of their conspicuous presence as part of the memory (i.e., anamnestic) immune response to antigen. We propose a role for eosinophils, along with Antigen Presenting Cells (APC), in the initiation of humoral immune responses to protein antigens by memory T Helper-2 (TH2) cells and memory B lymphocytes. Eosinophils have long been known as a component of the inflammatory response induced during prolonged or repeated exposure to diverse antigens. However, their precise contribution to secondary antibody responses remains to be fully clarified. These morphologically unique granulocytes possess a wide assortment of preformed cytokines, interleukins, chemokines and RNAses (collectively known as eokines), which can be rapidly released during an inflammatory response to foreign proteins. Their accumulation and release of diverse eokines during the inflammation that accompanies an anamnestic response suggests an immunoregulatory function for eosinophils. Based on published results, we propose that preformed eokines attract dendritic cells and other APC to the inflammatory site and contribute to their maturation and activation. In addition, eosinophil RNAses and other enzymes catabolize cellular products (e.g., dsRNA complexes) released by antigen-activated memory TH2 cells undergoing apoptosis. As shown for other antigeninduced responses, B memory cells could bind components of RNA and other molecules present in the inflammatory exudate via their cell surface Toll Like Receptors (TLR), contributing to activation, clonal expansion, and differentiation into antibody-producing plasma cells. Overall, we present a unifying hypothesis to account for the presence and immunomodulatory role of eosinophils in the humoral immune response.