Chapter 6. Function of Siglec-8 on human eosinophils

Eosinophil recruitment and activation are regarded as central to the pathophysiology of allergic diseases, including asthma. An improved understanding of the mechanisms involved in these responses is therefore of great relevance to asthma pathogenesis and the development of new therapeutics. As part of ongoing efforts to discover novel eosinophil-specific molecules, we recently cloned Siglec-8 (formerly called sialoadhesin family member-2) from a human eosinophil cDNA library. Siglecs (sialic acid binding Ig-like lectins) are a family of transmembrane, I-type lectins characterized by an N-terminal V-set Ig domain that binds sialic acid. We now know that Siglec-8 is expressed only on human eosinophils, basophils and mast cells, giving it a unique expression pattern on effector cells of allergic disease. We have determined that in eosinophils, Siglec-8 exists in two isoforms, one of which contains two putative cytoplasmic tyrosine-based signalling motifs, including an ITIM (immunoreceptor tyrosine-based inhibitory motif) sequence. Because of the ITIM sequence, we hypothesized that Siglec-8 ligation would inhibit eosinophil functions. Initial studies found that incubation of eosinophils with Siglec-8 binding monoclonal antibodies under cross-linking conditions caused rapid and profound caspase-dependent apoptosis, and this response could not be rescued by the survival-promoting cytokine interleukin (IL)-5. In fact, IL-5 enhanced the ability of Siglec-8 cross-linking to induce eosinophil apoptosis. Activation via Siglec-8 could potentially be used to inhibit eosinophil survival in vivo, providing a novel strategy for reducing or inhibiting these cells in allergic and other diseases.