CD93 and related family members: their role in innate immunity

CD93 belongs to a newly described family of transmembrane glycoproteins which also includes endosialin and thrombomodulin. These cell surface proteins are grouped into a family based on similar ectodomain architecture consisting of a C-type lectin-like domain, a series of EGF-like repeats and a highly glycosylated mucin-like domain. However, recent studies suggest overlapping functions in the regulation of processes involving innate immunity and inflammation. CD93 regulates phagocytosis of apoptotic cells in vivo, a function critical to development, tissue repair and maintenance of tissue homeostasis. In addition, in vitro studies have demonstrated a role for CD93 in phagocytosis of antibody and complement opsonized particles and also in leukocyte and endothelial cell adhesion. Analysis of CD93 expression on endothelial cells in the developing mouse embryo correlates with the remodeling of blood vessels suggesting a role for CD93 in regulating angiogenesis, a process tightly linked to acute and chronic inflammation and also required for tumor metastasis. Endosialin has been characterized as a tumor specific antigen and functions in angiogenesis. Thrombomodulin is best characterized as a natural anticoagulant; however, more recent reports have illuminated the importance of thrombomodulin in the regulation of inflammation. This review discusses similarities and differences in the family members, and focuses on known and speculated functions in regulation of innate immunity.