Immunomodulatory Roles of the Carcinoembryonic Antigen Family of Glycoproteins

 One of the most remarkable aspects of the immune system is its ability to fashion an immune response most appropriate to the activating stimulus. Although the immune system possesses a number of adaptations to accomplish this, an important theme is local immune regulation by site‐specific expression of receptors and ligands. One family of molecules that is gaining attention as modulators of the immune system is the carcinoembryonic antigen cell‐adhesion molecule family (CEACAM). Functionally, the carcinoembryonic antigen family can mediate cell–cell contact, host–pathogen interactions, and immune regulation. For example, biliary glycoprotein (CEACAM1) can have direct activity on T cells, leading to the inhibition of helper or cytotoxic T cell function. The expression of carcinoembryonic antigen (CEACAM5) on intestinal epithelial cells is involved in the activation of populations of regulatory CD8 + T cells, while a distinct subset of regulatory CD8 + T cells is activated by nonspecific cross‐reacting antigen (CEACAM6) on placental trophoblasts. Interestingly, the function and phenotype of these cells depend upon the specific member of the carcinoembryonic antigen family expressed, as well as the antigen‐presenting molecule with which it associates. Thus, these glycoproteins comprise a family of molecules whose functions can depend on their nature and context.