CEACAM3: An innate immune receptor directed against human-restricted bacterial pathogens

Carcinoembryonic antigen-related cell adhesion molecule 3 (CEACAM3) is an immunoglobulin-related glycoprotein exclusively expressed on granulocytes. In contrast to other members of the CEACAM family, CEACAM3 does not support cell–cell adhesion, but rather mediates the opsonin-independent recognition and elimination of a restricted set of human-specific Gram-negative bacterial pathogens including Neisseria gonorrhoeae, Haemophilus influenzae, and Moraxella catarrhalis. Within the last 4 years, molecular determinants of CEACAM3 function and CEACAM3-initiated signaling pathways have been elucidated. Sequence comparison between CEACAM3 and other CEACAM family members points to a chimeric origin of this receptor with the bacteria-binding extracellular domain and the function-promoting intracellular domain derived from different genes. This review summarizes the current knowledge about the structure–function relationship of CEACAM3 and tries to combine these molecular aspects with a plausible scenario concerning the evolutionary origin of this phagocyte receptor in the light of host–pathogen adaptation.