Evolution of CEACAM pathogen decoy receptors in primates

CEACAM1 in leukocytes controls cell activation during inflammation. This and its expression in epithelial cells led to frequent independent appropriation of CEACAM1 as receptor by pathogens in humans and other species to gain host access and to downregulate its immune response. As a countermeasure, decoy receptors with CEACAM1-like pathogen-binding domains evolved. The granulocyte-specific human CEACAM3 endocytic receptor diverts CEACAM1-binding pathogens to neutrophils for internalization and destruction. The role of the glycosylphosphatidylinositol-anchored CEACAM5 and CEACAM6 which can also bind CEACAM1-targeting pathogens in humans is less clear.

We analyzed the selection of CEACAMs to avoid pathogen binding and to maintain similarity between pathogen receptors and decoy receptors in 148 primate species.

Notably, functional CEACAM3 genes were not found in gibbons and New World monkeys. Interestingly, CEACAM6 in these primates exhibits similar high ratios of rates of nonsynonymous and synonymous substitution (dN/dS) in their pathogen-binding N domain exons as found for CEACAM1. High dN/dS ratios are indicative of selection for diversification typically seen in pathogen receptors. Human CEACAM6 is expressed on granulocytes and epithelial cells. Therefore, CEACAM6 could substitute for the missing endocytic receptor CEACAM3. In nearly all investigated primate groups also N exons of the epithelially expressed CEACAM5 exhibit selection for diversification. In African populations, five high-frequency polymorphisms are observed in the pathogen-binding region of CEACAM5 (I80V, V83A, I100T, I112V, I113T) with 3-4 polymorphisms combined in the same individual. These polymorphisms correspond to CEACAM1 pathogen-binding domain sequences.

The glycosylphosphatidylinositol-anchored CEACAM5 and CEACAM6 are under selection to maintain similarity to the pathogen receptor CEACAM1 in most primate species, indicating a function as decoy receptors.