Evolution of host-microbe cell adherence by receptor domain shuffling

Stable adherence to epithelial surfaces is required for colonization by diverse host-associated microbes. Successful attachment of pathogenic microbes via surface adhesin molecules is also the first step in many devastating infections. Despite the primacy of epithelial adherence in establishing host-microbe associations, the evolutionary processes that shape this crucial interface remain enigmatic. Carcinoembryonic antigen associated cell adhesion molecules (CEACAMs) encompass a multifunctional family of vertebrate cell surface proteins which are recurrent targets of bacterial surface adhesins at epithelial surfaces. Here we show that multiple members of the primate CEACAM family exhibit evidence of repeated natural selection at protein surfaces targeted by bacteria, consistent with pathogen-driven evolution. Inter-species diversity of CEACAM proteins, between even closely-related great apes, determines molecular interactions with a range of bacterial adhesins. Phylogenetic analyses reveal that repeated gene conversion of CEACAM extracellular domains during primate divergence plays a key role in limiting bacterial adhesin tropism. Moreover, we demonstrate that gene conversion has continued to shape CEACAM diversity within human populations, with abundant CEACAM1 variants mediating evasion of adhesins from Neisseria gonorrhoeae, the causative agent of gonorrhea. Together this work reveals a mechanism by which gene conversion shapes first contact between microbes and animal hosts.