Dynamic Evolution of Bacterial Ligand Recognition by Formyl Peptide Receptors

Abstract

The detection of invasive pathogens is critical for host immune defense. Cell surface receptors play a key role in the recognition of diverse microbe-associated molecules, triggering leukocyte recruitment, phagocytosis, release of antimicrobial compounds, and cytokine production. The intense evolutionary forces acting on innate immune receptor genes have contributed to their rapid diversification across plants and animals. However, the functional consequences of immune receptor divergence are often unclear. Formyl peptide receptors (FPRs) comprise a family of animal G protein-coupled receptors which are activated in response to a variety of ligands including formylated bacterial peptides, pathogen virulence factors, and host-derived antimicrobial peptides. FPR activation in turn promotes inflammatory signaling and leukocyte migration to sites of infection. Here we investigate patterns of gene loss, diversification, and ligand recognition among FPRs in primates and carnivores. We find that FPR1, which plays a critical role in innate immune defense in humans, has been lost in New World primates. Amino acid variation in FPR1 and FPR2 among primates and carnivores is consistent with a history of repeated positive selection acting on extracellular domains involved in ligand recognition. To assess the consequences of FPR divergence on bacterial ligand interactions, we measured binding between primate FPRs and the FPR agonist Staphylococcus aureus enterotoxin B, as well as S. aureus FLIPr-like, an FPR inhibitor. We found that few rapidly evolving sites in primate FPRs are sufficient to modulate recognition of bacterial proteins, demonstrating how natural selection may serve to tune FPR activation in response to diverse microbial ligands.

Keywords: bacteria; immunity; molecular evolution; pathogen; positive selection; receptors.