Monoclonal Antibody 5F1 Modulates Formyl Peptide Receptor 1 Conformation for Transmembrane Signaling

Formyl peptide receptor 1 (FPR1) is a G protein-coupled receptor (GPCR) that mediates chemotaxis and bactericidal activities in phagocytes. The monoclonal antibody 5F1 is generated against full-length FPR1 and used widely for detection of FPR1 expression. This study aimed to characterize 5F1 for its functions. We found that 5F1 is highly selective for human FPR1 over the homologous FPR2. Epitope mapping led to the identification of extracellular loop 2 (ECL2) as a major epitope, and the synthetic peptide of ECL2 interfered with 5F1 binding to FPR1. Using a NanoLuc Bioluminescence Resonance Energy Transfer approach, we found that 5F1 binding induced FPR1 conformational changes. Although less potent than fMLF, 5F1 binding induced FPR1 internalization, Gi protein dissociation, and β-arrestins membrane translocation. Alanine substitution of F110 and R205 markedly reduced 5F1 binding without affecting FPR1 cell surface expression, suggesting that 5F1 is sensitive to conformational changes in FPR1 as these residues are not present in ECL2. Altogether, mAb 5F1 can alter FPR1 conformation and modulate transmembrane signaling, features that may be explored for potential use beyond the detection of FPR1 expression.