Assessment of neutrophil N ‐formyl peptide receptors by using antibodies and fluorescent peptides

Enrichment of chemoattractant receptors on the neutrophil surface has been difficult to assess, primarily because of limitations in sensitivity of visualization. Using an ultrasensitive, cooled charge‐coupled device camera, we investigated spatial‐temporal relationships between N ‐formyl peptide receptor distribution and directional motility of human neutrophils. Live cells were labeled with fluorescent receptor ligands, i.e., fluoresceinated tert ‐butyl‐oxycarbonyl‐Phe‐( d )‐Leu‐Phe‐( d )‐Leu‐Phe‐OH (Boc‐FLFLF) and formyl‐Nle‐Leu‐Phe‐Nle‐Tyr‐Lys (fnLLFnLYK), while fixed cells were labeled with either fluorescent peptides or monoclonal antibodies. Double labeling of receptors and filamentous actin (F‐actin) was done to investigate possible colocalization. N ‐Formyl peptide receptors on unstimulated cells were randomly distributed. However, on polarized neutrophils, the receptors accumulated toward regions involved in motility and distributed nonuniformly. In fixed neutrophils, antibody‐labeled receptors colocalized with the F‐actin‐rich leading edge whereas peptide‐labeled receptors lagged behind this region. We suggest that neutrophils use an asymmetric receptor distribution for directional sensing and sustained migration. A separation between receptors labeled with peptides and those labeled with antibodies reflects two functionally distinct receptor populations at the membrane of motile neutrophils.