A fluorescent interleukin-8 receptor probe produced by targetted labelling at the amino terminus

Interleukin-8 is the most extensively characterised member of the structurally related chemotactic and pro-inflammatory proteins collectively called chemokines. It binds to two closely related members of the seven transmembrane chemokine receptor family found on a variety of leukocyte cell types. In order to study the interaction of interleukin-8 with its receptors, and their distribution, we have produced a fluorescently labelled protein as an alternative to the radioactive 125I-interleukin-8 ligand. Interleukin-8 is naturally produced as two forms, a 72-residue polypeptide by monocytes and a 77-residue form produced by endothelial cells which has an extension of five amino acids at the amino terminal. Both forms are active at nanomolar concentrations, implying that chemical modification to the amino terminus of the 72-residue form will not destroy activity. The 72-residue interleukin-8 sequence starts with a serine residue, which can be oxidised under mild conditions to give a reactive glyoxylyl function which is then reacted with a nucleophilic fluorescein derivative. The site-specifically labelled protein was easily isolated by reverse-phase HPLC. The dissociation constant of the fluorescently labelled interleukin-8 from its receptors on neutrophils was measured by displacement of 125I-interleukin-8 and found to be 10 nM compared to 1 nM for the unmodified protein. The modified protein is highly active in in vitro bioassays using human neutrophils, giving an EC50 of 7 nM in chemotaxis and an EC50 of 0.62 nM for shape change. The binding of the fluorescent protein to neutrophils can also be measured by fluorescent automatic cell sorter (FACS) analysis, and can be competed by unlabelled interleukin-8. The amino-terminal modification of interleukin-8 has produced a reagent which is useful for the quantification of interleukin-8 receptor expression, and will also be useful in monitoring the fate of the ligand after receptor binding.