Interaction of Recombinant Interleukin-2 with Liposomal Bilayers

Liposomes have been employed as a delivery system for recombinant interleukin-2 (rIL-2) in cancer immunotherapy. In this study the effects of the rIL-2-bilayer interaction on protein structure were investigated. It was shown that rIL-2 adsorbs to liposomal membranes when added to preformed liposomes. Polarized fluorescence decay studies showed that the single tryptophan in “native” rIL-2 has a relatively large motional freedom, although iodide quenching of this residue’s fluorescence was relatively ineffective. However, adsorption of rIL-2 to liposomes alters this situation dramatically— fluorescence intensity increased 2-fold and the residue became more susceptible to iodide quenching. At the same time, the average fluorescence lifetime of the fluorophore is extended. Interestingly, circular dichroism studies showed that no major conformational changes occurred in rIL-2’s secondary structure upon adsorption. These observations support the hypothesis that intramolecular quenching takes place in the native rIL-2 molecule, which is abrogated upon adsorption to the liposomal membrane, resulting in a higher fluorescence intensity. Fluorescence anisotropy decay experiments indicate that the protein forms self-aggregates under the low-ionic strength conditions used, confirming the earlier observations on the tendency of the protein to precipitate in salt-containing media.