PHAGE PRESENTATION AND AFFINITY SELECTION OF A DELETION MUTANT OF HUMAN INTERLEUKIN-3

A deletion derivative of the cytokine human interleukin-3 (hIL-3(15-12 5), comprising amino acids 15-125 of the native protein) was produced as a fusion to the filamentous phage surface protein pIII. The cytokin e was detected in association with phage particles by protein immunobl otting. Compared to an equivalent quantity of soluble cytokine, phage- presented hIL-3(15-125) exhibited reduced biological activity in a hIL -3-dependent cell proliferation assay. The reduction in activity was a ttributable to presence of phage particles in the assay, rather than d irectly owing to physical incorporation of the cytokine into the phage particle. Owing to the position of the amber codon in the phagemid ve ctor, the phagemid-produced free hIL-3(15-125) species (designated hIL -3(15-125) epsilon) had 20 amino acids appended to its C-terminus; hIL -3(15-125) epsilon did not exhibit reduced bioactivity. hIL-3(15-125)- presenting phage were affinity-selected with either a hIL-3-reactive p olyclonal antibody or with cells expressing the heterodimeric hIL-3 re ceptor. These data are consistent with the use of phage-display techno logy for the affinity selection of hIL-3 variants with modified biolog ical properties.