Monoclonal antipeptide antibodies recognize IL-3 and neutralize its bioactivity in vivo

Mixtures of synthetic peptides corresponding to segments of murine IL-3 or synthetic IL-3 were used to raise murine mAb. Several mAb able to recognize synthetic IL-3 were obtained, two of which exhibited significant cross-reactivity with native IL-3 as shown by precipitation of biosynthetically 35S-labeled IL-3 and their effectiveness as affinity reagents for the purification of IL-3 from conditioned medium. The amino acid sequence recognized by the two mAb was determined by using synthetic peptide segments of IL-3. In both cases binding of the mAb to synthetic IL-3 was inhibited best with a hexapeptide corresponding to the amino acid residues 130-135 of IL-3, although the mAb differed in other characteristics. Neither mAb neutralized IL-3 bioactivity in vitro. However, we observed that in vivo administration of one mAb abrogated the increase in splenic mast cells and their precursors that normally occurred in mice bearing a s.c. IL-3-producing tumor, WEHI-3B.