ENHANCED PROLIFERATIVE ACTIVITY OF PIXY-321, THE GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR AND INTERLEUKIN-3 FUSION PROTEIN

Previous studies with the granulocyte-macrophage colony-stimulating fa ctor (GM-CSF) and interleukin-3 (IL-3) fusion protein, PIXY-321, demon strated the enhanced biological activity of this molecule in compariso n with GM-CSF or IL-3 alone or in combination. Here, we per formed exp eriments to determine the proliferative effect of PIXY-321 on 13 const itutively growth factor-dependent human leukemia cell Lines in compari son with GM-CSF, IL-3 and their combination using as read-out paramete rs the 48-hour H-3-thymidine incorporation assay and viable cell count s after in vitro culture for 7-8 days. Whereas one cell Line was not r esponsive to any of these three cytokines, the other 12 cell lines sho wed variable degrees of growth in response to these effector molecules . PIXY-321 increased proliferation as measured by thymidine uptake rel ative to IL-3, GM-CSF or GM-CSF + IL-3 by 34% (range 5-448%), 12% (ran ge 0-122%), and 6% (range 1-13%), respectively. PIXY-321 induced a mea n increase of 32%, 30% and 11% in cell counts relative to the values o btained with IL-3, GM-CSF or GMCSF + IL-3, respectively. Altogether, t hese data indicate that PIXY-321 stimulates proliferation of immature hematopoietic cells substantially better than equivalent concentration s of the single growth factors GM-CSF and IL-3. This hybrid growth fac tor showed a marginal to modest, but definite and reproducible increas e in proliferation compared to the combination of GM-CSF plus IL-3. In summary, the fusion cytokine protein PIXY-321 appears to have biologi cal effects superior to those elicited by its components, singly or in combination. This unique molecule should represent a useful tool in s tudies on the mechanisms underlying cytokine ligand-receptor interacti on and the subsequent signal transduction. The use of PIXY-321 provide s an opportunity for taking greater advantage in vitro and in vivo of the hematopoietic stimulatory activities of GM-CSF and IL-3.