NEW INSIGHTS INTO THE NEGATIVE REGULATION OF HEMATOPOIESIS BY CHEMOKINE PLATELET-FACTOR-4 AND RELATED PEPTIDES

Platelet factor 4 (PF4) has been recognized as an inhibitor of myeloid progenitors. However, the mechanism of action of this chemokine remai ns poorly understood, The present study was designed to determine its structure/function relationship. A series of peptides overlapping the C-terminal and central regions of PF4 were analyzed in vitro for their action on murine hematopoietic progenitor growth to assess the minima l sequence length required for activity. The peptides p17-58 and p34-5 8 possessed an increased hematopoietic inhibitory activity when compar ed with PF4, whereas the shorter peptides p47-58 and p47-70 were equiv alent to the native molecule and the peptide p58-70 was inactive. The PF4 functional motif DLQ located in 54-56 was required for the activit y of these peptides, The peptide p34-58 impaired to a similar extent t he growth of colony-forming unit-megakaryocyte (CFU-MK) as well as bur st-forming unit-erythroid (BFU-E) and colony-forming unit-granulocyte- macrophage (CFU-GM), whereas PF4 was more active on CFU-MK. In the exp eriments using purified murine CD34(+) marrow cells, statistically sig nificant inhibition induced by p34-58 was shown at concentrations of 2 .2 nmol/L or greater for progenitors of the three lineages, whereas th at induced by PF4 was seen at 130 nmol/L for CFU-MK and 650 nmol/L for CFU-GM and BFU-E, indicating that the p34-58 acts directly on hematop oietic progenitors and its activity is approximately 60- to 300-fold h igher than PF4, The p34-58, unlike PF4, lacked affinity for heparin an d its inhibitory activity could not be abrogated by the addition of he parin, In addition, an antibody recognizing p34-58 neutralized the act ivity of p34-58 but not whole PF4 molecule, These results demonstrate that PF4 contains a functional domain in its central region, which is independent of the heparin binding properties, and provide evidence fo r a model of heparin-dependent and independent pathways of PF4 in inhi biting hematopoiesis. (C) 1998 by The American Society of Hematology.