The molecular basis for the cytokine-induced defect in homing and engraftment of hematopoietic stem cells

Objective. Hematopoietic stem cell homing and engraftment is dramatically a ltered by cytokine exposure. These studies address the molecular mechanisms responsible for the observed changes in transplantation biology. Methods. Primitive murine hematopoietic stem cells were isolated by fluores cence-activated cell sorting of lineage depleted (Lin(-)) cells exhibiting low staining of Hoechst 33342 and rhodamine 123 dyes or Lin(-) cells bearin g Sea. Adhesion receptor expression was examined by immunofluorescence and reverse transcriptase polymerase chain reaction. In vitro adhesion assays w ere employed to define binding interactions between stem cells and stroma o r extracellular matrix proteins. Results. Adhesion of Lin(-)Sca(+) cells to Dexter stroma could be blocked b y about 90% with antibodies to PECAM-1, alphaa(4), or beta (1), and partial ly blocked by antibodies to alpha (5), CD44, or L-selectin. By immunofluore scence, about 30% of purified Lin(-)Ho(lo)Rho(lo) cells expressed alpha (4) , alpha (5), beta (1), and L-selectin, about 15% expressed alpha (L) and al pha (6) half expressed PECAM-1, and none expressed alpha (1) or alpha2. Aft er 48 hours in expansion cytokines, only 9% of the cells expressed alpha (4 ) and none expressed beta (1), whereas alpha (L) expression was fully resto red, PECAM-1 and L-selectin partially restored, CD44 expression was newly i nduced, and adhesion to both fibronectin and laminin was reduced. Adhesion to purified collagen, fibronectin, or laminin enhanced expression of beta ( 1) integrins. Conclusion. Expansion cytokines that move quiescent primitive hematopoietic stem cells into S phase markedly altered adhesion receptor expression and reduced their functional binding to extracellular matrix, which could reduc e engraftment after transplant. (C) 2001 International Society for Experime ntal Hematology. Published by Elsevier Science Inc.