Antibodies to VLA4 integrin mobilize long-term repopulating cells and augment cytokine-induced mobilization in primates and mice.

Although the use of cytokine-mobilized peripheral blood stem cells has gained a significant momentum in clinical transplantation, the mobilization schemes practiced are guided by a great deal of empiricism. The mechanism(s) by which cytokines or chemokines, alone or in combination, bring about redistribution of stem/progenitor cells from bone marrow to peripheral blood are poorly understood. Likewise the fate of mobilized stem/progenitor cells and their biological properties are incompletely defined. One of the leading hypotheses to explain the mechanism of cytokine-induced mobilization encompasses the view that cytokines disrupt, directly or indirectly, cytoadhesive interactions of stem/progenitor cells with their bone marrow stroma. Compatible with this view are changes in the expression and/or function of several cytoadhesion molecules, especially integrins, postmobilization, and extensive in vitro experimentation supporting the concept of cytokine/integrin interactions. To provide a further insight on the cytokine/integrin interplay in vivo, we have combined cytokine treatments with anti-integrin treatments for mobilization in primates and mice. We found that anti-VLA4 treatment combined with either granulocyte colony-stimulating factor (G-CSF ) treatment or kit ligand treatment leads to significant enhancement of mobilization efficiency (fivefold to eightfold) well above the levels produced by either cytokine alone or anti-VLA4 treatment alone. Similar enhancement was seen when combinations of cytokines, ie, G-CSF plus kit ligand or G-CSF plus Flt3-ligand were used with anti-VLA4 in primates and mice. Furthermore, when anti-VLA4 was given in 5-Fluorouracil-treated primates, significant numbers of progenitor cells were circulating for several days during the recovery period only in the anti-VLA4 treated animals. These data suggest that (1) the effect of anti-VLA4 on mobilization, when used alone, is unlikely to be mediated by secondary cytokine elaboration in vivo; (2) three different cytokines and their combinations do not appear to influence the in vivo responsiveness to anti-VLA4 in coadministration schemes; (3) even if cytokine treatments on their own exert downmodulation of VLA4 function, the target progenitor cells influenced by anti-VLA4 or by cytokines may not necessarily overlap; and (4) augmentation of mobilization in cytokine/anti-VLA4 treatments is most likely caused by an amplification of the pool of target cells on which anti-VLA4 exerts its effects. Because cytokines or anti-VLA4 are each capable of mobilizing long-term repopulating cells and because we show with the present studies that anti-VLA4 in an autologous bone marrow cell transplantation setting does not cause any delay in engraftment, the combination of cytokine/anti-integrin treatment enhancing mobilization may have a clinical use.