Surface-bound stem cell factor and the promotion of hematopoietic cell expansion

In vivo, stem cell factor (SCF) exists in both a bound and soluble isoform. It is believed that the bound form is more potent and fundamentally required for the maintenance of hematopoietic stem cells (HSCs). This theory is supported by the observation that steel-Dickie mice lacking the bound isoform of SCF are unable to maintain hematopoiesis and by the fact that bound SCF displayed on the surface of transgenic cells is better able to maintain c-kit activation than soluble SCF. Further work has shown that recombinant SCF molecules, which include a surface-binding domain, are more potent than their soluble equivalent. It is generally assumed that such an elegant approach is necessary to provide the correct molecular orientation and avoid the pitfalls of random cross-linking or the denaturation associated with the adsorption of proteins to surfaces. However, in this work we demonstrate that SCF physisorbed to tissue culture plastic (TCP) is not only bioactive, but more potent than the soluble equivalent. By contrast, cross-linking of SCF via free amines is shown to compromise its bioactivity. These observations demonstrate that simple surface modification solutions cannot be discounted and with the advent of low-cost pharmaceutical grade proteins, they should not be.