Conditioning Regimens in Stem Cell Transplantations Facilitate Homing by Increasing Bone Marrow SDF-1 through Induction of SDF-1 Gene Transcription.

Stromal Derived Factor-1 (SDF-1) is a key regulator of engraftment. The clinical relevance of SDF-1 is suggested by the relation between in vitro migratory capacity of CD34-positive cells towards SDF-1 and engraftment after stem cell transplantation (SCT). For better insight in engraftment after SCT clarifying the determinants of SDF-1 is therefore important. We investigated SDF-1 responses after chemotherapy and irradiation of the murine stromal M210B4 cell line and in allogeneic SCT patients. SDF-1 protein levels were measured in conditioned medium (CM) of the M210B4 cell line, in peripheral blood plasma and bone marrow plasma by ELISA. The migration supportive capacity of CM was tested by allowing CD34-positive cells to migrate over fibronectin coated transwells for 4 hours. Furthermore, sdf-1 gene expression was investigated by means of a quantitative PCR using mRNA from M210B4 cells or from buffy coats of peripheral blood and bone marrow cells.

Both irradiation and cytarabine treatment of M210B4 cells resulted in a significant ~2-fold increase in SDF-1 levels and a significant ~1.5-fold increase in migration of CD34-positive cells. Changes in protein levels correlated with those of mRNA: both treatments resulted in a significant ~2-fold increase in SDF mRNA copies.

In agreement with in vitro results, a significant ~3-fold increase in SDF-1 protein levels was induced by in vivo myeloablative and non-myeloablative conditioning regimens. The increase in SDF-1 was higher in BM plasma as compared to PB plasma, thereby significantly increasing BM-PB gradients (BM minus PB SDF-1 levels: 11.0±3.5 after versus 4.0±2.8 ng before conditioning). Corresponding with protein levels, sdf-1 mRNA increased in bone marrow cells of 4/5 patients (~40-fold increase, from 0.3±0.3% to 9.7±15.2% relative to GAPDH expression, n=4 of whom 2 received myeloablative and 2 non-myeloablative regimens). Wide ranges of increase in SDF-1 message (ranging from 0.26 to 104-fold increase) and protein levels (1.26 to 5.16-fold increase) were observed, which may be accounted for by differences in conditioning regimens.

We conclude that generation of an SDF-1 gradient over the blood-bone marrow barrier by conditioning regimens plays a role in engraftment. The introduction of reduced intensity stem cell transplantations challenged the idea that sublethal conditioning is a prerequisite for engraftment by preparing vacant niches. Our observation that both non-myeloablative and myeloablative regimens result in an increase in SDF-1 gradient, can be proposed as an explanation. Interestingly, the variation in increase of SDF-1 after different conditioning regimens indicate that homing efficiency may be improved by reconsidering the optimal moment of stem cell infusion and the agents used in conditioning regimens.