Purpose: Effector and regulatory T cells (Tregs) are main cell populations in the context of cardiac allograft vasculopathy (CAV). The immunostimulant plerixafor mobilizes hematopoietic stem cells and may boost T cell. Therefore, the aim of this study was to evaluate if a single or repeated application as well as continuous treatment with plerixafor increases peripheral bone marrow derived stem cells including Tregs leading to tolerance induction and thus reducing chronic rejection in a mouse allograft transplant model.

Methods: Fully allogeneic mouse aortas from C57BL/6 mice (H2b) were abdominally transplanted into CBA mice (H2k). The continuous plerixafor application [1mg/kg/d] was carried out for 14 days using implanted small osmotic pumps. Single dose (s.c.) injection of 1mg/kg/d or 5mg/kg/d were done at day 1 after transplantation (Tx.) and pulsed injections [1mg/kg/d] were conducted on days 1, 7, 14, and 21 after Tx. The control allograft group received vehicle loaded osmotic pumps. Stem cell mobilization was monitored by FACS. Recipients were sacrificed on day 14 for intragraft gene expression analysis or on day 30 for histological measurements.

Results: Murine aortic grafts with pulsed plerixafor injections showed significantly reduced neointima proliferation compared to control allografts (33.65%±8.84% vs.53.13% ±12.41%; p<0.05). The single shot and continuous treatment groups exhibited no improvement concerning neointima formation vs. the untreated group. First FACS analysis revealed significantly less hematopoietic stem cells (HSC) in the bone marrow of plerixafor treated mice vs. the control at day 14 after Tx. Though, there were significantly more HSCs in the peripheral blood on day 30 after Tx, with the pulsed injection even doubling HSCs [0.0152%±0.008; p < 0.005 (pulsed); 0.0046%±0.002%; p < 0.01 (pump); 0.0076%±0.001%; p < 0.01 (single dose of 1mg/kg); 0.0039%±0.0017%; p < 0.1 (single dose of 5mg/kg) vs. 0.0018%±0.0016% (control)]. Intragraft gene expression results showed clearly reduced IFNγ expression and a significantly increase of IL-4,  IL-10 and TGFβ.

Conclusion: The data suggests that pulsed, continued and single dose application of plerixafor leads to potent stem cell mobilization and hereby the repeated treatment with plerixafor reduces neointima formation in a mouse aortic transplant model.