Background: Many studies have demonstrated mesenchymal stem cells (MSCs) could inhibit transplant rejection. However, the agreement on the correlation of transfused time or dose with its effect has not been reached. The aim of this study was to investigate the correlation of transfusion timing and dose of MSCs with its effect on inhibition of transplant rejection.

Methods: Human MSCs (hMSCs) were intravenously administered to Balb/c recipients at gradient doses, 1×10⁶ (Low dose), 2×10⁶ (Medium dose) and 5×10⁶ cells (High dose)/mouse, 24 h before or after cardiac transplantation from C57BL/6 donors. Allograft survival was compared, and T cells and B cells were monitored 7 days after transplant.

Results: Pre-transplant infusion of hMSCs prolonged graft survival to 10 days at high dose, while low or medium dose didn't show beneficial effect. Post-transplant infusion of hMSCs prolonged graft survival to 9.5 (low dose), 12 (medium dose) and 21 days (high dose) respectively. Post-transplant MSCs enhanced longer graft survival than pre-transplant MSCs at high dose (21 vs 10 days, p<0.05). Lymphocyte infiltration to cardiac grafts was reduced by hMSCs treatment, especially in the post-transplant high dose group. Compared to the control group, CD4⁺ and CD8⁺T cells in the post-transplant high dose group respectively decreased from 20.2% to 11.5% and from 10.6% to 6.3% in spleens (p<0.01), and T cells activation (CD25⁺ expression) was also inhibited from 4.0% to 1.6% (p<0.01). Moreover, CD4+Foxp3+regulatory T cells increased by 10% in spleens and 25% in draining lymph nodes. Of note, CD5+CD19+B cells also increased by 20-25% in spleens.

Conclusions: Transfusion time and dose of hMSCs did affect transplant outcome in mice. hMSCs may take effect through inhibition of Th1 immune response and increase of regulatory T and B cells, while the very primary initiative mechanism deserves further investigation.