Control by myeloid-derived suppressor cells and CCL5 of transplant tolerance and intragraft regulatory T cell localization.

Nahzli Dilek¹, Nicolas Poirier¹, Claire Usal¹, Bernard Martinet¹, Gilles Blancho¹, Bernard Vanhove¹

¹Institute of Transplantation Urology Nephrology, INSERM UMR 643, University of Nantes, Nantes, France

Myologie-derived suppressor cells (MDSCs) are a heterogeneous population of immature cells that are known to inhibit immune responses in tumor microenvironment. We previously demonstrated their role also in the control of kidney transplant tolerance in association with intragraft Treg cells.

In this work, we analyzed by DNA microarray gene expression in blood MDSCs from tolerant LEW.1A recipients of allogeneic class I and II missmatched LEW.1W kidney allografts and fond 525 significant differences compared to syngeneic graft recipients. CCL5 (Rantes), a chemotactic cytokine playing an active role in recruiting leukocytes into inflammatory sites, and specially attracting Treg cells, was the most down-regulated gene in blood MDSCs from tolerant rats (160-fold, p=0<0.001). This downregulation was confirmed by qRT-PCR in independent samples and translated into a 2-fold decrease of CCL5 protein plasmatic level (0.60±0.17 vs 1.17±0.48 ng/ml; p<0.01). In contrast, intragraft levels of CCL5 were higher in tolerated allografts as compared to control syngeneic grafts. Therefore we hypothesized that a gradient of CCL5 between the graft itself and peripheral blood could contribute to the intragraft localization of Treg cells in tolerant animals. To test the hypothesis, we treated tolerant rats with recombinant rat CCL5 so as to restore normal plasma concentration. This led to an increase of creatinine and urea concentrations and to kidney graft rejection after about a month. This rejection was accompanied by a strong reduction of intragraft Treg cells monitored by immunohistofluorescence and by Foxp3 mRNA assessed by qRT-PCR (decrease of 24.40±5.47 fold; p<0.05). Kidney function of syngeneic grafts was not affected by a similar administration of CCL5.

In conclusion, our data demonstrate that peripheral MDSCs repress CCL5 gene expression in tolerant kidney graft recipients which contributes to the establishment of a graft-to-periphery gradient and to the recruitment of Treg cells into the graft where they obviously maintain tolerance.