Background: MDSC increase in cancer and associate with poor prognosis, however their role in transplantation (Tx) is unknown. We aimed to study the MDSC effects on the evolution of kidney transplant recipients (KTR).
Methods: A cohort of229 KTR was prospectively studied at pre- and day 7 and 14,  month 1, 3, 6 and 12 post-Tx.Myeloid cells CD11b+CD33+CD14+CD15-HLA-DR-/low(M-MDSC),  CD33+CD11b+CD14-HLA-DRlow/-(early stage MDSC, eMDSC) and CD11b+CD33+CD14+CD15-HLA-DR+(monocytes) were enumerated, and their suppressive capacity was tested in cocultures with autologous lymphocytes. Anti-HLA antibodies, total antioxidant capacity (TAC), arginase-1 (Arg-1), matrix metalloproteinase 9 (MMP9) and suppressive cytokines were measured in serum and supernatants. In some experiments, IL2 receptor (IL2R) expression of chains α (CD25), β (CD122) and γ (CD132) and STAT5 phosphorylation (pSTAT5) after incubation with IL2 were tested. The induction of myeloid cell death by basiliximab (Bx, anti-CD25) after adding complement (C´) was also analyzed.
Results: Before Tx, HLA-DR-/low  and HLA-DR+ myeloid subsets were similar among KTR and healthy volunteers. After Tx, CD11b+CD33+CD14+CD15-HLA-DR-/lowcells expanded and remained high for the first year. In vitro, these cells inhibited CD4 and CD8 T lymphocytes proliferation and IFNγ, TNFα and GM-CSF synthesis, thus confirming them as M-MDSC. The expansion of M-MDSC was not related with post-Tx events excepting for the development of cancer. Patients with M-MDSC>45.2% and >179.2 cell/µl at post-Tx day 14 had a significantly lower cancer-free survival in the 2 years after Tx (p<0.05), and showed a higher risk for cancer than patients with low M-MDSC (p=0.029 and 0.031 respectively). Multivariate Cox regression showed that M-MDSC>179.2 cell/µl was a risk factor for cancer in KTR independent of age, sex and immunosuppression (HR=6.98, p=0.02). Circulating M-MDSCs high counts correlated with high Arg-1 (p<0.01) and IL-10 (p=0.01), and low TAC (p=0.03) in serum. M-MDSC were lower in patients with enhanced alloimmune response as represented by anti-HLA sensitization.
eMDSC levels also increased after Tx, excepting in patients who received Bx. CD25+eMDSC counts and the CD25per cell expression (MFI CD25) increased after Tx, but were not detected in patients who received Bx. eMDSC were found to express all three IL2R chains (high affinity IL2R), while M-MDSC and monocytes only expressed β and γ (intermediate affinity IL2R). After incubation of eMDSC with IL2, pSTAT5 was detected. Preincubation with Bx blocked both detection of CD25 and pSTAT5, but did not result in cell death after adding C´.
Conclusions: Early post-transplant mobilisation of M-MDSC predicts cancer and adds risk as an independent factor in KTR. M-MDSC may favor an immunosuppressive environment that promotes the tumoral development. A CD25+eMDSC subset has been identified which is under study.