['Immune dysfunction reported in renal cell carcinoma (RCC) patients may contribute to tumor progression. Myeloid-derived suppressor cells (MDSC) represent one mechanism by which tumors induce T-cell suppression. Several factors pivotal to the accumulation of MDSC are targeted by the tyrosine kinase inhibitor, sunitinib. The effect of sunitinib on MDSC-mediated immunosuppression in RCC patients has been investigated.', 'Patient peripheral blood levels of MDSC and regulatory T-cell (Treg) and T-cell production of IFN-gamma were evaluated before and after sunitinib treatment. Correlations between MDSC and Treg normalization as well as T-cell production of IFN-gamma were examined. The in vitro effect of sunitinib on patient MDSC was evaluated.', 'Metastatic RCC patients had elevated levels of CD33(+)HLA-DR(-) and CD15(+)CD14(-) MDSC, and these were partially overlapping populations. Treatment with sunitinib resulted in significant reduction in MDSC measured by several criteria. Sunitinib-mediated reduction in MDSC was correlated with reversal of type 1 T-cell suppression, an effect that could be reproduced by the depletion of MDSC in vitro. MDSC reduction in response to sunitinib correlated with a reversal of CD3(+)CD4(+)CD25(hi)Foxp3(+) Treg cell elevation. No correlation existed between a change in tumor burden and a change in MDSC, Treg, or T-cell production of IFN-gamma. In vitro addition of sunitinib reduced MDSC viability and suppressive effect when used at >/=1.0 microg/mL. Sunitinib did not induce MDSC maturation in vitro.'] Does sunitinib mediate reversal of myeloid-derived suppressor cell accumulation in renal cell carcinoma patients?