AIM:To investigate the frequency and clinical significance of the myeloid-derived suppressor cells(MDSC) in human colorectal carcinoma(CRC).METHODS:Samples of peripheral blood and tumor tissue from 49 CRC patients were analyzed.Mononuclear cells were isolated by Ficoll-Hypaque density gradient centrifugation and were subjected to a flow cytometrybased immunophenotypic analysis.RESULTS:A considerable increase in the percentage of CD33+HLA-DR-MDSCs was observed in the peripheral blood(1.89% ± 0.75%) and tumor tissues(2.99% ± 1.29%) of CRC patients as compared with that in theperipheral blood of healthy controls(0.54% ± 0.35%).This expanded CD33+HLA-DR-subset exhibited immature myeloid cell markers,but not lineage markers,and showed up-regulation of CD18/CD11b expression as compared with the MDSCs from healthy donors.Further studies showed that the MDSC proportion in CRC peripheral blood was correlated with nodal metastasis(P = 0.023),whereas that in tumor tissues was correlated with nodal/distant metastasis(P = 0.016/P = 0.047) and tumor stage(P = 0.028),suggesting the involvement of MDSCs in CRC tumor development.CONCLUSION:Characterization of MDSCs in CRC suggests the clinical significance of circulating and tumorinfiltrating MDSCs and may provide new insights into the CRC immunotherapy targeting MDSCs. AIM: To investigate the frequency and clinical significance of the myeloid-derived suppressor cells (MDSC) in human colorectal carcinoma (CRC). METHODS: Samples of peripheral blood and tumor tissue from 49 CRC patients were isolated. Mononuclear cells were isolated by Ficoll- RESULTS: Hypaque density gradient centrifugation and were subjected to a flow cytometry based immunophenotypic analysis. RESULTS: A considerable increase in the percentage of CD33 + HLA-DR-MDSCs was observed in the peripheral blood (1.89% ± 0.75% 1.29%) of CRC patients as compared with that in theperipheral blood of healthy controls (0.54% ± 0.35%). This expanded CD33 + HLA-DR-subset demonstrated immature myeloid cell markers, but not lineage markers, and showed up-regulation of CD18 / CD11b expression as compared with the MDSCs from healthy donors. Further studies showed that the MDSC proportion in CRC peripheral blood was correlated with nodal metastasis (P = 0.023), then that in tumor tissues was correlated with n suggesting that involvement of MDSCs in CRC tumor development. CONCLUSION: Characterization of MDSCs in CRC suggests the clinical significance of circulating and tumor infiltrating MDSCs and may provide new insights into the CRC immunotherapy targeting MDSCs.