Functional characterization of human Cd33+ And Cd11b+ myeloid-derived suppressor cell subsets induced from peripheral blood mononuclear cells co-cultured with a diverse set of human tumor cell lines

Background: Tumor immune tolerance can derive from the recruitment of suppressor cell populations, including myeloid-derived suppressor cells (MDSC). In cancer patients, MDSC accumulation correlates with increased tumor burden, but the mechanisms of MDSC induction remain poorly understood. Methods: This study examined the ability of human tumor cell lines to induce MDSC from healthy donor PBMC using in vitro co-culture methods. These human MDSC were then characterized for morphology, phenotype, gene expression, and function. Results: Of over 100 tumor cell lines examined, 45 generated canonical CD33(+)HLA-DR(low)Lineage(-) MDSC, with high frequency of induction by cervical, ovarian, colorectal, renal cell, and head and neck carcinoma cell lines. CD33(+) MDSC could be induced by cancer cell lines from all tumor types with the notable exception of those derived from breast cancer (0/9, regardless of hormone and HER2 status). Upon further examination, these and others with infrequent CD33(+) MDSC generation were found to induce a second subset characterized as CD11b(+)CD33(low)HLA-DR(low)Lineage(-). Gene and protein expression, antibody neutralization, and cytokine-induction studies determined that the induction of CD33(+) MDSC depended upon over-expression of IL-1 beta, IL-6, TNF alpha, VEGF, and GM-CSF, while CD11b(+) MDSC induction correlated with over-expression of FLT3L and TGF beta. Morphologically, both CD33(+) and CD11b(+) MDSC subsets appeared as immature myeloid cells and had significantly up-regulated expression of iNOS, NADPH oxidase, and arginase-1 genes. Furthermore, increased expression of transcription factors HIF1 alpha, STAT3, and C/EBP beta distinguished MDSC from normal counterparts. Conclusions: These studies demonstrate the universal nature of MDSC induction by human solid tumors and characterize two distinct MDSC subsets: CD33(+)HLA-DR(low)HIF1 alpha(+)/STAT3(+) and CD11b(+)HLA-(DRC)-C-low/EBP beta(+), which should enable the development of novel diagnostic and therapeutic reagents for cancer immunotherapy.