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-1β, IL-6, TNFα, VEGF, and GM-CSF, while CD11b(+ )MDSC induction correlated with over-expression of FLT3L and TGFβ. 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 HIF1α, STAT3, and C/EBPβ 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)HIF1α(+)/STAT3(+ )and CD11b(+)HLA-DR(low)C/EBPβ(+), which should enable the development of novel diagnostic and therapeutic reagents for cancer immunotherapy.