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.

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.

Of over 100 tumor cell lines examined, 45 generated canonical CD33⁺HLA-DR^lowLineage^- 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^lowHLA-DR^lowLineage^-. 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.

These studies demonstrate the universal nature of MDSC induction by human solid tumors and characterize two distinct MDSC subsets: CD33⁺HLA-DR^lowHIF1α⁺/STAT3⁺ and CD11b⁺HLA-DR^lowC/EBPβ⁺, which should enable the development of novel diagnostic and therapeutic reagents for cancer immunotherapy.