Obesity is a risk factor for cancer incidence and cancer mortality. The association of obesity and cancer is attributed to multiple factors, but the tightest linkage is with the chronic, low-grade inflammation that accompanies obesity. Myeloid-derived suppressor cells (MDSC) are known facilitators of cancer progression that act by suppressing the activation and function of tumor-reactive T cells. Because MDSC quantity and function are driven by chronic inflammation, we hypothesized that MDSC may accumulate in obese individuals and facilitate tumor growth by suppressing antitumor immunity. To test this hypothesis, tumor-bearing mice on a high fat or low fat diet (HFD or LFD) were assessed for tumor progression and the metabolic dysfunction associated with obesity. HFD enhanced the accumulation of MDSC, and the resulting MDSC had both beneficial and detrimental effects. HFD-induced MDSC protected mice against diet-induced metabolic dysfunction and reduced HFD-associated inflammation, but also increased the accumulation of fat, enhanced tumor progression, and spontaneous metastasis and reduced survival time. HFD-induced MDSC facilitated tumor growth by limiting the activation of tumor-reactive CD8 T cells. Leptin, an adipokine that regulates appetite satiety and is overexpressed in obesity, undergoes crosstalk with MDSC in which leptin drives the accumulation of MDSC while MDSC down-regulate the production of leptin. Collectively, these studies demonstrate that although MDSC protect against some metabolic dysfunction associated with HFD they enhance tumor growth in HFD mice and that leptin is a key regulator linking HFD, chronic inflammation, immune suppression, and tumor progression.

During successful pregnancy, a woman is immunologically tolerant of her genetically and antigenically disparate fetus, a state known as maternal-fetal tolerance. How this state is maintained has puzzled investigators for more than half a century. Diverse, immune and nonimmune mechanisms have been proposed; however, these mechanisms appear to be unrelated and to act independently. A population of immune suppressive cells called myeloid-derived suppressor cells (MDSCs) accumulates in pregnant mice and women. Given the profound immune suppressive function of MDSCs, it has been suggested that this cell population may facilitate successful pregnancy by contributing to maternal-fetal tolerance. We now report that myeloid cells with the characteristics of MDSCs not only accumulate in the circulation and uterus of female mice following mating but also suppress T cell activation and function in pregnant mice. Depletion of cells with the phenotype and function of MDSCs from gestation d 0.5 through d 7.5 resulted in implantation failure, increased T cell activation, and increased T cell infiltration into the uterus, whereas induction of MDSCs restored successful pregnancy and reduced T cell activation. MDSC-mediated suppression during pregnancy was accompanied by the down-regulation of L-selectin on naïve T cells and a reduced ability of naïve T cells to enter lymph nodes and become activated. Because MDSCs regulate many of the immune and nonimmune mechanisms previously attributed to maternal-fetal tolerance, MDSCs may be a unifying mechanism promoting maternal-fetal tolerance, and their induction may facilitate successful pregnancy in women who spontaneously abort or miscarry because of dysfunctional maternal-fetal tolerance.