Abstract B52: Characterizing the MDSCs phenotypes, expansion, and trafficking pattern and associated tumor-induced immune dysfunction

Among the prerequisites to be effective, immunotherapy must work in the presence of the host immunosuppression. Myeloid-derived suppressor cells (MDSCs) are one of the cellular mediators that have a role in limiting the induction of adaptive immune response and host responses to immunotherapy. MDSCs are an immature, heterogeneous cellular population, including hematopoietic precursors for granulocytes, macrophages, dendritic cells, and endothelial cells. They are infrequent in healthy hosts, but increase during tumor progression as a result of dysregulated myeloid cell expansion and differentiation. Tumor growth is frequently associated with an increased frequency of MDSCs that results in a loss in T-cell number and function through increased ROS and peroxynitrites production and arginase 1 (ARG1) and nitric oxide synthase 2 (NOS-2) upregulation. A better understanding of MDSCs expansion and the regulation of biodistribution and arrest would improve interventional strategies and cancer immunotherapy outcomes. Using five mammary tumor models, we have examined tumor-associated expansion of MDSCs and observed that this occurs in an organ-specific manner. Murine MDSCs can be subdivided into phenotypically and functionally distinct sub-populations according to Gr-1 expression levels; the CD11b+Gr-1 dim MDSCs with monocytic nuclei and strong T-cell suppressive activity, and CD11b+Gr-1bright having ring-shaped nuclei (polymorphonuclear cells) and a lower T-cell suppression profile. These cells can also be subset based on the expression of Ly-6C which is highly expressed on immature, bone marrow-derived cells and combined this Gr-1 expression Our studies revealed a significant increase in the Ly-6Cbright expression on the CD11b+Gr-1dim MDSCs in all organs studied in tumor-bearing mice as compared to non-tumor-bearing mice. In association with the increase in CD11b+Gr-1dimLy-6Cbright cells in the tumor non-parenchymal cells of 4T1 tumor bearing mice, increased transcripts levels of cytokines associated with MDSCs expansion (granulocyte-colony stimulating factor [G-CSF]) and MDSCs-associated immunosuppression (ARG-1, NOS-2, and cyclooxygenase 2 [COX-2]) while lower levels of these cytokines were observed in tumor NPCs from Cl66 tumor-bearing mice and non-tumor-bearing mice. In association with the increase in MDSCs and loss of T-cells in the 4T1 tumor-bearing mice there was a significant decrease in cytokine transcript levels that can enhance immune response (interferon gamma [IFN-γ] and interleukin 12 [IL-12]) from tumor non-parenchymal cells (NPCs) in 4T1 tumor bearing mice as compared to Cl66 tumor bearing mice and non-tumor bearing mice. Studies to assess where the MDSCs were expanding revealed that in 4T1 tumor bearing mice, frequency of the proliferating MDSCs is increased only in the spleen and liver compared to the non-tumor-bearing mice. Our studies also examined the affect of tumor burden on the trafficking of MDSCs. We observed that, after adoptive transfer of MDSCs isolated from 4T1 tumor-bearing mice, they arrest initially within the lungs in non-tumor-bearing mice. While in 4T1 tumor-bearing mice they rapidly transverse the lungs with arrest within the tumor tissue. We and others reported that the increase in MDSCs frequencies is associated with a decrease in CD3+ cells function and numbers. We observed a decrease in frequency of all T-cell subsets in the thymus of tumor bearing mice tested compared to the non-tumor-bearing mice suggesting defects in T-cell maturation associated with tumor burden. We investigated the effect of tumor burden on the phenotype of the CD3+ cells using the Annexin V staining and found that 4T1 tumor bearing mice have increased frequency of Annexin V positive CD3+ T-cells that are infiltrating tumor tissue compared to those in other organs. Citation Information: Clin Cancer Res 2010;16(14 Suppl):B52.