Myeloid-derived suppressor cells (MDSC) are a heterogeneous group of myeloid cells with T-cell suppressive activity. While the identity and function of circulating MDSC is fairly well established in human cancer patients, the nature of potential MDSC in tissues remains largely enigmatic. However, understanding the biology of MDSC in human tissues is of pivotal importance, because MDSC are believed to regulate the activity of antitumor T cells and may serve as a major resistance mechanism in modern cancer immunotherapy. We developed a whole-mount tissue processing and staining procedure to mark PMN-MDSC subsets and T-cell effector function in situ in an intact 3-dimensional tissue architecture. We then used ultramicroscopy, digital image analysis and 3-D image reconstruction of original data to analyze and quantify the spatial distribution of MDSC and T cells. We identified intratumoral hot spots with high prevalence for both MDSC and T cells. T-cell effector function was reduced in these hot spots. Multicolor immunofluorescence and high-resolution image analysis also revealed the identity and frequency of cellular conjugates between MDSC and T cells. Quantitative spatial analyses indicated that immunosuppressive activity of MDSC requires close proximity of MDSC and T cells. Obtained data were applied to 2-D multicolor immunofluorescence analysis in a larger cohort of head and neck cancer patients with well-defined follow-up. Patients with evidence for strong downregulation of T-cell activity by PMN-MDSC presented with a significantly impaired survival, suggesting a clinical relevance of our findings. Our data suggest that co-targeting of PMN-MDSC and T cells could be beneficial in a subgroup of cancer patients. We also identified parameters useful to select those patients, who are likely to benefit from such immuno-combinations.

Citation Format: Yu Si, Anthony Squire, Simon Merz, Kirsten Bruderek, Stephan Lang, Matthias Gunzer, Sven Brandau. Quantitative high-resolution tissue analysis defines intratumoral hot spots of PMN-MDSC activity in situ [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B12.