A Paradoxical Role for Regulatory T Cells in the Tumor Microenvironment of Pancreatic Cancer

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is considered to be a poorly immunogenic cancer type that combines a low mutation burden with a strong immunosuppressive tumor microenvironment. Regulatory T cells (Tregs) are major drivers of immune suppression but their prognostic role, particularly in gastrointestinal malignancies, remains controversial. Lymphocytic infiltration in 122 PDAC samples was assessed by multispectral immunofluorescence with anti-Keratin, -CD3, -CD8, -FOXP3 and -CD163 antibodies. Differential infiltration by Tregs was analyzed in the context of transcriptomic profiles that were available for 65 tumors. High infiltration of CD3⁺CD8^- (mainly CD4⁺) T cells and, especially, of the subset expressing FOXP3 (Tregs) was associated with improved patient survival, whilst cytotoxic CD3⁺CD8⁺ T cell infiltration did not have an impact on overall survival. Transcriptomic analysis revealed three signatures in PDAC tumors comprising of epithelial-mesenchymal transition (EMT)/stromal, metabolic, and secretory/pancreatic signature. However, none of these signatures explained differences in Treg infiltration. We show that Tregs associate with improved overall survival in PDAC patients. This effect was independent of cytotoxic T cell infiltration and the transcriptomic profiles of their respective tumors. These findings provide a new layer of complexity in the study of PDAC tumor microenvironment that must be considered when developing immunotherapeutic interventions for this disease.

Keywords: pancreatic ductal adenocarcinoma (PDAC); prognosis; regulatory T cells (Treg); tumor infiltrating lymphocytes (TIL); tumor microenvironment (TME).

Figure 1

PDAC samples display heterogeneous amounts of T cell infiltration with CD3⁺CD8⁻(CD4⁺) T cells (including Tregs) being the most abundant population. Distribution and frequencies of analyzed phenotypes in PDAC samples (a). Representative images of the PDAC tumor microenvironment displaying high- and low lymphocytic infiltrate (b,c). Correlation analyses between cytotoxic CD3⁺CD8⁺ T cell and CD3⁺CD8⁻(CD4⁺) T cell counts (d), CD3⁺CD8⁻(CD4⁺) T cell and Treg counts (e), and cytotoxic CD3⁺CD8⁺ T cell and Treg counts (f). Spearman correlation coefficients (r) are presented per analysis.

Figure 2

CD3⁺CD8⁻(CD4⁺) T cells (including Tregs) and Treg infiltration positively associate with improved overall survival of PDAC patients. Survival analyses of all 122 PDAC patients based on high and low infiltration by CD3⁺CD8⁻(CD4⁺) T cells (including Tregs) (a), CD3⁺CD8⁻FOXP3⁺ Tregs (b), CD3⁺CD8⁻FOXP3⁻ cells (CD4⁺ T helper cells) (c), cytotoxic CD3⁺CD8⁺ T cells (d). High and low infiltrated groups were formed on the basis of median counts. Significance was determined by using the Log rank Mantel-Cox test.