Exclusion of T Cells From Pancreatic Carcinomas in Mice Is Regulated by Ly6Clow F4/80+ Extratumoral Macrophages

Abstract

Background & Aims Immunotherapies that induce T-cell responses have shown efficacy against some solid malignancies in patients and mice, but these have little effect on pancreatic ductal adenocarcinoma (PDAC). We investigated whether the ability of PDAC to evade T-cell responses induced by immunotherapies results from the low level of immunogenicity of tumor cells, the tumor's immunosuppressive mechanisms, or both. Methods KrasG12D/+; Trp53R172H/+; Pdx-1-Cre (KPC) mice, which develop spontaneous PDAC, or their littermates (controls) were given subcutaneous injections of a syngeneic KPC-derived PDAC cell line. Mice were then given gemcitabine and an agonist of CD40 to induce tumor-specific immunity mediated by T cells. Some mice were also given clodronate-encapsulated liposomes to deplete macrophages. Tumor growth was monitored. Tumor and spleen tissues were collected and analyzed by histology, flow cytometry, and immunohistochemistry. Results Gemcitabine in combination with a CD40 agonist induced T cell-dependent regression of subcutaneous PDAC in KPC and control mice. In KPC mice given gemcitabine and a CD40 agonist, CD4+ and CD8+ T cells infiltrated subcutaneous tumors, but only CD4+ T cells infiltrated spontaneous pancreatic tumors (not CD8+ T cells). In mice depleted of Ly6Clow F4/80+ extra-tumor macrophages, the combination of gemcitabine and a CD40 agonist stimulated infiltration of spontaneous tumors by CD8+ T cells and induced tumor regression, mediated by CD8+ T cells. Conclusions Ly6Clow F4/80+ macrophages that reside outside of the tumor microenvironment regulate infiltration of T cells into PDAC and establish a site of immune privilege. Strategies to reverse the immune privilege of PDAC, which is regulated by extra-tumor macrophages, might increase the efficacy of T cell immunotherapy for patients with PDAC.

Stromal cues regulate the pancreatic cancer epigenome and metabolome

“…This cellular compartment not only cooperates with oncogenic alterations to drive pancreatic tumorigenesis (7,8) but compromises the efficacies of cytotoxic and immune-targeted therapies (9)(10)(11)(12). Stromal alterations during pancreatic tumorigenesis include activation of tissue-resident stellate cells: Although stellate cells contribute to tissue homeostasis and maintenance of the basement membrane under normal conditions (13), these cells transdifferentiate into myofibroblast-like cells in the context of tissue damage or during pancreatic tumor progression (14).…”

Immunotherapy and Combination Strategies in Pancreatic Cancer: Current Status and Emerging Trends

“…Increasing evidence suggests that dysregulated polarization of TAM, the most abundant immune cell population in the tumor microenvironment, facilitates tumor growth and metastasis [9,30,31]. TAMs are a heterogeneous population of myeloid cells that differentiate in the tumor microenvironment and become sensitized to tumor-derived suppressive signals [32].…”

“…Besides, pancreatic cancer is characterized by a highly immunosuppressive microenvironment in which dense desmoplasia with prominent infiltration of tumor-promoting tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) is present [8], interfering with T-cell infiltration into the tumor microenvironment. As a result, sufficient and effective Tcell responses cannot be elicited against pancreatic tumors [9].…”

The role of the tumour microenvironment in immunotherapy

“…The immune excluded phenotype of PCs is partially regulated by macrophages residing outside of the tumour microenvironment. Depletion of these macrophages restored T cell immunotherapy efficacy (Beatty et al 2015). Other major contributors of the immunosuppressive tumour microenvironment in PCs include suppressive myeloid cells and regulatory T cells (Tregs) (Morse et al 2009, Delitto et al 2016.…”