From the Spring 2013 Penn Medicine Gastroenterology Division NewsletterThis edition of the Gastroenterology Newsletter features a selection of the groundbreaking research examining the pathophysiology, genetics, diagnostics and treatment of pancreatic cancer and cystic disease. These studies represent a cooperative effort between the Divisions of Gastroenterology and Hematology-Oncology, the Abramson Cancer Center and the Departments of Medicine, Surgery, Pathology and Laboratory Medicine, and Oncology and Clinical Pharmacology at Penn Medicine.
Myeloid Inflammation and T Cell Immunity in Pancreatic Cancer
The tumor microenvironment of pancreatic ductal adenocarcinoma (PDA) is defined by active suppression of the immune response concomitant to inflammatory cell-associated tumor development and progression. Recently, a team of researchers at Penn Medicine identified granulocyte-macrophage colony-stimulating factor (GM-CSF) as an important regulator of inﬂammation and immune suppression in PDA.
The team, comprised of specialists from the Abramson Cancer Center, the Divisions of Hematology-Oncology and Gastroenterology, and the Departments of Medicine and Pathology and Laboratory Medicine, was led by Robert H. Vonderheide, MD, DPhil, with contributions from Ben Z. Stanger MD PhD.
The GM-CSF finding was the result of a wider investigation into the role of the antigens Gr-1 and CD11b in PDA. Both antigens are expressed as markers on myeloid-derived suppressor cells (MDSCs), which contribute to immunosuppression in PDA. Numerous in vitro studies have reported the expansion of Gr-1+ CD11b+ cells in implantable tumor models. However, the in vivo relevance of the T cell suppressive qualities remains controversial.
The Penn researchers focused on the KPC mouse model of spontaneous PDA to evaluate a mechanism of tumor-induced immune modulation critical to maintaining the local immune suppressive network characteristic of the disease. KPC mice develop primary PDA lesions that faithfully recapitulate features of the human disease, including progression from preinvasive pancreatic intraepithelial neoplasia (PanIN) to invasive cancer to metastatic disease.
The Penn researchers found that the dense desmoplasia and leukocytic inﬁltration classically observed in the tumor stroma of patients with PDA is reproduced in tumors of KPC mice. In addition, Gr-1+ CD11b+ cells were shown to accumulate in the spleen as well as the tumor in this model, where these cells maintained a close proximity to tumor cells and were prominently associated with metastatic lesions.
The authors noted that Gr-1+ CD11b+cells derived from tumor-bearing KPC mice suppressed the proliferation of splenic T cells from normal mice and that the cells exhibited high levels of arginase activity and nitrite, suggesting expression of inducible nitric oxide synthase (iNOS); both arginase and iNOS have been previously linked to immunosuppression by Gr-1+ CD11b+ cells in tumorbearing mice.
Splenic cells proved to be the link to the potential origin of Gr-1+ CD11b+ cells in PDA. In the KPC murine model, splenocytes from tumor-bearing KPC mice exhibited a c-kit+ population similar in percentage to that of c-kit+ precursors found in bone marrow and higher than that found in splenocytes from normal mice. C-kit is a cell surface marker used to identify some types of hematopoietic progenitors in bone marrow.
When the researchers isolated c-kit+ Gr-1+ CD11b+ lineage cells from the spleens of tumor-bearing KPC mice and incubated them with conditioned media obtained from previously isolated cultured PDA tumor cells, c-kit+ cells expressed high levels of Gr-1 and CD11b, exhibited arginase and iNOS activity, and potently suppressed T cell proliferation in the OT-1 T cell suppression assay.
The Penn researchers hypothesized that a tumor-derived factor might drive the generation of Gr-1+ CD11b+ cells from c-kit+ cells in the spleen. To identify this factor, a set of secreted proteins from a panel of PDA tumor cell lines was measured in conditioned media and the results compared to those for conditioned media from benign pancreatic ductal cells from normal control mice. Conditioned media from every PDA line supported proliferation of c-kit+ cells into Gr-1+ CD11b+ cells, whereas media from none of the normal pancreatic ductal cells supported c-kit+ cell proliferation.
Among 11 proteins examined, only granulocyte-macrophage colony-stimulating factor was expressed at high levels by every PDA line but by none of the normal pancreatic ductal lines, suggesting that tumor-derived GM-CSF might be linked to Gr-1+ CD11b+ cell generation.
When recombinant GM-CSF was tested in in vitro assays, the researchers found that GM-CSF drove proliferation and differentiation of c-kit+ Gr-1+ CD11b+ splenocytes isolated from tumor-bearing mice into functional myeloid-derived suppressor cells. Further investigation concluded that GMCSF
is both necessary and sufficient for in vitro generation of functional, immunosuppressive Gr-1+ CD11b+cells, and that in vivo GM-CSF secreted by transformed pancreatic epithelial cells is critically involved in the regulation of inflammation associated with PDA.
When tumor-derived GM-CSF was abrogated in vivo, tumors failed to grow, rejected by an T cell response. Importantly, the investigators showed that GM-CSF is expressed by more than 95% of pancreatic tumors from patients, providing further rationale for novel strategies to inhibit GM-CSF in clinical trials.
This study was published in Cancer Cell.
Bayne LJ, Beatty GL, Jhala N, Clark CE, Rhim AD, Stanger BZ, Vonderheide
RH. Tumor-Derived Granulocyte-Macrophage Colony-Stimulating Factor
Regulates Myeloid Inﬂammation and T Cell Immunity in Pancreatic Cancer.
Cancer Cell. 2012 Jun 12;21(6):822-35.