Suppressing the rejection of pancreatic tumours

Credit: Rubber ball

The immune system is often co-opted by developing tumours to promote an immunosuppressive microenvironment that prevents rejection. This is particularly evident for pancreatic ductal adenocarcinoma (PDA), which initially develops as pancreatic intraepithelial neoplasia (PanIN; a premalignant lesion) and is associated with the infiltration of immunosuppressive leukocytes. Two papers in Cancer Cell now characterize a mechanism of immunosuppression in pancreatic tumorigenesis.

MDSCs are recruited by KRAS-G12D-transformed pancreatic tumour cells ... to suppress CD8⁺ T cell-mediated antitumour immune responses

To investigate immunomodulation in PDA development, Bayne and colleagues used KPC genetically engineered mice, which have pancreas-specific Kras^G12D and Trp53^R172H mutations. Pylayeva-Gupta and colleagues used an allograft model in which green fluorescent protein (GFP)⁺ pancreatic ductal epithelial cells (PDECs) expressing KRAS-G12D were injected into the pancreata of syngeneic mice. Both mouse models developed PanINs and PDAs with features of the human disease, particularly the infiltration of myeloid-derived suppressor cells (MDSCs). MDSCs (which are GR1⁺CD11b⁺) seem to be immature myeloid cells with similarities to macrophages, dendritic cells and granulocytes and are thought to suppress T cell-mediated antitumour immune responses. Bayne and colleagues found that MDSCs derived from KPC mice suppressed T cell proliferation and the production of interferon-γ in vitro. These authors noted that KPC mice developed splenomegaly as MDSCs and their KIT⁺ precursors accumulated in the spleen, indicating that a tumour-derived factor might promote the development of MDSCs. Indeed, they found that culturing KIT⁺ splenocytes isolated from tumour-bearing KPC mice with conditioned media from PDA cells promoted their proliferation and the expression of GR1 and CD11b; these cells suppressed T cell proliferation in vitro.

Based on the splenomegaly and the recruitment of MDSCs to PanINs and PDAs, both groups looked for a tumour cell-derived secreted factor that could be responsible. Analyses of supernatants from allograft tumour tissue and from the culture of a panel of pancreatic tumour cell lines showed that granulocyte–macrophage colony-stimulating factor (GM-CSF) was consistently highly expressed. Tumour cells in human PanIN and PDA samples also expressed GM-CSF, but Pylayeva-Gupta and colleagues found that other types of pancreatic lesion that are not associated with oncogenic KRAS did not. Transcripts of Csf2 (which encodes GM-CSF) were also increased in GFP⁺ KRAS-G12D PDECs, indicating that KRAS-G12D causes the upregulation of GM-CSF expression. This cytokine induced the proliferation and differentiation into MDSCs of bone marrow-derived haematopoietic precursor cells isolated from the allograft mouse model, and of KIT⁺ splenocytes from KPC mice.

Does GM-CSF secretion by pancreatic tumour cells promote an immunosuppressive microenvironment in vivo? Bayne and colleagues implanted GM-CSF-suppressed PDA1 cells into wild-type mice and found that the cells failed to grow and that the implant sites appeared necrotic. Moreover, the number of infiltrating CD45⁺GR1⁺CD11b⁺ cells at these sites was substantially decreased compared with sites of control PDA1 cell implantation; depletion of GR1⁺CD11b⁺ cells also inhibited the growth of PDA1 cells in vivo. Pylayeva-Gupta and colleagues knocked down GM-CSF expression in GFP⁺ KRAS-G12D PDECs, which reduced their engraftment and specifically decreased the infiltration of GR1⁺CD11b⁺ cells. These GFP⁺ cells were absent from the graft sites 2 weeks after implantation (although the grafts were similar to controls 1 week after implantation). Moreover, the graft sites had infiltrates of CD8⁺ T cells 2 weeks after implantation, but not after 1 week, indicating that GM-CSF-mediated recruitment of MDSCs probably occurs when T cells begin to infiltrate graft sites. Both groups then showed that implanting GM-CSF-suppressed PDA1 cells or GFP⁺ KRAS-G12D PDECs into mice lacking CD8⁺ T cells restored engraftment but did not rescue the infiltration of GR1⁺CD11b⁺ cells.

Together, these papers show that GR1⁺CD11b⁺ MDSCs are recruited by KRAS-G12D-transformed pancreatic tumour cells in the early stages of tumour development to suppress CD8⁺ T cell-mediated antitumour immune responses and thereby allow tumour growth.