Pancreatic cancer is characterized by the accumulation of a fibro-inflammatory stroma. Accumulation of the stroma is already evident surrounding Pancreatic Intraepithelial Neoplasias (PanINs), common precursor lesions to pancreatic cancer (Hezel et al., 2006). The stroma is abundantly infiltrated by immune cells, and myeloid cells are a predominant population (Clark et al., 2007). Different myeloid subsets have been correlated with tumor promotion and unmasking of anti-tumor immunity (Liou et al., 2015; Long et al., 2016; Mitchem et al., 2013; Stromnes et al., 2014). Both PanINs and pancreatic cancer and commonly associated with oncogenic mutations in the Kras gene (Biankin et al., 2012; Jones et al., 2010; Kanda et al., 2012). Expression of oncogenic Kras in the pancreas of genetically engineered mice recapitulates the PanIN to pancreatic cancer progression, including the accumulation of fibrotic stroma (Hingorani et al., 2003). We have described a mouse model that allows inducible and reversible expression of oncogenic Kras in the pancreas, the iKras* mouse. Inactivation of oncogenic Kras during the PanIN stage or in cancer leads to regression of the epithelial lesions as well as to remodeling of the stroma, indicating that the accumulation of the stroma is regulated by signals derived from oncogenic Kras-expressing epithelial cells (Collins et al., 2012a).

In the current study, we have investigated the interaction between epithelial cells and myeloid cells that infiltrate the pancreas. For this purpose, we have used a combination of genetically engineered mice (iKras*p53* mice (Collins et al., 2012b)) and transplantation approaches into CD11b-DTR mice (Duffield et al., 2005), that allow depletion of myeloid cells upon administration of Diphtheria Toxin. Our results show that the infiltration and polarization of macrophages in the pancreas depends on signals derived from oncogenic Kras-expressing epithelial cells, either directly or through activation of a pro-inflammatory subset of stromal fibroblasts. Conversely, myeloid cells infiltration is required for the progression of PanINs and pancreatic cancer. Depletion of myeloid cells prevented KrasG12D driven pancreatic cancer initiation. In pre-established tumors, myeloid cell depletion resulted in arrest of growth or tumor regression. We observed that tumor progression was dependent on myeloid cell-mediated blockade of CD8+ T cell anti-tumor activity. Furthermore, myeloid cells regulate the expression of the Programmed death-ligand 1 (PD-L1) in tumor cells in an EGFR/MAPK dependent manner.

Our results show that myeloid cells regulate a complex network of signals that ensure immune suppression within the pancreatic cancer microenvironment. Moreover, we show that depletion of the myeloid cell population restores anti-tumor immunity mediated by CD8+ T cells, a finding with implications for the design of immune therapies for pancreatic cancer.

References:

Biankin, A. V., Waddell, N., Kassahn, K. S., Gingras, M. C., Muthuswamy, L. B., Johns, A. L., Miller, D. K., Wilson, P. J., Patch, A. M., Wu, J., et al. (2012). Pancreatic cancer genomes reveal aberrations in axon guidance pathway genes. Nature 491, 399-405.

Clark, C. E., Hingorani, S. R., Mick, R., Combs, C., Tuveson, D. A., and Vonderheide, R. H. (2007). Dynamics of the immune reaction to pancreatic cancer from inception to invasion. Cancer Res 67, 9518-9527.

Collins, M. A., Bednar, F., Zhang, Y., Brisset, J. C., Galban, S., Galban, C. J., Rakshit, S., Flannagan, K. S., Adsay, N. V., and Pasca di Magliano, M. (2012a). Oncogenic Kras is required for both the initiation and maintenance of pancreatic cancer in mice. J Clin Invest 122, 639-653.

Collins, M. A., Brisset, J. C., Zhang, Y., Bednar, F., Pierre, J., Heist, K. A., Galban, C. J., Galban, S., and di Magliano, M. P. (2012b). Metastatic pancreatic cancer is dependent on oncogenic Kras in mice. PLoS One 7, e49707.

Duffield, J. S., Forbes, S. J., Constandinou, C. M., Clay, S., Partolina, M., Vuthoori, S., Wu, S., Lang, R., and Iredale, J. P. (2005). Selective depletion of macrophages reveals distinct, opposing roles during liver injury and repair. J Clin Invest 115, 56-65.

Hezel, A. F., Kimmelman, A. C., Stanger, B. Z., Bardeesy, N., and Depinho, R. A. (2006). Genetics and biology of pancreatic ductal adenocarcinoma. Genes Dev 20, 1218-1249.

Hingorani, S. R., Petricoin, E. F., Maitra, A., Rajapakse, V., King, C., Jacobetz, M. A., Ross, S., Conrads, T. P., Veenstra, T. D., Hitt, B. A., et al. (2003). Preinvasive and invasive ductal pancreatic cancer and its early detection in the mouse. Cancer Cell 4, 437-450.

Jones, S., Wang, T. L., Shih Ie, M., Mao, T. L., Nakayama, K., Roden, R., Glas, R., Slamon, D., Diaz, L. A., Jr., Vogelstein, B., et al. (2010). Frequent mutations of chromatin remodeling gene ARID1A in ovarian clear cell carcinoma. Science 330, 228-231.

Kanda, M., Matthaei, H., Wu, J., Hong, S. M., Yu, J., Borges, M., Hruban, R. H., Maitra, A., Kinzler, K., Vogelstein, B., and Goggins, M. (2012). Presence of somatic mutations in most early-stage pancreatic intraepithelial neoplasia. Gastroenterology 142, 730-733 e739.

Liou, G. Y., Doppler, H., Necela, B., Edenfield, B., Zhang, L., Dawson, D. W., and Storz, P. (2015). Mutant KRAS-induced expression of ICAM-1 in pancreatic acinar cells causes attraction of macrophages to expedite the formation of precancerous lesions. Cancer Discov 5, 52-63.

Long, K. B., Gladney, W. L., Tooker, G. M., Graham, K., Fraietta, J. A., and Beatty, G. L. (2016). IFNgamma and CCL2 Cooperate to Redirect Tumor-Infiltrating Monocytes to Degrade Fibrosis and Enhance Chemotherapy Efficacy in Pancreatic Carcinoma. Cancer Discov.

Mitchem, J. B., Brennan, D. J., Knolhoff, B. L., Belt, B. A., Zhu, Y., Sanford, D. E., Belaygorod, L., Carpenter, D., Collins, L., Piwnica-Worms, D., et al. (2013). Targeting tumor-infiltrating macrophages decreases tumor-initiating cells, relieves immunosuppression, and improves chemotherapeutic responses. Cancer Res 73, 1128-1141.

Stromnes, I. M., Brockenbrough, J. S., Izeradjene, K., Carlson, M. A., Cuevas, C., Simmons, R. M., Greenberg, P. D., and Hingorani, S. R. (2014). Targeted depletion of an MDSC subset unmasks pancreatic ductal adenocarcinoma to adaptive immunity. Gut.

Citation Format: Yaqing Zhang, Esha Mathew, Ashley Velez-Delgado, Kristen B. Long, Dongjun Li, Flor M. Mendez, Kevin Flannagan, Andrew D. Rhim, Diane M. Simeone, Gregory L. Beatty, Marina Pasca di Magliano.{Authors}. Myeloid cells are required for PD-1/PD-L1 checkpoint activation and the establishment of an immune-suppressive environment in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr IA21.