Elucidating Novel Immunomodulatory Mechanisms in the Metastatic Cascade

Abstract

Every step of the metastatic cascade, from invasion to colonization, is modulated by the immune system. Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive malignancy of the pancreas, and the mechanisms regulating its propensity to metastasize remain mostly elusive. We sought to investigate the contribution of the fibroinflammatory responses in the PDAC tumor microenvironment (TME) to metastasis. Type 2 (Th2) immune responses evolved for anti-parasite defense and cause allergic disease, but thus far have been minimally linked to cancer. We found that eosinophils, effector cells of a Th2 response, are abundant in pancreatic cancer in mice and humans. Genetic loss of eosinophils in mice does not affect primary tumor growth but increases rates of spontaneous metastasis. Treatment with the Th2 alarmin IL-33 or the allergen papain increases intratumoral eosinophil activation and decreases metastasis. Tumor-resident eosinophils in humans and mice produce IL-4, which prevent epithelial-to-mesenchymal transition, preserving tumor cells in an epithelial state and thus stopping their egress from the primary tumor. Systemic delivery of extended half-life IL-4 or a computationally designed IL-4 mimic ameliorates PDAC prognosis, by reducing primary tumor burden and preventing metastasis to liver and lung. Besides metastasis in major organs, lymphatic colonization has been linked to worse prognosis in several cancers, as lymph nodes act as a reservoir of malignant cells, allowing their dissemination to distant organs. More recently, lymph node metastasis was found to promote systemic immunosuppression, further highlighting the need for a better understanding of the mechanisms supporting its formation. To this end, we investigated the transcriptional profiling of immune cells in pre-metastatic tumor-draining lymph nodes and observed a phenotypic change in the activation state of B cells. We developed a mouse model of trackable PDAC highly metastatic to the draining lymph node and investigated the contribution of B cells to the metastatic cascade. We observed that metastatic PDAC cells fail to successfully colonize lymph nodes when B cells are depleted either genetically, in μMT-/- mice, or following αCD20 therapy. Similar findings replicated in metastatic melanoma. We found that a role for B cells in supporting metastasis is through antigen presentation to CD4+ T cells, as absence of MHCII on B cells led to a reduction in lymph node metastasis. We analyzed CD4+ T cells in the tumor-draining lymph node of μMT-/- mice and observed a deficit in T regulatory cells, which correlated with stronger systemic anti-tumor responses. Collectively, these findings elucidate novel immunomodulatory mechanisms in the suppression or support of the metastatic cascade.