Mast cells (MC) are a bone marrow-derived, long-lived, heterogeneous cellular population that function both as positive and negative regulators of immune responses. They are arguably the most productive chemical factory in the body and influence other cells through both soluble mediators and cell-to-cell interaction. MC are commonly seen in various tumors and have been attributed alternatively with tumor rejection or tumor promotion. Tumor-infiltrating MC are derived both from sentinel and recruited progenitor cells. MC can directly influence tumor cell proliferation and invasion but also help tumors indirectly by organizing its microenvironment and modulating immune responses to tumor cells. Best known for orchestrating inflammation and angiogenesis, the role of MC in shaping adaptive immune responses has become a focus of recent investigations. MC mobilize T cells and antigen-presenting dendritic cells. They function as intermediaries in regulatory T cells (Treg)-induced tolerance but can also modify or reverse Treg-suppressive properties. The central role of MC in the control of innate and adaptive immunity endows them with the ability to tune the nature of host responses to cancer and ultimately influence the outcome of disease and fate of the cancer patient.

Purpose To assess the clinical and pathological significance of mast cell infiltration in human pancreatic cancer and evaluate crosstalk between mast cells and cancer cells in vitro. Experimental Design Immunohistochemistry for tryptase was performed on 53 pancreatic cancer specimens. Mast cell counts were correlated with clinical variables and survival. Serum tryptase activity from cancer patients was compared to patients with benign pancreatic disease. In vitro, the effect of pancreatic cancer conditioned media on mast cell migration was assessed. The effect of conditioned media from the human mast cell line, LAD-2, on cancer and normal ductal cell proliferation was assessed by thymidine incorporation. Matrigel invasion assays were used to evaluate the effect of mast cell conditioned media on cancer cell invasion in the presence and absence of a matrix metalloproteinase inhibitor, GM6001. Results Mast cell infiltration was significantly increased in pancreatic cancer compared to normal pancreatic tissue [11.4±6.7vs.2.0±1.4(p<0.001)]. Increased infiltrating mast cells correlated with higher grade tumors (p<0.0001) and worse survival. Patients with pancreatic cancer had elevated serum tryptase activity (p<0.05). In vitro, AsPC1 and PANC-1 cells induced mast cell migration. Mast cell conditioned media induced pancreatic cancer cell migration, proliferation and invasion but had no effect on normal ductal cells. Furthermore, the effect of mast cells on cancer cell invasion was in large part MMP-dependent. Conclusions Tumor infiltrating mast cells are associated with worse prognosis in pancreatic cancer. In vitro, the interaction between mast cells and pancreatic cancer cells promote tumor growth and invasion.

T-regulatory cells (Treg) and mast cells (MC) are abundant in colorectal cancer (CRC) tumors. Interaction between the two is known to promote immune suppression or loss of Treg functions and autoimmunity. Here, we demonstrate that in both human CRC and murine polyposis the outcome of this interaction is the generation of potently immune suppressive but proinflammatory Treg (ΔTreg). These Treg shut down IL10, gain potential to express IL17, and switch from suppressing to promoting MC expansion and degranulation. This change is also brought about by direct coculture of MC and Treg, or culture of Treg in medium containing IL6 and IL2. IL6 deficiency in the bone marrow of mice susceptible to polyposis eliminated IL17 production by the polyp infiltrating Treg, but did not significantly affect the growth of polyps or the generation of proinflammatory Treg. IL6-deficient MC could generate proinflammatory Treg. Thus, MC induce Treg to switch function and escalate inflammation in CRC without losing T-cell–suppressive properties. IL6 and IL17 are not needed in this process.

Pancreatic cancer is associated with a pronounced fibrotic reaction that was recently shown to limit delivery of chemotherapy. To identify potential therapeutic targets to overcome this fibrosis, we examined the interplay between fibrosis and the key proteinase membrane type 1-matrix metalloproteinase (MT1-MMP, MMP-14), which is required for growth and invasion in the collagen-rich microenvironment. In this report we show that compared to control mice (Kras+/MT1-MMP-) that express an activating KrasG12D mutation necessary for pancreatic cancer development, littermate mice that express both MT1-MMP and KrasG12D (Kras+/MT1-MMP+) developed a greater number of large, dysplastic mucin-containing papillary lesions. These lesions were associated with a significant amount of surrounding fibrosis, increased α-smooth muscle actin(+) cells in the stroma, indicative of activated myofibroblasts, and increased Smad2 phosphorylation. To further understand how MT1-MMP promotes fibrosis, we established an in vitro model to examine the effect of expressing MT1-MMP in pancreatic ductal adenocarcinoma (PDAC) cells on stellate cell collagen deposition. Conditioned media from MT1-MMP-expressing PDAC cells grown in 3D collagen enhanced Smad2 nuclear translocation, promoted Smad2 phosphorylation and increased collagen production by stellate cells. Inhibiting the activity or expression of the TGF-β type I receptor in stellate cells attenuated MT1-MMP conditioned media-induced collagen expression by stellate cells. Additionally, a function-blocking anti-TGF-β antibody also inhibited MT1-MMP conditioned media-induced collagen expression in stellate cells. Overall, we demonstrate that the bona fide collagenase MT1-MMP paradoxically contributes to fibrosis by increasing TGF-β signaling and that targeting MT1-MMP may thus help to mitigate fibrosis.