In this study, we showed that CCR2 regulates the production of CXC chemokines and IFN-α in macrophages, thereby inducing PDAC invasion and apoptosis, and that inhibition of CCR2 can improve the prognosis of PDAC.
CCR2, a receptor of the major chemokine family, CC chemokines, has been reported to play important roles in many malignancies, including PDAC (19)−(22). The efficacy of CCR2 inhibitors against PDAC has also been reported in humans and mice (23), (34), (35); however, clinical trials on human PDAC are still in phase 1, and the mouse models previously used were transplanted models. In this study, we investigated the phenotype of whole-body Ccr2 KO mice using a genetically engineered PDAC model, PKF mice, which can most closely recapitulate clinical PDAC histology. We also performed in vitro assays to understand the underlying mechanisms of action of Ccr2 in macrophages.
The possible TME of PDAC, based on the present results, is shown in Fig. 7. In the tumor-stromal interaction, PDAC cells stimulate CAFs to produce CCL2 and CCL7, which stimulate macrophages via CCR2. Macrophages produce CXCL2 and CXCL5, which promote PDAC cell invasion (Fig. 7A). CCR2 inhibition also increases IFN-α production in macrophages, which induces apoptosis of PDAC cells. In the TME, CCR2 inhibition changes the phenotype of macrophages from M2-dominant to M1-dominant and upregulates IFN-α expression. As a result, inhibition of vascular invasion and an increase in apoptosis of PDAC cells may suppress PDAC progression and improve prognosis (Fig. 7B). Although there been reported that TAMs migrate into the TME via CCR2 and contribute to tumor promotion (20)−(22), there are no reports on the CCR2-dependent production of CXCLs in macrophages. With regard to CXCL production by macrophages, it has been reported that CXCL9 and CXCL10 produced by macrophages play a cancer-promoting role in the TME of breast cancer (36), but there are no reports on CXCL2 or CXCL5.
It has been reported that IFN-α production in macrophages is downregulated via CCR2 (37), which is consistent with our results that IFN-α production is increased in Ccr2-KO macrophages. Fourteen subtypes of IFN-have been reported α in mice, and IFN-α4, IFN-α11, and IFN-α12 are reported to be highly bioactive (38). In this study, we observed elevated levels of IFN-α4 and IFN-α12 in macrophages following CCR2 inhibition.
The Anti-tumor effects of type I IFNs, including IFN-α, have been confirmed in several cancers (38)−(42), but they are not currently the standard therapy due to their adverse effects. However, in recent years, owing to advances in cancer immunotherapy, a combination of immunotherapy and type I IFNs has been explored (43)−(46).
Almost all cells in the body, including macrophages, are considered to produce type I IFN (42); therefore, it is unclear which cells in the TME primarily produce type I IFN. The present study indicates that Ccr2-mediated regulation of IFN-α in macrophages may play an important role in PDAC. CCR2 inhibition increases the production of IFN-α in macrophages, and the activated IFN-α signaling pathway in PDAC cells may induce apoptosis in PDAC cells.
IFN-α binds to its receptor and activates various downstream signals. Among these, there are many reports on the JAK-STAT pathway (40). IFN-α inhibits proliferation and angiogenesis in cancer cells (42). It has also been reported to induce apoptosis and inhibit cancer cell invasion by suppressing EMT induction (47), (48). In the present study, an increase in IFN-α production by macrophages and STAT1 expression in PDAC cells were observed in PKFC mice. Taken together, IFN-α may be involved in the underlying mechanisms of PDAC control by inducing apoptosis.
The combination of CCR2 inhibitors and FOLFIRINOX has been demonstrated in humans and mice, and it has been reported that in human PDAC, the combination with FOLFIRINOX suppressed the induction of monocytes from the bone marrow and reduced TAM infiltration into the TME (34). In an orthotopic transplantation mouse model, CCR2 inhibition also suppressed TAM infiltration; however, an increase in TAN infiltration has also been reported (35). In the present study, there was no change in the infiltration of neutrophils and macrophages into the TME, but CCR2 inhibition caused a shift from M2-like dominant to M1-like dominant change in the macrophage phenotype, which was considered important for increased apoptosis, suppression of vascular invasion, and prolonged survival observed in this study. There might be differences, especially in the immune microenvironment, between the transplantation and genetically engineered models. In contrast, in comparison to our previous study in which Cxcr2 heterozygous KO was performed in PKF mice, both observed increased apoptosis of cancer cells, suppression of cancer microinvasion, and prolonged survival of mice, as well as increased M1/M2 ratio of macrophages, while Cxcr2 heterozygous KO mice showed decreased neutrophil infiltration and increased macrophage infiltration into the TME (16). This may also be a difference between Ccr2 KO and Cxcr2 KO mice. It is also likely that CCR2 KO and CXCR2 KO macrophages differ in their altered gene expression profiles. The possibility that simultaneous knockout of CCR2 and CXCR2 may synergistically inhibit PDAC progression remains an issue for future investigation.
This study has some limitations. First, this study examined mouse PDAC, but not human PDAC, although the TME was intact compared to the transplantation models. In mouse pancreatic tumor tissues, we found Ccr2-expressing cells other than macrophages, and we were unable to examine the role of these cells in the PDAC TME. In this study, we observed significant changes in lymphoid cells reported to have CCR2, such as Tregs, although the number of infiltrated cells in the TME was very small. We observed that CXCLs produced by macrophages promoted PDAC cell invasion; however, other cytokines detected in the cytokine array might also contribute to PDAC cell invasion.
In conclusion, in the PDAC microenvironment, macrophages promote PDAC invasion in a CCR2-dependent manner partly by producing CXC chemokines. CCR2 inhibition changes the macrophage profile from M2-dominant to M1-dominant and increases IFN-α production, thereby inhibiting invasion and increasing apoptosis in PDAC, resulting in prolonged survival. Thus, CCR2 inhibition may be a potent treatment option for PDAC that contributes to prognosis.