Anti-tumor immunity depends mainly on the immune cells in vivo. Tumor tissues are often infiltrated by immune cells, mainly lymphocytes [5]. Previous studies [24] considered the hypothesis that lymphocytes attack tumor cells; however, tumor-infiltration lymphocytes have also been shown to promote tumor metastasis, resulting in poor prognosis of pancreatic cancer patients [25]. Different types of lymphocytes and different locations of tumor-infiltrating lymphocytes indicate the different survival outcome. Treg cells, an integral component of tumor-infiltrating lymphocytes, have been widely studied recently. Treg is thought to play an important role in maintaining immune balance and immune tolerance in the body [26].
FoxP3+, the most representative marker of Treg cells, plays a critical role in immune tolerance and the suppression of anti-tumor immunity [27–29]. However, some studies [30] have shown that activated T cells can express FoxP3+, and it was also thought to not be the determine marker of Treg cells. A majority of studies revealed that the level of FoxP3 + Treg cells had a negative impact on the prognosis of pancreatic cancer [31, 32]. However, other studies showed that Foxp3 + Treg cells also play a dual function of inhibiting or promoting in different tumors [33]. The accurate significance of prognosis is still unclear, and the information is imited. However, until now, there has been no meta-analysis of the prognostic significance of TILs in pancreatic cancer. Furthermore, no meta-analysis has been undertaken to evaluate FoxP3 + Treg cells as a prognostic marker in pancreatic cancer.
In our meta-analysis, 8 studies involving 972 patients were analyzed. The HRs for the association between intratumoral FoxP3 + Treg cell level and OS and the indicators of recurrence (DFS/PFS/RFS) are 2.13 and 1.70, respectively. Similarly, the HRs for the association between the peritumoral FoxP3 + Treg cell level and OS is 2.17. These pooled results showed that a high density of tumor-infiltrating FoxP3 + Treg cells are associated with poor survival and high recurrence, regardless of whether they are found in the intratumoral or peritumoral tissue of pancreatic cancer. The results are consistent with the initial hypothesis that FoxP3 + Treg cells inhibit anti-tumor immunity. Tumor-infiltrating lymphocytes in our meta-analysis refer to the lymphocytes infiltrating the tumor bed but not peripheral circulating blood. This is more likely to reflect the tumor's immune microenvironment and the interaction of immune cells. These results are potentially important for the prognosis and treatment of pancreatic cancer.
The prognosis of patients with pancreatic cancer is very poor. The NEJM [34] and other magazines have reported that the monoclonal antibody drug blocking immunoassay (such as PD-1 and PD-L1) is effective for treating non-small cell lung cancer, malignant melanoma, renal cell carcinoma, ovarian cancer, stomach cancer, and breast cancer. However, the effect of the treatment effect in pancreatic cancer was not reported. This may be because there was little or no infiltration of cytotoxic T cells in pancreatic cancer tissue.
One of the possible reasons for this finding is that FoxP3 + Treg cells suppressed the function of cytotoxic T cells to destroy the function of anti-tumor, which resulted in the escape of immunological surveillance [35]. It is reported FoxP3 + Treg cells secrete TGF-β, which means that the suppression of anti-tumor immunity of FoxP3 + Treg cells may be cytokine-dependent [36]. FoxP3 + Treg cells also secreted IL-10 to suppress Th1/2 cell proliferation and down-regulating MHC class II in monocytes [37]. However, some studies have shown that FoxP3 + Treg down-modulate immune function by generates adenosine. These mechanisms may account for the poor survival outcomes in pancreatic cancer patients with high expression of FoxP3 + Treg cells.
The initial view that FoxP3 + Treg cells always suppress tumor immunity was challenged in the case of gastrointestinal tumors. The discrepancy in some studies may result from different research methods or the biological characteristics of specific tumor types. We need to better understand the function of FoxP3 + Treg cells and their different biological characteristics in the tumor. Our study elucidates the effect of FoxP3 + Treg on patient prognosis in the pancreatic cancer tumor microenvironment. Meta-analysis is useful to integrate the results from all single studies for an uncertain outcome.
However, due to the limitations of our study, we should be careful when dealing with these results. There are several limitations that need to be considered. First, this study was constrained to studies published in the English language, and we may be missing studies published in other languages. Second, the patients from the 8 studies included in our analysis did not necessarily have consistent treatments and the category of TNM and histologic types of pancreatic cancer varied. Fortunately, the sensitivity analysis showed that individual studies had little effect on the overall outcome. Third, although the data indicate that there is no significant publication bias in the studies that used OS as a survival outcome, the potential publication bias was unavoidable and some data could still be missing. Fourth, experimental reagents, immunohistochemical scoring strategies, and truncated values were not exactly the same in all 8 studies. Finally, the HR and 95% CIs extracted from survival curves but directly obtained from the articles may be less reliable.