Introduction
Pancreatic ductal adenocarcinoma (PDAC) is a lethal disease with a devastating 5-year overall survival of only ~ 7%. Although just 4% of all malignant diseases is accounted to PDAC, it will become the second leading cause of cancer-related deaths before 2030 [
1]. A major cause for the aggressiveness and dismal prognosis of this malignant disease is the abundance of immune suppressive mechanisms displayed by PDAC, which is thought to support tumor growth and promote metastasis. The former perception of PDAC as a poorly immunogenic tumor has now been replaced by the notion of an inflamed tumor that evades the immune control by exerting immunosuppressive mechanisms in its microenvironment [
2]. For this reason, kinetics of blood-based markers might be used to track the footprint that is left by the inflammatory response in patients with PDAC that reflects the current state of the tumor in terms of tumor growth and progression.
Interleukin-33 (IL-33) is a pro-inflammatory cytokine and it is involved in the development of chronic inflammation and cancer [
3‐
6]. It is negatively regulated by several mechanisms and sequestration by the “decoy” receptor soluble suppression of tumorigenicity 2 (sST2) is likely to be crucial in this process [
7]. IL-33 is upregulated in PDAC and nonmalignant cells in the tumor microenvironment of PDAC and lesions from chronic pancreatitis: pancreatic stellate cells and pancreatic myofibroblasts have been identified as important sources of this cytokine [
8‐
10].
Therefore, we assessed the plasma levels of IL-33 and sST2 by blood sampling in patients with PDAC before start of their systemic cytotoxic therapy to determine its role as a prognostic marker in PDAC.
Discussion
This is the first study that evaluated circulating IL-33 and sST2 as a potential prognostic marker in patients with advanced PDAC.
There is increasing evidence that the IL-33/sST2 axis plays a crucial role in tumorigenesis and tumor progression in various malignancies such as colon, head and neck, breast, gastric, ovarian, lung, renal and pancreas [
10,
17‐
25]. IL-33 has diverse context-dependent functions but originally it has been found that it mediates its biological effects via IL-1 family receptor transmembrane ST2 (ST2L) and activates NF-kB and MAP kinases. Its administration leads to significant increases in gene expression of prominent T helper (Th)
2-associated cytokines IL-4, IL-5, and IL-13 in vivo in mice [
26]. In PDAC, there is a strong imbalance between Th
1 and Th
2 response toward a Th
2-type response and tumor infiltration with Th
2-lymphocytes correlates with an increased expression pattern of Th
2-related cytokines in the blood of PDAC patients [
27,
28]. A predominant Th
2-infiltrate in the tumor is an independent predictive marker of poor survival in PDAC patients [
29]. To date, all studies except of one have linked increased serum levels of IL-33 and sST2 in cancer patients to negative prognosis [
30‐
36]. For example, decreased survival has been demonstrated in non-small cell lung cancer patients with high serum levels of IL-33 and in hepatocellular carcinoma patients with high serum levels of sST2 [
37,
38]. Our results support that high plasma levels of sST2 are also associated with inferior survival of PDAC patients undergoing systemic chemotherapy.
Regarding our results for IL-33, we found a significant negative effect of low IL-33 plasma levels on survival; however, in 35% of all patients, the measured IL-33 levels where below the detectable limit of the assay. Therefore, more caution to the interpretation of linking IL-33 plasma levels to survival has to be paid. If the patients are split up into two groups (IL-33 plasma levels < 0.001 vs. ≥ 0.001 pg/ml), we observed a statistically significant negative effect on survival in the univariate as well as in the multivariable treatment adjusted model. However, in the same model, there was no statistically significant difference in survival in patients with IL-33 plasma levels over the detectable limit in relation to the continuous levels (HR 0.93, 95% CI 0.76–1.18,
p = 0.54) neither in the univariate nor in the multivariable model. Therefore, we found no statistical evidence suggesting that IL-33 plasma levels over the detectable limit have any effect on the survival in our patients. It must also be mentioned, that currently available IL-33 ELISA assays still lack the necessary reliability that would be needed for valid survival analysis [
39]. In connection to that, we also just found a moderate negative correlation of IL-33 and sST2 levels. Recent studies have described pancreatic stellate cells and pancreatic myofibroblasts as sources of IL-33 production [
8,
9]. As an intracellular cytokine, IL-33 is released after cell or tissue damage [
40]. When released, IL-33 could enhance the expression of chemokines and cytokines in pancreatic myofibroblasts and stimulate the proliferation and migration of these cells, which further contributes to the pathogenesis of pancreatic desmoplasia, a hallmark of PDAC [
8,
41,
42]. However, some studies also propose a possible protective role of the IL33/sST2 axis in melanoma, breast, colorectal, prostate and hepatocellular cancer tumor models by stimulating antitumor immunity [
43‐
48]. Therefore, further work is required to clarify the role of IL-33 in cancer.
The greatest limitations of our study are the small sample size, its retrospective character, and the current lack of a reliable IL-33 ELISA assay. If IL-33 is to be studied in the serum of patients, a more sensitive and specific assay method will be required, which is vital for further understanding and targeting of the IL-33/IL-1RL1 axis in human disease. This issue has been addressed in a recent study comparing currently available IL-33 ELISA assays [
39]. With a larger patient cohort and a more reliable IL-33 assay, we would also not only expect to clarify the prognostic relevance of IL-33 plasma levels in PDAC patients but we could also divide patients into four different groups according to IL-33 and sST2 levels (IL-33
low/sST2
low, IL-33
high/sST2
low, IL-33
low/sST2
high, IL-33
high/sST2
high) to further elucidate the role of the relation between IL-33 and sST2 in malignant disease. However, it has to be highlighted that our data demonstrates a clear clue for the prognostic potential of sST2 levels for mortality prediction in our cohort and the statistical significance of the observed effect is also maintained in the treatment adjusted multivariable model.
In conclusion, we here report the first study that measured circulating IL-33 and sST2 in advanced PDAC patients that underwent systemic chemotherapy. Our results suggest a negative impact of high sST2 plasma levels on survival.