1. Introduction
Bladder cancer (BCa) is the 9th most commonly diagnosed cancer and the 13th leading cause of cancer-related death worldwide [
1]. It originates from the urothelium of the ureter, urethra (upper urinary tract), and bladder [
1,
2]. Clinical management of BCa [
2,
3], etiology, and diagnostic, prognostic, or predictive biomarkers for BCa have been described extensively [
4,
5]. Treatment options are available for both superficial and invasive BCa; however, metastatic disease still presents a serious clinical problem with limited therapeutic options. Interestingly, BCa and breast cancer can be subdivided into basal and luminal subtypes that harbor prognostic and predictive relevance [
6,
7,
8,
9,
10,
11,
12,
13]. However, this classification is mostly based on RNA expression levels. Recently, Sjödahl et al. proposed an algorithm for immunohistochemical (IHC) subtyping of muscle-invasive bladder cancers [
14]. In addition, there are several classifications by the presence of immune cells. These classifications are the NLR (neutrophil-lymphocyte ratio), PLR (platelet-lymphocyte ratio), CAR (C-reactive protein/albumin ratio), and SII (systemic immune-inflammation index: platelet count × neutrophil count/lymphocyte count) [
15,
16,
17,
18]. Several inflammatory biomarkers and their association with BCa prognosis have been described (reviewed in [
19]). For example, the abundance of cytotoxic T lymphocyte markers, such as CD3 and CD8, was associated with improved survival, while the macrophage marker CD68 was associated with worse survival, recurrence, and progression in bladder cancer [
19,
20]. Recently, an association between immune cells, such as CD8 tumor-infiltrating lymphocytes, in the tumor microenvironment and the survival of BCa patients was further confirmed [
21,
22]. In addition, the molecular characteristics of immune cells were considered by an immune classification based on T cell infiltration, as in the TMIT classification (tumor microenvironment immune type: PD-L1 and CD8) or a classification published by our group with a unique immune evasion phenotype with constitutive overexpression of PD-L1 on tumor cells [
23,
24]. However, it would be helpful to identify a few or a single protein marker expressed in tumor cells and immune cells that is associated with prognosis in BCa.
An interesting candidate for marking both tumor cells and immune cells at the protein level is the chemokine CCL2 (chemokine, CC motif, ligand 2; synonym monocyte chemotactic protein 1 (MCP-1)). CCL2 production in tumors facilitates the accumulation of immune-suppressive tumor-promoting/tumor-associated macrophages, but CCL2 can also activate monocytes for tumor cell killing [
25]. The role of CCL2 in different tumors is controversial [
25,
26]. High levels of CCL2 have been associated with cancer progression in several types of cancer, including breast, prostate, colorectal, kidney, and thyroid cancers [
27,
28,
29,
30,
31,
32]. Recently, Chen et al. showed that high CCL2 mRNA expression in a TCGA cohort of bladder cancer patients was associated with poor OS and DFS [
33]. However, according to human protein atlas, CCL2 protein is not detected in nonmalignant urothelium. However, BCa patients with a higher tumor stage, higher tumor grade, or metastasis have a significantly higher CCL2 concentration in their urine than patients with a lower tumor stage, with a lower tumor grade, or without metastasis [
34]. On the other hand, in resected pancreatic cancer patients, CCL2 is an independent favorable prognostic factor with indirect effects on pancreatic cancer cell growth via cytokines, such as IL1B [
35]. Furthermore, in soft tissue sarcoma patients, high CCL2 mRNA expression is associated with a good prognosis [
36], and higher CCL2 expression was significantly associated with better OS in non-small-cell lung cancer (NSCLC) patients [
37]. In addition, a protective role of the CCR2/CCL2 chemokine pathway has been demonstrated by cytotoxic T lymphocytes’ migration toward melanoma cells that secrete CCL2, which results in tumor cell apoptosis [
38], and further by the recruitment of subsets of tumor-infiltrating lymphocytes (type 1 cytotoxic gamma-delta T lymphocytes) to tumor beds in a melanoma model [
39]. The aim of our study was to analyze for the first time the expression of CCL2 in BCa tumor cells and immune cells by immunohistochemistry and to correlate the results with clinicopathological and survival data to assess the impact of CCL2 as a prognostic factor in BCa.
3. Discussion
We analyzed, for the first time, the protein expression of CCL2 in TCs and ICs in BCa patients (N = 168) and assessed its association with clinicopathological and survival data. The expression of CCL2 in TCs was considered positive or negative, and expression in the ICs was considered as the percentage of CCL2-expressing cells of all ICs; this scoring system resulted in a negative group with ≤6% CCL2-expressing ICs and a positive group with >6% CCL2-expressing ICs. CCL2 staining in the TCs was not correlated with staining in the ICs. Remarkably, CCL2 staining in the TCs was positively correlated with the age at cystectomy and negatively correlated with OS and time to recurrence. In contrast, CCL2 staining of ICs was negatively correlated with tumor stage, lymph node stage, and molecular subtype and positively correlated with OS, DSS, RFS, and CK5 staining. In addition, as expected, CCL2 staining of the ICs was significantly positively correlated with the presence of different immune cells/cell markers, such as CD3 (T cells), CD8 (T cells), CD68 (macrophages), PD-L1 in TCs and ICs, and the percentage of stromal tumor-infiltrating lymphocytes (sTILs).
The role of CCL2 in cancers is still controversial [
25,
26]. Mostly, the detection of CCL2 in tumors is considered a marker for tumor progression/metastasis, e.g., in cancers of the breast, prostate, colon, and thyroid glands [
28,
29,
30,
31,
33]. Our finding of a correlation of CCL2 staining with lymph node status supports previous results, as CCL2 expression is correlated with lymph node metastasis in papillary thyroid carcinoma [
29]. On the other hand, CCL2 production confirmed in neoplastic ducts of the pancreas could be a relevant negative regulator of pancreatic cancer progression [
35]. Furthermore, serum CCL2 levels are positively correlated with tumor macrophage infiltration in pancreatic cancers [
35].
We showed for the first time that CCL2 expression in TCs is an independent negative predictor for OS in muscle invasive BCa patients. Comparably, patients with diffuse large B cell lymphoma (DLBCL) and a higher CCL2 expression had a shorter OS and progression-free survival [
40]. Breast cancer patients with high CCL2 expression in their tumor cells possessed a shorter RFS [
27], and for clear cell renal cell cancer patients, elevated CCL2 expression was correlated with clinical stage, OS, and macrophage infiltration [
32]. However, NSCLC patients with higher CCL2 expression showed a significantly better OS [
37]. Furthermore, in line with our findings, another CC chemokine ligand (CCL18) has been shown to play a critical role in the progression of bladder cancer [
41]. After stratifying our patients’ cohort further, we found that CCL2 positivity in TCs was an independent negative prognostic factor for OS for patients with pT2 stage, with chemotherapy treatment, and with the luminal subtype
Furthermore, we described for the first time that the expression of CCL2 in ICs is generally a positive independent prognostic factor for DSS in muscle invasive BCa patients. After molecular stratification, in the basal subtype, positive CCL2 staining in the ICs remained a positive independent prognostic factor for DSS. A role of CCL2 mRNA in the basal subtype has been described in a breast cancer mouse model recently; however, tumor-associated macrophages induce CCL2 expression in tumor cells [
42].
Remarkably, after stratifying the BCa patients according to their lymph node stage (N0 vs. N1+2), CCL2 staining in ICs gave contradictory results for prognosis. For N0 patients, CCL2 staining was a positive independent prognostic factor for OS, DSS, and RFS. However, for N1+2 patients, CCL2 staining was a negative independent prognostic factor for OS and RFS. Such a contradictory effect for CCL2 in ICs according to lymph node status has not yet been described in tumors. When testing whether sTILs, macrophages, and PD-L1 in ICs all show a similar behavior, we observed an association between sTILs, macrophages, and PD-L1 on ICs with a good prognosis in N0 patients, but we did not see an association between sTILs, macrophages or PD-L1 in ICs and prognosis in N1+2 patients. Our result that sTILs and PD-L1 expression are associated with a good prognosis in BCa is in accordance with the literature [
19,
20,
43]. The finding that macrophages are associated with a good prognosis in BCa patients is controversial in the literature [
19]. However, the results may depend on cut-offs and on the application of different antibodies and they are not easily comparable (Eckstein et al. 2019). In addition, the ICs expressing CCL2 may comprise different ICs [
25], e.g., macrophages have been classified as classical M1 (antitumor macrophages) and alternative M2 (protumor macrophages) polarized subtypes [
26]. Since CD68 is a pan-macrophage marker, a more M2-specific macrophage marker, i.e., CD163 can be applied [
44]. In a mouse tumor model from a breast metastasis cell line (MDA-MB-231), stimulation of CCL2 expression by M2 macrophages was recently shown [
42]. Recently, Xu et al. could show that increased levels of CCL2 polarize macrophages in multiple myelomas toward the M2-like phenotype that generally suppresses antitumor immunity [
45]. They suggest as a mechanism that CCL2 upregulates the expression of the immunosuppressive molecular MCP-1-induced protein (MCPIP1) in macrophages [
45]. However, we did not see differences in the number of M2-like macrophages (CD163 positive) between N0 and N1+2 patients. Possibly, CCL2 could rather affect macrophages qualitatively than quantitatively.
Our findings suggest that CCL2 could mark a population of ICs that may be antitumorigenic in N0 but protumorigenic in N1+2 muscle invasive BCa patients. This fits the general thinking that a tumor, such as bladder cancer, itself is a key immunological player that can shape immune responses to favor itself [
44]. Another aspect is that high-grade BCa tumors could recruit more mast cells, which are also ICs, to produce CCL2 via estrogen receptor β signaling [
46]. However, mast cells were not characterized in this study.
In addition, when comparing patients receiving chemotherapy or not, high CCL2 expression in ICs was associated with a better prognosis in patients without chemotherapy, but here no association of CCL2 with prognosis was observed as high CCL2 expression in TC was associated with a shorter OS and DSS in patients receiving a chemotherapy. Although, we cannot compare patients treated with chemotherapy with those not treated with chemotherapy since the tumor biology and prognosis are quite different, we saw no positive effect of the application of chemotherapy in the N0 patients, at least in our patient cohort. Recently, a study of muscle invasive BCa in Sweden showed that patients treated with radio/chemotherapy or radiotherapy had a better OS and DSS than untreated patients [
47]. However, treated patients were not further stratified by their lymph node status. Thus, we think that before suggesting chemotherapy to muscle invasive BCa patients, the lymph node stage should be considered. In addition, an effect of chemotherapy on the presence of ICs might also be considered. However, Waidhauser et al. showed that chemotherapy markedly reduces B cells but not T cells and natural killer cells in cancer patients [
48]. The future aim is to develop tumor therapies that kill cancer cells without harming the cells that constrain tumor growth, particularly T cells and other immune cells [
49]. A possible way could be the application of glutamine antagonists that affect tumor cells but not immune cells [
49,
50].
Our study has some limitations. This was a retrospective study, and for a comprehensive statistical analysis of three parameters in eight groups, the number of study patients was too low. In addition, only 27.4% (47/168) of our patients were treated with chemotherapy, which is again a rather small cohort. Therefore, our results must be evaluated in a larger prospective study. However, altogether, the number of study patients (n = 168) allowed reasonable multivariate analysis of the association of one parameter, such as CCL2 staining, with prognosis in muscle invasive BCa patients.
Altogether, CCL2 positivity in TCs is a negative prognostic factor for OS. For ICs, the prognosis depends on the lymph node status, i.e., CCL2 can mark ICs associated with good prognosis in N0 patients, whereas it marks ICs associated with poor prognosis in N1+2 patients. Altogether, CCL2 staining on TCs and on ICs is suggested as a prognostic biomarker for muscle invasive BCa patients.