A causal association between exposure to at least some types of asbestos and lung carcinomas and malignant pleural mesothelioma (MPM) has been long recognized , and in 2012 the WHO/International Agency for Research on Cancer (IARC, Lyon) evaluated all forms of asbestos (chrysotile, crocidolite, amosite, tremolite, actinolite, and anthophyllite) as carcinogenic to humans . The 2014 updated Helsinki Criteria notes that while the use of asbestos is banned in many industrialized countries, the global production of asbestos remains at over two million metric tons a year, with an estimated 125 million people being exposed to asbestos in the workplace . Furthermore, workers engaged in cleaning debris at sites of natural disasters and workers involved in demolition work may be exposed to asbestos. For example, asbestos-related disease is predicted to be significant in workers engaged in debris cleaning operations after the Great Hanshin Earthquake that occurred in Japan in 1995. Worldwide, asbestos exposure results in an estimated 255,000 deaths annually, with a significant fraction (over 30,000 in 2016) of these deaths due to mesothelioma . In Japan, the number of patients that die of MPM is currently 1,500 a year (Vital Statistics, Ministry of Health Labour and Welfare, Japan, 2015) and the incidence of MPM is predicted to remain relatively high in the coming years due to past exposure to asbestos.
Macrophages are essential constituents of solid tumors [5, 6], and mesotheliomas are heavily infiltrated by macrophages [7-10]. Macrophages have the capability to support the transformation of a normal cell into an initiated preneoplastic cell and to promote malignant progression [5, 6, 11-17]. Macrophages have a continuum of functional phenotypes, ranging from macrophages with the functions of classically activated M1 macrophages, such as phagocytosis and destruction of invading pathogens and promotion of inflammation, activities that can result in tissue damage, to macrophages with functions such as resolution of inflammation and protection and repair of tissues, functions commonly referred to as M2-like functions [18-21]. Macrophages with M1-like functions can target and kill transformed cells, and a high M1/M2 macrophage ratio has been found to be beneficial in patients with non-small cell lung carcinoma . In general, while the subtypes of macrophages within a tumor is heterogeneous , tumor development is associated with the presence of macrophages with M2-like characteristics, particularly in patients with a poor prognosis [8, 23-25]. One of the basic functions of M2-like macrophages that is associated with tissue protection and repair is immunosuppression , and tumors have generally been found to contain macrophages with immunosuppressive characteristics [5, 11, 12, 14, 26].
Another important myeloid cell population that is associated with tumors are myeloid-derived suppressor cells. Immature myeloid cells are released from the bone marrow during the process of myelopoiesis. Usually, these cells migrate to peripheral organs and differentiate into dendritic cells, monocytes, or granulocytes . During pregnancy and in inflamed tissues, immature myeloid cells can acquire an immunosuppressive phenotype, whereupon they are referred to as myeloid-derived suppressor cells (MDSCs), and protect the fetus/tissue from damage by the host immune system [28-35]. MDSCs can differentiate into immunosuppressive macrophages , possibly into immunosuppressive neutrophils [36-39], or they can retain their immunosuppressive capability without further differentiation; whether these so-called undifferentiated MDSCs are actually immature cells or terminally differentiated cells with immunosuppressive properties is not certain . Regardless of their state of differentiation, when these cells accumulate at sites of inflammation, they exert an immunosuppressive effect. Inflammation is a hallmark of cancer , and when MDSCs accumulate in the inflammatory environment associated with tumorigenesis they protect the tumor from attack by the host immune system [27, 41-44]. Determination of the exact role of MDSCs in tumorigenesis is complicated by the lack of specific markers that unequivocally differentiate between MDSCs and myeloid cells without an immunosuppressive phenotype [45-47]. This makes functional suppression assays essential for identification and characterization of MDSCs, however, these assays can be difficult or impossible to carry out, especially in tissue samples. Consequently, the parameters used to identify MDSCs oftentimes differ between studies. Nevertheless, there is almost universal agreement that accumulation of myeloid cells with MDSC-like phenotypes in the blood or tumor correlates with disease progression, poor prognosis, poor response to therapy, and decreased overall survival [41, 48-53]. MDSCs are associated with tumor progression in mouse models of mesothelioma [54-56], and MDSCs are believed to be associated with mesotheliomas in human patients [57, 58].
C-C motif chemokine ligand 2 (CCL2), also known as monocyte chemotactic protein-1 (MCP-1), is expressed in most human cancers [59-61], and plays a key role in the recruitment of macrophages and MDSCs [59, 60, 62-64]. In the tumor, CCL2 is also expressed by tumor associated macrophages [60, 65]. Several studies have reported that increased expression of CCL2 in tumor tissue is associated with advanced tumor stage and worse prognosis: These studies include patients with breast cancer [66-69], prostate cancer [70, 71], gastric cancer , colorectal cancer [73, 74], esophageal squamous cell carcinoma , head and neck squamous cell carcinoma , and glial tumors . However, there are also reports that increased expression of CCL2 in tumor tissue is associated with better prognosis: These studies include patients with gastric cancer , colorectal cancer , liver cancer , and non-small cell lung cancer .
In general agreement with the findings that tumors accumulate macrophages and MDSCs that have pro-tumorigenic properties and express CCL2 and that CCL2 expression in tumor tissue is associated with advanced tumor stage and worse prognosis, there are several studies that report elevated levels of CCL2 in the serum of cancer patients and/or an association between elevated serum CCL2 and poor prognosis: Moogooei et al.  and Pan et al.  report elevated levels of serum CCL2 in patients with glial tumors and lung cancer. Lu et al.  and Sharma et al.  report an association between elevated serum CCL2 levels and poor prognosis in patients with prostate cancer, and Lu et al.  report an association between elevated serum CCL2 levels and poor prognosis in patients with nasopharyngeal cancer. Cai et al. , Wang et al. , Wu et al. , Lubowicka et al. , and Hefler et al.  report elevated levels of serum CCL2 in patients with lung, liver, gastric, breast, and ovarian cancer and that increased serum CCL2 was associated with poor prognosis. Lebrecht et al.  did not find a difference in serum CCL2 levels between breast cancer patients and normal donors, but they did find an association between serum CCL2 and poor prognosis.
The findings of several other studies, however, differ from those noted above. Tas et al. , Tsaur et al. , and Monti et al.  found elevated serum CCL2 levels in patients with gastric, prostate, and pancreas cancer. However, Tas et al. report that while gastric cancer patients who responded to chemotherapy had lower serum CCL2 than non-responders, there was no association between serum CCL2 and any measured clinical variables; Tsuar et al. report that elevated serum CCL2 was negatively correlated with PSA value in prostate cancer patients; and Monti et al. report that elevated serum CCL2 was associated with increased survival in pancreas cancer patients. Sullivan et al.  report that there was no difference in serum CCL2 levels between pancreas cancer patients and normal donors and that serum CCL2 did not correlate with any measured clinico-pathological parameters. Koper et al. , Ding et al. , and Tonouchi et al. report that serum CCL2 levels were decreased in patients with astrocytic brain tumors, oral squamous cell carcinoma, and gastric cancer, and Tonouchi et al. report CCL2 levels tended to decrease in accordance with disease progression and that decreased serum CCL2 levels was associated with poor survival. Dehqanzada et al.  report that elevated serum CCL2 levels correlated with favorable prognostic variables in patients with breast cancer, and Farren et al.  report that elevated serum CCL2 levels correlated with increased survival in pancreas cancer patients.
Whether the disparate findings of the studies cited above are due to differences in tumor stage, CCL2 being associated with a tumorigenic response in some cases and to a tumoricidal response in others, differing immune suppression mechanisms in different tumor types or the patient cohorts studied, or to some other factor is not known. It is clear, however, that the role of CCL2 in tumorigenesis is likely to be affected by tumor-specific factors. The current study was undertaken to investigate serum CCL2 levels in mesothelioma patients. We found that serum CCL2 levels were increased in mesothelioma patients and that this increase was dependent on advancing mesothelioma stage.