Abstract
It is common knowledge that asbestos exposure causes asbestos-related diseases such as asbestosis, lung cancer and malignant mesothelioma (MM) not only in people who have handled asbestos in the work environment, but also in residents living near factories that handle asbestos. These facts have been an enormous medical and social problem in Japan since the summer of 2005. We focused on the immunological effects of asbestos and silica on the human immune system. In this brief review, we present immunological changes in patients with MM and outline their experimental detection. For example, there is over-expression of
Introduction
It is common knowledge that asbestos exposure causes asbestos-related diseases such as asbestosis, lung cancer and malignant mesothelioma (MM) not only in people who have handled asbestos in the work environment, but also in residents living near factories that handle asbestos. These facts have been an enormous medical and social problem in Japan since the summer of 2005 (Kanazawa et al. 2006; Murayama et al. 2006; Nakano, 2006). Several patients with MM living in Amagasaki, Hyogo prefecture, Japan have featured in news reports. These patients resided 1 km from an asbestos factory and had no identifiable occupational exposure to asbestos. Given that MM is an incurable disease and prognosis is not promising (Vogelzang and Pass, 2006; Zucali and Giaccone, 2006; Tsiouris and Walesby, 2007), and considering the absence of effective government legislation concerning the usage of asbestos, people in Japan have become concerned about social and medical issues related to asbestos.
Asbestos is categorized as a silicate (mineralogical complexes containing metals, such as iron and magnesium) and includes forms such as chrysotile, crocidolite, and amosite. Patients exposed to asbestos develop pulmonary fibrosis known as asbestosis, mesothelial plaque and malignant diseases such as lung cancer and MM (Niklinski et al. 2004; Becklake et al. 2007; O'Reilly et al. 2007). The mechanisms of asbestos-induced carcinogenesis are thought to produce an accumulation of DNA damage due to asbestos-induced production of reactive oxygen/nitrogen species (ROS/RNS) and an escape from the asbestos-induced activation of the mitochondrial apoptotic pathway (Shukla et al. 2003; Upadhyay and Kamp, 2003). In addition, we believe that some of these malignancies may be caused by a decline in tumor immunity owing to exposure of immunocompetent cells to asbestos.
Silica is known as one of the strongest environmental substances that cause autoimmunity dysfunction (Hess, 2002; Cooper and Parks, 2004). Silicosis patients often develop immunological complications such as rheumatic arthritis (known as Caplan syndrome (Caplan, 1953)), systemic sclerosis (SSc), and systemic lupus erythematoses (SLE). The effects of silica on autoimmunity have also been recognized following the discovery that patients who receive plastic surgery with implants containing silicone ([SiO2-O-]n) show frequent complications involving autoimmune disorders (Shons and Schubert, 1992; Hirmand et al. 1993). These findings clearly indicate that crystalline silica causes dysregulation and/or disturbance of the human immune system, particularly autoimmunity.
The overall evidence suggests that asbestos may influence human immunocompetent cells and that such alterations may affect the occurrence and progression of asbestos-related malignant diseases. Thus, we have focused on the immunological effects of asbestos. Among the many types of asbestos, chrysotile has mainly been used in our experiments. It is known that magnesium, the main compartment of chrysotile as silicate, usually dissociates from the chrysotile core (SiO2) in the human body after inhalation, and chrysotile is known to induce malignant transformation. However, its carcinogenic capacity is lower than that of other forms of iron-containing asbestos such as crocidolite and amosite (Harington, 1991).
In this article, we present immunological changes in MM patients with our experimental model. These changes may have resulted from the immunological effects of asbestos on human immunocompetent cells, and may offer some suggestions for the immunological prevention of the occurrence and progression of asbestos-induced malignant diseases.
bcl-2 expression of peripheral CD4+ T cells in MM patients
As shown in the upper panel of Figure 1-A, peripheral CD4+ T cells from MM patients showed a significantly higher expression of
Comparison of
Experiments that exposed a high dose of chrysotile to peripheral fresh T cells or T cell-derived cell lines for a short time revealed that a human T-cell leukemia virus type-1 (HTLV-1)-immortalized human polyclonal T-cell line, MT-2, underwent apoptosis with ROS production via activation of the mitochondrial apoptotic pathway with the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK) signaling molecules. In addition, we observed a shift of the Bax-dominant Bax/Bcl-2 balance, the release of cytochrome-c from mitochondria into the cytosol, and the activation of caspases 9 and 3 upon short-term, high-level exposure to chrysotile (Hyodoh et al. 2005). However, we thought that an
Thus, we established a chrysotile-induced apoptosis-resistant subline of MT-2 (MT-2Rst), and characterized the cell biological differences between the original MT-2 cell line (MT-2Org) and MT-2Rst. MT-2Rst cells were characterized by (i) an enhanced expression of
Another interesting finding was obtained from analyses using
Factor analysis of clinical parameters in mesothelioma patients with relative
IL-10 and TGF-β concentrations in MM patients and the experimental model
As shown in the upper panels of Figures 1-B and 1-C, plasma concentrations of IL-10 and transforming growth factor (TGF)-β were significantly higher in MM patients than in healthy volunteers. TGF-β is known as one of the mesothelioma cell-producing cytokines (Gerwin et al. 1987; Maeda et al. 1994). However, the above-mentioned MT-2Rst cells, representing the outcome of the experimental low-dose and long-term asbestos-exposure T-cell model, showed significantly higher secretion of TGF-β than MT-2Org cells (lower panel of Fig. 1-C). As we mentioned previously, the IL-10 concentration in culture supernatants of MT-2Rst was higher than that of MT-2Org (lower panel of Fig. 1-B). Thus, the source of the elevated TGF-β and IL-10 concentrations in MM patients is not only tumor cells, but also immunocompetent cells.
It is interesting to note that IL-10 and TGF-β are the important soluble factors necessary for the function of CD4+25+FoxP3+ regulatory T cells (Treg), even though cell-cell contact is the main route for the manifestation of Treg function (Wahl et al. 2004; Romagnasi, 2006). If circulating Treg and tumor-infiltrated Treg have enhanced function as a result of these elevated concentrations of IL-10 and TGF-β, further aggressive progression of asbestos-induced cancer cells may have occurred. It may be important to analyze the Treg function using the experimental model that we have developed.
T-cell receptor (TcR) Vβ expression
We reported previously that asbestos may act on peripheral T cells as a superantigen (Aikoh et al. 1998; Ueki, 2001). The effects of a superantigen such as staphylococcal enterotoxin B (SEB) may modify TcRVβ on peripheral T cells to enhance multiple, but not clonal, TcRVβ expression (Schubert, 2001; Li et al. 1999). As shown in Figure 2, various TcRVβs were over-expressed in MM and asbestosis patients. TcRVβs from most patients showed a higher expression, exceeding the average plus 2SD (standard deviation) limit. In addition, several TcRVβs such as VB 1, 4 and 9 among the 24 kinds of TcRVβ were strongly over-expressed in many patients. This phenomenon was also observed from the comparison of TcRVβ expression in MT-2Org and MT-2Rst cell lines. As a result, MT-2Rst cells over-expressed various TcRVβs. Although TcRVβ-overexpressing MT-2Org cells underwent apoptosis due to their first contact with chrysotile, MT-2Rst cells showed no significant changes when they again came in contact with chrysotile (Nishimura et al. 2006). These findings may suggest that the over-expression of various TcRVβs may be the result of contact between cells and chrysotile, an asbestos fiber, during the acquisition of resistance to CB-induced apoptosis caused by a long-term and low-dose exposure to CB (Nishimura et al. 2006).
The multiple over-expression of TcRVβ in CD3+ peripheral T cells derived from asbestos-exposed patients may be one of the candidates to detect previous asbestos exposure. Although these data were obtained from a limited number of patients, it is worth continuing these analyses using samples from many patients in an effort to explore the biological mechanisms involved in these findings.
Conclusion
A summarized schematic presentation of various aspects of this investigation is shown in Figure 3. This schema only shows the experimental and clinical findings related to asbestos exposure of T cells. We have been investigating the effects of asbestos on the function of natural killer (NK) cells from cellular and molecular viewpoints. TGF-β is similar to PDGF in that it is also known as a mesothelioma-producing growth factor (Gerwin et al. 1987; Maeda et al. 1994; Langerak et al. 1996; Klominek et al. 1998). Thus, the effects of TGF-β1 on asbestos-exposed MT-2Rst cells are being investigated and compared with MT-2Rst and MT-2Org cells that have not been exposed to TGF-β1. These examinations are on-going and will be presented in the near future.
Experimental findings of immunological effects of chrysotile, a form of asbestos, induced by long-term and low-dose exposure using MT-2, an HTLV-1 immortalized human polyclonal T-cell line.
Recent advances in immunomolecular studies have led to detailed analyses of the immunological effects of asbestos. Asbestos affects immunocompetent cells and these effects may be associated with the pathophysiological development of complications in asbestos-exposed patients such as malignant tumors. In addition, immunological analyses may lead to the discovery of new clinical tools for the modification of pathophysiological aspects of diseases, such as the regulation of tumor immunity using cell-mediated therapies, various cytokines and molecule-targeting therapies. As the incidence of asbestos-related malignancies increases against the growing concern in Japan since the summer of 2005 for medical and social problems created by such malignancies, efforts should be focused on developing a cure for these diseases in order to eliminate the nationwide anxiety concerning these malignancies.
Footnotes
Acknowledgements
The authors thank Ms. Misao Kuroki, Keiko Kimura, Tomoko Sueishi, Yoshiko Yamashita, Keiko Kimkura, Satomi Hatada, Yumika Isozaki and Haruko Sakaguchi for technical assistance.
The data obtained in the Department of Hygiene, Kawasaki Medical School and published by the authors were supported by Special Coordination Funds for Promoting Science and Technology (H18-1-3-3-1), JSPS KAKENHI (17790375, 18390186, 19689153, 19790431 and 19790411), Kawasaki Medical School Project Grants (16-212, 16-401N, 17-210S, 7-404M, 17-611O, 18-209T, 18-403 and 18-601), a Sumitomo Foundation Grant (053027), and a Yasuda Memorial Foundation Grant (H18).