**5. Findings in MT-2CEs regarding antitumor immunity**

The purpose of establishing a cell line model involving continuous and relatively low-dose exposure of human T cells to asbestos fibers was to investigate cellular and molecular alterations that may reflect the immune function in human populations exposed to asbestos as well as patients exhibiting PP or MM.

A consideration of the development of cancer in asbestos-exposed patients suggested that focus within investigations should be placed on antitumor immunity.

Our investigations showed that the CXC chemokine receptor 3 (CXCR3) exhibited reduced expression in MT-2 CEs compared to MT-2Org [38, 39]. CXCR3 is known to play an important role in antitumor immunity because it summons antitumor T cells with IFN-γ. As shown in the cell line model, investigation of freshly isolated CD4-positive T cells revealed decreased expression of CXCR3 when activated *in vitro* with CH fibers. Furthermore, peripheral blood CD4-positive cells from patients with PP or MM showed a reduction of CXCR3 and an inhibited potential for IFN-γ production when stimulated *in vitro* [31, 38, 39]*.* These investigations indicated that the cell line model for continuous and low-dose exposure to asbestos using the MT-2 cell line was suitable for analysis of immune alteration in asbestos-exposed human populations and patients with PP or MM [31, 38, 39].

As the MT-2 cell line was known to possess a Treg function [40, 41], the Treg function was estimated in MT-2Org and MT-2 CE. In regard to cell-cell contact, MT-2 CE enhanced its suppressive function onto Tresp cells [42]. In addition, MT-2 CE produced higher TGF-β and IL-10 in comparison to MT-2Org as described earlier. These two cytokines are the typical soluble factors for Treg in order to manifest its function. Following the knockdown of each cytokine in MT-2 CE, the suppressive function was reduced relative to that in MT-2 CE [42]. These results indicated that asbestos exposure enhanced Treg function by cell-cell contact and an increase of soluble factors [42]. In addition, FoxO1 reduced its expression in MT-2 CE as described earlier, and is known to regulate the cell cycle to suppress the accelerating genes, such as cyclins, as well as to enhance the breaking genes, such as cyclin-dependent kinase inhibitors (CDK-Is) [43]. As a consequence, cyclins were enhanced and the expression of CDK-I2 was reduced in MT-2 CE because of the reduced expression of FoxO1. Cell cycle progression was, therefore, enhanced in MT-2 CE [43]. These overall results suggest that Treg volume may also be enhanced in asbestos-exposed human populations and patients exhibiting PP or MM [42, 43]. These findings indicated that asbestos exposure causes reduction of antitumor immunity.
