In this study, we have focused on bladder-tumor-cell surface antigens (MHC Class II, CD1, CD80 and ICAM-1) that are considered important for antigen presentation. MHC Class II antigens serve as restriction elements for cells that present antigens to CD4+ helper T cells, and it has been reported that intravesical BCG treatment induces the expression of MHC Class II on normal urothelium and bladder tumor cells in patients.8–12
The CD1 family are class I-like antigen presentation molecules, and it is reported that glycolipids of mycobacteria, including BCG, serve as their ligands and activate NKT cells and may have an important role in the antitumor effect.13
Considered most important for T cell activation as a major ligand of CD28 is CD80 (B7-1), which is a major costimulatory molecule.14 ICAM-1 is known to play an important role in the immune response,15,16 and it was reported that either IFN-γ or TNF-α, cytokines induced by BCG, enhanced the expression of ICAM-1 on transitional cell carcinoma.17 An important role of ICAM-1 in the immune response against bladder cancer after intravesical BCG therapy was also suggested.18
In the bladder tumor cell lines used in this study, the expression of all these molecules which characterize APC were augmented by BCG treatment, although augmentations of CD1b and CD80 of J82 were not quite as strong. We report here, for the first time, that the expressions of MHC Class II, CD1, CD80 and ICAM-1 were augmented directly by BCG in vitro, not via the host immune mechanisms. When lymphocytes from BCG-immunized mice (L(BCG)) were cocultured with murine bladder tumor cells treated with BCG (MBT-2(BCG)), IL-2 and IFN-γ were produced in the culture supernatant, but lymphocytes from SRC-immunized mice (L(SRC)) did not produce the cytokines under the same conditions. These cytokine productions were abolished by the pretreatment of MBT-2(BCG) with anti-IAk mAb, thus suggesting that the lymphocytes recognized the BCG antigen presented with MHC Class II on MBT-2(BCG). Taken together, these results indicate that BCG-treated bladder tumor cells, by internalization of BCG, can function as APC. The ineffectiveness of killed BCG seems to imply that a prolonged supply of secretory antigens to the immune system by live BCG is crucial for the expression of antitumor activity, as in the case of protective immunity against tuberculosis. Recently, cells other than professional phagocytes, such as Langerhans cells,19 keratinocytes,20 B cells,21 T cells,22 endothelial cells23 and epithelial cells24,25 were also shown to have APC functions, so it seems natural that this is also the case in MBT-2, T-24 and J82 used in the present study. As for bladder tumor cells, Lattime et al. (1992) observed MHC Class II-restricted, BCG-specific antigen presentation in murine bladder carcinoma cell MB49. However, in their experiment, the expression of MHC Class II(IAb) on the tumor cell surface was induced by the IFN-γ treatment.26 We found that neither IFN-γ nor tumor necrosis factor (TNF)-α were produced when MBT-2 and BCG were cocultured (data not shown), so it is conceivable that the augmentation of surface molecule expression observed in our study is the consequence of the direct effect of (internalized) BCG alone. In vivo, however, this expression may be further augmented by IFN-γ or other cytokines produced by the host immune system. We also investigated the subsets of murine draining lymph node cells immunized with BCG and found that CD4+, CD8+, IAK+, NK1.1+ and NK+ subsets exist, but the production of IL-2 and IFN-γ was not induced by each subset alone when cocultured with MBT-2(BCG) in preliminary experiments (data not shown). Further study about this is required. Recently, it was reported that the transfection of the CD80 gene into tumor cells made the tumor cells recognizable by existing CTL.27 The induction of CD80 expression on the surface of tumor cells by BCG treatment might render the tumor cells more susceptible to the cytotoxicity by CTL and this might be a part of the mechanism of the antitumor effect of BCG. The elucidation of the nature of putative effector cell needs further investigation.