The immune system may facilitate the growth of some tumors. Several studies have suggested a role for tumor-associated DCs or macrophages as a driver of growth in breast cancer, lymphoma and myeloma.11, 18
As malignant myeloma cells are B cells or plasma cells, we thought that Th2 cells probably promote myeloma growth. Prior studies reported enhanced myeloma growth after immunotherapy with tumor Ag-pulsed DCs.3, 11
However, our in vitro
data indicated that Th2 cells do not promote proliferation and clonogenicity of myeloma cells directly. We think that the absence of MHC class II molecules and/or other molecules on myeloma cell membrane may account for this limitation. Promotion of tumor growth Th2 cells should require myeloma cell expression of MHC class II molecules, costimulatory molecules B7 (CD80, CD86), CD40, adhesion molecules (CD54) and so on, which are required in Th2 cell-induced proliferation of B cells.19, 20, 21, 22, 23, 24, 25, 26
Our studies showed that myeloma cells express membrane markers of B cells including high levels of CD86 and CD54 and detectable levels of HLA-DR, CD40 and CD80, which can be upregulated by IFN-γ. These data are consistent with the findings of prior studies.17
Surprisingly, we found that microbial Ags, such as BCGV, TTA and so on, could induce expression of MHC-II and CD40 on the myeloma cell surface. In our studies, BCGV-specific Th2 cells induced by BCGV-loaded-DCs significantly enhanced the clonogenicity of myeloma cells induced to express MHC-II, CD40 and CD80 by IFN-γ and BCGV. These data suggest that a small proportion of malignant plasma cells retain B-cell functions, may possess stem cell characteristics, and must interact with Th2 cells to form colonies. This malignant plasma cell subpopulation may account for the development and recrudescence of malignant plasma cell tumors.
In our studies, blockade of T-cell receptor (TCR)–MHC-II interaction by anti-MHC-II MoAb or blockade of the CD40L–CD40 interaction by anti-CD40 MoAb inhibited Th2 cell-mediated enhancement of the clonogenicity of human myeloma cells, suggesting that the Th2 cell–myeloma cell interaction is like the Th2 cell–B-cell interaction in that CD40L ligands and TCR on the Th2-cell surface recognize the specific Ag peptide borne by the MHC-II on the myeloma cell. Prior studies have confirmed that myeloma cells can function as APCs and can present Ag peptide to CD4+
T cells, which are MHC class-II restricted.17
In our studies, promotion of tumor colony formation by Th2 cells only occurred if TCR recognized the specific Ag peptide bound to the MHC-II of the tumor cell. The fact that anti-MHC-II MoAb abolished the enhancement of tumor clonogenicity but anti-CD40 MoAb merely reduced the enhancement indicates that tumor colony growth promotion by Th2 cells is mainly MHC class-II restricted. However, our data do not exclude the possibility that other molecules such as adhesion molecules CD54 (CD54–CD11 a interaction) or costimulatory molecules B7 (B7–CD28 interaction) on myeloma cells may also be important in Th2 cell–myeloma interactions. Th2-mediated regulation of myeloma growth may also subvert Th1-cell-mediated immunity and require the participation of Th2-cell-derived cytokines, such as IL-4, IL-6 and IL-10.
In most cases, myeloma cells, like U266 cells, express almost no Bcl-6.11, 27, 28, 29
However, in our study, Bcl-6 was highly expressed in cells from Th2 cell-induced MM colonies. Normally, Bcl-6 contributes to the definition of the germinal center phenotype by repressing multiple DNA damage and cell proliferation checkpoint genes.30, 31, 32
The level of Bcl-6 is downregulated during the differentiation of B cells from memory B cells and plasmablastic cells.33
The Bcl-6-mediated loss of checkpoint function may result in a potentially hazardous state of physiological genomic instability, which may cause B cells to undergo malignant transformation.34, 35, 36
These findings are consistent with the findings of a previous study that showed the reactivation of the B-cell program after Bcl-6 was exogenously expressed in myeloma cell lines.37
So, it is concluded that upregulation of Bcl-6 in myeloma cells may protect the cells from apoptosis and idiovariability and thereby facilitate the formation of malignant clones. We further observed that anti-CD40 reduced but failed to abolish the upregulation of Bcl-6. Therefore, other pathways besides the CD40 signaling pathway should exist to promote Bcl-6 expression in Th2 cell-induced tumor colonies. In our study, Bcl-6 expression was upregulated and the percentages of HLA-DR+
cells and CD40+
cells were higher in Th2 cell-induced tumor colonies, suggesting that the differentiation state of myeloma cells is plastic and can be modified by Th2 cell–myeloma cell interaction.
Microbial Ags are regarded as a potential cause of some tumors. Prior studies have argued that carcinogenesis results from indirect or direct interaction of inflammatory cells and mediators with epithelial cells, stromal cells and extracellular matrix components, which in turn stimulate angiogenesis.38, 39, 40
Nevertheless, the direct effects of microbial Ags on tumor cells have not been well studied. Unlike previous studies, our study showed that microbial Ags, such as BCGV and TTA, enhance myeloma clonogenicity, not just the level of humoral immunity.12, 17, 41, 42
To our knowledge, our findings provide the first evidence that microbial Ags presented by APCs (DCs and myeloma cells) to Th2 cells cause malignant plasma cell disease and participate in MM pathogenesis. In our studies, we analyzed whether allogeneic Th2 cells or autogeneic Th2 cells could drive myeloma cell colony formation, indicating that Th2 cells may be one of the risk factors for patients with MM. The result is consistent with previous reports that early stage and non-advanced MM patients were with Th1 high proportion, whereas advanced MM patients were with a predominated Th2-like response in PBMC after stimulation by monoclonal IgG or microbial Ags.43, 44, 45
So, microbial Ags-Th2 cells may deeply involve in pathogenesy, recurrence and development of MM. However, our findings do not exclude other mechanisms of MM pathogenesis. In conclusion, our study has presented evidence for a novel mechanism of MM pathogenesis (that is, tumor generation from a small number of MHC-II+
malignant plasma cells with B cell and stem cell characteristics that can form colonies efficiently upon Th2-cell induction). Specifically, when the MHC-II+
malignant plasma cells take up microbial Ag they act as APCs. DCs presenting the same microbial Ag activate Th2 cells to recognize malignant plasma cells and induce them to proliferate and form colonies. Thus, every factor involved in Ag presentation, such as microbial Ags, DCs, Th2 cells and so on, may be potential targets to be further studied for therapeutic intervention.